Presence of Skin Tissue-Resident Memory T Cells in Human Nonmalignant and Premalignant Melanocytic Skin Lesions and in Melanoma

The infiltration of tissue-resident memory (T _RM) cells in melanoma correlates with improved survival, suggesting an important role for T _RMcells in immunity against melanoma. However, little is known about the presence of T _RMcells in nonmalignant and premalignant melanocytic lesions. This study aimed to evaluate the presence of T _RMcells in human skin melanocytic lesions, representing the spectrum from healthy skin to metastatic melanoma. FFPE sections from healthy skin, sun-exposed skin, benign nevi, lentigo maligna (LM), primary LM melanoma, and primary cutaneous and metastatic melanoma were analyzed by immunohistochemistry. The number of infiltrating cells expressing T _RM-associated markers, CD3, CD4, CD8, CD69, CD103, and CD49a, was quantified by digital analyses. Multiplex immunofluorescence was performed to analyze coexpression of T _RMcell markers. More T cells and CD69 ⁺cells were found in melanoma lesions, as compared with healthy skin and nevi. CD103 ⁺and CD49a ⁺cell numbers did not significantly differ. More importantly, no differences were seen in expression of all markers between healthy skin and benign nevi. Similar results, except for CD69, were observed in LM melanoma, as compared with LM and sun-exposed skin. Interestingly, multiplex immunofluorescence showed that nevi tissues have comparable CD103 ⁺T cell numbers with healthy skin but comprise more CD103 ⁺CD8 ⁺cells. Expression of T _RMcell markers is significantly increased in melanoma, as compared with nonmalignant skin. Our data also show that T _RMcells are not abundantly present already in premalignant tissues. Further studies on the specificity of T _RMcells for melanocyte/melanoma antigens may reveal their significance in cancer immunosurveillance.