Accumulation of CD103(+) CD8(+) T cells in a cutaneous melanoma micrometastasis

OBJECTIVE: The immune system can halt cancer progression by suppressing outgrowth of clinically occult micrometastases in a state of cancer‐immune equilibrium. Cutaneous melanoma provides a unique opportunity to study the immune contexture of such lesions, as miniscule skin metastases are accessible to clinical inspection and diagnostic biopsy. METHODS: Here, we analysed by multiplex immunofluorescence microscopy samples from a melanoma patient presenting with an overt and an occult in‐transit metastasis (ITM), the latter of which appeared as a small erythematous papule. RESULTS: Microarchitecture and immune composition in the two lesions were vastly different. CD4(+) and CD8(+) T cells accumulated around the margin of the overt SOX10(+) Melan A(+) ITM but were largely excluded from the tumor centre. By contrast, the occult micrometastasis contained only few SOX10(+) Melan A(−) melanoma cells which were scattered within a dense infiltrate of T cells, including a prominent population of CD103(+) CD8(+) T cells resembling tissue‐resident memory T (T(RM)) cells. Notably, almost every single melanoma cell in the micrometastasis was in close proximity to these T(RM)‐like cells. CONCLUSION: Such results support the emerging concept that CD103(+) CD8(+) T(RM) cells are key mediators of cancer surveillance and imply an important function of these cells in controlling clinically occult micrometastases in humans.