Cell-intrinsic Tim-3 is Required for Optimal CD8 Response to Acute LCMV Infection

Tim-3, or transmembrane immunoglobulin and mucin domain-3, is a type I membrane protein expressed by various immune cell types and which is 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. CD8+ T cell effectors were analyzed by flow cytometry after acute infection with LCMV Armstrong. At the effector stage, endogenous Tim-3+ T cells had increased expression of effector CD8 markers, compared with Tim-3 negative cells. Tim-3 knockout mice also showed a significantly lower number of LCMV-specific T cells. Additionally, endogenous Tim-3+ T cells were also significantly better at cytokine production and had increased cytotoxicity. Tim-3 expression correlated with better cell survival in homeostatic conditions after the virus had been cleared. 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.