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International Immunology, Vol. 16, No. 4, pp. 539-547, April 2004
© 2004 Japanese Society for Immunology

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

Lou Ann Yi1, Soheila Hajialiasgar1 and Ellen Chuang1,2

1 Division of Hematology and Medical Oncology, Department of Medicine and 2 Graduate Program in Immunology, Weill Medical College of Cornell University, New York, NY 10021, USA

Correspondence to: E. Chuang; E-mail: elc2007{at}med.cornell.edu
Transmitting editor: K. Murphy

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.

Keywords: co-stimulation, T cell homeostasis, Th1/Th2


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