Purpose: The cross talk between tumour cells, myeloid cells, and T cells play a critical role in tumour pathogenesis and response to immunotherapies. Although the aetiology of mesothelioma is well understood the impact of mesothelioma on the surrounding immune microenvironment is less well studied. In this study the effect of the mesothelioma microenvironment on circulating and infiltrating granulocytes and T cells is investigated. Experimental Design: Tumour and peripheral blood from mesothelioma patients were evaluated for presence of granulocytes, which were then tested for their T cell suppression. Co-cultures of granulocytes, mesothelioma cells, T cells were used to identify the mechanism of T cell inhibition. Results: Analysis of tumours showed that the mesothelioma microenvironment is enriched in infiltrating granulocytes, which inhibit T cell proliferation and activation. Characterisation of the blood at diagnosis identified similar, circulating, immunosuppressive CD11b+CD15+HLADR- granulocytes at increased frequency compared to healthy controls. Culture of healthy-donor granulocytes with human mesothelioma cells showed that GM-CSF upregulates NOX2 expression and the release of Reactive Oxygen Species (ROS) from granulocytes, resulting in T cell suppression. Immunohistochemistry and transcriptomic analysis revealed that a majority of mesothelioma tumours express GM-CSF and that higher GM-CSF expression correlated with clinical progression. Blockade of GM-CSF with neutralising antibody, or ROS inhibition, restored T cell proliferation suggesting that targeting of GM-CSF could be of therapeutic benefit in these patients. Conclusions: Our study presents the mechanism behind the cross-talk between mesothelioma and the immune micro-environment and indicates that targeting GM-CSF could be a novel treatment strategy to augment immunotherapy.
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