Tumor Cell Repopulation between Cycles of Chemotherapy is Inhibited by Regulatory T-Cell Depletion in a Murine Mesothelioma Model

Introduction: Malignant pleural mesothelioma is a highly aggressive cancer with poor prognosis. We have previously demonstrated that regulatory T cells (Treg) depletion can impact tumor microenvironment when combined with chemotherapy. The aim of this study is to analyze the impact of Treg depletion on tumor cell repopulation during cycles of chemotherapy in a murine mesothelioma model. Methods: Tumor-bearing mice were treated with chemotherapy once weekly to mimic clinical settings and with PC61 to cause Treg depletion after each cycle of chemotherapy. Tumor cell repopulation was evaluated by BrdU labeling index with immunohistochemistry and flow cytometry, and Ki67 gene expression was determined by real-time reverse-transcribed polymerase chain reaction. The proportion of CD4+ CD25+ Foxp3+ Tregs, CD4+, and CD8+ T cells in the tumor, spleen, draining lymph node, and peripheral blood from tumor-bearing mice was determined by using flow cytometry, and gene expression of activated T-cell-related cytokines was quantified by enzyme-linked immunosorbent assay and reverse-transcribed polymerase chain reaction. Results: Tumor growth delay was achieved by cisplatin followed by PC61 or cyclophosphamide. The BrdU labeling index indicated that tumor cell repopulation between cycles of cisplatin was significantly inhibited by PC61. The CD4+ CD25+ Foxp3+ Tregs in tumor and lymphoid organs were almost completely depleted, whereas the CD4+ or CD8+ T cells did not change. PC61 after chemotherapy resulted in an increase of gene expression of interferon-gamma, granzyme B, perforin, and IP-10, thus leading to tumor cell lysis in cytotoxic lymphocyte assay. Nevertheless, cell killing induced by cyclophosphamide combined with cisplatin was due to cytotoxicity rather than specific immune response. Conclusion: Treg depletion between cycles of chemotherapy could improve the outcome of mesothelioma. Nevertheless, this effect seems limited, and more effective approaches need to be developed.