Different antigens result in different immune responses. Soluble, monovalent antigens such as food are tolerogenic; whereas microbes which generally have repeating epitopes are immunogenic. We therefore hypothesized that epitope valency of the antigen plays a role in deciding the immune outcome. B cells are antigen presenting cells that specifically recognize antigenic epitopes and become activated in the presence of multivalent antigen. We studied the effect of different epitope valencies on B cell activation and consequently T cell activation. Using the same antigen, having either monovalent epitopes or divalent epitopes, we determined B cell activation markers by flow cytometry; and T cell activation by proliferation assays and cytokine analysis (ELISA). We found that like unstimulated B cells, monovalent epitope stimulated B cells were not activated but could expand CD4⁺CD25⁺Foxp3 ⁺T cells (Tregs); whereas antigens with divalent epitopes could activate B cells and consequently T cells, but did not induce Tregs. We further identified TGFβ3 to be the cytokine required for the expansion of Tregs by resting B cells. We also found that activation of B cells via Toll-like receptors resulted in T effector cell expansion rather than expansion of Tregs. Our results imply that the activation status of the B cell plays a critical role in triggering the immune response. However, it is the epitope valency of the antigen that decides the activation status of the B cell.

To determine if resting B cells were important in maintaining peripheral Treg populations in vivo, we determined Treg percentages in μMT B cell deficient mice and found them to be decreased. This signifies the importance of B cells in maintaining peripheral tolerance to self antigens and potentially preventing autoimmunity.