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.