The effects of the carbocyclic nucleoside MDL 201,112 and the purine nucleoside adenosine on the interferon-gamma (IFN-gamma)-induced priming of macrophages (m phi s) for the respiratory burst and major histocompatibility class II (MHC class II) Ia+ antigen expression were compared. Priming of purified, peritoneal m phi s from Lewis (LEW/N) rats for 18 h with recombinant rat IFN-gamma (rRaIFN-gamma) in the presence of either adenosine (100 microM) or MDL 201,112 (10 microM) resulted in a fourfold decrease in superoxide anion (O2-) production after stimulation with opsonized zymosan. Both agents were effective even when added 2 or 4 h after rRaIFN-gamma treatment. Peritoneal m phi s from LEW/N rats stimulated with LPS/rRaIFN-gamma were observed to secrete immunoreactive and bioactive TNF-alpha over 18 h in vitro and this cytokine could be dose-dependently inhibited by MDL 201,112. MDL 201,112 did not bind to classical A1 or A2 receptors on rat brain homogenates. Physiological levels of adenosine deaminase, or treatment with the nucleoside transport inhibitor dipyridamole, reversed the effects of adenosine; however, these agents at physiological concentrations had little or no effect on the inhibition of O2- release mediated by MDL 201,112. Furthermore, incubation of LEW/N m phi s for 18 h in vitro with rRaIFN-gamma resulted in significant enhancement of MHC class II Ia+ antigen expression, and these levels could be blocked by nearly 50% by either MDL 201,112 (10 microM) or adenosine (100 microM). MDL 201,112 and adenosine were also effective in decreasing m phi opsonized zymosan-stimulated O2- levels and MHC class II Ia+ antigen expression in vivo. The effects of MDL 201,112 on the down-regulation of heat-killed M. tuberculosis-activated LEW/N m phi MHC class II Ia+ antigen expression in vitro appear to be mediated by a novel pathway, because there was no rank order of potency of ADO A1 or A2 agonist/antagonists (CCPA, NECA, XAC, or CPT) in our in vitro system. In summary, our data provide compelling evidence that immunoregulatory carbocyclic nucleoside analogues such as MDL 201,112 or adenosine appear to regulate LEW/N rat m phi activation through novel molecular mechanisms and may have important therapeutic implications for acute and chronic inflammatory diseases.