Tigit Expression Defines a Subset of Activated T_reg Cells with Prognostic Relevance in Follicular Lymphoma

T cell Ig and ITIM domain (TIGIT) is an immune checkpoint molecule that is expressed on a variety of cell types including NK cells, effector and memory T cells, and T_reg cells. Upon ligation with CD155, TIGIT delivers an inhibitory signal and negatively regulates anti-tumor responses. While important in normal T-cell biology, the pathological significance of TIGIT expression and function in the tumor microenvironment of the patients with follicular lymphoma (FL) is largely unknown. The present study sought to phenotypically and functionally characterize TIGIT⁺ T cell subsets in FL. While its expression is not detected on resting T cells in peripheral blood, we found that TIGIT is highly expressed on intratumoral T cells from FL. Treatment with cytokines such as IL-4 and TGF-β downregulated TIGIT expression on T cells. We found that TIGIT is predominantly expressed on effector memory T cells (T_EM) with an activation/exhausted phenotype, and TIGIT⁺ T cells have higher expression levels of CD69 and PD-1 when compared to TIGIT^- T cells. Functionally, TIGIT⁺ T cells displayed reduced capacity of cell proliferation and cytokine production (IFN-γ and IL-2). Using mass cytometry (CyTOF), we observed that TIGIT is abundantly expressed on some intratumoral T_reg (CD4⁺CD25⁺Foxp3⁺) cells, while other T_reg cells lack TIGIT expression in FL, forming two subsets of T_reg cells: CD4⁺CD25⁺TIGIT⁺ and CD4⁺CD25⁺TIGIT^-. These two subsets are phenotypically distinct in that CD25⁺TIGIT⁺ cells exhibited increased expression of activation/costimulatory markers such as CD69, CD27 and CD28 compared to CD25⁺TIGIT^-, suggesting an activated T_reg subset of CD25⁺TIGIT⁺ cells. This CD25⁺TIGIT⁺ subset had increased suppressive function and could more effectively inhibit activation and proliferation of CD8⁺ T cells than CD25⁺TIGIT^- cells. Furthermore, we found that lymphoma B cells promoted the development of TIGIT-expressing T cells as TGF-β-mediated downregulation of TIGIT on T cells was inhibited when CD19⁺ lymphoma cells were present. Using a cohort of 31 FL patients, we found that intratumoral TIGIT-expressing T cells were associated with a favorable prognosis. Patients with TIGIT⁺ T cell numbers greater than 50% had better overall survival than patients with TIGIT⁺ T cell numbers less than 50%. Taken together, our results reveal a role of TIGIT in defining T_reg cell subsets with different immune function and TIGIT expression may be useful in predicting patient outcome in FL.