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Abstract

MDSC play a major role in immunosuppression and tumor progression in melanoma. Their recruitment to the tumor is mediated by chemokine receptors and their ligands, in particular by chemokine receptor CCR5. The aims of this thesis were to study the mechanisms of CCR5 upregulation on murine MDSC in melanoma and of the previously observed stronger immunosuppressive phenotype of CCR5+ MDSC as compared to their CCR5- counterparts. IL-6, GM-CSF and IFN-g upregulated Ccr5 mRNA expression in murine myeloid cells, whereas the CCR5 ligands, tumor-derived extracellular vesicles, toll-like receptor ligands and IL-1b failed to affect Ccr5 expression. IL-6 and GM-CSF were able to induce CCR5 surface expression during MDSC in vitro differentiation for four days by a STAT3 dependent mechanism. Importantly, we found four putative STAT3 binding sites in the murine Ccr5 promoters. In addition, the STAT3 inhibitor Stattic abrogated Ccr5 upregulation induced by IL-6 and GM-CSF. In the RET transgenic mouse model of malignant melanoma, IL-6 levels in the tumors correlated with the frequency of tumor-infiltrating CCR5+ MDSC. CCR5+ MDSC showed increased phosphorylated STAT3 levels. In addition to the upregulation of CCR5, IL-6 was responsible for stimulation of Arginase (Arg)1 activity and ROS production upon MDSC in vitro differentiation, inducing increased capacity of MDSC to suppress CD8+ T cell proliferation in a co-culture assay. The upregulation of Arg1 by IL-6 was also STAT3 dependent. In contrast to IL-6, CCR5 ligands failed to induce increased immunosuppressive capacity of MDSC. Altogether, IL-6 upregulated CCR5 and immunosuppressive capacity of MDSC in vitro in parallel, which could explain the elevated expression of immunosuppressive factors Arg1 and ROS on CCR5+ MDSC and their increased ability to suppress CD8+ T cell proliferation. However, we found only a slight increase in tumor-infiltrating MDSC and no increase in CCR5 expression or immunosuppressive factors detectable upon s.c. injection of IL-6 overexpressing Ret cells into wild type mice. In the same model, the tumor growth and mouse survival remained unaltered. Finally, we blocked IL-6 in vivo in RET transgenic melanoma-bearing mice by an anti-IL-6 antibody to decrease CCR5+ MDSC recruitment to the tumor and to inhibit IL-6-induced increase in MDSC immunosuppressive capacity, thereby neutralizing the immunosuppression in the tumor and preventing melanoma progression. Unexpectedly, the anti-IL-6 therapy resulted in accelerated tumor progression and earlier death of mice, which was most likely due to the negative effect of anti-IL-6 on T cell activation which was reflected by decreased CD4+ conventional T cells in the tumor. Altogether, we found IL-6 to upregulate CCR5 and immunosuppressive capacity of MDSC in vitro making this cytokine an interesting target for immunotherapy. However, further research should be performed to understand the delicate balance of IL-6 signaling in melanoma immunity in vivo and the challenges of IL-6 blocking immunotherapy for melanoma treatment.