Our study has showed that CD138+ T cells apoptosis in MRL/MPJ mice was in Fas dependent way. But CD138 expression in CD3+ T cells of MRL/lpr mice significantly prevented the apoptosis of CD3+ T cells, contributed to CD3+ T cells and DN T cells subsets accumulation and simultaneoulsy promoted CD3+ T cells activation. Importantly CD138 expression could also increase FasL expression in DN T cells promoting the cytotoxicity of DN T cells. Moreover, decreased apoptotic and increased live numbers of CD138+ T cells led to the accumulation of CD138+ T cells in spleen of MRL/lpr mice. Our results demonstrated PI stimulation prevented CD138+ T cells accumulation and decreased CD138+ cells frequencies in DN and CD4+ T cells by inducing the specific apoptosis of CD138+ T cells. Our results demonstrated PI stimulation caused specific apoptosis in CD138+ T cells via increasing CD69 expression in them. Our results also suggested CD138 expression in CD3+ T cells was probably caused by the failure of activation in autoreactive T cells before self-antigens exposure to immune system.
Fas (CD95) is the member of the tumor necrosis factor receptor family and interacts with Fas ligand (FasL) after T cell receptor (TCR) activation to induce apoptosis 16. Fas deficiency leads to DN T cells accumulation in MRL/lpr lupus mice resulting in lymphadenectasis and splenomegaly 23,24. In addition to DN T cells, our results also showed apoptosis of CD138+ T cells was in a Fas-dependent way and indicated Fas deficiency also led to CD138+ T cells accumulation in MRL/lpr mice as the age increased and the lupus developed 12. Our results showed CD138+ T cells had a high level of CD69 expression but a low level of CD25 expression. Importantly, the majority of CD138+ T cells were CD4 and CD8 double negative. CD138 expression significantly increased FasL expression in CD3+ T cells and their subsets including DN T cells. However, DN T cells in MRL/lpr mice are strongly cytotoxic, overexpressing FasL, which results in autoimmune injuries of multiple tissues that express small amounts of Fas receptor 16,25. That indicated CD138 expression increased DN T cells cytotoxicity which could promote the lupus development and tissue injuries in MRL/lpr mice.
Our results showed CD3+ T cells accumulated but had a defective proliferation in Fas-deficiency MRL/lpr mice. Moreover the accumulated CD138+ T cells had a significant decrease in apoptotic number of cells and simultaneously had a higher number of live cells compared with CD138- T cells demonstrating CD138+ T cells have a defective apoptosis in MRL/lpr mice. Importantly, previous study has shown that CD138+ T cells had a lower level of proliferation compared with CD138- T cells subsets 12,14. According to these results, we conclude that CD138+ T cells accumulation was caused by Fas deficiency leading to their defective apoptosis but not hyper proliferation. In addition, our results also demonstrated CD138 expression greatly contributes to the accumulation of T cells and DN T cells in MRL/lpr mice by significantly decreasing the apoptotic number of T cells and DN T cells.
Production of autoantibodies has a detrimental effect on multiple organs and plays a key role in the progress of SLE 26. We know immature T cells will experience positive selection and negative selection to be mature single positive T cells that cannot recognize self-antigens 27,28. Autoreactive T cells will be deleted by Fas-mediated apoptosis during negative selection in the thymus 29. Fas deficiency give the chance of autoreactive T cells to pass through the negative selection 30,31. Autoreactive B cells may also avoid the apoptosis in negative selection induced by Fas-dependent apoptosis 32. Our results has shown Fas deficiency results in the accumulation of CD138+ T cells in MRL/lpr mice. In addition, CD138+ T cells including CD4+CD138+ T cells commonly express B220 which has been demonstrated to express on those autoreactive T cells such as nonselected CD8+ T cells and DN T cells 29,33. Importantly, CD138+ T cells have been demonstrated to be key in the progression of anti-dsDNA antibody secretion with a CD4 receptor dependent way 12. CD138+ T cells have been also reported to promote the tissue injuries in condition that self-antigens are exposed to the immune system 7,12. These results demonstrate accumulated CD138+ T cells had large numbers of the auto-reactive T cells that had avoided Fas-dependent apoptosis in negative selection 12,14. CD4+CD138+ T cells in MRL/lpr mice had a significant decrease in CD4 expression compared with CD4+CD138- T cells. It had been reported that CD4+ T cells could convert into DN T cells 34,35. CD4+CD138+ T cells in MRL/lpr mice may be the precursor of DN T cells which were originated from CD4+ T cells that have the potential of conversion into DN T cells.
CD138 expression in T cells plays a key role in the progression of lupus in MRL/lpr mice. Our results showed CD138 expression could contribute to the accumulation of CD3+ T cells by significantly preventing their apoptosis. Morever, CD138+ T cells showed to be activated more easily compared with CD138- T cells indicating CD138 expression could promote the activation of T cells in MRL/lpr mice. That suggested CD138 expression could contribute to autoreactive T cells accumulation and simultaneously promote autoreactive T cells to be activated by autoreactive B cells, by which abnormal plasma cells formation was increased 12. However, the mechanisms of CD138 expession in these abnormal T cells were still undeciphered. Our results showed CD138- T cells in MRL/MPJ mice were more easily activated compared with CD138- T cells in MRL/lpr mice. PI stimulation also failed to significantly activate CD3+CD138- T cells in splenocytes of MRL/lpr mice. These results indicated CD138- T cell had an evidently defective activation in MRL/lpr mice. Our results simultaneously showed PI stimulation significantly decreased CD138 expression in CD138+ T cells. PI stimulation was able to significantly reduce frequency of CD138+ cell in CD3+ T cell and its cell subsets with increased activation level of CD3+ T cell and CD138+ T cells. Previous research also has showed TCR activation negatively regulated CD138 expression frequency in DN T cells 14. These results demonstrated CD138 expression in CD3+ T cells could be prevented by the stimulation to activate CD3+ T cells.
Isolated splenocytes from MRL/lpr mice were cultured in vitro in the medium without cellular stimulation. We observed CD138+ T cells frequency increased in the 48 h accompanied with decreased activation level of CD3+ T cells and CD138+ T cells, and then decreased gradually after 48 hours accompanied with increased activation level of CD3+ T cells and CD138+ T cells. That indicated CD138+ T cells frequency was inversely correlated with the activation level of CD3+ T cells and CD138 + T cells. Previous research has demonstrated CD138+ T cells had large numbers of autoreactive T cells that promote autoantibody production in the presence of dsDNA, and enhanced disease progression in SLE by rapidly activating autoreactive B cells when self-antigens are exposed to the immune system 12. These evidences suggest that CD138 expression in CD3+ T cells was probably due to failure of activation in these abnormal T cells in absence of self-antigens exposure.
In our present study, PI stimulation reduced CD138+ T cells frequency in splenocytes of MRL/lpr mice. Moreover, our results showed PI stimulation led to specific apoptosis of CD138+ T cells. PI stimulation simultaneously promoted the activation of CD138+ T cells with increased CD69 cells frequency and FasL expression in them. What is important, CD69+ cells in CD138+ T cells had a significant increase in apoptosis level compared with CD69- cells in CD138+ T cells. According to our results, we confirmed that the increased apoptosis of CD138+ T cells and subsequent decreased accumulation of CD138+ T cells were induced by PI stimulation via increasing CD69 expression in CD138+ T cells. Our research uncovered the role of CD138 expression in CD3+ T cells, and why to activate CD3+ T cells significantly increased the apoptosis of CD138+ T cells and prevented the accumulation of CD138+ T cells in MRL/lpr mice. We also provided the novel insight into the mechansims preventing the autoreactive T cells accumulation in SLE.