hsBAFF regulates proliferation and response in cultured CD4+ T lymphocytes by upregulation of intracellular free Ca2+ homeostasis

B cell activating factor belonging to the TNF family (BAFF, also called BLyS, TALL-1, THANK, or zTNF4) is an important survival factor for B cells, and is able to regulate T-cell activation. Recently, we have demonstrated that treatment of mice with human soluble BAFF (hsBAFF) causes a significant increase of percentages of splenic CD4+ T lymphocytes dose-dependently, but the CD8+ T lymphocyte percentages maintained unchanged. Here, we show that hsBAFF significantly enhanced CD4+ T lymphocyte response of cultured mouse splenic cells, and hsBAFF induced the proliferation and IL-2/IFN-γ secretion of purified CD4+ T lymphocytes suboptimally stimulated through anti-CD3. Of importance, we observed that IL-2 or IFN-γ cytokine has additive effect on the proliferation and activity of hsBAFF-stimulated CD4+ T lymphocytes. Using Flow cytometry with fluorescent probe, Fluo-3/AM, we found that hsBAFF elicited [Ca2+]i elevation contributing to CD4+ T cell proliferation. This is evidenced by our finding that pretreatment with BAPTA/AM, an intracellular Ca2+ chelator, significantly attenuated the proliferation of hsBAFF-stimulated CD4+ T lymphocytes. Subsequently, we revealed that hsBAFF-stimulated CD4+ T cell proliferation was markedly suppressed after pretreatment with EGTA, an extracellular Ca2+ chelator, or with 2-APB, an inhibitor of Ca2+ influx through CRAC channels, respectively, suggesting that extracellular Ca2+ influx due to hsBAFF is closely associated with [Ca2+]i elevation contributing to CD4+ T cell proliferation. In addition, we noticed that hsBAFF-treated cells conferred partial resistance to decrease of cellular viability induced by thapsigargin (Tg), an endoplasmic reticulum (ER) Ca2+-ATPase inhibitor. Taken together, our data indicate that hsBAFF may promote CD4+ T cell proliferation and response by upregulation of [Ca2+]i homeostasis.