Defining the Role of B Cells in Graft Rejection Following Lung Transplantation

PURPOSE: Humoral immunity is maintained by long lived memory B and plasma cells, both of which arise from the activation and differentiation of naïve B cells, and require T cell help. In the transplantation setting, humoral immune responses that target the donor tissue are undesirable and can lead to antibody mediated rejection (AMR). AMR has remained challenging to prevent and overcome, with current immunosuppressive therapies predominantly focused on T cells. However, the presence of antibodies to donor tissue human leukocyte antigen (HLA), aka donor-specific antibody (DSA), are associated with a poorer prognosis in transplantation. DSA can be either pre-existing (generated through previous transplants, blood transfusions or multiple pregnancies) or generated de novo after transplantation. Thus, understanding and characterizing B cell responses in transplantation may reveal novel immune monitoring and/or treatment. METHODS: This study is a retrospective longitudinal and comparative analysis of B and T cells in the peripheral blood of lung transplant recipients at the Alfred Hospital (Melbourne, Australia), and aims to measure HLA Class II antibody responses. In addition to healthy control donors, a total of 22 lung transplant recipients (DSA- n=11; DSA+ (de novo) n=11) have had cryopreserved peripheral blood mononuclear cells immunophenotyped at several time points, including pre-transplant. RESULTS: Comparative analysis of lymphocytes between the two lung transplant cohorts revealed that patients who developed DSA had a higher frequency of B cells at 3 months post-transplantation in compared than those who did not. Further to this, at 6 weeks and 3 months pre-transplant, CD4+ T cells were more abundant than CD8+ T cells in patients with DSA, and these frequencies deviated from both their DSA- counterparts and healthy blood donors. In addition to B and T cell ex vivo characterization, we have developed a unique in vitro memory B cell culture system, design for low cell seed numbers that expand and differentiate memory B cells into antibody secreting cells. From these cultures, we are able to collect and characterize antibody secreted in the supernatant. To date, we have completed this work assessing tetanus toxoid antibody responses and are now assessing HLA II antibody responses. CONCLUSION: This study provides impetus to study the role of humoral immunity in lung transplantation.