Immunological roles of antigen-presenting cancer-associated fibroblasts and mesothelial cells in colorectal cancer

Cancer-associated fibroblasts (CAFs) play crucial roles in tumor progression and metastasis through their interactions with cancer and immune cells in the tumor microenvironment. Among CAF subsets, antigen-presenting CAFs (apCAFs), characterized by their expression of MHC class II molecules and antigen-presenting ability, have been shown to exert immunosuppressive effects and are thought to originate from mesothelial cells. In this study, we investigated the immune-related functions of apCAFs and mesothelial cells in colorectal cancer (CRC). Using an orthotopic transplantation mouse model, human CRC surgical specimens, and established CAF and mesothelial cell lines, we analyzed their interactions with colorectal cancer cells. Immunohistochemical analysis of CRC tissues revealed that patients with HLA-DR-positive stromal cells had significantly poorer recurrence-free survival compared to HLA-DR-negative patients. The proportion of apCAF marker-positive stromal cells increased with tumor stage. In orthotopically transplanted CRC tumors, flow cytometric analysis demonstrated that the proportion of apCAFs peaked six weeks after transplantation, followed by an increase in myofibroblastic CAFs (myCAFs). Gene expression analysis of mesothelial cells, which are proposed to give rise to apCAFs, showed elevated expression of apCAF-related genes in cancer-associated mesothelial cells (CAmeso) compared with normal mesothelial cells. In an allogeneic mixed lymphocyte reaction using human mesothelial cells, the proliferation of CD4 + T cells was significantly reduced when co-cultured with CAmeso compared to dendritic cells alone, indicating that CAmeso exert immunosuppressive effects despite their antigen-presenting potential. In addition, single-cell RNA sequencing identified CAF subsets with high SLPI expression, suggesting the existence of immunomodulatory CAF populations within the tumor stroma. Furthermore, immunofluorescence staining confirmed the presence of stromal cells co-expressing alphaSMA and SLPI, and patients with alphaSMA + SLPI + stromal cells had significantly worse prognosis. Notably, alphaSMA + SLPI + stromal cells were more abundant in liver metastases and peritoneal dissemination lesions than in primary colorectal cancer tissues, suggesting that these cells may be involved in metastatic progression. Taken together, these findings demonstrate that apCAFs are associated with poor clinical outcomes in CRC, and that cancer-associated mesothelial cells acquire antigen-presenting but immunosuppressive properties under the influence of cancer cells, thereby contributing to tumor progression and unfavorable prognosis.