We observed accumulation of macrophages and mesothelial precursor cells (MPC) in peritoneal lavage during mesothelioma development after exposure to asbestos in Nf2 heterozygous mice, suggesting that mesothelioma-associated macrophages (MAM) and MPC are critical to promote tumorigenesis. Depletion of tumor-induced macrophages reduced sphere formation in ip murine mesothelioma model. We hypothesize that MAM may adopt MPC phenotypes thus to initiate mesothelioma tumorigenesis in the immunosuppressive microenvironment.
Methods: The MAM and lymphocytes in association with MPC were evaluated at different times after ip injection of RN5 mouse malignant mesothelioma cells. The characteristics of MAM and MPC were identified using flow cytometry and immunofluorescence. Lavage cells, spleen and blood were used to determine gene expression by RT-PCR and microarray. Tumorigenesis was assessed by ip mesosphere-forming assay and sc injection. Sorted M2-polarized macrophages were injected sc or ip into mice to evaluate tumorigenesis.
Results: MAM and MPC significantly increased in the peritoneal lavage after tumor challenge. Transcriptome analysis showed that TCR- and BCR-associated gene expressions were down-regulated. M2-specific gene was significantly up-regulated while co-stimulatory genes were down-regulated. Gene expression of pluripotency pathways was up-regulated. More strikingly, solid tumors grew in mice at 8wk after sc injection, and tumor spheroids were visible at 10wk after ip injection of sorted M2 macrophages.
Conclusion: Accumulation of MAM is correlated with MPC in tumor microenvironment, suggesting that both are indispensable for mesothelioma tumorigenesis. A subpopulation of MAM may share MPC property to initiate tumorigenesis in mouse model. This finding may provide a new mechanism explaining why tumorigenesis occurs in immunosuppressive microenvironment.