Uncovering the Role of Cancer Associated Fibroblasts in Tumor Immunosuppression

Current advances in cancer immunotherapy are hindered by the immunosuppressive nature of the tumor microenvironment (TME). Cancer-associated fibroblasts (CAFs) are a key component of the TME that contribute to suppressing the immune response. Chemokines and cytokines released by CAFs, such as IL-6, attract circulating monocytes which are converted into myeloid-derived suppressor cells (MDSCs). MDSCs suppress T-cells and natural killer cell proliferation which inactivates the immune response and deters immunotherapy. Both Prostaglandin E2 (PGE2) and granulocyte-macrophage colony-stimulating factor (GM-CSF) have been implicated in the CAF immunosuppressive pathway, but a causal link has yet to be established. We hypothesized that PGE2 and GM-CSF secreted from CAFs are responsible for the conversion of monocytes into MDSCs which suppress the immune response in the TME. CAFs were induced via coculture of normal fibroblasts with various cancer cell lines utilizing transwell plates (500,000 cells). PGE2 ELISA, GM-CSF ELISA, and cytokine array were performed with the resulting supernatant after a 72-hour incubation. One-way ANOVA and subsequent Tukey tests were employed for data analysis of the ELISAs (significance: p\u3c0.05). Primary CAFs derived from adenocarcinoma obtained from a lung cancer patient exhibited increased levels of both PGE2 (p\u3c0.01) and GM-CSF (p\u3c0.001) as compared to the WI-38 normal fibroblast control. It is evident that PGE2 and GM-CSF are upregulated in CAFs isolated from adenocarcinoma but the magnitude of upregulation is not consistent across CAFs derived from other tumor types such as melanoma. The results suggest that there may be other factors involved in the CAFs and MDSC interaction such as TNF-ɑ, CXCL10, and CD40 ligand, so it is not yet clear which factors are directly responsible for the induction of MDSCs