INTERLEUKIN-12 PRODUCTION BY HUMAN MONOCYTES INFECTED WITH MYCOBACTERIUM-TUBERCULOSIS - ROLE OF PHAGOCYTOSIS

Mycobacterium tuberculosis and its antigens are potent inducers of cyt okine expression by mononuclear phagocytes. In this study, the ability of live M. tuberculosis to stimulate interleukin-12 (IL-12) expressio n by human monocytes was examined. Monocytes were purified from periph eral blood mononuclear cells by adherence and either infected with M. tuberculosis or exposed to soluble protein antigens of M. tuberculosis (purified protein derivative [PPD]). Live M. tuberculosis (10(6) to 1 0(7) CFU/ml) was a potent stimulus for interleukin-12 (IL-12). By usin g reverse transcription-PCR, p40 mRNA was detected at 3 h, peaked at 6 to 12 h, and decayed to baseline levels at 18 to 24 h following infec tion, Bioactive IL-12 (p70) was measured by the phytohemagglutinin bla st proliferation assay and confirmed the p40 mRNA results. In contrast , soluble PPD at concentrations known to readily induce IL-1 and tumor necrosis factor alpha expression by monocytes (10 to 100 mu g/ml) was a poor stimulus for IL-12 p40 mRNA expression. The different efficien cies of M. tuberculosis bacilli and PPD for IL-12 expression by monocy tes was in part due to a requirement for phagocytosis. Induction of IL -12 in response to M. tuberculosis was reduced by cytochalasin D. Furt hermore, phagocytosis of dead M. tuberculosis or inert 2-mu m-diameter polystyrene beads by monocytes induced IL-12 p40 mRNA. In contrast, 0 .5-mu m-diameter beads, which can enter cells through pinocytosis, did not stimulate IL-12 expression. Functionally, IL-12 readily enhanced PPD-stimulated IFN-gamma production and CD4(+) T-cell-mediated cytotox icity by peripheral blood mononuclear cells from healthy tuberculin-po sitive donors but induced less enhancement when live M. tuberculosis w as the antigen. These results suggest that IL-12 is upregulated as par t of the early cytokine response of mononuclear phagocytes to M. tuber culosis and that the cellular events associated with phagocytosis are themselves a potent signal for IL-12 production. IL-12 released by inf ected macrophages in turn can further upregulate M. tuberculosis-speci fic CD4(+) T-cell effector function.