Targeting Delivery Of MicroRNA Let-7b For Cancer Immunotherapy

Malignant tumor is one of diseases which seriously threatened to the human health, harnessing the patient’s own immune system is one of research hotspots in oncotherapy. In the oncogenesis and development of tumor, there are a large number of immune cells infiltrating in tumor tissues, both tumor associated macrophages (TAMs) and tumor infiltrating dendritic cells (TIDCs) are important components in the tumor microenvironment that play an central role in promoting cancer progression. Macrophages could be classified into a classically (M1) activated phenotype with anti-tumoral activities or an alternatively (M2) activated phenotype with pro-tumoral activities. In tumor microenvironment, TAMs exhibit predominantly M2-like phenotype with pro-tumoral and immunosuppressive activities. Meanwhile, TIDCs are the immature phenotype, which have no the ability of presenting antigen and triggering immune response. TAMs/TIDCs could secrete a large number of growth factors and immunosuppressive cytokine which involved in maintaining the immunosuppression of tumor microenvironment. Hence, targeting both cells (re-polarizing M2-like macrophages to M1-like macrophages and promoting TIDCs from immature phenotype to mature phenotype) may become a new strategy to recover their anti-tumor activities.In this study, we constructed a nucleic acid delivery system (named as Let-7b&vector) that consists of Cationic Bletilla Striata polysaccharide (cBSP) which has a rich source of mannose, His-modified alginate (PHA, a pH sensitive material) and let-7b, which could effectively release the let-7b&cBSP in the low pH tumor microenvironment. Due to the surface of TAMs or TIDCs have a rich source of mannose receptors, cBSP may transfer let-7b into TAMs and TIDCs possibly through mannose-mediated endocytosis. In previous report, Let-7b could act as a Toll-like receptors-7(TLR-7) agonist and interleukin (IL)-10 inhibitor to stimulate TAMs/TIDCs, in vitro, Let-7b Inhibited the production of IL-10 and up-regulated the expression of IL-12. Real-time Quantitative PCR (qRT-PCR) assay observed the increased M1 marker (iNOS) and decreased M2 marker (Arg-1 and MR). Meanwhile, let-7b also up-regulated markers(CD40, CD80, CD86 and MHC-Ⅱ) of TIDCs, prompted TIDCs mature, and stimulated T cells to secrete INF-γ. In the 4T1 tumor-bearing mice model, let-7b&vector treatment could effectively inhibit the growth of tumors and obviously prolong the survival time of mice. Then we verified the distribution of let-7b in cells, the results showed us, let-7b mainly gathered in TAMs/TIDCs. We also tested the status of TAMs/TIDCs after let-7b & vector treatment, similar to phenomenon obtained in vitro, let-7b&vector reversed phenotype of TAMs, prompted TIDCs to upregulate the expression of mature marker, and inhibited the production of IL-10 and raised the expression of IL-12. After therapy by Let-7b&vector, we also examined the cytokine of tumor microenvironment, we found that the expression levels of IL-10, vascular endothelial growth factor (VEGF) and Matrix metallopeptidase 9(MMP9) were significantly decreased; whereas it increased the expression of IL-12. In conclusion, let-7b delivered by this system efficiently reversed immunosuppressive status of tumor microenvironment and achieved good therapeutic effect of tumor-bearing mice. Designed based upon TAMs/TIDCs-targeting delivery and the dual biological functions of let-7b may provide a new approach for cancer immunotherapy.