CD8+ T cells at the earliest stage of effector generation have not been identified at tumor site of melanoma patients. Such early effectors, if present, should be characterized by a specific phenotype, distinct from that expressed at later stages of the antigen-induced differentiation program, by short-lived effector cells, memory precursors, and terminal effectors. Here, we show that neoplastic tissues from primary and metastatic lesions of melanoma patients contain a subset of CD8+ T cells expressing FOXP3. CD8+ FOXP3+ CD25+ T lymphocytes were found in tumor-invaded lymph nodes (TILN), s.c. metastases, and advanced primary lesions. Their frequency was significantly higher in TILN compared with tumor-free lymph nodes or with peripheral blood and in primary tumors compared with TILN. CD8+ FOXP3+ T cells did not express markers of regulatory [CTLA-4, CCL4, interleukin-10 (IL-10), transforming growth factor-β1], exhausted (PD-1), or senescent (CD57) CD8+ T lymphocytes. Instead, this subset showed an antigen-experienced "EM1" phenotype (CCR7– CD45RA– CD28+ CD27+) and exhibited a CD127–, KLRG1–, HLA-DR+, CD38+, T-bet+, perforin+ "early effector" profile predicted by current models. CD8+ FOXP3+ T cells produced IFN- on short in vitro activation, recognized autologous tumor by CD107a mobilization, and expressed Ki-67 on ex vivo analysis. In response to autologous tumor plus IL-2/IL-15, the CD8+ FOXP3+ T cells proliferated promptly and showed competence for differentiation (downregulation of CD27 and upregulation of T-bet). These results suggest development of early phases of antitumor immunity even in advanced melanoma. Moreover, the CD8+ FOXP3+ "early effector" subset may be an invaluable tool for monitoring immunity at tumor site. Cancer Res; 70(21); 8378–87. 2010 AACR. cancerres.aacrjournals.org
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