Tim-3 marks human natural killer cell maturation and suppresses cell-mediated cytotoxicity

Natural killer (NK) cells are innate lymphocytes that play an important role against viral infections and Cancer. This effect is achieved through a complex mosaic of inhibitory and activating receptors expressed by NK cells that ultimately determine the magnitude of the NK-cell response. The T-cell immunoglobulin- and Mucin domain-containing (Tim)-3 receptor was initially identified as a T-helper 1-specific type I membrane protein involved in regulating T-cell responses. Human NK cells transcribe the highest amounts of TIM-3 among lymphocytes. TIM-3 protein is expressed on essentially all mature CD56(dim)CD16(+) NK cells and is expressed heterogeneously in the immature CD56(bright)CD16(-) NK-cell subset in blood from healthy adults and in cord blood. TIM-3 expression was induced on CD56(bright)CD16(-) NK cells after stimulation with IL-15 or IL-12 and IL-18 in vitro, suggesting that TIM-3 is a maturation marker on NK cells. Whereas TIM-3 has been used to identify dysfunctional T cells, NK cells expressing high amounts of TIM-3 are fully responsive with respect to cytokine production and cytotoxicity. However, when TIM-3 was cross-linked with Antibodies it suppressed NK cell-mediated cytotoxicity. These findings suggest that NK-cell responses may be negatively regulated when NK cells encounter target cells expressing cognate ligands of TIM-3.