New insights in the pathogenesis of type 1 and type 2 lepra reaction.

In the current scenario of leprosy elimination, lepra reactions (LRs) remain a major persistent problem. Type 1 LR (T1LR) and type 2 LR (T2LR) are the major causes of nerve damage and permanent disabilities. The immunopathogenesis of LR have recently become an important field of research, since it may provide the relevant targets for the early detection and control of these episodes. Presently, there are no uniformly acceptable laboratory markers for LR. Genetic and serum markers in human host may predict susceptibility to reactions as well as progression of nerve damage in leprosy. Therefore, a deeper understanding of the molecular mechanisms involved in LR may provide a rational strategy for early diagnosis and prevention of the catastrophic consequences of LR.

Mycobacterium leprae phenolglycolipid-1 expressed by engineered M. bovis BCG modulates early interaction with human phagocytes.

The species-specific phenolic glycolipid 1 (PGL-1) is suspected to play a critical role in the pathogenesis of leprosy, a chronic disease of the skin and peripheral nerves caused by Mycobacterium leprae. Based on studies using the purified compound, PGL-1 was proposed to mediate the tropism of M. leprae for the nervous system and to modulate host immune responses. However, deciphering the biological function of this glycolipid has been hampered by the inability to grow M. leprae in vitro and to genetically engineer this bacterium. Here, we identified the M. leprae genes required for the biosynthesis of the species-specific saccharidic domain of PGL-1 and reprogrammed seven enzymatic steps in M. bovis BCG to make it synthesize and display PGL-1 in the context of an M. leprae-like cell envelope. This recombinant strain provides us with a unique tool to address the key questions of the contribution of PGL-1 in the infection process and to study the underlying molecular mechanisms. We found that PGL-1 production endowed recombinant BCG with an increased capacity to exploit complement receptor 3 (CR3) for efficient invasion of human macrophages and evasion of inflammatory responses. PGL-1 production also promoted bacterial uptake by human dendritic cells and dampened their infection-induced maturation. Our results therefore suggest that M. leprae produces PGL-1 for immune-silent invasion of host phagocytic cells.

TLR activation triggers the rapid differentiation of monocytes into macrophages and dendritic cells.

Leprosy enables investigation of mechanisms by which the innate immune system contributes to host defense against infection, because in one form, the disease progresses, and in the other, the infection is limited. We report that Toll-like receptor (TLR) activation of human monocytes induces rapid differentiation into two distinct subsets: DC-SIGN+ CD16+ macrophages and CD1b+ DC-SIGN- dendritic cells. DC-SIGN+ phagocytic macrophages were expanded by TLR-mediated upregulation of interleukin (IL)-15 and IL-15 receptor. CD1b+ dendritic cells were expanded by TLR-mediated upregulation of granulocyte-macrophage colony-stimulating factor (GM-CSF) and its receptor, promoted T cell activation and secreted proinflammatory cytokines. Whereas DC-SIGN+ macrophages were detected in lesions and after TLR activation in all leprosy patients, CD1b+ dendritic cells were not detected in lesions or after TLR activation of peripheral monocytes in individuals with the progressive lepromatous form, except during reversal reactions in which bacilli were cleared by T helper type 1 (TH1) responses. In tuberculoid lepromatous lesions, DC-SIGN+ cells were positive for macrophage markers, but negative for dendritic cell markers. Thus, TLR-induced differentiation of monocytes into either macrophages or dendritic cells seems to crucially influence effective host defenses in human infectious disease.

The possible role of interleukin (IL)-12 and interferon-gamma-inducing factor/IL-18 in protection against experimental Mycobacterium leprae infection in mice.

Cell-mediated immunity participates in host defense against mycobacterial infection. Both interleukin 12 (IL-12) and interferon-gamma-inducing factor (IGIF/IL-18), produced mainly by macrophages, play a critical role in expression of cell-mediated immunity. To investigate the role of IL-12 and IGIF/IL-18 in vivo, we examined cytokine profile, bacterial growth, and the potential benefit of cytokine therapy in susceptible and resistant mice infected with Mycobacterium leprae. The early expression of IL-12 p40 and IGIF/IL-18 at the site of inoculation was found in resistant mice 3-72 h after the infection, but not in susceptible mice. Both strains of mice did not show expression of IFN-gamma and IL-4. IL-12 administration resulted in a significant reduction of bacterial counts in mice with established M. leprae infection. The results imply that susceptible mice exhibit decreased expression of type 1 helper T (Th1) response without reciprocal increased Th2 response and show responsiveness to exogenous IL-12. IL-12 therapy may be a possible rationale for treatment of M. leprae infection.

Macrophage NRAMP1 and its role in resistance to microbial infections.

The identification and characterization of genetic factors influencing natural susceptibility to infectious diseases in humans and in model organisms, such as the laboratory mouse, can provide new insight into the basic mechanisms of host defense against infections. In the mouse, resistance or susceptibility to infection with intracellular pathogens such as Salmonella, Mycobacterium and Leishmnania is controlled by the Natural resistance associated macrophage protein (Nramp1) gene on chromosome 1, which influences the rate of intracellular replication of these parasites in macrophages. Nramp1 codes for an integral membrane protein, which is expressed exclusively in macrophage/monocytes and polymorphonuclear leukocytes. The protein is localized to the endosomal/lysosomal compartment of the macrophage and is rapidly recruited to the membrane of the particle-containing phagosome upon phagocytosis. Nramp defines a novel family of functionally related membrane proteins including Nramp2, which was recently shown to be the major transferrin-independent uptake system of the intestine in mammals. This observation supports the hypothesis that the phagocyte-specific Nramp1 protein may regulate the intraphagosomal replication of antigenically unrelated bacteria by controlling divalent cation concentrations at that site. Recent genetic studies have found that allelic variants at the human NRAMP1 locus are associated with susceptibility to leprosy (Mycobacterium leprae) and tuberculosis (Mycobacterium tuberculosis) and possibly with the onset of rheumatoid arthritis.