The CD200-CD200R axis promotes squamous cell carcinoma metastasis via regulation of Cathepsin K

The CD200-CD200R immunoregulatory signaling axis plays an etiological role in the survival and spread of numerous cancers primarily through suppression of anti-tumor immune surveillance. Our previous work outlined a pro-metastatic role for the CD200-CD200R axis in cutaneous squamous cell carcinoma (cSCC) that is independent of direct T cell suppression but modulates the function of infiltrating myeloid cells. To identify effectors of the CD200-CD200R axis important for cSCC metastasis, we conducted RNA-Seq profiling of infiltrating CD11b+Cd200r+ cells isolated from Cd200+ versus Cd200 null cSCCs and identified the cysteine protease Cathepsin K (Ctsk) to be highly upregulated in Cd200+ cSCCs. CD11b+Cd200r+ cells, which expressed phenotypic markers associated with myeloid-derived suppressor cell-like cells and tumor-associated macrophages, were the primary source of Ctsk expression in cSCC. Using a Cd200r+ myeloid cell-cSCC co-culture system, we observed that induction of Ctsk in Cd200r+ tumor-infiltrating cells was dependent on engagement of the CD200-CD200R axis, indicating that Ctsk is a target gene of this pathway in the cSCC tumor microenvironment. Inhibition of Ctsk, but not matrix metalloproteinases (MMPs), significantly blocked cSCC cell migration through collagen gels in vitro. Finally, both targeted Cd200 disruption in tumor cells and Ctsk pharmacological inhibition dramatically reduced cSCC metastasis in vivo. Collectively, our findings identify a novel direct role for Cd200r+ infiltrating myeloid lineages in tumor metastasis and support the conclusion that CD200 stimulates cSCC invasion and metastasis via induction of Ctsk in CD200r+ infiltrating cells. Profiling Cd200-dependent genes in Cd200r+ cSCC-infiltrating myeloid cells in Cd200r+ myeloid cells purified from orthotopic Wt (CD200+) or EpiKO(CD200-) cSCCs.