Stereological quantification of tumor infiltrating immune cells as predictor of immunotherapy in metastatic melanoma.

Immunotherapy by immune checkpoint inhibitors (ICIs) revolutionized the treatment of melanoma patients. Tumor-infiltrating immune cells (TIICs) play a crucial role in antitumor immunity activated by ICIs. However, ICIs treatment may be associated with serious immune-related adverse events (irAEs). The aim of the study was to identify the key immune cells and molecules of the tumor microenvironment responsible for the treatment effects and risk of irAEs through immunohistochemical quantification of TIICs. We analyzed metastases (FFPE) of 28 melanoma patients treated with ICIs. Multilevel sampling and stereological quantification were used to assess TIICs identified immunohistochemically by the markers CD1a, CD1d, CD3, CD4, CD8, CD20, CD56, CD68, FOXP3, including immune checkpoint molecules LAG3, PD1, PD-L1. In lymph node metastases, higher infiltration of PD-L1, CD8-positive cells and lower infiltration of CD1a-positive cells predicted response to ICIs (P ≤ 0.05, P ≤ 0.05, P ≤ 0.05, resp.). In all metastasis's subtypes, higher expression of PD-L1 was predictor of response to immunotherapy (P ≤ 0.05). Lower PD-L1 expression (P ≤ 0.05) and lower CD3 expression (P ≤ 0.001) were associated with irAEs. Higher infiltration of CD8-positive T lymphocytes was associated with longer progression-free survival (P = 0.0166) as well as overall survival (P = 0.0454). Stereological quantification of specific immune cells in melanoma metastases, such as T-lymphocytes (CD3), cytotoxic T-lymphocytes (CD8), dendritic cells (CD1a) and PD-L1-positive cells, may predict ICIs treatment efficacy or the risk of irAEs. High infiltration of metastatic tissue by CD8-positive T cells is important for long-term favorable therapeutic response to ICIs.