Pasteurella haemolytica exotoxin : chemical, biological, and immunological characterization of a leukotoxin produced by P. haemolytica

Typescript (photocopy).Pasteurella haemolytica pneumonia is recognized as an economically important constituent of the group of bovine respiratory disease problems collectively referred to as the bovine respiratory disease complex, and as an important cause of economic losses to the sheep and goat industries. Metabolically active P. haemolytica cells synthesize and secrete an exotoxin into the culture supernatant which is toxic for ruminant, but not for nonruminant leukocytes. The term leukotoxin has been suggested as a descriptive name for the exotoxin since its action is on leukocytes but not against other ruminant cell types. A luminol-dependent chemiluminescence inhibition (LDCLI) assay was developed for use in chemical, biological and immunological studies directed at characterizing this molecule. Leukotoxin, which is both released into the culture supernatant and retained in a form associated with the cell, has the ability to biologically incapacitate the oxygen-dependent activation of the hexose monophosphate shunt in ruminant neutrophils (PMN) and thereby render inoperative an important antimicrobial system of these cells. The LDCL response of ruminant PMN to unopsonized living P haemolytica was characterized by the development of a peak response at 10 minutes of incubation followed by a precipitous decline and a subsequent complete cessation of chemiluminescence. Opsonization of living P haemolytica with serum only temporarily spared ruminant PMN preparations from the detrimental effects of leukotoxin. Regardless whether leukotoxin was presented with opsonized or unopsonized heat-killed Staphylacoccus aureus or P haemolytica, it was able to inhibit the LDCL response of ruminant, but not nonruminant PMN. The LDCLI, trypan blue dye exclusion (TBDE), and ('51)chromium release assays were compared for sensitivity in detecting leukotoxin activity. The LDCLI assay was approximately 17 and 2480 times more sensitive than the ('51)Cr release and TBDE assays, respectively. Leukotoxin production is a stable phenotypic characteristic of pathogenic P haemolytica isolates and the genetic information coding for leukotoxin activity is apparently located on the bacterial host chromosome. Current scientific evidence indicates that P haemolytica exotoxin is a heat-labile, oxygen-stable, pH-stable, nondialyzable, nonhemolytic, water-soluble, antigenic glycoprotein, which is toxic to ruminant leukocytes and may be an important virulence factor of the organism