Exopolysaccharide from Rhizopus nigricans Modulates Antitumor Immune Responses by Promoting Tumor-Associated Macrophage Polarization toward the M1 Phenotype in C57BL/6J Mice

Objective: Exopolysaccharide (EPS1-1) exhibits immunomodulatory and antitumor activities; however, its mechanism of action remains unknown. We determined the antitumor activity and whether EPS1-1 could enhance the shift of tumor-associated macrophages (TAMs) toward the M1 phenotype. Methods: The viability of MC-38 was assessed by the CCK-8 assay. The phagocytic activity of primary peritoneal macrophages (PEMs) was assayed by immunofluorescence microscopy and the levels of TNF-α, IL-6, and NO were tested by ELISA kits, and NO assay kit. M1 polarization was analyzed by flow cytometry and Immunofluorescence staining. Results and Discussion: The viability of MC-38 cells did not exhibit a significant difference (p > 0.05). EPS1-1 significantly upregulated IL-6, TNF-α, and NO production as well as phagocytic activity. Furthermore, it significantly inhibited tumor growth (85.32%), increased the spleen and thymus indices, and elevated the proportion of M1 cells in vitro and in vivo. Conclusions: It indicated that EPS1-1 had no direct inhibitory effect on MC-38 cell viability, exhibited remarkable anti-tumor activity against MC-38 transplanted tumors, reduced the volume of transplanted tumors, and significantly activated TAMs polarization to an immunostimulatory M1 phenotype to modulate antitumor immune responses in C57BL/6J mice.