Abstract 1530: Polymorphonuclear myeloid derived suppressor cells secrete IL6 to promote colon tumorigenesis in an inflammatory vicious cycle
Authors: Ibrahim Mohammed L. , Klement John D. , Redd Priscilla S. , Lu Chunwan , Liu Kebin 
Published: August 2019

Abstract
It is widely accepted now that chronic inflammation can create a favorable environment for neoplastic transformation and tumor progression. Tumor Microenvironment consists of various innate and adaptive immune cells, which secrete numerous inflammatory mediating and immunoregulatory cytokines. Among all immune cells infiltrated to the tumor site, myeloid cells have been considered as major players in the tumor associated inflammation. Recently, a subpopulation named myeloid-derived suppressor cells (MDSCs) have gain a great attention as a potential link between inflammation and tumor progression. MDSCs are heterogeneous population of immature myeloid cells that is subdivided into monocytic (M-MDSC) and polymorphonuclear (PMN-MDSC) subsets depending on different markers expressed differentially on their surface. Both PMN-MDSC and M-MDSC have been shown to be immune-suppressive and to exert an immunoregulatory role in the tumor microenvironment. However, a clear distinction in their functional and molecular properties has remained a matter of debate. Here we report that PMN-MDSC but not M-MDSC is the major secreting immune subset of the proinflammatory and tumor-promoting cytokine Interleukin 6 (IL6) in colon cancer. IL6 has been shown in literature to function as a tumor promoter through enhancing tumor growth, invasion and chemoresistance. We observed that IL6 upregulates DNMT1 and DNMT3b in colon carcinoma cells to boost their growth and survival in a STAT3 dependent mechanism. We also observed that tumor-expressed IL6 acts as a key modulator of the immune profile in colon tumor microenvironment. We generated an IL6-overexpressing colon carcinoma cell lines. We observed that IL6-overexpressing tumor triggers a significant increment in PMN-MDSCs but not M-MDSCs infiltration and accumulation in mouse colon carcinoma xenograft model. This remarkable PMN-MDSC tumor enrichment is associated with less effector T lymphocytes infiltration and more progressive and proliferative tumor. Moreover, DSS-induced colitis murine model is characterized with higher infiltration of PMN-MDSC but not M-MDSC, which secrete significant amount of IL6. Therefore, inflammation-derived IL6 induces neoplastic transformation of colonic epithelial cells through upregulation of DNMT1 and DNMT3b. Collectively; our data indicate that PMN-MDSC is the primary IL6 producer among all tumor-infiltrated leukocytes. PMN-MDSC-derived IL6 has a dual tumor favoring function through nurturing colon tumor cells in DNA-methylation dependent mechanism, and raising an immunosuppressive context enriched with more PMN-MDSCs, which secrete more IL6 in an endless vicious cycle. Thus, targeting this circuit, may have a promising implication in colon cancer immunotherapy.
Citation Format: Mohammed L. Ibrahim, John D. Klement, Priscilla S. Redd, Chunwan Lu, Kebin Liu. Polymorphonuclear myeloid derived suppressor cells secrete IL6 to promote colon tumorigenesis in an inflammatory vicious cycle [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1530.