*Purpose: B regulatory cells (B regs) represent a subset of B cells with non-canonical regulatory function. The majority of effort has been focused on the paracrine effects on T cells through the secretion of immunosuppressive cytokines such as IL-10 and TGF-β. However, B cells are also professional antigen presenting cells which is well known to be important for B cell as well as T cell activation. While interactions between B and T cells are critical for the modulation of immunity, the role antigen presentation plays in the function of B regs in allograft tolerance is unknown. We hypothesize that B regs exert their regulatory activity through antigen presentation in a manner that promotes Tregs.

*Methods: Balb/c islets were transplanted underneath the kidney capsule of a mouse that constitutively expresses ovalbumin (OVA) in all tissues or a C57BL/6 mouse. These mice then underwent dual (anti-CD45RB and anti-TIM1) antibody treatment (DAT) to induce B regs to Balb/C islet antigens as we have demonstrated in prior work. We then isolated these B cells from the spleens of animals (B regs) and adoptively transferred them into μMT mice that were skin grafted with skin from OVA expressing animals or congenic BM12 mice.

*Results: Adoptively transferred B cells from mice made tolerant to Balb/C islets in an OVA mouse had a significant delay in the rejection of OVA skin grafts compared to B cells isolated from a naïve non-DAT OVA mouse (p< 0.05). In addition, mice adoptively transferred with B cells isolated from DAT C57Bl/6 mice had similar OVA skin graft survival as mice either adoptively transferred with B cells from a naïve OVA mouse or animals that were not adoptively transferred with B cells. Importantly, μMT mice adoptively transferred with B cells from DAT OVA animals rejected skin grafts from a mouse with third party antigens (BM12) similar to μMT animals that were transferred with B cells from either naïve OVA, DAT C57/BL6, or no B cell groups.

*Conclusions: These data suggest that the observed significant delay in OVA skin graft rejection seen in μMT animals adoptively transferred with B cells from OVA mice made tolerant to Balb/C islets is antigen specific and likely reliant upon antigen presentation. Further, they give insight into the role antigen presentation may play in B reg function

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