Phenotypic and functional characterizations of CD8⁺ T cell populations in malignant pleural effusion

Malignant pleural effusions (MPE) are a common terminal pathway for many types of cancer, especially non-small cell lung cancer (NSCLC). However, the phenotype and differentiation status of MPE-infiltrating CD8⁺ T cells have not yet been systematically addressed. In this study, the surface molecules and cytokine secretion of T cells in MPE and peripheral blood (PB) were analyzed using flow cytometry. We found an increased frequency of CD8⁺ T cells in MPE compared to PB among lung cancer patients, of which the effector memory subset (Tem, CCR7^- CD45RA^-) and central memory subset (Tcm, CCR7⁺ CD45RA^-) were upregulated. MPE-derived Tem and Tcm subsets expressed more PD1 or CD39, and there was a greater population of cells in these subsets that co-expressed them. In addition, Tem and Tcm cells from MPE had higher cytokine production than terminally differentiated effector memory cells (TemRA, CCR7^- CD45RA⁺) and naïve cells (Tnaive, CCR7⁺CD45RA⁺). Our results demonstrate that the Tem and Tcm cells in MPE may have advantages in both tumor reactivity and immune functionality. Altogether, these findings help to characterize the phenotype of MPE-derived CD8⁺ T cells in terms of differentiation and tumor reactivity and reveal their potential as a target for immunotherapy.