Opsonizing antibodies (IgG1) up-regulate monocyte proinflammatory cytokines tumour necrosis factor-alpha (TNF-α) and IL-6 but not anti-inflammatory cytokine IL-10 in mycobacterial antigen-stimulated monocytes—implications for pathogenesis

Abstract

SUMMARY Cachexia is one of the prominent features of advanced tuberculosis (TB) seen in association with increased expression of the monokine TNF-α. Several mycobacterial proteins, including PPD, stimulate TNF-α secretion from monocytes. Host factors that may play a role in cytokine expression from monocytes remain largely unknown. One such factor is the opsonizing antibodies. Monocytes have high-affinity receptors (FcγI and FcγIII) for IgG1 and IgG3 antibodies that mediate antigen uptake. We have reported selective up-regulation of IgG1 (which bind to Fcγ receptors) in advanced TB and have recently shown the ability of PPD-specific IgG1 antibodies to augment TNF-α expression in PPD-stimulated monocytes. These observations have now been extended to other cytokines with semipurified fractions from secreted antigens of Mycobacterium tuberculosis (containing 30 kD and 58 kD) that were devoid of lipids, glycolipids and carbohydrates. In the presence of heat-inactivated TB plasma containing known amounts of antigen-specific IgG1 antibodies, these fractions induced significantly increased TNF-α, IL-6 and IL-10 secretion. Absorption of IgG1 with Protein ‘A’ removed the augmenting activity for TNF-α and IL-6 secretion from the TB plasma samples. In the case of IL-10, removal of IgG1 resulted in increased rather than decreased IL-10 secretion. These results suggest a possible pathogenic role for antibodies in TB by enhancing proinflammatory and blocking down-regulatory cytokines such as IL-10 cytokines during the chronic phase of TB.