Comparison of the abilities of Mycobacterium avium and Mycobacterium intracellulare to infect and multiply in cultured human macrophages from normal and human immunodeficiency virus-infected subjects.

Patients with AIDS commonly develop disseminated infections with Mycobacterium avium (MA) but not its close relative, M. intracellulare (MI). In non-AIDS patients who have these infections, the two species are about equally distributed. The higher incidence of infection with MA than with MI in AIDS patients might be due to the selective susceptibility of these patients to MA. This possibility was tested by comparing the abilities of MA and MI to infect and replicate in cultured macrophages from normal subjects and from patients with AIDS-related complex or AIDS. The macrophages were cultured in medium supplemented with 1 or 5% normal or patient sera or with 1% defined serum substitute. Replication of MA (serovar 4) or MI (serovars 16 and 17) in the macrophages was measured by CFU counts made from lysed samples of the macrophages taken at 0,4, and 7 days after macrophage infection. MA and MI in infected normal macrophages which were cultured in normal serum replicated in these macrophages at similar rates. MA but not MI multiplied abnormally rapidly in patient macrophages cultured in either normal serum or patient serum. The accelerated growth of MA in patient macrophages was macrophage dependent, because patient sera did not change the rate of MA replication in culture medium lacking macrophages. However, patient sera did increase the permissiveness of normal macrophages to MA but not MI. These results suggest that a selective increased susceptibility to MA compared with a retained normal resistance to MI in human immunodeficiency virus-infected patients as they progress from AIDS-related complex to AIDS accounts for the higher prevalence of MA than MI infection in AIDS patients. The results also indicate that the mechanisms of native resistance in human macrophages to MA and MI are different.