- Impairment of neutrophil functions and high levels of apoptotic neutrophils have been reported in human immunodeficiency virus (HIV) patients. The aim of the present study was to investigate the direct in vitro effects of the different HIV protease inhibitors (PIs) on neutrophil functions and apoptosis and to explore their mechanisms of action. The effects of nelfinavir (NFV), saquinavir (SQV), lopinavir (LPV), ritonavir (RTV), and amprenavir (APV) in the range of 5 to 100 microg/ml on neutrophil function, apoptosis, and mu-calpain activity were studied. The neutrophil functions studied included superoxide production stimulated by 5 ng/ml phorbol myristate acetate, 5 x 10(-7) M N-formyl-methionyl-leucyl-phenylalanine, and 1 mg/ml opsonized zymosan; specific chemotaxis; random migration; and phagocytosis. Apoptosis was determined by DNA fragmentation, fluorescein isothiocyanate-annexin V binding, and nuclear morphology. All three neutrophil functions, as well as apoptosis, were similarly affected by the PIs. SQV and NFV caused marked inhibition and LPV and RTV caused moderate inhibition, while APV had a minor effect. mu-Calpain activity was not affected by the PIs in neutrophil lysate but was inhibited after its translocation to the membranes after cell stimulation. SQV, which was the most potent inhibitor of neutrophil functions and apoptosis, caused significant inhibition of calpain activity, while APV had no effect. The similar patterns of inhibition of neutrophil functions and apoptosis by the PIs, which coincided with inhibition of calpain activity, suggest the involvement of calpain activity in the regulation of these processes.