Role of inducible nitric oxide synthase in resistance to Mycobacterium leprae in mice

PGL-III, a Rare Intermediate of Mycobacterium leprae Phenolic Glycolipid Biosynthesis, Is a Potent Mincle Ligand

Although leprosy (Hansen's disease) is one of the oldest known diseases, the pathogenicity of Mycobacterium leprae (M. leprae) remains enigmatic. Indeed, the cell wall components responsible for the immune response against M. leprae are as yet largely unidentified. We reveal here phenolic glycolipid-III (PGL-III) as an M. leprae-specific ligand for the immune receptor Mincle. PGL-III is a scarcely present trisaccharide intermediate in the biosynthetic pathway to PGL-I, an abundant and characteristic M. leprae glycolipid. Using activity-based purification, we identified PGL-III as a Mincle ligand that is more potent than the well-known M. tuberculosis trehalose dimycolate. The cocrystal structure of Mincle and a synthetic PGL-III analogue revealed a unique recognition mode, implying that it can engage multiple Mincle molecules. In Mincle-deficient mice infected with M. leprae, increased bacterial burden with gross pathologies were observed. These results show that PGL-III is a noncanonical ligand recognized by Mincle, triggering protective immunity.

Fusarium Mycotoxins Zearalenone and Deoxynivalenol Reduce Hepatocyte Innate Immune Response after the Listeria monocytogenes Infection by Inhibiting the TLR2/NFκB Signaling Pathway

Zearalenone (ZEA) and deoxynivalenol (DON) are two common mycotoxins produced by the genus Fusarium and have potential immunotoxic effects that may lead to a weak immune response against bacterial infections. Listeria monocytogenes (L. monocytogenes), a food-borne pathogenic microorganism ubiquitous in the environment, actively multiplies in the liver, where hepatocytes are capable of resistance through mediated innate immune responses. At present, it is not clear if ZEA and DON affect hepatocyte immune responses to L. monocytogenes infection or the mechanisms involved. Therefore, in this study, in vivo and in vitro models were used to investigate the effects of ZEA and DON on the innate immune responses of hepatocytes and related molecules after L. monocytogenes infection. In vivo studies revealed that ZEA and DON inhibited the toll-like receptors 2 (TLR2)/nuclear factor kappa-B (NFκB) pathway in the liver tissue of L. monocytogenes-infected mice, downregulating the expression levels of Nitric oxide (NO), in the liver and repressing the immune response. In addition, ZEA and DON inhibited Lipoteichoic acid (LTA)-induced expression of TLR2 and myeloid differentiation factor 88 (MyD88) in Buffalo Rat Liver (BRL 3A) cells in vitro, downregulating the TLR2/NFκB signaling pathway and resulting in the decreased expression levels of NO, causing immunosuppressive effects. In summary, ZEA and DON can negatively regulate NO levels through TLR2/NFκB, inhibiting the innate immune responses of the liver, and aggravate L. monocytogenes infections in mouse livers.

Susceptibility and resistance in leprosy: Studies in the mouse model

Leprosy is a chronic granulomatous infectious disease caused by the pathogen, Mycobacterium leprae, and the more recently discovered, M. lepromatosis. Described in 1873, M. leprae was among the first microorganisms to be proposed as a cause of a human infectious disease. As an obligate intracellular bacterium, it has still not thus far been reproducibly cultivated in axenic medium or cell cultures. Shepard's mouse footpad assay, therefore, was truly a breakthrough in leprosy research. The generation of immunosuppressed and genetically engineered mice, along with advances in molecular and cellular techniques, has since offered more tools for the study of the M. leprae–induced granuloma. While far from perfect, these new mouse models have provided insights into the immunoregulatory mechanisms responsible for the spectrum of this complex disease.

Murine experimental leprosy: Evaluation of immune response by analysis of peritoneal lavage cells and footpad histopathology

This study evaluated the immune response of nude and BALB/c mice inoculated in the footpads (FP) with Mycobacterium leprae after 3, 5 and 8 months. At each timepoint peritoneal cells, peripheral blood, FP and popliteal lymph nodes (PLN) were collected. Peritoneal cell cultures were performed to measure the H₂ O₂ , O₂ ^- , NO, IL-2, IL-4, IL-10, IL-12, IFN-γ and TNF levels. Serum levels of anti-PGL-I antibodies were also quantified. The results showed that the infection was progressive in nude mice with bacterial multiplication, development of macroscopic lesions in the FP and presence of bacilli in the PLN at 8 months. In BALB/c mice, the infection reached a plateau of bacillary multiplication at 5 months and regressed at 8 months. Histopathological analysis of FP revealed a mononuclear inflammatory infiltrate with a large number of neutrophils at 5 months, with a higher number in nude mice. At 8 months, the number of neutrophils decreased and the infiltrate was predominantly mononuclear in both mouse strains. There was no H₂ O_2, O₂ ^- , IL-2, IL-4, IL-10 and IFN-γ production in the course of infection in nude mice; however, in BALB/c, O₂ ^- and IL-12 production was higher at 5 months and NO, IFN-γ and TNF production was higher at 8 months when there was a decrease in the number of bacilli. The level of anti-PGL-I antibodies was higher in BALB/c mice. Thus, nude and BALB/c mice can be used as experimental models for the study of various aspects of leprosy.

Experimental models of leprosy

Leprosy (Hansen’s disease) is a chronic granulomatous bacterial disease which mainly affects skin and peripheral nervous system. Leprosy is caused by the obligate intercellular pathogen known as Mycobacterium leprae. Creating experimental models of leprosy is associated with serious problems due to biological characteristics of the pathogen. Numerous attempts to develop experimental models on different types of animals resulted in a few reproducible models on mice and nine-banded armadillos. Strains of knockout mice with genetic defects caused by site-directed mutagenesis are used as a basis for different leprosy models. Experimental models of leprosy are used for screening of anti-leprosy drugs, detection of drug resistance, studies on the pathogenesis of leprosy, production and evaluation of viability of M. leprae, developing of anti-leprosy vaccines.

IL-10 and NOS2 modulate antigen-specific reactivity and nerve infiltration by T cells in experimental leprosy

Background

Although immunopathology dictates clinical outcome in leprosy, the dynamics of early and chronic infection are poorly defined. In the tuberculoid region of the spectrum, Mycobacterium leprae growth is restricted yet a severe granulomatous lesion can occur. The evolution and maintenance of chronic inflammatory processes like those observed in the leprosy granuloma involve an ongoing network of communications via cytokines. IL-10 has immunosuppressive properties and IL-10 genetic variants have been associated with leprosy development and reactions.

Methodology/Principal Findings

The role of IL-10 in resistance and inflammation in leprosy was investigated using Mycobacterium leprae infection of mice deficient in IL-10 (IL-10−/−), as well as mice deficient in both inducible nitric oxide synthase (NOS2−/−) and IL-10 (10NOS2−/−). Although a lack of IL-10 did not affect M. leprae multiplication in the footpads (FP), inflammation increased from C57Bl/6 (B6)<IL-10−/−<NOS2−/−<10NOS2−/−. While IL-10−/− mice exhibited modest FP induration compared to B6, NOS2−/− and 10NOS2−/− mice developed markedly enlarged FP marking distinct phases: early (1 month), peak (3–4 months), and chronic (8 months). IFN-γ-producing CD4+CD44+ cells responding to M. leprae cell wall, membrane, and cytosol antigens and ML2028 (Ag85B) were significantly increased in the evolved granuloma in NOS2−/− FP compared to B6 and IL-10−/− during early and peak phases. In 10NOS2−/− FP, CD4+CD44+ and especially CD8+CD44+ responses were augmented even further to these antigens as well as to ML0380 (GroES), ML2038 (bacterioferritin), and ML1877 (EF-Tu). Moreover, fragmented nerves containing CD4+ cells were present in 10NOS2−/− FP.

Conclusions/Significance

The 10NOS2−/− strain offers insight on the regulation of granuloma formation and maintenance by immune modulators in the resistant forms of leprosy and presents a new model for investigating the pathogenesis of neurological involvement.

Despite effective antimicrobial therapy, 30–50% of leprosy patients develop immunological complications called leprosy reactions before, during or even years after being cured. Leprosy reactions are a major risk for neuritis that leads to peripheral nerve damage, disfigurement and disability. Unfortunately, why and how leprosy reactions occur is not well understood. Based on the latest human genetic leprosy susceptibility research and mouse infection models, we generated a double knockout mouse strain (10NOS2−/−) which has deficiencies in two key immune factors, interleukin-10 (IL-10) and inducible nitric oxide synthase (NOS2). We investigated the dynamics of the immune response to Mycobacterium leprae infection and chronicled the types of immune cells recruited to the site of infection. 10NOS2−/− mice developed a substantial induration in response to infection, as well as an increased interferon-gamma response to components of the leprosy bacillus. Interestingly, these animals also exhibited CD4+ T cell infiltration into the nerves, a phenomenon which has not been previously reported in leprosy mouse models. This new model provides insight into potential mechanisms whereby immune modulators may regulate leprosy reactions and neuritis and could aid the development of tests for monitoring and treatment of leprosy patients.

Analysis of polymorphic sites in the promoter of the nitric oxide synthase 2 gene in Brazilian patients with leprosy

Leprosy is one of the most neglected infectious tropical diseases of the skin and the nerves caused by the intracellular pathogen Mycobacterium leprae. The inducible NOS isoform encoded by NOS2A plays a vital role in host defence against bacterial infections. The functional promoter polymorphisms in NOS2A are associated with various autoimmune and infectious diseases. We investigated the association of NOS2A variants with progression of leprosy in a Brazilian cohort including 221 clinically classified patients and 103 unrelated healthy controls. We observed a novel variant ss528838018A/G in the promoter region at position -6558. The other functional variants were observed with low frequency of minor allele (<0.005). NOS2A promoter variant (-954G/C) was not observed in Brazilian populations, and the new observed promoter variant (ss528838018A/G) as well as other promoter variants were not associated with any clinical forms of leprosy in the Brazilian populations.

Insights from animal models on the immunogenetics of leprosy: a review

A variety of host immunogenetic factors appear to influence both an individual's susceptibility to infection with Mycobacterium leprae and the pathologic course of the disease. Animal models can contribute to a better understanding of the role of immunogenetics in leprosy through comparative studies helping to confirm the significance of various identified traits and in deciphering the underlying mechanisms that may be involved in expression of different disease related phenotypes. Genetically engineered mice, with specific immune or biochemical pathway defects, are particularly useful for investigating granuloma formation and resistance to infection and are shedding new light on borderline areas of the leprosy spectrum which are clinically unstable and have a tendency toward immunological complications. Though armadillos are less developed in this regard, these animals are the only other natural hosts of M. leprae and they present a unique opportunity for comparative study of genetic markers and mechanisms associable with disease susceptibility or resistance, especially the neurological aspects of leprosy. In this paper, we review the recent contributions of genetically engineered mice and armadillos toward our understanding of the immunogenetics of leprosy.

Immunohistochemical evaluation of macrophage activity and its relationship with apoptotic cell death in the polar forms of leprosy

The objective of the present study was to investigate the correlation between macrophage activity and apoptosis in the polar forms of leprosy because the immunopathological phenomena involved in these forms are still poorly understood. For this purpose, 29 skin biopsy samples obtained from patients with the polar forms of leprosy were analyzed. Macrophage activity and apoptosis were evaluated by immunohistochemistry using lysozyme, CD68, iNOS and caspase 3 as markers. The nonparametric Mann-Whitney test and Spearman's linear correlation test were used for statistical analysis. The results suggest that the apoptosis rate is under the direct influence of macrophage activity in lesions of patients with the tuberculoid form. In contrast, in lepromatous lesions other factors seem to induce programmed cell death, possibly TGF-beta. Further studies are necessary to identify additional factors involved in the immunopathogenesis of leprosy.

Lymphotoxin-alpha and TNF have essential but independent roles in the evolution of the granulomatous response in experimental leprosy

Recent studies identified an association between genetic variants in the lymphotoxin-alpha (LTalpha) gene and leprosy. To study the influence of LTalpha on the control of experimental leprosy, both low- and high-dose Mycobacterium leprae foot pad (FP) infections were evaluated in LTalpha-deficient chimeric (cLTalpha(-/-)) and control chimeric (cB6) mice. Cellular responses to low-dose infection in cLTalpha(-/-) mice were dramatically different, with reduced accumulation of CD4(+) and CD8(+) lymphocytes and macrophages and failure to form granulomas. Growth of M. leprae was contained for 6 months, but augmented late in infection. In contrast, tumor necrosis factor knockout and tumor necrosis factor receptor 1 knockout FPs exhibited extensive inflammatory infiltration with an increase in M. leprae growth throughout infection. Following high-dose infection, cB6 FP induration peaked at 4 weeks and was maintained for 12 weeks. Induration was not sustained in cLTalpha(-/-) FPs that contained few lymphocytes and no granulomas. There was a reduction in the expression levels of inflammatory cytokines, chemokines, and chemokine receptors, including nitric oxide synthase 2, vascular cell adhesion molecule, and intercellular cell adhesion molecule. Furthermore, cLTalpha(-/-) popliteal lymph nodes contained a higher proportion of naïve CD44(lo)CD62L(hi) T cells than cB6 mice, suggestive of reduced T cell activation. Therefore, both LTalpha and tumor necrosis factor are essential for the regulation of the granuloma, but they have distinctive roles in the recruitment of lymphocytes and maintenance of the granulomatous response during chronic M. leprae infection.

Expression and characterization of recombinant interferon gamma (IFN-gamma) from the nine-banded armadillo (Dasypus novemcinctus) and its effect on Mycobacterium leprae-infected macrophages

Armadillos (Dasypus novemcinctus) manifest the full histopathological spectrum of leprosy, and are hosts of choice for in vivo propagation of Mycobacterium leprae. Though potentially useful as a model of leprosy pathogenesis, few armadillo-specific reagents exist. We have identified a region of high homology to the interferon gamma (IFN-gamma) of other mammals within the recently published armadillo whole genomic sequence. cDNA was made from ConA-stimulated armadillo peripheral blood mononuclear cells (PBMC), amplified, and cloned into a pET expression vector for transformation and over-expression in Escherichia coli. The recombinant protein (rDnIFN-gamma) was characterized by western blot and its biological function confirmed with bioassays including intracellular killing of Toxoplasma gondii and induction of indoleamine 2, 3-dioxygenase activity. In using rIFN-gamma to activate macrophages from mice, humans or armadillos, similar to humans, rIFN-gamma-activated armadillo MPhi did not produce nitrite and or inhibit the viability of M. leprae in vitro. Conversely, murine rIFN-gamma-activated mouse MPhi produced high levels of nitrite and killed intracellular M. leprae in vitro. These data indicate that the response of armadillo MPhi to rDnIFN-gamma is similar to that which occurs in humans, and demonstrates a potentially important value of the armadillo as a model in leprosy research.

Antigen-specific cellular and humoral responses are induced by intradermal Mycobacterium leprae infection of the mouse ear

Leprosy is caused by infection with Mycobacterium leprae. The immune response of leprosy patients can be highly diverse, ranging from strong cellular responses accompanied by an apparent deficit of M. leprae-specific antibodies to strong humoral responses with a deficit of cell-mediated responses. Leprosy takes many years to manifest, and this has precluded analyses of disease and immune response development in infected humans. In an attempt to better define development of the immune response during leprosy we have developed an M. leprae ear infection model. Intradermal inoculation of M. leprae into the ear supported not only infection but also the development of a chronic inflammatory response. The inflammatory response was localized, comprising a T-cell infiltration into the ear and congestion of cells in the draining lymph nodes. The development of local chronic inflammation was prevented by rifampin treatment. Importantly, and in contrast to subcutaneous M. leprae footpad infection, systemic M. leprae-specific gamma interferon and antibody responses were detected following intradermal ear infection. These results indicate the utility of intradermal ear infection for both induction and understanding of the immune response during M. leprae infection and the identification or testing of new leprosy treatments.

Emergence of an effective adaptive cell mediated immune response to Mycobacterium leprae is not impaired in reactive oxygen intermediate-deficient mice

Cytokine-activated macrophages (MPhi) employ reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI) to combat pathogens. The requirement for ROI for an effective host response to experimental leprosy using mice which have a disruption in the 91-kD subunit of the NAPDH oxidase cytochrome b (phox91-/-) was examined. Mycobacterium leprae multiplication in phox91-/- foot pads (FP) was elevated early in infection but subsequently arrested similarly to control mice within a noninvasive granuloma. Using a modified lepromin test model, a similar cellular composition in the M. leprae-induced FP granuloma in both strains with lymphocyte infiltration consisting primarily of CD4+CD44(hi)CD62L(lo) effector cells was found. Of great interest was the disparity in the T cell population between the granuloma and the draining lymph node which contained predominantly naïve CD4+CD44(lo)CD62L(hi) cells and was, therefore, not representative of the infection site. TH1 cytokines, chemokines and inducible nitric oxide synthase were comparably expressed in the FP of both strains. When infected in vitro, normal MPhi from B6 and phox91-/- mice supported bacterial viability, whereas IFNgamma-activated MPhi killed M. leprae in a RNI-dependent manner, emphasizing that ROI was dispensable. These data show that phox91-/- mice generate a strong adaptive immune response and control long-term infection with M. leprae.

NO dissociation represents the rate limiting step for O2-mediated oxidation of ferrous nitrosylated Mycobacterium leprae truncated hemoglobin O

Mycobacterium leprae truncated hemoglobin O (trHbO) protects from nitrosative stress and sustains mycobacterial respiration. Here, kinetics of M. leprae trHbO(II)-NO denitrosylation and of O(2)-mediated oxidation of M. leprae trHbO(II)-NO are reported. Values of the first-order rate constant for *NO dissociation from M. leprae trHbO(II)-NO (k(off)) and of the first-order rate constant for O(2)-mediated oxidation of M. leprae trHbO(II)-NO (h) are 1.3 x 10(-4) s(-1) and 1.2 x 10(-4) s(-1), respectively. The coincidence of values of k(off) and h suggests that O(2)-mediated oxidation of M. leprae trHbO(II)-NO occurs with a reaction mechanism in which *NO, that is initially bound to heme(II), is displaced by O(2) but may stay trapped in a protein cavity(ies) close to heme(II). Next, M. leprae trHbO(II)-O(2) reacts with *NO giving the transient Fe(III)-OONO species preceding the formation of the final product M. leprae trHbO(III). *NO dissociation from heme(II)-NO represents the rate limiting step for O(2)-mediated oxidation of M. leprae trHbO(II)-NO.

Peroxynitrite scavenging by ferrous truncated hemoglobin GlbO from Mycobacterium leprae

Mycobacterium leprae GlbO has been proposed to represent merging of both O(2) uptake/transport and scavenging of nitrogen reactive species. Peroxynitrite reacts with M. leprae GlbO(II)-NO leading to GlbO(III) via the GlbO(III)-NO species. The value of the second order rate constant for GlbO(III)-NO formation is >1x10(8)M(-1)s(-1) in the absence and presence of CO(2) (1.2x10(-3)M). The CO(2)-independent value of the first order rate constant for GlbO(III)-NO denitrosylation is (2.5+/-0.4)x10(1)s(-1). Furthermore, peroxynitrite reacts with GlbO(II)-O(2) leading to GlbO(III) via the GlbO(IV)O species. Values of the second order rate constant for GlbO(IV)O formation are (4.8+/-0.5)x10(4) and (6.3+/-0.7)x10(5)M(-1)s(-1) in the absence and presence of CO(2) (=1.2x10(-3)M), respectively. The value of the second order rate constant for the peroxynitrite-mediated GlbO(IV)O reduction (= (1.5+/-0.2)x10(4)M(-1)s(-1)) is CO(2)-independent. These data argue for a role of GlbO in the defense of M. leprae against nitrosative stress.

The continuing challenges of leprosy

Leprosy is best understood as two conjoined diseases. The first is a chronic mycobacterial infection that elicits an extraordinary range of cellular immune responses in humans. The second is a peripheral neuropathy that is initiated by the infection and the accompanying immunological events. The infection is curable but not preventable, and leprosy remains a major global health problem, especially in the developing world, publicity to the contrary notwithstanding. Mycobacterium leprae remains noncultivable, and for over a century leprosy has presented major challenges in the fields of microbiology, pathology, immunology, and genetics; it continues to do so today. This review focuses on recent advances in our understanding of M. leprae and the host response to it, especially concerning molecular identification of M. leprae, knowledge of its genome, transcriptome, and proteome, its mechanisms of microbial resistance, and recognition of strains by variable-number tandem repeat analysis. Advances in experimental models include studies in gene knockout mice and the development of molecular techniques to explore the armadillo model. In clinical studies, notable progress has been made concerning the immunology and immunopathology of leprosy, the genetics of human resistance, mechanisms of nerve injury, and chemotherapy. In nearly all of these areas, however, leprosy remains poorly understood compared to other major bacterial diseases.

Truncated hemoglobin GlbO from Mycobacterium leprae alleviates nitric oxide toxicity

As a consequence of reductive genome evolution, the obligate intracellular pathogen Mycobacterium leprae has minimized the repertoire of genes implicated in defense against reactive oxygen and nitrogen species. Genes for multiple hemoglobin types coexist in mycobacterial genomes, but M. leprae has retained only glbO, encoding a group-II truncated hemoglobin. Mycobacterium tuberculosis GlbO has been involved in oxygen transfer and respiration during hypoxia, but a role in protection from nitric oxide (NO) has not been documented yet. Here, we report that the in vitro reaction of oxygenated recombinant M. leprae GlbO with NO results in an immediate stoichiometric formation of nitrate, concomitant with heme-protein oxidation. Overexpression of GlbO alleviates the growth inhibition of Escherichia colihmp (flavohemoglobin gene) mutants in the presence of NO-donors, partly complementing the defect in Hmp synthesis. A promoter element upstream of glbO was predicted in silico, and confirmed by using a glbO::lacZ transcriptional fusion in the heterologous Mycobacterium smegmatis system. The glbO::lacZ fusion was expressed through the whole growth cycle of M. smegmatis, and moderately induced by NO. We propose that M. leprae, by retaining the unique truncated hemoglobin GlbO, may have coupled O2 delivery to the terminal oxidase with a defensive mechanism to scavenge NO from respiratory enzymes. These activities would help to sustain the obligate aerobic metabolism required for intracellular survival of leprosy bacilli.

Nitric oxide scavenging by Mycobacterium leprae GlbO involves the formation of the ferric heme-bound peroxynitrite intermediate

Ferrous oxygenated (Fe(II)O2) hemoglobins (Hb's) and myoglobins (Mb's) have been shown to react very rapidly with NO, yielding NO3(-) and the ferric heme-protein derivative (Fe(III)), by means of the ferric heme-bound peroxynitrite intermediate (Fe(III)OONO), according to the minimum reaction scheme: Fe(II)O2 + NO (k(on))--> Fe(III)OONO (h)--> Fe(III) + NO3(-). For most Hb's and Mb's, the first step (indicated by k(on)) is rate limiting, the overall reaction following a bimolecular behavior. By contrast, the rate of isomerization and dissociation of Fe(III)OONO (indicated by h) is rate limiting in NO scavenging by Fe(II)O2 murine neuroglobin, thus the overall reaction follows a monomolecular behavior. Here, we report the characterization of the NO scavenging reaction by Fe(II)O2 truncated Hb GlbO from Mycobacterium leprae. Values of k(on) (=2.1x10(6) M(-1) s(-1)) and h (=3.4 s(-1)) for NO scavenging by Fe(II)O2 M. leprae GlbO have been determined at pH 7.3 and 20.0 degrees C, the rate of Fe(III)OONO decay (h) is rate limiting. The Fe(III)OONO intermediate has been characterized by optical absorption spectroscopy in the Soret region. These results have been analyzed in parallel with those of monomeric and tetrameric globins as well as of flavoHb and discussed with regard to the three-dimensional structure of mycobacterial truncated Hbs and their proposed role in protection from nitrosative stress.

Genetic study of ICAM1 in clinical malaria in Senegal

Many genes have been implicated in the risk of severe malaria, generally based on candidate gene studies in case/control populations. Among these genes, there has been conflicting reports for the implication of a variant of the intercellular adhesion molecule 1 (ICAM1), ICAM1(Kilifi), in the risk of severe malaria, while in vitro studies provided independent support for a functional role of this variant. In order to explore the possible implication of ICAM1 in the susceptibility/resistance to malaria and to try to understand its clinical relevance in the disease process, we have conducted linkage and association studies of ICAM1 in two Senegalese villages located in regions of endemic malaria. We explored the full genetic variability of ICAM1, and tested it on several clinical malarial traits which are under genetic control, focusing principally on variables related to the parasite density and the number of malarial attacks. Our study provides no evidence for a role of ICAM1 variability on the malarial phenotypes studied.

Effects of thalidomide on intracellular Mycobacterium leprae in normal and activated macrophages

Thalidomide is an effective drug for the treatment of erythema nodosum leprosum (ENL). ENL is an inflammatory reaction that may occur in multibacillary leprosy patients. Its cause(s) as well as the mechanism of thalidomide in arresting this condition are not fully understood. It has been suggested that ENL is an immune complex-mediated hypersensitivity precipitated by the release of Mycobacterium leprae from macrophages. The released antigen may complex with precipitating antibodies, initiating complement fixation and the production of inflammatory cytokines like tumor necrosis factor alpha (TNF-alpha). Thalidomide has been shown in vitro to reduce antigen- or mitogen-activated macrophage production of TNF-alpha. We investigated if thalidomide could also influence the viability of intracellular M. leprae. Mouse peritoneal macrophages were infected with M. leprae, activated with gamma interferon and endotoxin, or nonactivated, and treated with thalidomide. Intracellular bacilli were recovered, and metabolic activity was assessed by a radiorespirometric procedure. Thalidomide did not possess antimicrobial action against M. leprae in normal and activated host macrophages. This suggests that thalidomide does not retard the release of mycobacterial antigens, a possible prelude or precipitating factor for ENL. A distinct sequence of events explaining the mechanism of action for thalidomide's successful treatment of ENL has yet to be established.

Application of a viability-staining method for Mycobacterium leprae derived from the athymic (nu/nu) mouse foot pad

Mycobacterium leprae cannot be cultured, so ascertaining viability of the organism remains a major obstacle, impeding many avenues of investigation. This study tested a two-colour, Syto9 and propidium iodide, fluorescence assay, which scores for membrane damage in individual bacilli, to determine if a rapid direct-count viability-staining technique can be reliably applied to M. leprae. A variety of experimental conditions were employed to validate this technique. This technique was also used to correlate the viability of M. leprae with the course of athymic mouse foot pad infection to optimize the provision of viable M. leprae as a research reagent. The data show that in untreated suspensions of M. leprae there is a good correlation between the metabolic activity of leprosy bacilli and their membrane damage. Fixation of M. leprae with ethanol, paraformaldehyde and gluteraldehyde completely suppressed their metabolic activity but showed little effect on their membrane integrity. The present study also showed that the metabolic activity of M. leprae declines more than the extent of membrane damage at 37 degrees C within 72 h, but that they are not significantly affected at 33 degrees C. Irradiation at 10(4) Gy showed high numbers of dead bacilli by the staining method. The results show that the reliability of metabolic-activity data as well as viability-staining data is dependent on the method by which M. leprae is killed. This staining method helped us predict reliably that the smaller M. leprae-infected athymic mouse foot pad seen early in infection, between 4 and 5 months, yields markedly better quality leprosy bacilli than older, larger foot pad infections, as defined by their metabolic activity and membrane integrity.

An in vitro model for the lepromatous leprosy granuloma: fate of Mycobacterium leprae from target macrophages after interaction with normal and activated effector macrophages

The lepromatous leprosy granuloma is a dynamic entity requiring a steady influx of macrophages (Mphi) for its maintenance. We have developed an in vitro model to study the fate of Mycobacterium leprae in a LL lesion, with and without immunotherapeutic intervention. Target cells, consisting of granuloma Mphi harvested from the footpads of M. leprae-infected athymic nu/nu mice, were cocultured with normal or IFN-gamma-activated (ACT) effector Mphi. The bacilli were recovered and assessed for viability by radiorespirometry. M. leprae recovered from target Mphi possessed high metabolic activity, indicating a viable state in this uncultivable organism. M. leprae recovered from target Mphi incubated with normal effector Mphi exhibited significantly higher metabolism. In contrast, bacilli recovered from target Mphi cocultured with ACT effector Mphi displayed a markedly decreased metabolic activity. Inhibition by ACT Mphi required an E:T ratio of at least 5:1, a coculture incubation period of 3-5 days, and the production of reactive nitrogen intermediates, but not reactive oxygen intermediates. Neither IFN-gamma nor TNF-alpha were required during the cocultivation period. However, cell-to-cell contact between the target and effector Mphi was necessary for augmentation of M. leprae metabolism by normal effector Mphi as well as for inhibition of M. leprae by ACT effector Mphi. Conventional fluorescence microscopy and confocal fluorescence microscopy revealed that the bacilli from the target Mphi were acquired by the effector Mphi. Thus, the state of Mphi infiltrating the granuloma may markedly affect the viability of M. leprae residing in Mphi in the lepromatous lesion.

Effect of var gene disruption on switching in Plasmodium falciparum

The molecular mechanisms underpinning switching of variant antigens on the surface of Plasmodium falciparum-infected erythrocytes are poorly understood. We tested the hypothesis that insertional disruption of the A4var gene, one of two var genes located within the subtelomeric region of one end of chromosome 13, would result in a preferential switch in transcription to the adjacent R29var gene upon rosette selection. In this way, we aimed to mimic the preferential transcription of R29var in rosetting R29 parasites, a parasite line in which the A4var gene is deleted through a chromosome end truncation. Initial analysis of the knock-out parasite lines shows that the insertional disruption of the A4var gene prevents A4 PfEMP1 expression, but that switching transcription to other var gene variants is unaffected. Furthermore, analysis of var transcription in the knock-out parasite line during rosette selection shows that, rather than facilitating a switch to R29var gene transcription, this event was suppressed in the transfectants. These data, and the implications for epigenetic transcriptional control of var genes, are discussed.

The truncated hemoglobin from Mycobacterium leprae

Truncated hemoglobins (trHb's) form a family of low molecular weight O2 binding hemoproteins distributed in eubacteria, protozoa, and plants. TrHb's branch in a distinct clade within the hemoglobin (Hb) superfamily. A unique globin gene has recently been identified from the complete genome sequence of Mycobacterium leprae that is predicted to encode a trHb (M. leprae trHbO). Sequence comparison and modelling considerations indicate that monomeric M. leprae trHbO has structural features typical of trHb's, such as 20-40 fewer residues than conventional globin chains, Gly-based sequence consensus motifs, likely assembling into a 2-on-2 alpha-helical sandwich fold, and hydrophobic residues recognized to build up the protein matrix ligand diffusion tunnel. The ferrous heme iron atom of deoxygenated M. leprae trHbO appears to be hexacoordinated, like in Arabidopsis thaliana trHbO-3 (A. thaliana trHbO-3). Accordingly, the value of the second-order rate constant for M. leprae trHbO carbonylation (7.3 x 10(3) M(-1) s(-1)) is similar to that observed for A. thaliana trHbO-3 (1.4 x 10(4) M(-1) s(-1)) and turns out to be lower than that reported for carbon monoxide binding to pentacoordinated Mycobacterium tuberculosis trHbN (6.7 x 10(6) M(-1) s(-1)). The lower reactivity of M. leprae trHbO as compared to M. tuberculosis trHbN might be related to the higher susceptibility of the leprosy bacillus to toxic nitrogen and oxygen species produced by phagocytic cells.

IFN-gamma and NO in mycobacterial disease: new jobs for old hands

Granulomatous disease following exposure to Mycobacterium tuberculosis, Mycobacterium leprae or Mycobacterium avium is correlated with strong inflammatory and protective responses. The mouse model of mycobacterial infection provides an excellent tool with which to examine the inter-relationship between protective cell-mediated immunity and tissue-damaging hypersensitivity. It is well established that T cells and interferon (IFN)-gamma are necessary components of anti-bacterial protection. We propose that IFN-gamma also modulates the local cellular response by downregulating lymphocyte activation and by driving T cells into apoptosis, and that the events that limit excessive inflammation are largely mediated by IFN-gamma-induced nitric oxide (NO). In several murine models of mycobacterial infection, the absence of IFN-gamma and/or NO results in dysregulated granuloma formation and increased lymphocytic responses, which, in the case of M. avium infection, even leads to reduced bacterial growth.

The study of Mycobacterium leprae infection in interferon-gamma gene--disrupted mice as a model to explore the immunopathologic spectrum of leprosy

Mycobacterium leprae infection was evaluated in interferon-gamma knockout (GKO) mice. At 4 months, growth of the bacilli in the footpads of GKO mice plateaued a log(10) higher than that in control mice. Control mice exhibited mild lymphocytic and histiocytic infiltrates, whereas GKO mice developed large, unorganized infiltrates of epithelioid macrophages and scattered CD4 and CD8 T cells. Flow cytometric analysis of popliteal lymph node cells demonstrated similar profiles of T cells; however, GKO cells exhibited an elevated proliferative response to M. leprae antigen. Expression of inducible nitric oxide synthase mRNA was decreased in GKO mice, whereas macrophage inflammatory protein-1alpha and interleukin-4 and -10 mRNA expression were augmented. Control and GKO activated macrophages inhibited bacterial metabolism and produced nitrite. Thus, although deficient in an important Th1 cytokine, GKO mice possess compensatory mechanisms to control M. leprae growth and feature elements resembling mid-borderline leprosy in humans.

Nitric oxide and the immune response

During the past two decades, nitric oxide (NO) has been recognized as one of the most versatile players in the immune system. It is involved in the pathogenesis and control of infectious diseases, tumors, autoimmune processes and chronic degenerative diseases. Because of its variety of reaction partners (DNA, proteins, low-molecular weight thiols, prosthetic groups, reactive oxygen intermediates), its widespread production (by three different NO synthases (NOS) and the fact that its activity is strongly influenced by its concentration, NO continues to surprise and perplex immunologists. Today, there is no simple, uniform picture of the function of NO in the immune system. Protective and toxic effects of NO are frequently seen in parallel. Its striking inter- and intracellular signaling capacity makes it extremely difficult to predict the effect of NOS inhibitors and NO donors, which still hampers therapeutic applications.

Cerebral malaria and immunogenetics

Cerebral malaria depends largely on the capacity of Plasmodium falciparum infected red blood cells to adhere to the endothelia of microvessels, leading to their occlusion. The most important players include receptors expressed on the surface of the endothelial cell and known to interact with the parasite, cytokines modulating the expression of these adhesion molecules and nitric oxide (NO). Platelets, monocytes and lymphocytes have the ability to adhere to these endothelial receptors and to one another, leading to a more complex situation and an increase in the degree of vessel occlusion. The polymorphism of all these molecules, implicated either in adhesion, in modulation of this adhesion or activation of the expression of diverse endothelial mediators should be an important field of study. Polymorphism of five of these molecules has been explored so far: ICAM-1, TNF-alpha, IL-1-beta, inducible NOS and complement receptor-1 (CR-1). To these studies can be added those concerning mannose binding protein (MBP), a protein playing a role in innate immunity, and the class-I antigen HLA-B53. To date, the only clear cut result concerns TNF-alpha. With the other polymorphisms, either no association is found (IL-1RA, CR-1, MBP), or the results are geographically heterogeneous (ICAM-1, HLA-B53), or contradictory (iNOS2). Most often, the candidate gene approach has been followed, as part of case control studies. One of the main problems in this approach is the difficulty of establishing the control cohort. This difficulty disappears in family studies, which include their own controls. So far, the only results based on complex segregation analysis have been focused on parasite multiplication and not on cerebral malaria.