Effect of liposome on gene expression of splenic macrophages

Myeloid-derived suppressor cells (MDSC) are induced in various situations. Generation of MDSC has been investigated extensively. Several cytokines are involved in regulating MDSC. Cellular microvesicles, which comprise cellular membrane (single lipid bilayer) are involved in the induction of MDSC. Especially, proteins or nucleic acids encapsulated in them or expressed on their surface are thought to play a role in the induction of MDSC. Nevertheless, the role of lipid components remains to be investigated. Our earlier study using WKAH rat revealed that MDSC-like cells can be induced in the rat spleen by intravenous injection of liposomal microparticles. They comprise a single lipid bilayer of approximately 250 nm diameter, with no cellular component. Therefore, they resemble cellular microvesicles, Cells capable of suppressing T cell proliferation were liposome-internalized cells. The cell surface phenotype was class II low/-, CD11b/c+. Nitric oxide was responsible for their suppression. In this study, we showed that they express B7-H3 molecule on their surface, removal of B7-H3 positive cells restored T cell proliferation. Western blot analysis confirmed that they are positive for iNOS and NFkB was activated 4 hr after liposomal injection. These results indicate that these cells in question belongs to a subset of MDSCs. Furthermore, we elucidated that vesicular form of lipid is important for the induction of T cell suppressive function. Based on these data, we concluded that liposomal microparticles can induce B7-H3 positive MDSC in vivo and the vesicular formation of lipid is essential for the induction of MDSC.髓系来源抑制性细胞（Myeloid-derived suppressor cells, MDSC）可在多种情境下被诱导产生。目前学界已对髓系来源抑制性细胞的生成机制开展了广泛研究，多种细胞因子参与调控其生理过程。由细胞膜（单层脂质双分子层）构成的细胞微泡，同样参与了髓系来源抑制性细胞的诱导过程；其中包裹于微泡内或表达于其表面的蛋白质与核酸，被认为在该诱导过程中发挥了关键作用。然而，脂质组分在髓系来源抑制性细胞诱导中的具体功能仍有待进一步探究。本团队此前利用WKAH大鼠开展的研究显示，经静脉注射脂质体微粒，可在大鼠脾脏中诱导出髓系来源抑制性细胞样细胞，这类微粒直径约250nm，由单层脂质双分子层构成，不含任何细胞组分，因此其结构与细胞微泡高度相似；能够抑制T细胞增殖的细胞为脂质体内化细胞，其表面表型为MHC II类分子低表达/不表达、CD11b/c阳性，且该类细胞的免疫抑制活性依赖于一氧化氮。本研究证实，该类细胞表面表达B7-H3分子；清除B7-H3阳性细胞后，T细胞增殖能力得以恢复。蛋白质免疫印迹（Western blot）分析进一步证实，该类细胞诱导型一氧化氮合酶（iNOS）表达阳性，且脂质体注射后4小时，核因子κB（NF-κB）即被激活。上述结果表明，本研究中所关注的这类细胞属于髓系来源抑制性细胞的一个亚群。此外，本研究明确了脂质囊泡形式对于诱导细胞产生T细胞抑制功能至关重要。基于上述实验结果，本研究得出结论：脂质体微粒可在体内诱导产生B7-H3阳性的髓系来源抑制性细胞，且脂质的囊泡化形成是诱导髓系来源抑制性细胞产生的必要条件。