Development of Highly Organized Lymphoid Structures in Buruli Ulcer Lesions after Treatment with Rifampicin and Streptomycin

Abstract

Buruli ulcer caused by Mycobacterium ulcerans is an infection of the subcutaneous tissue leading to chronic necrotising skin ulcers. The pathogenesis is associated with the cytocidal and immunosuppressive activities of a macrolide toxin. Histopathological hallmark of progressing disease is a poor inflammatory response despite of clusters of extracellular bacilli. While traditionally wide excision of the infected tissue was the standard treatment, provisional WHO guidelines now recommend an eight week pre-treatment with streptomycin and rifampicin. We conducted a detailed immunohistochemical analysis of tissue samples from Buruli patients who received antibiotic treatment. Cellular immune response along with bacterial load and distribution were monitored. We demonstrate that this treatment leads to the development of highly organized cellular infiltration surrounding areas of coagulative necrosis. Diffuse infiltrates, granulomas and dense lymphocyte aggregation close to vessels were observed. Mycobacterial material was primarily located inside mononuclear phagocytes and microcolonies consisting of extracellular rod-shaped mycobacteria were no longer found. In observational studies some patients showed no clinical response to antibiotic treatment. Corresponding to that, one of five lesions analysed presented with huge clusters of rod-shaped bacilli but no signs of infiltration. Results signify that eight weeks of antibiotic treatment reverses local immunosuppression and leads to an active inflammatory process in different compartments of the skin. Structured leukocyte infiltrates with unique signatures indicative for healing processes developed at the margins of the lesions. It remains to be analysed whether antibiotic resistance of certain strains of M. ulcerans, lacking patient compliance or poor drug quality are responsible for the absent clinical responses in some patients. In future, analysis of local immune responses could serve as a suitable surrogate marker for the efficacy of alternative treatment strategies. Buruli ulcer (BU) is a debilitating disease of the skin presenting with extensive tissue destruction and suppression of local host defence mechanisms. Surgical removal of the affected area has been the standard therapy until in 2004 WHO recommended eight weeks' treatment with the anti-mycobacterial drugs rifampicin and streptomycin. We performed a detailed histological analysis of the local immune response in biopsies from five children medicated according to WHO provisional guidelines. One patient still revealed all histopathological signatures of an active BU lesion with huge bacterial clusters in areas of fatty tissue necrosis. Different factors can contribute to treatment failure, such as poor patient compliance and resistant bacterial strains. In four patients, different compartments of the skin presented active immune processes with only limited residues of bacterial material persisting. We demonstrated that antibiotic treatment not only directly controls the infectious agent but is also associated with fulminant host immune responses. Characterization of the healing process in BU due to therapy is highly relevant to increase our knowledge of the impact of treatment strategies to fight the disease.