MECHANISM OF SUPPRESSION OF MACROPHAGE NITRIC-OXIDE RELEASE BY IL-13 - INFLUENCE OF THE MACROPHAGE POPULATION

IL-13 is a cytokine produced by T lymphocytes, mast cells, basophils, and certain B cell lines that up-regulates or inhibits various macroph age functions. In the present study we analyzed the mechanisms of supp ression of nitric oxide (NO) release by IL-13 in the macrophage cell l ine J774A.1 and in thioglycolate-elicited mouse peritoneal macrophages . In both cell types efficient reduction (> 80%) of NO production requ ired treatment of the macrophages with IL-13 for at least 7 h before s timulation with IFN-gamma and LPS. In J774A.1 cells, increasing concen trations of IFN-gamma partially antagonized the suppression mediated b y IL-13, whereas in peritoneal macrophages, the inhibitory effect of I L-13 was largely independent of the concentrations of IFN-gamma and LP S. In J774A.1 cells, IL-13 strongly reduced both the mRNA and protein levels of inducible nitric oxide synthase (iNOS, NOS-2), as determined by Northern blot analysis and immunoprecipitation. In peritoneal macr ophages, in contrast, IL-13 decreased iNOS protein and enzyme activiti es after 8 to 48 h of stimulation, without altering the expression of iNOS mRNA. Pulse labeling with [S-35] methionine revealed that IL-13 c aused a 4.7-fold reduction of the de novo synthesis of iNOS protein in these cells. These data demonstrate for the first time that IL-13 is capable of regulating iNOS at both the mRNA and translational levels a nd underline the important influence of the macrophage population when studying mechanisms of cytokine functions.