Early and sustained immunosuppressive macrophage in rat mesothelioma

Malignant mesothelioma (MM) is a rare cancer caused by carcinogenic fibers such as asbestos or carbon nanotubes (CNT) exposure and affecting serous membrane of pleural and peritoneal cavities. It is accepted that fiber-induced chronic inflammation, M1-macrophage and neutrophil infiltration are crucial in tumor establishment. However, there is growing evidence that immunosuppressive leukocytes preventing efficient anti-tumor immune responses are also implicated in development of MM. Our experimental study aims to determine if the carcinogenic responses to asbestos and Mitsui-7 CNT are associated with the accumulation of immunosuppressive macrophages. We used a Wistar rat peritoneum model which allows directly exposing mesothelial cells to asbestos or CNT, and easily sampling the mesothelial cavity for monitoring macrophage responses during the carcinogenic process. We show that FACS-sorted eosinophilic macrophages present in CNT-7-induced mesothelioma microenvironment suppress polyclonal activation of T lymphocytes in vitro, during the early response to carcinogenic CNT or asbestos (day 1 to 30), well before the cancer establishment. They also express CD163, a marker of M2 phenotype, known to inhibit human T lymphocytes activation and proliferation in vitro. Peritoneal macrophages purified from CNT-treated rats (day 1) highly express the immunosuppressive mediators IL-10 and Arginase-1. The levels of TGF-beta in peritoneal fluids were also increased by CNT-7 or asbestos injection. Altogether, our data demonstrate that carcinogenic CNT possess the intrinsic capacity to induce a rapid and sustained accumulation of immunosuppressive macrophages and their related mediators before mesothelioma is established.