Mycobacterium leprae alters classical activation of human monocytes in vitro.

BACKGROUND: Macrophages play a central role in the pathogenesis of leprosy, caused by Mycobacterium leprae. The polarized clinical presentations in leprosy are associated with differential immune activation. In tuberculoid leprosy, macrophages show a classical activation phenotype (M1), while macrophages in lepromatous disease display characteristics of alternative activation (M2). Bacille Calmette-Guérin (BCG) vaccination, which protects against leprosy, can promote sustained changes in monocyte response to unrelated pathogens and may preferentially direct monocytes towards an M1 protective phenotype. We previously reported that M. leprae can dampen the response of naïve human monocytes to a strong inducer of pro-inflammatory cytokines, such as BCG. Here, we investigated the ability of the pathogen to alter the direction of macrophage polarization and the impact of BCG vaccination on the monocyte response to M. leprae. FINDINGS: We show that in vitro exposure of monocytes from healthy donors to M. leprae interferes with subsequent M1 polarization, indicated by lower levels of M1-associated cytokine/chemokines released and reduced expression of M1 cell surface markers. Exposure to M. leprae phenolic glycolipid (PGL) 1, instead of whole bacteria, demonstrated a similar effect on M1 cytokine/chemokine release. In addition, we found that monocytes from 10-week old BCG-vaccinated infants released higher levels of the pro-inflammatory cytokines TNF-α and IL-1ß in response to M. leprae compared to those from unvaccinated infants. CONCLUSION: Exposure to M. leprae has an inhibitory effect on M1 macrophage polarization, likely mediated through PGL-1. By directing monocyte/macrophages preferentially towards M1 activation, BCG vaccination may render the cells more refractory to the inhibitory effects of subsequent M. leprae infection.

Modulation of the cytokine response in human monocytes by mycobacterium leprae phenolic glycolipid-1.

Leprosy is a chronic but treatable infectious disease caused by the intracellular pathogen Mycobacterium leprae. M. leprae cell wall is characterized by a unique phenolic glycolipid-1 (PGL-1) reported to have several immune functions. We have examined the role of PGL-1 in the modulation of monocyte cytokine/chemokine production in naive human monocytes. PGL-1 in its purified form or expressed in a recombinant Mycobacterium bovis Bacillus Colmette-Guérin (BCG) background (rBCG-PGL-1) was tested. We found that PGL-1 selectively modulated the induction of specific monocyte cytokines and chemokines and, when used as prestimulus, exerted priming and/or inhibitory effects on the induction of selected cytokines/chemokines in response to a second stimulus. Taken together, the results of this study support a modulatory role for PGL-1 in the innate immune response to M. leprae. Thus, PGL-1 may play an important role in the development of the anergic clinical forms of disease and in tissue damage seen in lepromatous patients and during the reactional states of leprosy.

Mycobacterium leprae actively modulates the cytokine response in naive human monocytes.

Leprosy is a chronic but treatable infectious disease caused by the intracellular pathogen Mycobacterium leprae. Host immunity to M. leprae determines the diversity of clinical manifestations seen in patients, from tuberculoid leprosy with robust production of Th1-type cytokines to lepromatous disease, characterized by elevated levels of Th2-type cytokines and a suboptimal proinflammatory response. Previous reports have indicated that M. leprae is a poor activator of macrophages and dendritic cells in vitro. To understand whether M. leprae fails to elicit an optimal Th1 immune response or actively interferes with its induction, we have examined the early interactions between M. leprae and monocytes from healthy human donors. We found that, in naïve monocytes, M. leprae induced high levels of the negative regulatory molecules MCP-1 and interleukin-1 (IL-1) receptor antagonist (IL-1Ra), while suppressing IL-6 production through phosphoinositide-3 kinase (PI3K)-dependent mechanisms. In addition, low levels of proinflammatory cytokines were observed in association with reduced activation of nuclear factor-kappaB (NF-kappaB) and delayed activation of IL-1beta-converting enzyme, ICE (caspase-1), in monocytes stimulated with M. leprae compared with Mycobacterium bovis BCG stimulation. Interestingly, although in itself a weak stimulator of cytokines, M. leprae primed the cells for increased production of tumor necrosis factor alpha and IL-10 in response to a strongly inducing secondary stimulus. Taken together, our results suggest that M. leprae plays an active role to control the release of cytokines from monocytes by providing both positive and negative regulatory signals via multiple signaling pathways involving PI3K, NF-kappaB, and caspase-1.

The phenolic glycolipid of Mycobacterium tuberculosis differentially modulates the early host cytokine response but does not in itself confer hypervirulence.

Mycobacterium tuberculosis possesses a diversity of potential virulence factors including complex branched lipids such as the phenolic glycolipid PGL-tb. PGL-tb expression by the clinical M. tuberculosis isolate HN878 has been associated with a less efficient Th1 response and increased virulence in mice and rabbits. It has been suggested that the W-Beijing family is the only group of M. tuberculosis strains with an intact pks1-15 gene, required for the synthesis of PGL-tb and capable of producing PGL-tb. We have found that some strains with an intact pks1-15 do not produce PGL-tb while others may produce a variant of PGL-tb. We examined the early host cytokine response to infection with these strains in vitro to better understand the effect of PGL-tb synthesis on immune responses. In addition, we generated a PGL-tb-producing H37Rv in order to determine the effect of PGL-tb production on the host immune response during infection by a strain normally devoid of PGL-tb synthesis. We observed that PGL-tb production by clinical M. tuberculosis isolates affected cytokine production differently depending on the background of the strain. Importantly, while ectopic PGL-tb production by H37Rv suppressed the induction of several pro- and anti-inflammatory cytokines in vitro in human monocytes, it did not lead to increased virulence in infected mice and rabbits. Collectively, our data indicate that, while PGL-tb may play a role in the immunogenicity and/or virulence of M. tuberculosis, it probably acts in concert with other bacterial factors which seem to be dependent on the background of the strain.

Mycobacterium tuberculosis inhibits maturation of human monocyte-derived dendritic cells in vitro.

To induce effector immunity, dendritic cells (DCs) must differentiate into fully mature cells. We show that, after human monocyte-derived DCs were infected with virulent Mycobacterium tuberculosis, up-regulation of cellular-surface maturation markers was minimal and reversible. In the presence of a potent stimulus for maturation (tumor necrosis factor [TNF]-alpha, interleukin [IL]-1beta, and prostaglandin E2 [PGE2]), M. tuberculosis inhibited phenotypic DC maturation. M. tuberculosis-infected DCs had an impaired ability to induce allogeneic lymphoproliferation and activated autologous memory CD4+ and CD8+ T cells optimally only in the presence of TNF-alpha, IL-1beta, and PGE2. Thus, virulent M. tuberculosis inhibits phenotypic and functional maturation of human monocyte-derived DCs. This mechanism, which has been described elsewhere for various viruses and for the virulent mycobacterium M. leprae, may be a novel mechanism that this pathogen uses to evade the host's immune response.