FVB/N Mice Spontaneously Heal Ulcerative Lesions Induced by Mycobacterium ulcerans and Switch M. ulcerans into a Low Mycolactone Producer

Buruli ulcer, a debilitating disease, is caused by Mycobacterium ulcerans. The incidence of this neglected tropical disease is steadily increasing. As a rule, without treatment, skin ulcers occur and a lengthy healing process may be observed associated with severe functional disabilities. Mouse models are already available to study establishment of lesions or evaluation of therapy but a lack of a suitable animal model, mimicking all clinical stages, in particular the healing process, remains an obstacle to understand the pathophysiology of M. ulcerans infection. M. ulcerans was s.c. inoculated in three consanguine mouse strains, that is, BALB/c and C57BL/6, classically used to study mycobacterial infection, and FVB/N. Strikingly, FVB/N mice, although as sensitive as all other mouse strains with respect to M. ulcerans infection, presented a spontaneous healing after the ulcerative phase despite stable bacterial load, and mycolactone toxin was not detected in the healed tissues. The spontaneous healing process was accompanied by an activation of the innate immune system. The adaptive response initiated by FVB/N mice was not involved in the healing process and did not confer protection against M. ulcerans. Our work highlights the importance of innate immune responses to control M. ulcerans infection. This in vivo model of M. ulcerans infection now paves the way for new avenues of research toward the elucidation of critical stages of this disease, such as the characterization of the regulation of mycolactone production, a better understanding of the pathophysiology of M. ulcerans infection, and the development of new therapeutic strategies.