The role of B lymphocytes in acute allograft rejection

Abstract

The role of B lymphocytes in the progression of acute allograft rejection has been assumed to be negligible; this, despite the well-established capacity of B cells for: 1) specific alloantigen uptake and presentation, 2) activation and clonal expansion upon alloantigen encounter and 3) rapid recirculation in vivo. Here, a two-phased translational study addressing the role of B lymphocytes in acute allograft rejection is described. Chimeric mice (H-2b) with an MHC class II deficiency confined to the B lymphocyte compartment were genetically engineered and utilized as allograft recipients in order to assess the requirement of B cell antigen presentation for the progression of acute rejection. These mice were transplanted with allogeneic heterotopic cardiac allografts from H-2d or H-2k strain donors. In the absence of B cell mediated antigen presentation, acute rejection of MHC disparate cardiac allografts was disrupted (MST<>100 days; n=30) as compared to that seen in control chimeras (MST=12d;n=15). To determine whether this enhanced survival was the result of inefficient CD4 T cell priming, we utilized an alloreactive TCR transgenic system to quantify the extent of alloreactive CD4 T cell activation. In the absence of B cell MHC class II presentation, alloreactive CD4 T cell activation following transplantation was 3-6 fold reduced as compared to control counterparts. This latter finding was based on an vivo tracking experiment using CFSE to archive the division history of alloreactive CD4 T cells. Overall, the mechanistic phase of this study led to the conclusion that B cell mediated antigen presentation is a key participant in the progression of acute allograft rejection. It was, therefore, hypothesized that B lymphocyte directed immunotherapy might improve the outcome of clinical organ transplantation. A preclinical translational model of islet transplantation in cynomolgus monkeys was utilized to address this possibility. An anti-B lymphocyte mAb (Rituxan) was combined with a routinely utilized T cell specific induction agent (Thymoglobulin) to determine if transient B cell depletion early following transplantion would promote immunological tolerance. Two cohorts of monkeys were rendered diabetic and transplanted with allogeneic islets. The experimental cohort was treated with an induction course consisting of a combination of Rituxan and Thymoglobulion while the control cohort was treated with Thymoglobulin alone. The experimental regimen led to a near complete depletion of peripheral B cells, which lasted for 60-90 days. All recipients were treated with a maintenance dose of Rapamycin. Islet allograft survival was limited in control recipients (MST=20 days; n=5), whereas experimental monkeys were free of diabetes for a markedly prolonged period (<>48dx2, 60d, <>210d, 266d, <>320, <>830, <>837; n=8). These results led to the conclusion that the addition of a B lymphocyte specific agent to the standard T cell specific induction agents currently in clinical use, might promote a state of immunological tolerance to allografts. Overall, the present study: 1) delineated B lymphocyte mediated antigen presentation as an important mechanistic participant in acute allograft rejection and 2) established the potential clinical utility of an induction regimen containing a B cell specific mAb for the establishment of transplantation tolerance.