Naturally produced opsonizing antibodies restrict the survival ofMycobacterium tuberculosisin human macrophages by augmenting phagosome maturation

Abstract

This study investigated the hypothesis that serum antibodies against Mycobacterium tuberculosis present in naturally infected healthy subjects of a tuberculosis (TB) endemic area could create and/or sustain the latent form of infection. All five apparently healthy Indian donors showed high titres of serum antibodies against M. tuberculosis cell membrane antigens, including lipoarabinomannan and alpha crystallin. Uptake and killing of bacilli by the donor macrophages was significantly enhanced following their opsonization with antibody-rich, heat-inactivated autologous sera. However, the capability to opsonize was apparent for antibodies against some and not other antigens. High-content cell imaging of infected macrophages revealed significantly enhanced colocalization of the phagosome maturation marker LAMP-1, though not of calmodulin, with antibody-opsonized compared with unopsonized M. tuberculosis. Key enablers of macrophage microbicidal action—proinflammatory cytokines (IFN-γ and IL-6), phagosome acidification, inducible NO synthase and nitric oxide—were also significantly enhanced following antibody opsonization. Interestingly, heat-killed M. tuberculosis also elevated these mediators to the levels comparable to, if not higher than, opsonized M. tuberculosis. Results of the study support the emerging view that an efficacious vaccine against TB should, apart from targeting cell-mediated immunity, also generate ‘protective’ antibodies.

Antibodies and tuberculosis

“…Although greater phagolysosomal fusion did not have an effect on viable intracellular bacterial numbers, this process was still thought to benefit immunity. This finding of enhanced phagolysosomal been replicated in more recent studies [80], [81]. Serum from individuals who received two booster vaccinations with BCG showed high titres of anti-LAM antibodies and was used to pre-treat bacilli prior to monocyte or neutrophil uptake [81].…”

“…Opsonizing antibodies isolated from healthy donors in India were able to restrict the growth of M.tb h37Rv in macrophages, and showed enhancement of TNF-α and IL-12 secretion [80]. These antibodies, but not non-opsonizing antibodies, were able to enhance intracellular killing by increasing LAMP-1 and iNOS localization to the phagosome as well as phagolysosome acidification [80].…”

“…These antibodies, but not non-opsonizing antibodies, were able to enhance intracellular killing by increasing LAMP-1 and iNOS localization to the phagosome as well as phagolysosome acidification [80]. FcγR − / - mice are more susceptible to infection with M.tb , and loss of function of the inhibitory FcγRIIb improves TB outcome by enhancing IL-12, granuloma formation and IFN-γ levels [82].…”

Immune responses to Mycobacterium tuberculosis membrane-associated antigens including alpha crystallin can potentially discriminate between latent infection and active tuberculosis disease

“…Recently, Li et al [12] used a panel of Mtb membrane proteins to identify antigens which produced stronger T cell responses in TB patients than did ESAT6 (a component of IGRA). With respect to B cell responses also, we have shown that sera of healthy Indians contain significantly higher levels of antibodies to membrane-associated than cytosolic antigens of Mtb [13,14]. In another study, serum antibodies of TB patients reacted strongly with Mtb membrane (MtM) proteins [15].…”

“…A previously described protocol [13,14] was used after necessary modifications. The used antigen concentrations and serum dilutions had provided optimal results during protocol development.…”