Growth of Mycobacterium leprae and M. marinum in congenitally athymic (nude) mice

Abstract

THE failure to grow the leprosy bacillus, Mycobacterium leprae, in vitro means that there is a requirement for a highly susceptible experimental animal model. The only animal confirmed so far as naturally highly susceptible is the nine-banded armadillo, of which at least some specimens develop, after inoculation, a disseminated disease which resembles human lepromatous leprosy¹. The inoculation of M. leprae into the footpads of mice results in a localised, self-limiting infection, with a maximum bacillary yield of about 10^6.5 organisms per footpad². Suppression of the immune response of the mouse, by thymectomy and irradiation, results in enhancement of the footpad infection, and dissemination of M. leprae bacilli to internal organs and peripheral tissue such as ears, nose and tail³. This ability to enhance infection by nonspecific depression of T-cell function suggests that the congenitally athymic, or nude, mouse might provide a suitable, naturally susceptible host for M. leprae. In spite of the severe impairment of the cellular immune response of nude mice^4–7, the only report of the growth of M. leprae in such mice concluded that there was no significant increase in bacillary proliferation, and no tendency to promote a generalised infection⁸. This conclusion was based, however, on observations made only 6 months after inoculation—an interval that is almost certainly too short for detection of significant enhancement or dissemination.