Targeting B Cells and Antibody in Transplantation

Abstract

There has been increasing interest in the role played by B cells, plasma cells and their associated antibody in the immune response to an allograft, driven by the need to undertake antibody-incompatible transplantation and evidence suggesting that B cells play a role in acute cellular rejection and in acute and chronic antibody-mediated rejection. A number of immunosuppressive agents have emerged which target B cells, plasma cells and/or antibody, for example, the B cell-depleting CD20 antibody rituximab. This review describes recent developments in the use of such agents, our understanding of the role of B cells in alloimmunity and the application of this knowledge toward novel therapies in transplantation. It also considers the evidence to date suggesting that B cells may act as regulators of an alloimmune response. Thus, future attempts to target B cells will need to address the problem of how to inhibit effector B cells, while enhancing those with regulatory capacity.

Introduction

Over the past two decades, immunosuppressive strategies in solid organ transplantation have focused on depleting T cells or inhibiting their function. However, there has been increasing interest in the role played by B cells, plasma cells and their associated antibody, in the immune response to an allograft, initially driven by the need to reexplore the feasibility of antibody-incompatible transplantation. ABO-incompatible living donor renal transplantation is now widely offered, leading to the notion that the deleterious effects of B cells and antibody on allograft survival can be overcome. However, human leucocyte antigen (HLA)-incompatible transplantation remains a challenge, particularly in recipients of deceased donor organs, where a timely desensitization programme may not be feasible. This is a pressing issue for heart and lung transplant recipients, many of whom are sensitized prior to transplantation. The appearance of de novo donor-specific antibodies (DSA) and the development of acute antibody-mediated rejection (AMR) can also negatively impact on allograft survival, particularly if this occurs after the early post-transplant period. In addition, there is an increasing appreciation that B cells may play a role in acute cellular rejection (ACR) and perhaps more significantly, in chronic allograft attrition, in the guise of chronic AMR. In response to these clinical needs, a number of immunosuppressive agents have emerged which target B cells, plasma cells or antibody. Many of these agents were initially used in hemo-oncology for the treatment of B cell or plasma-cell malignancies, and were subsequently adopted for the treatment of B-cell-mediated autoimmune diseases and in transplantation. In this review, I will outline recent developments in our understanding of the processes involved in B-cell activation and the generation of alloantibody and how this can be applied to identify new therapeutic targets in transplantation. I will also consider the growing body of evidence demonstrating that B cells can not only act as effectors, but may also negatively regulate or modulate immune responses. Thus, the therapeutic goal is no longer simply one of B-cell depletion, as this may have deleterious effects on long-term transplant outcomes, but may require more subtle approaches to manipulate different B-cell subsets.

The B Cell and Its Activation

When considering B-cell-directed therapy in transplantation, it is important to appreciate that the lineage contains a variety of cells, with differing functions and surface markers (Figure 1). B1 cells reside principally in the peritoneal and pleural cavities, are characterized by the expression of CD5, and produce low-affinity natural antibody independent of T-cell help. B2 cells are formed in the bone marrow, released as immature B cells, and continuously circulating through secondary lymphoid organs (spleen and lymph nodes) until they encounter antigen. Once activated, a B cell interacts with its cognate T cell, through the presentation of antigen displayed on major histocompatibility complex (MHC) class II molecules which are recognized by the T cell via its T-cell receptor (TCR). B cells are important antigen presenting cells (APCs), due to their ability to clonally expand, and efficiently take up antigen via their B-cell receptor (BCR). B cells can also produce cytokines which support T cells (1) (Figure 2). Hence, B cells are critical for optimal T-cell activation (2), and the development of T-cell memory (3) in alloimmune responses.

Figure 1. B-cell ontogeny and differentiation.

Most peripheral B cells are produced in bone marrow, and referred to as B2 cells. A minor B-cell population, known as B1 cells (initially identified in mice, and very recently in humans), are found mainly in the pleural and peritoneal cavities, but also in small numbers in the spleen. B1 cells express CD19, high levels of CD5 and produce low affinity natural antibody (mainly IgM), without T-cell help. It is currently unclear whether B1 cells arise from a unique progenitor or from a progenitor common to both B1 and B2 cells. B2 cells are formed in the bone marrow and develop from pro-B cells, to pre-B cells, though to immature B cells which are released into the periphery. Following antigen encounter, B cells obtain T-cell help and enter the germinal center. Here they undergo class switch recombination and affinity maturation, a process involving iterative cycles of somatic hypermutation and proliferation. A specific subset of T cells, known as T follicular helper cells (Tfh) are critical for germinal center formation and are thought to provide both contact and cytokine (IL21) signals for GC B cells. Short-lived plasmablasts and memory cells arise from GC B cells. Some plasmablasts circulate (depending on the expression of (CXCR3 and CXCR4) and a small proportion find a suitable niche for long-term survival within bone marrow and inflamed tissue, e.g. rejecting allografts. A variety of molecules are expressed by B2 cells during their maturation and activation, as shown; Fcγ RIIB is expressed throughout development, whereas most other markers are expressed either on B cells or on antibody-producing plasma cells. Dark blue bars represent high expression, light blue bars intermediate expression. Regulatory B cells are a recently described subset which inhibit T-cell responses via the production of IL10. These cells are likely to arise from multiple B-cell subsets (B1, transitional and marginal zone). Regulatory B cells have recently been described in humans and are characterized by surface expression of CD5, CD1d, CD24, CD27 and CD38. There are also thought to be regulatory B cells which act independently of IL-10 (not shown).

Figure 2. Balancing the potential beneficial and deleterious effects of B-cell depletion.

B-cell depletion (e.g. using rituximab) has been undertaken with the aim of reducing the potential deleterious contribution of B cells to an alloimmune response (left panel). These include antigen presentation to T cells, cytokine production facilitating T-cell polarization, antibody production, and tertiary lymphoid organ generation. In addition, there may be an expansion of regulatory B cells in the reconstituted B-cell compartment post-B-cell depletion that might potentially promote tolerance (see Ref. 33). However, this must be balanced with the potential adverse effect of depleting regulatory B cells (right panel). Regulatory B cells modulate T cell-responses through the production of IL-10 (B10 cells), or TGF-β or contact-mediated inhibition. Thus, B cells may promote tolerance through inhibiting effector T cells (T eff), dendritic cells and macrophages and facilitating the expansion of regulatory T cells (T reg). The timing of B-cell depletion is likely to be critical in determining whether B-cell depletion exacerbates or modulates alloimmunity.

Activated B cells may form extrafollicular plasmablasts, producing early, low affinity antibody or may enter the germinal center where they undergo somatic hypermutation and class switch recombination. Germinal center B cells with higher affinity for antigen are positively selected and differentiate into either memory B cells or plasma cells. Recent studies have demonstrated that a Bcl-6-expressing T-cell subset found within B-cell follicles (T follicular helper [Tfh] cells) are essential for the development of germinal center B cells (4). Specific inhibition of Tfh cells may represent a useful strategy in future attempts to inhibit humoral alloimmunity. A small proportion of plasma cells arising from the germinal center become established as long-lived plasma cells in the bone marrow. They reside within a number of limited niches, do not proliferate, but act as long-term antibody factories, producing IgG. Plasma cells have also been described in inflamed tissues in autoimmunity and within allografts (5-7), suggesting that inflammatory lesions in peripheral tissues can provide additional niches for plasma cells (Figure 1). Furthermore, tertiary lymphoid organs have been observed in animal models of transplantation (5) and in human renal and cardiac allograft (5-7) raising the possibility that B-cell activation may occur directly in the graft. B cells can produce lymphotoxin-β and VEGF-A, driving lymphoid organ formation and lymphangiogenesis respectively (8), and may therefore play a role in orchestrating the development of these structures within allografts.

BAFF (B-cell-activating factor belonging to the tumor necrosis factor family), also known as BLys, TALL-1, and THANK) is a cytokine which enhances survival of B cells, particularly näive B cells (9,10). It exists in both membrane-bound and soluble forms and is produced by monocytes, macrophages, and dendritic cells (DCs). There are three BAFF receptors; BAFF-R (also known as BR3), TACI (trans-membrane activator and calcium modulator and cyclophyllin ligand interactor) and BCMA (B-cell maturation antigen). These are principally expressed on follicular, germinal center and memory B cells (Figure 1), but BCMA is preferentially expressed on plasma cells while BAFF-R is also expressed on activated T cells and regulatory T cells. Over-expression of BAFF leads to abnormal B-cell survival, hypergammaglobulinaemia, and a lupus-like autoimmune disease in mice (10).

The antibodies produced by terminally differentiated B cells are critical mediators of pathogenic alloimmunity. Antibodies have a variable, antigen-binding F(ab)2 region and an Fc region responsible for mediating many effector functions of antibody, for example, complement activation and Fc receptor cross-linking. Activatory Fcγ Rs cross-linking by IgG may result in DC maturation, macrophage phagocytosis and pro-inflammatory cytokine production, and neutrophil activation as well as NK cell-mediated antibody-dependent cellular cytotoxicity (ADCC) (11). These activatory effects of antibody are controlled by a single inhibitory receptor Fcγ RIIB, which is also expressed on most immune cells, modulating immune-complex-driven activatory Fcγ R activity and B-cell activation via the BCR (11).

IgG has a far longer half-life than other serum proteins, and this attribute is critically dependent on the neonatal Fc receptor, FcRn. FcRn belongs to the MHC class I family, and binds to IgG at low pH after it has been endocytosed and transported to acidic vesicles. FcRn-bound IgG is recycled to the cell surface, rather than being degraded, and is released back into the circulation at physiological pH. Mice deficient in FcRn have reduced serum IgG levels and a significantly shortened IgG half-life (12). Saturation of FcRn is thought to be the mechanism by which therapeutic IVIG mediates an acute reduction in alloantibody titers (13). In addition, IgG engineered to have an increased affinity for FcRn can potently inhibit the interaction of FcRn with endogenous IgG, leading to a rapid reduction in IgG titers (14). These ‘AbDegs’ (antibodies which enhance degradation), may represent a potentially useful therapeutic future strategy to reduce circulating alloantibody (Figure 3).

Figure 3. Current and potential B-cell therapy in transplantation.

The main mechanisms of action of agents are: 1. B-cell depletion or inhibition of B-cell activation; 2. plasma-cell depletion; 3. antibody removal; 4. inhibition of antibody effector function. B-cell depleting agents currently in use in transplantation are ATG, alemtuzumab, rituximab. Those with future potential include the CD20 antibodies ocrelizumab and ofatumumab and the CD19 antibody MDX1342. Agents which may reduce B-cell activation or limit survival include those targeting the BAFF pathway (belimumab and atacicept) and epratuzumab. ATG and alemtuzumab may also limit B-cell survival though removal of T cell help. The proteosome inhibitor bortezomib has been used to deplete plasma cells, other potential strategies include blockade of BAFF and APRIL (e.g. with atacicept) or cross-linking of Fcγ RIIB to induce apoptosis. Plasmapharesis allows the removal of antibody, but is a short-term solution, as titers usually rebound. IVIG can also modulate IgG titers, probably by blocking FcRn-mediated salvage of circulating IgG. AbDegs (IgG modified to increase affinity for FcRn) might also be useful to reduce alloantibody. IVIG may also act to block the effector functions of antibody, including activatory Fcγ R ligation on neutrophils and macrophages (Fc mediated) as well as neutralizing complement components (F(ab)2 mediated).

It has recently been appreciated that B cells are not only immune activators, but that subpopulations may act as inhibitory cells (Figure 1). These regulatory B cells were first described in the mouse and subsequently in humans (15-17) and mediate their regulatory function via the production of IL-10, leading some authors to describe them as B10 cells (17). In mice, several groups have identified IL-10-producing regulatory B cells within a number of B-cell compartments including CD5+ B1 cells, CD1d^hi marginal zone B cells, and CD21^hiCD23^hi transitional two marginal zone B cells. In humans, IL-10-producing B cells have also been identified and comprise around 5% of circulating B cells, although cells with the potential to produce IL-10 may be found at higher frequencies. Human IL-10-producing B cells characterized by surface expression of CD24, CD27, and CD38 and also express high levels of CD5 and CD1d. Studies suggest that peripheral B cells may be manipulated ex vivo via CD40 and TLR ligation to harness all potential IL-10-producing B cells (17). This raises the possibility that regulatory B cells generated ex vivo may be used therapeutically. It will also be of interest to determine whether regulatory B cells play a role in transplant tolerance, given the increasing evidence of a B-cell signature in microarray studies of tolerant transplant recipients, and the presence of a higher percentage of B cells expressing CD1d and CD5 in the peripheral blood of tolerant patients (18,19). In mice, transplant tolerance can be induced by the administration of an anti-CD45RB antibody. This phenomenon is dependent upon B cells, but not on IL-10 (20). Thus, there are likely to be additional regulatory B-cell subsets which suppress in an IL-10-independent manner, via production of TGF-β or via contact-dependent inhibition (17) (Figure 2).

B-Cell Targeted Therapies

In transplantation, there are a number of clinical scenarios in which one might envisage efficacy for B cell/plasma cell/antibody inhibition; desensitization to allow antibody-incompatible transplantation, acute or chronic AMR and ACR resistant to T-cell therapies. In contrast, it may be useful to augment B cells to induce transplant tolerance (reviewed by (21)). Current and potential therapeutic strategies include B-cell depletion, inhibition of B-cell activation, plasma-cell depletion and antibody removal or inhibition antibody effector function (Figure 3).

B-cell depletion

B-cell depletion has been achieved through splenectomy or via the administration of cytotoxic antibodies which bind antigens expressed on the surface of B cells. Agents currently used for B-cell depletion are the anti-CD52 antibody, alemtuzumab (CAMPATH-1H), anti-thymocyte globulin (ATG) (both of which deplete T cells in addition to B cells), and the anti-CD20 antibody rituximab (discussed below). Future B-cell depleting agents which may be useful in transplantation include a potentially more potent anti-CD20 antibody, ofatumumab (22) and anti-CD19 monoclonal antibodies (23), the latter showing promise in the treatment of some autoimmune diseases. CD19 is expressed earlier in the B-cell lineage than CD20, and in memory B cells, short-lived plasma cells and B1 cells (Figure 1), thus, CD19-targeted therapy has the advantage of depleting additional B-cell subsets, including 50% of murine bone-marrow resident, long-lived plasma cells (23,24). Thus, in contrast to CD20 antibody treatment, a single treatment with a CD19 monoclonal antibody, resulted in a reduction in circulating allograft-specific IgG in an acute cardiac allograft model (24). Furthermore, in a murine model of renal chronic allograft rejection (DBA/2 to C57BL/6), B-cell depletion using a CD19 antibody significantly enhanced allograft survival, was associated with a reduction in IgM and IgG alloantibody titers, and was shown to reduce alloantibody titers in sensitized animals (24).

Rituximab is a chimeric murine–human monoclonal antibody directed against the B–cell surface molecule CD20. CD20 is not found on pro–B cells or mature plasma cells (the latter produce 90% of circulating IgG), thus, rituximab eliminates peripheral B cells without preventing the regeneration of B cells from precursors, and does not directly affect immunoglobulin levels. A number of reports also suggest that rituximab may deplete B-cell aggregates within allografts (25). Despite a lack of effect against long-lived plasma cells, and therefore serum IgG, some (26,27) (but not all (25)) groups have reported that ritxuimab treatment is associated with a reduction in DSA titers. This suggests that the alloantibody-producing cell in such cases is not a long-lived plasma cell and continues to express CD20, or more likely reflects the effects of high-dose intravenous immunoglobulin (IVIG) which was used concomitantly with rituximab in some of these studies (see later discussion) (26,27).

Rituximab has been used in renal transplantation for the treatment of AMR in more than 150 patients (reviewed in (25)) and in conjunction with IVIG for the prevention of AMR (27,28). In such patients, rituximab-mediated depletion may well prevent the generation of additional alloantibody-producing cells from the naive B-cell pool, and may also target short-lived plasma cells still expressing CD20 (Figure 1). Some, but by no means all (29), studies have shown an association between CD20+ infiltrates and steroid-resistant ACR, and rituximab has demonstrated efficacy for this indication (25). More recently there has been interest in the use of rituximab for the treatment of chronic AMR in renal transplantation (25,26), and a randomized control trial is currently on-going in the United Kingdom to test its efficacy in this context (http://clinicaltrials.gov/ct2/show/NCT00476164).

The data linking B cells with ACR, as well as acute and chronic AMR has prompted the use of rituximab as an induction agent in transplantation in nonsensitized patients. Two randomized controlled trials (RCT) have been undertaken for this indication. Tyden and colleagues used a single dose of rituximab in combination with steroids, tacrolimus and mycophenolate mofetil and showed a reasonable rate of ACR (11.6% at 6 months compared with 17.6% in the control group) (30). We undertook a similar trial, but were forced to halt recruitment, due to an excess rate of ACR in the rituximab group (83% vs. 14% in the control group) (31). In contrast to the study by Tyden et al., we used two doses of rituximab, at day 0 and day 7 and did not use corticosteroids for maintenance. The explanation for the observed differences may be that the timing of B-cell depletion is critical in determining whether regulatory or effector B cells are targeted and that regulatory B cells were inadvertently depleted. This study emphasizes the concept that B cells may play an important role in modulating the immune response to an allograft, and that pan-B-cell depletion is therefore a potentially risky strategy (Figure 2). Recent data showing accelerated allograft rejection, and enhanced CD4 T-cell activation in a skin graft model following CD20 antibody-mediated B-cell depletion supports such a view (24). In addition to the removing the direct beneficial effects of regulatory B cells on an alloimmune response, B-cell depletion may also negate the indirect effect of B cells in expanding regulatory T cells (reviewed in (1)). The effects of B-cell depletion on immune regulatory cells are likely to explain the less well-publicized, paradoxical, exacerbation of existing autoimmunity or the occurrence of new autoimmune disease in patients treated with rituximab (32). However, there is some evidence that the reconstituted B-cell compartment following B-cell depletion may have a bias toward a regulatory phenotype, promoting long-term islet allograft survival in a nonhuman primate model (33).

Modulation of B-cell activation and survival

Antigen-specific activation of B cells via the BCR can be modulated by engagement of costimulatory receptors, such as CD19 and CD21 (CR2), or B-cell inhibitory receptors, such as Fcγ RIIB and CD22. Epratuzumab, a humanized anti-CD22 antibody, induces some depletion of naive and transitional B cells, producing a 35% reduction in total B-cell numbers but can also inhibit B-cell activation and proliferation (34).

BAFF is an important costimulator of B-cell survival and expansion. There are a number of small studies implicating BAFF in the acute and chronic AMR, including the presence of BAFF in C4d positive renal transplant biopsies with acute rejection and BAFF-R positive B and T cells in allografts with chronic rejection (10). A number of agents have been developed to target this pathway, including belimumab and atacicept. Belimumab, a humanized monoclonal antibody that specifically inhibits BAFF, has been shown to be effective in SLE (35). When considering the use of this agent in transplantation, it is worth noting that B cells are not universally dependent on BAFF for survival. Naive B cells are more sensitive to BAFF depletion than memory B cells (9). Thus, BAFF-blockade may be more useful for the depletion of B cells in AMR in the context of de novo antibody production, rather than in sensitized individuals with a large alloantigen-specific memory pool.

Atacicept is a fusion protein formed from the extra cellular domain of one of the receptors for BAFF, TACI. It binds BAFF and APRIL (a proliferation inducing ligand), which also promotes B cell and plasma-cell survival. Atacicept has been trialed in SLE and induced a modest depletion in peripheral B cells, but a significant (30%) reduction in serum IgG levels, reflecting the sensitivity of plasma cells to TACI blockade (35). It may therefore be useful in sensitized patients with established alloantibody-producing plasma cells.

Plasma-cell depletion

ATG contains antibodies which can bind syndecan (CD138), a plasma-cell-specific molecule (36), although in vivo ATG treatment is not associated with a reduction in either splenic or bone-marrow plasma cells. However, clinical studies suggest that ATG may reduce the risk of AMR in patients with preformed donor specific antibody (DSA) (37), presumably by removing T-cell help for alloreactive B cells and via B-cell depletion due to antibodies which directly bind B cells (36).

Bortezomib is a proteosome inhibitor which is licensed for the treatment of multiple myeloma. Plasma cells produce large quantities of protein (IgG), a proportion of which will be misfolded and require degradation and disposal by the proteasome. Inhibition of proteasome activity results in the accumulation of misfolded proteins and consequent apoptosis of plasma cells. In addition, proteosome inhibition reduces NF-κB activity, due to a reduction in proteosome-mediated breakdown of IκB. This leads to a reduction in NF-κB-driven transcription of a number of cytokines including IL-6, a potent plasma-cell survival factor (38). In vitro, bortezomib leads to apoptosis of alloantibody-producing plasma cells and may lead to a modest reduction in anti-HLA antibodies in sensitized patients (39) (previous reports reviewed in (25)), but may not be sufficiently efficacious to be used as a single agent for desensitization (40). In addition, one center has also used bortezomib in nonsensitized patients as part of a more complex induction/tolerization strategy (41). Bortezomib has also been used to treat acute AMR in kidney transplantation (38,42,43). Experimental transplant models suggest possible efficacy in chronic AMR (44,45). Although bortezomib is thought to target long-lived plasma cells, there may be a preferential effect on alloantibody-producing cells; Everly et al. noted a conservation of protective antibody titers, despite the reduction in DSA (46). In summary, the evidence to date (comprising case reports and series) suggests bortezomib may be useful in transplantation, particularly for the treatment of AMR but given associated toxicity, a (RCT) for this indication is urgently needed.

The inhibitory receptor, Fcγ RIIB, is expressed on plasma cells, as well as B cells. A monoclonal antibody against Fcγ RIIB (hu2B6–35) has been used to direct monocyte- or macrophage-induced cytotoxicity against plasma cells from patients with systemic light-chain amyloidosis (47). In addition, Fcγ RIIB cross-linking on B cells, plasma cells and myeloma cell lines (48) can induce apoptosis in vitro. Thus Fcγ RIIB-crosslinking may potentially provide a mechanism by which alloantibody-producing plasma cells can be depleted.

Antibody removal and the inhibition of antibody effector function

Plasma exchange/plasmapheresis (PP) is effective in depleting DSA prior to transplantation and in patients with AMR. However, antibody titers rapidly rebound unless additional measures are used to prevent further DSA production. IVIG has been used with, or instead of, PP to reduce DSA titers during desensitization protocols and in the treatment of AMR (49).

IVIG is a preparation of human polyclonal IgG obtained from pooled plasma samples. There is little direct experimental evidence addressing the mechanism of action of IVIG in transplantation, but studies in autoimmunity suggest that it is likely to have multiple modes of action, that target different aspects of the humoral immune response and perhaps also cellular immunity (13) (Figure 3); For example, high-dose IVIG given in place of PP during desensitization or in the treatment of AMR rapidly reduces DSA titer, an effect likely mediated by altering IgG half life through the blockade of FcRn. In the treatment of AMR, IVIG may also dampen the inflammatory response to deposited alloantibody by inhibiting complement activation and complement-mediated B costimulation via CD21 (CR2), and blockade of neutrophil and macrophage activation via Fcγ Rs. In addition, IVIG may also induce an upregulation of the inhibitory receptor Fcγ RIIB on monocytes and B cells, an effect dependent on the glycosylation state of IgG, specifically the degree of sialylation (50). In an era where demand for IVIG significantly outweighs supply, altering sialylation may be a means of substantially improving efficacy. In addition to its effects on antibody and Fcγ Rs, there is evidence to suggest that IVIG might also induce an expansion of regulatory T cells (51).

There are only three RCT examining the efficacy of IVIG in transplantation, two in the context of desensitization prior to HLA-incompatible transplantation and one in the context of treatment of steroid resistant rejection (reviewed in (49)). Thus, despite the fact that IVIG has been used in thousands of transplant recipients, particularly for desensitization, RCT data are available for just 159 patients. These studies, together with additional data from nonrandomized studies and case series suggest efficacy for the use of low-dose (100 mg/kg) IVIG with PP perioperatively, high-dose IVIG (2 g/kg) prior to transplantation to reduce DSA (27,49) or for its use in conjunction with ATG as induction therapy in sensitized patients (37). IVIG also has some efficacy in the treatment of AMR, and the addition of rituximab may provide further benefit. Furthermore, the administration of IVIG to sensitized patients post-transplantation, again, in conjunction with rituximab may lower DSA and chronic AMR (28).

More recently, there has been interest in the use of eculizumab, an antibody directed against the complement component C5, and currently licensed for use in paroxysmal nocturnal hemoglobinuria. Eculizumab appears efficacious in patients with recurrent atypical hemolytic uremic syndrome post-transplant, has been used to treat AMR (52,53) and is also effective in preventing AMR in renal transplant recipients transplanted despite a positive cross-match (54). A single center, open-label study is underway to tests its efficacy in this context (http://clinicaltrials.gov/ct2/show/NCT01106027).

Concluding Comments

The removal of DSA through PP is both expensive and invasive, and difficult to apply in a timely manner to sensitized patients awaiting deceased donor transplantation. This situation highlights the need for alternative strategies to limit alloantibody and its effects. There is also firm evidence demonstrating that B cells play an important role in the immune response to an allograft beyond antibody production, particularly in supporting T cells. We are entering an era in which there is a huge expansion of agents available to target the humoral immune response, and a willingness to use them in transplantation, as demonstrated by the widespread application of rituximab. However, a review of the paucity of high-grade evidence for IVIG in transplantation provides a cautionary tale and encourages us as a transplant community to initiate and support RCT of these novel agents in order to produce appropriate evidence on which to base our clinical practice. Such studies are particularly pertinent given recent data demonstrating the existence of regulatory B cells in humans, and their possible importance in transplant tolerance. Thus, the challenge will be to determine how to use these new agents, perhaps in combination, to achieve more subtle modulation of B cells.

Abbreviations

ACR

acute cellular rejection

ADCC

antibody-dependent cellular cytotoxicity

AMR

antibody-mediated rejection

APC

antigen presenting cells

APRIL

a proliferation inducing ligand

ATG

anti-thymocyte globulin

BAFF

B-cell-activating factor belonging to the tumor necrosis factor family

BCR

B-cell receptor

DC

dendritic cell

DSA

donor-specific antibody

HLA

human leucocyte antigen

IVIG

intravenous immunoglobulin

MHC

major histocompatibility complex

PP

plasmapheresis

TACI

transmembrane activator and calcium modulator and cyclophyllin ligand interactor

Tfh

T follicular helper cell