Accumulation of TNFR2-expressing regulatory T cells in malignant pleural effusion of lung cancer patients is associated with poor prognosis

BACKGROUND: Regulatory T cells (Tregs) may represent a major cellular mechanism in immune suppression by dampening the anti-tumor response in malignant pleural effusion (MPE). Tumor necrosis factor receptor type II (TNFR2) has emerged as a novel identification for the maximally suppressive subset of Tregs in the tumor environment. At present, the significance of TNFR2 expression on Tregs in MPE remains unclear. METHODS: The distribution of TNFR2(+)cells in Tregs and effector T cells (Teffs) in MPE, peripheral blood (PB), and tuberculosis pleural effusion (TPE) were determined. The associations between TNFR2(+)Tregs frequencies present in MPE and the clinical and laboratorial characteristics of patients with lung cancer were investigated. The immunosuppressive phenotype of TNFR2(+)Tregs in MPE was analyzed. The effects of the TNF-TNFR2 interaction on the immunosuppressive function of Tregs was explored. The efficacy of targeting TNFR2 for MPE therapy was examined. The source of TNF in MPE was identified. RESULTS: We observed that markedly higher levels of TNFR2 were expressed in MPE Tregs compared with the levels expressed in MPE Teffs, PB Tregs, or in TPE Tregs. The frequencies of TNFR2(+)Tregs were positively correlated with the number of tumor cells in MPE, as well as the volume of MPE. High frequencies of TNFR2(+)Tregs in MPE indicated short survival time and poor performance status for MPE patients. Compared to TNFR2(-)Tregs, TNFR2(+)Tregs expressed higher levels of immunosuppressive molecules cytotoxic T lymphocyte-associated protein 4 (CTLA-4), programmed cell death-ligand 1 (PD-L1), and replicating marker Ki-67. Consequently, the proportions of interferon gamma (IFN-γ)-producing cytotoxic T lymphocytes (CTLs) were significantly increased after TNFR2 blockade. Furthermore, tumor necrosis factor (TNF), through interaction with TNFR2, enhanced the suppressive capacity of Tregs by up-regulating CTLA-4 and PD-L1 expression. Interestingly, T helper 1 (Th1) and T helper 17 (Th17) cells are the major source of TNF in MPE, suggesting that MPE Teffs may paradoxically promote tumor growth by boosting MPE Treg activity via the TNF-TNFR2 pathway. CONCLUSIONS: Our data expanded the immunosuppressive mechanism present in MPE induced by Tregs, and provides novel insight for the diagnosis, disease evaluation, and treatment of MPE patients.