Systemic suppression of interferon-γ responses in Buruli ulcer patients resolves after surgical excision of the lesions caused by the extracellular pathogen Mycobacterium ulcerans

Buruli ulcer (BU), caused by Mycobacterium ulcerans, is the third most common mycobacterial infection in immunocompetent humans besides tuberculosis and leprosy. We have compared by ex vivo enzyme-linked immunospot analysis interferon-γ (IFN-γ) responses in peripheral blood mononuclear cells (PBMC) from BU patients, household contacts, and individuals living in an adjacent M. ulcerans nonendemic region. PBMC were stimulated with purified protein derivative (PPD) and nonmycobacterial antigens such as reconstituted influenza virus particles and isopentenyl-pyrophosphate. With all three antigens, the number of IFN-γ spot-forming units was reduced significantly in BU patients compared with the controls from a nonendemic area. This demonstrates for the first time that M. ulcerans infection-associated systemic reduction in IFN-γ responses is not confined to stimulation with live or dead mycobacteria and their products but extends to other antigens. Interleukin (IL)-12 secretion by PPD-stimulated PBMC was not reduced in BU patients, indicating that reduction in IFN-γ responses was not caused by diminished IL-12 production. Several months after surgical excision of BU lesions, IFN-γ responses of BU patients against all antigens used for stimulation recovered significantly, indicating that the measured systemic immunosuppression was not the consequence of a genetic defect in T cell function predisposing for BU but is rather related to the presence of M. ulcerans bacteria.