Characterization of the cellular immune defect in lepromatous leprosy: a specific lack of circulating Mycobacterium leprae-reactive lymphocytes.

The blastogenic response of leucocyte cultures from patients with tuberculoid and lepromatous leprosy has been studied. The leucocytes from the two groups were studied simultaneously and cultivated in the same pool of normal human serum. While the leucocytes from twenty-eight tuberculoid patients responded quite strongly to Mycobacterium leprae after 7 days of culture (average lymphocyte transformation 11·1%), there was a complete lack of response in similar cultures from twenty-seven lepromatous patients (average 0·1% transformed cells). These results were confirmed by studies on cellular incorporation of 3H-thymidine in the cultures from four tuberculoid and four lepromatous patients. This lack of response was quite specific as leucocytes from several lepromatous patients responded to BCG. Furthermore, four patients with both lepromatous leprosy and tuberculosis responded as strongly to BCG and PPD as tuberculous patients without leprosy. In the mixed leucocyte reaction, between two lepromatous or two tuberculoid patients respectively, the lepromatous cells responded well (average 15·0%) and comparably to tuberculoid cells (average 12·1%). The blastogenic response of purified lymphocytes to M. leprae revealed a similar pattern, i.e. the tuberculoid cells responded well, while again there was a lack of response in the lepromatous group. It is concluded that the lepromatous patients lack circulating lymphocytes responding to M. leprae, indicating that their immunological defect as observed in the present study has features in common with immunological tolerance.