In the article by Jaiswal et al,1 the authors describe that the abnormal increase of NKG2A-positive natural killer (NK) cell subsets following Epstein-Barr virus (EBV) reactivation may be associated with the occurrence of chronic graft versus host disease (GVHD). The authors have applied a novel approach to allogeneic hematopoietic stem cell transplantation (HSCT) combining posttransplant cyclophosphamide (PT-Cy) and T-cell costimulation blockade with CTLA-4-Ig that has lowered the overall incidence of acute GVHD. However, with prospective data collection and analysis, the authors have observed a phenomenon in which EBV reactivation subsequently upregulates NKG2A+ NK cell subsets and ultimately increases the risk for chronic GVHD in patients. While the sample size of EBV reactivated patients is small to draw conclusions, the authors provide an interesting perspective on the role of NK cells following allogeneic HSCT.
NKG2A is an inhibitory receptor that belongs to the C-type lectin-like superfamily and recognizes human leukocyte antigen (HLA)-E, a nonclassic major histocompatibility complex class I molecule that is mainly expressed on NK and CD8+ T cells. When the NKG2A/CD94 complex binds to its cognate ligand, an inhibitory signal is sent to the immune effector cells.2 In an allogeneic HSCT setting, NKG2A receptor is initially elevated during the early reconstitution period and later lost throughout the course of posttransplantation period. Subsequently, the expression of the activating NKG2C receptor is upregulated to establish a balanced NKG2A to NKG2C ratio. This process is similar to peripheral NK cell development and maturation.3 However, HSCT recipients who develop severe acute and chronic GVHD have a higher NKG2A to NKG2C NK cell ratio, especially in an HLA-mismatched setting.4 Similarly, Jaiswal et al1 show that patients who developed chronic GVHD have an upsurge in NKG2A levels up to 150 days post-transplantation; the NKG2C receptors are downregulated and only recovered following GVHD treatment. On the other hand, patients who did not develop chronic GVHD show a similar NKG2A to NKG2C ratio.
The direct cause of this difference in NKG2A levels in GVHD versus non-GVHD patients remains to be elucidated. Jaiswal et al1 suggest that EBV reactivation may be associated with the alterations in NKG2A and NKG2C subsets. In all patients with EBV reactivation, there was an increase in NKG2A population during and following EBV reactivation which eventually lead to the onset of chronic GVHD. Recently, the relationship between NK cells and viral infections in the HSCT setting has been widely studied in relation to human cytomegalovirus (CMV) infections. CMV reactivation or infection drives the proliferation of NKG2C+ memory-like NK cell subsets that exhibit antiviral effects against CMV-infected cells.5 However, more evidence is now suggesting that the expansion of NKG2C+ NK cell populations is specific to CMV and is not a generalized response to human herpesvirus infection.6 In support of this observation, Jaiswal et al1 also show that the NKG2C+ NK cell subsets remained unaltered during EBV infection. However, the possibility of the interaction between the NKG2A and NKG2C subsets needs to be considered. It has been previously demonstrated that the frequency of NKG2A+CD57+ NK cells was higher following EBV infection only in CMV-seropositive individuals.6 In this study, all patients were CMV seropositive and 7 out of 8 patients had CMV reactivation preceding the EBV reactivation. Unfortunately, the baseline levels of NKG2A and NKG2C levels before day 60 post-transplantation, during CMV reactivation and preceding EBV reactivation, were not analyzed in detail.1
In addition to the increase in quantity, whether NKG2A+ NK cells, like NKG2C+ NK cells, directly function and play an antiviral role against EBV-infected cells remains to be determined. While some in vitro studies have revealed that NKG2A+ NK cells can effectively lyse EBV-infected cells, these cells often coexpress NKG2D which may play a more major and direct role against EBV-transformed cells than NKG2A itself.7 Therefore, it is possible that NKG2A is simply a marker of the EBV-responsive NK cell population but plays no direct role in NK cell function. In another study, the use of anti-NKG2A monoclonal antibodies has shown to enhance the cytotoxic effects of NKG2A+ NK cells against EBV-positive cell lines both in vitro and in vivo suggesting NKG2A as an inhibitory checkpoint receptor.8 In fact, anti-NKG2A monoclonal antibody monalizumab is being used as an immune checkpoint inhibitor to enhance antitumor effects in clinical trials to enhance graft versus tumor effects and prevent relapse in HLA-matched allogeneic HSCT (NCT02921685). In addition to the disease-free survival, the incidence of GVHD as well as EBV reactivation in this study will also be of particular interest to further understand the role of NKG2A+ NK cells post-transplantation.
Finally, it is important to note that the authors have applied an unique GVHD prophylactic regimen using a combination of PT-Cy and CTLA-4-Ig which may have an influence on the NK cell reconstitution patterns. It has been previously reported that PT-Cy elevates interleukin-15 level in the environment which promotes subsequent NK cell recovery and expansion.9 Furthermore, CTLA-4-Ig infusion or CTLA-4–primed donor lymphocyte infusion can also induce in vivo NK cell proliferation as well as enhance NK cell cytotoxicity.10 Taken together, the NK cells within the transplanted graft or CTLA-4-Ig–primed donor lymphocyte infusion may utilize the elevated IL-15 created by PT-Cy for NK cell reconstitution. However, in conjunction with the occurrence of EBV reactivation, the immature NKG2A+ NK cell type may have been selectively expanded reducing the beneficial effects of CTLA-4-Ig–primed DLI and leading to a possible pathogenic effect.
In conclusion, NK cell immunity plays a critical role following HSCT and its subpopulations may be altered due to various factors such as EBV reactivation. While it is clear in this study that the changes in NKG2A NK cell subsets preceded the onset of chronic GVHD, the pathogenesis of chronic GVHD is complex and may involve numerous complex mechanisms and pathways. However, taken together, the alterations in NKG2A NK cell subsets postallogeneic HSCT need to be taken into more careful consideration by researchers and clinicians as it is a potential target for therapeutic approaches and may also be a predictive marker of clinical outcome.
1. Jaiswal SRB, Bhagwati P, Aiyar G, et al. Alterations in NKG2A and NKG2C subsets of natural killer cells following Epstein Barr virus reactivation in CTLA4-Ig based haploidentical transplantation is associated with increased chronic graft-versus-host disease. Transplantation. [Epub ahead of print. May 1, 2018]. doi: 10.1097/TP.0000000000002941
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