Project Summary: We have demonstrated T cell activation by immune complexes (ICs) and sublytic amounts of the membrane attack complex (MAC). Abnormal T cell activation along with elevated levels of ICs and complement activation are observed in systemic lupus erythematosus (SLE) and other autoimmune diseases. The complement anaphylotoxin C5a modulates CD4 mediated T cell responses. MAC is formed by binding of C5b to C6, C7, C8 and multiple molecules of C9, which triggers pleiotropic responses such as DNA synthesis, expression of transcription factors, and generates inflammatory mediators such as archidonic acid. The presence of complement receptors and regulators on T cells is well documented. ICs and the complement system interaction with T cells and specifically their participation in the T cell activation has not been studied. T cells treated with sublytic MAC, demonstrated aggregation of the membrane rafts (MRs) and T cell receptor (TCR) signaling complex CD3. Such events also occur in T cells of SLE patients. We established that 5 to 10% of CD4+ SLE T cells demonstrate binding to aggregated human -globulin (AHG) and labeled ICs, a two to three fold increase over normal. This binding was facilitated by the presence of FcRIIIA on nave CD4+ T cells. The increased expression of FcRIIIA was also observed in, in vitro activated nave cells. IC ligation in the presence of sublytic MAC resulted in the recruitment of FcR chain and induced Syk phosphorylation, which then was recruited to the membrane receptor. The engagement of FcRIIIA by ICs triggered differentiation of these cells into IFN-+ population in the presence of IL-2, IL-1, and IL-23. Also a subset of cells became positive for IL17A, and IL-22. In addition, this T cell signal generated CD25+FoxP3high population in the presence of IL-2 and TGF-1. Thus, the formation of ICs, which results in complement activation can provide an inflammatory milieu and costimulation, two out of three requirements of peripheral tolerance breakdown. The third being a TCR signal from the engagement by the antigen-MHC complex. Thus, in this proposal we will investigate the role of ICs and MAC in generation of TH1 and TH17 phenotypes that correlate them with disease pathology. Since the TH17high IFN- cells generated by IL-23 are more pathogenic, we would further investigate the contribution of signals from ICs and MAC in generation of these phenotypes. Additionally, we would investigate the role of Syk in ICs and MAC mediated activation of CD4+ T cells and in its contribution to TH1 and TH17 responses. Since Syk is a therapeutic target for autoinflammatory disease, our studies will further enhance the development of such therapeutic targets for SLE and other autoimmune diseases.