Immune checkpoint proteins CTLA-4, PD-1 and PD-L1 in melanoma as predictive biomarkers to guide clinical decisions

Abstract

Background CTLA-4, PD-1 and PD-L1 are potential targets in cancer immunotherapy. However, to predict patients who are likely to respond to treatment with PD-1/PD-L1/CTLA-4 blockers still remains a challenge. Unfortunately, it is often ignored that CTLA-4 exists also in cells other than lymphocytes. This study aimed to assess the expression and diagnostic significance of predictive biomarkers for melanoma. Methods To address this issue, we performed an immunophenotyping of CTLA4/PD1/PD-L1-positive cells in melanoma employing multiple immunofluorescent immunolabeling. For immunolabeling, we used primary antibodies to PD-1 and PD-L1, a panel of anti–CTLA-4 antibodies of different clones and a panel of CD antibodies against diverse cell types (CD1a, CD3, CD8 and CD68). Results In contrast to the currently accepted view, immunolabeling with anti-CTLA-4 antibodies of different clones permitted to reveal CTLA-4 in cells other than lymphocytes. CTLA-4 and PD-L1 are both expressed in the tumor microenvironment cells including antigen-presenting cells ÷ macrophages and dendritic cells. PD-1 is expressed solely in T-lymphocytes. CTLA-4 and PD-L1 in melanoma are not expressed in malignant cells. Conclusion Accordingly, PD-L1 expression by tumor cells cannot serve as a biomarker of clinical response to checkpoint blockade in melanoma immunotherapy, but PD-L1 and CTLA-4 expressed in the tumor microenvironment cells are valuable biomarkers to guide clinical decisions. The expression of CTLA-4 also in cells other than T-lymphocytes suggests the involvement of this molecule in alternative CTLA-4 signaling pathways together with the well-known classical scheme for the regulation of T-lymphocyte activity.