Radiolabelled M. leprae resident in human macrophage cultures as an in vitro indicator of effective immunity in human leprosy.

Twelve strains of human derived, freshly extracted M. leprae maintained within human macrophages showed a 2.1-13.2-fold increase in the incorporation of 3H-thymidine compared to parallel cultures containing heat-killed bacilli of the same strain. The addition of antigen stimulated lymphokines from five paucibacillary, tuberculoid leprosy patients resulted in the inhibition of the uptake of the radiolabel by 49-87%. Minimal, or no, inhibition was noted in the presence of similar culture supernatants from five bacilliferous lepromatous leprosy individuals. The results indicate that in contrast to lepromatous leprosy, tuberculoid patients possess antigen reactive lymphocytes which modulate macrophage function through soluble products. Attention is drawn to a rapid and sensitive in vitro method with potential for studying the immunological mechanisms leading to bacterial killing in human leprosy.