Characterization of Pleural Mesothelioma by Hierarchical Clustering Analyses Using Immune Cells within Tumor Microenvironment

<b><i>Introduction:</i></b> Over the past decade, classifications using immune cell infiltration have been applied to many types of tumors; however, mesotheliomas have been less frequently evaluated. <b><i>Methods:</i></b> In this study, 60 well-characterized pleural mesotheliomas (PMs) were evaluated immunohistochemically for the characteristics of immune cells within tumor microenvironment (TME) using 10 immunohistochemical markers: CD3, CD4, CD8, CD56, CD68, CD163, FOXP3, CD27, PD-1, and TIM-3. For further characterization of PMs, hierarchical clustering analyses using these 10 markers were performed. <b><i>Results:</i></b> Among the immune cell markers, CD3 (<i>p</i> &lt; 0.0001), CD4 (<i>p</i> = 0.0016), CD8 (<i>p</i> = 0.00094), CD163+ (<i>p</i> = 0.042), and FOXP3+ (<i>p</i> = 0.025) were significantly associated with an unfavorable clinical outcome. Immune checkpoint receptor expressions on tumor-infiltrating lymphocytes such as PD-1 (<i>p</i> = 0.050), CD27 (<i>p</i> = 0.014), and TIM-3 (<i>p</i> = 0.0098) were also associated with unfavorable survival. Hierarchical clustering analyses identified three groups showing specific characteristics and significant associations with patient survival (<i>p</i> = 0.016): the highest number of immune cells (IC^High); the lowest number of immune cells, especially CD8+ and CD163+ cells (IC^Low); and intermediate number of immune cells (IC^Int). IC^High tumors showed significantly higher expression of PD-L1 (<i>p</i> = 0.00038). Cox proportional hazard model identified IC^High [hazard ratio (HR) = 2.90] and IC^Int (HR = 2.97) as potential risk factors compared with IC^Low. Tumor CD47 (HR = 2.36), tumor CD70 (HR = 3.04), and tumor PD-L1 (HR = 3.21) expressions were also identified as potential risk factors for PM patients. <b><i>Conclusion:</i></b> Our findings indicate immune checkpoint and/or immune cell-targeting therapies against CD70-CD27 and/or CD47-SIRPA axes may be applied for PM patients in combination with PD-L1-PD-1 targeting therapies in accordance with their tumor immune microenvironment characteristics.