Abstract： Objective To investigate the immunological effects of thymoglobulin (RATG) on human CD4+and CD8+cells for costimulatory molecule gene expression and the production ofimmune-regulatory cytokines. Methods CD4+and CI8+T cells were isolated and purified fromnormal human peripheral blood mononuclear cells (PBMC) followed by incubation with RATG at37℃. Cells and culture supematants were collected at 24, 48, and 72 h after incubation, and analyzedby real-time quantitative polymerase chain reaction (RT-PCR) for CTLA-4, CD154, forkhead box P3(Foxp3), OX40, IFN-γ, IL-2, IL-10 and CD25 gene expression, and multiplex cytokine detectionassay for IFN-y, IL-2, IL-10, and IL-4 production. Untreated and rabbit isotype Ig-treated cells wereused as negative controls. Results RT-PCR demonstrated that RATG pre-treated CI+and CD8+cells upregulated the expression of CTLA-4, OX40, Foxp3, CD25, IFN-γ, IL-10 and IL-2 genes, anda dramatic increase of supernatant IFN-γ, IL-10, IL-2 and IL-4 was revealed 24 h after treatment asdetermined by multiplex cytokine detection assay when compared with negative controls. Theupre gulation of CTLA-4, Foxp3, OX40, IL-10 and CD25 was reduced, and a down-regulation ofCD154 and IL-2 gene expression was revealed 48 h after treatment. Cells, treated with RATG for 72h, demonstrated up-regulation of CTLA-4, Foxp3, OX40, IFN-y and CD25 gene expression, and theexpression of IL-2 and IL-10 genes was down-regulated. Additionally, supernatant IFN-γ, IL-2,IL-10 and IL-4 levels were decreased. Conclusion RATG stimulates CI4/CD8 T cells to up-regulatecostimulatory molecules and release immune regulation associated cytokines IF'N-γ, IL-2, IL-10in vitro. These results suggest that the unique effect of RATG on CD4+CD8+T cells may be animportant mechanism for its action in inducing immunoregulation, immunosuppression and transplanttolerance.