B-Cell Adapter for Phosphoinositide 3-Kinase Is a Signaling Adapter in the Toll-Like Receptor/Interleukin-1 Receptor Superfamily




Troutman, Ty Dale

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Toll-like receptor (TLR)/Interleukin-1 receptor (IL1R) superfamily members share signaling components and (with the exception of TLR3) depend on the adapter myeloid differentiation primary response gene 88 (MyD88) for engagement of downstream pathways. Signals from the receptor to the adapter are transmitted through homotypic interaction of TIR (Toll-Interleukin-1 receptor) homology domains found in all TLR/IL1R family members and their adapters. The present work defines a novel TLR/IL1R signaling adapter, termed BCAP (B-cell adapter for PI3K), which was identified based on the presence of a cryptic N-terminal TIR domain. I show here that BCAP (B-cell adapter for PI3K) contains a functional TIR domain enabling its participation in the TLR signaling pathway. Through its TIR domain, BCAP associates with the TLR/IL1R signaling adapter MyD88, as well as the TLR signaling adapter toll-interleukin 1 receptor domain containing adapter protein (TIRAP). Importantly, BCAP plays an obligate role in linking TLRs to activation of phosphoinositide 3-kinase (PI3K) through recruitment of PI3K to the signaling complex and relief of inhibitory influences on PI3K activity. Importantly, BCAP selectively mediates TLR signaling towards the PI3K branch without affecting signaling to NFκB nor MAP kinases. In this capacity, BCAP inhibits secretion of inflammatory cytokines and regulates susceptibility to inflammatory colitis. Because the TLR/IL1R family shares signaling components, BCAP may also function in IL1R family signaling. To test this hypothesis, T cells were chosen as a model cell type responding to IL1R family signals. T helper cells utilize IL18 and IL1 (which engage the IL18R or the IL1R respectively, both IL1R family members) cytokines provided by myeloid cells to achieve optimal Th1 and Th17 effector capacities. I show here that BCAP intrinsically regulates differentiation of naïve T cells towards Th1 and Th17 effector lineages by participation in the IL1R family signaling pathways. Further, BCAP intrinsically regulates both T cell proliferation and survival during priming. The significance of this work lies in the revelation of a TLR signaling adapter serving as a node connecting TLRs to PI3K. Further, the findings here will increase the understanding of key signaling pathways involved in disease and inflammation.

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