Leukotoxin As a Virulent Factor of Pasteurella Haemolytica

Abstract

Pasteurella haemolytica biotype A, serotype 1 (Al) has been established as the primary agent responsible for the clinical disease and pathophysiologic events leading to acute lobar fibrinonecrotizing pneumonia associated with bovine pneumonic pasteurellosis(1,2,3).

The bacteria produces several potential virulent factors, of which the leukotoxin (LKT) has received the most attention(4). The LKT is a heat-labile proteinaceous exotoxin, that is oxygen-stable, non-dialyzable, water soluble, and is produced in high concentrations by P.haemolytica during the logarithmic phase of growth(5). The genes that code for the synthesis and secretion of this LKT have been cloned(6). All 15 serotypes of P. haemolytica produce LKT(7). It has a molecular weight of 101-105kDa when determined by sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis(8). Attempts to purify the LKT has been futile because it is tightly bound to the LPS and there is no efficient way to remove the LPS without inactivating the LKT. It is one of a family of RTX(repeat toxins)-pore forming cytolysin which has a unique specificity, in that it is only cytocidal to ruminant leukocytes(9). This cytotoxicity is caused by the formation of pores in the cell membrane which allows the influx of calcium and results in a sequence of cell-damaging events(lO). Since this organism produces disease only in ruminants, this would support the role of LKT as a virulence factor. Indeed, several observations point to the central role that LKT has in the pathogenesis of pneumonic pasteurellosis. For example, in experimental pasteurellosis, the clinical and pathophysiologic events are dose-dependently reproduced by the intatracheal administration of live logarithmic phase P. haemolytica and not by stationary phase organisms(ll). This enhanced pathogenicity may be related to the amount of LKT produced by these different populations. Indeed, we (5) and others(12)* have shown that the logarithmic phase cells produce far greater amounts of this LKT than stationary cells. In other studies, cattle with high LKT neutralizing antibody titers have higher survival rates in the natural disease and experimental pasteurellosis than animals with low antibody titers(13).

Although advances have been made in describing the cytolytic properties of the P.haemolytica LKT, the mechanisms by which it brings about lung injury in cattle are poorly understood. Our laboratory has long been interested in elucidating the contributions and mechanisms by which the LKT induce this acute lung injury in pneumonic pasteurellosis. Three experiments were designed to study if the LKT contributed in the genesis of lung injury in pneumonic pasteurellosis.