Expression of ALCAM (CD166) and PD-L1 (CD274) Independently Predicts Shorter Survival in Malignant Pleural Mesothelioma

Diffuse malignant mesothelioma of the pleura is a highly aggressive tumor typically associated with short survival. ALCAM (CD166), a type I transmembrane protein, is a member of the immunoglobulin superfamily. In normal cells, ALCAM regulates physiological processes such as angiogenesis and immune response. In cancer, it is associated with neoplastic progression, including invasion, migration, and metastasis. Furthermore, ALCAM is considered one of the cancer stem cell markers such as ALDH1 (ALDH1A1) and SALL4. The PD-L1 (CD274)/PD-1 (PDCD1, CD279) pathway is crucial for the modulation of immune responses in normal cells. Nevertheless, pathologic activation of the PD-L1/PD-1 pathway participates in immune evasion by tumor cells. Many PD-L1-expressing tumor cells have been identified in different types of cancer, including malignant mesothelioma. In this study, 175 well-characterized primary diffuse pleural mesotheliomas, including the epithelioid (n = 148), biphasic (n = 15), and sarcomatoid (n = 12) histotypes, were evaluated immunohistochemically for cancer stem cell markers (ALCAM, ALDH1, and SALL4) and PD-L1 expression. Twenty-five percent of the mesotheliomas (43/175) expressed ALCAM, whereas ALDH1 and SALL4 positivity was seen in 1% to 2% of cases. Thirty-three percent of the analyzed tumors (57/175) contained PD-L1-positive cells. Overall survival was significantly decreased in the cohort of patients with ALCAM- or PD-L1-positive tumors (both P < .01). Furthermore, the multivariate Cox hazards regression analysis identified ALCAM and PD-L1 (both P < 0.01) as potential independent risk factors. Thus, a combination of these 2 markers might be useful for prognostication and planning the treatment of patients with malignant pleural mesothelioma.