Research Article: Mycolactone Diffuses from Mycobacterium ulcerans–Infected Tissues and Targets Mononuclear Cells in Peripheral Blood and Lymphoid Organs

Date Published: October 22, 2008

Publisher: Public Library of Science

Author(s): Hui Hong, Emmanuelle Coutanceau, Marion Leclerc, Laxmee Caleechurn, Peter F. Leadlay, Caroline Demangel, Pamela L. C. Small

Abstract: BackgroundBuruli 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-γ responses in BU patients remains unclear.Methodology/Principal FindingsHere 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.ConclusionIn 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.

Partial Text: Buruli ulcer (BU) is a cutaneous disease caused by Mycobacterium ulcerans, leading to the formation of progressive ulcers, with extensive skin and soft tissue destruction. The presence of a coagulative necrosis forming a nidus for colonies of bacilli, accompanied by minimal inflammation, are considered the most reliable features for the histopathological diagnosis of BU disease [1]. These hallmarks of BU lesions in fact reflect the dual properties of a macrolide toxin produced by M. ulcerans, mycolactone, which plays a critical role in bacterial virulence [2],[3]. This unique polyketide has been shown to be a potent cytocidal molecule in vitro and in vivo[4],[5],[6],[7]. In addition, mycolactone displays significant immunosuppressive properties at non-cytotoxic doses towards a wide range of immune cells [5],[8],[9],[10].

BU is a tropical disease receiving far less attention than TB and leprosy, although it is more common in some endemic regions of West Africa. In contrast to these other two mycobacterioses, BU is acquired from the environment following inoculation of M. ulcerans in the dermis by a mechanism involving aquatic niches and insect vectors although the exact mode of transmission remains unknown [3]. BU starts as a painless subcutaneous nodule, oedema or plaque, enlarging over time. As lesions progress, ulcers eventually form that are characterized by an extensive necrosis of subcutaneous tissues accompanied by minimal inflammation [1],[23]. The pathology of the disease is closely associated with the production of a lipophilic toxin, mycolactone. This macrocyclic polyketide is highly cytotoxic to a variety of mammalian cells in vitro, and the injection of mycolactone into the skin of guinea pigs is sufficient to provoke ulcers [2]. Although the presence of mycolactone is reflected locally by its damaging effects on infected tissues any investigation of its diffusion outside the ulcerative lesion has been rendered difficult by the lack of a detection tool for this poorly immunogenic compound. In the present study, we have used a radiolabeled mycolactone to show that the toxin diffuses far beyond the sphere of its cytocidal action at the site of inoculation, or at the point of M. ulcerans infection.



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