Objective:To study the mechanisms of immune suppression mediated by Gr-1+CDllb+ myeloid derived suppressor cells (Gr-1~+CDllb~+MDSC)from tumor-bearing mice.Methods:Gr-1~+CDllb~+MDSC recruited into spleen and bone marrow of tumor-bearing mice were purified by Percoll,and suppression mediated by MDSC on T cell proliferation from spleen of naive mice was detected by flow cytometry with CSFE and FTTC-anti CD3 staining,and NO,ROS,IL-10 and TGF-β in the supematant of MDSC were detected by Griess and ELISA.Results:There were much more Gr-1~+CDllb~+MDSCs in spleen and bone marrow from tumor-bearing mouse than those of naive mouse,and suppression on T cell proliferation mediated by MDSC from tumor beating mouse was significantly increased,and there were much more NO,ROS,IL-10 and TGF-βin the supematant of these MDSC than that from naive mouse.Conclusion:MDSC from tumor-bearing mice secreted hish level of NO,ROS,IL-10 and TGF-βto induce immune suppression,and inhibite the proliferation of T cells.

Combination of passive targeting with active targeting is a promising approach to improve the therapeutic efficacy of nanotherapy. However, most reported polymeric systems have sizes above 100 nm, which limits effective extravasation into tumors that are poorly vascularized and have dense stroma. This will, in turn, limit the overall effectiveness of the subsequent uptake by tumor cells via active targeting. In this study, we combined the passive targeting via ultra-small-sized gemcitabine (GEM)-based nanoparticles (NPs) with the active targeting provided by folic acid (FA) conjugation for enhanced dual targeted delivery to tumor cells and tumor-associated macrophages (TAMs). We developed an FA-modified prodrug carrier based on GEM (PGEM) to load doxorubicin (DOX), for co-delivery of GEM and DOX to tumors. The co-delivery system showed small particle size of ∼10 nm in diameter. The ligand-free and FA-targeted micelles showed comparable drug loading efficiency and a sustained DOX release profile. The FA-conjugated micelles effectively increased DOX uptake in cultured KB cancer cells that express a high level of folate receptor (FR), but no obvious increase was observed in 4T1.2 breast cancer cells that have a low-level expression of FR. Interestingly, in vivo, systemic delivery of FA-PGEM/DOX led to enhanced accumulation of the NPs in tumor and drastic reduction of tumor growth in a murine 4T1.2 breast cancer model. Mechanistic study showed that 4T1.2 tumor grown in mice expressed a significantly higher level of FOLR2, which was selectively expressed on TAMs. Thus, targeting of TAM may also contribute to the improved in vivo targeted delivery and therapeutic efficacy.