Collagen Density Modulates the Immunosuppressive Functions of Tumor-Associated Macrophages

Tumor-associated macrophages (TAMs) support tumor growth by suppressing the activity of tumor infiltrating T cells. Consistently, the number of TAMs has been correlated with a poor prognosis of cancer. The immunosuppressive TAMs are also considered a major limitation for the efficacy of cancer immunotherapy. However, the molecular reason behind the acquisition of an immunosuppressive TAM phenotype is still not completely understood. During solid tumor growth, the extracellular matrix (ECM) is degraded and substituted with a tumor specific collagen-rich ECM. The collagen density of this tumor ECM has been associated with a poor prognosis of several cancers, but the underlying reason for this correlation is not well understood. Here, we have investigated whether the collagen density could modulate the immunosuppressive activity of TAMs and thereby promote tumor progression.\n\nIn this study, the macrophage cell line RAW 264.7 was 3D cultured in collagen matrices of low- and high collagen densities mimicking healthy and tumor tissue, respectively. The effects of collagen density on macrophage phenotype and function were investigated by confocal microscopy, flow cytometry, RNA sequencing, qRT-PCR, and ELISA analysis. To investigate the effect of collagen density on the immune modulatory activity of macrophages, co-culture assays with primary T cells to assess T cell chemotaxis and proliferation were conducted. Lastly, the effects of collagen density on primary cells were investigated using murine bone-marrow derived macrophages (BMDMs) and TAMs isolated from murine 4T1 breast tumors.\n\nCollagen density did not affect the proliferation, viability or morphology of macrophages. However, whole-transcriptome analysis revealed a striking response to the surrounding collagen density including the differential regulation of many immune regulatory genes and genes encoding chemokines. The transcriptional changes in RAW 264.7 macrophages were shown to be similar in murine BMDMs and TAMs. Strikingly, the collagen density-induced changes in the gene expression profile had functional consequences for the macrophages. Specifically, macrophages cultured in high density collagen were less efficient at attracting cytotoxic T cells and also capable of inhibiting T cell proliferation to a greater extent than macrophages cultured in low density collagen.\n\nOur study demonstrates that a high collagen density can instruct TAMs to acquire an immunosuppressive phenotype. This could be one of the mechanisms decreasing the efficacy of immunotherapy and linking increased collagen density to poor patient prognosis.