Aire-dependent thymic B-1a cells play a key role in neonatal tolerance induction - CD5+ B-1a cells are innate-like B cells in mouse and man. They emerge during fetal/neonatal development independent of the bone marrow (BM)-derived progenitors that later give rise to follicular B-2 cells. In essence, we see B-1a and B-2 (follicular B cell) as having evolved sequentially to create B cell layers that provide progressively more complex immune capabilities, including, for B-1a, the induction of self-tolerance. Thus, we show here that 1) B-1a express high levels of Aire, which promotes the expression/presentation of self-antigens by thymic APCs and hence promotes the induction of T cell tolerance; 2) B-1a activation and Aire induction in neonatal thymus depends on CD4+ T cell-mediated CD40 signaling, and requires T cells expressing a special repertoire; 3) Partial ablation of Foxp3+ CD4+ regulatory T (Treg) cells in neonatal thymus results in decreased numbers of thymic B-1a; and, reciprocally, 4) Depletion of thymic B-1a cells during neonatal life decreases Treg generation. Together, these findings demonstrate that B-1a cells interact with CD4+ thymocytes in neonatal thymus to generate cross talk that activates B-1a cells to differentiate to Aire+IgMnegIgDnegCD5+ APCs; and, 5) these cells in turn participate in the selection of neonatal Treg cells that play a key role in inducing/maintaining self-tolerance (21-23). In earlier studies, we have shown that T cell tolerance to B cells is impaired in B cell-deficient µMT mice, i.e., CD4+ T cells in µMT mice inhibit B cell reconstitution in adult µMT hosts. Most significantly, we find that B-1a cells in neonatal µMT mice condition CD4+ T cells to enable establishment of follicular B cells (24). Further, we find that Treg generation in neonatal µMT mice is decreased and, most strikingly, that Treg cells from normal neonatal mice suppress the inhibitory function of µMT CD4+ T cells, as these cells enable B cell reconstitution in µMT hosts when co-transferred with BM whereases co-transfer of Treg cells from µMT mice fails to do so. Taken together, these findings lead us to propose that B-1a cells play a key role in inducing T cell tolerance to B cells in neonatal thymus, where the B-1a cells interact with CD4+ thymocytes, internalize surface IgM, differentiate to Aire+IgMnegIgDnegCD5+ APCs, and then, select a population of neonatal Treg cells that confer B cell-tolerance. Studies proposed here will test this hypothesis and will clarify the B cell tolerance induction mechanism that is induced by B-1a and regulated by Treg in neonatal thymus. Findings from this study will thus shed key light on the fundamental mechanisms that governs the induction of neonatal immunological self-tolerance, and will offer new insights into the autoimmune diseases to which Aire-mediated B-1a cell function contributes.
