Cytotoxic T Cells in PD-L1-Positive Malignant Pleural Mesotheliomas Are Counterbalanced by Distinct Immunosuppressive Factors

PD-L1 immunohistochemical staining does not always predict whether a cancer will respond to treatment with PD-1 inhibitors. We sought to characterize immune cell infiltrates and the expression of T-cell inhibitor markers in PD-L1-positive and PD-L1-negative malignant pleural mesothelioma samples. We developed a method for immune cell phenotyping using flow cytometry on solid tumors that have been dissociated into single-cell suspensions and applied this technique to analyze 43 resected malignant pleural mesothelioma specimens. Compared with PD-L1-negative tumors, PD-L1-positive tumors had significantly more infiltrating CD45⁺ immune cells, a significantly higher proportion of infiltrating CD3⁺ T cells, and a significantly higher percentage of CD3⁺ cells displaying the activated HLA-DR⁺/CD38⁺ phenotype. PD-L1-positive tumors also had a significantly higher proportion of proliferating CD8⁺ T cells, a higher fraction of FOXP3⁺/CD4⁺ Tregs, and increased expression of PD-1 and TIM-3 on CD4⁺ and CD8⁺ T cells. Double-positive PD-1⁺/TIM-3⁺ CD8⁺ T cells were more commonly found on PD-L1-positive tumors. Compared with epithelioid tumors, sarcomatoid and biphasic mesothelioma samples were significantly more likely to be PD-L1 positive and showed more infiltration with CD3⁺ T cells and PD-1⁺/TIM-3⁺ CD8⁺ T cells. Immunologic phenotypes in mesothelioma differ based on PD-L1 status and histologic subtype. Successful incorporation of comprehensive immune profiling by flow cytometry into prospective clinical trials could refine our ability to predict which patients will respond to specific immune checkpoint blockade strategies. Cancer Immunol Res; 4(12); 1038-48. ©2016 AACR.