Is there a role for nitric oxide in regulation of T cell secretion of IL-2?

Nitric oxide (NO) can have both effector (cytotoxic) and regulatory roles in immune function. In this study, we have re-examined the potential role of nitric oxide in mediating the macrophage-dependent suppression of IL-2 synthesis. In our model, TNP-specific CD4+ T cells are cocultured with Ag and either peritoneal or alveolar macrophages. Both populations of macrophages after in vitro stimulation with IFN-gamma can inhibit IL-2 release. In vitro stimulation also induces substantial levels of NO release by these macrophages, as well as high levels of prostaglandin E2 (PGE2). However, there was no correlation between NO levels and inhibitory activity. Furthermore, NG-monomethyl-L-arginine monoacetate, a specific inhibitor of NO release had no effect on IL-2 release, while indomethacin, which blocked prostaglandin synthesis, largely abrogated the suppressor activity of both macrophage populations. Although the addition of exogenous NO donors at high concentrations could inhibit IL-2 release by T cells, our data does not support the hypothesis that NO is a major macrophage mediator of suppression in this model.