Characterization and role of CCR8+regulatory T-cells in mouse models of malignant melanoma

Melanoma cells disseminate through lymphatic vasculature into the regional lymph nodes early in disease progression. We found that a large subset of metastatic melanomas express chemokine receptor CCR8. Its principal ligand, CCL1, is constitutively expressed by lymphatic endothelial cells in the lymph node and is further upregulated by inflammation. The CCL1-CCR8 axis is an important checkpoint for melanoma lymph node metastasis, as an inhibition of CCR8 leads to arrest of melanoma cells in collecting lymphatic vessels and prevents lymph node metastasis. Among immune cells, CCR8 is predominantly expressed by regulatory T-cells (Treg) and by activated Th2 cells, and has been implicated mainly in allergic inflammation. Here we characterized CCR8+ immune cells and examined the role of CCR8 in mouse models of melanoma. Using inducible, genetically engineered (Tyr::CreER;BrafCAPtenloxPCtnnb1loxex3) and syngeneic mouse models of melanoma (B16F10) we characterized CCR8 expression on different immune cell subsets in primary tumors and in sentinel lymph nodes by flow cytometry. In primary tumors, CCR8 was exclusively expressed by CD4+ T-cells, with the highest percentage being FOXP3+CCR8+ Tregs. Increased numbers of CD4+FOXP3+CCR8+ Tregs were found in sentinel lymph nodes with metastases, compared to non-tumor-draining lymph nodes and lymph nodes from tumor-naive mice. This also corresponded with an increase in the total number of FOXP3+ T-regs in sentinel lymph nodes, compared to non-tumor draining lymph nodes. We then evaluated the role of CCR8 on B16F10 tumor growth. Tumor growth was significantly reduced in CCR8-/- mice compared to WT mice when anti-tumor immunity was stimulated with Poly(I:C). However, B16F10 tumor growth did not differ between untreated wild-type (WT) and CCR8 -/- mice. These results suggest that targeting CCR8+ Tregs may increase antitumor immune response. Further studies are required to determine the mechanism by which CCR8+ immune cells facilitate melanoma growth and to explore CCR8 as a therapeutic target in melanoma immunotherapy. Citation Format: Andrew K. Edwards, Mihaela Skobe, Anita Rogic, Marcus Bosenberg. Characterization and role of CCR8+ regulatory T-cells in mouse models of malignant melanoma [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr B156.