Strain- and donor-related differences in the interaction of Mycobacterium avium with human monocytes and its modulation by interferon-gamma.

Mycobacterium avium is a cause of disseminated infection in AIDS patients. The pathogenicity of M. avium for human monocytes was examined in an in vitro model. Peripheral blood monocytes obtained from 13 healthy donors were precultured for 2 days before infection. Monocytes were infected with six AIDS-associated and three non-AIDS-associated strains and four strains of M. avium selected on the basis of colonial morphology. Uptake of M. avium detected by counting intracellular acid-fast bacilli differed according to colonial morphology: Bacteria with round and opaque colony forms were phagocytosed more readily than those with flat colonies. Virulence as defined by intracellular growth was also partly associated with colonial morphology. Some but not all bacilli with flat colony forms multiplied in human monocytes; strains of the round opaque colonial form did not. The effects of recombinant human interferon-gamma on M. avium infection also were examined. Pretreatment of monocytes suppressed phagocytosis. After infection, coculturing usually augmented mycobacterial growth inhibition by human monocytes, but these effects were variable from strain to strain. Overall, interferon-gamma produced a small but statistically significant inhibition of intracellular growth in three of four strains tested.