Salvia mitiorrhiza Bunge aqueous extract attenuates infiltration of tumor-associated macrophages and potentiates anti-PD-L1 immunotherapy in colorectal cancer through modulating Cox2/PGE2 cascade.

Ethnopharmacological Relevance: Based on the notion of traditional Chinese medicine, the theory of invigorating the circulation of blood is a prominent treatment for cancer in clinic. Therefore, Salvia miltiorrhiza Bunge, as a representative of Chinese medicine of invigorating the circulation of blood, has been proved to be an effective medicinal herb for treating cancer.

Aim Of The Study: To clarify the anti-cancer effect of Salvia miltiorrhiza Bunge aqueous extract (SMAE) on colorectal cancer (CRC) and investigate whether the therapeutic effect of SMAE was mediated by attenuating the infiltration of tumor-associated macrophages (TAMs) into the tumor microenvironment (TME).

Materials And Methods: High-performance liquid chromatography (HPLC) was used for determined the main compounds of SMAE. MC38 cells were subcutaneously injected into the mice to establish the mouse model of CRC. Tumor growth curve was detected by tumor volume measurement. The model group received distilled water irrigation once a day. SMAE-treated group received 5 g/kg or 10 g/kg SMAE once a day. Anti-PD-L1 treated group received 5 mg/kg anti-PD-L1 once every three days. Protein expression of Cox2 and PD-L1 was determined by Western blot assay. The secretion levels of PGE2, IL-1β, IL-6, MCP-1, and GM-CSF were evaluated through ELISA. The mRNA expression of CSF1, CCL2, CXCL1, CXCL2, and CXCL3 was measured by using RT-qPCR. Staining of Ki67, TUNEL and Caspase3 was used to investigate cell proliferation and apoptosis. Immunohistochemical staining was used to determine CD8 T cell distribution. H&E staining was used to confirm histopathological changes. The expressions of F4/80 and CD68 were measured by flow cytometry to identify macrophages in tumors and lymph nodes. The number of CD8 T cells and the expression of PD-1, IFN-γ, and Granzyme B (GZMB) were determined by flow cytometry.

Results: SMAE significantly retarded the growth of MC38 mouse colorectal cancer. SMAE strikingly inhibited the expression of Cox2 and impaired the secretion of PGE2 in tumors, contributing to the attenuated intra-tumoral infiltration of TAMs via Cox2/PGE2 cascade. Meanwhile, SMAE augmented anti-tumor immunity by the elevated proportion of IFN-γ CD8 T cells and GZMB CD8 T cells, which decreased the tumor load. Furthermore, the combination of SMAE and anti-PD-L1 showed a higher therapeutic efficacy than either monotherapy in controlling tumor growth in MC38 xenograft model.

Conclusions: SMAE attenuated the infiltration of TAMs into tumors and synergized with anti-PD-L1 to treat CRC via modulating Cox2/PGE2 cascade.