Autoreactive T cells, specifically CD138(+) (syndecan-1) T cells produced in Fas-deficient systemic lupus erythematosus (SLE) mouse models, were shown to significantly promote the generation of autoantibodies. In the present study, Murphy Roths Large lymphoproliferative (MRL/lpr) lupus mice were used to investigate the role of CD138 protein expression in T cells in the progression of SLE. Measurement of flow cytometry, immunofluorescence and Luminex were performed to determine the effect of CD138 on T cells in MRL/lpr mice. The results demonstrate that CD138(+ ) cells induce apoptosis via a Fas-dependent pathway. CD138 protein expression in T cells of MRL/lpr mice significantly reduced T cell apoptosis and contributed to the accumulation of T cells and double negative (DN) T cells, whilst simultaneously promoting T cell activation in Fas-deficient lupus mice. CD138 protein expression in DN T cells also significantly increased the protein expression of Fas ligand to enhance the cytotoxicity of DN T cells. Furthermore, phorbol 12-myristate 13-acetate and ionomycin (PI) stimulation reduced CD138 protein expression in CD3(+) T cells and prevented CD138+ T cell accumulation by inducing specific apoptosis. PI stimulation also activated T cells in MRL/lpr mice to increase CD69 protein expression. CD69 protein expression in CD138(+) T cells significantly increased the frequency of apoptotic CD138(+ )T cells. In addition, results from the present study demonstrated that CD138- T cells of MRL/lpr lupus mice had an activation defect. CD138 protein expression in T cells significantly reversed the defective activation and activating T cells could significantly reduce CD138 protein expression in CD3(+) T cells of MRL/lpr mice. This suggests that CD138 protein expression in CD3(+)CD138(-) T cells of MRL/lpr mice may be a consequence of the impaired activation in autoreactive T cells prior to exposure to self-antigens by the immune system. CD138 expression in autoreactive T cells has a central role in promoting the progression and development of autoimmune response in MRL/lpr mice.