Recent studies indicate that immune-associated aplastic anemia(AA)resembles such autoimmune diseases as insulin-dependent diabetes and chronic autoimmune thyroiditis that belong to organ-specific autoimmune diseases.Many independent investigation groups have successfully isolated rnthe pathopoiesis-associated T cell clone causing hematopoiesis failure with a CD4 phenotype from peripheral blood and bone marrow(BM)in AA patients.In the current study,BM CD4+ T cells were isolated from AA patients and healthy controls with immunomagnetic beads sorting,and proliferation capability,apoptosis features and the impacts of their secreted cytokines on hematopoiesis stem/progenitor cells were compared between them.By 3H-TdR method,CD4+ T cells in AA group presented more enhanced proliferative activity.The stimulation index in control group and AA group was 1.47±0.24,and 2.51±0.34 respectively(P<0.01).After BM CD4+ T cells were induced by high concentration of CD3 monoclonal antibody for 18h,evident apoptosis cells could be seen under the electron microscope in both control group and AA group.Flow cytometry revealed that apoptosis rates in the early and late stages of AA group were significantly higher than in control group(P<0.01).Early-stage apoptosis rate in control and AA groups was(6.85±1.48)% and(16.98±4.40)%,and late-stage apoptosis rate in control group and AA group was(2.654±1.57)% and(7.74±0.83)%,respectively(P<0.01).The CFU-GM count in AA group and control group was(74.50±9.50)/104 cells and(124.25±19.80)/104 cells respectively under an inverted microscope(P<0.01),and the expression levels of CyclinD3 mRNA and protein in cord blood CD34+ cells were both down-regulated induced by BM CD4+ T cell culture supatant in AA patients.These results indicate that BM CD4+ T cells of AA patients are likely in an abnormally proliferative,and activated state which can correlate intimately with AA hematopoiesis damage.BM CD4+ T cells in AA patients can secret some soluble cytokines that can inhibit proliferation of hematopoietic stem cells by suppressing the expression of Cyclin D3,resulting in hematopoiesis failure.