Nitric oxide mediation of active immunosuppression associated with graft-versus-host reaction

In the immunosuppression accompanying the lethal systemic graft-versus- host reaction (GVHR) directed against minor histocompatibility antigens in irradiated adult mice, we previously determined that non-T, non-B, L-leucine methyl ester (LME)-sensitive cells were implicated via two different mechanisms: one, which is interferon-γ (IFN-γ)-dependent and affects both T-cell proliferative responses and thymusin-dependent antibody production by CD5⁺ B cells; and a second, which is IFN-γ-independent and affects B-cell proliferative responses. Because IFN-γ, induces the production of nitric oxide (NO), a potent immunosuppressive molecule, we investigated the involvement of NO in the suppression mediated by the LME-sensitive cells. Inducible NO synthase (iNOS) mRNA, iNOS protein, and the stable end products of iNOS pathway, L-citrulline and nitrite, were detected early in GVHR in LME-sensitive spleen cells taken ex vivo and could be amplified in vitro by T and B mitogens. Inhibition of NO production with arginine analogs (aminoguanidine, N(G)-monomethyl-L-arginine [LMMA]), like anti-IFN-γ antibodies, reversed suppression of both T-cell responses to concanavalin A and CD5⁺ B-cell responses, but not of B-cell response to lipopolysaccharides (LPS). The GVHR-associated, IFN-γ-dependent immunosuppression of T-cell proliferation and of antibody synthesis by CD5⁺ B cells is the consequence of NO production by LME-sensitive cells. Immunohistochemical analyses indicate that these cells belong to the macrophage lineage.