Evidence for an Intramacrophage Growth Phase ofMycobacterium ulcerans

Abstract

Mycobacterium ulcerans is the etiologic agent of Buruli ulcer (BU), an emerging tropical skin disease. Virulent M. ulcerans secretes mycolactone, a cytotoxic exotoxin with a key pathogenic role. M. ulcerans in biopsy specimens has been described as an extracellular bacillus. In vitro assays have suggested a mycolactoneinduced inhibition of M. ulcerans uptake by macrophages in which its proliferation has not been demonstrated. Therefore, and uniquely for a mycobacterium, M. ulcerans has been classified as an extracellular pathogen. In specimens from patients and in mouse footpad lesions, extracellular bacilli were concentrated in central necrotic acellular areas; however, we found bacilli within macrophages in surrounding inflammatory infiltrates. We demonstrated that mycolactone-producing M. ulcerans isolates are efficiently phagocytosed by murine macrophages, indicating that the extracellular location of M. ulcerans is not a result of inhibition of phagocytosis. Additionally, we found that M. ulcerans multiplies inside cultured mouse macrophages when low multiplicities of infection are used to prevent early mycolactone-associated cytotoxicity. Following the proliferation phase within macrophages, M. ulcerans induces the lysis of the infected host cells, becoming extracellular. Our data show that M. ulcerans, like M. tuberculosis, is an intracellular parasite with phases of intramacrophage and extracellular multiplication. The occurrence of an intramacrophage phase is in accordance with the development of cell-mediated and delayed-type hypersensitivity responses in BU patients.Mycobacterium ulcerans is the etiologic agent of Buruli ulcer (BU), an emerging, devastating, difficult-to-treat skin disease reported in many countries, mostly in tropical areas (16), and BU has become the third most common mycobacterial infection in humans after tuberculosis and leprosy. BU assumes various nonulcerative clinical forms that can progress to ulcers.M. ulcerans (26, 41, 47, 59), M. tuberculosis (24, 38, 42), M. marinum (25,57), and M. haemophilum (23) are known to have cytotoxic activity, M. ulcerans being the most cytotoxic of all known mycobacteria. The M. ulcerans toxin is a mycolactone, a unique polyketide lipid exotoxin that has a potent destructive activity for cells that provokes the extensive necrotic lesions characteristic of BU (26). Mycolactone is considered to be the major virulence factor of this pathogen (1, 26, 41).Intracellular parasites are defined on the basis of their lifestyles in the infected hosts and of the types of immune responses elicited (7, 37): they live and multiply predominantly within host cells, typically macrophages, and therefore are able to survive and grow within cultured macrophages. The host immune response against intramacrophage parasites involves mechanisms of cell-mediated immunity (CMI) accompanied by delayed-type hypersensitivity (DTH).Genetic analysis places M. ulcerans very close to M. marinum and M. tuberculosis (71), two species of mycobacteria that are typical intracellular p...

L’ulcèrede Buruli

“…Cette dernière réponse est dominée par les cytokines anti-inflammatoires IL-4 et IL-10 qui favoriseraient la progression des infections mycobactériennes [12]. Dans l'ulcère de Buruli, les M. ulcerans semblent passer par une phase à la fois intra-et extracellulaire [5,13,14]. En effet, ils sont ingérés dans un premier temps par les phagocytes [5,13,14] puis relargués dans l'espace extracellulaire après la lyse des phagocytes par la mycolactone [5,[13][14][15].…”

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

“…Although large clusters of toxin-producing M. ulcerans bacteria are found in established BU lesions, there are indications that multiplication of the pathogen in phagocytes plays a role in the early steps of the infection. 25,26 Furthermore, BU disease leads to reduced interferon (IFN)-γ release. 27 Management of HIV/AIDS over the years has seen progressive improvement in drug therapy and clearer guidelines, which has dramatically decreased mortality and incidence of AIDSdefining opportunistic infections.…”

Buruli ulcer

“…1,8,24,30,60 This has led to improved therapy, 61,62 as well as identification of possible candidates for vaccination and diagnosis. 26,28,51 However, many questions remain to be answered, such as the relative importance of humoral and cellular immune responses in controlling the infection.…”

“…23 Following a proliferation phase within macrophages, M. ulcerans induces the lysis of the infected host cells, and the bacteria become extracellular. 24 This extracellular phase of infection means that humoral responses may be more important for protection than in the other mycobacterial infections, with obvious implications for vaccine development.…”