Abstract
Buruli ulcer (BU), one of the skin-related neglected tropical diseases (skin NTDs), is a necrotizing and disabling cutaneous disease caused by subcutaneous infection with Mycobacterium ulcerans. Leading on from the World Health Organization’s (WHO) establishment of a global BU initiative in 1998, >67,000 cases of BU have been reported from over 32 countries, mostly from West Africa and Australia. While treatment is currently in the transition period from rifampicin plus streptomycin (injection) to an all-oral regimen, it cannot hope to eradicate this opportunistic environmental pathogen. M. ulcerans is genetically very similar to related pathogenic organisms M. marinum, M. leprae and M. tuberculosis. However, M. ulcerans carries a unique megaplasmid, pMUM001, encoding the biosynthetic machinery responsible for production of a lipid-like exotoxin virulence factor, mycolactone. This diffusible compound causes the substantial divergence in BU’s pathogenic aetiology from other mycobacterial infections. Hence, mycolactone is cytotoxic and immunosuppressive and causes vascular dysfunction in infected skin. A major recent advance in our understanding of BU pathogenesis has been agreement on the mycolactone’s mechanism of action in host cells, targeting the Sec61 translocon during a major step in secretory and membrane protein biogenesis. While vaccine development for all mycobacteria has been challenging, mycolactone production likely presents a particular challenge in the development of a BU vaccine. The live-attenuated vaccine BCG is known to provide only partial and transient protection in humans but provides a convenient baseline in mouse preclinical studies where it can delay, but not prevent, disease progression. No experimental vaccine strategy has yet conferred greater protection than BCG. However, there is now the prospect of developing a vaccine against mycolactone itself, which may provide hope for the future.
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Keywords
1 Buruli Ulcer
The neglected tropical disease (NTD) Buruli ulcer (BU) is caused by subcutaneous infection with Mycobacterium ulcerans, resulting in necrosis of subcutaneous fatty tissue and the formation of ulcers with undermined edges which can extend to 15% of body surface area [1]. Much of its obscurity may be attributed to the fact that it predominantly affects the poor [2, 3], usually in remote rural areas with limited access to health services [4, 5]. Hence, while BU is considered a rare disease on a global scale, its impact on endemic communities should not be underestimated. BU is associated with social stigma [6] and presents a large financial [7, 8] and psychological [9] burden to patients and their care-givers, especially since most patients are young teenagers. A major global intervention came in 1998 when the WHO launched its Global BU Initiative (GBUI). This served as a forum for disease control and research efforts. Its success is clear from the global decrease in BU prevalence since 2010 [4], although this does not take into account under-reporting in countries without effective national control programmes [5]. Moreover, it brought about much of the research described in this chapter, and its success has underpinned the WHO’s most recent integrated approach to control all skin NTDs [10].
1.1 Epidemiology and Transmission
Infections that were most likely BU were first described by Sir Albert Cook at the turn of the twentieth century [11]. However, it was not until 1948 that the causative organism was identified by Peter MacCallum [12], due to the fortuitous breakdown of an incubator. To date, a total of >67,000 cases of BU have been reported worldwide in 32 countries including Japan, Papua New Guinea and Central and South America. At present, the highest prevalence of BU is in West Africa although there has recently been a worrying increase in cases in Australia’s state of Victoria [13]. The disease burden is difficult to objectively assess in many endemic countries (especially those that are lower-middle-income countries or least developed countries) due to the remote location of affected communities and lack of credible health system data [5] (Fig. 5.1).
The exact mode of transmission of M. ulcerans is unknown; however it seems almost certain that this opportunistic environmental pathogen enters the body by mechanical transfer. No incidences of person-to-person transmission have been reported, with the notable exception of a case involving a human bite [14]. Cases linked to other types of minor trauma, such as abrasions and even snake bite [15, 16], suggest that skin surface contamination may be important. Insects have been implicated in both Australia and West Africa, but this topic remains controversial and may vary between different environments (recently reviewed in [17,18,19]).
1.2 Clinical Presentations and Current Treatments
BU presents clinically as painless skin lesion(s) in one of five forms, nodule, papule, plaque or oedema and ulcers (Fig. 5.2), and, in some cases, bone involvement can result in osteomyelitis [20]. Nodules/papules are the first sign of (localised) infection, and the WHO has categorised more advanced lesions according to severity, with Category I including single small ulcers <5 cm diameter; Category II including larger ulcers of 5–15 cm, as well as plaques and oedema; and Category III including large ulcers >15 cm, multiple ulcers or ulcers that have spread to include particularly sensitive sites such as the eyes, bones, joints or genitals [20]. The more serious manifestations are much more common in African countries than Australia, most likely due to differences in health infrastructure.
Hence, the most common presentation of BU is a necrotising skin ulcer [1, 20]. Typically, the edges of these ulcers are ‘undermined’ due to subcutaneous necrosis, meaning that ulcers are frequently larger than the area of dermal breakdown. Remarkably, given the extensive tissue loss that can occur, BU patients are usually otherwise well, rarely experiencing the severe pain that might be expected based on the physical appearance of the lesions.
Until 2004, the only medical intervention available was radical surgery, either in the form of wide excision and debridement some 10 cm beyond the extent of affected tissue, or even limb amputation [21]. Although M. ulcerans was known to be sensitive to a range of antimycobacterial antibiotics from an early stage [22,23,24,25], a key success of the WHO GBUI was the testing [26] and introduction [20] of effective antibiotic regimens. Initially, a combination of rifampicin and streptomycin (for 8 weeks) was used [27,28,29]. To tackle the poor compliance and ototoxicity from injectable streptomycin [30], this is now transitioning to an all-oral combination including clarithromycin [31]. While antibiotic therapy can cause so-called paradoxical reactions, where lesions can appear to worsen or appear in new locations, this should not be confused as treatment failure [1, 32]. Fortunately, antimicrobial resistance has not yet been reported in Buruli ulcer, which supports its classification as an opportunistic environmental pathogen and argues against ‘re-seeding’ of environmental niches from patient lesions.
Since antibiotics were introduced, there has been a significant reduction in surgical intervention for BU [33, 34]. Indeed, antibiotic treatment of BU at an early nodule/papule stage can result in healing before ulceration [35]. Therefore, surgery is now usually reserved for patients with severe disease [36], although clinical decision-making varies from clinic to clinic [37]. With or without surgery, BU comes with a high burden of disability and deformity due to the extensive tissue damage caused and the risk of contractures [20]. Careful wound management and physiotherapy are critical to minimise these risks. Consequently, improved diagnostic tools and public health measures aimed at early detection of BU are now a key goal of the WHO.
2 Mycobacterium ulcerans
The closest genetic relative of M. ulcerans is M. marinum, another pathogenic mycobacterium that causes ‘fish tank granuloma’, to which its genome is 98% identical [38, 39]. Despite this phylogenetic similarity, major changes in the M. ulcerans genome have altered its interaction with the host [38, 39]. First, ‘reductive evolution’ has occurred with pseudogene accumulation and gene deletion due to the accumulation of single nucleotide polymorphisms (SNPs). Second, two different insertion sequences (IS2404 and IS2606 [40]) have proliferated throughout the genome leading to disruption and loss of virulence regions. These include the well-characterised Early Secreted Antigenic Target 6 kDa (ESAT-6) secretion system 1 (ESX-1) that allows other mycobacteria to escape the phagosome [41, 42]. Third, it has acquired a plasmid, pMUM, which carries the only virulence genes identified to date [43]. These genes encode the polyketide synthases and accessory proteins that manufacture mycolactone. Notably, there are two lineages of M. ulcerans, which may explain some of the divergence between findings in Africa and Australia [44], including subtle differences in mycolactone structure and function [45].
2.1 Mycolactone
The identification of mycolactone [46], and the subsequent understanding of its effects on host cells and tissues, has been critical to the understanding of BU pathogenesis [47]. Mycolactone is a lipid-like molecule with a 12-membered lactone ring that can vary in the hydroxylation and methylation pattern on the longer polyketide side chain. The most potent congener found in most African strains is known as mycolactone A/B (Fig. 5.3a). Purified mycolactone can replicate the ulceration caused by M. ulcerans [46, 49], and strains that cannot produce it lose their virulence [50]. To date, the best characterised consequences of mycolactone exposure are cytopathic/cytotoxic effects and immune suppression, although vascular dysfunction has also recently been described [51, 52]. All of these have now been shown to be dependent on activity of mycolactone against the normal function of the Sec61 translocon [53,54,55,56,57], which is the main entry point to the canonical secretory pathway of secreted proteins, type I and type II transmembrane proteins and multi-pass membrane proteins [58, 59] (Fig. 5.3b). Indeed, the structure of mycolactone bound to Sec61 has recently been solved (Fig. 5.3c) [57].
Mycolactone has a cytopathic effect on cultured mammalian cells characterized by cytoskeletal rearrangement, followed by rounding up and detachment from tissue culture plates [50, 60, 61]. It is also cytotoxic and induces apoptosis several days after exposure [50, 62,63,64] as well as cell cycle arrest in G0/G1 phase [46, 65]. We now know that the pathway to apoptosis involves changes in intracellular Ca2+ gradients [66, 67], the so-called integrated stress response [64, 68], and autophagy [69]. Cells carrying mutations in the gene encoding the major Sec61 subunit, Sec61α, are highly resistant to the cytopathic and cytotoxic effects of mycolactone and can proliferate in its presence [54, 57, 64, 67].
Mycolactone’s immunosuppressive effects are wide-ranging, which is unsurprising considering its inhibition of Sec61 and the consequent loss of secretory proteins (most cytokines and chemokines) and receptors (constitutive and induced), which normally act in elegant concert to mediate both innate and adaptive immune responses [48, 70]. Mycolactone has been shown to strongly suppress innate immunity by limiting phagocytosis [71] and inflammatory responses by monocytes, macrophages and dendritic cells [53, 71,72,73,74]. It limits adaptive immunity by suppressing both antigen presentation by dendritic cells and T cell activation [75,76,77]. Specific evidence demonstrated Sec61-dependent effects on TNF, IL-6 and Cox-2 production, antigen processing mediated by invariant chain during MHC class II processing and T cell activation [53, 54, 78]. A notable exception here is the recent discovery that mycolactone can induce the production of the cytokine IL-1β, by acting as the ‘second signal’ during inflammasome activation [48, 79]. This observation is entirely in line with Sec61 inhibition by mycolactone, since IL-1β does not use the canonical secretory pathway for its production.
Yet, there are no drugs described that can counteract the effect of mycolactone on the Sec61 translocon. Indeed, other inhibitors of Sec61 recapitulate the effects of mycolactone [80, 81] (R Simmonds, unpublished observation), and so this is not a viable treatment option. However, inhibitors of apoptosis such as Z-VAD-FMK, or genetic deletion of Bim, are able to at least delay cytotoxicity, both in vitro and in vivo [63, 82].
2.2 Immune Response to M. ulcerans Infection
The immunosuppressive properties of mycolactone described above are thought to explain the histopathology of BU lesions. Here, the lesions display coagulative necrosis, with clusters of extracellular acid-fast bacilli visible at the base of the subcutaneous tissue, and epidermal hyperplasia [3]. The cellular infiltrate of immune cells, normally expected in a microbial infection, is reduced and limited to the periphery of the lesion. In their elegant work, Ruf and Pluschke have shown that, in both humans and pigs, the infiltrating leukocytes are restricted to a ‘belt’ outside the necrotic core of the early ulcerative lesions [83, 84]. This contains T cells, CD68-positive macrophages and neutrophils, as well as clusters of B cells [83]. However, the immune cells are not able to access the necrotic core containing M. ulcerans, which contains neutrophilic debris and stains strongly and diffusely for apoptotic markers [83, 84]. Notably this picture changes remarkably during antibiotic therapy [85,86,87], which is presumed to be a result of a drop in mycolactone production.
Despite this, there is considerable evidence that both human and animal hosts can mount an immune response to M. ulcerans [88]. Critically, spontaneous recovery from BU without treatment reported in both humans [89,90,91,92] and animal models [93, 94] shows that the immune system can contain the infection in some circumstances. Both T cell [95,96,97,98,99] and serological [100,101,102] responses to M. ulcerans antigens have been demonstrated in the blood of BU patients. Moreover, their household contacts also display similar responses although they had never experienced clinical disease [100, 102,103,104]. Experimentally, IFN-γ protects against M. ulcerans infection in mice [105], and similarly a human genome SNP in the IFNG gene increases susceptibility to BU [106]. Such genetic studies in BU patients have identified a range of disease-modifying SNPs in genes involved in the cellular response to infection, including iNOS, the inducible nitric oxide synthase that generates bactericidal NO in macrophages [106]. Although the intra-macrophage stage of M. ulcerans infection is thought to be transient [71], SNPs in genes involved in this response also impact BU, including in PARK2, NOD2 and ATG16L1 [106,107,108].
3 Vaccine Candidates
Notwithstanding the obvious serious sequelae of infection, the motivation for a BU vaccine also encompasses the origins of the infection from the environment. It is now clear that there are certain environments where M. ulcerans is highly prevalent, especially those disturbed by human activity, such as mining or agricultural land use [109, 110]. Unfortunately those living in such environments are at high risk of developing BU [111], even if they adhere to risk-reducing guidance [112,113,114,115]. Therefore, a vaccine may be the only realistic hope of BU eradication.
Although studies aimed at developing a vaccine against M. ulcerans infection date back to the 1950s and the work of the Australian microbiologist Frank Fenner [116, 117] (https://www.science.org.au/learning/general-audience/history/interviews-australian-scientists/professor-frank-fenner), there is still currently no effective vaccine that provides long-term protection from BU [118]. Early attention focussed on the Bacillus Calmette-Guérin (BCG) strain of M. bovis that is primarily known as the vaccine for M. tuberculosis [119]. Indeed, most countries endemic for BU have a current national BCG vaccination policy for all citizens (www.bcgatlas.org), although Australia now only vaccinates special groups.
3.1 Human Studies with BCG
In two early randomised controlled trials using BCG in Uganda, there was evidence that BCG did confer some protection against BU even though this was thought to be short-lived [120, 121]. However, it should be noted that these studies were confounded by many factors. For example, in the first randomised trial with Rwandan refugees [120], participants were selected based on their tuberculin skin test (TST) negativity, which ruled out TB and latent TB infection, but almost certainly included both BCG-vaccinated and unvaccinated individuals (as TST in response to BCG wanes dramatically over time [122]). Moreover, this trial could not be fully completed, as the participants were lost to follow-up due to relocation of refugees. The second [121] was more successful in that the trial aims were fully achieved, but the outcomes were similar, in that partial and short-term protection was observed. Thus, an overall efficacy of BCG vaccination of 47% was reported, which declined sharply after 12 months, and was also notably highly variable depending on the immune status of participants on the outset. In that study, included participants had a broad spectrum of immune status, including those with known previous BU disease, presence or absence of BCG scar and even individuals with latent TB infection [121].
Since then, multiple other clinical studies found no evidence that BCG confers any long-term protection. For example, an observational study by Phillips et al. [123] found no association between BCG (presence of scar) and BU disease incidence amongst participants recruited from Congo, Ghana and Togo, replicating results from Benin [124]. For further reading on these studies, we refer the readers to two excellent recent reviews on the subject [125, 126].
The poor efficacy of BCG from these human studies is most likely due to insufficient immune cross-reactivity with M. ulcerans and suboptimal performance of BCG in countries with high exposure to non-tuberculous mycobacteria (NTM). In the case of cross-protection, this is likely the result of the divergent pathophysiologies of the infections they cause despite a high degree of genetic homology between different mycobacterial species. Thus, similarly to M. ulcerans, BCG vaccine offers only partial protection against M. leprae in human clinical trials [127, 128]. Furthermore, while multiple other environmental and non-environmental factors are undoubtedly involved, it is well known that BCG efficacy against TB is drastically reduced in geographical settings with high burden of non-pathogenic mycobacteria due to immunological interference (reviewed in [129]). In other words, the reasons for failure of BCG to impart better, longer-lasting protection against BU disease may be the same as those that also undermine its efficacy against TB.
3.2 Mouse Studies of BU Vaccine Candidates
Development of BU vaccines that offer improved protection over BCG has frequently involved in vivo models of M. ulcerans infection [130]. The mouse hind footpad model of M. ulcerans infection was originally developed by Fenner [116] and continually refined over many decades, predominantly in the BALB/c and C57BL/6 strains of mice. Today, it is the gold standard for studying treatment interventions and new vaccine candidates against BU disease. M. ulcerans bacteria are injected subcutaneously into the footpad, and (depending on the injected dose and mouse strain) the initial signs of swelling may appear over the metatarsal area approximately 2–5 weeks later. If untreated, the swelling progresses and then extends into the leg, finally leading to onset of ulceration. These stages of experimental pathogenesis of M. ulcerans infection in mouse footpads can be graded according to their physical appearance, according to a process originally proposed by Stanford [23] and later refined by Converse [34] in line with modern animal welfare legislation (Fig. 5.4). This allows for experimental humane points to be achieved without causing undue suffering to animals, usually before the point of ulceration. Notably, the analgesic effects of mycolactone [131, 132] mean that the animals do not experience inflammatory, hypoxic or tissue pressure pain even at the more severe grades. Objective measures of the intervention can also be taken in terms of physical parameters (footpad diameter), enumeration of bacteria in footpad via either culturable bacilli or quantification of bacterial DNA and measurement of inflammatory markers in blood or tissue.
An alternative model involving subcutaneous injection of M. ulcerans into the central portion of the tail has also been described [71]. The main outcome measure here was time to ulceration in days (60–70 days in unvaccinated C57BL/6 and BALB/c mice, respectively) [133, 134]. A low-dose infection model using a recombinant bioluminescent strain of M. ulcerans allowing for bacteria enumeration in live animals [135, 136] has also been used [137].
Here, we have categorised the various vaccine candidates tested in mice under two broad arms: whole bacteria and subunit vaccines, including those based on mycolactone (Table 5.1). Despite its lack of efficacy in human clinical trials, BCG has proven useful as a baseline to compare the efficacy of other vaccine candidates (recently reviewed in [125, 126]), as it provides short-lived but measurable protection against mouse footpad infections. This has been reproducible since Fenner’s first attempts at a vaccine in the 1950s [116, 117] and is seen even when BCG booster approaches are used [148]. These studies showed that vaccine-mediated protection from M. ulcerans infection may be Th1-mediated, via sustained levels of IFN-γ and TNF and the absence of IL-4, IL-10 and IL-17 [139].
3.2.1 Whole Bacteria Vaccines
In concert with the earliest BCG studies in mice, several reports have attempted to use M. marinum as a vaccine against M. ulcerans infection. Early attempts showed increased efficacy over BCG, but these were also still short-lived and waned with time [116, 117]. More recently, there has been some interest in overexpressing antigens in BCG and M. marinum and using these recombinant strains as whole bacteria vaccines. By this design, M. ulcerans-specific antigens were presented in a vaccine which lacked the virulent and immunomodulatory potential of mycolactone. These studies have focused on antigens that are known to be immunodominant in M. tuberculosis including EsxH, the M. ulcerans ortholog of M. tuberculosis TB10.4 antigen and proteins of the Ag85 complex. The latter is made up of Ag85A, Ag85B and Ag85C and is known to be secreted from BCG and to elicit strong Th1 responses [149]. Each of these 30–32 kDa proteins is highly conserved between different species of mycobacteria, being involved in the synthesis of cord factor and the organisation of mycolic acids in the bacterial cell wall. Notably Ag85A induced measurable, but relatively weak, IFN-γ responses during whole blood restimulations of BU patients and their household contacts [103].
Hart et al. [142] used recombinant M. marinum expressing M. ulcerans Ag85A (MU-Ag85A). Although this did not seem to delay the onset of ulceration (the experimental endpoint), it did significantly reduce the bacterial load of the challenged footpads. Hart et al. applied this same technology to generate BCG expressing M. ulcerans Ag85B with and without a fusion with EsxH. Mice challenged with M. ulcerans following a single subcutaneous vaccination with BCG MU-Ag85B-EsxH [144] or BCG MU-Ag85B [143] displayed significantly less bacterial burden at 6 and 12 weeks post-infection, reduced histopathological tissue damage and significantly delayed (but not prevented) onset in ulceration compared to vaccination with BCG.
Others have attempted vaccines using various doses and strains of M. ulcerans itself. Once again, Fenner paved the way and found that low, but not high, doses of M. ulcerans (1615E) provided protection against footpad infections [117]. Though not explained, this may have been due to the immunomodulatory action of mycolactone. In an attempt to bypass this, Fraga et al. [139] used a mycolactone-deficient strain of M. ulcerans (5114) that had lost the MUP038 gene involved in mycolactone biosynthesis [150]. This strain delayed the onset of footpad swelling post-challenge similarly to BCG. Finally, an interesting approach was taken by Watanabe et al. [138], who inactivated and dewaxed M. ulcerans by organic solvent treatments, prior to using it as a vaccine in mice. This candidate conferred complete protection against swelling at 28 days post-challenge, though the authors did not investigate if this protection was long-lasting.
3.2.2 Subunit Vaccines for BU
An alternative approach has been the use of acellular/subunit vaccines formulated with adjuvants and delivered as proteins or DNA. Tanghe et al. [140] demonstrated that a DNA vaccine based on BCG-Ag85A was able to confer partial protection (like BCG) against M. ulcerans infection in mice, as measured by reduced bacterial load. This was further improved on with MU-Ag85A, particularly when used as a DNA-prime protein-boost regimen, with a 100-fold reduction of bacterial load compared to unvaccinated mice [141]. These experiments also demonstrated that the protective immune responses were localised and Th1-mediated, with strong roles for IL-2 and IFN-γ. However, while this vaccine delayed the onset of footpad ulceration, it was less effective than BCG, a finding later replicated by Roupie et al. [147].
Other immunodominant antigens of M. ulcerans that have been investigated as vaccine candidates include MUL_2232 (also known as Hsp18, homologous to an immunodominant cell wall antigen of M. leprae that is reactive with the sera of patients with BU [100]) and MUL_3720 (a highly expressed 21 kDa protein with unknown function [151, 152]). However, despite their strong induction of IgG antibodies, they failed to provide any protection in either the footpad or tail infection models [133, 145]. No further improvement was reported when vesicular stomatitis virus-based RNA replicon particles encoding these proteins were used [146]. Prior to this, Coutanceau et al. [134] had tried a DNA vaccine using M. leprae Hsp65 antigen, but this did not confer any protection despite inducing strong IgG antibody responses. These studies give credence to the thinking that T cell responses, rather than antibodies, may have a more significant role in M. ulcerans immunity.
Moreover, different domains of the three large mycolactone polyketide synthases mlsA1, mlsA2 and mlsB encoded by pMUM001 and found associated with the M. ulcerans cell wall [150] have been investigated as vaccine candidates. These included the acyl carrier protein type 1, 2 and 3 (ACP-1, ACP-2 and ACP-3), type 1 and type 2 acyltransferases (acetate) (ATac-1 and ATac-2), acyltransferase (propionate) (ATp), enoylreductase (ER), ketoreductase A (KR-A) and the load module ketosynthase domain (KS). Many of these domains have been shown to induce humoral or cellular responses, supporting their immunogenicity. Of these, ER, ATp and KR-A have been shown to discriminate serological responses between BU patients and controls in non-endemic regions [102]. Other domains, particularly ER and KS, were able to successfully induce IFN-γ and IL-5 during whole blood restimulations of BU patients and their household contacts [103].
Unfortunately, vaccine trials using this strategy have been disappointing. Roupie et al. [147] used a DNA prime/protein boost protocol and found that the antibody and cellular (IL-2 and IFN-γ) immune responses to these antigens varied, with ATp providing the strongest response amongst the nine domains in line with, or better than, the MU-Ag85A control. However, this did not significantly extend the time for mice to display 4 mm footpad swelling or reduce bacterial numbers in infected feet. More recently, an approach that involved electrostatically coupling the ER domain to the Toll-Like receptor 2 (TLR-2) agonist adjuvant R4Pam2Cys was tested [137]. In this low-dose challenge tail model, this vaccine provided reduced protection compared to BCG and was associated with ER-specific serum IgG titres and IL-2/IL-4 in the draining lymph nodes.
With limited success so far with both whole bacteria and subunit protein candidates, it has been postulated that a vaccine design based on mycolactone could provide the much sought-after protection against BU. Evidence that such a toxin-blocking vaccine might be fruitful comes from the successful generation of mycolactone-neutralising antibodies using a truncated and non-cytotoxic mycolactone derivative. This compound (PG-203) lacking the so-called ‘Southern’ chain and conjugated to BSA via a diethylene glycol-based linker, it elicited protein-based immune responses as determined by ELISA and other neutralisation assays [153]. The vaccine potential of mycolactone has also been demonstrated using in vitro display methods comprising both phage and yeast [154].
4 Prospects
So, what are the prospects of a BU vaccine in the future? Based on the available evidence with BCG, a BU-specific vaccine is needed. While none of the promising preclinical candidates described here fully meet the criteria to be advanced to human studies, these partial successes strongly suggest that, with further improvements, such a vaccine may yet be achievable.
To that end, we would like to conclude this review with a preliminary report from our own attempts of developing a subunit-based vaccine against BU. Using our expertise from BCG-boost subunit vaccines studies for TB [155,156,157], we have recently developed several formulations that were tested in the mouse footpad model of M. ulcerans infection. These formulations contain individual or combinations of M. ulcerans antigens, as well as mycolactone itself, mixed with different types of adjuvants and delivery systems. While the data are yet to be published, we were very encouraged to observe that one of these formulations, which we have termed ‘BuruliVac’, was particularly effective in preventing swelling and ulceration of the mouse footpad and completely prevented footpad swelling in all experimental animals. This was corroborated by absence of C-reactive protein and other inflammatory markers in the tissue (Boakye-Appiah and Reljic, unpublished).
These ongoing proof-of-principle vaccine studies demonstrate that it is feasible to prevent M. ulcerans infection in this experimental model and that future efforts should be concentrated on further optimising and advancing such second-generation vaccine candidates against BU. Recent developments in vaccination strategies that allow specific targeting of skin resident memory T cells may be of value here [158]. However, it should also be noted that unlike BCG, a new BU-specific vaccine will come with a significant caveat, in that its clinical development and eventual licensure will depend on it being able to attract sufficient interest from pharmaceutical industry. BU, despite being the most significant mycobacterial disease after TB and leprosy, is an NTD that affects a relatively small proportion of population, mostly in the endemic areas in Western Africa. Vaccine development is an extremely costly undertaking for the pharmaceutical industry, amounting to hundreds of millions of US dollars. This investment can only be recouped by selling enough doses and over a prolonged period. The battle to develop a BU vaccine will therefore be fought on two separate fronts, in research laboratories and in the commercial arena. We, the scientific community, have the responsibility to ensure that if it comes to that second battle, we have something to fight with, a vaccine that has a real chance to eradicate the terrible affliction that is BU.
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Boakye-Appiah, J., Hall, B., Reljic, R., Simmonds, R.E. (2023). Current Progress and Prospects for a Buruli Ulcer Vaccine. In: Christodoulides, M. (eds) Vaccines for Neglected Pathogens: Strategies, Achievements and Challenges . Springer, Cham. https://doi.org/10.1007/978-3-031-24355-4_5
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