B cells licensed by DCs are primary antigen presenting cells for DNA plasmid upon passive uptake

Abstract Dendritic cells are considered the primary cells involved in professional antigen presentation. However, our group and others have shown that B cells are similarly capable of presenting antigens leading to activation and proliferation of antigen-specific T cells. In this study, we aimed to understand the mechanism of presentation of antigen, encoded by plasmid DNA when delivered to APCs via passive uptake. We used plasmid DNA encoding ovalbumin, splenic B cells from Bl6 mice, DCs from Flt3L treated Bl6 mice, and CD8 T cells from OT1 mice that are restricted for the SIINFEKL-specific T-cell receptor. We have previously reported that only B cells are capable of transcribing the encoded antigen, whereas DCs and macrophages degrade the plasmid DNA upon passive uptake. In our current study we show that B cells also translate the encoded antigen, but can present the antigen to CD8 T cells only in the presence of DCs. We utilized a trans-well assay to show that physical contact between B cells and DCs is required for successful antigen presentation and that live DCs are essential, as replacing them with DC lysate or DC supernatant or both was not adequate. We also show that the presence of DCs promotes B cell survival, mostly through CD40-CD40L signaling by using CD40 KO mice and that B cells are primary antigen presenting cells in this co-culture by using MHC I KO mice. We also show that exposure of B cells to CD40 or CD40L, which is one of the identified interactions between B cells and DCs, was not sufficient for B cells to present antigen in the absence of DCs. In summary, our data suggests that B cells and DCs communicate through a currently unknown cell-cell interaction that licenses B cells for antigen presentation after passive uptake of plasmid DNA.