Objectives Hyperhomocysteinemia (HHcy) accelerates atherosclerosis in apoE-/-mice by promoting T-cell activation, proliferation and inflammatory cytokine secretion, but the mechanism is still unknown.Cytotoxic T lymphocyte antigen 4 (CTLA4) delivers inhibitory effects by competition with CD28 costimulatory signal to avoid T-cell overactivation.In this study, we investigated the effects of CTLA4-IgG on homocysteine (Hcy)-induced T-cell activation and potential signal pathways that have an impact on atherosclerosis.Materials and Methods Female C57BL/6 mice, 6 weeks old, were fed normal chow supplemented with or without Hcy (1.8 g/L)in drinking water for 2 weeks.T cells were isolated from splenocytes of mice.CTLA4 surface expression and endocytosis were explored by use of flow cytometric analysis (FACS).Using confocol microscope, the distribution of endocytosed CTLA4 in subcellular organelle was investigated.Furthermore, block experiment was used to show the potential signaling pathway which mediated the Hcy-induced CTLA4 endocytosis.In co-culture system, the role of CTLA4 in the interaction between T cell and macrophage was also investigated.Results CD28 signal was significantly amplified by Hcy treatment in splenic T cells and HHcy-accelerated plaques in apoE-/-mice.As a major competitor of CD28, 100 lμg/mouse/week of CTLA4-IgG (abatacept) pretreatment in apoE-/-mice could reverse HHcy-accelerated atherosclerosis development.The membrane level of CTLA4 was decreased and its endocytosis level notably increased by HHcy.Cellular organelle tracker showed that much more endocytosed CTLA4 molecules were located in lysosome and endosome with Hcy treatment.Moreover, Hcy-increased CTLA4 endocytosis and secretion of inflammatory cytokines in T cells were blocked by CD28 costimulatory signal blocker (0.25 μg/ml) and PI3K inhibitor LY294002 (50 μM), but not by mTOR inhibitor rapamycin (5 μM).The data from T cell-macrophage coculture system showed that blocking CD28 signal pathway in T cell decreased Hcypromoted macrophage migration and cytokine secretion significantly.Conclusions These results provided a novel mechanism that promoted CD28-dependent T-cell costimulation is involved in HHcy-accelerated atherosclerosis,implicating that this study may extend the pharmacologic application of CTLA4-IgG for atherosclerosis.