TIM3 is a context-dependent co-regulator of cytotoxic T cell function

TIM3 is a co-regulatory receptor that is highly expressed on multiple immune cell types, including on T cells after prolonged exposure to antigen. It marks functionally suppressed cytotoxic T lymphocytes (CTL) in the tumor microenvironment. However, it is unresolved whether TIM3 acts directly on suppressed CTL. Moreover, the nature of TIM3 ligands remains controversial. Paradoxically, TIM3 combines inhibitory function in vivo with costimulatory signaling capability in vitro. Here we have investigated TIM3 in the direct interaction of suppressed murine and human CTL with tumor target cell using spheroids. TIM3 directly inhibited the function of such CTL. TIM3 regulated the ability of suppressed CTL to polarize their cytoskeleton as a required step in cytolysis. Expression of CEACAM1 in cis, on the CTL, blocked TIM3 function, expression of CEACAM1 and galectin9 in trans, on the tumor target cells, enhanced TIM3 function. TIM3 only functioned as an inhibitory receptor on the spheroid-suppressed CTL, not on active CTL in a two-dimensional tissue culture model. These data suggest that TIM3 amplifies T cell function, serving as a co-inhibitory or co-stimulatory receptor depending on the functional context of the T cell it is expressed on. ### Competing Interest Statement CJH is a full-time employee and shareholder at Immunocore.