The Role of CCR5 in the Recruitment of MDSC to the Tumor Microenvironment

The aim of the study was to analyze the recruitment and accumulation of MDSC in the tumor microenvironment via C-C chemokine receptor type 5 (CCR5)/CRR5 ligand interaction, as well as immunosuppressive activity of CCR5-expressing tumor-infiltrating MDSC. We demonstrated that CCR5+ Mo-MDSC and CCR5+ Gr-MDSC accumulated during tumor progression in melanoma lesions of ret transgenic mice, as well as in melanoma patients at different stages. In the mouse model, the frequency of CCR5+ cells was higher among Gr-MDSC than among Mo-MDSC, whereas in melanoma patients, Mo-MDSC were characterized by higher frequency of CCR5+ cells. CCR5+ MDSC demonstrated an increased immunosuppressive phenotype reflected by enhanced NO and ROS production, as well as ARG-1 and PD-L1 expression, as compared to their CCR5- counterpart. Both in mouse model and in melanoma patients, the concentration of CCR5 ligands such as CCL3 (MIP-1α), CCL4 (MIP-1β) and CCL5 (RANTES) and inflammatory factors such as granulocyte/macrophage colony-stimulating factor (GM-CSF), vascular endothelial growth factor (VEGF), interleukin (IL)-6 and interferon-γ (IFN-γ) were increased in skin tumors as compared to the serum that allows the migration of these cells to the tumor microenvironment. Furthermore, we demonstrated that CCR5+ MDSC inhibited T cell proliferation in a dose-dependent manner and showed a tendency to stronger inhibition of T cell proliferation than their CCR5- counterparts. In addition, we detected the CCR5 expression on CD4+ and CD8+ T cells in ret transgenic melanoma bearing mice. Interestingly, the frequency of CCR5+ regulatory T cells (Treg) was significantly higher than that among other T cell subsets. Antigen-experienced CCR5+ Treg accumulated in advanced stage melanoma patients as compared to early stage patients and healthy donor (HD). After intraperitoneal injection of mCCR5-Ig, which blocks CCR5-CCR5 ligand interactions into tumor bearing mice, we observed a significantly increase in mouse survival as compared to the control group. Moreover, the frequency of MDSC and Treg decreased in skin tumors after mCCR5-Ig therapy. NO production by MDSC in skin tumors and the frequency of CD69 expression, reduced activated Treg in metastatic lymph nodes (LN), which indicates the therapeutic effect of mCCR5-Ig. In summary, we demonstrated that during melanoma progression, CCR5 expressing MDSC accumulated in the tumor microenvironment and exerted strong immunosuppressive activity. This accumulation was mediated by CCR5 ligands and other inflammatory factors. Blockage of CCR5-CCR5 ligand interactions induced the prolongation of the survival of tumor bearing mice mediated by a reduced migration and immunosuppressive activity of MDSC and Treg at the tumor site. Our findings define a critical role for CCR5 in recruiting MDSC and Treg,which can be used for the development of novel immunotherapeutic strategies for melanoma patients