Metastasis founder cells activate immunosuppression early in human melanoma metastatic colonization

The earliest steps of lethal metastasis in patients are incompletely understood. To dissect them, we prospectively searched for the earliest detectable disseminated cancer cells (DCC) in sentinel lymph node biopsies of 492 stage I-III patients. By visually-controlled, micromanipulator-assisted isolation and single cell transcriptome analysis of these extremely rare DCC, we identified MCSP+ melanoma cells as strong candidates for metastasis founder cells (MFC) in lymph nodes. Based on a median follow-up time of 6 years, their detection was the strongest predictor of systemic metastasis and death upon multivariable analysis. During transition from single cells to metastasis-initiating clusters, melanoma DCC were exposed to CD8 T cell attack, activated the extracellular vesicular exosomal pathway, and expressed the immunomodulatory proteins CD155 and CD276, but rarely PD-L1. CD155 and CD276-positive extracellular vesicles from patient-derived DCC models exhibited an immunosuppressive activity on CD8 T cells. Our data indicate that either direct targeting of MFC employing MCSP or their immune escape mechanisms might be key for cure of early-stage melanoma. ### Competing Interest Statement The authors have declared no competing interest.