Tumor PD-L1 expression in malignant pleural and peritoneal mesothelioma by Dako PD-L1 22C3 pharmDx and Dako PD-L1 28-8 pharmDx assays.

Malignant mesothelioma (MM) is an aggressive neoplasm with poor prognosis. The Dako PD-L1 22C3 and 28-8 pharmDx assays are approved by the US Food and Drug Administration (FDA) as companion and complementary diagnostics for the anti-PD-1 drugs pembrolizumab and nivolumab, respectively. Data from multiple clinical trials indicate that immunotherapy has antitumor activity in advanced malignant pleural (MPM) and peritoneal mesothelioma (MPeM). However, large studies of PD-L1 expression in MM using the FDA-approved anti-PD-L1 assays are lacking. We stained tissue microarray sections (N = 125; 112 MPM and 13 MPeM) using 2 FDA-approved clinical immunohistochemical makers for PD-L1 expression: Dako PD-L1 22C3 pharmDx and Dako PD-L1 28-8 pharmDx. Overall, 22% (27/125) of MMs were positive using the 22C3 assay, whereas 27% (32/117) were positive using the 28-8 assay, using a tumor proportion score cutoff of 1%. Tumor cell PD-L1 expression was strongly correlated with PD-L1 expression on tumor-associated immune cells. No significant difference in PD-L1 expression was observed by patient sex, age, treatment history, pathologic stage, or histologic subtype. However, the proportion of cases positive for PD-L1 expression was higher among MPeM compared to MPM (P = .007 for 22C3 assay; P = .04 for 28-8 assay). PD-L1 is expressed in a substantial proportion of MM cases, as measured by FDA-approved companion assays for widely used immunotherapeutic drugs. PD-L1 expression is particularly prevalent in MPeM. These findings support large clinical studies to further examine PD-L1 as a biomarker for a subset of MM patients that may benefit from immunotherapy.