Behavior of Mycobacterium leprae in human macrophages in vitro

Attempts have been made to cultivate Mycobacterium leprae in human macrophages in vitro. In 27 out of 55 experiments a two- to ninefold increase (mean 2.31 +/- 1.46) in acid-fast organisms were observed over a period of 1.5 to 3 months of cultivation. No such increase was observed with heat-killed bacilli (mean fold increase 0.88 +/- 0.19). Macrophages were necessary for obtaining increases. No multiplication was observed on artificial media. A close correlation between increases of acid-fast organisms and changes in viability as determined by the morphology of the bacilli (morphologic index) was found. The increases in acid-fast organisms could be inhibited by anti-leprosy drugs. It is concluded that multiplication of M. leprae may take place inside human macrophages in vitro. Multiplication appears not to be dependent on whether the macrophages are derived from lepromatous or tuberculoid patients or healthy individuals. Moreover, multiplication took place both at 33 and 37 C. The applicability of this method is at present limited by the restricted survival of human macrophages in vitro.