Killing by antimycobacterial agents of AIDS-derived strains of Mycobacterium avium complex inside cells of the mouse macrophage cell line J774.

The murine macrophage continuous cell line J774 was used to measure the ability of antimicrobial agents, either singly or in combination, to kill intracellular Mycobacterium avium complex. All of 14 strains of M. avium complex, isolated from patients with AIDS, grew inside J774 cells during an incubation period of 7 days. The susceptibility of macrophage-ingested M. avium complex to antimicrobial agents was determined by comparing the number of colony-forming units (cfu) of M. avium complex inside untreated macrophages at the time of drug addition with the number of cfu present in macrophages after treatment with drugs for 7 days. Simultaneous experiments were carried out in broth medium without macrophages in order to compare killing of free mycobacteria with killing of macrophage-ingested mycobacteria. Antimicrobial agents (rifampin, rifabutin [Ansamycin], ethambutol, ciprofloxacin, clofazimine, isoniazid, and amikacin) were tested using concentrations that are achievable in the serum of patients. Among drugs known to penetrate macrophages, there was 96.2% agreement in susceptibility test results between the broth experiments and the J774 experiments when single drugs were tested, but only 74% agreement when combinations of drugs were tested. Killing of M. avium complex inside J774 cells by any single drug was uncommon. However, killing in J774 cells occurred against 10 of 11 (91%) strains with the combination of rifabutin + ethambutol + ciprofloxacin and against all of seven strains tested with the combination of rifabutin + ethambutol + amikacin. Interpretive criteria of in vitro susceptibility data need to be developed so that these interpretations correlate with a predictable clinical response in patients.