Interleukin-12 production by human monocytes infected with Mycobacterium tuberculosis: role of phagocytosis

Abstract

Mycobacterium tuberculosis and its antigens are potent inducers of cytokine expression by mononuclear phagocytes. In this study, the ability of live M. tuberculosis to stimulate interleukin-12 (IL-12) expression by human monocytes was examined. Monocytes were purified from peripheral 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 10 7 CFU/ml) was a potent stimulus for interleukin-12 (IL-12). By using 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 infection. Bioactive IL-12 (p70) was measured by the phytohemagglutinin blast 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 g/ml) was a poor stimulus for IL-12 p40 mRNA expression. The different efficiencies of M. tuberculosis bacilli and PPD for IL-12 expression by monocytes was in part due to a requirement for phagocytosis. Induction of IL-12 in response to M. tuberculosis was reduced by cytochalasin D. Furthermore, phagocytosis of dead M. tuberculosis or inert 2-m-diameter polystyrene beads by monocytes induced IL-12 p40 mRNA. In contrast, 0.5-m-diameter beads, which can enter cells through pinocytosis, did not stimulate IL-12 expression. Functionally, IL-12 readily enhanced PPD-stimulated IFN-␥ production and CD4 ؉ T-cell-mediated cytotoxicity by peripheral blood mononuclear cells from healthy tuberculin-positive donors but induced less enhancement when live M. tuberculosis was the antigen. These results suggest that IL-12 is upregulated as part of the early cytokine response of mononuclear phagocytes to M. tuberculosis and that the cellular events associated with phagocytosis are themselves a potent signal for IL-12 production. IL-12 released by infected macrophages in turn can further upregulate M. tuberculosis-specific CD4 ؉ T-cell effector function. Mycobacterium tuberculosis is an intracellular bacterial pathogen, which readily survives within human mononuclear phagocytes. The majority (Ͼ90%) of healthy individuals infected with M. tuberculosis develop a cellular immune response to this pathogen and successfully control its growth (10). The interaction of T cells and mononuclear phagocytes is critical in this acquired, protective immune response to M. tuberculosis and is associated with a proinflammatory cytokine environment, which includes interleukin-1 (IL-1) IL-6, IL-8, granulocyte-macrophage colony-stimulating factor, tumor necrosis factor alpha (TNF-␣) and gamma interferon (IFN-␥) (38). Recent studies have demonstrated that upon exposure to M. tuberculosis bacilli or its constituents, monocytes/macrophages can secrete both proinflammatory cytokines such as IL-1, IL-6, and TNF-␣ and inhibitory cytokines such as IL-10 and transforming growth factor ␤ (TGF-␤) (4, 49, 54). Cyto...

Assessment ofin vitroimmunity toM ycobacterium tuberculosisin a human peripheral blood infection model using a luciferase reporter construct ofM. tuberculosisH37Rv

“…Several cytokines have been suggested previously to have an inhibitory or enhancing effect on the growth of M. tuberculosis in human macrophages [15][16][17][18][19][20][21][22][23][24][25]. Among these, TNFα and IFN-γ have been studied extensively [15][16][17][18][19][20][21][22][23][24][25]. In the present work, to study the effect of TNF-α, IFN-γ, IL-12 and IL-10 on M. tuberculosis growth in whole blood cultures, we have used two approaches.…”

Interleukin‐15 in Mycobacterial Infection of Antigen‐Presenting Cells

“…The specific factors capable of inducing biologically meaningful IL-15 secretion have still to be identified. Candidate molecules, such as trehalose dimycolate (TDM), a glycolipid present in the cell wall of mycobacteria, which have been demonstrated to induce the Th1-associated cytokine IL-12 [29,30] failed to induce IL-15 secretion ( Table 3). It has been suggested that the release of mycobacterial components within the mycobacterium-containing host vacuole, and their subsequent trafficking and processing, may be crucial for pathways leading to successful cytokine secretion [31].…”

Host Responses and Antigens Involved in Protective Immunity to Mycobacterium tuberculosis

“…Interleukin-12 is of crucial importance for murine Th1 cell development [71] and administration of exogeneous IL-12, as well as neutralization of endogenously produced cytokine by monoclonal antibodies (MoAbs) have in murine models been used to demonstrate a crucial role for this cytokine in immunity to TB [72,73]. The main source of IL-12 during infection is the macrophages which are stimulated by the phagocytic event [74] and by the presence of TNF-a and IFN-g in the local environment [75]. The balance between Th1 and Th2 cells is decisive for the outcome of mycobacterial disease, a fact very clearly illustrated by the spectrum of disease manifestations found in leprosy.…”