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Altered T-cell homeostasis, such as expansion of CD8⁺ T cells to the secondary lymphatic compartments, has been suggested as a mechanism of HIV/simian immunodeficiency virus (SIV)-pathogenesis. However, the role of immune activation of CD8⁺ T cells in the CD4/CD8 turnover and viral replication in these tissues is not completely understood. In this study, we compared the expression of immune activation markers (CD69 and HLA-DR) on CD8⁺ T cells in the peripheral blood and lymph nodes (LNs) of SIV-infected/uninfected Chinese rhesus macaques. SIV-infected macaques had significantly higher percentages of CD8⁺CD69⁺ and CD8⁺HLA-DR⁺ T cells in all these anatomical compartments than uninfected macaques. LNs that located close to the gastrointestinal (GI) tract (colon, mesenteric, and iliac LNs) of SIV-infected macaques had profoundly lower numbers of CD4⁺ T cells, but no significant difference in expression of activation marker (CD8⁺CD69⁺ and CD8⁺HLA-DR⁺) as compared with the peripheral lymphatic tissues (axillary and inguinal LNs). The CD4/CD8 ratios were negatively correlated with the activation of CD8⁺ T cells in the overall LNs, with further associations with CD8⁺HLA-DR⁺ in GI LNs while CD8⁺CD69⁺ in peripheral LNs. These observations demonstrate that the increase of CD8⁺ T cell activation is a contributing factor for the decline of CD4/CD8 ratios in GI system.