Single-Cell Immune Mapping of Melanoma Sentinel Lymph Nodes Reveals an Actionable Immunotolerant Microenvironment

Purpose: Improving our understanding of the immunologic response to cancer cells within the sentinel lymph nodes (SLN) of primary tumors is expected to identify new approaches to stimulate clinically meaningful cancer immunity. Experimental Design: We used mass cytometry by time-of-flight (CyTOF), flow cytometry, and T-cell receptor immunosequencing to conduct simultaneous single-cell analyses of immune cells in the SLNs of patients with melanoma. Results: We found increased effector-memory ab T cells, TCR clonality, and g d T cells selectively in the melanoma-bearing SLNs relative to non–melanoma-bearing SLNs, consistent with possible activation of an antitumor immune response. However, we also observed a markedly immunotolerant environment in the melanoma-bearing SLNs indicated by reduced and impaired NK cells and increased levels of CD8^þCD57^þPD-1^þ cells, which are known to display low melanoma killing capabilities. Other changes observed in melanoma-bearing SLNs when compared with non–melanoma-bearing SLNs include (i) reduced CD8^þCD69^þ T cell/T regulatory cell ratio, (ii) high PD-1 expression on CD4^þ and CD8^þ T cells, and (iii) high CTLA-4 expression on gd T cells. Conclusions: Our data suggest that these immunologic changes compromise antimelanoma immunity and contribute to a high relapse rate. We propose the development of clinical trials to test the neo-adjuvant administration of anti–PD-1 antibodies prior to SLN resection in patients with stage III melanoma.