Our previous work showed that epicutaneous (EC) immunization of mice with different protein Ags applied on the skin in the form of a patch induces a state of subsequent Ag-nonspecific unresponsiveness due to suppressor CD4+8+ T cells (Ts) that inhibit Th1-mediated contact sensitivity (CS) reactions via released TGF-beta. In the present work we show that EC immunization with Ag together with the TLR4 ligand LPS induced cells that could prevent suppression by the Ag-nonspecific Ts. These up-regulatory cells, called contrasuppressor T cells (Tcs), belong to a population of Ag-specific TCRalphabeta CD4+ lymphocytes and are different from Th1 CD4+ cells that mediate the CS reaction. Experiments using knockout mice showed that EC induced contrasuppression is MyD88, INF-gamma, and IL-12 dependent, whereas IL-6 is not involved in this phenomenon. Additional experiments with anti-IFN-gamma mAb showed that IFN-gamma is required for induction of Tcs cells but does not play a crucial role in the effector phase of contrasuppression. Additionally, treatment of CS effector cells with rIL-12 makes them resistant to EC induced suppression without affecting Ts cells, whereas IL-12 neutralization in vitro abrogates contrasuppression. These data show that IL-12 is indeed involved in the effector phase of EC induced contrasuppression and that this cytokine does not act directly on Ts cells. The mechanism of action of Tcs protects Th1 effector cells mediating CS from the nonspecific Ts, leaving suppression to other Ags intact. Ts and Tcs cells do not influence each other and can be induced simultaneously in the same animal.