The B7-CD28/CTLA-4 costimulatory pathway has a critical role in T cell activation, expansion and differentiation. For this reason, manipulation of this pathway has received considerable attention for the treatment of autoimmunity. Although the B7-CD28 pathway provides essential signals for T cell activation, this pathway does not account for all costimulatory activity. Recent studies have pointed to the ICOS- B7h and Ox40-Ox40L pathways as being particularly important for CD4 T cell activation, differentiation and effector function following B7-CD28 activation. The goal of this project is to define the interplay among the CD28, ICOS and Ox40 costimulatory pathways for T cells in both the afferent arm of the autoimmune response where T cells become activated in the periphery, and in the affector arm, where potentially pathogenic T cells are driven to mediate tissue destruction. Because ICOS and Ox40 expression are induced after T cell activation and depend in part on B7-CD28 signals for their upregulation, we hypothesize the some of the functions attributed to CD28 may be mediated b ICOS and Ox40 costimulation, and that there may be synergy between the ICOS and Ox40 pathways, particularly for effector T cells responses. To address these issues, we have generated mice that lack or overexpress these costimulatory molecules. Furthermore, we have generated MOG 35-55/Iab specific TcR transgenic mice to analyze the role of these three costimulatory pathways in the pathogenesis of experimental autoimmune encephalomyelitis (EAE). We will: 1. Characterize the MOG TcR transgenic mice for their T cell responses to encephalitogenic MOG peptides and development of EAE. 2. Investigate the role of ICOS during the induction and effector phases of EAE using transgenic mice constitutively overexpressing ICOS on T cells. We also will investigate how interactions between ICOS and the B7-CD28/CTLA-4 pathway affect the initiation and progression of EAE. 3. We will investigate the role of ox40-Ox40L pathway during the induction and effector phases of EAE. We will also investigate how interactions among ICOS-B7h, Ox40-Ox40L and B7-CD28/CTLA-4 pathways influence the initiation and effector phases of EAE. Our B7-1/B7-2 deficient mice, Ox40L deficient mice and transgenic mice constitutively expressing ICOSc on T cells are definitive tools for analyzing the interplay among these pathways. Our newly generated a MOG 35-55 specific TcR transgenic mouse on the B6 background will enable visualization of the activation, migration and expansion of naive and activated MOG-specific T cell responses in the presence or absence of these costimulators in vivo. Taken together, these studies should lead to new insights into how the B7-CD28/CTLA-4, B7h-ICOS, and Ox40-Ox40L pathways interact with each other and regulate the responses of self-reactive T cells. The results from these studies will provide insight for manipulation of these pathways for treating autoimmune diseases.