BCL-2 expression promotes immunosuppression in chronic lymphocytic leukemia by enhancing regulatory T cell differentiation and cytotoxic T cell exhaustion.

BACKGROUND: Chronic lymphocytic leukemia (CLL) results in increased susceptibility to infections. T cell dysfunction is not associated with CLL in all patients; therefore, it is important to identify CLL patients with T cell defects. The role of B-cell lymphoma-2 (BCL-2) in CLL has been explored; however, few studies have examined its role in T cells in CLL patients. Herein, we have investigated the regulatory role of BCL-2 in T cells in the CLL tumor microenvironment. METHODS: The expression of BCL-2 in T cells was evaluated using flow cytometry. The regulatory roles of BCL-2 were investigated using single-cell RNA sequencing (scRNA-seq) and verified using multi-parameter flow cytometry on CD4 and CD8 T cells. The clinical features of BCL-2 expression in T cells in CLL were also explored. RESULTS: We found a significant increase in BCL-2 expression in the T cells of CLL patients (n = 266). Single cell RNA sequencing (scRNA-seq) indicated that BCL-2(+)CD4(+) T cells had the gene signature of increased regulatory T cells (Treg); BCL-2(+)CD8(+) T cells showed the gene signature of exhausted cytotoxic T lymphocytes (CTL); and increased expression of BCL-2 was associated with T cell activation and cellular adhesion. The results from scRNA-seq were verified in peripheral T cells from 70 patients with CLL, wherein BCL-2(+)CD4(+) T cells were enriched with Tregs and had higher expression of interleukin-10 and transforming growth factor-β than BCL-2(-)CD4(+) T cells. BCL-2 expression in CD8(+)T cells was associated with exhausted cells (PD-1(+)Tim-3(+)) and weak expression of granzyme B and perforin. T cell-associated cytokine profiling revealed a negative association between BCL-2(+) T cells and T cell activation. Decreased frequencies and recovery functions of BCL-2(+)T cells were observed in CLL patients in complete remission after treatment with venetoclax. CONCLUSION: BCL-2 expression in the T cells of CLL patients is associated with immunosuppression via promotion of Treg abundance and CTL exhaustion.