BOVINE PLATELET-ADHESION IS ENHANCED BY LEUKOTOXIN AND SIALOGLYCOPROTEASE ISOLATED FROM PASTEURELLA-HAEMOLYTICA A1 CULTURES

Platelet and fibrin deposits are among characteristic changes observed in lung alveoli of cattle with pasteurellosis induced by Pasteurella haemolytica (biotype A, serotype 1). To determine whether the platelet function could be directly affected by protein products produced by t he bacterium, the effects of leukotoxin and O-sialoglycoprotease, cult ure supernatant antigen secreted by Pasteurella haemolytica Al, on bov ine platelet activation were examined by evaluating the enhancement of platelet adhesion to a negatively charged surface relative to untreat ed control samples. The glycoprotease, or the leukotoxin, was added to plasma free suspensions of bovine platelets and platelet adhesion ass essed by two parameters: (i) the number of H-3-adenine-labeled adheren t platelets and (ii) the morphology of unlabeled platelets adhering to the charged surface under scanning electron microscopy (SEM). In the presence of calcium, the glycoprotease produced a dose-dependent incre ase in adhesion. At a concentration of 4.0 mu g glycoprotease extract protein per 10(7) platelets, a 2-fold increase in adhesion was observe d which was similar to the increase in adhesion induced by 0.10 units of thrombin, a known platelet agonist. Both increased platelet adhesio n and platelet aggregation were observed with 0.8 mu g glycoprotease e xtract protein in the presence of calcium. The response of the bovine platelet suspensions to leukotoxin extract protein was dependent on th e dosage of the leukotoxin. Adhesion was enhanced at dosages of 25 mu g leukotoxin protein per 10(7) platelets and below, while at dosages o f 50 mu g and above adhesion was suppressed. Thus, the two proteins se creted by P, haemolytica may interact directly with bovine platelets t o initiate platelet aggregation and fibrin formation in alveolar tissu e in pneumonic pasteurellosis. (C) 1998 Elsevier Science B.V.