Induction of thymocyte apoptosis by systemic administration of concanavalin A in mice:: role of TNF-α, IFN-γ and glucocorticoids

Administration of concanavalin A (Con A) is a well-established model of acute immunemediated hepatitis. Here, we demonstrate that intravenous injection of Con A in mice induces profound thymic atrophy. Compared to liver damage, the kinetics of Con A- induced thymic atrophy is slower and more prolonged; the nadir in thymocyte number is reached 4 days after Con A injection, whereas peak transaminase levels are observed at 12-24 h. Marked alterations in the ratio of CD4(+) and CD8(+) cells in the thymus and spleen and significantly increased rates of thymocyte and splenocyte apoptosis are observed. Neutralization of the cytokines TNF-alpha or IFN-gamma, which protects mice from Con A- induced hepatitis, prevents thymic atrophy as well as alterations in CD4(+) and CD8(+) cell numbers and apoptosis rates. However, neither TNF-alpha nor IFN-gamma are detectable in thymocyte lysates after Con A injection, whereas both cytokines are present in liver, spleen and serum. Administration of the glucocorticoid receptor antagonist mifepristone does not prevent thymic atrophy, thus ruling out a possible contribution of endogenous glucocorticoids. Con A-induced thymic atrophy is accompanied by down-regulation of Bcl-2 expression in the thymus, which is prevented by neutralization of TNF-alpha or IFN-gamma. These data demonstrate that the thymus is a critical target organ of Con A-induced inflammation; the effects of Con A on the thymus are mediated by extrathymic production of TNF-alpha and IFN-gamma, but not by glucocorticoids.