Project Summary Initiation of innate immune responses depends on cognate interaction between germline-encoded pattern recognition receptors and their ligands (expressed by microbes). Following recognition, the receptors initiate activation of downstream signal transduction pathways that often involve recruitment of downstream adapters and kinases. The Toll-like receptor family of PRRs, which are the subject of current investigation, are expressed both on the plasma membrane and in the endosomes. Several recent studies have demonstrated that endocytosis of the plasma membrane TLRs (especially TLR4) plays a critical role in regulating both quality and magnitude of inflammatory responses in a responding macrophage. Other studies have demonstrated that TLR signaling enhances phagocytosis of microbial cargo but not of apoptotic cell cargo suggesting a degree of specificity that is not understood. In addition, although endocytosis of TLR4 and the events following endocytosis of TLR4 that influence signal transduction are very well studied, it is not entirely clear if and how endocytosis influences signaling downstream of other plasma membrane and endosomal TLRs. In our studies, we find that a protein called Vps33B regulates handling of the phagocytic and endocytic cargo following pattern recognition receptor activation. More importantly, Vps33B directly influences the outcome of signaling downstream of TLRs in mice and Toll- and IMD pathways in Drosophila. Mutations in the genes VPS33B and VPS16B are linked to a rare human disease called ARC (Arthrogryposis-renal dysfunction-cholestasis) syndrome. Both of these ARC genes encode paralogs of HOPS complex subunits suggesting a role in membrane fusions but how perturbation of function of these proteins results in a diverse spectrum of disease symptoms in ARC patients is not entirely clear. It has however been documented that ARC patients suffer from sepsis and recurrent bacterial infections and we were therefore investigated the role of these proteins in influencing immune responses. We find that in the absence of VPS33B, Drosophila respond vigorously to microbial insult. Exaggerated immune responses are generated in response to live or dead bacteria and purified ligands of the Toll and IMD pathway results in death of Vps33B mutant, but not wild-type flies. This function of Vps33B is conserved in vertebrates and we find that mouse macrophages lacking Vps33B secrete very high quantities of inflammatory cytokines, when stimulated by either plasma membrane or endosomal TLR ligands. We therefore hypothesize that activation of pattern recognition receptors and specifically TLRs leads to formation of specialized endosomes that depend on Vps33B for lysosomal fusion. Lack of Vps33B is likely to affect several aspects of innate and adaptive immunity and to test this hypothesis, we propose to 1. Define the molecular events that regulate Vps33B function in endosomal maturation, 2. Define the role of Vps33B-regulated ...