Background: B cells have regulatory functions and promote tolerance by inducing deletion or anergy of CD8⁺ T cells, increasing Foxp3⁺Treg, inhibiting proliferation of CD4 T cells in a CD40/CD40L dependent manner, and producing IL-10. We previously showed that B cell depletion prevented costimulatory blockade induced tolerance, and hypothesized that the tolerogenic function of B cells resided in germinal center subsets that produce immunomodulatory cytokines.

Methods: Wild type C57BL/6, or mice with a B cell specific deficiency in IL-10 (CD19-Cre^+/-::IL-10^fl/fl), received vascularized BALB/c cardiac allografts and were tolerized with BALB/c donor-specific splenocyte transfusion (DST) and anti-CD40L mAb. Various B cell subsets were purified using specific mAbs or flow sorting, and their immunologic functions characterized by flow, RT-PCR, culture, and adoptive transfer.

Results: The tolerogenic regimen of DST plus anti-CD40L specifically increased the percentage of MZP B cells compared to control mice (28.2 ± 3.8 vs 46.3 ± 3.4%), without altering other B cell subsets. The tolerogenic regimen induced increased IL-21R on MZP B cells. IL-21R⁺ MZP cells, but not CD19⁺ CD5⁺ CD1d⁺ B cells, had increased expression of IL-10 mRNA in tolerized mice; and MZP B cells did not show increased surface TIM1 or TIM4 expression. Purified IL-21R⁺ MZP B cells suppressed the proliferation of effector CD4 T cells. Deficiency of IL-10 in B cells prevented the generation of tolerance, but did not lead to changes in the percentages of various B cell subsets or in Foxp3⁺ Treg. Graft rejection was characterized by increased expression of IL-17, IL-21 and CCR6 on the effector CD4 T cells in the lymph nodes and spleen. Neutralization of IL-6 or IL-21, or adoptive transfer of wild type MZP B cells into CD19-Cre^+/-::IL-10^fl/fl, mice rescued co-stimulatory blockade induced allograft survival.

Conclusion: We identified a unique subset of B cells (IL-21R⁺IL-10⁺ MZP B cells) that is required for the generation of allogeneic tolerance. Deficiency of IL-10 in MZP B cells lead to disruption of cellular interactions in the germinal center required for tolerance. The MZP B cells were distinct from CD5⁺CD1d⁺ and TIM⁺ regulatory B cells. The MZP subset can be manipulated therapeutically to enhance tolerance, and shows that B cell depletion may prevent graft survival.

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