Investigating the role of germline TIM-3 mutations in CD8 T cell malignancy and autoinflammation

T cell immunoglobulin and mucin domain-containing protein 3 (TIM-3) is an inhibitory receptor expressed on T cells and myeloid cells. In humans, germline loss-of-function mutations of TIM-3 have been associated with subcutaneous panniculitis-like T cell lymphoma (SPTCL), a type of cutaneous T cell lymphoma composed of CD8+ T cells, with hemophagocytic lymphohistiocytosis (HLH). The identified TIM-3 mutations, such as Tyr82Cys (Y82C), result in intracellular aggregates of misfolded TIM-3 and a loss of TIM-3 surface expression. However, whether TIM-3 mutations are responsible for the disease and the mechanism by which TIM-3 germline loss-of-function leads to immune overactivation and CD8+ T cell malignancy remains unclear. Here, using CRISPR/Cas9 technology, we developed a mouse model bearing the TIM-3 Y83C mutation, the murine equivalent of the SPTCL-associated TIM-3 Y82C mutation in humans. Surprisingly, we found that the TIM-3 Y83C mutation caused T cell developmental abnormalities in the thymus, leading to increased CD8+ T cells. Through mixed bone marrow chimera experiments, we further demonstrated that the effect of the TIM-3 Y83C mutation on T cell development was cell-intrinsic. Examination of the TIM-3 Y83C mutant thymus revealed that the TIM-3 Y83C mutation affected multiple stages of thymocyte development, with the most significant alterations observed during β selection and positive selection. Together, these findings suggest a mechanism by which germline TIM-3 mutations predispose individuals to SPTCL and provide novel insights into the function of TIM-3.