A key aspect of the adaptive immune response is the production of antigen-specific antibodies of high specificity and affinity that ought to be available as fast as possible in the situation of a first, but especially, repeat encounter with an antigen. The capacity to retain information about specific antigens after a first encounter and primary response is dependent on memory cells that allow an immediate differentiation into antibody-secreting cell (ASC) or plasma cell (PC) in case of a repeat encounter. The presence of isotype-switched antibodies, which PCs produce following their differentiation from B cells activated in a T-cell–dependent (TD) manner, is widely regarded as the cornerstone of antigen-induced humoral immunity1 and the typical response to peptide and protein antigens such as the HLA on transplanted organs.2 The resulting PCs and memory B cells (MBCs) are class switched, meaning that they produce, express, and secrete IgG-type immunoglobulins. In addition, antibody production can arise from T-cell–independent (TI) B-cell activation, typically supported by an interplay with the innate immune system via the B-cell coreceptor component complement receptor 2 (CD21 in humans) and its ligand complement complexes.3 These antibodies are more commonly directed toward less complex antigens such as polysaccharides (including the ABO blood group antigens on transplanted organs), lipids, or mixed antigens and show lower affinity with the producing clones and have undergone less somatic hypermutation (SHM). The PCs and B cells arising from this response typically do not experience a class switch (are IgM expressing), may persist for a shorter time period, and are mostly represented in the splenic marginal zone (MZ) cells in humans and the B1/B cells that were found in lavages from the peritoneal cavity of rodents.4,5 Reactivation and differentiation of MBCs into newly formed and relatively short-lived PCs in the secondary lymphatic tissue surrounding the zone of antigen encounter and in the spleen is the prototype of memory response to previously encountered antigens.6-8 However, there is now increasing evidence that along with this mechanism, long-lived plasma cells (LLPCs) home to specific areas of the bone marrow to produce antibodies for an indeterminate time period and possibly intensify this production at recurrent antigen contact. Accordingly, long-term, low-level antibody presence in plasma is likely the result of bone marrow–located LLPCs. A repeat encounter with the antigen can ramp up the antibody production by LLPC,9 in addition to the rapid reactivation of MBC in the spleen, lymph nodes, and circulating in peripheral blood, resulting in the generation of new short-lived and highly productive PC with previously optimized antigen-specific response. Therefore, LLPCs and MBCs represent redundant pathways, synergistically provided by the adaptive immune systems to support fast and effective antibody response. A schematic overview is shown in Figure 1. The contribution of either pathway may change with age and type of antigen.
In the context of solid organ transplantation and antibody-directed therapies, it is essential to understand some of the basic concepts of these pathways and the resulting consequences for the design of effective therapeutic regimens. In the following, we will outline the current understanding of the pathways of B-cell differentiation, PC persistence, and reactivation with the resulting antibody response to foreign human antigens.
Any attempt to position MBC or LLPC hierarchically along the B-cell maturation pathways requires a brief summary of current concepts of these pathways. B-cell precursors undergo substantial changes in morphological and functional characteristics as they give rise to mature B cells. The stages of differentiation begin in the bone marrow, which exports to the peripheral blood a developmentally intermediate, heterogeneous population of B cells, termed transitional B cells. Most of these cells are self-tolerant, with remaining autoreactive cells undergoing clonal deletion or receptor editing in the periphery before finally developing into naïve B cells.10-14 It has also been proposed that a small proportion of naïve B cells emerge directly from the bone marrow.15 Naïve cells circulate through peripheral blood and lymphoid tissues and die within days if they do not encounter a cognate antigen.15 The majority of mature B cells are follicular B cells, located in the lymphoid follicles of the spleen and lymph nodes. In humans, an additional mature B-cell population is formed by the splenic MZ B cells, which seems to fulfill the role that was attributed to the B1 cells in rodents. These B1 cells can be found largely in the peritoneal and pleural cavities and at mucosal sites after lavage with sodium chloride solutions and may reflect a local B-cell population or B cells specifically responsive to the irritation of fluid injection in these areas. They are identified by high expression of CD5, and a similarly distinguishable B-cell population was never found in humans. Whether a small subgroup of lymphocytes identified by surface expression of CD20, CD27, and CD43 that was found to show a high CD5 expression16 actually represents B1/B cells in human peripheral blood remains controversial.17,18 Beside the classical activation pathways described below, some B cells were also found to develop regulatory capacity, mostly mediated by interleukin (IL)-10 production.19 Whether these cells represent a distinct B-cell phenotype or a pathway of maturation remains unclear, as the surface expression of CD1d and CD5 first described in mice does not seem to distinguish a clear regulatory population among B cells.20 While fascinating, this cell group will not be further discussed here given the scope of this review.
The sequence of events triggered by antigen encounter ultimately shapes B-cell fates. The 2-phase TD response begins with an extrafollicular stage, where B lymphoblasts develop as a result of antigen receptor stimulation, rapidly proliferate, and terminally differentiate into short-lived, antigen-specific, ASCs: plasmablasts (dividing) and PCs (nondividing). This process yields the majority of the early protective antibodies, which have a moderate antigen affinity. In the second phase, activated B cells that have not differentiated into plasmablasts may reenter the B-cell follicle, proliferate, and together with follicular T helper cells that have been activated by antigen-presenting cells via the inducible T-cell costimulatory receptor (ICOS), form a unique structure, the germinal center (GC). These veritable hubs for affinity-based selection and B-cell clonal expansion were described as early as 1884 by Walther Flemming, who noted that microanatomical regions of secondary lymphoid organs contained dividing cells.21 Here, follicular T helper and B cells reciprocally promote cell maturation through a multifaceted interaction that includes direct cell-to-cell signaling via ICOS and CD40 as well as a number of cytokines, among which IL-4 and IL-21, programmed cell death-1, and interferon-γ.22 This rapport with T cells has traditionally been considered to have a tremendous contribution to engendering B-cell memory features.
As research sought to clarify the mechanisms underlying the essential attributes of memory responses, such as enhanced production of antigen-specific antibodies due to affinity maturation, the lack of unequivocal markers of B-cell memory has been a significant limitation to reliably distinguish between naïve and antigen-experienced B cells. Several surrogate markers of antigenic experience have been suggested: immunoglobulin class (isotype)-switch recombination (CSR), SHM,23 and surface expression of CD27. It should be noted that CSR/SHM alone cannot distinguish between a true memory or long-lasting cell and a recently activated cell. This distinction can usually be made only if the time of the original antigen encounter is known, as it is the case for vaccinations or in experimental settings.23
The immunoglobulin class refers to the isotype of the immunoglobulin component of the B-cell receptor (BCR). The immunoglobulin component of the BCR changes with B-cell activation during adaptive immune response.24 CSR may occur in B lymphoblasts, but the extent of SHM is limited in this phase. The most extensive SHM and CSR occur in GC,25 which has been a strong argument in favor of T cells ultimately driving the imprinting of memory characteristics in B cells.
MBC: PHENOTYPE AND FUNCTION
MBCs form the basis of lifelong protection. In humans, MBCs specific for smallpox antigens have been detected >50 years after vaccination.26,27 MBCs lack any effector function and need restimulation to contribute to the memory response,23 but they are able to proliferate more rapidly than naïve B cells to form plasmablasts and can also reenter GC to give rise to new memory populations.28-30 It is yet unclear if the constant PC generation required for the maintenance of potentially decade-spanning humoral immunity is driven in an antigen-dependent31 or antigen-independent fashion.32 It is also possible that the source of antibodies could be long-lived cell populations, such as LLPCs.33
Some of the challenges in reliably defining the phenotype of MBC populations are detailed further. As in many aspects of physiology, the use of mouse models has offered clear advantages but equally significant limitations in regards to natural differences to humans, especially in the case of MBCs. Not only are MBCs relatively few (≈5% of peripheral B cells), which still translates into relatively low numbers expected in mouse spleen, but there is no marker for MBCs in mice. Nevertheless, techniques such as bromodeoxyuridine labeling of B cells proliferating at a certain time point or the use of engineered mouse models that allow drug-driven activation of B cells34,35 affords long-term tracking of B cells responding to controlled antigen exposure.
Immunoglobulin Class as a Memory Marker
Traditionally, MBCs have been described as GC derived, class switched, and somatically hypermutated. In recent decades, most aspects of this definition have been under scrutiny. Early findings in rodents suggested that the lack of IgM/IgD as an indication of CSR could be hallmarks of MBC.36,37 However, the absence of either IgM or IgD or both is not a satisfactory marker, as it was revealed later by the identification of somatically mutated IgM+ and IgD+ B cells.38-40 By the late 1980s, several populations of MBCs had been distinguished based on their BCR expression. More recently, insights gathered using mouse models41-49 suggest that when considering immunoglobulin class, as well as several surface markers associated with T-cell interaction, MBCs exist along a continuum, a concept that is further supported when taking population-specific plasticity and lifespan into account.
- IgG+ MBCs (class switched), the quintessential MBC according to the original definition, have undergone extensive SHM and often exhibit ≥2 of the T-cell interaction–associated molecules CD73, CD80, and programmed cell death 1 ligand (PDL)-2 on their surface. Furthermore, cells with this phenotype are more likely to form plasmablasts during secondary responses than IgM+ and IgD+ MBCs.50
- IgM+ MBCs (IgM+IgD− or IgMhiIgDlo) also display SHM,51-53 but to a lesser degree than IgG+ MBC, and are less likely to present CD73, CD80, and PDL-2.41-43,48,49
- IgD+ MBCs (IgM−IgD+ or IgMloIgDhi) are usually CD73−CD80−PDL2− but may still display SHM,49,54 have a greater plasticity, most closely resembling naïve B cells, than IgG+ or IgM+ populations, and have been shown to be GC independent.55
CD27 as a Memory Marker
CD27, a tumor necrosis factor receptor family member, has been widely used in recent years as a marker of memory on human B cells, based on accumulating evidence. In 1988, Klein et al56 reported that a significant subset (≈15%) of IgM+IgD+ B cells, the phenotype generally ascribed to naïve B cells, also expressed CD27 and resembled class-switched and IgM+-only memory cells. CD27+ cells were found to harbor the majority of immunoglobulin sequence mutations, with the molecule being minimally expressed on cord blood and IgD+ B cells, but present on most IgD− cells and generally upregulated on B cells on activation and GC entry, while remaining expressed as cells leave the GC.57 Based solely on CD27 expression, B cells in adult blood and secondary lymphoid organs contain almost equal proportions of naïve (CD27−) cells and memory (CD27+ cells), of which double-negative, class-switched IgM−IgD− and double-positive, unswitched IgM+IgD+ cells each comprise ≈40%; the remaining 20% of CD27+ cells are chiefly IgM only, with a small proportion of highly mutated IgD-only cells that are similar to the IgD-only cells found in GCs and tonsils.23
However, the use of CD27 as the universal marker for memory cells was challenged by the finding that some CD27+ cells are not bona fide MBCs, as well as by the identification of new subsets of CD27− true MBCs. Subsets of B cells sharing either lack of adenosine triphosphate–binding cassette B1 transporter (ABCB1)58,59 or expression of the Fc receptor homolog 4 (FcRH4) family member,60 but not necessarily expressing CD27, were found to exhibit somatically mutated immunoglobulin genes, although the ability of CD27−FcRH4+ B cells, which concentrated mostly in tonsils, to produce antibodies against previously encountered antigens was not investigated. Notably, CD27−ABCB1− B cells produced antibodies against some previously encountered antigens such as tetanus toxoid and influenza virus, albeit they did not react against pneumococcal polysaccharide antigens, unlike their CD27− ABCB1− counterparts.59
PLASMA CELLS AND LONG-LIVED PLASMA CELLS: PHENOTYPE and FUNCTION
Sometimes termed memory PC,23 LLPCs are nondividing cells that secrete large amounts of antibodies and persist for extended periods of time in the bone marrow33,61 independently of antigen.62 Following their identification as ASCs in 1948,63 PCs have been generally considered short-lived based on their rapid decline in secondary lymph organs after peak response,64 rapid death in vitro,65 and characterization as apoptotic cells,66 all of which were arguments in favor of constant production of PC from precursors, rather than prolonged survival of cells past initial response. Although this prolonged survival was proven to be possible,67,68 the contribution of either mechanism to PC maintenance is still unclear, as well as whether LLPCs are simply a vestige of the original responder cells or a discrete entity on their own.
Although ASCs (plasmablasts, PCs, and LLPCs) are the products of terminal differentiation of activated B cells, they constitute a unique, distinct lineage on their own. The details of the processes determining which B cells are committed to the ASC lineage, as well as the exact relationships between ASC types, remain to be elucidated. It has been proposed that higher-affinity BCR is a more common feature in PCs compared with GC B cells and that it may be a driving factor for PC commitment.69 From a transcriptional point of view, the switch from a B-cell program to ASC involves numerous changes, perhaps most notably the repression of broad complex-tramtrack-bric a brac and Cap'n'collar homology 2 (BACH2), an essential transcription factor associated with B cells throughout their development and paired box protein 5, with the upregulation of B lymphocyte-induced maturation protein 1.70 Furthermore, PCs in different compartments were found to differentially express factors such as the antiapoptotic B-cell lymphoma-2, which is increased in bone marrow PCs compared with circulating PCs,23 possibly facilitating long-term survival for these cells.
In terms of surface marker expression, PCs most notably lose their surface immunoglobulin and are typically identified based on their coexpression of CD138 and CD38.71 CD20, a B-cell specific molecule acquired during late pre–B-cell development is also presumed lost on ASC lineage commitment in vivo.72 This has proven therapeutically and experimentally useful in certain settings, as CD20 targeting (eg, rituximab) allows depletion of mature, GC B cells and MBCs without significant reduction of serum immunoglobulin levels, especially due to bone marrow–residing ASC preservation28 providing persistent humoral immunity in the setting of B-cell depletion. However, it represents a specific limitation if anti-CD20 antibodies are used with the goal to reduce antibody concentrations, in the setting of antibody-mediated rejection (AMR) in transplantation as detailed below. Similarly, CD19 expression is diminished in ASC, and there is recent evidence that a population of CD19− LLPC may contribute to long-term humoral immunity in patients with persistent B-cell aplasia following CD19-targeted depletion.73
It is generally believed that the key to LLPC persistence lies in extrinsic signals favoring the survival of these cells. Despite indications that transition to LLPC mandates homing to the bone marrow,70 there is still no consensus on the presence of a physical survival niche (regulatory microenvironments); 2 conflicting models supported by experimental findings were recently discussed by Wilmore and Allman.74 Recent data from long-term studies on primates suggest that LLPCs survived for up to 7.3 years, continuously producing specific antibodies against viral, tetanus, and pertussis antigens despite repeated depletion of B cells and MBCs via anti-CD20 antibodies.27 In this study, the antigen-specific LLPCs were primarily found in the long bones (of upper and lower limb and much less in the ribs, vertebrae, and iliac crest, with the latter representing the typical region of access for all human studies). A variety of cell types, humoral, and genetic factors have been identified in animal models to be essential parts of the microniche in the bone marrow promoting long-term persistence of LLPCs including megakaryocytes,75 mesenchymal stroma cells,76 eosinophils,77 and fat cells.78 However, while factors of the microenvironment composition became better elucidated,79 their origin and detailed contribution remain unclear.
ROLE OF T-CELL INTERACTIONS
The crucial role of GCs, and more specifically interaction with T cells and SHR driving affinity selection inside GCs, in generating B cells with memory attributes was intuited as early as the 1960s.80 The association between B-cell memory and GCs/T-cell help was further strengthened by findings in patients with deficiencies related to factors essential to T/B-cell interactions, such as null mutations in the genes encoding CD40L (CD154)81-83 or ICOS,84 who are unable to establish GCs and long-lasting humoral immunity. However, these patients may still be capable of developing MBC populations, such as the IgM+IgD+CD27+ cells in null-mutated CD154 patients. Also, there is now growing evidence from mouse models that normal organization or even presence of GCs may not be an absolute requirement in the development of memory in B cells, although these cells may not be as numerous or persist as long as in phenotypically normal subjects.81,85-89 Furthermore, the formation of GCs in a TI scenario is also possible.90-92
The contribution of T cells to B-cell responses ties back to antigen structure. Classically, protein and nonprotein antigens differ in that they dictate TD and TI responses, respectively, but the intricacies of the B-cell responses extend beyond this division. All mature B cells are capable of mounting a TI antigen response, with follicular B cells appearing more likely to respond in a TD fashion. LLPCs and persistent antibody levels can be successfully induced by TI antigens even in the absence of significant GC formation.93-95
Recognizing the interplay and redundancy between T-cellular and humoral immune response is crucial to explain some of the clinically observed phenomena in organ transplantation, such as the strong correlation between recurrent acute cellular rejection and the development of donor-specific HLA antibodies. It also explains differences in the observed posttransplant antibody development toward peptide (HLA) and polysaccharide (ABO) antigens on the transplanted organ and age-related differences in the effectiveness of desensitizing interventions. These aspects will be discussed in the following.
The impact of MEMORY B CELLS AND LONG LIVED PLASMA CELLS on antibody-reducing strategies in transplantation
Antibody development toward the donors HLA or ABO surface markers has been progressively identified as a key factor limiting the longevity of transplanted organs, resulting in acute AMR and contributing to chronic vasculopathy and progressive subacute destruction of virtually any transplanted organ.96 Medical interventions include direct reduction of plasma antibody levels by plasmapheresis97 or immune adsorption,98 as well as interventions to reduce pathological downstream effects of the antibodies by hampering complement activation99 showing some clinical effect, mostly in reducing the incidence of AMR. However, antibody removal alone appears to be a transient solution only, as long as the reproduction of new donor-directed antibodies by newly maturating PCs or persisting LLPCs is not affected. In addition, B cells are known to have antigen-presenting function and the capacity to directly facilitate activation of other immune cells and therefore harmful effects on the graft beyond the differentiation into new PCs. An overview of targets of currently commonly used interventions and their impact on different cells involved in humoral immune response is schematically shown in Figure 2 and some specific clinical aspects are outlined in Table 1.
In recognition of the transient character of sole antibody removal, therapy protocols for AMR or pretransplant desensitization mostly include B-cell–directed agents, usually anti-CD20 antibodies with a B-cell depleting effect. Rituximab has been the first clinically available anti-CD20 antibody and has been used in the management of autoimmune disorders and hematologic diseases since 1997. The available literature in solid organ transplant almost uniformly used rituximab for B-cell depletion. Newer agents include the humanized antibody ofatumumab, with greater efficiency toward cells with lower CD20 expression,100 obinutuzumab, which shows less lipid raft relocalization of CD20 resulting in less complement dependent but more direct cytotoxic B-cell depleting effects,101 and ublituximab, which provides enhanced antibody-dependent cellular cytotoxicity through glycoengineering.102 While extensive comparison studies are underway, mostly in various settings of the therapy of hematological neoplastic disorders, none of these agents has thus far been evaluated in a structured study for AMR in solid organ transplant and the available published data are limited to case reports. Whether potential benefits on rituximab resistance or low CD20-expressing lymphoma cells are relevant in the setting of depletion of healthy B cells in the setting of AMR is unclear but could be target of future studies.
When differentiating into plasmablasts and PCs, CD20 expression is lost on the B-cell surface. Accordingly, PCs and LLPCs will not be depleted by rituximab therapy, and following isolated rituximab therapy, a measurable reduction of allospecific antibodies is expected with at least 3 months delay after natural demise of the short-lived PCs with no new supply following. Persistence of some degree of new antibody production has been observed in both animal models27 and human trials and is likely associated with continuous low-level antibody production by LLPC. In human therapy trials, the picture is heterogenous because anti-CD20 therapy is almost always combined with intravenous immunoglobulin therapy (IVIG), often with antibody removal therapies and sometimes with PC-directed therapies.
It becomes obvious that rituximab therapy as a sole intervention will rarely be a suitable strategy in the clinical setting due to delayed and limited effectiveness. The addition of high-dose (2 g/kg body weight every 4 wk) IVIG does not only provide protection from infections to the recipient but also provides immune modulation via negative feedback, alteration of the cytokine milieu, and blockade of the Fc receptors of many immune cells. Whether the IVIG also has a regulatory effect on LLPC remains unclear, however, appears plausible. Interestingly, single time pretransplant therapy with rituximab was found to result in long-term persistent reduction of antibodies directed toward the AB blood group antigens in the setting of ABO-incompatible solid organ transplant in adults103 and older children.104 This is in keeping with the hypothesis that the mostly TI activation of B cells toward these polysaccharide antigens105 is predominantly provided by the splenic MZ B cells and IgM-expressing MBC106 and possibly no bone marrow–resident LLPC are toward AB antigens.
Anti-CD19 antibodies would provide a broader therapeutic application because CD19 expression persists throughout B-cell activation and in PCs. Treatment with anti-CD19 has been found to inhibit the production of allospecific IgG in animal trials107; however, clinical application thus far has not gone beyond phase II trials, mostly in hematologic–oncologic settings. More recent trials have included CD19 into chimeric antigen receptors and may result in promising therapeutic options for the future.108
Antithymocyte globulins (ATGs) are generated by injection of human thymic tissue and cell lysates into animals. Consequently, they are polyclonally directed against a variety of human lymphocyte surfaces including common leucocyte antigens (CD52), HLA epitopes, and other surface markers shared between many lymphocytes including B cells such as CD5 and CD27. Accordingly, some degree of B-cell depletion is expected in the context of ATG therapy either as induction agent or for acute cellular rejection. However, the effect seems much less pronounced on B cells than T cells, and lymphocyte subtype analysis post-ATG therapy commonly shows B cells as the predominantly remaining cell type. At present, there is no evidence of a significant antibody-reducing effect of ATG; however, a clinical benefit for the graft may arise in the individual situation from limiting the downstream effects of antibodies as well as the T-cell support of activated B cells.
Proteasome inhibitors were initially developed for therapy of neoplastic proliferation of PC-derived tumor cells (multiple myeloma) but found to also induce apoptosis in regular PCs, resulting in reduced anti-HLA antibody production.109 Clinical efficacy against AMR was shown in small studies as part of combination therapies with antibody removal, IVIG, and B-cell depletion.110 However, subsequent trials including a recent placebo-controlled study111 suggest that bortezomib therapy lacks long-term efficacy on HLA antibodies and is not effective to prevent graft deterioration in AMR late after transplant.112 This lack of long-term efficacy is observed despite mechanistic studies showing that proteasome inhibitors (bortezomib, carfilzomib, and new agents under development) in vitro not only reduce immunoglobulin production and survival of peripheral blood PCs but also induce apoptosis in B cells including MBC. The discrepancy between in vitro effects and limited efficiency in vivo may be due to microenvironmental factors in secondary lymphatic organs and especially a lack of effect on LLPC resident in the bone marrow; however, there are no available experimental data to confirm this hypothesis, and given the lack of access to theses, LLPC in a human setting is unlikely to be generated. In mice, bortezomib showed a limited effect on antiplatelet antibody-producing LLPC in vitro, but these cells had been isolated from their bone marrow environment and stimulated in isolated cultures.113
ROLE OF CONCOMITANT ANTIBODY-DIRECTED THERAPIES
Teasing out individual effects of antibody-directed therapies in the clinical setting is extremely difficult because the majority of trials concomitantly use multiple modalities. There is good rationale for this approach; however, some considerations have to go into planning of a desensitization or AMR treatment protocol:
- Antibody removal via plasmapheresis and immune adsorption appears to have a feedback mechanism on PC and B cells resulting in higher proportions of activated cells and consequently better responsiveness to proteasome inhibitors. Accordingly, performing these therapies before proteasome inhibitor application may increase their efficacy. In the oncological setting, proteasome inhibitor efficacy appears enhanced by parallel steroid application.114
- Depleting antibodies such as rituximab rely on downstream effects of antibody-mediated immune response including complement activation. Therefore, sufficient time intervals for them to unfold their effect have to be provided before using complement activation inhibitors (eculizumab, C1 esterase inhibitors) or immune-modulating doses of IVIG.
- A recently published technique uses cleavage of IgG using a streptococcal endopeptidase (immunoglobulin-depleting enzyme of Streptococcus pyogenes)115 to separate Fc and Fab fragments of IgG in 2 steps thus reducing complement activation in the first step and any type of Fc-mediated antibody effect. In a pilot study, it effectively depleted or reduced HLA antibodies in 24 of 25 highly sensitized patients in preparation for kidney transplantation in combination with rituximab and IVIG.116 This may represent a new approach enhancing antibody removal with a less invasive method compared with the current mechanical removal techniques.
- Several novel therapeutic interventions target the downstream effects of antibodies by blocking complement activation (eculizumab)117 or antibody production via intervention in the cytokine communication (tocilizumab)118 and have been used as additional or escalating steps to the above-discussed combinations. Their detailed discussion would exceed the focus of this review.
Memory reactivation resulting in newly formed PCs and persistence of antibody production by bone marrow–resident, immune-privileged LLPCs reflect synergistic and redundant systems providing highly effective antibody-mediated immune response to previously encountered antigens. Accordingly, no single target therapeutic strategy is likely to fully succeed in providing complete resolution of antibody-mediated effects on a transplanted organ. Modalities that are effective in vitro are often limited by microenvironment factors in vivo. Therapeutic combination approaches targeting B-cell memory, PCs, antibody concentration, and downstream effects are currently the most promising, albeit not perfect, approach to prevent antibody-mediated harm from the graft.
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