Local activation of the innate immune system in Buruli ulcer lesions

Genetics in the Host-Mycobacterium ulcerans interaction

Buruli ulcer is an emerging infectious disease associated with high morbidity and unpredictable outbreaks. It is caused by Mycobacterium ulcerans, a slow-growing pathogen evolutionarily shaped by the acquisition of a plasmid involved in the production of a potent macrolide-like cytotoxin and by genome rearrangements and downsizing. These events culminated in an uncommon infection pattern, whereby M. ulcerans is both able to induce the initiation of the inflammatory cascade and the cell death of its proponents, as well as to survive within the phagosome and in the extracellular milieu. In such extreme conditions, the host is sentenced to rely on a highly orchestrated genetic landscape to be able to control the infection. We here revisit the dynamics of M. ulcerans infection, drawing parallels from other mycobacterioses and integrating the most recent knowledge on its evolution and pathogenicity in its interaction with the host immune response.

Vaccine-Specific Immune Responses against Mycobacterium ulcerans Infection in a Low-Dose Murine Challenge Model

The neglected tropical disease Buruli ulcer (BU) is an infection of subcutaneous tissue with Mycobacterium ulcerans There is no effective vaccine. Here, we assessed an experimental prime-boost vaccine in a low-dose murine tail infection model. We used the enoyl reductase (ER) domain of the M. ulcerans mycolactone polyketide synthases electrostatically coupled with a previously described Toll-like receptor 2 (TLR-2) agonist-based lipopeptide adjuvant, R₄Pam₂Cys. Mice were vaccinated and then challenged via tail inoculation with 14 to 20 CFU of a bioluminescent strain of M. ulcerans Mice receiving either the experimental ER vaccine or Mycobacterium bovis bacillus Calmette-Guérin (BCG) were equally protected, with both groups faring significantly better than nonvaccinated animals (P < 0.05). To explore potential correlates of protection, a suite of 29 immune parameters were assessed in the mice at the end of the experimental period. Multivariate statistical approaches were used to interrogate the immune response data to develop disease-prognostic models. High levels of interleukin 2 (IL-2) and low gamma interferon (IFN-γ) produced in the spleen best predicted control of infection across all vaccine groups. Univariate logistic regression revealed vaccine-specific profiles of protection. High titers of ER-specific IgG serum antibodies together with IL-2 and IL-4 in the draining lymph node (DLN) were associated with protection induced by the ER vaccine. In contrast, high titers of IL-6, tumor necrosis factor alpha (TNF-α), IFN-γ, and IL-10 in the DLN and low IFN-γ titers in the spleen were associated with protection following BCG vaccination. This study suggests that an effective BU vaccine must induce localized, tissue-specific immune profiles with controlled inflammatory responses at the site of infection.

Susceptibility to Mycobacterium ulcerans Disease (Buruli ulcer) Is Associated with IFNG and iNOS Gene Polymorphisms

Buruli ulcer (BU) is a chronic necrotizing disease of the skin and subcutaneous fat tissue. The causative agent, Mycobacterium ulcerans, produces mycolactone, a macrolide toxin, which causes apoptosis of mammalian cells. Only a small proportion of individuals exposed to M. ulcerans develop clinical disease, as surrounding macrophages may control the infection by bacterial killing at an early stage, while mycolactone concentration is still low. Otherwise, bacterial multiplication leads to in higher concentrations of mycolactone, with formation of necrotizing lesions that are no more accessible to immune cells. By typing a cohort of 96 Ghanaian BU patients and 384 endemic controls without BU, we show an association between BU and single nucleotide polymorphisms (SNPs) in iNOS (rs9282799) and IFNG (rs2069705). Both polymorphisms influence promoter activity in vitro. A previously reported SNP in SLC11A1 (NRAMP, rs17235409) tended to be associated with BU. Altogether, these data reflect the importance of IFNG signaling in early defense against M. ulcerans infection.

Mycolactone-mediated neurite degeneration and functional effects in cultured human and rat DRG neurons: Mechanisms underlying hypoalgesia in Buruli ulcer

Background

Mycolactone is a polyketide toxin secreted by the mycobacterium Mycobacterium ulcerans, responsible for the extensive hypoalgesic skin lesions characteristic of patients with Buruli ulcer. A recent pre-clinical study proposed that mycolactone may produce analgesia via activation of the angiotensin II type 2 receptor (AT₂R). In contrast, AT₂R antagonist EMA401 has shown analgesic efficacy in animal models and clinical trials for neuropathic pain. We therefore investigated the morphological and functional effects of mycolactone in cultured human and rat dorsal root ganglia (DRG) neurons and the role of AT₂R using EMA401. Primary sensory neurons were prepared from avulsed cervical human DRG and rat DRG; 24 h after plating, neurons were incubated for 24 to 96 h with synthetic mycolactone A/B, followed by immunostaining with antibodies to PGP9.5, Gap43, β tubulin, or Mitotracker dye staining. Acute functional effects were examined by measuring capsaicin responses with calcium imaging in DRG neuronal cultures treated with mycolactone.

Results

Morphological effects: Mycolactone-treated cultures showed dramatically reduced numbers of surviving neurons and non-neuronal cells, reduced Gap43 and β tubulin expression, degenerating neurites and reduced cell body diameter, compared with controls. Dose-related reduction of neurite length was observed in mycolactone-treated cultures. Mitochondria were distributed throughout the length of neurites and soma of control neurons, but clustered in the neurites and soma of mycolactone-treated neurons. Functional effects: Mycolactone-treated human and rat DRG neurons showed dose-related inhibition of capsaicin responses, which were reversed by calcineurin inhibitor cyclosporine and phosphodiesterase inhibitor 3-isobutyl-1-Methylxanthine, indicating involvement of cAMP/ATP reduction. The morphological and functional effects of mycolactone were not altered by Angiotensin II or AT₂R antagonist EMA401.

Conclusion

Mycolactone induces toxic effects in DRG neurons, leading to impaired nociceptor function, neurite degeneration, and cell death, resembling the cutaneous hypoalgesia and nerve damage in individuals with M. Ulcerans infection.

Local Cellular Immune Responses and Pathogenesis of Buruli Ulcer Lesions in the Experimental Mycobacterium Ulcerans Pig Infection Model

Background

Buruli ulcer is a neglected tropical disease of the skin that is caused by infection with Mycobacterium ulcerans. We recently established an experimental pig (Sus scrofa) infection model for Buruli ulcer to investigate host-pathogen interactions, the efficacy of candidate vaccines and of new treatment options.

Methodology/Principal Findings

Here we have used the model to study pathogenesis and early host-pathogen interactions in the affected porcine skin upon infection with mycolactone-producing and non-producing M. ulcerans strains. Histopathological analyses of nodular lesions in the porcine skin revealed that six weeks after infection with wild-type M. ulcerans bacteria extracellular acid fast bacilli were surrounded by distinct layers of neutrophils, macrophages and lymphocytes. Upon ulceration, the necrotic tissue containing the major bacterial burden was sloughing off, leading to the loss of most of the mycobacteria. Compared to wild-type M. ulcerans bacteria, toxin-deficient mutants caused an increased granulomatous cellular infiltration without massive tissue necrosis, and only smaller clusters of acid fast bacilli.

Conclusions/Significance

In summary, the present study shows that the pathogenesis and early immune response to M. ulcerans infection in the pig is very well reflecting BU disease in humans, making the pig infection model an excellent tool for the profiling of new therapeutic and prophylactic interventions.

Buruli ulcer is a necrotizing ulcerative disease of the skin and underlying tissue caused by infection with Mycobacterium ulcerans. Because patients often present late to health facilities, early stages of Buruli ulcer are only insufficiently described by histopathology. To study early host-pathogen interactions, we recently established an experimental pig infection model for Buruli ulcer. Here we used the model to study the pathogenesis and the local cellular immune responses upon infection with mycolactone-producing and non-producing M. ulcerans strains. Infection with toxin-producing bacteria led to the development of nodular lesions six weeks after infection, in which extracellular clumps of acid fast bacilli were surrounded by distinct layers of leukocytes. Ulceration of the nodular lesions subsequently led to the loss of most of the bacterial burden. In contrast, after infection with toxin-deficient M. ulcerans bacteria increased granulomatous cellular infiltration was observed, and massive tissue necrosis was absent. Pathogenesis as well as early immune responses to M. ulcerans infection in the pig is very well reflecting the human disease, making it a good model for the evaluation of the efficacy of new treatment options and candidate vaccines.

Interferon-γ Is a Crucial Activator of Early Host Immune Defense against Mycobacterium ulcerans Infection in Mice

Buruli ulcer (BU), caused by infection with Mycobacterium ulcerans, is a chronic necrotizing human skin disease associated with the production of the cytotoxic macrolide exotoxin mycolactone. Despite extensive research, the type of immune responses elicited against this pathogen and the effector functions conferring protection against BU are not yet fully understood. While histopathological analyses of advanced BU lesions have demonstrated a mainly extracellular localization of the toxin producing acid fast bacilli, there is growing evidence for an early intra-macrophage growth phase of M. ulcerans. This has led us to investigate whether interferon-γ might play an important role in containing M. ulcerans infections. In an experimental Buruli ulcer mouse model we found that interferon-γ is indeed a critical regulator of early host immune defense against M. ulcerans infections. Interferon-γ knockout mice displayed a faster progression of the infection compared to wild-type mice. This accelerated progression was reflected in faster and more extensive tissue necrosis and oedema formation, as well as in a significantly higher bacterial burden after five weeks of infection, indicating that mice lacking interferon-γ have a reduced capacity to kill intracellular bacilli during the early intra-macrophage growth phase of M. ulcerans. This data demonstrates a prominent role of interferon-γ in early defense against M. ulcerans infection and supports the view that concepts for vaccine development against tuberculosis may also be valid for BU.

Mycobacterium ulcerans is the causative agent of Buruli ulcer (BU), a slow progressing ulcerative skin disease. The mode of transmission of M. ulcerans remains unknown and only little is known about the early stages of the disease and the nature of protective immune responses against this pathogen. Given the increasing evidence for an early intracellular growth phase of M. ulcerans, we aimed at evaluating the impact of cell-mediated immunity for immunological defense against M. ulcerans infections. By comparing wild-type and interferon-γ-deficient mice in a BU mouse model, we could demonstrate that interferon-γ is a critical regulator of early host immune defense against M. ulcerans infections, indicative for an important role of early intracellular multiplication of the pathogen. In mice lacking interferon-γ the bacterial burden increased faster, resulting in accelerated pathogenesis. The observed differences between the two mouse strains were most likely due to differences in the capacity of macrophages to kill intracellular bacilli during the early stages of infection.

Challenges Associated with Management of Buruli Ulcer/Human Immunodeficiency Virus Coinfection in a Treatment Center in Ghana: A Case Series Study

The synergy between Mycobacterium tuberculosis infection and human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome is well established but not so in Buruli ulcer (BU). We screened confirmed BU cases for HIV infection and followed seven BU/HIV-coinfected patients. Management of BU/HIV was based on the World Health Organization guidelines and patient condition. The HIV positivity among BU patients (8.2%; 11/134) was higher compared with that of general patients attending the facility (4.8%; 718/14,863; P = 0.07) and that of pregnant women alone (2.5%; 279/11,125; P = 0.001). All seven BU/HIV-coinfected cases enrolled in the study presented with very large (category III) lesions with four having multiple lesions compared with 54.5% of category III lesions among HIV-negative BU patients. During the recommended BU treatment with streptomycin and rifampicin (SR) all patients developed immune infiltrates including CD4 T cells in their lesions. However, one patient who received antiretroviral therapy (ART) 1 week after beginning SR treatment developed four additional lesions during antibiotic treatment, while two out of the four who did not receive ART died. Further evidence is required to ascertain the most appropriate time to commence ART in relation to SR treatment to minimize paradoxical reactions.

Experimental infection of the pig with Mycobacterium ulcerans: a novel model for studying the pathogenesis of Buruli ulcer disease

Background

Buruli ulcer (BU) is a slowly progressing, necrotising disease of the skin caused by infection with Mycobacterium ulcerans. Non-ulcerative manifestations are nodules, plaques and oedema, which may progress to ulceration of large parts of the skin. Histopathologically, BU is characterized by coagulative necrosis, fat cell ghosts, epidermal hyperplasia, clusters of extracellular acid fast bacilli (AFB) in the subcutaneous tissue and lack of major inflammatory infiltration. The mode of transmission of BU is not clear and there is only limited information on the early pathogenesis of the disease available.

Methodology/Principal Findings

For evaluating the potential of the pig as experimental infection model for BU, we infected pigs subcutaneously with different doses of M. ulcerans. The infected skin sites were excised 2.5 or 6.5 weeks after infection and processed for histopathological analysis. With doses of 2×10⁷ and 2×10⁶ colony forming units (CFU) we observed the development of nodular lesions that subsequently progressed to ulcerative or plaque-like lesions. At lower inoculation doses signs of infection found after 2.5 weeks had spontaneously resolved at 6.5 weeks. The observed macroscopic and histopathological changes closely resembled those found in M. ulcerans disease in humans.

Conclusion/Significance

Our results demonstrate that the pig can be infected with M. ulcerans. Productive infection leads to the development of lesions that closely resemble human BU lesions. The pig infection model therefore has great potential for studying the early pathogenesis of BU and for the development of new therapeutic and prophylactic interventions.

Buruli ulcer caused by Mycobacterium ulcerans infection is a necrotizing disease of the skin and the underlying subcutaneous tissue. Since the skin of pigs (Sus scrofa) has striking structural and physiological similarities with human skin, we investigated whether it is possible to develop an experimental M. ulcerans infection model by subcutaneous injection of the mycobacteria into pig skin. Injection of 2×10⁶ or 2×10⁷ colony forming units of M. ulcerans led to the development of lesions that were both macroscopically and microscopically very similar to human Buruli ulcer lesions. In particular for the characterization of the pathogenesis of Buruli ulcer and of immune defence mechanisms against M. ulcerans, the pig model appears to be superior to the mouse foot pad model commonly used for the evaluation of the efficacy of chemotherapeutic regimens.

History, biology and chemistry of Mycobacterium ulcerans infections (Buruli ulcer disease)

Mycobacterium ulcerans infections (Buruli ulcer disease) have a long history that can be traced back 150 years. The successive discoveries of the mycobacteria in 1948 and of mycolactone A/B in 1999, the toxin responsible for this dramatic necrotic skin disease, resulted in a paradigm shift concerning the disease itself and in a broader sense, delineated an entirely new role for bioactive polyketides as virulence factors. The fascinating history, biology and chemistry of M. ulcerans infections are discussed in this review.

Microbiological, histological, immunological, and toxin response to antibiotic treatment in the mouse model of Mycobacterium ulcerans disease

Mycobacterium ulcerans infection causes a neglected tropical disease known as Buruli ulcer that is now found in poor rural areas of West Africa in numbers that sometimes exceed those reported for another significant mycobacterial disease, leprosy, caused by M. leprae. Unique among mycobacterial diseases, M. ulcerans produces a plasmid-encoded toxin called mycolactone (ML), which is the principal virulence factor and destroys fat cells in subcutaneous tissue. Disease is typically first manifested by the appearance of a nodule that eventually ulcerates and the lesions may continue to spread over limbs or occasionally the trunk. The current standard treatment is 8 weeks of daily rifampin and injections of streptomycin (RS). The treatment kills bacilli and wounds gradually heal. Whether RS treatment actually stops mycolactone production before killing bacilli has been suggested by histopathological analyses of patient lesions. Using a mouse footpad model of M. ulcerans infection where the time of infection and development of lesions can be followed in a controlled manner before and after antibiotic treatment, we have evaluated the progress of infection by assessing bacterial numbers, mycolactone production, the immune response, and lesion histopathology at regular intervals after infection and after antibiotic therapy. We found that RS treatment rapidly reduced gross lesions, bacterial numbers, and ML production as assessed by cytotoxicity assays and mass spectrometric analysis. Histopathological analysis revealed that RS treatment maintained the association of the bacilli with (or within) host cells where they were destroyed whereas lack of treatment resulted in extracellular infection, destruction of host cells, and ultimately lesion ulceration. We propose that RS treatment promotes healing in the host by blocking mycolactone production, which favors the survival of host cells, and by killing M. ulcerans bacilli.

Mycobacterium ulcerans infection causes Buruli ulcer (BU), a disfiguring skin disease now found principally in poor rural areas of West Africa. M. ulcerans produces a toxin called mycolactone (ML), which destroys fat cells in skin tissue. BU typically first shows as a nodule that eventually ulcerates. The lesions may continue to spread over limbs or occasionally the trunk. The current standard treatment is 8 weeks of daily rifampin and injections of streptomycin (RS). The treatment kills the bacilli and wounds gradually heal. We tried to determine if RS treatment actually stops mycolactone production before killing bacilli. Using a mouse footpad model of M. ulcerans infection where the time of infection and lesion development can be followed in a controlled manner before and after antibiotic treatment, we found that RS treatment rapidly reduced footpad swelling, M. ulcerans numbers, and ML production. Microscopic analysis of footpads revealed that RS treatment resulted in bacilli being destroyed by host cells whereas lack of treatment resulted in extracellular infection, destruction of host cells, and lesion ulceration. We propose that RS treatment promotes healing in the host by blocking mycolactone production, which favors the survival of host cells, and by killing M. ulcerans.

Mycobacterium ulcerans triggers T-cell immunity followed by local and regional but not systemic immunosuppression

Buruli ulcer is a neglected infectious disease caused by Mycobacterium ulcerans and is characterized by necrotic cutaneous lesions induced by the exotoxin mycolactone. Despite evidence of Th1-mediated protective immunity, M. ulcerans infection has been associated with systemic immunosuppression. We show that early during mouse infection with either mycolactone-positive or negative strains, pathogen-specific gamma interferon (IFN-γ)-producing T cells developed in the draining lymph node (DLN). CD4(+) cells migrated to the infection foci, but progressive infection with virulent M. ulcerans led to the local depletion of recruited cells. Moreover, dissemination of virulent M. ulcerans to the DLN was accompanied by extensive DLN apoptotic cytopathology, leading to depletion of CD4(+) T cells and abrogation of IFN-γ expression. Advanced footpad infection with virulent M. ulcerans did not induce increased susceptibility to systemic coinfection by Listeria monocytogenes. These results show that infection with M. ulcerans efficiently triggers a mycobacterium-specific T-cell response in the DLN and that progression of infection with highly virulent M. ulcerans leads to a local and regional suppression of that immune response, but without induction of systemic immunosuppression. These results suggest that prophylactic and/or therapeutic interventions to prevent dissemination of M. ulcerans to DLN during the early phase of infection would contribute for the maintenance of protective immunity and disease control.

IFN-gamma-dependent activation of macrophages during experimental infections by Mycobacterium ulcerans is impaired by the toxin mycolactone

Buruli ulcer, caused by Mycobacterium ulcerans infections, is a necrotizing skin disease whose pathogenesis is associated with the exotoxin mycolactone. Despite the relevance of this emergent disease, little is known on the immune response against the pathogen. Following the recent demonstration of an intramacrophage growth phase for M. ulcerans, we investigated the biological relevance of IFN-gamma and the antimycobacterial mechanisms activated by this cytokine in M. ulcerans-infected macrophages. Three M. ulcerans strains were tested: 5114 (mutant mycolactone-negative, avirulent strain); 94-1327 (intermediate virulence); and 98-912 (high virulence). We show in this study that IFN-gamma is expressed in mouse-infected tissues and that IFN-gamma-deficient mice display increased susceptibility to infection with strains 5114 and, to a lesser extent, 94-1327, but not with the highly virulent strain. Accordingly, IFN-gamma-activated cultured macrophages controlled the proliferation of the avirulent and the intermediate virulent strains. Addition of mycolactone purified from strain 98-912 to cultures of IFN-gamma-activated macrophages infected with the mycolactone-negative strain led to a dose-dependent inhibition of the IFN-gamma-induced protective mechanisms, involving phagosome maturation/acidification and increased NO production, therefore resulting in increased bacterial burdens. Our findings suggest that the protection mediated by IFN-gamma in M. ulcerans-infected macrophages is impaired by the local buildup of mycolactone.

Pathogenetic mechanisms of the intracellular parasite Mycobacterium ulcerans leading to Buruli ulcer

The necrotising skin infection Buruli ulcer is at present the third most common human mycobacteriosis worldwide, after tuberculosis and leprosy. Buruli ulcer is an emergent disease that is predominantly found in humid tropical regions. There is no vaccine against Buruli ulcer and its treatment is difficult. In addition to the huge social effect, Buruli ulcer is of great scientific interest because of the unique characteristics of its causative organism, Mycobacterium ulcerans. This pathogen is genetically very close to the typical intracellular parasites Mycobacterium marinum and Mycobacterium tuberculosis. We review data supporting the interpretation that M ulcerans has the essential hallmarks of an intracellular parasite, producing infections associated with immunologically relevant inflammatory responses, cell-mediated immunity, and delayed-type hypersensitivity. This interpretation judges that whereas M ulcerans behaves like the other pathogenic mycobacteria, it represents an extreme in the biodiversity of this family of pathogens because of its higher cytotoxicity due to the secretion of the exotoxin mycolactone. The acceptance of the interpretation that Buruli ulcer is caused by an intracellular parasite has relevant prophylactic and therapeutic implications, rather than representing the mere attribution of a label with academic interest, because it prompts the development of vaccines that boost cell-mediated immunity and the use of chemotherapeutic protocols that include intracellularly active antibiotics.

Mycolactone inhibits monocyte cytokine production by a posttranscriptional mechanism

The virulence and immunosuppressive activity of Mycobacterium ulcerans is attributed to mycolactone, a macrolide toxin synthesized by the bacteria. We have explored the consequence and mechanism of mycolactone pretreatment of primary human monocytes activated by a wide range of TLR ligands. The production of cytokines (TNF, IL-1beta, IL-6, IL-10, and IFN-gamma-inducible protein-10), chemokines (IL-8), and intracellular effector molecules (exemplified by cyclooxygenase-2) was found to be powerfully and dose dependently inhibited by mycolactone, irrespective of the stimulating ligand. However, mycolactone had no effect on the activation of signaling pathways that are known to be important in inducing these genes, including the MAPK and NF-kappaB pathways. Unexpectedly, LPS-dependent transcription of TNF, IL-6, and cyclooxygenase-2 mRNA was found not to be inhibited, implying that mycolactone has a novel mechanism of action and must function posttranscriptionally. We propose that mycolactone mediates its effects by inhibiting the translation of a specific subset of proteins in primary human monocytes. This mechanism is distinct from rapamycin, another naturally occurring immunosuppressive lactone. The current findings also suggest that monocyte-derived cytokine transcript and protein levels may not correlate in Buruli ulcer lesions, and urge caution in the interpretation of RT-PCR data obtained from patient biopsy samples.

Buruli ulcer: reductive evolution enhances pathogenicity of Mycobacterium ulcerans

Buruli ulcer is an emerging human disease caused by infection with a slow-growing pathogen, Mycobacterium ulcerans, that produces mycolactone, a cytotoxin with immunomodulatory properties. The disease is associated with wetlands in certain tropical countries, and evidence for a role of insects in transmission of this pathogen is growing. Comparative genomic analysis has revealed that M. ulcerans arose from Mycobacterium marinum, a ubiquitous fast-growing aquatic species, by horizontal transfer of a virulence plasmid that carries a cluster of genes for mycolactone production, followed by reductive evolution. Here, the ecology, microbiology, evolutionary genomics and immunopathology of Buruli ulcer are reviewed.

Phagocytosis of Mycobacterium ulcerans in the course of rifampicin and streptomycin chemotherapy in Buruli ulcer lesions

BACKGROUND

Infection with Mycobacterium ulcerans involves a devastating skin disease called Buruli ulcer (BU). Currently, dual therapy with rifampicin and streptomycin (R/S) for 8 weeks as well as surgery are the standard treatments.

OBJECTIVES

To elucidate the processes taking place in BU lesions in the course of chemotherapy we performed an in-depth histological analysis of lesions after 4 weeks of R/S treatment and compared results with findings in untreated lesions and lesions treated for 8 weeks.

METHODS

Tissue specimens were collected from patients who had no treatment and from patients after 4 and 8 weeks of R/S treatment. The main features evaluated were local immune responses, histopathological alterations and bacterial distribution.

RESULTS

After 4 weeks of R/S treatment we observed a large proportion of mycobacteria inside macrophages, occasionally forming globus-like aggregations. While distinct bands of inflammatory leucocytes surrounded the necrotic core in an ulcer and early granuloma formation was apparent in the healthy-appearing margins, acute cellular infiltration covering the whole lesion had developed in a nodular lesion. In contrast, ulcerative lesions after 8 weeks of chemotherapy showed intra- and extracellular bacterial debris as well as the presence of extensive chronic infiltrates forming huge granulomas.

CONCLUSIONS

R/S treatment of BU results in a rapid onset of local cellular immune responses associated with phagocytosis of the extracellular M. ulcerans. This may be related to declining levels of the macrolide toxin mycolactone in the tissue, thus leading to an enhanced chemotherapy-induced clearance of the infection.

Mycolactone diffuses from Mycobacterium ulcerans-infected tissues and targets mononuclear cells in peripheral blood and lymphoid organs

BACKGROUND

Buruli ulcer (BU) is a progressive disease of subcutaneous tissues caused by Mycobacterium ulcerans. The pathology of BU lesions is associated with the local production of a diffusible substance, mycolactone, with cytocidal and immunosuppressive properties. The defective inflammatory responses in BU lesions reflect these biological properties of the toxin. However, whether mycolactone diffuses from infected tissues and suppresses IFN-gamma responses in BU patients remains unclear.

METHODOLOGY/PRINCIPAL FINDINGS

Here we have investigated the pharmacodistribution of mycolactone following injection in animal models by tracing a radiolabeled form of the toxin, and by directly quantifying mycolactone in lipid extracts from internal organs and cell subpopulations. We show that subcutaneously delivered mycolactone diffused into mouse peripheral blood and accumulated in internal organs with a particular tropism for the spleen. When mice were infected subcutaneously with M. ulcerans, this led to a comparable pattern of distribution of mycolactone. No evidence that mycolactone circulated in blood serum during infection could be demonstrated. However, structurally intact toxin was identified in the mononuclear cells of blood, lymph nodes and spleen several weeks before ulcerative lesions appear. Importantly, diffusion of mycolactone into the blood of M. ulcerans-infected mice coincided with alterations in the functions of circulating lymphocytes.

CONCLUSION

In addition to providing the first evidence that mycolactone diffuses beyond the site of M. ulcerans infection, our results support the hypothesis that the toxin exerts immunosuppressive effects at the systemic level. Furthermore, they suggest that assays based on mycolactone detection in circulating blood cells may be considered for diagnostic tests of early disease.

Localised Mycobacterium ulcerans infection in a cat in Australia

A 10-year-old castrated male domestic cat domiciled in eastern Victoria (Australia) was presented for a subcutaneous mass on its nasal bridge in November 2006. Cytological examination of an aspirate demonstrated pyogranulomatous inflammation. At surgery, the lesion consisted of an encapsulated mass containing viscid fluid. Histological examination of the resected lesion revealed pyogranulomatous inflammation surrounding a central zone of necrosis. Sections stained with the Ziehl–Neelsen method revealed numerous acid-fast bacilli, intracellularly within macrophages and extracellularly. Molecular studies established the infection was caused by Mycobacterium ulcerans. As histology demonstrated that the infection extended to the margin of the excised tissues, the cat was treated subsequently with clarithromycin (62.5 mg orally once daily for 7 days, then twice daily for 3 months). The surgical wound healed unremarkably. The infection has not recurred at the time of writing, 1 year following discontinuation of treatment. Although M ulcerans infections have been recorded in variety of mammals, this is the first known case in a cat.

Development of highly organized lymphoid structures in Buruli ulcer lesions after treatment with rifampicin and streptomycin

BACKGROUND

Buruli ulcer caused by Mycobacterium ulcerans is an infection of the subcutaneous tissue leading to chronic necrotising skin ulcers. The pathogenesis is associated with the cytocidal and immunosuppressive activities of a macrolide toxin. Histopathological hallmark of progressing disease is a poor inflammatory response despite of clusters of extracellular bacilli. While traditionally wide excision of the infected tissue was the standard treatment, provisional WHO guidelines now recommend an eight week pre-treatment with streptomycin and rifampicin.

METHODOLOGY/PRINCIPAL FINDINGS

We conducted a detailed immunohistochemical analysis of tissue samples from Buruli patients who received antibiotic treatment. Cellular immune response along with bacterial load and distribution were monitored. We demonstrate that this treatment leads to the development of highly organized cellular infiltration surrounding areas of coagulative necrosis. Diffuse infiltrates, granulomas and dense lymphocyte aggregation close to vessels were observed. Mycobacterial material was primarily located inside mononuclear phagocytes and microcolonies consisting of extracellular rod-shaped mycobacteria were no longer found. In observational studies some patients showed no clinical response to antibiotic treatment. Corresponding to that, one of five lesions analysed presented with huge clusters of rod-shaped bacilli but no signs of infiltration.

CONCLUSIONS/SIGNIFICANCE

Results signify that eight weeks of antibiotic treatment reverses local immunosuppression and leads to an active inflammatory process in different compartments of the skin. Structured leukocyte infiltrates with unique signatures indicative for healing processes developed at the margins of the lesions. It remains to be analysed whether antibiotic resistance of certain strains of M. ulcerans, lacking patient compliance or poor drug quality are responsible for the absent clinical responses in some patients. In future, analysis of local immune responses could serve as a suitable surrogate marker for the efficacy of alternative treatment strategies.

Mycolactone-mediated inhibition of tumor necrosis factor production by macrophages infected with Mycobacterium ulcerans has implications for the control of infection

The pathogenicity of Mycobacterium ulcerans, the agent of Buruli ulcer, depends on the cytotoxic exotoxin mycolactone. Little is known about the immune response to this pathogen. Following the demonstration of an intracellular growth phase in the life cycle of M. ulcerans, we investigated the production of tumor necrosis factor (TNF) induced by intramacrophage bacilli of diverse toxigenesis/virulence, as well as the biological relevance of TNF during M. ulcerans experimental infections. Our data show that murine bone marrow-derived macrophages infected with mycolactone-negative strains of M. ulcerans (nonvirulent) produce high amounts of TNF, while macrophages infected with mycolactone-positive strains of intermediate or high virulence produce intermediate or low amounts of TNF, respectively. These results are in accordance with the finding that TNF receptor P55-deficient (TNF-P55 KO) mice are not more susceptible than wild-type mice to infection by the highly virulent strains but are more susceptible to nonvirulent and intermediately virulent strains, demonstrating that TNF is required to control the proliferation of these strains in animals experimentally infected by M. ulcerans. We also show that mycolactone produced by intramacrophage M. ulcerans bacilli inhibits, in a dose-dependent manner, but does not abrogate, the production of macrophage inflammatory protein 2, which is consistent with the persistent inflammatory responses observed in experimentally infected mice.