T cell infiltration on local CpG-B delivery in early-stage melanoma is predominantly related to CLEC9A + CD141 + cDC1 and CD14 + antigen-presenting cell recruitment

Background We previously reported CpG-B injection at the primary tumor excision site prior to re-excision and sentinel node biopsy to result in immune activation of the sentinel lymph node (SLN), increased melanoma-specific CD8 + T cell rates in peripheral blood, and prolonged recurrence-free survival. Here, we assessed recruitment and activation of antigen-presenting cell (APC) subsets in the SLN and at the injection site in relation to T cell infiltration. Methods Re-excision skin specimens from patients with clinical stage I-II melanoma, collected 7 days after intradermal injection of either saline (n=10) or 8 mg CpG-B (CPG7909, n=12), were examined by immunohistochemistry, quantifying immune subsets in the epidermis, papillary, and reticular dermis. Counts were related to flow cytometric data from matched SLN samples. Additional in vitro cultures and transcriptional analyses on peripheral blood mononuclear cells (PBMCs) were performed to ascertain CpG-induced APC activation and chemokine profiles. Results Significant increases in CD83 +, CD14 +, CD68 +, and CD123 + APC were observed in the reticular dermis of CpG-B-injected skin samples. Fluorescent double/triple staining revealed recruitment of both CD123 + BDCA2 + plasmacytoid dendritic cells (DCs) and BDCA3/CD141 + CLEC9A + type-1 conventional DC (cDC1), of which only the cDC1 showed considerable levels of CD83 expression. Simultaneous CpG-B-induced increases in T cell infiltration were strongly correlated with both cDC1 and CD14 counts. Moreover, cDC1 and CD14 + APC rates in the reticular dermis and matched SLN suspensions were positively correlated. Flow cytometric, transcriptional, and chemokine release analyses of PBMC, on in vitro or in vivo exposure to CpG-B, indicate a role for the activation and recruitment of both cDC1 and CD14 + monocyte-derived APCs in the release of CXCL10 and subsequent T cell infiltration. Conclusion The CpG-B-induced concerted recruitment of cDC1 and CD14 + APC to the injection site and its draining lymph nodes may allow for both the (cross-)priming of T cells and their subsequent homing to effector sites.