Data from Low Exposures to Amphibole or Serpentine Asbestos in Germline <i>Bap1</i>-mutant Mice Induce Mesothelioma Characterized by an Immunosuppressive Tumor Microenvironment

Abstract

<div>Abstract<p>Asbestos and <i>BAP1</i> germline mutations are risk factors for malignant mesothelioma (MM). While it is well accepted that amphibole asbestos is carcinogenic, the role of serpentine (chrysotile) asbestos in MM has been debated. To address this controversy, we assessed whether minimal exposure to chrysotile could significantly increase the incidence and rate of MM onset in germline <i>Bap1</i>-mutant mice. With either crocidolite or chrysotile, and at each dose tested, MMs occurred at a significantly higher rate and earlier onset time in <i>Bap1</i>-mutant mice than in wild-type littermates. To explore the role of gene–environment interactions in MMs from <i>Bap1</i>-mutant mice, we investigated proinflammatory and protumorigenic factors and the tumor immune microenvironment (TIME). IHC and immunofluorescence staining showed an increased number of macrophages in granulomatous lesions and MMs. The relative number of CD163-positive (CD163<sup>+</sup>) M2 macrophages in chrysotile-induced MMs was consistently greater than in crocidolite-induced MMs, suggesting that chrysotile induces a more profound immunosuppressive response that creates favorable conditions for evading immune surveillance. MMs from <i>Bap1</i>-mutant mice showed upregulation of CD39/CD73-adenosine and C-C motif chemokine ligand 2 (Ccl2)/C-C motif chemokine receptor 2 (Ccr2) pathways, which together with upregulation of IL6 and IL10, promoted an immunosuppressive TIME, partly by attracting M2 macrophages. Interrogation of published human MM RNA sequencing (RNA-seq) data implicated these same immunosuppressive pathways and connections with CD163<sup>+</sup> M2 macrophages. These findings indicate that increased M2 macrophages, along with upregulated CD39/CD73-adenosine and Ccl2/Ccr2 pathways, contribute to an immunosuppressive TIME in chrysotile-induced MMs of <i>Bap1</i>-mutant mice, suggesting that immunotherapeutic strategies targeting protumorigenic immune pathways could be beneficial in human <i>BAP1</i> mutation carriers who develop MM.</p>Significance:<p>We show that germline <i>Bap1</i>-mutant mice have enhanced susceptibility to MM upon minimal exposure to chrysotile asbestos, not only amphibole fibers. Chrysotile induced a more profound immune tumor response than crocidolite in <i>Bap1</i>-mutant mice by upregulating CD39/CD73-adenosine and Ccl2/Ccr2 pathways and recruiting more M2 macrophages, which together contributed to an immunosuppressive tumor microenvironment. Interrogation of human MM RNA-seq data revealed interconnected immunosuppressive pathways consistent with our mouse findings.</p></div>