Absence of functional inducible NO synthase enhances the efficacy of tolerance induced by high dose antigen feeding.

Recent studies have suggested that IL-12 and IFN-gamma may impair the ability of fed Ag to induce systemic tolerance. Because both of these cytokines can function to directly or indirectly induce inducible NO synthase (iNOS) expression, we have investigated whether the functional expression of iNOS regulates oral tolerance. C57BL/6J wild-type or C57BL/6J NOS2(-/-) mice were gavaged with a single dose of 20 mg of keyhole limpet hemocyanin (KLH), followed by s.c. immunization with KLH/CFA. In the absence of feeding Ag, several parameters of the immune response were more robust in C57BL/6J NOS2(-/-) mice following KLH/CFA immunization, including the magnitude of the delayed-type hypersensitivity response, the proliferative response, and the production of IFN-gamma and IL-2 by Ag-activated draining lymph node cells. These heightened responses in the C57BL/6J NOS2(-/-) mice are still effectively inhibited by feeding KLH. Feeding KLH to the C57BL/6J NOS2(-/-) mice elicited heightened TGF-ss1 production by Ag-activated lymphocytes, as well as augmented total IgG, IgG1, and IgG2a responses to KLH/CFA compared with that seen in Ag-fed wild-type mice. Feeding Ag to the NOS2(-/-) mice suppressed proliferative responses and IFN-gamma production, while increasing IL-4 production and the IgG1/IgG2a ratio even following a booster immunization of KLH/CFA. Administrating L-N:(6)-(1-iminoethyl)-lysine. 2HCl to wild-type mice during the period of Ag feeding reproduced the high TGF-ss1 production seen in Ag-activated lymphocytes from Ag-fed NOS2(-/-) mice. Feeding KLH is followed by transient up-regulation of NOS2 mRNA expression in the Peyer's patches of wild-type mice. Selective inhibition of NOS2 may be a simple way to augment tolerogenic mucosal immune responses.