Evidence that Human Leukocyte Inhibitory Factor (LIF) Is an Esterase

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Abstract
There are known to be two distinct lymphocyte mediators which inhibit cell migration in vitro, migration inhibitory factor (MIF), which inhibits macrophage migration, and leukocyte inhibitory factor (LIF), which influences the migration of polymorphonuclear leukocytes (PMNL). The present studies were carried out to clarify conflicting reports concerning the sensitivity of MIF activity to a serine esterase inhibitor, diisopropyl phosphofluoridate (DFP). Human MIF and LIF activities were generated by stimulating blood lymphocytes with concanavalin A, filtering the supernatants on Sephadex G-100 gels, and collecting the active fractions of each. MIF and LIF preparations were incubated for 30 min at 37°C with varying concentrations of DFP, dialyzed extensively to remove the inhibitor, and tested on their respective indicator cells. The results showed that LIF activity, but not MIF activity, was irreversibly abolished by DFP (10-3 to 10-5 M). Another irreversible terase inhibitor m-[o-(2-chloro-5-fluorosulfonyl-phenylureido)-phenoxybutoxy] benzamidine, in a dose-dependent fashion, abolished LIF activity. The inactivation of LIF by DFP was concentration, time, pH, and temperature dependent, indicating that DFP served as a substrate for LIF. In addition, LIF preparations (but not MIF) were found to contain hydrolytic activity for certain artificial substrates such as benzoyl arginine ethyl ester and tosyl arginine methyl ester but not glysyl lysyl methyl ester. The response of human monocytes to MIF was not altered by pretreatment with DFP or soybean trypsin inhibitor (STI). In contrast, pretreatment of PMNL with DFP and STI prevented their response to LIF. These studies suggest that LIF, but not MIF, is an esterase.

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