Phagocytosis and killing of Mycobacterium avium complex by human neutrophils

Organisms belonging to the Mycobacterium avium complex (MAC) cause life-thr eatening bacteremia in immunocompromised patients. Monocytes and macrophage s are thought to be responsible for ingestion and killing of MAC. However, it has been suggested that neutrophils may play a role in the early immune response to MAC infection. Here, neutrophils in autologous plasma were incu bated (at 0 and 37 degreesC) with M. avium labeled with Auramine O, a poten t fluorochrome. Neutrophil phagocytosis was measured by dow cytometry. Neut rophils incubated at 37 degreesC showed an increase in fluorescence over ti me with a maximum at 15 min, whereas neutrophils on ice showed no time-depe ndent increase in FL1. At 15 min Fl 1 at 37 degreesC was twice as high as F L1 at 0 degreesC. Examination under the fluorescent microscope showed multi ple intracellular fluorescent mycobacteria, Results in nine independent exp eriments showed time-dependent decrease of colony-forming units inn neutrop hil-associated live M. avium. Significant killing was observed within 30 mi n and was complete by 120 min. Observation by electron microscopy clearly c onfirmed the presence of intraphagosomal MAC, both intact and with evidence of degradation. These data demonstrate that MAC is rapidly phagocytized an d killed by human neutrophils, The newly established flow cytometry method should be useful in further studies of neutrophil function and of the role of G-CSF and oilier cytokines in MAC disease.