Phenotypic and functional characterization of tumor-reactive T cells in malignant pleural effusions.

Adoptive cell therapy using tumor-infiltrating lymphocytes (TIL) is effective for treating advanced melanoma but requires surgical tumor resection. Malignant pleural effusions (MPE) may provide a more accessible source of tumor-reactive T cells. Synchronously collected MPE, lung metastasis, and blood from a patient with metastatic melanoma were analyzed using high-dimensional flow cytometry, and single-cell RNA/T cell receptor (TCR) sequencing. TCR reactivity to autologous tumor was tested in vitro. The proliferative and cytotoxic capacity of ex vivo expanded T cells was assessed in vitro. MPE contained a higher fraction of CD3+ T cells compared with tumor and was enriched for effector CD8+ T cells and effector memory CD4+ T cells. Compared with TIL, MPE T cells exhibited lower features of T cell exhaustion and higher cytotoxicity signatures. The clonal repertoire of MPE and tumor highly overlapped, including 62.2% of predicted neoantigen-specific (NeoTCR) clonotypes. MHC class I-restricted reactivity was functionally confirmed in two of four selected NeoTCR clonotypes. MPE T cells demonstrated higher proliferative capacity under high-dose IL-2 expansion relative to TIL and achieved comparable MHC class I-dependent tumor killing. Overall, MPE contains polyclonal, tumor-reactive T cells with favorable functional features, supporting MPE as an accessible source for TIL therapy.