TGF-β1 regulates lymphocyte homeostasis by preventing activation and subsequent apoptosis of peripheral lymphocytes

TGF-β1 plays an important role in the maintenance of immune homeostasis and self-tolerance. To determine the mechanism by which TGF-β1 prevents autoimmunity we have analyzed T cell activation in splenic lymphocytes from TGF-β1 -deficient mice. Here we demonstrate that unlike wild-type splenic lymphocytes, those from Tgfb1^-/- mice are hyporesponsive to receptor-mediated mitogenic stimulation, as evidenced by diminished proliferation and reduced IL-2 production. However, they have elevated levels of IFN-γ and eventually undergo apoptosis. Receptor-independent stimulation of Tgfb1^-/- T cells by PMA plus ionomycin induces IL-2 production and mitogenic response, and it rescues them from anergy. Tgfb1^-/- T cells display decreased CD3 expression; increased expression of the activation markers LFA-1, CD69, and CD122; and increased cell size, all of which indicate prior activation. Consistently, mutant CD4⁺ T cells have elevated intracellular Ca²⁺ levels. However, upon subsequent stimulation in vitro, increases in Ca²⁺ levels are less than those in wild-type cells. This is also consistent with the anergic phenotype. Together, these results demonstrate that the ex vivo proliferative hyporesponsiveness of Tgfb1^-/- splenic lymphocytes is due to prior in vivo activation of T cells resulting from deregulated intracellular Ca²⁺ levels.