Cellular interactions in the tumour microenvironment during inflammation-associated carcinogenesis
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The colon tumour microenvironment is highly stromal in composition and a greater stromal cell density correlates with a poor prognosis for patients. The majority of these stromal cells are of mesenchymal origin (MSCs) and are known contributors to tumour angiogenesis and invasiveness. Little is known about the role of their immunosuppressive potential in the TME. We investigated the molecular regulation of the induced immunosuppressive, tumour-promoting phenotype of tumour-associated MSCs, and the effect of inflammation on this process. Balb/c bone marrow derived MSCs were treated with conditioned medium from untreated CT26 tumour cells (MSCTCM) or TNF-α treated CT26 cells (MSCTNF-TCM), resulting in increased expression of TCR ligands MHC-I, MHC-II and PD-L1 compared to MSCControl. This was significantly enhanced by TNF-α induced tumour cell inflammation. MSCTCM co-cultured with syngeneic activated T cells displayed an enhanced ability to suppress CD8+ T cell proliferation, which was further potentiated by inflammatory activation of CT26 (MSCTNF-TCM). This effect was dependent on induced PD-L1 expression on MSCs as PD-1 blockade restored CD8+ T cell proliferation and granzyme B secretion. In an immunocompetent Balb/c syngeneic model, we assessed tumour growth and anti-tumour immune responses following sub-cutaneous injection of CT26 cells alone or co-injection with MSCControl or MSCTNF-TCM. Co-injection of MSCControl significantly promoted tumour growth, and this was further potentiated by the co-injection of MSCTNF-TCM. We showed that this stromal cell mediated tumour promotion could be reversed by administration of a PD-1 blocking antibody, via restoration of granzyme B secreting CD8+ T cells. Furthermore, we validated our findings by microarray profiling of clinical samples from human colon cancer patients, and we show that the phenomenon of stromal cell PD-L1 induction in response to the inflammatory tumour secretome holds true in the human system. Together, this data shows for the first time that stromal cells in the tumour microenvironment directly modulate anti-tumour immune responses via PD-L1. This data will lead to better stratification of patients for immunotherapeutic regimens resulting in more targeted and durable responses.
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