High lights
- The different immune states in COVID-19 are summarized.
- The possible pathophysiological mechanisms of different immune states are analyzed.
- A theoretical basis for the selection of immunotherapy strategies based on different immune states is provided.
INTRODUCTION
At the end of 2019, Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-Cov-2) was found to be responsible for a cluster of patients with unknown pneumonia named subsequently as coronavirus disease 2019 (COVID-19) (1). On March 11, 2020, COVID-19 was declared a worldwide pandemic and soon swept over 235 countries, areas, and territories with more than 41 million confirmed cases up to October 22, 2020 (2, 3).
Similar to the severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV) and the Middle East respiratory syndrome (MERS) coronavirus (4–6), SARS-CoV-2 is a highly pathogenic β-coronavirus virus in humans, with strong infectivity among individuals (1, 7, 8). SARS-CoV-2 and SARS-CoV have been shown to share the same binding receptor on host cells, which has been identified as angiotensin-converting enzyme-2 (ACE2) (9). Subsequent studies have revealed that SARS-CoV-2 cell entry depends on combination of the viral spike (S) protein with the ACE2 receptor on host cells, achieved with the assistance of the cellular serine protease transmembrane serine protease 2 (9–11). The exact further mechanisms by which SARS-CoV-2 invades host cells and causes a pathological state are still under investigation.
Patients with COVID-19 are characterized by fever and respiratory system manifestations including cough, dyspnea, and typical chest radiographs with pulmonary ground-glass opacification (12–14). Severe patients often experience respiratory failure, coagulation dysfunction, shock, and multiple organ dysfunction (15–17). In addition, a decrease in lymphocytes has been observed in the laboratory, suggesting an immunosuppression state (14–16, 18). On the other hand, the elevated level of inflammatory cytokines observed among patients with COVID-19 indicates a hyperinflammation state (16, 18). These studies demonstrate a paradoxical immunity status that complicates treatment of COVID-19 and causes a delay in recovery. Additionally, the relevant immunopathological mechanisms have yet not been completely determined, and effective immunological treatments for COVID-19 are still being explored. As a result, discussing the specific immunity status of COVID-19 patients and exploring proper treatments under various immune states is of great significance.
Critically ill patients with COVID-19 first go through a rapid period of inflammation in response to infection, followed by a period of coexistence of hyperinflammation and immunosuppression that eventually leads to multiple organ failures and even death. In this review, all available information about immune states of COVID-19 in severe patients and the underlying pathophysiological mechanisms will be presented and discussed. Moreover, we also look at the association between immune states and the development of disease as well as the impact on the selection of immunotherapy strategies.
IMMUNE STATES OF PATIENTS WITH COVID-19
A status of immunosuppression
Lymphocyte decrease and depletion
In the course of virus infection, both the innate and adaptive immune systems cooperatively take part in the antiviral response (19). As a vital part of the adaptive immune system, lymphocytes play an important role in clearance of the invading virus and provide effective protection of human health under the virus threat. Studies have shown that lymphopenia is a common feature in patients with COVID-19, with significant decreases in CD4+ T cells, CD8+ T cells, and B cells (14, 16, 18, 20, 21). Total T cell, CD8+ T cell, or CD4+ T cell counts have been found to be negatively correlated with patient survival (20). A similar conclusion was confirmed in another recent study demonstrating a positive correlation between lower counts of T lymphocyte subsets and in-hospital death from COVID-19 (22). An analysis of lymphocyte subgroups in hospitalized patients with COVID-19 indicated that SARS-CoV-2 primarily affects T lymphocytes, including helper T cells (CD3+ CD4+ cells), cytotoxic suppressor T cells (CD3+ CD8+ cells), and regulatory T cells (18). In severe cases, helper T cells and regulatory T cells have a more distinct decrease, along with an increase in the naive-to-memory T cell ratio (18). Differentiation of naive T cells into effector and memory T cells is regarded as the foundation of T cell-regulated immunity, and the balance between naive T cells and memory T cells has a profound impact on effective induction of the adaptive immune response (23, 24). Furthermore, CD4+ T cell counts have shown the most significant decrease in patients with severe disease compared with patients with mild symptoms (18). Another study demonstrated that a significant decrease in CD8+ T cells occurred in critically ill patients (25). Generally, both CD4+ T cells and CD8+ T cells are significantly reduced in severe cases, and larger studies are still needed to clarify which type of T cell SARS-CoV-2 mainly acts on. Moreover, interferon-&gamma (IFN-γ) secreted by CD4+ T cells, CD8+ T cells, and Natural Killer cells (NK cells) was expressed at lower levels in severe patients, suggesting a state of immunosuppression in patients with severe COVID-19 (26).
To investigate T cell dysfunction in the course of COVID-19, studies have also examined the exhaustion marker programmed death 1 (PD-1), an immune inhibitor responsible for delivering inhibitory signals after ligation with PD-1 ligands on activated T cells (27). The results demonstrated noticeably higher PD-1 expression on CD8+ and CD4+ T cells in COVID-19 cases, especially in critically ill cases, compared with healthy controls, indicating the presence of T cell dysfunction and exhaustion in COVID-19 (20).
A status of hyperinflammation
Neutrophilia
Neutrophils, a type of polymorphonuclear leukocyte, are typically the first to be recruited and to engage in the acute inflammation caused by invading pathogens (28, 29). Apart from lymphopenia, studies have also shown an increase in neutrophils in patients with COVID-19 (18, 25). Neutrophil infiltration was observed in autopsy samples from patients with COVID-19 (30–32). Furthermore, neutrophilia has been confirmed to be associated with poor outcomes of patients with COVID-19 (12). The neutrophil-to-lymphocyte ratio (NLR) index is an inflammation biomarker of infection and has been verified to be of great significance in bacterial pneumonia and sepsis (33, 34). Relatively, the NLR is also suggested to be an independent risk factor for severe patients and the most powerful prognostic factor for COVID-19, with the ability to predict COVID-19 severity and outcome (25, 35–37). A previous study demonstrated that neutrophils play a key role in the release of inflammatory mediators and the regulation of cytokines in patients with influenza, suggesting that a significant increase in neutrophil number is closely related to the occurrence of cytokine storms (38). Correspondingly, an elevation in neutrophils and white blood cells is thought to enhance cytokine release and promote cytokine storms in patients with COVID-19 (12).
Cytokine elevation
To protect the body from an invading virus, immune cells are recruited and release cytokines that activate more immune cells through positive feedback loops, thereby inducing the release of more pro-inflammatory cytokines (39, 40). Remarkably cytokine production has been observed in response to SARS-CoV-2 infection, including IL-1β, IL-1RA, IL-2, IL-6, IL-8, monocyte chemotactic protein-1 (MCP-1), granulocyte-colony stimulating factor, and TNF-α (16, 18). Particularly, plasma concentrations of IL-2, IL-6, IL-8, and TNF-α have been found to be significantly elevated in patients with severe disease in comparison with patients with mild or moderate disease (18, 20). Cytokine levels are found positively correlated with disease severity and are supposed to be useful predictors for COVID-19 severity (16, 21, 41). A recent study also reported that an increased serum IL-6 level is closely correlated with a larger area of pulmonary infiltration in computed tomography scan images (42). In addition, during SARS-CoV-2 infection, severe cases are usually characterized by the inevitable development of a cytokine storm into acute respiratory distress syndrome (ARDS) (16, 43), and respiratory failure due to ARDS is considered the leading cause of death from COVID-19 (44). Innate immunity is known to be on the front line of defense against virus invasion and to restrain viral replication, but sometimes, it may turn out to be more harmful than helpful.
Endothelial dysfunction and altered coagulation
ACE2 receptor is reported to be widely expressed on endothelial cells of multiple organs throughout the body (45). Viral inclusion structures found in endothelial cells and accumulation of inflammatory cells around endothelium of deceased patients with COVID-19 revealed that SARS-CoV-2 could directly infect endothelial cells and promote the occurrence of endotheliitis (46). One typical characteristic of COVID-19 is the strong activation of coagulation system, manifested by a significant increase in D-dimer and the development of disseminated intravascular coagulation (DIC) (47, 48). This altered coagulation biology is thought to be associated with endothelial dysfunction. In response to infection, inflammatory cytokines facilitate blood clotting by damaging the endothelial cell-mediated anticoagulative pathways including protein C system and tissue factor pathway inhibitor (49). IL-1β and TNF accelerate the initiation of coagulation by promoting the expression of P-selectin, von Willebrand factor, and fibrinogen by endothelial cells (50). In addition, it has been reported in SARS infection that autoantibodies against endothelial cells could mediate complement-dependent cytotoxicity (51). This development also occurs in COVID-19 and conduces to endothelial dysfunction and abnormal activation of coagulation (52). Studies proved that disruption of endothelial barrier integrity and strongly activated coagulation contribute a lot to the development and aggravation of ARDS in COVID-19 (46, 53).
MECHANISM OF IMMUNE DISORDERS
Mechanisms of immunosuppression
Up to now, the cause and underlying mechanisms of lymphopenia are still under study. It might result from a direct viral attack of SARS-CoV-2 or be a consequence of redistribution by chemokines and apoptosis. Through quantitative transcriptome analysis of bronchoalveolar lavage fluid and peripheral blood mononuclear cells from patients with COVID-19, one study has shown that genes associated with apoptosis are significantly increased, including Cathepsin L, Cathepsin B, DNA damage inducible transcript 4, RRAS, Cathepsin D, Baculoviral IAP repeat containing 5, tumor protein P53 inducible protein 3, and so on (54). In addition, as an important part of the P53 signaling pathway and the apoptosis process, the P53 gene has been found to be overexpressed in COVID-19 patients (54). These results indicate that the occurrence of lymphopenia might be caused by apoptosis. Although ACE2 is absent in lymphocytes, evidence from a recent study demonstrated that SARS-CoV-2 could contaminate T cells through combination of S protein with an unknown receptor on T cells, suggesting that the apoptosis of T cells may be due to direct attack by SAR-CoV-2 (9, 54–56) (Fig. 1). Moreover, TNF-α has been shown to be able to promote T cell apoptosis (57, 58), and elevated Il-2 levels in local environments with high antigen level can mediate T cell apoptosis by promoting Fas signaling (59–61) (Fig. 1). Therefore, increased cytokine levels might mediate T cell apoptosis in COVID-19, which may also explain the negative correlation between cytokine levels and lymphocyte counts discovered in other studies (20, 25, 62, 63). Furthermore, as demonstrated in previous studies on SARS-CoV, an increase in cytokines such as IL-6 and IL-8 tends to weaken the ability of T cells to activate dendritic cells, resulting in an inability of the immune system to accomplish an effective adaptive immune response (64).
;)
Fig. 1: Possible mechanisms of immune disorders. Lymphopenia is thought to result from depletion of lymphocytes mediated by direct viral attack, elevated cytokines, and MDSCs. The occurrence of cytokine storm might be caused by several factors associated with overactivated innate immune response and suppressive adaptive response. ADE indicates antibody-dependent enhancement; IL-2, interleukin-2; MDSC, myeloid-derived suppressor cell; NK cell, natural killer cell; TNF-α, tumor necrosis factor-α.
In addition, studies showed that emergency myelopoiesis might also be involved in the mechanisms of lymphopenia and immunosuppression in COVID-19. The emergency myelopoiesis is characterized by release of plenty of immature and dysfunctional myeloid cells which are termed myeloid-derived suppressor cells (MDSC) (65, 66). The onset of emergency myelopoiesis is thought to be primarily caused by prolonged and persistent inflammatory stimuli including granulocyte-macrophage colony stimulating factor, S100A8, S100A9, IL-1β, IL-4, IL-6, TNF-α, etc (67, 68). Generally, MDSCs are detected and discussed in cancer and recently in some infectious diseases, found to be associated with the immunosuppression of T cells through inhibition of T cell proliferation and promotion of lymphocytic apoptosis (69–75). These immunosuppressive activities of MDSC are reported to be carried out by the close cell–cell contact with T cells as well as increased expression of Arginase 1 and inducible nitric oxide synthase (76–80). Similarly, emergency myelopoiesis has also been observed in COVID-19 presented as an increase in immature neutrophils expressing PD-L1, indicating a potential immunosuppressive effect of MDSC on lymphocytes (81–87). It is important to note that this pathological process occurs only in severe patients with COVID-19; therefore, the long duration of inflammatory stimuli in severe patients contributes a lot to the occurrence of emergency myelopoiesis, leading to a state of immunosuppression.
Mechanisms of hyperinflammation
A significant increase in neutrophils was observed at illness onset in severe COVID-19 patients compared with those with mild illness, which indicates an early activated innate immune response to SARS-CoV-2 and explains the increased cytokine levels in the early phase of disease (25).
However, neutrophil counts are back to normal within a few days, whereas cytokines are continuously elevated throughout the disease (25). There could be several reasons for the continuous inflammatory response in the late stage of the disease. Studies have shown that in severe cases of COVID-19, CD4+ T cells differentiate into pathogenic type 1 T helper cells, which can generate cytokines, such as granulocyte-macrophage colony stimulating factor, followed by activated CD14+CD16+ monocytes that highly express IL-6 and promote systemic inflammation (88). Another study confirmed a higher percentage of CD14+ monocytes accompanied by amplified expression of proinflammatory cytokines such as IL-1β in patients in the early stage of COVID-19 recovery (89). The overactivated inflammatory responses conducted by pathogenic T helper cells and CD14+ monocytes could result in immune impairment with pathological lung injury and high mortality (88). Consistent with this, postmortem studies have confirmed that the exudative cells in the alveolar cavity mainly comprised monocytes and macrophages (31). On the other hand, previous studies have shown that the presence of T cells can inhibit inflammatory cytokine overexpression by blocking activation of antigen-presenting cells, such as NK cells, and therefore, a lack of normal functional T cells could lead to an overreaction of the innate immune system in response to viral stimulation, which would present as a cytokine storm (62, 63). As stated previously, T cell apoptosis might be the predominant contributor to lymphopenia in COVID-19. However, lymphocyte apoptosis could also release cytokines in bulk. Different from cytokines involved in the innate immune response to virus infection, apoptosis-associated cytokines are always characterized as IL-1β and IL-18, which can enhance the inflammatory response through mediation of immune cell maturation and pro-inflammatory signaling, such as the NF-κB signaling pathway (90–93). Furthermore, increases in White blood cells and neutrophils are observed in some cases at the end stage of COVID-19 infection, which might indicate a secondary infection caused by microbes and contribute to aggravated systemic inflammation and mortality (12, 25). Clinical trials have reported that secondary infection can occur in 5% to 27% of patients with COVID-19, 50% to 100% of whom died (47, 94, 95). Most secondary infections are caused by fungi or bacteria and are characterized by a significant increase in White blood cells, which is not very common in COVID-19, whereas either bacterial or viral infection can lead to cytokine storms (96, 97).
Some scientists have considered the cytokine storms observed in COVID-19 patients as macrophage activation syndrome (MAS) or secondary hemophagocytic lymphohistocytosis (sHLH) (44). sHLH is primarily triggered by viral infection and is an underrecognized hyperinflammation syndrome characterized by explosive inflammatory responses and multiple organ failure (98). Common features shared by COVID-19 and sHLH include fever, hyperferritinemia, excessive inflammation, and primary pulmonary involvement (14, 98–100). Markedly elevated C-reactive protein and cytokines like IL-6, TNF are reported in COVID-19, which resembles MAS (101, 102). As a result, studies have speculated whether the inflammatory responses in COVID-19 are actually caused by overactivation of macrophages mediated by antibody-dependent enhancement (ADE), especially in the lungs (103). When antibodies produced by immune cells cannot fully neutralize the virus, virus–antibody complexes can bind to Fc receptors or other receptors on host cells and assist virus invasion, which is called antibody-dependent enhancement (104). Binding of the complex to macrophages can promote pro-inflammatory polarization of macrophages, contributing to the production and release of cytokines and resulting in tissue damage (103). ADE-mediated hyperinflammation through skewing of macrophages has been confirmed in SARS, indicating that ADE might play the same role in COVID-19 (105).
In short, the bulk production of cytokines and neutrophilia in the early phase of COVID-19 is thought to be associated with an activated innate immune response directly due to viral infection, while a continuous hyperinflammation status in the late stage of disease might be caused by several possible mechanisms, including pathogenic activation of immune cells, suppression of adaptive immune response, T cell apoptosis, secondary infection, and the occurrence of ADE. (Fig. 1) To further understand the underlying mechanisms of hyperinflammation in COVID-19, more investigations are required.
DEVELOPMENT AND SEVERITY OF COVID-19
The increase in neutrophil and cytokine levels might reflect the intensity of the inflammatory response, whereas the reduction in lymphocytes might reflect the disruption of the immune system caused by viral infection (25).
As stated above, neutrophil levels increase early in the course of COVID-19, accompanied by increased cytokine levels (25). A previous study showed that a change in serum IL-6 levels is an antecedent to falling CD4+ and CD8+ T cell counts and can be considered an early biomarker of COVID-19, indicating that robust cytokine production precedes the occurrence of lymphocyte apoptosis (42). In addition, the expression of the exhaustion marker PD-1 and of T cell Ig- and mucin-domain-containing molecule-3 (TIM-3) was found to be extremely low in the prodromal stage of COVID-19 and to increase progressively in the overtly systemic and ICU-hospitalized stages, suggesting a developing immunosuppression status in patients with COVID-19 (20).
Consequently, in the early phase of COVID-19, the innate immune system is rapidly activated and produces large amounts of cytokines, followed by the release of MDSC. Subsequently, inhibition of T cell proliferation and massive apoptosis of lymphocytes occur, leading to an immunosuppression state in the late stage of disease. As a result of the several underlying mechanisms stated above, the innate immune system remains overactivated, with uncontrolled production and release of cytokines, causing extraordinary inflammatory damage and resulting in multiple organ failure (Fig. 2).
Fig. 2: Development of COVID-19 associated with immune states. Patients with COVID-19 experience a rapid immune response to viral infection with elevated cytokines and end up with a coexistence of immunosuppression and hyperinflammation. COVID-19 indicates coronavirus disease 2019.
Given the immune disorders presented in sepsis patients, the immunopathological characteristics between sepsis and COVID-19 can be compared. Effective immune responses to both infection types are activated in the early stages of disease, manifested as elevated pro-inflammatory cytokine levels (106). Resembling COVID-19, hyperinflammation and immunosuppression are thought to coexist in the late stage of sepsis (107). Endothelial dysfunction and excessively activated coagulation as a result of inflammation are observed in sepsis and culminate in the occurrence of DIC (49). In addition, lymphocyte depletion and MDSCs release are both detected in sepsis, contributing to immune suppression (107, 108). Therefore, sepsis and severe COVID-19 share similar immunologic features. What's more, the clinical manifestations of COVID-19 coincide with the diagnosis of sepsis, which is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection according to the Sepsis-3 International Consensus (109, 110). Thus we could also refer to COVID-19 as viral sepsis.
The disease severity of COVID-19 predominantly depends on viral load and host immunity, which is closely associated with age, gender and pre-existing comorbidities. Studies have found a positive correlation between detectable serum SARS-CoV-2 RNA (RNAaemia) and high levels of IL-6, which portends a poor prognosis (111). In addition, the SARS-CoV-2 viral load detected in the respiratory tract of patients is closely correlated with cytokine levels and NTR, effectively predicting the severity of disease (112, 113). Some studies have found that males account for more of the severe cases than females (12, 16, 114, 115). The reduced susceptibility of women to viral infection can be attributed to sex hormones, which play an important role in body defenses conducted by the innate and adaptive immune systems (116, 117). In addition, ACE2 expression and distribution have been found to be more widespread in male patients, which might also explain the difference in susceptibility between sexes (118). Elderly patients and patients with preexisting disease are more likely to develop severe cases and suffer from a significantly higher mortality (12, 16, 113). Male sex, advanced age, and accompanying hypertension have been found to be correlated with prolonged viral RNA shedding. Moreover, T cell levels are lower in elderly patients with COVID-19 in an age-dependent manner, suggesting increased susceptibility and a weaker adaptive immune response to SARS-CoV-2 in the elderly (20, 119).
On the whole, in the course of COVID-19, patients with a weak immune response or infected with higher viral loads may be unable to efficiently remove the virus and are more likely to progress to aggravated lymphopenia and sustained inflammation. (Fig. 2)
IMMUNOTHERAPY
Immunoenhancement therapy
Convalescent plasma
Convalescent plasma (CP), recognized as a passive immunotherapeutic strategy, contains plenty of antibodies, ant-inflammatory cytokines, and other beneficial molecules. CP therapy is thought to ameliorate infectious disease through virus neutralization and regulation of the inflammatory response (120). Recognized risks of CP therapy include inadvertent infection and possible reactions to plasma components (121). ADE is regarded as a potential risk of CP therapy since some antibodies could facilitate virus entry and enhance viral infection through secondary combination with Fc fragment of immunoglobulin G receptors on immune cells (122–124). Data have shown that CP administration can reduce the mortality of SARS and influenza compared with placebo or no therapy (125, 126). Improved clinical outcomes after CP therapy have also been recently observed in patients with COVID-19, along with increased lymphocyte counts and reduced viral load (127, 128). Despite the small sample sizes, these positive results suggest the potential therapeutic effect and safety of CP therapy in treatment of COVID-19. Further multicentral studies with more patients are still required to determine the specific role of CP therapy in COVID-19.
Intravenous immunoglobulin (IVIg)
Intravenous immunoglobulin is a blood product consisting of polyclonal immunoglobulin G derived from the plasma of healthy donors. IVIg can provide passive immune protection against many pathogens and is mainly used for severe infections, especially patients with primary or secondary immunosuppression (129, 130). A previous study confirmed that IVIg can effectively neutralize pathogens and improve host defense during infection (131). Recent studies evaluated the efficacy of intravenous immunoglobulin and showed that the administration of IVIg in the treatment of critically ill patients with COVID-19 can significantly reduce the mortality and time of hospitalization, returning temperatures back to normal and alleviating breathing difficulties (132, 133). Although IVIg treatment is expensive, it is a recommended therapeutic option for early serum conversion.
Immunosuppressive therapy
Corticosteroid
Corticosteroids are widely used to treat severe community-acquired pneumonia and sepsis due to the immunosuppressive effects of corticosteroids on dysfunctional systemic inflammation (134, 135). However, the effects of corticosteroids in SARS-CoV-2 infection remain controversial. Studies on SARS and MERS indicated that systemic corticosteroid therapy could account for delayed viral clearance and an increase in plasma viral load (136, 137). Studies on COVID-19 showed similar findings (119). Moreover, corticosteroid treatment tends to aggravate lung injury and showed no survival advantage even with the increased mortality of COVID-19 (138–140). On the other hand, some studies revealed that steroids may play a beneficial role in severe COVID-19. A small clinical trial has reported decreased risk of admission to ICU or requirement of noninvasive ventilation or death in severe patients treated with methylprednisolone (141). Afterward, results from a large-scale randomized controlled trial demonstrate that administration of dexamethasone is associated with lower 28-day mortality among severe patients with COVID-19 (142). Similar results were observed by a meta-analysis conducted by the World Health Organization whereas a recent randomized trial showed no significant difference between severe patients treated with low-dose hydrocortisone and placebo (143, 144). Consequently, further researches are still needed to clarify the effects of steroids.
Cytokine inhibitors
As significant increases in cytokines have been observed in COVID-19, cytokine antagonists may play a positive role in the treatment of the disease. Tocilizumab, an IL-6 inhibitor, has been found to improve clinical outcomes and reduce mortality in severe and critically ill patients with COVID-19, with no adverse reactions observed (145). Similar results were shown in an another study, indicating that tocilizumab could be a safe option for patients with severe COVID-19 (146). Subsequently, tocilizumab was included in the guidelines for COVID-19 treatments in China and Italy (147, 148). In the Chinese guidelines, treatment with tocilizumab can be attempted for severe patients with extensive and bilateral lung disease and increased IL-6 levels (147). In the Italian guidelines, tocilizumab is only allowed at the end of the initial phase for severe patients with a high viral load, rapid deterioration of respiratory exchange, and high levels of Il-6 or D-dimer/C-reactive protein/ferritin/fibrinogen (148). However, a recent study showed no significant differences in ICU admission or mortality rate between the tocilizumab group and SOC (a combination of hydroxychloroquine, azithromycin and a prophylactic dose of low-weight heparin) group (149). Therefore, more data are still needed to evaluate the therapeutic effects of tocilizumab in COVID-19 and to further identify the optimal dose and timing of tocilizumab treatment.
Anakinra, a well-known IL-1 receptor antagonist, has been demonstrated to improve the survival of septic patients with MAS while showing no obvious adverse events (150). Similarly, in recent studies of COVID-19, rapidly resolved systemic inflammation and highly improved respiratory functions have been observed in patients treated with anakinra, especially in patients with sHLH, suggesting the safety and possible therapeutic effect of anakinra (151, 152). However, additional randomized controlled clinical trials are indispensable to verify the treatment effect and determine adverse reactions.
JAK inhibitors
Baricitinib, a selective JAK inhibitor, is now widely approved for inflammatory and autoimmune diseases as a result of its effects on restraining systemic inflammatory responses and alleviating cytokine storms by hindering the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway, which is associated with several cytokine signals (153, 154). In addition, by inhibiting viral entry and impairing viral endocytosis, baricitinib has been found to have potential antiviral bioactivity (155, 156). Therefore, baricitinib might be a therapeutic option for COVID-19. Results from a recent study showed that patients treated with baricitinib achieved significant improvements in clinical parameters and reduced ICU admissions, with no adverse events (157). More clinical trials of baricitinib are in progress with the aim of clarifying its therapeutic effects.
Mesenchymal stem cells (MSCs)
MSCs are hemopoietic cells derived from certain tissues, such as bone marrow, adipose tissue, the lungs, and placenta. MSCs act as immunoregulators by moderating immune responses through direct interactions with a series of immune cells, including lymphocytes, natural killer cells, dendritic cells, macrophages, and neutrophils (158). MSCs contribute to restricting immune cell activity and elevating expression of anti-inflammatory factors (159). MSCs have been approved for the treatment of autoimmune disease and ARDS and have been shown to effectively control inflammation (160, 161). Therefore, MSCs may also be a good therapeutic option for treatments of COVID-19. A recent pilot study of MSC transplantation indicated that MSC treatments can significantly improve the clinical outcomes of patients with COVID-19 (162). Considering that high anti-inflammatory factor expression has been detected in MSCs, remarkably decreased levels of pro-inflammatory cytokines and increases in lymphocytes have been observed in patients after treatment with MSCs (162). All these results suggest that MSCs might have powerful therapeutic effects in COVID-19. Furthermore, the absence of ACE2 and transmembrane serine protease 2 on MSCs confirmed through RNA-seq analysis assures the safety and effectiveness of MSC therapy for SARS-CoV-2 infection (162).
SELECTION OF IMMUNOTHERAPY STRATEGIES
With the number of patients rapidly increasing, unprecedented pressure is placed on the healthcare system. Because there is currently no vaccine or specifically effective drug against SARS-CoV-2, timely and efficient therapeutic strategies to avoid the deterioration of disease condition have become a top priority.
Based on the theoretical assumptions and assessments presented above, we recommend early use of immunoenhancement therapy, such as CP and IVIg treatments, with COVID-19 patients at illness onset, with the purpose of assisting the immune system in timely removal of the virus and mediating immune responses. Observational studies on other infectious diseases have identified the early therapeutic effects of CP, whereas CP therapy administered in mid-stage or late-stage infection showed no benefits (163, 164) (Fig. 3).
Fig. 3: Selection of immunotherapy strategies. Immunoenhancement therapy such as CP and IVIg is recommended for early stage of disease to assist in viral clearance. And immunosuppressive therapy is recommended around the 7th day after illness onset. Treatments with MSCs and IVIg can be good options for patients in late stage of COVID-19, considering their immunoregulatory effects. ARDS indicates acute respiratory distress syndrome; COVID-19, coronavirus disease 2019; CP, convalescent plasma; IL-1β, interleukin-1β; IL-6, interleukin-6; IVIg, intravenous immunoglobulin; JAK inhibitor, Janus kinase inhibitor; MODS, Multiple Organ Dysfunction Syndrome; MSC, mesenchymal stem cell; SIRS, Systematic Inflammation Response Syndrome; TCZ, tocilizumab; TNF-α, tumor necrosis factor-α.
Conversely, due to possible effects of immunosuppressive therapy on the host virus clearance, premature use of cytokine inhibitors and JAK inhibitors is considered inadvisable, with the possibility of aggravating immune system impairment and increasing the risk of infections that lead to deterioration of disease condition. Thus, the time for immunosuppressive therapy should be at the end of the early stage, when the viral load has possibly decreased and patients are in a state of excessive inflammation. Accordingly, SARS-CoV-2 persists in patients for at least 10 days after illness onset, indicating that the optimal time for immunosuppressive treatment might be approximately 7 days after appearance of the initial symptoms (165, 166). The use of immunosuppressive therapy at this time point can help intervene early in the inflammatory responses and prevent lethal, irreversible, immune-mediated injuries, and the development of ARDS or multiple organ failure (Fig. 3).
Because MSCs have the potential to elevate lymphocytes while suppressing inflammation and IVIg may be able to reduce inflammatory responses, MSCs and IVIg can also be used in the later stage of COVID-19, when immunosuppression and hyperinflammation coexist (162, 167) (Fig. 3). As for the use of steroids, the optimal dose and duration of treatment still remain controversial because two recent randomized controlled trials come to opposite results (141, 144). More trials are needed to estimate the effect of steroids.
Recommended immunotherapy strategies are shown in Figure 3. Our recommendations are provided in general, and the status of each individual should be assessed carefully before administering immunotherapy.
CONCLUSION
In severe patients with COVID-19, innate immune responses are rapidly initiated on the front line of resistance to viral infection, manifested by increase and recruitment of neutrophils, massive production of inflammatory cytokines, endothelial injury, and aberrant coagulation. Attributed to the persistent viral attack and sustained excessive inflammation, the adaptive immune responses are impaired presented as lymphocyte apoptosis and dysfunction. In the struggle against SARS-CoV-2, the immune system fails to reach homeostasis and comes to a state in which hyperinflammation and immunosuppression coexist. As for the selection of immunotherapy, early use of immunoenhancement therapy including CP, IVIg treatments are thought to be beneficial whereas immunosuppressive therapy like cytokine inhibitors is supposed to be applied during the window phase of viral load reduction.
Acknowledgments
The authors acknowledge all participants involved in this work.
REFERENCES
1. Zhu N, Zhang D, Wang W, Li X, Yang B, Song J, Zhao X, Huang B, Shi W, Lu R, et al. A Novel Coronavirus from Patients with Pneumonia in China, 2019.
N Engl J Med 382 (8):727–733, 2020.
2. World Health Organization: Coronavirus disease (COVID-19) outbreak situation. Available at:
https://www.who.int/emergencies/diseases/novel-coronavirus-2019. Accessed October 22, 2020.
3. WHO Director-General's opening remarks at the media briefing on COVID-19-11 March 2020. Available at:
https://www.who.int/dg/speeches/detail/who-director-general-s-opening-remarks-at-the-media-briefing-on-covid-19---11-march-2020. Accessed October 22, 2020.
4. Ksiazek TG, Erdman D, Goldsmith CS, Zaki SR, Peret T, Emery S, Tong S, Urbani C, Comer JA, Lim W, et al. A novel coronavirus associated with severe acute respiratory syndrome.
N Engl J Med 348 (20):1953–1966, 2003.
5. Drosten C, Gunther S, Preiser W, van der Werf S, Brodt HR, Becker S, Rabenau H, Panning M, Kolesnikova L, Fouchier RA, et al. Identification of a novel coronavirus in patients with severe acute respiratory syndrome.
N Engl J Med 348 (20):1967–1976, 2003.
6. Zaki AM, van Boheemen S, Bestebroer TM, Osterhaus AD, Fouchier RA. Isolation of a novel coronavirus from a man with pneumonia in Saudi Arabia.
N Engl J Med 367 (19):1814–1820, 2012.
7. Chan JF, Yuan S, Kok KH, To KK, Chu H, Yang J, Xing F, Liu J, Yip CC, Poon RW, et al. A familial cluster of pneumonia associated with the 2019 novel coronavirus indicating person-to-person transmission: a study of a family cluster.
Lancet 395 (10223):514–523, 2020.
8. Li Q, Guan X, Wu P, Wang X, Zhou L, Tong Y, Ren R, Leung KSM, Lau EHY, Wong JY, et al. Early transmission dynamics in Wuhan, China, of novel coronavirus-infected pneumonia.
N Engl J Med 382 (13):1199–1207, 2020.
9. Zhou P, Yang XL, Wang XG, Hu B, Zhang L, Zhang W, Si HR, Zhu Y, Li B, Huang CL, et al. A pneumonia outbreak associated with a new coronavirus of probable bat origin.
Nature 579 (7798):270–273, 2020.
10. Hoffmann M, Kleine-Weber H, Schroeder S, Kruger N, Herrler T, Erichsen S, Schiergens TS, Herrler G, Wu NH, Nitsche A, et al. SARS-CoV-2 cell entry depends on ACE2 and TMPRSS2 and is blocked by a clinically proven protease inhibitor.
Cell 181 (2):271.e8–280.e8, 2020.
11. Wrapp D, Wang N, Corbett KS, Goldsmith JA, Hsieh CL, Abiona O, Graham BS, McLellan JS. Cryo-EM structure of the 2019-nCoV spike in the prefusion conformation.
Science 367 (6483):1260–1263, 2020.
12. Wang D, Hu B, Hu C, Zhu F, Liu X, Zhang J, Wang B, Xiang H, Cheng Z, Xiong Y, et al. Clinical characteristics of 138 hospitalized patients with 2019 novel coronavirus-infected pneumonia in Wuhan, China.
JAMA 323 (11):1061–1069, 2020.
13. Pan Y, Guan H, Zhou S, Wang Y, Li Q, Zhu T, Hu Q, Xia L. Initial CT findings and temporal changes in patients with the novel coronavirus pneumonia (2019-nCoV): a study of 63 patients in Wuhan, China.
Eur Radiol 30 (6):3306–3309, 2020.
14. Guan WJ, Ni ZY, Hu Y, Liang WH, Ou CQ, He JX, Liu L, Shan H, Lei CL, Hui DSC, et al. Clinical characteristics of Coronavirus Disease 2019 in China.
N Engl J Med 382 (18):1708–1720, 2020.
15. Richardson S, Hirsch JS, Narasimhan M, Crawford JM, McGinn T, Davidson KW. the Northwell C-RC. Presenting characteristics, comorbidities, and outcomes among 5700 patients hospitalized with COVID-19 in the New York City Area.
JAMA 323 (20):2052–2059, 2020.
16. Huang C, Wang Y, Li X, Ren L, Zhao J, Hu Y, Zhang L, Fan G, Xu J, Gu X, et al. Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China.
Lancet 395 (10223):497–506, 2020.
17. Bhatraju PK, Ghassemieh BJ, Nichols M, Kim R, Jerome KR, Nalla AK, Greninger AL, Pipavath S, Wurfel MM, Evans L, et al. Covid-19 in critically ill patients in the Seattle region—case series.
N Engl J Med 382 (21):2012–2022, 2020.
18. Qin C, Zhou L, Hu Z, Zhang S, Yang S, Tao Y, Xie C, Ma K, Shang K, Wang W, et al. Dysregulation of immune response in patients with coronavirus 2019 (COVID-19) in Wuhan, China.
Clin Infect Dis 71 (15):762–768, 2020.
19. Zinkernagel RM. Immunology taught by viruses.
Science 271 (5246):173–178, 1996.
20. Diao B, Wang C, Tan Y, Chen X, Liu Y, Ning L, Chen L, Li M, Liu Y, Wang G, et al. Reduction and functional exhaustion of T cells in patients with coronavirus disease 2019 (COVID-19).
Front Immunol 11 (827):827, 2020.
21. Wan S, Yi Q, Fan S, Lv J, Zhang X, Guo L, Lang C, Xiao Q, Xiao K, Yi Z, et al. Characteristics of lymphocyte subsets and cytokines in peripheral blood of 123 hospitalized patients with 2019 novel coronavirus pneumonia (NCP).
medRxiv 2020; 2020.02.10.20021832.
22. Xu B, Fan CY, Wang AL, Zou YL, Yu YH, He C, Xia WG, Zhang JX, Miao Q. Suppressed T cell-mediated immunity in patients with COVID-19: a clinical retrospective study in Wuhan, China.
J Infect 81 (1):e51–e60, 2020.
23. Moro-Garcia MA, Alonso-Arias R, Lopez-Larrea C. When aging reaches CD4+ T-cells: phenotypic and functional changes.
Front Immunol 4 (107):107, 2013.
24. Sallusto F, Lenig D, Forster R, Lipp M, Lanzavecchia A. Two subsets of memory T lymphocytes with distinct homing potentials and effector functions.
Nature 401 (6754):708–712, 1999.
25. Liu J, Li S, Liu J, Liang B, Wang X, Wang H, Li W, Tong Q, Yi J, Zhao L, et al. Longitudinal characteristics of lymphocyte responses and cytokine profiles in the peripheral blood of SARS-CoV-2 infected patients.
EBioMedicine 55:102763, 2020.
26. Chen G, Wu D, Guo W, Cao Y, Huang D, Wang H, Wang T, Zhang X, Chen H, Yu H, et al. Clinical and immunological features of severe and moderate coronavirus disease 2019.
J Clin Invest 130 (5):2620–2629, 2020.
27. Ishida Y, Agata Y, Shibahara K, Honjo T. Induced expression of PD-1, a novel member of the immunoglobulin gene superfamily, upon programmed cell death.
EMBO J 11 (11):3887–3895, 1992.
28. Brinkmann V, Reichard U, Goosmann C, Fauler B, Uhlemann Y, Weiss DS, Weinrauch Y, Zychlinsky A. Neutrophil extracellular traps kill bacteria.
Science 303 (5663):1532–1535, 2004.
29. Sadik CD, Kim ND, Luster AD. Neutrophils cascading their way to inflammation.
Trends Immunol 32 (10):452–460, 2011.
30. Barnes BJ, Adrover JM, Baxter-Stoltzfus A, Borczuk A, Cools-Lartigue J, Crawford JM, Dassler-Plenker J, Guerci P, Huynh C, Knight JS, et al. Targeting potential drivers of COVID-19: neutrophil extracellular traps.
J Exp Med 217 (6):e20200652, 2020.
31. Yao XH, Li TY, He ZC, Ping YF, Liu HW, Yu SC, Mou HM, Wang LH, Zhang HR, Fu WJ, et al. A pathological report of three COVID-19 cases by minimal invasive autopsies.
Zhonghua Bing Li Xue Za Zhi 49 (5):411–417, 2020.
32. Fox SE, Akmatbekov A, Harbert JL, Li G, Quincy Brown J, Vander Heide RS. Pulmonary and cardiac pathology in African American patients with COVID-19: an autopsy series from New Orleans.
Lancet Respir Med 8 (7):681–686, 2020.
33. Liu X, Shen Y, Wang H, Ge Q, Fei A, Pan S. Prognostic significance of neutrophil-to-lymphocyte ratio in patients with sepsis: a prospective observational study.
Mediators Inflamm 2016:8191254, 2016.
34. Curbelo J, Luquero Bueno S, Galvan-Roman JM, Ortega-Gomez M, Rajas O, Fernandez-Jimenez G, Vega-Piris L, Rodriguez-Salvanes F, Arnalich B, Diaz A, et al. Inflammation biomarkers in blood as mortality predictors in community-acquired pneumonia admitted patients: importance of comparison with neutrophil count percentage or neutrophil-lymphocyte ratio.
PLoS One 12 (3):e0173947, 2017.
35. Liu J, Liu Y, Xiang P, Pu L, Xiong H, Li C, Zhang M, Tan J, Xu Y, Song R, et al. Neutrophil-to-lymphocyte ratio predicts critical illness patients with 2019 coronavirus disease in the early stage.
J Transl Med 18 (1):206, 2020.
36. Zhang B, Zhou X, Zhu C, Song Y, Feng F, Qiu Y, Feng J, Jia Q, Song Q, Zhu B, et al. Immune phenotyping based on the neutrophil-to-lymphocyte ratio and IgG level predicts disease severity and outcome for patients with COVID-19.
Front Mol Biosci 7:157, 2020.
37. Wang F, Nie J, Wang H, Zhao Q, Xiong Y, Deng L, Song S, Ma Z, Mo P, Zhang Y. Characteristics of peripheral lymphocyte subset alteration in COVID-19 pneumonia.
J Infect Dis 221 (11):1762–1769, 2020.
38. Teijaro JR. The role of cytokine responses during influenza virus pathogenesis and potential therapeutic options.
Curr Top Microbiol Immunol 386:3–22, 2015.
39. Kolaczkowska E, Kubes P. Neutrophil recruitment and function in health and inflammation.
Nat Rev Immunol 13 (3):159–175, 2013.
40. Lin JX, Leonard WJ. Fine-tuning cytokine signals.
Annu Rev Immunol 37 (1545–3278):295–324, 2019.
41. Gao Y, Li T, Han M, Li X, Wu D, Xu Y, Zhu Y, Liu Y, Wang X, Wang L. Diagnostic utility of clinical laboratory data determinations for patients with the severe COVID-19.
J Med Virol 92 (7):791–796, 2020.
42. Wenjun W, Xiaoqing L, Sipei W, Puyi L, Liyan H, Yimin L, Linling C, Sibei C, Lingbo N, Yongping L, et al. The definition and risks of Cytokine Release Syndrome-Like in 11 COVID-19-infected pneumonia critically ill patients: disease characteristics and retrospective analysis.
medRxiv 2020; 2020.02.26.20026989.
43. Chen L, Liu HG, Liu W, Liu J, Liu K, Shang J, Deng Y, Wei S. Analysis of clinical features of 29 patients with 2019 novel coronavirus pneumonia.
Zhonghua Jie He He Hu Xi Za Zhi 43 (3):203–208, 2020.
44. Mehta P, McAuley DF, Brown M, Sanchez E, Tattersall RS, Manson JJ. Hlh Across Speciality Collaboration UK. COVID-19: consider cytokine storm syndromes and immunosuppression.
Lancet 395 (10229):1033–1034, 2020.
45. Ferrario CM, Jessup J, Chappell MC, Averill DB, Brosnihan KB, Tallant EA, Diz DI, Gallagher PE. Effect of angiotensin-converting enzyme inhibition and angiotensin II receptor blockers on cardiac angiotensin-converting enzyme 2.
Circulation 111 (20):2605–2610, 2005.
46. Varga Z, Flammer AJ, Steiger P, Haberecker M, Andermatt R, Zinkernagel AS, Mehra MR, Schuepbach RA, Ruschitzka F, Moch H. Endothelial cell infection and endotheliitis in COVID-19.
Lancet 395 (10234):1417–1418, 2020.
47. Zhou F, Yu T, Du R, Fan G, Liu Y, Liu Z, Xiang J, Wang Y, Song B, Gu X, et al. Clinical course and risk factors for mortality of adult inpatients with COVID-19 in Wuhan, China: a retrospective cohort study.
Lancet 395 (10229):1054–1062, 2020.
48. Levi M, Thachil J, Iba T, Levy JH. Coagulation abnormalities and thrombosis in patients with COVID-19.
Lancet Haematol 7 (6):e438–e440, 2020.
49. Levi M, van der Poll T. Coagulation and sepsis.
Thromb Res 149:38–44, 2017.
50. Pober JS, Sessa WC. Evolving functions of endothelial cells in inflammation.
Nat Rev Immunol 7 (10):803–815, 2007.
51. Yang YH, Huang YH, Chuang YH, Peng CM, Wang LC, Lin YT, Chiang BL. Autoantibodies against human epithelial cells and endothelial cells after severe acute respiratory syndrome (SARS)-associated coronavirus infection.
J Med Virol 77 (1):1–7, 2005.
52. Noris M, Benigni A, Remuzzi G. The case of complement activation in COVID-19 multiorgan impact.
Kidney Int 98 (2):314–322, 2020.
53. Ackermann M, Verleden SE, Kuehnel M, Haverich A, Welte T, Laenger F, Vanstapel A, Werlein C, Stark H, Tzankov A, et al. Pulmonary vascular endothelialitis, thrombosis, and angiogenesis in Covid-19.
N Engl J Med 383 (2):120–128, 2020.
54. Xiong Y, Liu Y, Cao L, Wang D, Guo M, Jiang A, Guo D, Hu W, Yang J, Tang Z, et al. Transcriptomic characteristics of bronchoalveolar lavage fluid and peripheral blood mononuclear cells in COVID-19 patients.
Emerg Microbes Infect 9 (1):761–770, 2020.
55. Wang X, Xu W, Hu G, Xia S, Sun Z, Liu Z, Xie Y, Zhang R, Jiang S, Lu L. RETRACTED ARTICLE: SARS-CoV-2 infects T lymphocytes through its spike protein-mediated membrane fusion.
Cell Mol Immunol 2020; 1–3.
56. Hamming I, Timens W, Bulthuis ML, Lely AT, Navis G, van Goor H. Tissue distribution of ACE2 protein, the functional receptor for SARS coronavirus. A first step in understanding SARS pathogenesis.
J Pathol 203 (2):631–637, 2004.
57. Aggarwal S, Gollapudi S, Gupta S. Increased TNF-alpha-induced apoptosis in lymphocytes from aged humans: changes in TNF-alpha receptor expression and activation of caspases.
J Immunol 162 (4):2154–2161, 1999.
58. Gupta S, Bi R, Kim C, Chiplunkar S, Yel L, Gollapudi S. Role of NF-kappaB signaling pathway in increased tumor necrosis factor-alpha-induced apoptosis of lymphocytes in aged humans.
Cell Death Differ 12 (2):177–183, 2005.
59. Lenardo MJ. Interleukin-2 programs mouse alpha beta T lymphocytes for apoptosis.
Nature 353 (6347):858–861, 1991.
60. Lenardo MJ. The molecular regulation of lymphocyte apoptosis.
Semin Immunol 9 (1):1–5, 1997.
61. Refaeli Y, Van Parijs L, London CA, Tschopp J, Abbas AK. Biochemical mechanisms of IL-2-regulated Fas-mediated T cell apoptosis.
Immunity 8 (5):615–623, 1998.
62. Kim KD, Zhao J, Auh S, Yang X, Du P, Tang H, Fu YX. Adaptive immune cells temper initial innate responses.
Nat Med 13 (10):1248–1252, 2007.
63. Maloy KJ, Salaun L, Cahill R, Dougan G, Saunders NJ, Powrie F. CD4+CD25+ T(R) cells suppress innate immune pathology through cytokine-dependent mechanisms.
J Exp Med 197 (1):111–119, 2003.
64. Yoshikawa T, Hill T, Li K, Peters CJ, Tseng CT. Severe acute respiratory syndrome (SARS) coronavirus-induced lung epithelial cytokines exacerbate SARS pathogenesis by modulating intrinsic functions of monocyte-derived macrophages and dendritic cells.
J Virol 83 (7):3039–3048, 2009.
65. Loftus TJ, Mohr AM, Moldawer LL. Dysregulated myelopoiesis and hematopoietic function following acute physiologic insult.
Curr Opin Hematol 25 (1):37–43, 2018.
66. Schultze JL, Mass E, Schlitzer A. Emerging principles in myelopoiesis at homeostasis and during infection and inflammation.
Immunity 50 (2):288–301, 2019.
67. Condamine T, Gabrilovich DI. Molecular mechanisms regulating myeloid-derived suppressor cell differentiation and function.
Trends Immunol 32 (1):19–25, 2011.
68. Condamine T, Mastio J, Gabrilovich DI. Transcriptional regulation of myeloid-derived suppressor cells.
J Leukoc Biol 98 (6):913–922, 2015.
69. Tebartz C, Horst SA, Sparwasser T, Huehn J, Beineke A, Peters G, Medina E. A major role for myeloid-derived suppressor cells and a minor role for regulatory T cells in immunosuppression during Staphylococcus aureus infection.
J Immunol 194 (3):1100–1111, 2015.
70. du Plessis N, Loebenberg L, Kriel M, von Groote-Bidlingmaier F, Ribechini E, Loxton AG, van Helden PD, Lutz MB, Walzl G. Increased frequency of myeloid-derived suppressor cells during active tuberculosis and after recent mycobacterium tuberculosis infection suppresses T-cell function.
Am J Respir Crit Care Med 188 (6):724–732, 2013.
71. Tacke RS, Lee HC, Goh C, Courtney J, Polyak SJ, Rosen HR, Hahn YS. Myeloid suppressor cells induced by hepatitis C virus suppress T-cell responses through the production of reactive oxygen species.
Hepatology 55 (2):343–353, 2012.
72. Zhai NC, Li HJ, Song HX, Yang Y, Cui A, Li TY, Niu JQ, Crispe IN, Su LS, Tu ZK. Hepatitis C virus induces MDSCs-like monocytes through TLR2/PI3K/AKT/STAT3 Signaling.
PloS One 12 (1):e0170516, 2017.
73. Tumino N, Turchi F, Meschi S, Lalle E, Bordoni V, Casetti R, Agrati C, Cimini E, Montesano C, Colizzi V, et al. In HIV-positive patients, myeloid-derived suppressor cells induce T-cell anergy by suppressing CD3 ζ expression through ELF-1 inhibition.
AIDS 29 (18):2397–2407, 2015.
74. Zhang ZN, Yi N, Zhang TW, Zhang LL, Wu X, Liu M, Fu YJ, He SJ, Jiang YJ, Ding HB, et al. Myeloid-derived suppressor cells associated with disease progression in primary HIV infection: PD-L1 blockade attenuates inhibition.
J Acquir Immune Defic Syndr 76 (2):200–208, 2017.
75. Gabrilovich DI, Nagaraj S. Myeloid-derived suppressor cells as regulators of the immune system.
Nat Rev Immunol 9 (3):162–174, 2009.
76. Rodriguez PC, Ochoa AC. Arginine regulation by myeloid derived suppressor cells and tolerance in cancer: mechanisms and therapeutic perspectives.
Immunol Rev 222:180–191, 2008.
77. Bronte V, Zanovello P. Regulation of immune responses by L-arginine metabolism.
Nat Rev Immunol 5 (8):641–654, 2005.
78. Ochoa AC, Zea AH, Hernandez C, Rodriguez PC. Arginase, prostaglandins, and myeloid-derived suppressor cells in renal cell carcinoma.
Clin Cancer Res 13 (2 pt 2):721s–726s, 2007.
79. Kusmartsev SA, Li Y, Chen SB. Gr-1(+) myeloid cells derived from tumor-bearing mice inhibit primary T cell activation induced through CD3/CD28 costimulation.
J Immunol 165 (2):779–785, 2000.
80. Gabrilovich DI, Velders MP, Sotomayor EM, Kast WM. Mechanism of immune dysfunction in cancer mediated by immature Gr-1+ myeloid cells.
J Immunol 166 (9):5398–5406, 2001.
81. Schulte-Schrepping J, Reusch N, Paclik D, Bassler K, Schlickeiser S, Zhang B, Kramer B, Krammer T, Brumhard S, Bonaguro L, et al. Severe COVID-19 is marked by a dysregulated myeloid cell compartment.
Cell 2020; 182 (6): 1419–1440.e23.
82. Silvin A, Chapuis N, Dunsmore G, Goubet AG, Dubuisson A, Derosa L, Almire C, Henon C, Kosmider O, Droin N, et al. Elevated calprotectin and abnormal myeloid cell subsets discriminate severe from mild COVID-19.
Cell 2020; 182 (6): 1401–1418.e18.
83. Bowers NL, Helton ES, Huijbregts RP, Goepfert PA, Heath SL, Hel Z. Immune suppression by neutrophils in HIV-1 infection: role of PD-L1/PD-1 pathway.
PLoS Pathog 10 (3):e1003993, 2014.
84. Castell SD, Harman MF, Moron G, Maletto BA, Pistoresi-Palencia MC. Neutrophils which migrate to lymph nodes modulate CD4(+) T cell response by a PD-L1 dependent mechanism.
Front Immunol 10:105, 2019.
85. de Kleijn S, Langereis JD, Leentjens J, Kox M, Netea MG, Koenderman L, Ferwerda G, Pickkers P, Hermans PW. IFN-gamma-stimulated neutrophils suppress lymphocyte proliferation through expression of PD-L1.
PLoS One 8 (8):e72249, 2013.
86. Langereis JD, Pickkers P, de Kleijn S, Gerretsen J, de Jonge MI, Kox M. Spleen-derived IFN-gamma induces generation of PD-L1(+)-suppressive neutrophils during endotoxemia.
J Leukoc Biol 102 (6):1401–1409, 2017.
87. Vadillo E, Taniguchi-Ponciano K, Lopez-Macias C, Carvente-Garcia R, Mayani H, Ferat-Osorio E, Flores-Padilla G, Torres J, Gonzalez-Bonilla CR, Majluf A, et al. A Shift towards an immature myeloid profile in peripheral blood of critically ill COVID-19 patients.
Arch Med Res 2020; 1873–5487.
88. Wei H, Xu X, Tian Z, Sun R, Qi Y, Zhao C, Wang D, Zheng X, Fu B, Zhou Y. Pathogenic T-cells and inflammatory monocytes incite inflammatory storms in severe COVID-19 patients.
National Sci Rev 7 (6):998–1002, 2020.
89. Wen W, Su W, Tang H, Le W, Zhang X, Zheng Y, Liu X, Xie L, Li J, Ye J, et al. Immune cell profiling of COVID-19 patients in the recovery stage by single-cell sequencing.
Cell Discov 6 (1):31, 2020.
90. Miwa K, Asano M, Horai R, Iwakura Y, Nagata S, Suda T. Caspase 1-independent IL-1beta release and inflammation induced by the apoptosis inducer Fas ligand.
Nat Med 4 (11):1287–1292, 1998.
91. Puren AJ, Fantuzzi G, Gu Y, Su MS, Dinarello CA. Interleukin-18 (IFNgamma-inducing factor) induces IL-8 and IL-1beta via TNFalpha production from non-CD14+ human blood mononuclear cells.
J Clin Invest 101 (3):711–721, 1998.
92. Yamada N, Katz SI. Generation of mature dendritic cells from a CD14+ cell line (XS52) by IL-4, TNF-alpha, IL-1 beta, and agonistic anti-CD40 monoclonal antibody.
J Immunol 163 (10):5331–5337, 1999.
93. Rosenwasser LJ. Biologic activities of IL-1 and its role in human disease.
J Allergy Clin Immunol 102 (3):344–350, 1998.
94. Cao J, Tu WJ, Cheng W, Yu L, Liu YK, Hu X, Liu Q. Clinical features and short-term outcomes of 102 patients with Coronavirus Disease 2019 in Wuhan, China.
Clin Infect Dis 71 (15):748–755, 2020.
95. Clancy CJ, Nguyen MH. Coronavirus Disease 2019, superinfections, and antimicrobial development: what can we expect?
Clin Infect Dis 71 (10):2736–2743, 2020.
96. Zhou P, Liu Z, Chen Y, Xiao Y, Huang X, Fan XG. Bacterial and fungal infections in COVID-19 patients: a matter of concern.
Infect Control Hosp Epidemiol 41 (9):1124–1125, 2020.
97. Rawson TM, Moore LSP, Zhu N, Ranganathan N, Skolimowska K, Gilchrist M, Satta G, Cooke G, Holmes A. Bacterial and fungal coinfection in individuals with coronavirus: a rapid review to support COVID-19 antimicrobial prescribing.
Clin Infect Dis 71:2459–2468, 2020.
98. Ramos-Casals M, Brito-Zeron P, Lopez-Guillermo A, Khamashta MA, Bosch X. Adult haemophagocytic syndrome.
Lancet 383 (9927):1503–1516, 2014.
99. Seguin A, Galicier L, Boutboul D, Lemiale V, Azoulay E. Pulmonary involvement in patients with hemophagocytic lymphohistiocytosis.
Chest 149 (5):1294–1301, 2016.
100. Ruan Q, Yang K, Wang W, Jiang L, Song J. Clinical predictors of mortality due to COVID-19 based on an analysis of data of 150 patients from Wuhan, China.
Intensive Care Med 46 (5):846–848, 2020.
101. McGonagle D, Sharif K, O’Regan A, Bridgewood C. The role of cytokines including interleukin-6 in COVID-19 induced pneumonia and macrophage activation syndrome-like disease.
Autoimmun Rev 19 (6):102537, 2020.
102. Wan S, Yi Q, Fan S, Lv J, Zhang X, Guo L, Lang C, Xiao Q, Xiao K, Yi Z, et al. Characteristics of lymphocyte subsets and cytokines in peripheral blood of 123 hospitalized patients with 2019 novel coronavirus pneumonia (NCP).
medRxiv 2020.
103. Fu Y, Cheng Y, Wu Y. Understanding SARS-CoV-2-mediated inflammatory responses: from mechanisms to potential therapeutic tools.
Virol Sin 35 (3):266–271, 2020.
104. Takada A, Kawaoka Y. Antibody-dependent enhancement of viral infection: molecular mechanisms and in vivo implications.
Rev Med Virol 13 (6):387–398, 2003.
105. Liu L, Wei Q, Lin Q, Fang J, Wang H, Kwok H, Tang H, Nishiura K, Peng J, Tan Z, et al. Anti-spike IgG causes severe acute lung injury by skewing macrophage responses during acute SARS-CoV infection.
JCI Insight 4 (4):e123158, 2019.
106. Hotchkiss RS, Karl IE. The pathophysiology and treatment of sepsis.
N Engl J Med 348 (2):138–150, 2003.
107. van der Poll T, van de Veerdonk FL, Scicluna BP, Netea MG. The immunopathology of sepsis and potential therapeutic targets.
Nat Rev Immunol 17 (7):407–420, 2017.
108. Hotchkiss RS, Monneret G, Payen D. Sepsis-induced immunosuppression: from cellular dysfunctions to immunotherapy.
Nat Rev Immunol 13 (12):862–874, 2013.
109. Singer M, Deutschman CS, Seymour CW, Shankar-Hari M, Annane D, Bauer M, Bellomo R, Bernard GR, Chiche JD, Coopersmith CM, et al. The Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3).
JAMA 315 (8):801–810, 2016.
110. Li H, Liu L, Zhang D, Xu J, Dai H, Tang N, Su X, Cao B. SARS-CoV-2 and viral sepsis: observations and hypotheses.
Lancet 395 (10235):1517–1520, 2020.
111. Chen X, Zhao B, Qu Y, Chen Y, Xiong J, Feng Y, Men D, Huang Q, Liu Y, Yang B, et al. Detectable Serum Severe Acute Respiratory Syndrome Coronavirus 2 Viral Load (RNAemia) is closely correlated with drastically elevated interleukin 6 level in critically ill patients with coronavirus disease 2019.
Clin Infect Dis 71 (8):1937–1942, 2020.
112. Liu Y, Yang Y, Zhang C, Huang F, Wang F, Yuan J, Wang Z, Li J, Li J, Feng C, et al. Clinical and biochemical indexes from 2019-nCoV infected patients linked to viral loads and lung injury.
Sci China Life Sci 63 (3):364–374, 2020.
113. Yang Y, Shen C, Li J, Yuan J, Yang M, Wang F, Li G, Li Y, Xing L, Peng L, et al. Exuberant elevation of IP-10, MCP-3 and IL-1ra during SARS-CoV-2 infection is associated with disease severity and fatal outcome.
medRxiv 2020; 2020.03.02.20029975.
114. Yang X, Yu Y, Xu J, Shu H, Xia J, Liu H, Wu Y, Zhang L, Yu Z, Fang M, et al. Clinical course and outcomes of critically ill patients with SARS-CoV-2 pneumonia in Wuhan, China: a single-centered, retrospective, observational study.
Lancet Respir Med 8 (5):475–481, 2020.
115. Cai H. Sex difference and smoking predisposition in patients with COVID-19.
Lancet Respir Med 8 (4):e20, 2020.
116. Jaillon S, Berthenet K, Garlanda C. Sexual dimorphism in innate immunity.
Clin Rev Allergy Immunol 56 (3):308–321, 2019.
117. Klein SL, Flanagan KL. Sex differences in immune responses.
Nat Rev Immunol 16 (10):626–638, 2016.
118. Zhao Y, Zhao Z, Wang Y, Zhou Y, Ma Y, Zuo W. Single-cell RNA expression profiling of ACE2, the putative receptor of Wuhan 2019-nCov.
bioRxiv 2020; 2020.01.26.919985.
119. Xu K, Chen Y, Yuan J, Yi P, Ding C, Wu W, Li Y, Ni Q, Zou R, Li X, et al. Factors associated with prolonged viral RNA shedding in patients with coronavirus disease 2019 (COVID-19).
Clin Infect Dis 71 (15):799–806, 2020.
120. Casadevall A, Pirofski LA. Antibody-mediated regulation of cellular immunity and the inflammatory response.
Trends Immunol 24 (9):474–478, 2003.
121. Casadevall A, Pirofski LA. The convalescent sera option for containing COVID-19.
J Clin Invest 130 (4):1545–1548, 2020.
122. Taylor A, Foo SS, Bruzzone R, Dinh LV, King NJ, Mahalingam S. Fc receptors in antibody-dependent enhancement of viral infections.
Immunol Rev 268 (1):340–364, 2015.
123. Tirado SM, Yoon KJ. Antibody-dependent enhancement of virus infection and disease.
Viral Immunol 16 (1):69–86, 2003.
124. Wan Y, Shang J, Sun S, Tai W, Chen J, Geng Q, He L, Chen Y, Wu J, Shi Z, et al. Molecular mechanism for antibody-dependent enhancement of coronavirus entry.
J Virol 2020; 94 (5): e02015–19.
125. Mair-Jenkins J, Saavedra-Campos M, Baillie JK, Cleary P, Khaw FM, Lim WS, Makki S, Rooney KD, Nguyen-Van-Tam JS, Beck CR, et al. The effectiveness of convalescent plasma and hyperimmune immunoglobulin for the treatment of severe acute respiratory infections of viral etiology: a systematic review and exploratory meta-analysis.
J Infect Dis 211 (1):80–90, 2015.
126. Luke TC, Kilbane EM, Jackson JL, Hoffman SL. Meta-analysis: convalescent blood products for Spanish influenza pneumonia: a future H5N1 treatment?
Ann Intern Med 145 (8):599–609, 2006.
127. Duan K, Liu B, Li C, Zhang H, Yu T, Qu J, Zhou M, Chen L, Meng S, Hu Y, et al. Effectiveness of convalescent plasma therapy in severe COVID-19 patients.
Proc Natl Acad Sci U S A 117 (17):9490–9496, 2020.
128. Shen C, Wang Z, Zhao F, Yang Y, Li J, Yuan J, Wang F, Li D, Yang M, Xing L, et al. Treatment of 5 critically ill patients with COVID-19 with convalescent plasma.
JAMA 323 (16):1582–1589, 2020.
129. Galeotti C, Kaveri SV, Bayry J. IVIG-mediated effector functions in autoimmune and inflammatory diseases.
Int Immunol 29 (11):491–498, 2017.
130. De Ranieri D, Fenny NS. Intravenous immunoglobulin in the treatment of primary immunodeficiency diseases.
Pediatr Ann 46 (1):e8–e12, 2017.
131. Tagami T, Matsui H, Fushimi K, Yasunaga H. Intravenous immunoglobulin and mortality in pneumonia patients with septic shock: an observational nationwide study.
Clin Infect Dis 61 (3):385–392, 2015.
132. Cao W, Liu X, Bai T, Fan H, Hong K, Song H, Han Y, Lin L, Ruan L, Li T. High-dose intravenous immunoglobulin as a therapeutic option for deteriorating patients with coronavirus disease 2019.
Open Forum Infect Dis 7 (3):ofaa102, 2020.
133. Xie Y, Cao S, Dong H, Li Q, Chen E, Zhang W, Yang L, Fu S, Wang R. Effect of regular intravenous immunoglobulin therapy on prognosis of severe pneumonia in patients with COVID-19.
J Infect 81 (2):318–356, 2020.
134. Moran JL, Graham PL, Rockliff S, Bersten AD. Updating the evidence for the role of corticosteroids in severe sepsis and septic shock: a Bayesian meta-analytic perspective.
Crit Care 14 (4):R134, 2010.
135. Jiang S, Liu T, Hu Y, Li R, Di X, Jin X, Wang Y, Wang K. Efficacy and safety of glucocorticoids in the treatment of severe community-acquired pneumonia: a meta-analysis.
Medicine (Baltimore) 98 (26):e16239, 2019.
136. Arabi YM, Mandourah Y, Al-Hameed F, Sindi AA, Almekhlafi GA, Hussein MA, Jose J, Pinto R, Al-Omari A, Kharaba A, et al. Corticosteroid therapy for critically ill patients with Middle East respiratory syndrome.
Am J Respir Crit Care Med 197 (6):757–767, 2018.
137. Hui DS. Systemic corticosteroid therapy may delay viral clearance in patients with middle east respiratory syndrome coronavirus infection.
Am J Respir Crit Care Med 197 (6):700–701, 2018.
138. Zha L, Li S, Pan L, Tefsen B, Li Y, French N, Chen L, Yang G, Villanueva EV. Corticosteroid treatment of patients with coronavirus disease 2019 (COVID-19).
Med J Aust 212 (9):416–420, 2020.
139. Russell CD, Millar JE, Baillie JK. Clinical evidence does not support corticosteroid treatment for 2019-nCoV lung injury.
Lancet 395 (10223):473–475, 2020.
140. Lu X, Chen T, Wang Y, Wang J, Yan F. Adjuvant corticosteroid therapy for critically ill patients with COVID-19.
Crit Care 24 (1):241, 2020.
141. Corral L, Bahamonde A, Arnaiz delas Revillas F, Gomez-Barquero J, Abadia-Otero J, Garcia-Ibarbia C, Mora V, cerezo-hernandez A, Hernandez JL, Lopez-Muniz G, et al. GLUCOCOVID: a controlled trial of methylprednisolone in adults hospitalized with COVID-19 pneumonia.
medRxiv 2020; 2020.06.17.20133579.
142. Group RC, Horby P, Lim WS, Emberson JR, Mafham M, Bell JL, Linsell L, Staplin N, Brightling C, Ustianowski A, et al. Dexamethasone in hospitalized patients with Covid-19—preliminary report.
N Engl J Med 2020; Online ahead of print.
143. Sterne JAC, Murthy S, Diaz JV, Slutsky AS, Villar J, Angus DC, Annane D, Azevedo LCP, Berwanger O, Cavalcanti AB, et al. WHO Rapid Evidence Appraisal for COVID-19 Therapies (REACT) Working Group. Association between administration of systemic corticosteroids and mortality among critically ill patients with COVID-19: a meta-analysis.
JAMA 324 (13):1330–1341, 2020.
144. Dequin PF, Heming N, Meziani F, Plantefeve G, Voiriot G, Badie J, Francois B, Aubron C, Ricard JD, Ehrmann S, et al. Effect of hydrocortisone on 21-day mortality or respiratory support among critically ill patients with COVID-19: a randomized clinical trial.
JAMA 324 (13):1298–1306, 2020.
145. Xu X, Han M, Li T, Sun W, Wang D, Fu B, Zhou Y, Zheng X, Yang Y, Li X, et al. Effective treatment of severe COVID-19 patients with tocilizumab.
Proc Natl Acad Sci U S A 117 (20):10970–10975, 2020.
146. Sciascia S, Apra F, Baffa A, Baldovino S, Boaro D, Boero R, Bonora S, Calcagno A, Cecchi I, Cinnirella G, et al. Pilot prospective open, single-arm multicentre study on off-label use of tocilizumab in patients with severe COVID-19.
Clin Exp Rheumatol 38 (3):529–532, 2020.
147. Novel Coronavirus Pneumonia Diagnosis and Treatment Plan (Provisional 7th Edition). Available at:
http://www.gov.cn/zhengce/zhengceku/2020-03/04/content_5486705.htm.
149. Colaneri M, Bogliolo L, Valsecchi P, Sacchi P, Zuccaro V, Brandolino F, Montecucco C, Mojoli F, Giusti EM, Bruno R, et al. Tocilizumab for Treatment of Severe COVID-19 Patients: Preliminary Results from SMAtteo COvid19 REgistry (SMACORE).
Microorganisms 8 (5):695, 2020.
150. Shakoory B, Carcillo JA, Chatham WW, Amdur RL, Zhao HQ, Dinarello CA, Cron RQ, Opal SM. Interleukin-1 receptor blockade is associated with reduced mortality in sepsis patients with features of macrophage activation syndrome: reanalysis of a prior phase III trial.
Crit Care Med 44 (2):275–281, 2016.
151. Pontali E, Volpi S, Antonucci G, Castellaneta M, Buzzi D, Tricerri F, Angelelli A, Caorsi R, Feasi M, Calautti F, et al. Safety and efficacy of early high-dose IV anakinra in severe COVID-19 lung disease.
J Allergy Clin Immunol 146 (1):213–215, 2020.
152. Dimopoulos G, de Mast Q, Markou N, Theodorakopoulou M, Komnos A, Mouktaroudi M, Netea MG, Spyridopoulos T, Verheggen RJ, Hoogerwerf J, et al. Favorable anakinra responses in severe Covid-19 patients with secondary hemophagocytic lymphohistiocytosis.
Cell Host Microbe 28 (1):117. e1–123.e1, 2020.
153. Serra Lopez-Matencio JM, Morell Baladron A, Castaneda S. JAK-STAT inhibitors for the treatment of immunomediated diseases.
Med Clin (Barc) 152 (9):353–360, 2019.
154. Rawlings JS, Rosler KM, Harrison DA. The JAK/STAT signaling pathway.
J Cell Sci 117 (pt 8):1281–1283, 2004.
155. Stebbing J, Phelan A, Griffin I, Tucker C, Oechsle O, Smith D, Richardson P. COVID-19: combining antiviral and anti-inflammatory treatments.
Lancet Infect Dis 20 (4):400–402, 2020.
156. Richardson P, Griffin I, Tucker C, Smith D, Oechsle O, Phelan A, Rawling M, Savory E, Stebbing J. Baricitinib as potential treatment for 2019-nCoV acute respiratory disease.
Lancet 395 (10223):e30–e31, 2020.
157. Cantini F, Niccoli L, Matarrese D, Nicastri E, Stobbione P, Goletti D. Baricitinib therapy in COVID-19: a pilot study on safety and clinical impact.
J Infect 81 (2):318–356, 2020.
158. Le Blanc K, Mougiakakos D. Multipotent mesenchymal stromal cells and the innate immune system.
Nat Rev Immunol 12 (5):383–396, 2012.
159. Weiss ARR, Dahlke MH. Immunomodulation by mesenchymal stem cells (MSCs): mechanisms of action of living, apoptotic, and dead MSCs.
Front Immunol 10:1191, 2019.
160. Wilson JG, Liu KD, Zhuo H, Caballero L, McMillan M, Fang X, Cosgrove K, Vojnik R, Calfee CS, Lee JW, et al. Mesenchymal stem (stromal) cells for treatment of ARDS: a phase 1 clinical trial.
Lancet Respir Med 3 (1):24–32, 2015.
161. Kamen DL, Nietert PJ, Wang H, Duke T, Cloud C, Robinson A, Gilkeson GS. CT-04 Safety and efficacy of allogeneic umbilical cord-derived mesenchymal stem cells (MSCs) in patients with systemic lupus erythematosus: results of an open-label phase I study.
Lupus Sci Med 2018; 5: DOI: 10.1136/lupus-2018-lsm.76.
162. Leng Z, Zhu R, Hou W, Feng Y, Yang Y, Han Q, Shan G, Meng F, Du D, Wang S, et al. Transplantation of ACE2(-) mesenchymal stem cells improves the outcome of patients with COVID-19 pneumonia.
Aging Dis 11 (2):216–228, 2020.
163. Casadevall A, Scharff MD. Serum therapy revisited: animal models of infection and development of passive antibody therapy.
Antimicrob Agents Chemother 38 (8):1695–1702, 1994.
164. Cheng Y, Wong R, Soo YOY, Wong WS, Lee CK, Ng MHL, Chan P, Wong KC, Leung CB, Cheng G. Use of convalescent plasma therapy in SARS patients in Hong Kong.
Eur J Clin Microbiol Infect Dis 24 (1):44–46, 2005.
165. Wolfel R, Corman VM, Guggemos W, Seilmaier M, Zange S, Muller MA, Niemeyer D, Jones TC, Vollmar P, Rothe C, et al. Virological assessment of hospitalized patients with COVID-2019.
Nature 581 (7809):465–469, 2020.
166. Ferro F, Elefante E, Baldini C, Bartoloni E, Puxeddu I, Talarico R, Mosca M, Bombardieri S. COVID-19: the new challenge for rheumatologists.
Clin Exp Rheumatol 38 (2):175–180, 2020.
167. Diebel LN, Liberati DM, Diglio CA, Brown WJ. Immunoglobulin a modulates inflammatory responses in an in vitro model of pneumonia.
J Trauma 59 (5):1099–1106, 2005.
