The interacting role of physical stiffness and tumor cells on the macrophages polarization

Matrix stiffness enhances cancer-macrophage interactions and M2-like macrophage accumulation in the breast tumor microenvironment

The role of intratumor heterogeneity is becoming increasingly apparent in part due to expansion in single cell technologies. Clinically, tumor heterogeneity poses several obstacles to effective cancer therapy dealing with biomarker variability and treatment responses. Matrix stiffening is known to occur during tumor progression and contribute to pathogenesis in several cancer hallmarks, including tumor angiogenesis and metastasis. However, the effects of matrix stiffening on intratumor heterogeneity have not been thoroughly studied. In this study, we applied single-cell RNA sequencing to investigate the differences in the transcriptional landscapes between stiff and compliant MMTV-PyMT mouse mammary tumors. We found similar compositions of cancer and stromal subpopulations in compliant and stiff tumors but differential intercellular communication and a significantly higher concentration of tumor-promoting, M2-like macrophages in the stiffer tumor microenvironments. Interestingly, we found that cancer cells seeded on stiffer substrates recruited more macrophages. Furthermore, elevated matrix stiffness increased Colony Stimulating Factor 1 (CSF-1) expression in breast cancer cells and reduction of CSF-1 expression on stiffer substrates reduced macrophage recruitment. Thus, our results demonstrate that tissue phenotypes were conserved between stiff and compliant tumors but matrix stiffening altered cell-cell interactions which may be responsible for shifting the phenotypic balance of macrophages residing in the tumor microenvironment towards a pro-tumor progression M2 phenotype. STATEMENT OF SIGNIFICANCE: Cells within tumors are highly heterogeneous, posing challenges with treatment and recurrence. While increased tissue stiffness can promote several hallmarks of cancer, its effects on tumor heterogeneity are unclear. We used single-cell RNA sequencing to investigate the differences in the transcriptional landscapes between stiff and compliant MMTV-PyMT mouse mammary tumors. We found similar compositions of cancer and stromal subpopulations in compliant and stiff tumors but differential intercellular communication and a significantly higher concentration of tumor-promoting, M2-like macrophages in the stiffer tumor microenvironments. Using a biomaterial-based platform, we found that cancer cells seeded on stiffer substrates recruited more macrophages, supporting our in vivo findings. Together, our results demonstrate a key role of matrix stiffness in affecting cell-cell communication and macrophage recruitment.

Effect of RGD functionalization and stiffness of gellan gum hydrogels on macrophage polarization and function

Macrophages, the primary effector cells in the immune response, respond rapidly to the physical or chemical properties of biomaterial implants. Balanced macrophage polarization, phagocytosis, and migration would be beneficial for implant success and tissue regeneration. Here, we investigated macrophage phenotypic changes, phagocytosis, and migration in response to RGD functionalized surfaces and changes in stiffness of gellan gum hydrogels. We also inhibited the RhoA pathway. The compressive moduli ranged from ~5 to 30 kPa. Cell population and cell spreading area of classically activated macrophages (M(LPS)) and alternatively activated macrophages (M(IL-4)) are promoted on RGD modified hydrogel. ROCK inhibitor induced the opposite effect on the cell spreading of both M(LPS) and M(IL-4) macrophages on RGD modified hydrogels. Macrophage polarization was found to be stiffness-driven and regulated by the RGD motif and blocked by the RhoA pathway. RGD functionalized hydrogel shifted M(IL-4) cells toward a more pro-inflammatory phenotype, while ROCK inhibition shifted M(LPS) cells to a more anti-inflammatory phenotype. Both M(LPS) and M(IL-4) cells on untreated hydrogels shifted to a more pro-inflammatory phenotype in the presence of aminated-PS particles. The RGD motif had a significant impact on cellular uptake, whereas cellular uptake was stiffness driven on untreated hydrogels. Cell migration of M(LPS) and M(IL-4) cells had ROCK-dependent migration. The stiffness of gellan gum hydrogels had no influence on macrophage migration rate. Collectively, our results showed that gellan gum hydrogels can be used to direct immune response, macrophage infiltration, and phagocytosis.

The Influence of Polysaccharides-Based Material on Macrophage Phenotypes

Macrophage polarization is a key factor in determining the success of implanted tissue engineering scaffolds. Polysaccharides (derived from plants, animals, and microorganisms) are known to modulate macrophage phenotypes by recognizing cell membrane receptors. Numerous studies have developed polysaccharide-based materials into functional biomaterial substrates for tissue regeneration and pharmaceutical application due to their immunostimulatory activities and anti-inflammatory response. They are used as hydrogel substrates, surface coatings, and drug delivery carriers. In addition to their innate immunological functions, the newly endowed physical and chemical properties, including substrate modulus, pore size/porosity, surface binding chemistry, and the mole ratio of polysaccharides in hybrid materials may regulate macrophage phenotypes more precisely. Growing evidence indicates that the sulfation pattern of glycosaminoglycans and proteoglycans expressed on polarized macrophages leads to the changes in protein binding, which may alter macrophage phenotype and influence the immune response. A comprehensive understanding of how different types of polysaccharide-based materials alter macrophage phenotypic changes can be beneficial to predict transplantation/implantation outcomes. This review focuses on recent advances in promoting wound healing and balancing macrophage phenotypes using polysaccharide-based substrates/coatings and new directions to address the limitations in the current understanding of macrophage responses to polysaccharides.