Mannheimia haemolyticaLeukotoxin Activates a Nonreceptor Tyrosine Kinase Signaling Cascade in Bovine Leukocytes, Which Induces Biological Effects

Abstract

The leukotoxin (LktA) produced by Mannheimia haemolytica binds to bovine lymphocyte function-associated antigen 1 (LFA-1) and induces biological effects in bovine leukocytes in a cellular and species-specific fashion. We have previously shown that LktA also binds to porcine LFA-1 without eliciting any effects. These findings suggest that the specificity of LktA effects must entail both binding to LFA-1 and activation of signaling pathways which are present in bovine leukocytes. However, the signaling pathways leading to biological effects upon LktA binding to LFA-1 have not been characterized. In this context, several reports have indicated that ligand binding to LFA-1 results in activation of a nonreceptor tyrosine kinase (NRTK) signaling cascade. We designed experiments with the following objectives: (i) to determine whether LktA binding to LFA-1 leads to activation of NRTKs, (ii) to examine whether LktA-induced NRTK activation is target cell specific, and (iii) to determine whether LktA-induced NRTK activation is required for biological effects. We used a biologically inactive mutant leukotoxin ( Bovine pneumonic pasteurellosis (BPP) caused by Mannheimia (Pasteurella) haemolytica serotype 1 remains a major economic problem for the beef and dairy cattle industries in North America and Western Europe (2, 10, 14, 47). The leukotoxin (LktA) produced by this bacterium is the primary virulence factor that contributes to the pathogenesis of the fibrinonecrotizing pleuropneumonia and death characteristic of this disease (7,8,43,44). A large body of evidence indicates that much of the lung injury in this disease is caused by inflammatory mediators released from alveolar leukocytes by LktA-induced activation and cytolysis (11,34,41,48).LktA is a member of a family of gram-negative RTX (repeats in toxin) cytolysins (12,20). Unlike most other RTX cytolysins, leukotoxins produced by Actinobacillus actinomycetemcomitans (LtxA) and M. haemolytica (LktA) demonstrate cell-type-specific and species-specific biological effects. The LtxA of A. actinomycetemcomitans, which causes dental caries in humans, interacts only with cells of the lymphocytic and monomyelocytic lineages of humans and some nonhuman primates and provokes biological effects (29); the LktA of M. haemolytica interacts only with ruminant leukocytes and platelets and induces biological effects (6,25,37). A study by Lally et al. (30) reported that human lymphocyte function-associated antigen 1 (LFA-1), a member of the ␤2 integrins, is a target cell receptor for LtxA of A. actinomycetemcomitans. Three studies have identified bovine CD18, the ␤ subunit of all three bovine ␤2 integrins (LFA-1, Mac-1, and p150,95), as a receptor for M. haemolytica LktA (3,33,42). However, in these studies no specific member of the ␤2 integrin family was identified as an LktA receptor. We have extended these observations and have shown that bovine LFA-1, but not other members of the ␤2 integrin family, is a receptor for M. haemolytica LktA (23).LFA-1 is a heterodimeric glycoprotein co...

Phylogenetic relationship of equine Actinobacillus species and distribution of RTX toxin genes among clusters

“…Differences in the toxic activities are most probably due to specific binding of the toxin to receptors on the target cells and/or differences in the signaling cascade after toxin binding. This was shown for other RTX toxins like the leukotoxin Lkt of Mannheimia haemolytica which after binding to the LFA-1 receptor is toxic for bovine leukocytes but has no effect on porcine or equine lymphocytes [10,11,19].…”

How Mannheimia haemolytica defeats host defence through a kiss of death mechanism

“…Indeed, inactivation of M. haemolytica leukotoxin by a gene knockout hardly causes any further pulmonary lesions although the wild-type and mutant strains were equally capable of colonising the upper respiratory tracts of the calves [165]. Moreover, necrosis of neutrophils can be reproduced in vitro with purified LKT [5,34,40,46,88,89,161,162,176]. Biologically, it is worth noting that even if LKT is able to bind leukocytes from various animal species, it is only cytotoxic for ruminant leukocytes, suggesting that the interaction specificity between LKT and ruminant leukocytes could be responsible for the ruminant-specificity of M. haemolytica [92,149,161].…”

“…1 to 4), but not in porcine leukocytes (LKT binds to porcine LFA-1 without eliciting any effects). This binding is known to involve G proteins [79,90] and to cause, in a dose dependent way, sustained elevation in intracellular calcium in bovine leukocytes [36,89,139] that results mainly from an incoming flux from the extracellular medium via voltage-gated channels (Fig. 2) [61,79,80,139].…”

Mannheimia haemolytica

“…57,84 Toxins and Extracellular Enzymes Leukotoxin M haemolytica LKT and LPS are well-characterized virulence factors with respect to their pathogenic role in the SF. 52,60 Since its initial discovery, LKT has been the subject of intense investigation relative to its role in the pathogenesis of SF. LKT is an exotoxin and a member of the RTX (Repeats in ToXin) family of toxins.…”