Both
Mycobacterium ulcerans and the leprosy bacillus have a low optimal growth temperature of 30–33 °C, which is considered a major factor for the skin tropism and the limited systemic dissemination of the infections they cause. Both pathogens cause chronic infections with serious skin manifestations but have developed two very distinct survival strategies in the human host. In advanced BU lesions, clusters of extracellular
M. ulcerans reside in necrotic tissue areas largely devoid of infiltrating leukocytes and thus mostly inaccessible to the immune system [
1]. Immune evasion of
M. ulcerans is facilitated by mycolactone and in addition by the loss of immunodominant antigens during the course of reductive evolution [
2,
3]. Also
M. leprae has adapted to mammalian host cells via rigorous genome reduction and loss of gene function, but in contrast to
M. ulcerans,
M. leprae has evolved to become an obligate intracellular microorganism, able to invade, and replicate in phagocytic cells and Schwann cells [
4]. Polarization of the immune response toward either T
H1-type cytokine profiles restricting growth of the bacilli or T
H2-type responses resulting in progressive infection has been identified as one of the key determinants for the outcome of an exposure to
M. leprae in mouse infection models. However, other T lymphocyte subsets identified in recent years in humans may cause effects that go beyond this classical T
H1/T
H2 paradigm [
5]. Considering evidence of an intracellular stage of
M. ulcerans early after infection [
6], cell-mediated immunity may also be important for the containment of BU.
Conclusion: Intracellular vs necrotic hideout—a comparison of lifestyles
The current strategy to fight the neglected tropical skin diseases BU and leprosy relies on active case finding and antibiotic treatment, which in early stages of the diseases is curative and can prevent the disabling and stigmatizing long-term effects of the diseases. Both diseases have certain features in common but differ also profoundly in pathogenesis and transmission (Table
1).
M. ulcerans and
M. leprae both cause various types of skin lesions that can be relatively unspecific. This complicates the diagnosis of the two diseases, as no sufficiently sensitive and specific diagnostic point of care tests are available. If untreated, early lesions may evolve to serious dermatological and neurologic (leprosy) manifestations. The imminent threat of antibiotic resistance calls for new therapeutic options for both diseases.
Table 1Comparison of BU and leprosy: common features and profound differences
Causative agent | Mycobacterium ulcerans (first isolated in 1948) | Mycobacterium leprae (first isolated in 1873); Mycobacterium lepromatosis (first isolated in 2008) |
Clinical manifestations | Skin lesions |
Pre-ulcerative nodules, papules, plaques, and edema; in advanced stages chronic, necrotizing ulcers affecting skin, subcutaneous tissue, and sometimes bones (osteomyelitis) | Papules, macules, nodules, plaques, hypochromic or not, scattered or disseminated; in specific forms: necrotizing ulcers, nerve lesions: thickness, tenderness, loss of sensation, hypo or total anesthesia, in advanced stages: deformities of the hands and feet, impairment of the eyes, soft tissues, and bones, endocrine dysfunction (sterility, osteoporosis, hypothyroidism) |
WHO categories | Category I: single lesion < 5 cm in diameter. Category II: single lesion ≥ 5 to 15 cm in diameter, plaque, and edematous forms. Category III: single lesion > 15 cm in diameter, multiple lesions, lesion at critical sites, osteomyelitis | Paucibacillary: 5 or less skin lesions Multibacillary: more than 5 skin lesions |
Diagnosis | Clinical signs and IS2404 PCR in reference laboratories as gold standard for laboratory diagnosis; microscopy has limited sensitivity | Clinical signs and identification of bacteria in lesions; positive RLEP PCR in difficult to diagnose single lesions or pure neural leprosy cases |
Sensitive and specific point of care diagnostic tests needed |
Immunodiagnosis | No specific test available. In a large proportion of healthy individuals living in BU endemic areas serological responses and response to whole-cell lysate of M. ulcerans, most noticeable in people with healed lesions | None specific to all leprosy forms. Anti-PGL-I serological response is specific for multibacillary cases |
Treatment | Combination therapy with rifampicin and clarithromycin for 8 weeks | Multibacillary: rifampicin, dapsone, and clofazimine, 12 months. Paucibacillary: rifampicin and dapsone, 6 months |
Shorter regimens with less severe side effects needed |
Genome | 5.6 Mb, ~ 4160 CDS, 771 pseudogenes; 213 copies of IS2404, and 91 copies of IS2606 (in strain Agy99); compared to M. marinum M (6.6 Mb), genome decay including loss of genes associated with intracellular lifestyle in other mycobacteria; acquisition of virulence plasmid (pMUM; 174 kb) | 3.2 Mb, ~ 1600 CDS, 1100 pseudogenes; up to 37 copies of the specific repetitive element RLEP in M. leprae, absent in M. lepromatosis |
Genome reduction combined with loss of function mutations; niche-adapted pathogens |
Virulence factors | Cytotoxic and immunosuppressive effects of mycolactone; immune evasion facilitated by loss of immunodominant antigens | Unknown; probably ESX-1 system and PGL-I for neuropathy |
Burden | More than 30,000 cases reported to WHO in the past 10 years; 2713 new cases in 2018; most likely substantial underreporting | > 200,000 new cases/year reported for the last 10 years; 208,613 new cases in 2018 |
Geographical distribution | Highly focal occurrence; most cases caused by classical lineage in West and Central Africa, Australia, PNG; sporadic cases by ancestral lineage in the Americas and Asia. Strong association of BU with stagnant water bodies | M. leprae: worldwide with 85% of new cases in India, Brazil, and Indonesia. M. lepromatosis: Mexico and the Caribbean region |
Prevention | No vaccine candidate ready for clinical testing. Limited knowledge of transmission pathways and preventable risks | Vaccine in phase 1 clinical trial |
Mode(s) of transmission | Unknown |
Entry of mycobacteria into hosts presumably via penetration of skin through injuries and/or insect vectors; low probability of person-to-person transmission | Entry of mycobacteria into hosts presumably via the nose or skin; possible transmission via nasal droplets. Probable person-to-person transmission; zoonotic spread through contact with armadillos in the Southern United States and Brazil |
Reservoir(s) | Unknown; presumed involvement of environmental and animal reservoirs; possums identified as animal reservoir in Australia | Mainly human; animal reservoirs: armadillos (USA), red squirrels (Europe), non-human primates (Africa, Philippines) |
Incubation period | Can only be estimated due to limited knowledge on mode(s) of transmission. Mean incubation period of 4.5 months in Australia, may be shorter in Africa | 3 to 20 years |
Compartment | Extracellular in advanced lesions; early intracellular phase suspected | Obligate intracellular |
Protective immunity | Mechanisms unknown; probably important role for cell-mediated immunity (against M. ulcerans in early disease stages) |
Effect of HIV infection | HIV infection increases both risk and severity of disease | No increase in risk but in severity due to the immune reconstitution inflammatory syndrome (IRIS) after initiation of HIV treatment |
Immunological complications | Massive infiltration of lesions during antibiotic treatment; immune reconstitution inflammatory syndrome (IRIS)-like effects may in some patients cause delay in wound healing (“paradoxical reactions”) | Leprosy reactions (30–50% of cases) are characterized by an increased cellular response (reversal reaction). Mechanisms of ENL and Lucio’s phenomenon are unknown |
Age spectrum and gender distribution | In Africa bimodal age distribution; BU mainly affects children between 4 and 15 years and the elderly population; overall balanced male:female ratio | All age groups affected; children are mostly diagnosed with mild forms such as indeterminate or tuberculoid leprosy |
Host genetic factors | SNPs in the inducible nitric oxide synthase gene iNOS and in the interferon gamma gene IFNG, the natural resistance-associated macrophage protein gene SLC11A1 (NRAMP1), and the autophagy-related E3 ubiquitin protein ligase gene PARK2 have been linked to susceptibility to BU | SNPs in genes from the innate immune recognition, type I IFN, autophagy, lipid and energy metabolism, and the regulatory regions of PARK2 have been linked to susceptibility to leprosy |
Spontaneous healing; exposure and disease | Anecdotal descriptions of spontaneous healing; sero-epidemiological studies indicate that only a small proportion of exposed individuals develop disease |
Long-term consequences | Catastrophic household expenditure for treatment; social isolation of patients during treatment and thereafter (stigma); permanent disabilities |
M. ulcerans and
M. leprae patient isolates are characterized by limited genetic diversity. Both pathogens have gone through massive gene decay (loss of gene function through pseudogenization and genome reduction) in the course of their evolution, probably linked to the adaptation to new lifestyles and more stable niche environments [
2,
12,
136].
M. ulcerans has evolved from a common progenitor with
M. marinum, a process first and foremost enhanced by the acquisition of the virulence plasmid pMUM. It seems likely that the ability of
M. ulcerans to produce mycolactone was a first step in the emergence of a highly clonal new species (MPM, which are now all designated
M. ulcerans) with increased virulence compared with
M. marinum, which only occasionally causes granulomatous skin infections in humans [
137]. Additional changes in the chromosome, such as those leading to the loss of expression of the highly immunogenic proteins ESAT-6 and CFP-10 in
M. ulcerans, also appear to contribute to the increase in virulence by reducing immunogenicity of the bacteria. On the other hand, this gene decay most likely also limits the ability of this largely extracellular pathogen to have an intracellular lifestyle. In contrast, the obligate intracellular leprosy bacilli have a functional ESX-1 virulence system, and there is no report of plasmid acquisition [
136]. A key feature of the
M. leprae genome is the even larger extent of gene deletions and of pseudogenization (pseudogenes occupy about 40% of the
M. leprae chromosome). The resulting loss of gene function accounts for the failure to culture
M. leprae in vitro, and loss of gene function mutations are most likely also responsible for the exceptionally slow growth rate of both
M. leprae and
M. ulcerans. Genomic data revealed that
M. leprae strains have not drastically evolved in the past 4000 years, and the only breakthrough suggesting the existence of an
M. leprae complex is the discovery of
M. lepromatosis in Mexico [
82,
138]. Both pathogens are able to cause leprosy-like lesions, but they genetically diverged 14 million years ago [
82].
Leprosy is one of the oldest diseases known to mankind, having plagued humans for thousands of years. Despite the decrease in the prevalence of leprosy to less than 1 case per 100,000 population since the year 2000, the incidence has plateaued at 200,000 to 250,000 new reported cases per year globally over the past 10 years with the brunt of the burden falling on only a few countries in Asia, Africa, and Latin America. The first cases of BU were reported in the late nineteenth century, mainly in remote, rural areas of West and Central Africa, Papua New Guinea, and Australia. The disease may have been present for much longer periods of time in the remote endemic areas, and the apparent decline in the number of new BU cases reported in the past years is at least partly due to vast underreporting after a stark decline in financial support by key NGOs [
11].
A more effective strategy to disease control would obviously be prevention, not least because BU and leprosy affect mainly poor rural populations with limited access to diagnostic and treatment facilities. Transmission is considered to be mainly interhuman in leprosy, whereas people in BU endemic areas are most likely primarily infected with
M. ulcerans by contact with environmental reservoirs associated with aquatic ecosystems. Interhuman transmission of
M. ulcerans seems to be very rare. Despite the successful worldwide administration of multidrug therapy for leprosy, transmission rates have remained stagnant. Therefore, the feasibility and acceptability of a single-dose preventative treatment with rifampicin given to direct contacts of newly diagnosed leprosy patients is currently being evaluated [
139,
140]. The contribution of non-human reservoirs in the continuing transmission may be underestimated for leprosy, particularly in countries with unbroken high incidences. For
M. ulcerans, no preventable risks are clearly identified, and if environmental reservoirs play a key role in transmission, early diagnosis and treatment of patients can only reduce incidence, if contamination of the environment via chronic BU wounds plays an important role in transmission. Effective preventive vaccination may not only protect individuals from developing BU or leprosy but may also interfere—at least in the case of leprosy—with transmission.
Mycobacterial pathogens have acquired vastly diverse and highly successful survival strategies in the host, bolstered by the impermeable nature of their cell wall providing resistance to many toxins and drugs.
M. tuberculosis, one of the most prevalent human pathogens, is a facultative intracellular bacillus and has evolved to survive and replicate in host macrophages, to induce the formation of granulomas in which immune cells and bacteria colocalize, and to transit into a state of dormancy, which is extremely resistant to host defense [
141]. Dormancy in tuberculosis gives rise to a large reservoir of latently infected individuals, in whom disease can reactivate at any time. Coinfection with HIV leads to a particularly high reactivation rate [
142]. To date, dormancy in BU and leprosy has not been reported, but the incubation period of leprosy may be several years, constituting another reservoir for infection with unknown magnitude.
M. leprae is an obligate intracellular mycobacterium, able to survive and multiply in phagocytic cells and to modulate the host immune system. In contrast,
M. ulcerans replicates mainly extracellularly after a suspected early intracellular stage in the host, causing chronic infections by the mycolactone-mediated generation of necrotic infection foci, downregulation of host immune responses, and the killing of invading immune cells before they can reach the clusters of extracellular mycobacteria.
Depending on the polarization of T cell responses, a complex range of types of leprosy may occur. In tuberculoid leprosy patients, the growth of the mycobacteria is contained by TH1 cells that activate infected macrophages. As a consequence, few live bacteria are present, antibody production is limited, and local inflammation causes few skin lesions but peripheral nerve damage. Inhibition of the TH1 responses to M. leprae leads to the severe, disseminated lepromatous form of the disease, in which a predominant TH2 response is unable to control replication of the mycobacteria in macrophages, leading to severe damage to connective tissue and the peripheral nervous system, if untreated. Knowledge on cell-mediated protective immune responses to BU is limited. As cell-mediated immunity is likely to play an important role in a postulated early, intracellular stage of M. ulcerans infections, a scenario similar to leprosy may be envisaged, in which TH1 responses lead to protective immunity against BU, whereas TH2 responses may be detrimental. The polarization of T cell responses in BU and leprosy may be influenced by host factors, the route of infection, and/or the dose of the initial inoculum.
While HIV infection increases the risk and severity of BU, this is not the case for leprosy. But initiation of HIV treatment may for both diseases bring the danger of an immune reconstitution inflammatory syndrome. Massive leukocyte infiltrations are also observed in BU lesions after initiation of antimycobacterial chemotherapy, when mycolactone levels drop and infiltrating immune cells are no longer killed by the cytotoxin. Also, host genetic factors are likely to play an important role for the outcome of an exposure to M. ulcerans and M. leprae. Case-control studies have identified significant associations of susceptibility to BU and leprosy with polymorphisms that are relevant for cellular immunity. However, selection of candidate genes investigated so far was strongly biased toward host polymorphisms that have previously shown significant associations with susceptibility to intracellular mycobacteria, and genome-wide association studies may in future identify genes relevant for other immune effector functions.
Vaccine design for mycobacterial diseases is hampered by the distinct resistance of the bacilli to many immune defense mechanisms and a lack of knowledge on immune effector functions required for protective immunity. Contact to the mycobacterial pathogens does lead only in a minority of individuals to clinical disease, and spontaneous healing seems to occur in both BU and leprosy. Lessons may be learnt by identifying factors leading to this highly diverse individual outcome of exposure to the pathogens and by analyzing responses to BCG vaccination. Although the precise mechanisms of protection conferred by BCG are not clearly understood, studies have demonstrated that BCG vaccination induces cellular immune responses that can limit the bacterial burden at the site of infection [
143]. BCG vaccination has however been shown to convey only limited protection from pulmonary tuberculosis [
144] and leprosy [
145], and only short-lived protection against BU [
70]. On the contrary, BCG is effective at preventing childhood tuberculosis meningitis and disseminated tuberculosis [
146] as well as BU-associated osteomyelitis [
68,
69]. It is tempting to speculate that by reducing the bacterial burden through early cell-mediated immune responses elicited by BCG, severe forms of the infections may be prevented, whereas these responses are not rigorous enough to entirely contain the pathogenic effects of the mycobacterial pathogens. In this context, it has been postulated in recent years that BCG vaccination has the potential to induce a short-lived, non-specific, cross-protective memory-like response in innate immune cells, a phenomenon known as “trained immunity,” responsible for early clearance of the pathogens and disease prevention [
147‐
149].
Understanding the exact mechanism of the immunopathology of BU and leprosy will help in developing new strategies for the design of effective vaccines and specific and highly sensitive RDTs. Mycolactone is the most promising target for future vaccine design for BU and also for the development of a point of care diagnostic test. Whereas the complexity of clinical leprosy forms, immunological responses, and variations in bacillary load in lesions have hindered the development of a universal diagnostic test for leprosy, the subunit vaccine candidate LepVax has recently entered clinical testing [
126]. In comparison, the tuberculosis vaccine pipeline is much broader, and in view of similarities in the antigenic makeup of
M. tuberculosis,
M. leprae, and
M. ulcerans, there is hope that a newly developed effective TB vaccine may be cross-protective against leprosy and BU.