Polysaccharides from Sea Cucumber ( Stichopus japonicus ) Synergize with Anti-PD1 Immunotherapy to Reduce MC-38 Tumor Burden in Mice Through Shaping the Gut Microbiome.

Immune checkpoint inhibitors (ICIs) have revolutionized cancer treatment and significantly improved outcomes for patients with certain malignancies. However, immunotherapy with ICIs is only effective in a subset of patients and the gut microbiota have been identified as an important factor associated with response to ICI therapy. Polysaccharides from sea cucumber ( Stichopus japonicus ) (SCP) have been shown to modulate the gut microbiota and exhibit beneficial health functions, but whether SCP could synergize with anti-PD1 immunotherapy remains unexplored. In this study, mice with ICI-sensitive MC38 tumors were treated with anti-PD1 antibody after supplementation with or without SCP to examine the potential impact of SCP on the efficacy of immunotherapy. SCP strongly amplified the anti-tumor activity of anti-PD1 in MC38 tumor-bearing mice. Flow cytometry and immunohistological staining demonstrated that SCP treatment increased cytotoxic CD8 ⁺ T lymphocytes while decreasing regulatory Foxp3 ⁺ CD4 ⁺ T lymphocytes. Gut microbiota and metabolomic analysis revealed that SCP modulated the microbiota and increased the abundance of certain metabolites such as indole-3-carboxylic acid. Furthermore, fecal microbiota transplantation experiments justified that the synergistic effect of SCP with anti-PD1 was partially mediated through the gut microbiota. Mice receiving microbiota from SCP-treated mice showed a boosted response to anti-PD1, along with enhanced anti-tumor immunity. These findings indicate that SCP could be utilized as a dietary strategy combined with anti-PD1 therapy to achieve improved outcomes in patients.