Germline BAP1 mutation alter the tumor immune microenvironment and impact the progression of Malignant Mesothelioma (MM)

Abstract

Asbestos causes malignant transformation of primary human mesothelial cells (HM), leading to mesothelioma. The primary inherited risk factor linked to mesothelioma is the dysfunction of BRCA1 Associated Protein 1 (BAP1). People with inherited BAP1 mutations face an increased risk of developing MM. However, to date, we found that none of the mesothelioma patients carrying germline BAP1 mutations were professionally exposed to asbestos. Using a Bap1+/− mouse model, we found that, compared with their wild-type littermates, Bap1+/− mice exposed to low-dose asbestos fibres showed significant alterations of the peritoneal inflammatory response and lower levels of several chemokines and cytokines. Consistent with these data, Bap1+/ − mice had a significantly higher incidence of mesothelioma after exposure to very low doses of asbestos doses that rarely induced mesothelioma in wild-type mice. Additionally, our findings showed that tumors in Bap1+/- mice have more M2-type macrophages as compared to tumors in Bap1WT mice. This puzzled us even more since M2 macrophages promote immunosuppressive microenvironment and should have adverse effects on therapies, but we see the opposite response in BAP1 mutant patients. We speculated if asbestos had an effect on the observed results. We created a mouse model from mesothelioma-derived luciferase integrated cancer cell line (AN1) from Bap1+/- mice which were biallelically inactivated for BAP1 as found in 60% of BAP1 mutant MM patient tumors, to understand how BAP1 loss in mesothelioma impact tumour microenvironment without asbestos exposure. Here, I injected Bap1WT and Bap1+/- with AN1 (Bap1-/-) biallelic inactivated tumor cell line. Tumors were digested and tumor immune cells were collected. Harvested cells were immunoprofiled by flow cytometry. I did not find any significant differences in the immune cell population amongst the two genotypes i.e. Bap1+/- and Bap1WT. Additionally, I did not find any differences in the T cell population (CD8+, CD4+, their activation states and Tregs) and IFNY levels by flow cytometry. However, I found that tumor-associated macrophages (TAMs) in Bap1+/- showed reduced levels of MHCII expression as compared to Bap1WT. I did not find similar findings in other antigen-presenting cells i.e. dendritic cells, B cells and macrophages outside of the TME. Additionally, I found that tumor-promoting markers i.e. Trem2, CD9 and Ly6C were significantly reduced on TAMs in Bap1+/- as compared to Bap1WT mice. These findings imply that while alterations in BAP1 status i.e. partial or complete loss are associated with MM development, the relationship between BAP1 status and survival outcomes is more complex and possibly influenced by other factors or molecular pathways which need to be explored further.