The Role of an Activatable Esterase in Immune-Dependent Phagocytosis by Human Neutrophils

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Abstract
Diisopropylphosphofluoridate, cyclohexyl alkylphosphonofluoridates, and cyclohexyl phenylalkylphosphonofluoridates, which are potent, irreversible inactivators of serine esterases, inhibit the phagocytosis of opsonized sheep erythrocytes by human neutrophils. Two types of inhibition were observed: a) “cell-dependent” inhibition, which is determined by measuring the ingestion of neutrophils pretreated with the esterase inhibitors and washed; and b) “phagocytosis-dependent” inhibition, which is due to the presence of the inhibitors during phagocytosis. With the cyclohexyl alkylphosphonofluoridates, phagocytosis-dependent inhibition was always greater than cell-dependent inhibition. Cell-dependent inhibition was irreversible and dependent on the duration of the incubation of neutrophils with inhibitor. Both types of inhibition were dependent on the concentration of the inhibitor. Poorly or non-phosphorylating analogues of the cyclohexyl alkylphosphonofluoridates of DFP were not inhibitory; nor did fluoride, the hydrolysis product of these inhibitors, inhibit ingestion under either condition. In addition, neither method of treating the neutrophils resulted in a decrease in neutrophil viability. Furthermore, pretreating the EAC1423 with the inhibitors did not decrease ingestion. We conclude that cell-dependent inhibition is due to the inactivation of an esterase required for phagocytosis which is in or on the neutrophil in an active form, and thus is susceptible to inhibition by the esterase inactivators before contact of the neutrophil with the phagocytic stimulus. Phagocytosis-dependent inhibition is interpreted as being due to inactivation of an esterase required for phagocytosis which is normally in an inactive precursor proesterase form that is activated by the interaction of the neutrophil with the phagocytic stimulus. The distinctly different inhibition profiles of the active and activatable esterases indicate that they are two different activities.

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