Long-term allograft tolerance is characterized by the accumulation of B cells exhibiting an inhibited profile

Numerous reports have highlighted the central role of regulatory T cells in long-term allograft tolerance, but few studies have investigated the B-cell aspect. We analyzed the B-cell response in a rat model of long-term cardiac allograft tolerance induced by a short-term immunosuppression. We observed that tolerated allografts are infiltrated by numerous B cells organized in germinal centers that are strongly regulated in their IgG alloantibody response. Moreover, alloantibodies from tolerant recipients exhibit a deviation toward a Th2 isotype and do not activate in vitro donor-type endothelial cells in a pro-inflammatory way but maintained expression of cytoprotective molecules. Interestingly, this inhibition of the B-cell response is characterized by the progressive accumulation in the graft and in the blood of B cells blocked at the IgM to IgG switch recombination process and overexpressing BANK-1 and the inhibitory receptor Fcgr2b. Importantly, B cells from tolerant recipients are able to transfer allograft tolerance. Taken together, these results demonstrate a strong regulation of the alloantibody response in tolerant recipients and the accumulation of B cells exhibiting an inhibited and regulatory profile. These mechanisms of regulation of the B-cell response could be instrumental to develop new strategies to promote tolerance.