Rat Splenocytes Inhibit Antigen-Specific Lymphocyte Proliferation through a Reactive Nitrogen Intermediate (RNI)-Dependent Mechanism and Exhibit Increased RNI Production in Response to IFN-γ

Rat splenocytes inhibited antigen-specific proliferation of primed lymph node cells in vitro. This inhibition resided in the plastic-adherent splenocyte fraction and was radioresistant, suggesting that the effect was due to macrophages. While this suppression was more evident if spleen cells were derived from immunized rats, spleen cells from normal rats were just as suppressive when added to cocultures at higher numbers. Proliferative responses were greatly enhanced in the presence of NG -monomethyl-L-arginine, a specific inhibitor of the nitric oxide synthetic pathway, and significant levels of nitrite (NO-2), a product of this pathway, were detected in culture supernatants in association with suppressed responses, supporting the notion that suppression was mediated by the L-arginine-dependent production of reactive nitrogen intermediates (RNI). When the splenocytes were physically separated from the responding lymph node cell population, high levels of NO-2 were still detected but proliferative responses were no longer inhibited, suggesting that cell proximity or contact is necessary for delivery of the suppresslye signal. Adherent splenocytes cultured alone produced low levels of NO-2. Addition of 1 to 50 U/ml IFN-γ induced a dose-dependent increase in NO-2 production, with the maximal level approximating that found in suppressed cocultures; TNF-α, IL-2, or LPS did not synergize with IFN-γ to enhance NO-2 production. These findings suggest that by activating macrophages to upregulate RNI synthesis, IFN-γ-producing T cells may exert a negative influence over their own proliferation.