Tyrosine-mediated inhibitory signals contribute to CTLA-4 function in vivo

The ability of CTLA-4 to inhibit T cell activation may be either negatively or positively regulated by a critical tyrosine at position 201 (Y201) within the CTLA-4 cytoplasmic domain. By binding to the clathrin-associated adaptor complex AP-2 and inducing endocytosis, Y201 reduces the amount of CTLA-4 on the cell surface, thereby down-regulating CTLA-4 inhibitory function. Alternatively, Y201 may function to transmit CTLA-4 inhibitory signals, perhaps through binding to intracellular proteins that oppose TCR- and/or CD28-induced signal transduction. Results from studies performed in vitro have cast doubt on whether this second mechanism contributes significantly to CTLA-4 function. In order to determine if a role existed for Y201 in mediating CTLA-4 inhibitory signaling in vivo, we studied lymphocyte activation and homeostasis in CTLA-4(-/-) mice that were reconstituted with a transgenic CTLA-4 receptor in which Y201 was mutated to valine (Y201V/CTLA-4(-/-)). We found that despite augmented levels of CTLA-4 on the cell surface of T cells, Y201V/CTLA-4(-/-) mice developed a lymphoproliferative syndrome characterized by lymphadenopathy and the accumulation of T cells that secreted IL-4. Mutant T cells exhibited increased cell division when treated with suboptimal doses of mitogenic stimuli in vitro. These results demonstrate that in addition to down-modulating CTLA-4 expression on the cell surface of T cells, the Y201 residue also functions to transmit CTLA-4 inhibitory signals in vivo. Elucidating the biochemical pathways downstream of Y201 will be important for a full understanding of the molecular basis for CTLA-4 function.