Objective  To investigate the immunoregulatory effects of silk fibroin peptide (SFP) in S180 tumor-bearing mice and provide a solid basis for clinical research into SFP.

  Methods  An S180 subcutaneous tumor mouse model was established, and S180 tumor-bearing mice were divided into four groups: the blank control group received saline, the positive control group received thymopentin (TP-5, 1 mg/kg), and the low- and high-dose SFP groups (SFP-L and SFP-H) received SFP at 300 mg/kg and 600 mg/kg, respectively, via intraperitoneal injection daily for 28 days. Antitumor effects were evaluated first, after which immune response progression was assessed by calculating immune organ (spleen and thymus) indices. Hematoxylin and Eosin staining was used to observe tumor-infiltrating lymphocytes. To evaluate specific immune responses in S180-bearing mice, ratios of CD4⁺ and CD8⁺ T-lymphocyte subsets and levels of secreted IL-2, IL-4, IL-5, IFN-γ, and TNF-α were quantified by flow cytometry. An MTT assay was used to assess the proliferative ability of T lymphocytes and calculate their stimulation index; sheep red blood cells were used to immunize mice, and then the serum antibodies were compared. Non-specific immunity was evaluated by carbon particle clearance test, to assess macrophage phagocytic activity, and MTT assay to assess NK cell-mediated cytotoxicity.

  Results  SFP significantly inhibited S180 cell growth in mice and increased immune organ indices (P < 0.05). SFP-H increased lymphocyte infiltration of tumors. SFP stimulated IL-2, IL-4, and TNF-α secretion and increased the lymphocyte stimulation index. SFP-H also increased the ratio of T lymphocyte subsets (P < 0.05), thereby enhancing specific immunity. SFP also increased the macrophage phagocytic coefficient α in our evaluation of non- specific immunity in S180-bearing mice (P < 0.05).

  Conclusions  SFP may protect immune organ development and enhance immune defense systems in tumor microenvironments. Immunity in S180-bearing mice was promoted by upregulating both specific and non-specific immune responses, potentially consolidating these antitumor immunoregulatory effects in a single therapeutic entity.