BAP1 Loss in Pleural Mesothelioma Is Associated With Reduced Soluble CCL2 in Patient Effusion, Abrogated CCL2-Mediated Monocyte Recruitment In Vitro.

Objectives: Pleural mesothelioma is an incurable cancer of the cell layer lining the chest wall and lung. Patients frequently present with pleural effusion, which is often drained for symptom relief and enables minimally invasive sampling of the tumour environment, including immune cells and related soluble factors. Most of the mesothelioma tumours exhibit loss of BRCA1-associated protein 1 (BAP1), a multifunctional tumour suppressor protein. Here, we aim to elucidate the effect of BAP1 loss on the mesothelioma microenvironment through profiling soluble factors within pleural effusion. Methods: A custom panel of 22 soluble factors was measured by Luminex assay and enzyme-linked immunosorbent assay in an initial cohort of 40 patients with known BAP1 status. Validation was performed by enzyme-linked immunosorbent assay in an independent cohort of 100 cases. Secretion of soluble factors and chemoattraction of monocytes were characterised using a CRISPR-mediated BAP1 deletion model in a mesothelioma and a lung cancer cell line. Immune cell infiltration, estimated by CIBERSORT, was further explored in the Cancer Genome Atlas -MESO cohort. Results: Soluble C-C motif chemokine ligand 2 (CCL2) was approximately 55% to 60% lower in pleural effusion from BAP1-loss cases in both independent cohorts. Deletion of BAP1 reduced CCL2 secretion in vitro and abolished CCL2-mediated chemoattraction of monocytes in both mesothelioma and lung cancer cell lines. In the Cancer Genome Atlas -MESO cohort, BAP1-mutant tumours exhibited a reduction in estimated macrophage content. Conclusion: Loss of BAP1 impairs CCL2 secretion into pleural effusions, potentially influencing monocyte recruitment into the tumour microenvironment.