B lymphocytes contribute to indirect pathway T cell sensitisation via acquisition of extracellular vesicles

B cells have been implicated in transplant rejection via antibody‐mediated mechanisms and more recently by presenting donor‐antigens to T cells. We have shown in patients with chronic antibody‐mediated rejection that B cells control the indirect T cell alloresponses. To understand more about the role of B cells as antigen presenting cells for CD4⁺ T cell with indirect allospecificity, B cells were depleted in C57BL/6 mice, using an anti‐CD20 antibody, prior to receiving MHC‐class I‐mismatched (Kᵈ) skin. The absence of B cells at the time of transplantation prolonged skin graft survival. To study the mechanisms behind this observation, T cells with indirect allospecificity were transferred in mice receiving a Kᵈ skin transplant. T cell proliferation was markedly inhibited in the absence of recipient B cells, suggesting that B cells contribute to indirect pathway sensitisation. Furthermore, we have shown that a possible way in which B cells present alloantigens is via acquisition of MHC‐peptide complexes. Finally, we demonstrate that the addition of B cell depletion to the transfer of Tregs with indirect alloresponse further prolonged skin graft survival. This study supports an important role for B cells in indirect T cell priming and further emphasises the advantage of combination therapies in prolonging transplant survival.