Targeting Melanoma by CXCR4 Inhibition
Chemokine receptor 4 (CXCR4) is a G protein-coupled receptor that is overexpressed in many kinds of cancer, both in the tumor cells themselves and in the surrounding stroma. CXCR4-mediated signaling, initiated by its endogenous ligand, CXCL12, promotes tumor initiation, progression, and metastasis. Hence, CXCR4 antagonists are currently in clinical trials as promising new cancer chemotherapeutic agents. CXCR4 also plays an important role in immune modulation, however, and the effects of systemic suppression of this role are not fully understood. This led Vanderbilt Basic Sciences investigator Ann Richmond and her laboratory to explore the effects of CXCR4 antagonism on the immune system. They did so by creating mice (CXCR4-mKO) in which the gene for CXCR4 had been knocked out selectively in the myeloid lineage of immune cells. They then injected B16F0 melanoma cells intravenously into these mice, and found that they developed fewer and smaller lung metastases than identically treated wild-type litter mates (CXCR4-mWT). Similar findings resulted following injection of PyMT breast cancer cells, confirming that suppression of CXCR4 in myeloid cells has a tumor suppressive effect. Treatment of the CXCR4-mKO mice with antibodies to selectively deplete natural killer (NK) cells or CD8+cytotoxic T cells demonstrated that NK cells were responsible for the increased tumor resistance associated with ablation of myeloid CXCR4. This was consistent with the presence of higher numbers of NK cells in the melanoma lung metastases of CXCR4-mKO mice as compared to CXCR4-mWT mice. Furthermore, a higher percentage of the NK cells of CXCR4-mKO mice than CXCR4-mWT mice expressed interferon-γ (IFN-γ), a cytokine known to increase NK anti-tumor activity. To monitor NK cell activity in vivo, the investigators injected Yac-1 tumor cells expressing luciferase intravenously into the mice. These cells, which are sensitive to NK cell-mediated cytotoxicity, could be visualized by luminescence following injection of luciferin. This approach confirmed a higher level of NK cell-mediated anti-tumor activity in CXCR4-mKO mice as compared to their wild-type littermates. NK cells from CXCR4-mKO mice expressed higher levels of membrane-bound Fas ligand (FasL) than those from CXCR4-mWT mice. Treatment of CXCR4-mKO NK cells with an antibody against FasL reduced their cytotoxicity, suggesting that the Fas/FasL pathway plays a role in NK cell-mediated anti-tumor activity. Selective depletion of neutrophils in CXCR4-mKO mice revealed that these cells contribute to tumor resistance. Consistently, neutrophils from CXCR4-mKO mice expressed higher levels of interleukin-18 (IL-18) than those from CXCR4-mWT mice, suggesting that IL-18-mediated NK cell activation contributes to tumor resistance in the CXCR4-mKO mice. Treatment of wild-type mice with LY2510924, a cyclic peptide antagonist of CXCR4 led to similar immune system effects and tumor resistance as were observed in CXCR4-mKO mice. Of particular interest was the discovery that human melanoma patients who exhibited high levels of a myeloid cell marker in their tumors had reduced progression-free survival if they also expressed high tumor levels of CXCR4. Together, the findings suggest that increased NK cell activity, mediated by IFN-γ, FasL, and IL-18, contributes to the anti-tumor effects of CXCR4 antagonism and that the patients most likely to benefit from this therapeutic approach are those in which myeloid cells are a significant component of the tumor microenvironment. The work is published in Cancer Immunology Research [J. Yang, et al. (2018) Cancer Immunol. Res., published online August 14, DOI: 10.1158/2326-6006].