Cell-intrinsic Tim-3 is Required for an Optimal Acute CD8+ T Cell Response

Abstract Tim-3, or transmembrane immunoglobulin and mucin domain-3, is a type I membrane protein expressed by various immune cells, best known as a negative regulator of anti-tumor immunity. However, Tim-3 also has a co-stimulatory role in T cells under some conditions, via enhancement of PI3K signaling. We hypothesize that Tim-3 signaling enhances CD8+ T cell activation during acute infection and contributes to enhanced CD8+ effector function. To test this hypothesis, we used LCMV-specific TCR transgenic (P14) mice expressing a truncated, non-signaling, allele of Tim-3 or CD8-specific deletion of Tim-3 and used LCMV Armstrong as an acute infection model. At the effector stage, endogenous Tim-3+ T cells had increased expression of effector markers, and cytokine production compared with Tim-3- cells. Tim-3 knockout mice also showed a significantly lower number of LCMV-specific T cells. Bulk RNA sequencing and TCR sequencing revealed that Tim-3-expressing effector CD8+ T cells had a distinct pattern of effector gene upregulation, with increased expansion of a subset of TCR clones. Mechanistically, Tim-3 signals enhances phosphorylation of Foxo1, inhibiting its entry into the nucleus, conferring enhanced effector function. Thus, Tim-3 signaling contributes positively to an effective CD8+ T cell response during acute infection. These findings may lead to a better understanding of CD8+ T cell function in different settings, including in tumors.