Molecular and biochemical mechanisms of Pasteurella haemolytica leukotoxin-induced cell death

Interaction of RTX toxins with the host cell plasma membrane

Repeats in ToXins (RTX) protein family is a group of exoproteins secreted by Type 1 secretion system (T1SS) of several Gram-negative bacteria. The term RTX is derived from the characteristic nonapeptide sequence (GGxGxDxUx) present at the C-terminus of the protein. This RTX domain binds to calcium ions in the extracellular medium after being secreted out of the bacterial cells, and this facilitates folding of the entire protein. The secreted protein then binds to the host cell membrane and forms pores via a complex pathway, which eventually leads to the cell lysis. In this review, we summarize two different pathways in which RTX toxins interact with host cell membrane and discuss the possible reasons for specific and unspecific activity of RTX toxins to different types of host cells.

Mannheimia haemolytica Negatively Affects Bovine Herpesvirus Type 1.1 Replication Capacity In Vitro

Bovine Respiratory Disease (BRD) is a multifactorial condition affecting cattle worldwide resulting in high rates of morbidity and mortality. The disease can be triggered by Bovine Herpesvirus-1 (BoHV-1) infection, stress, and the subsequent proliferation and lung colonization by commensal bacteria such as Mannheimia haemolytica, ultimately inducing severe pneumonic inflammation. Due to its polymicrobial nature, the study of BRD microbes requires co-infection models. While several past studies have mostly focused on the effects of co-infection on host gene expression, we focused on the relationship between BRD pathogens during co-infection, specifically on M. haemolytica’s effect on BoHV-1 replication. This study shows that M. haemolytica negatively impacts BoHV-1 replication in a dose-dependent manner in different in vitro models. The negative effect was observed at very low bacterial doses while increasing the viral dose counteracted this effect. Viral suppression was also dependent on the time at which each microbe was introduced to the cell culture. While acidification of the culture medium did not grossly affect cell viability, it significantly inhibited viral replication. We conclude that M. haemolytica and BoHV-1 interaction is dose and time-sensitive, wherein M. haemolytica proliferation induces significant viral suppression when the viral replication program is not fully established.

Kingella kingae RtxA Cytotoxin in the Context of Other RTX Toxins

The Gram-negative bacterium Kingella kingae is part of the commensal oropharyngeal flora of young children. As detection methods have improved, K. kingae has been increasingly recognized as an emerging invasive pathogen that frequently causes skeletal system infections, bacteremia, and severe forms of infective endocarditis. K. kingae secretes an RtxA cytotoxin, which is involved in the development of clinical infection and belongs to an ever-growing family of cytolytic RTX (Repeats in ToXin) toxins secreted by Gram-negative pathogens. All RTX cytolysins share several characteristic structural features: (i) a hydrophobic pore-forming domain in the N-terminal part of the molecule; (ii) an acylated segment where the activation of the inactive protoxin to the toxin occurs by a co-expressed toxin-activating acyltransferase; (iii) a typical calcium-binding RTX domain in the C-terminal portion of the molecule with the characteristic glycine- and aspartate-rich nonapeptide repeats; and (iv) a C-proximal secretion signal recognized by the type I secretion system. RTX toxins, including RtxA from K. kingae, have been shown to act as highly efficient ‘contact weapons’ that penetrate and permeabilize host cell membranes and thus contribute to the pathogenesis of bacterial infections. RtxA was discovered relatively recently and the knowledge of its biological role remains limited. This review describes the structure and function of RtxA in the context of the most studied RTX toxins, the knowledge of which may contribute to a better understanding of the action of RtxA in the pathogenesis of K. kingae infections.

RTX Toxins Ambush Immunity's First Cellular Responders

The repeats-in-toxin (RTX) family represents a unique class of bacterial exoproteins. The first family members described were toxins from Gram-negative bacterial pathogens; however, additional members included exoproteins with diverse functions. Our review focuses on well-characterized RTX family toxins from Aggregatibacteractinomycetemcomitans (LtxA), Mannheimiahaemolytica (LktA), Bordetella pertussis (CyaA), uropathogenic Escherichia coli (HlyA), and Actinobacillus pleuropneumoniae (ApxIIIA), as well as the studies that have honed in on a single host cell receptor for RTX toxin interactions, the β₂ integrins. The β₂ integrin family is composed of heterodimeric members with four unique alpha subunits and a single beta subunit. β₂ integrins are only found on leukocytes, including neutrophils and monocytes, the first responders to inflammation following bacterial infection. The LtxA, LktA, HlyA, and ApxIIIA toxins target the shared beta subunit, thereby targeting all types of leukocytes. Specific β₂ integrin family domains are required for the RTX toxin’s cytotoxic activity and are summarized here. Research examining the domains of the RTX toxins required for cytotoxic and hemolytic activity is also summarized. RTX toxins attack and kill phagocytic immune cells expressing a single integrin family, providing an obvious advantage to the pathogen. The critical question that remains, can the specificity of the RTX-β₂ integrin interaction be therapeutically targeted?

In-vitro phagocytosis and intracellular killing of Pasteurella multocida B:2 by phagocytic cells of buffaloes

Pasteurella multocida B:2 is a Gram-negative organism causing haemorrhagic septicaemia (HS) in buffaloes. It causes severe pulmonary infection, leading to infiltration of numerous macrophages and neutrophils. Despite the inflammatory response, buffaloes succumb to HS. This study aims to evaluate the in-vitro efficacy of macrophages and neutrophils of buffalo following exposure to P. multocida B:2. In-vitro infections were done using 10⁷ cfu/ml of P. multocida B:2 for Group 1, Escherichia coli for Group 2 and Mannhaemia haemolytica A:2 for Group 3 cells. The inoculated cell cultures were harvested at 0, 30, 60 and 120 min post-exposure and the phagocytic, killing and cell death rates were determined. Both phagocytosis and killing rates of all bacteria increased over time. Phagocytosis involved between 71% and 73% neutrophils and between 60% and 64% macrophages at 120 min. Killing rate of all bacteria involved between 76% and 79% for neutrophils and between 70% and 74% for macrophages at 120 min. Death rate of neutrophils ranged between 67% in Group 3, and 88% in Group 1 at 120 min, significantly (p < 0.05) higher than Group 3 but insignificant (p > 0.05) than Group 2. Similar pattern was observed for death rate of macrophages. The phagocytosis and killing rates of P. multocida B:2 were similar to other bacterial species used in this study but more neutrophils and macrophages were dead following infection by P. multocida B:2 than M. haemolytica A:2.

Effects of Pasteurella multocida lipopolysaccharides on bovine leukocytes

Lipopolysaccharide (LPS) is a major virulence factor of Gram-negative bacteria playing a major role in stimulating protective immune response in mammalian host. However, in many gram-negative bacterial infections, LPS also elicits immunopathology by inducing excessive inflammatory changes. P. multocida (Pm), a gram-negative bacterium, causes acute lung inflammation and fatal septicemic disease in animals. However, the effects of Pm LPS on host cells are little known. In this study, LPS isolated from three different serotypes (B:2, A:1 and A:3) of Pm were individually tested in vitro to assess the response of bovine leukocytes. Pm LPS induced cell proliferation and cell death of leukocytes, in a dose- and time-dependent manner. In these cells, mitochondrial dysfunction and caspase activation mediate cell death.

FOXO3 signalling links ATM to the p53 apoptotic pathway following DNA damage

DNA damage as a result of environmental stress is recognized by sensor proteins that trigger repair mechanisms, or, if repair is unsuccessful, initiate apoptosis. Defects in DNA damage-induced apoptosis promote genomic instability and tumorigenesis. The protein ataxia-telangiectasia mutated (ATM) is activated by DNA double strand breaks and regulates apoptosis via p53. Here we show that FOXO3 interacts with the ATM-Chk2-p53 complex, augments phosphorylation of the complex and induces the formation of nuclear foci in cells upon DNA damage. FOXO3 is essential for DNA damage-induced apoptosis and conversely FOXO3 requires ATM, Chk2, and phosphorylated p53 isoforms to trigger apoptosis as a result of DNA damage. Under these conditions FOXO3 may also play a role in regulating chromatin retention of phosphorylated p53. These results suggest an essential link between FOXO3 and the ATM-Chk2-p53-mediated apoptotic program following DNA damage.

Leukotoxin from Aggregatibacter actinomycetemcomitans causes shrinkage and P2X receptor-dependent lysis of human erythrocytes

Leukotoxin (LtxA) is a virulence factor secreted by the bacterium Aggregatibacter actinomycetemcomitans, which can cause localized aggressive periodontitis and endocarditis. LtxA belongs to the repeat-in-toxin (RTX) family of exotoxins of which other members inflict lysis by formation of membrane pores. Recently, we documented that the haemolytic process induced by another RTX toxin [α-haemolysin (HlyA) from Escherichia coli] requires P2X receptor activation and consists of sequential cell shrinkage and swelling. In contrast, the cellular and molecular mechanisms of LtxA-mediated haemolysis are not fully understood. Here, we investigate the effect of LtxA on erythrocyte volume and whether P2 receptors also play a part in LtxA-mediated haemolysis. We observed that LtxA initially decreases the cell size, followed by a gradual rise in volume until the cell finally lyses. Moreover, LtxA triggers phosphatidylserine (PS) exposure in the erythrocyte membrane and both the shrinkage and the PS-exposure is preceded by increments in the intracellular Ca(2+) concentration ([Ca(2+)](i)). Interestingly, LtxA-mediated haemolysis is significantly potentiated by ATP release and P2X receptor activation in human erythrocytes. Furthermore, the LtxA-induced [Ca(2+)](i) increase and following volume changes partially depend on P2 receptor activation. Theseobservations imply that intervention against local P2-mediated auto- and paracrine signalling may prevent LtxA-mediated cell damage.

Leukotoxin (Leukothera®) targets active leukocyte function antigen-1 (LFA-1) protein and triggers a lysosomal mediated cell death pathway

Leukotoxin (LtxA) is a protein toxin that is secreted from the oral bacterium, Aggregatibacter actinomycetemcomitans. LtxA targets specifically the β(2) integrin, leukocyte function antigen-1 (LFA-1) on white blood cells (WBCs) and causes cell death. LtxA preferentially targets activated WBCs and is being developed as a therapeutic agent for the treatment of WBC diseases such as hematologic malignancies and autoimmune/inflammatory diseases. However, the mechanism by which interaction between LtxA and LFA-1 results in cell death is not well understood. Furthermore, how LtxA preferentially recognizes activated WBCs is not known. We show here that LtxA interacts specifically with LFA-1 in the active (exposed) conformation. In THP-1 monocytes, LtxA caused rapid activation of caspases, but LtxA could overcome the inhibition of caspases and still intoxicate. In contrast, inhibiting the vesicular trafficking pathway or cathepsin D release from the lysosome resulted in significant inhibition of LtxA-mediated cytotoxicity, indicating a more potent, lysosomal mediated cell death pathway. LtxA caused rapid disruption of the lysosomal membrane and release of lysosomal contents into the cytosol. Binding of LtxA to LFA-1 resulted in the internalization of both LtxA and LFA-1, with LtxA localizing specifically to the lysosomal compartment. To our knowledge, LtxA represents the first bacterial toxin shown to localize to the lysosome where it induces rapid cell death.

Molecular cloning of CD18 of bison, deer and elk, and comparison with that of other ruminants and non-ruminants

Pneumonia caused by Mannheimia haemolytica is an important disease of cattle, domestic sheep, bighorn sheep and goats. Leukotoxin (Lkt) produced by M. haemolytica is cytolytic to all leukocyte subsets of these species. Lkt utilizes CD18, the beta subunit of beta(2)-integrins, as its functional receptor on leukocytes of these species. Cytotoxicity assays revealed that leukocytes from bison, deer, and elk are also susceptible to Lkt-induced cytolysis. The availability of cDNA encoding CD18 of bison, deer and elk would facilitate the comparison of a greater number of ruminant CD18 cDNA with that of non-ruminants as a means of the elucidation of the molecular basis for the specificity of M. haemolytica Lkt for ruminant leukocytes. Herein, we report the cloning and characterization of bison, deer, and elk CD18. The full-length cDNA of bison and deer consists of 2310bp with an ORF encoding 769 amino acids while elk CD18 consists of 2313bp with an ORF encoding 770 amino acids. This gene is highly conserved among ruminants compared with non-ruminants. Phylogenetic analysis based on amino acid sequences showed that CD18 of bison is most closely related to that of cattle while CD18 of deer and elk are more closely related to each other.

Intact signal peptide of CD18, the beta-subunit of beta2-integrins, renders ruminants susceptible to Mannheimia haemolytica leukotoxin

Signal peptides of membrane proteins are cleaved by endoplasmic reticulum-resident signal peptidase, and thus, are not present on mature membrane proteins. Here, we report that, contrary to the paradigm, the signal peptide of ruminant CD18, the beta-subunit of beta(2)-integrins, is not cleaved. Intriguingly, the intact signal peptide of CD18 is responsible for the susceptibility of ruminant leukocytes to Mannheimia (Pasteurella) haemolytica leukotoxin (Lkt). Inhibition of Lkt-induced cytolysis of ruminant leukocytes by CD18 peptide analogs revealed that the Lkt-binding site is formed by amino acids 5-17 of CD18, which, surprisingly, comprise most of the signal sequence. Flow cytometric analysis of ruminant leukocytes indicated the presence of the signal peptide on mature CD18 molecules expressed on the cell surface. Analysis of transfectants expressing CD18 containing the FLAG epitope at the putative cleavage site confirmed that the signal peptide of bovine CD18 is not cleaved. Analysis of the signal sequence of CD18 of eight ruminants and five nonruminants revealed that the signal sequence of CD18 of ruminants contains "cleavage-inhibiting" Q, whereas that of nonruminants contains "cleavage-conducive" G at position -5 relative to the cleavage site. Site-directed mutagenesis of Q to G at position -5 of the signal peptide of bovine CD18 resulted in the cleavage of the signal peptide and abrogation of cytolysis of transfectants expressing bovine CD18 carrying the Q(-5)G mutation. We propose that engineering cattle and other ruminants to contain this mutation would provide a novel technology to render them less susceptible to pneumonic pasteurellosis and concomitant economic losses.

Mannheimia haemolytica leukotoxin-induced cytolysis of caprine (Capra hircus) leukocytes is mediated by the CD18 subunit of beta2-integrins

Mannheimiosis is the major respiratory disease among some ruminants, whereas it is not pathogenic for other mammals, an observation that has been attributed to a specific interaction between Mannheimia haemolytica leukotoxin (Lkt) and bovine or ovine CD18 subunit of lymphocyte function-associated antigen-1 (LFA-1) and Mac-1. We therefore hypothesized that Lkt utilizes CD18 as its receptor on caprine leukocytes as well. We have transiently transfected the beta2-integrins-deficient K-562 cell line with cDNAs encoding caprine CD11a and caprine CD18 to determine the susceptibility of the transfectants to Lkt-induced cytolysis. Flow cytometric analysis of the transfectants revealed surface expression of caprine LFA-1 and lysis by Lkt in a concentration-dependent manner whereas the parent cells were not. Moreover, K562 cells expressing caprine CD18 and human or bovine CD11a were also sensitive to Lkt whereas K-562 cells expressing caprine CD11a and human CD18 were not. Taken together, these results indicate that CD18 on caprine leukocytes serves as a receptor for Lkt.

Effect of LHRH-PE40 on target cells via LHRH receptors

OBJECTIVE

To detect the effect and cytotoxicity of luteinizing hormone-releasing hormone-Pseudomonas aeruginosa exotoxin 40 (LHRH-PE40) on target cells using LHRH receptors (LHRHR).

METHODS

The affinity of LHRH-PE40 and LHRH binding to LHRHR on the membrane surface of target cells were measured by enzyme linked immunosorbent assay. Morphological observations with light microscope were used to analyze its receptor pathway, with Spiegelmer, and cytotoxicity. IC(50) values of LHRH-PE40, which caused 50% inhibition of tumor cell growth were evaluated by MTT assay. The target cells were exposed to LHRH-PE40 and its cytotoxicity was analyzed by scanning and transmission electron microscopies, agarose gel electrophoresis, and flow cytometry.

RESULTS

LHRH-PE40 killed target cells by LHRHR pathway. The morphological changes in these cells showed decreased cell size, cytoplasmic membrane blebbing, and chromatin condensation and margination. At a certain concentration and time point, HeLa cells were also induced to undergo programmed cell death.

CONCLUSION

LHRH-PE40 induced target cells apoptosis via LHRHR.

Mannheimia haemolytica and bovine respiratory disease

Mannheimia haemolytica is the principal bacterium isolated from respiratory disease in feedlot cattle and is a significant component of enzootic pneumonia in all neonatal calves. A commensal of the nasopharynx, M. haemolytica is an opportunist, gaining access to the lungs when host defenses are compromised by stress or infection with respiratory viruses or mycoplasma. Although several serotypes act as commensals, A1 and A6 are the most frequent isolates from pneumonic lungs. Potential virulence factors include adhesin, capsular polysaccharide, fimbriae, iron-regulated outer membrane proteins, leukotoxin (Lkt), lipopolysaccharide (LPS), lipoproteins, neuraminidase, sialoglycoprotease and transferrin-binding proteins. Of these, Lkt is pivotal in induction of pneumonia. Lkt-mediated infiltration and destruction of neutrophils and other leukocytes impairs bacterial clearance and contributes to development of fibrinous pneumonia. LPS may act synergistically with Lkt, enhancing its effects and contributing endotoxic activity. Antibiotics are employed extensively in the feedlot industry, both prophylactically and therapeutically, but their efficacy varies because of inconsistencies in diagnosis and treatment regimes and development of antibiotic resistance. Vaccines have been used for many decades, even though traditional bacterins failed to demonstrate protection and their use often enhanced disease in vaccinated animals. Modern vaccines use culture supernatants containing Lkt and other soluble antigens, or bacterial extracts, alone or combined with bacterins. These vaccines have 50-70% efficacy in prevention of M. haemolytica pneumonia. Effective control of M. haemolytica pneumonia is likely to require a combination of more definitive diagnosis, efficacious vaccines, therapeutic intervention and improved management practices.

Immune evasion by pathogens of bovine respiratory disease complex

Bovine respiratory tract disease is a multi-factorial disease complex involving several viruses and bacteria. Viruses that play prominent roles in causing the bovine respiratory disease complex include bovine herpesvirus-1, bovine respiratory syncytial virus, bovine viral diarrhea virus and parinfluenza-3 virus. Bacteria that play prominent roles in this disease complex are Mannheimia haemolytica and Mycoplasma bovis. Other bacteria that infect the bovine respiratory tract of cattle are Histophilus (Haemophilus) somni and Pasteurella multocida. Frequently, severe respiratory tract disease in cattle is associated with concurrent infections of these pathogens. Like other pathogens, the viral and bacterial pathogens of this disease complex have co-evolved with their hosts over millions of years. As much as the hosts have diversified and fine-tuned the components of their immune system, the pathogens have also evolved diverse and sophisticated strategies to evade the host immune responses. These pathogens have developed intricate mechanisms to thwart both the innate and adaptive arms of the immune responses of their hosts. This review presents an overview of the strategies by which the pathogens suppress host immune responses, as well as the strategies by which the pathogens modify themselves or their locations in the host to evade host immune responses. These immune evasion strategies likely contribute to the failure of currently-available vaccines to provide complete protection to cattle against these pathogens.

Transfection of non-susceptible cells with Ovis aries recombinant lymphocyte function-associated antigen 1 renders susceptibility to Mannheimia haemolytica leukotoxin

Mannheimia haemolytica is an important etiological agent of pneumonia in domestic sheep (DS, Ovis aries). Leukotoxin (Lkt) produced by this organism is the principal virulence factor responsible for the acute inflammation and lung injury characteristic of M. haemolytica caused disease. Previously, we have shown that the leukocyte-specific integrins, beta(2) integrins, serve as the receptor for Lkt. Although it is certain that CD18, the beta subunit of beta(2) integrins, mediates Lkt-induced cytolysis of leukocytes, it is not clear whether CD18 of all three beta(2) integrins, LFA-1, Mac-1 and CR4, mediates Lkt-induced cytolysis of DS leukocytes. Since polymorphonuclear leukocytes, which express all three beta(2) integrins, are the leukocyte subset that is most susceptible to Lkt, we hypothesized that all three beta(2) integrins serve as the receptor for Lkt. The objective of this study was to determine whether DS LFA-1 serves as a receptor for M. haemolytica Lkt. We cloned the cDNA for DS CD11a, the alpha subunit of LFA-1, and co-transfected it along with the previously cloned cDNA for DS CD18, into a Lkt-non-suceptible cell line. Transfectants stably expressing DS LFA-1 were bound by Lkt. More importantly, Lkt lysed the DS LFA-1 transfectants in a concentration-dependent manner. Pre-incubation of Lkt with a Lkt-neutralizing monoclonal antibody (MAb), or pre-incubation of transfectants with MAbs specific for DS CD11a or CD18, inhibited Lkt-induced cytolysis of the transfectants. Exposure of LFA-1 transfectants to low concentrations of Lkt resulted in elevation of intracellular [Ca(2+)](i). Taken together, these results indicate that DS LFA-1 serves as a receptor for M. haemolytica Lkt.

Monomeric expression of bovine beta2-integrin subunits reveals their role in Mannheimia haemolytica leukotoxin-induced biological effects

The ruminant-specific leukotoxin (Lkt) of Mannheimia haemolytica is the key virulence factor contributing to the pathogenesis of lung injury in bovine pneumonic pasteurellosis. Previous studies by us and others indicate that M. haemolytica Lkt binds to CD18, the beta subunit of bovine beta(2)-integrins on leukocytes, and that the species specificity of Lkt-induced effects is resident in the beta subunit CD18 and not in the alpha subunit CD11. However, Lkt also binds to the CD11a subunit of LFA-1. Furthermore, antibodies specific for CD18 or CD11a inhibit signaling events leading to elevation of intracellular [Ca(2+)], tyrosine phosphorylation of the cytosolic domain of CD18, and cytolysis of bovine leukocytes. These observations underscore the need for further investigation to identify the precise subunit of bovine LFA-1 utilized by M. haemolytica Lkt as the functional receptor. For this purpose, monomeric bovine CD18 and CD11a and heterodimeric LFA-1 were expressed in the HEK-293 cell line by transfection, and the resulting transfectants were tested for susceptibility to Lkt-induced effects. All three transfectants effectively bound Lkt. However, Lkt-induced cytolysis was observed only with transfectants expressing monomeric bovine CD18 or LFA-1. Furthermore, intracellular [Ca(2+)] elevation following exposure to Lkt, which is a marker for postbinding signaling leading to cellular activation, was seen only with transfectants expressing monomeric bovine CD18 or LFA-1. These results clearly indicate that the bovine CD18 subunit of beta(2)-integrins is the functional receptor for M. haemolytica Lkt.

Aggregatibacter actinomycetemcomitans leukotoxin requires beta-sheets 1 and 2 of the human CD11a beta-propeller for cytotoxicity

Aggregatibacter actinomycetemcomitans leukotoxin (Ltx) is a repeats-in-toxin (RTX) cytolysin that kills human leukocyte function-associated antigen-1 (LFA-1; alpha(L)/beta(2))-bearing cells. In order to determine whether the alpha(L) portion of the heterodimer is involved in Ltx recognition, we transfected human, mouse and bovine alpha(L) cDNAs into J-beta(2).7, an alpha(L)-deficient cell line, and looked for restoration of Ltx susceptibility. Cells expressing either bovine or human alpha(L) in conjunction with human beta(2) were efficiently killed by Ltx, an indication that bovine alpha(L) could substitute for its human counterpart in critical regions used by Ltx for attachment to LFA-1. On the other hand, cells expressing murine alpha(L) and human beta(2) were not susceptible to the lethal effects of Ltx indicating that the toxin recognition sites are not present in the corresponding mouse sequence. To further identify the region(s) of alpha(L) recognized by Ltx, we constructed and evaluated a panel of chimeric human/murine alpha(L) genes in J-beta(2).7 cells. Analysis of the alpha(L) mutant panel showed that the presence of human N-terminal 128 amino acids on a mouse CD11a background, a region that includes beta-sheets 1 and 2 of the beta-propeller of the human alpha(L) chain, was sufficient for Ltx cytolysis.

Mannheimia haemolytica leukotoxin-induced cytolysis of ovine (Ovis aries) leukocytes is mediated by CD18, the β subunit of β2-integrins

Mannheimia (Pasteurella) haemolytica causes severe pneumonia in cattle, sheep and goats. Leukotoxin (Lkt) is the most important virulence determinant produced by this organism. Previously, we identified CD18, the beta subunit of beta(2)-integrins, as the receptor for Lkt on bovine leukocytes. Since Lkt is specific for leukocytes of cattle, sheep and goats, we hypothesized that Lkt utilizes CD18 as its receptor on ovine leukocytes as well. Therefore, the objective of this study was to transfect an Lkt-resistant murine cell line (P815) with cDNA encoding ovine CD18, and to determine the susceptibility of the transfectants to Lkt-induced cytolysis. cDNA for ovine CD18 cloned from polymorphonuclear leukocytes was transfected into P815 cells. Flow cytometric analysis of the transfectants revealed surface expression of ovine CD18, and Lkt binding. In a cytotoxicity assay, the transfectants were lysed by Lkt in a concentration-dependent manner, whereas the parent cells were not. Pre-incubation of Lkt with an anti-Lkt neutralizing antibody and pre-incubation of transfectants with an anti-CD18 antibody resulted in inhibition of cytolysis confirming the interaction between Lkt and CD18. Taken together, these results indicate that CD18 on ovine leukocytes serves as a receptor for Lkt, and that CD18 is sufficient to mediate Lkt-induced cytolysis of ovine leukocytes.

Complexities of the pathogenesis ofMannheimia haemolyticaandHaemophilus somnusinfections: challenges and potential opportunities for prevention?

Progress in producing improved vaccines against bacterial diseases of cattle is limited by an incomplete understanding of the pathogenesis of these agents. Our group has been involved in investigations of two members of the family Pasteurellaceae,Mannheimia haemolyticaandHaemophilus somnus, which illustrate some of the complexities that must be confronted. Susceptibility toM. haemolyticais greatly increased during active viral respiratory infection, resulting in rapid onset of a severe and even lethal pleuropneumonia. Despite years of investigation, understanding of the mechanisms underlying this viral–bacterial synergism is incomplete. We have investigated the hypothesis that active viral infection increases the susceptibility of bovine leukocytes to theM. haemolyticaleukotoxin by increasing the expression of or activating the β2integrin CD11a/CD18 (LFA-1) on the leukocyte surface.In vitroexposure to proinflammatory cytokines (i.e. interleukin-1β, tumor necrosis factor-α and interferon-γ) increases LFA-1 expression on bovine leukocytes, which in turn correlates with increased binding and responsiveness to the leukotoxin. Alveolar macrophages and peripheral blood leukocytes from cattle with active bovine herpesvirus-1 (BVH-1) infection are more susceptible to the lethal effects of the leukotoxinex vivothan leukocytes from uninfected cattle. Likewise,in vitroincubation of bovine leukocytes with bovine herpesvirus 1 (BHV-1) potentiates LFA-1 expression and makes the cells more responsive to leukotoxin. A striking characteristic ofH. somnusinfection is its propensity to cause vasculitis. We have shown thatH. somnusand its lipo-oligosaccharide (LOS) trigger caspase activation and apoptosis in bovine endothelial cellsin vitro. This effect is associated with the production of reactive oxygen and nitrogen intermediates, and is amplified in the presence of platelets. The adverse effects ofH. somnusLOS are mediated in part by activation of endothelial cell purinergic receptors such as P2X7. Further dissection of the pathways that lead to endothelial cell damage in response toH. somnusmight help in the development of new preventive or therapeutic regimens. A more thorough understanding ofM. haemolyticaandH. somnusvirulence factors and their interactions with the host might identify new targets for prevention of bovine respiratory disease.

Phylogenetic diversity of Pasteurellaceae and horizontal gene transfer of leukotoxin in wild and domestic sheep

Wild and domestic animal populations are known to be sources and reservoirs of emerging diseases. There is also a growing recognition that horizontal genetic transfer (HGT) plays an important role in bacterial pathogenesis. We used molecular phylogenetic methods to assess diversity and cross-transmission rates of Pasteurellaceae bacteria in populations of bighorn sheep, Dall's sheep, domestic sheep and domestic goats. Members of the Pasteurellaceae cause an array of deadly illnesses including bacterial pneumonia known as "pasteurellosis", a particularly devastating disease for bighorn sheep. A phylogenetic analysis of a combined dataset of two RNA genes (16S ribosomal RNA and RNAse P RNA) revealed remarkable evolutionary diversity among Pasteurella trehalosi and Mannheimia (Pasteurella) haemolytica bacteria isolated from sheep and goats. Several phylotypes appeared to associate with particular host species, though we found numerous instances of apparent cross-transmission among species and populations. Statistical analyses revealed that host species, geographic locale and biovariant classification, but not virulence, correlated strongly with Pasteurellaceae phylogeny. Sheep host species correlated with P. trehalosi isolates phylogeny (PTP test; P=0.002), but not with the phylogeny of M. haemolytica isolates, suggesting that P. trehalosi bacteria may be more host specific. With regards to populations within species, we also discovered a strong correlation between geographic locale and isolate phylogeny in the Rocky Mountain bighorn sheep (PTP test; P=0.001). We also investigated the potential for HGT of the leukotoxin A (lktA) gene, which produces a toxin that plays an integral role in causing disease. Comparative analysis of the combined RNA gene phylogeny and the lktA phylogenies revealed considerable incongruence between the phylogenies, suggestive of HGT. Furthermore, we found identical lktA alleles in unrelated bacterial species, some of which had been isolated from sheep in distantly removed populations. For example, lktA sequences from P. trehalosi isolated from remote Alaskan Dall's sheep were 100% identical over a 900-nucleotide stretch to sequences determined from M. haemolytica isolated from domestic sheep in the UK. This extremely high degree of sequence similarity of lktA sequences among distinct bacterial species suggests that HGT has played a role in the evolution of lktA in wild hosts.

Actinobacillus actinomycetemcomitans leukotoxin requires lipid microdomains for target cell cytotoxicity

Actinobacillus actinomycetemcomitans produces a leukotoxin (Ltx) that kills leukocyte function-associated antigen-1 (LFA-1)-bearing cells from man, the Great Apes and Old World monkeys. The unique specificity of Ltx for the beta2 integrin, LFA-1, suggests it is capable of providing insight into the pathogenic mechanisms of Ltx and other RTX toxins. Using the Jurkat T cell line and an LFA-1-deficient Jurkat mutant (Jbeta2.7) as models, we found the initial effect of Ltx is to elevate cytosolic Ca2+ [Ca2+]c, an event that is independent of the Ltx/LFA-1 interaction. [Ca2+]c increases initiate a series of events that involve the activation of calpain, talin cleavage, mobilization to, and subsequent clustering of, LFA-1 in cholesterol and sphingolipid-rich regions of the plasma membrane known as lipid rafts. The association of Ltx and LFA-1 within lipid rafts is essential for cell lysis. Jbeta2.7 cells fail to accumulate Ltx in their raft fractions and are not killed, while cholesterol depletion experiments demonstrate the necessity of raft integrity for Ltx function. We propose that toxin-induced Ca2+ fluxes mobilize LFA-1 to lipid rafts where it associates with Ltx. These findings suggest that Ltx utilizes the raft to stimulate an integrin signalling pathway that leads to apoptosis of target cells.

Contribution of respiratory burst activity to innate immune function and the effects of disease status and agent on chemiluminescence responses by ruminant phagocytes in vitro

The mechanisms of interaction between phagocytes and different bacteria that help resolve lung infections or contribute to lung pathology are poorly defined. Alveolar phagocytes (resident macrophages and recruited neutrophils) make a major contribution to innate immunity by mounting a respiratory burst that helps kill internalised bacteria. However, this ability may be altered during or after exposure to infection. This review considers the application and limitations of a variety of analytical methods for oxygen-dependent mechanisms of respiratory burst in phagocytes initiated by soluble and particulate activators. Particular reference is given to the study in vitro of phagocytes from healthy and diseased ruminants during either natural infection with Mycobacterium avium paratuberculosis or experimental infection with Pasteurella multocida or Mannheimia haemolytica.

Macrophage cytotoxicity produced by adenylate cyclase toxin from Bordetella pertussis: more than just making cyclic AMP!

The cytotoxic effect of adenylate cyclase (AC) toxin from Bordetella pertussis on host cells has been attributed to the production of supraphysiologic levels of cyclic AMP by the toxin. We have tested this hypothesis and show that at least two different mechanisms, cAMP accumulation/ATP depletion and oligomerization/pore formation, contribute, perhaps synergistically, to AC toxin-induced cytotoxicity. Wild-type (WT) AC toxin causes cell death associated with an increase in cAMP, a reduction in ATP, activation of caspases 3/7, and increased annexin V and TUNEL staining. In contrast, a non-acylated, enzymatically active, non-haemolytic form of AC toxin is able to increase cAMP, reduce ATP and elicit annexin V staining, but the decrease in ATP and the annexin staining are transient and there is minimal caspase activation, TUNEL staining and cell death. Mutant AC toxins defective in either enzymatic activity or the ability to deliver their enzymatic domain are able to kill J774 cells, without cAMP production, and with minimal caspase activation and TUNEL staining. Comparison of the potencies of WT toxin and those of mutants that only increase cAMP or only create transmembrane pores establishes that at least two mechanisms are contributory and that simply the production of cAMP is not enough to account for the cytotoxicity produced by AC toxin.

Mannheimia (Pasteurella) haemolytica leukotoxin binding domain lies within amino acids 1 to 291 of bovine CD18

Previously, we identified bovine CD18 as the receptor for leukotoxin secreted by Mannheimia (Pasteurella) haemolytica. In this study, we constructed bovine-murine CD18 chimeras to locate the leukotoxin binding domain on CD18. Leukotoxin specifically lysed transfectants expressing bovine CD18 fragment encompassing amino acids 1 to 291, indicating that leukotoxin binding domain lies within amino acids 1 to 291 of bovine CD18.

Mannheimia haemolytica leukotoxin induces apoptosis of bovine lymphoblastoid cells (BL-3) via a caspase-9-dependent mitochondrial pathway

Mannheimia haemolytica is a key pathogen in the bovine respiratory disease complex. It produces a leukotoxin (LKT) that is an important virulence factor, causing cell death in bovine leukocytes. The LKT binds to the beta(2) integrin CD11a/CD18, which usually activates signaling pathways that facilitate cell survival. In this study, we investigated mechanisms by which LKT induces death in bovine lymphoblastoid cells (BL-3). Incubation of BL-3 cells with a low concentration of LKT results in the activation of caspase-3 and caspase-9 but not caspase-8. Similarly, the proapoptotic proteins Bax and BAD were significantly elevated, while the antiapoptotic proteins Bcl-2, Bcl(XL) and Akt-1 were downregulated. Following exposure to LKT, we also observed a reduction in mitochondrial cytochrome c and corresponding elevation of cytosolic cytochrome c, suggesting translocation from the mitochondrial compartment to the cytosol. Consistent with this observation, tetramethylrhodamine ethyl ester perchlorate staining revealed that mitochondrial membrane potential was significantly reduced. These data suggest that LKT induces apoptosis of BL-3 cells via a caspase-9-dependent mitochondrial pathway. Furthermore, scanning electron micrographs of mitochondria from LKT-treated BL-3 cells revealed lesions in the outer mitochondrial membrane, which are larger than previous reports of the permeability transition pore through which cytochrome c is usually released.

Apoptotic activity and sub-cellular localization of a T4SS-associated CagE-homologue in Actinobacillus actinomycetemcomitans

A potent virulence factor, cagE homologue of Actinobacillus actinomycetemcomitans, was identified via an expression cloning strategy and periodontitis-associated CD4(+)T-cells of a humanized mouse model. Through the immuno-gold labeling with transmission electron microscopy, immunofluorescent staining, in vitro co-cultures and Western blot studies, the resulting data clearly demonstrate that: (i) in CagE-homologue treated human epithelia in vitro, there are ultra-structural features of plasma membrane blebbing, sub-cellular disorganization with condensed and marginalized chromatins along the nuclear membrane, consistent with the pro-apoptotic characteristics, (ii) the disturbed membrane integrity detected above is associated with localization of the CagE proteins on target cell surface, and (iii) CagE-homologue is located in the cytoplasm of A. actinomycetemcomitans and associated with a bacterial type-IV secretion system (T4SS), suggesting that its translocation is required for secretion. Thus, CagE-homologue may be critically involved in A. actinomycetemcomitans-induced tissue destruction, inflammation and subsequent adverse immunity in periodontal pathogenesis.

Mechanisms underlying Mannheimia haemolytica leukotoxin-induced oncosis and apoptosis of bovine alveolar macrophages

Mannheimia (Pasteurella) haemolytica leukotoxin (LktA) binds to the bovine beta2 integrins (such as LFA-1-CD11a/CD18) and leads to subsequent cellular effects in a dose dependent manner. The objectives of this study were to delineate the mechanisms that underlie LktA-induced oncosis and apoptosis and to examine the role of LktA/LFA-1 interaction in these events. The results demonstrate that LktA-induced oncosis proceeds through a LFA-1 and caspase-1 dependent pathway referred to as 'pyrotosis', as well as through a LFA-1- and caspase-1-independent pathway. LktA-induced apoptosis in alveolar macrophages involves activation of caspase-3 and engages the extrinsic and intrinsic pathways of apoptosis, with the extrinsic pathway being dependent on LFA-1 signaling and TNFalpha.

Recombinant expression of bovine LFA-1 and characterization of its role as a receptor for Mannheimia haemolytica leukotoxin

Mannheimia (Pasteurella) haemolytica leukotoxin (LktA) is the primary virulence factor contributing to the pathogenesis of lung injury in bovine pneumonic mannheimiosis (BPM), a disease which causes major economic loss to the US cattle industry annually. Recent work from our laboratory using an antibody-based approach has shown that LktA binds to bovine LFA-1 in target cells. While this study suggests that LFA-1 might be a specific receptor, it remains to be conclusively shown that LFA-1 is sufficient to induce susceptibility to LktA. It was of interest to determine if functionally active bovine LFA-1 could be reconstituted on a LFA-1 negative cell line and reconstitute susceptibility to LktA. Here, we report the successful recombinant expression of bovine LFA-1 on the cell surface of the human erythroleukemic K562 cell line. The BoLFA-1 transductant expresses bovine CD18 and CD11a as a heterodimer. We found that LktA binds to both the CD18 and CD11a subunits of BoLFA-1 cells. Exposure of BoLFA-1 cells to LktA, induced tyrosine phosphorylation of the CD18 tail, elevation of intracellular calcium, and cytolysis. This is the first report on recombinant expression of functionally active bovine LFA-1 by transduction into an LktA-non-susceptible human cell line.

Incubation of bovine PMNs with conditioned medium from BHV-1 infected peripheral blood mononuclear cells increases their susceptibility to Mannheimia haemolytica leukotoxin

Active infection with bovine herpesvirus-1 (BHV-1) increases the susceptibility of cattle to secondary bacterial pneumonia with Mannheimia (Pasteurella) haemolytica A1. In the present study we found that bovine PMNs incubated with conditioned media from BHV-1 infected peripheral blood mononuclear cells (PBMCs) exhibited increased LFA-1 expression, enhanced LKT binding and increased LKT cytotoxicity. These effects were abrogated when the conditioned medium was pre-incubated with an anti-IL-1beta Mab before being added to the PMNs. These findings suggest that BHV-1 infection, and the resulting release of IL-1beta and perhaps other inflammatory cytokines, can stimulate activation of LFA-1 in bystander bovine PMNs, thus enhancing the binding and biological effects of LKT.

BHV-1 infection and inflammatory cytokines amplify the interaction of Mannheimia haemolytica leukotoxin with bovine peripheral blood mononuclear cells in vitro

Bovine herpesvirus-1 (BHV-1) has been reported to increase the susceptibility of cattle to respiratory disease caused by Mannheimia (Pasteurella) haemolytica A1. The principal virulence factor of M. haemolytica is a leukotoxin (LKT) that can specifically kill ruminant leukocytes following its binding to the beta2-integrin CD11a/CD18 (lymphocyte function-associated antigen 1 (LFA-1)). In this study, we investigated the effects of experimental infection of bovine peripheral blood mononuclear cells (MNCs) with BHV-1 in vitro, on the subsequent interaction of these cells with the M. haemolytica LKT. We found that BHV-1 infection increased LFA-1 expression (as assessed by flow cytometry), and subsequently enhanced LKT binding and cytotoxicity to bovine MNCs. We also found that BHV-1 infection increased CD18, IL-1beta, and IFN-gamma mRNA expression by MNCs. As previously reported for bovine polymorphonuclear neutrophils (PMNs), MNCs increased their expression of LFA-1, and their LKT binding and cytotoxicity, following exposure to IL-1beta, TNF-alpha, and IFN-gamma. These findings suggest that BHV-1 infection, and the resulting release of inflammatory cytokines, can stimulate expression of LFA-1 in bovine MNCs, thus enhancing the binding and biological effects of LKT. If such a mechanism occurs in vivo it might explain, in part, the increased susceptibility of BHV-1 infected cattle to bovine pasteurellosis.

Localization of linear cytotoxic and pro-apoptotic epitopes in RTX toxin ApxIII of Actinobacillus pleuropneumoniae

RTX toxin ApxIII secreted by Actinobacillus pleuropneumoniae affects porcine pulmonary alveolar macrophages and neutrophils. The distribution of ApxIII linear cytotoxic and pro-apoptotic determinants was characterized by neutrophil protection assay, DNA fragmentation and caspase-3 activity using antisera produced against its N-terminus, hydrophobic domain, activation domain, calcium-binding domain and C-terminal half. Neutralization of ApxIII cytotoxicity and pro-apoptotic activities was found in antisera raised against its N- and C-termini whereas ApxIII activation domain-specific antiserum expressed cytotoxic-neutralizing activity. No neutralizing activities were detected in antisera against other structural domains of ApxIII suggesting that the putative linear cytotoxic and pro-apoptotic determinants of ApxIII were mapped to its terminal domains and activation domains.

Actinobacillus actinomycetemcomitansCytolethal Distending Toxin (Cdt): Evidence That the Holotoxin Is Composed of Three Subunits: CdtA, CdtB, and CdtC

We have shown the Actinobacillus actinomycetemcomitans produces an immunosuppressive factor encoded by the cytolethal distending toxin (cdt)B gene, which is homologous to a family of Cdts expressed by several Gram-negative bacteria. We now report that the capacity for CdtB to induce G(2) arrest in Jurkat cells is greater in the presence of the other Cdt peptides: CdtA and CdtC. Plasmids containing the cdt operon were constructed and expressed in Escherichia coli; each plasmid contained a modified cdt gene that expressed a Cdt peptide containing a C-terminal His tag. All three Cdt peptides copurified with the His-tagged Cdt peptide. Each of the peptides associated with the complex was truncated; N-terminal amino acid analysis of CdtB and CdtC indicated that the truncation corresponds to cleavage of a previously described signal sequence. CdtA was present in two forms in crude extracts, 25 and 18 kDa; only the 18-kDa fragment copurified with the Cdt complexes. Cdt complexes were also immunoprecipitated from A. actinomycetemcomitans extracts using anti-CdtC mAb. Exposure of Jurkat cells to 40 pg resulted in >50% accumulation of G(2) cells. CdtB and CdtC were detected by immunofluorescence on the cell surface after 2-h exposure to the holotoxin. CdtA was not detected by immunofluorescence, but all three peptides were associated with Jurkat cells when analyzed by Western blot. These studies suggest that the active Cdt holotoxin is a heterotrimer composed of truncated CdtA, CdtB, and CdtC, and all three peptides appear to associate with lymphocytes.

Prior exposure to Mannheimia haemolytica leukotoxin or LPS enhances beta(2)-integrin expression by bovine neutrophils and augments LKT cytotoxicity

Mannheimia (Pasteurella) haemolytica serotype1 produces a variety of virulence factors that play an important role during the pathogenesis of bovine pneumonic pasteurellosis. Among these, a leukotoxin (LKT) and lipopolysaccharide (LPS) are thought to be the primary virulence factors that contribute to the characteristic pathology of pasteurellosis. Recent evidence suggests that M. haemolytica LKT binding to bovine leukocytes is mediated by the beta(2)-integrin CD11a/CD18 (LFA-1), which subsequently induces activation and death of these cells. Exposure of bovine peripheral blood neutrophils (PMNs) to LKT or LPS induces expression of inflammatory cytokines, which in turn can increase LFA-1 expression and conformational activation. In this study we demonstrated, by flow cytometry and Western blot, that bovine PMNs increased their LFA-1 expression following in vitro exposure to M. haemolytica LKT and LPS. Increased LFA-1 expression by PMNs exposed to LKT and LPS was associated with increased LKT binding and cell death. The results of this study suggest that M. haemolytica LKT and LPS might cooperatively increase LFA-1 expression, and by so doing amplify the lung inflammation that characterizes bovine pasteurellosis.

Biological effects of two genetically defined leukotoxin mutants of Mannheimia haemolytica

Mannheimia(Pasteurella)haemolytica serotype 1 is the primary causative agent responsible for bovine pneumonic mannheimiosis, also known as shipping fever in cattle. The bacterium produces a variety of virulence factors, foremost of which is the exotoxic leukotoxin. The leukotoxin is a calcium-dependent cytolysin that is a member of the RTX (repeats in toxin) family and exhibits a narrow cell-type and species specificity and has biological effects only on ruminant leukocytes and platelets. The genetic organization of the leukotoxin is comprised of four genes: lktC, lktA, lktB and lktD. The lktA structural gene encodes the protoxin (pro-LktA) and lktC encodes a transacylase that post-translationally modifies the inactive pro-LktA to a biologically active wild-type leukotoxin (LktA). The LktA has been implicated as the key factor that contributes to the pathogenesis of lung injury associated with the disease and considerable efforts have been employed in abrogating toxin function while retaining immunogenicity, with an eye towards design of attenuated vaccines. We hypothesized that the pro-LktA retains the ability to cause biological effects on target cells as has been reported in the case of the closely related RTX toxin alpha-hemolysin (HlyA). We also examined the biological effects of an amino-terminal truncation mutant leukotoxin DeltaLktA on target cells. Thus the objectives of our study were to investigate whether two different mutant leukotoxins, one a nonacylated pro-LktA, and the other lacking 344 amino acids at the N-terminal end of the LktA protein; DeltaLktA, are capable of (i). binding to the beta2-integrin leukotoxin receptor, (ii). inducing the elevation of second messenger intracellular calcium ([Ca(2+)](i)), and (iii). inducing inflammatory gene expression, reactive oxygen metabolites (ROMs) and cytolysis in target cells. Our results demonstrate that neither acylation nor the amino terminal 344 amino acids are required for LktA binding but are essential for LktA-induced [Ca(2+)](i) elevation, generation of ROM, generation of the inflammatory cytokine IL-8 and cytolysis in target cells.

Role of Mannheimia haemolytica leukotoxin in the pathogenesis of bovine pneumonic pasteurellosis

Bovine pneumonic pasteurellosis continues to be a major respiratory disease in feedlot cattle despite the recent advances in our understanding of the underlying complexities of causation. The etiological agent, Mannheimia haemolytica, possesses several virulence factors, including capsule, outer membrane proteins, adhesins, neuraminidase, endotoxin and exotoxic leukotoxin. Accumulating scientific evidence implicates leukotoxin as the primary factor contributing to clinical presentation and lung injury associated with this disease. Unlike other virulence factors, leukotoxin shows cell-type- and species-specific effects on bovine leukocytes. Recent investigations have delineated the mechanisms underlying the target-cell-specificity of leukotoxin and how this contributes to the pathogenesis of lung damage. This review summarizes current understanding of the secretion, regulation, mechanisms of action and evolutionary diversity of leukotoxin of M. haemolytica. Understanding the precise molecular mechanisms of leukotoxin is critical for the development of more effective prophylactic and therapeutic strategies to control this complex disease.

Bovine CD18 is necessary and sufficient to mediate Mannheimia (Pasteurella) haemolytica leukotoxin-induced cytolysis

Leukotoxin (Lkt) secreted by Mannheimia (Pasteurella) haemolytica is an RTX toxin which is specific for ruminant leukocytes. Lkt binds to beta(2) integrins on the surface of bovine leukocytes. beta(2) integrins have a common beta subunit, CD18, that associates with three distinct alpha chains, CD11a, CD11b, and CD11c, to give rise to three different beta(2) integrins, CD11a/CD18 (LFA-1), CD11b/CD18 (Mac-1), and CD11c/CD18 (CR4), respectively. Our earlier studies revealed that Lkt binds to all three beta(2) integrins, suggesting that the common beta subunit, CD18, may be the receptor for Lkt. In order to unequivocally elucidate the role of bovine CD18 as a receptor for Lkt, a murine cell line nonsusceptible to Lkt (P815) was transfected with cDNA for bovine CD18. One of the transfectants, 2B2, stably expressed bovine CD18 on the cell surface. The 2B2 transfectant was effectively lysed by Lkt in a concentration-dependent manner, whereas the P815 parent cells were not. Immunoprecipitation of cell surface proteins of 2B2 with monoclonal antibodies specific for bovine CD18 or murine CD11a suggested that bovine CD18 was expressed on the cell surface of 2B2 as a heterodimer with murine CD11a. Expression of bovine CD18 and the Lkt-induced cytotoxicity of 2B2 cells were compared with those of bovine polymorphonuclear neutrophils and lymphocytes. There was a strong correlation between cell surface expression of bovine CD18 and percent cytotoxicity induced by Lkt. These results indicate that bovine CD18 is necessary and sufficient to mediate Lkt-induced cytolysis of target cells.

Fusobacterium necrophorum leukotoxin induces activation and apoptosis of bovine leukocytes

Fusobacterium necrophorum, a gram-negative, rod-shaped, anaerobic bacterium, is a primary or secondary etiological agent in a variety of necrotic, purulent infections in humans and animals. Its major virulence factor is leukotoxin, a high-molecular-weight secreted protein, primarily toxic to ruminant leukocytes. In this study, bovine peripheral blood leukocytes were exposed to various concentrations of immunoaffinity-purified leukotoxin and the cytotoxicity was analyzed by flow cytometry and scanning and transmission electron microscopy. At very low toxin concentrations, polymorphonuclear leukocytes (PMNs) showed activation, as indicated by translocation of primary and secondary granules to the periphery of the cytoplasm. Furthermore, these cells showed changes characteristic of apoptosis, including decreased cell size, organelle condensation, cytoplasmic membrane blebbing (zeiosis), and chromatin condensation and margination, and decrease in cellular DNA content. At moderately high concentrations of leukotoxin, bovine mononuclear cells were also induced to undergo programmed cell death. At very high concentrations, leukotoxin caused necrotic cell death of bovine peripheral leukocytes. The ability of F. necrophorum leukotoxin to modulate the host immune system by its toxicity, including cellular activation of PMNs and apoptosis-mediated killing of phagocytes and immune effector cells, represents a potentially important mechanism of its pathogenesis.

Inflammatory cytokines enhance the interaction of Mannheimia haemolytica leukotoxin with bovine peripheral blood neutrophils in vitro

Mannheimia (Pasteurella) haemolytica A1 produces several virulence factors that play an important role in the pathogenesis of bovine pneumonic pasteurellosis. Foremost among these is a leukotoxin (LKT) that specifically kills ruminant leukocytes. Recent evidence suggests that M. haemolytica LKT binding to bovine leukocytes is mediated by the beta(2)-integrin CD11a/CD18 (lymphocyte function-associated antigen 1 [LFA-1]), which subsequently induces activation and cytolysis of these cells. Inflammatory cytokines, which are released during viral and bacterial infection, are reported to increase LFA-1 expression and conformational activation. We investigated the effects of the inflammatory cytokines interleukin-1beta (IL-1beta), tumor necrosis factor alpha (TNF-alpha), and gamma interferon (IFN-gamma) on the interaction of M. haemolytica LKT with bovine peripheral blood neutrophils (PMNs). In this study we demonstrated, by flow cytometry, that bovine PMNs increased their binding to an anti-bovine LFA-1 monoclonal antibody (BAT75A) following in vitro incubation with IL-1beta, TNF-alpha, or IFN-gamma. Incubation with cytokines also increased CD18 expression, as assessed by real-time PCR and by Western blotting. Increased LFA-1 expression by PMNs exposed to cytokines was associated with increased LKT binding and cytotoxicity. The latter represented, at least in part, enhanced PMN apoptosis, as assessed by propidium iodine staining and caspase-3 activation. The results of this study suggest that inflammatory cytokines may play an important role in enhancing the biological response of bovine PMNs to M. haemolytica LKT.

Trialysin, a novel pore-forming protein from saliva of hematophagous insects activated by limited proteolysis

We have characterized a pore-forming lytic protein from the saliva of the hematophagous insect Triatoma infestans, a vector of Chagas disease. This protein, named trialysin, has 22 kDa and is present in the saliva at about 200 microg/ml. Purified trialysin forms voltage-dependent channels in planar lipid bilayers with conductance of 880 +/- 40 pS. It lyses protozoan parasites and bacteria indicating that it has a role in the control of microorganism growth in the salivary glands. At higher concentrations, but below those found in saliva, trialysin can also permeabilize and lyse mammalian cells, suggesting that it might also facilitate insect blood feeding by interfering with the cell response of the host. The translated cDNA sequence of trialysin shows a basic, lysine-rich protein in which the N-terminal region is predicted to form an amphipathic alpha-helical structure with positive charges on one side and hydrophobic amino acids on the opposite side. A synthetic peptide corresponding to this cationic amphipathic alpha-helix induces protozoan lysis and mammalian cell permeabilization, showing that this region is involved in lytic activity. However, the lytic peptide G6V32 is 10-fold less efficient than trialysin in lysing parasites and 100-fold less efficient in permeabilizing mammalian cells. Trialysin activity is about 10-fold reduced in salivary gland homogenates prepared in the presence of an irreversible serine-protease inhibitor. Since trialysin precursor contains an anionic pro-sequence of 33 amino acids contiguous to the cationic amphipathic putative alpha-helix, we propose that removal of the acidic pro-sequence by limited proteolysis activates trialysin by exposing this lytic basic amphipathic motif.

Effect of experimental infection of cattle with bovine herpesvirus-1 (BHV-1) on the ex vivo interaction of bovine leukocytes with Mannheimia (Pasteurella) haemolytica leukotoxin

Mannheimia (Pasteurella) haemolytica A1 produces an extracellular leukotoxin (LKT) that is reported to bind the beta(2)-integrin CD11a/CD18 (LEA-1) on ruminant leukocytes. LKT binding induces activation, and subsequent cytolysis, of these cells. It is well known that active viral infection greatly increases the susceptibility of cattle to pasteurellosis. To better understand the mechanism by which this occurs, we investigated the effects of experimental in vivo infection of cattle with bovine herpes virus-1 (BHV-1) on the ex vivo interaction of bovine leukocytes with the M. haemolytica LKT. In this study, we demonstrated that active BHV-1 infection increased the expression of the beta(2)-integrin CD11a/CD18 (as defined by the mAb BAT75) on bovine peripheral blood neutrophils, enhanced the binding of LKT to bronchoalveolar lavage (BAL) leukocytes and peripheral blood neutrophils, and increased the killing of BAL leukocytes and peripheral blood leukocytes by LKT. In addition, BHV-1 greatly increased the number of BAL, resulting in many more LKT-responsive cells being present in the lungs. These findings might explain in part the increased susceptibility of BHV-1 infected cattle to pneumonic pasteurellosis.

Maintenance of oxidative phosphorylation protects cells from Actinobacillus actinomycetemcomitans leukotoxin-induced apoptosis

Subnanomolar concentrations (3 x 10(-10) M) of Actinobacillus actinomycetemcomitans leukotoxin (Ltx) trigger apoptosis of JY cells, as shown by a decrease in mitochondrial transmembrane potential (DeltaPsim), hyperproduction of reactive oxygen species (ROS) and release of cytochrome c from the intermembrane space. When compared with heat-inactivated leukotoxin (DeltaI Ltx) controls, ATP levels in Ltx-treated JY cells continued to decrease during a 24 h experiment while cytoplasmic ADP concentrations were increasing. These results suggest that a blockage occurred in ATP/ADP exchange. To maintain ATP/ADP exchange, JY cells were transfected with bcl-2 and bcl-xL and incubated with Ltx. ATP levels of the transfected cells decreased to 67% (JY/bcl-2) and 73% (JY/bcl-xL) after the experiment. Furthermore, cytochrome c remained localized to the mitochondrial fraction of Ltx-treated JY/bcl-2 and JY/bcl-xL cells, whereas its presence in the cytoplasmic fraction of JY/gen cells suggests an uncoupling of electron transport. Expression of bcl-2 and bcl-xL in cells inhibited downstream apoptotic events such as cleavage of poly(ADP-ribose) polymerase, DNA fragmentation and activation of a family of caspases. The results indicate that Ltx induces apoptosis through a mitochondrial pathway that involves decreased levels of the ADP in the mitochondrial matrix, a lack of substrate for ATP synthetase and arrest of oxidative phosphorylation.

Mannheimia haemolytica leukotoxin activates a nonreceptor tyrosine kinase signaling cascade in bovine leukocytes, which induces biological effects

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 (DeltaLktA) for comparison with LktA. Our results indicate that LktA induces tyrosine phosphorylation (TP) of the CD18 tail of LFA-1 in bovine leukocytes. The DeltaLktA mutant does not induce TP of the CD18 tail, albeit binding to bovine LFA-1. LktA-induced TP of the CD18 tail was attenuated by an NRTK inhibitor, herbimycin A; a phosphatidylinositol 3'-kinase (PI 3-kinase) inhibitor, wortmannin; and a Src kinase inhibitor, PP2, in a concentration-dependent manner. Furthermore, LktA induces TP of the CD18 tail in bovine, but not porcine, leukocytes. Moreover, LktA-induced intracellular calcium ([Ca2+]i) elevation was also inhibited by herbimycin A, wortmannin, and PP2. Thus, our data represent the first evidence that binding of LktA to bovine LFA-1 induces a species-specific NRTK signaling cascade involving PI 3-kinase and Src kinases and that this signaling cascade is required for LktA-induced biological effects.

Leukotoxicity of pyoverdin, production of reactive oxygen species, and effect of UV radiation

Pyoverdin was purified by solvent extraction, gel filtration, and ionic exchange chromatography. Assays of cytotoxic of pyoverdin were done with human leukocytes and macrophages from the peritoneum of mice. Both cell quantities showed a significant reduction. Death was followed by lysis in a dose-dependent form. The mechanism of action of pyoverdin involved the stimulation of reactive oxygen species (ROS) measured by Nitroblue Tetrazolium (NBT) reaction and chemiluminescence (CL). UV radiation at 368 nm increased the leukotoxicity; expositions of 5 min were enough to photostimulate the effect of pyoverdin on cellular oxydative metabolism, which increased between 35.4 and 53.2%. Genestein, an inhibitor of tyrosine kinases, counteracted the ROS stimuli of pyoverdin, suggesting endocytic mechanism of action for this pigment. The little chloroquine interference on oxydative stress indicated that intraphagosomal pH and the stimuli of reactive nitrogen intermediaries (RNI) seem to be of less importance than ROS in pyoverdin action on leukocytes.

Lipopolysaccharide enhances cytolysis and inflammatory cytokine induction in bovine alveolar macrophages exposed to Pasteurella (Mannheimia) haemolytica leukotoxin

Pasteurella (Mannheimia) haemolytica leukotoxin (Lkt) and lipopolysaccharide (LPS) are the primary virulence factors contributing to the pathogenesis of lung injury in bovine pneumonic pasteurellosis. Previous studies have characterized in vitro responses of bovine alveolar macrophages (AMs) to Lkt and LPS. Activation of AMs with Lkt or LPS causes induction of proinflammatory cytokines, and Lkt causes cytolysis of AMs at higher concentrations. Since AMs are exposed to both of these bacterial virulence factors during disease, previous studies may have underestimated the possibility of functional interactions between Lkt and LPS. The purpose of this study was to characterize the effect of simultaneous exposure to both Lkt and LPS on AM cytolysis and proinflammatory cytokine expression. Using cellular leakage of lactate dehydrogenase as an indirect measure of cytolysis, we studied AM responses to Lkt alone, LPS alone and Lkt+LPS. We found that 80-200 pg/ml LPS, which does not itself cause cytolysis, synergistically enhanced the cytolysis induced by 2-5 Lkt units (LU)/ml Lkt. Northern blot analysis demonstrated that synergism between Lkt and LPS resulted in increased levels of IL-8 mRNA, and that the kinetic patterns of TNF-alpha and IL-8 mRNA expression induced by Lkt+LPS differed from those induced by each agent separately. Finally, the WEHI 164 (clone 13) bioassay was used to show that Lkt/LPS synergism resulted in enhanced secretion of biologically active TNF-alpha. These results provide direct evidence of synergism between Lkt and LPS in AM cytolysis and inflammatory cytokine expression. Additional studies to characterize the molecular basis of this phenomenon are indicated.