The T cell immunoglobulin mucin (TIM) family of novel receptor-ligand pairs plays important roles in T cell activation, differentiation and effector function, and in regulation of immune responses in autoimmunity and allergy/asthma, but its functions in alloimmune responses are poorly defined. Our central hypothesis, supported by extensive preliminary data, is that the TIM-1 :TIM-4 pathway, by affecting T helper cell differentiation, play important roles in alloimmune responses and tolerance in vivo. In addition, data from our group and others suggest a possible role of the TIM family of molecules in the function of regulatory T cells. The main goal of this proposal is to define the role and understand the mechanisms of action of the TIM-1:TIM-4 pathway in regulating alloimmune responses in vivo as a means of achieving durable and reproducible tolerance in murine transplant models. Unique tools, including anti-TIM agonistic and blocking antibodies and fusion proteins, and gene knockout and TCR transgenic animals will be used to dissect the functions of the TIM-1 :TIM-4 pathway in alloimmunity using vascularized transplant models of acute and chronic rejection, and stringent models of skin transplantation. More specifically, the aims of the project are: 1. To investigate the functions of the TIM-1 :TIM-4 pathway in alloimmune responses in vivo. 2. To investigate the functions of the TIM-1 :TIM-4 pathway in chronic rejection. 3. To investigate the mechanisms of action of targeting the TIM-1 :TIM-4 pathway in alloreactivity and tolerance via defining the fate of alloreactive T cells in vivo. 4. To study the role of the TIM-1 :TIM-4 pathway in generation/function of Foxp3 regulatory T cells in vivo. Overall, our specific aims describe complimentary approaches that should lead to the development of innovative strategies to permit solid organ graft acceptance without chronic rejection to translate to primates and humans.