TGFβ1 inhibits Ca²⁺-calcineurin-mediated activation in thymocytes

TGFβ1 is a polypeptide growth modulatory and differentiation factor involved in many biological processes including immune homeostasis and self-tolerance. Tgfb1 knockout mice die around weaning age due to severe inflammation in most major organ systems, but the mechanism underlying this disease is not understood. In this study we demonstrate that Tgfb1^-/- CD4⁺CD8⁺ and CD4⁺CD8^- thymocytes are hyperresponsive to receptor-mediated and receptor-independent mitogenic stimulation. A sub-optimal concentration of ionomycin in the presence of PMA fully activates Tgfb1^-/- thymocytes, whereas the inhibitors of Ca²⁺ influx and calcineurin, EGTA and FK506, eliminate the hyperresponsiveness. Hence, the hypersensitivity of Tgfb1^-/- thymocytes is due to a lowered threshold for Ca²⁺-dependent activation. Further, we demonstrate that the hypersensitivity of thymocytes results from the absence of TGFβ^1-/- and not from the inflammatory environment because the thymocytes are hyperresponsive in preinflammatory-stage Tgfb1^-/- mice. Our results suggest for the first time that TGFβ1 functions to inhibit aberrant T cell expansion by maintaining intracellular calcium concentration levels low enough to prevent a mitogenic response by Ca²⁺-independent stimulatory pathways alone. Consequently, TGFβ1 prevents autoimmune disease through a Ca²⁺ regulatory pathway that maintains the activation threshold above that inducible by self-MHC-TCR interactions.