REGULATION OF BIOACTIVE IL-12 PRODUCTION IN LIPOPOLYSACCHARIDE-STIMULATED HUMAN MONOCYTES IS DETERMINED BY THE EXPRESSION OF THE P35 SUBUNIT

IL-12 enhances IFN-gamma production by NK cells and T lymphocytes and plays a pivotal role in the development of Th1 cells. Bioactive IL-12 (p70) is composed of two subunits, p35, which is only secreted as part of the p70 dimer, and p40, which can also be secreted by itself. The IL-12 subunits are encoded by two separate genes. Therefore, we studie d to what extent the expression of each of the subunits contributes to the regulation of IL-12 production. We found that in LPS-stimulated w hole blood and purified monocytes, p70 and p40 production are enhanced by IFN-gamma and inhibited by IL-10 and IL-4. However, IFN-gamma and IL-10 had stronger effects on p70 production than on p40 production, a nd IL-4 affected p40 production more strongly. Concomitantly, in ail e xperimental conditions tested, p40 production greatly exceeded p70 pro duction, suggesting that p35 expression was limiting. Analysis of p35 and p40 mRNA expression by PCR confirmed this notion. Resting purified monocytes expressed neither p40 nor p35 mRNA. The effects of IFN-gamm a, IL-10, and IL-4 on the p35 mRNA expression in LPS-stimulated purifi ed monocytes were similar to the effects of these cytokines on p70 pro tein production, and the p40 mRNA expression corresponded to p40 prote in production. Our results imply that production of the two IL-12 subu nits is differently regulated for each subunit, mainly at the level of mRNA expression, and that the level of bioactive IL-12 production in monocytes in response to LPS and cytokines is determined by the level of p35 expression.