Evidence for inhibition of fusion of lysosomal and prelysosomal compartments with phagosomes in macrophages infected with pathogenic Mycobacterium avium

Bone marrow-derived cultured macrophages were infected with the pathogenic organism Mycobacterium avium. Immediately after infection and at 1 to 28 days later, cells either were stained for acid phosphatase activity or given horseradish peroxidase, which served as a pinocytotic marker. With the former, fusions between phagosomes and lysosomes exclusively were assessed; with the latter, those between phagosomes and both pinosomes and lysosomes were determined. As a control, similar experiments were undertaken by infecting macrophages with gamma ray-killed M. avium and the nonpathogenic live organisms Mycobacterium aurum and Bacillus subtilis. After infection with live M. avium, fusions between phagosomes and acid phosphatase-positive vesicles (lysosomes) were inhibited. The same inhibition was observed whether phagosomes contained damaged or structurally intact (presumed to be live) bacteria, except for the early time points. This inhibition was, however, partial, suggesting that some of the live bacteria are resistant to the hydrolytic enzymes of the phagolysosomal environment. Fusions between horseradish peroxidase-positive vesicles (pinosomes and lysosomes) and phagosomes depended upon the morphological state of the bacteria. Damaged bacteria did not inhibit fusions, whereas with intact bacteria, a partial inhibition which increased with time was observed. The two types of experiment suggest that viable M. avium can impair phagosome-pinosome fusions.