Response to ipilimumab therapy in metastatic melanoma patients: potential relevance of CTLA-4+ tumor infiltrating lymphocytes and their in situ localization

Immune checkpoint inhibitors, including ipilimumab (IPI), achieve a clinical benefit in a small proportion of melanoma patients highlighting the need to investigate predictive biomarkers. In this study, we characterized tumor infiltrating lymphocytes (TILs), focusing on the CTLA-4+ subset, and evaluated their possible predictive significance. We characterized TIL density, cell type, and localization in 40 melanoma lesions from 17 patients treated with IPI. Associations of TILs with IPI timing, tissue localization, and response to IPI were estimated using a linear mixed-effects modelling approach. We found that most of TIL subsets increased in situ upon IPI therapy, with particular reference to FoxP3+ cells. TILs and TIL subsets, such as CD3+, CD45RO+, CTLA-4+, CD4+, CD8+ T cells, CD20+ B cells, and NKp46+ NK cells, showed significantly different spatial distributions in the tumor microenvironment being higher at the invasive margin (IM) as compared to the tumor center (TC) (P value < 0.001 for TIL score and P value < 0.05 for all subsets). Remarkably, high TIL score and density of CD3+, CD8+ T cells, and CTLA-4+ immune cells were significantly associated with a better response to IPI (P values = 0.002, 0.023, 0.007, and 0.001, respectively, for responders vs non-responders). In conclusion, we provide a detailed analysis of CTLA-4+ TIL distribution in melanoma tissues taking into account localization, relationship with CD3+/CD8+ TILs, and changes in response to IPI treatment. We identified that CTLA-4+ TILs may represent a marker of IPI response, alone or with CD3+/CD8+ subsets, although this requires confirmation in larger studies.