Characterization of the caprine (Capra hircus) beta-2 integrin CD18-encoding cDNA and identification of mutations potentially responsible for the ruminant-specific virulence ofMannheimia haemolytica

RTX Toxins of Animal Pathogens and Their Role as Antigens in Vaccines and Diagnostics

Exotoxins play a central role in the pathologies caused by most major bacterial animal pathogens. The large variety of vertebrate and invertebrate hosts in the animal kingdom is reflected by a large variety of bacterial pathogens and toxins. The group of repeats in the structural toxin (RTX) toxins is particularly abundant among bacterial pathogens of animals. Many of these toxins are described as hemolysins due to their capacity to lyse erythrocytes in vitro. Hemolysis by RTX toxins is due to the formation of cation-selective pores in the cell membrane and serves as an important marker for virulence in bacterial diagnostics. However, their physiologic relevant targets are leukocytes expressing β2 integrins, which act as specific receptors for RTX toxins. For various RTX toxins, the binding to the CD18 moiety of β₂ integrins has been shown to be host specific, reflecting the molecular basis of the host range of RTX toxins expressed by bacterial pathogens. Due to the key role of RTX toxins in the pathogenesis of many bacteria, antibodies directed against specific RTX toxins protect against disease, hence, making RTX toxins valuable targets in vaccine research and development. Due to their specificity, several structural genes encoding for RTX toxins have proven to be essential in modern diagnostic applications in veterinary medicine.

Radiolabeled Cyclic RGD Peptide Bioconjugates as Radiotracers Targeting Multiple Integrins

Angiogenesis is a requirement for tumor growth and metastasis. The angiogenic process depends on vascular endothelial cell migration and invasion, and is regulated by various cell adhesion receptors. Integrins are such a family of receptors that facilitate the cellular adhesion to and migration on extracellular matrix proteins in the intercellular spaces and basement membranes. Among 24 members of the integrin family, αvβ3 is studied most extensively for its role in tumor angiogenesis and metastasis. The αvβ3 is expressed at relatively low levels on epithelial cells and mature endothelial cells, but it is highly expressed on the activated endothelial cells of tumor neovasculature and some tumor cells. This restricted expression makes αvβ3 an excellent target to develop antiangiogenic drugs and diagnostic molecular imaging probes. Since αvβ3 is a receptor for extracellular matrix proteins with one or more RGD tripeptide sequence, many radiolabeled cyclic RGD peptides have been evaluated as "αvβ3-targeted" radiotracers for tumor imaging over the past decade. This article will use the dimeric and tetrameric cyclic RGD peptides developed in our laboratories as examples to illustrate basic principles for development of αvβ3-targeted radiotracers. It will focus on different approaches to maximize the radiotracer tumor uptake and tumor/background ratios. This article will also discuss some important assays for preclinical evaluations of integrin-targeted radiotracers. In general, multimerization of cyclic RGD peptides increases their integrin binding affinity and the tumor uptake and retention times of their radiotracers. Regardless of their multiplicity, the capability of cyclic RGD peptides to bind other integrins (namely, αvβ5, α5β1, α6β4, α4β1, and αvβ6) is expected to enhance the radiotracer tumor uptake due to the increased integrin population. The results from preclinical and clinical studies clearly show that radiolabeled cyclic RGD peptides (such as (99m)Tc-3P-RGD2, (18)F-Alfatide-I, and (18)F-Alfatide-II) are useful as the molecular imaging probes for early cancer detection and noninvasive monitoring of the tumor response to antiangiogenic therapy.

The role of RTX toxins in host specificity of animal pathogenic Pasteurellaceae

RTX toxins are bacterial pore-forming toxins that are particularly abundant among pathogenic species of Pasteurellaceae, in which they play a major role in virulence. RTX toxins of several primary pathogens of the family of Pasteurellaceae are directly involved in causing necrotic lesions in the target organs. Many RTX toxins are known as haemolysins because they lyse erythrocytes in vitro, an effect that is non-specific, but which serves as a useful marker in bacteriological identification and as an easily measurable signal in vitro in experimental studies. More recent studies have shown that the specific targets of most RTX toxins are leukocytes, with RTX toxins binding to the corresponding β-subunit (CD18) of β2 integrins and then exerting cytotoxic activity. After uptake by the target cell, at sub-lytic concentrations, some RTX toxins are transported to mitochondria and induce apoptosis. For several RTX toxins the binding to CD18 has been shown to be host specific and this seems to be the basis for the host range specificity of these RTX toxins. Observations on two very closely related species of the Pasteurellaceae family, Actinobacillus suis, a porcine pathogen particularly affecting suckling pigs, and Actinobacillus equuli subsp. haemolytica, which causes pyosepticaemia in new-born foals (sleepy foal disease), have revealed that they express different RTX toxins, named ApxI/II and Aqx, respectively. These RTX toxins are specifically cytotoxic for porcine and equine leukocytes, respectively. Furthermore, the ApxI and Aqx toxins of these species, when expressed in an isogenetic background in Escherichia coli, are specifically cytotoxic for leukocytes of their respective hosts. These data indicate the determinative role of RTX toxins in host specificity of pathogenic species of Pasteurellaceae.

Radiolabeled Cyclic RGD Peptides as Radiotracers for Imaging Tumors and Thrombosis by SPECT

The integrin family is a group of transmembrane glycoprotein comprised of 19 α- and 8 β-subunits that are expressed in 25 different α/β heterodimeric combinations on the cell surface. Integrins play critical roles in many physiological processes, including cell attachment, proliferation, bone remodeling, and wound healing. Integrins also contribute to pathological events such as thrombosis, atherosclerosis, tumor invasion, angiogenesis and metastasis, infection by pathogenic microorganisms, and immune dysfunction. Among 25 members of the integrin family, the α(v)β(3) is studied most extensively for its role of tumor growth, progression and angiogenesis. In contrast, the α(IIb)β(3 )is expressed exclusively on platelets, facilitates the intercellular bidirectional signaling ("inside-out" and "outside-in") and allows the aggregation of platelets during vascular injury. The α(IIb)β(3) plays an important role in thrombosis by its activation and binding to fibrinogen especially in arterial thrombosis due to the high blood flow rate. In the resting state, the α(IIb)β(3) on platelets does not bind to fibrinogen; on activation, the conformation of platelet is altered and the binding sites of α(IIb)β(3 )are exposed for fibrinogen to crosslink platelets. Over the last two decades, integrins have been proposed as the molecular targets for diagnosis and therapy of cancer, thrombosis and other diseases. Several excellent review articles have appeared recently to cover a broad range of topics related to the integrin-targeted radiotracers and their nuclear medicine applications in tumor imaging by single photon emission computed tomography (SPECT) or a positron-emitting radionuclide for positron emission tomography (PET). This review will focus on recent developments of α(v)β(3)-targeted radiotracers for imaging tumors and the use of α(IIb)β(3)-targeted radiotracers for thrombosis imaging, and discuss different approaches to maximize the targeting capability of cyclic RGD peptides and improve the radiotracer excretion kinetics from non-cancerous organs. Improvement of target uptake and target-to-background ratios is critically important for target-specific radiotracers.

RGD motif of lipoprotein T, involved in adhesion of Mycoplasma conjunctivae to lamb synovial tissue cells

Lipoprotein T (LppT), a membrane-located 105-kDa lipoprotein of Mycoplasma conjunctivae, the etiological agent of infectious keratoconjunctivitis (IKC) of domestic sheep and wild Caprinae, was characterized. LppT was shown to promote cell attachment to LSM 192 primary lamb joint synovial cells. Adhesion of M. conjunctivae to LSM 192 cells is inhibited by antibodies directed against LppT. The RGD (Arg-Gly-Asp) motif of LppT was found to be a specific site for binding of M. conjunctivae to these eukaryotic host cells. Recombinant LppT fixed to polymethylmethacrylate slides binds LSM 192 cells, whereas LppT lacking the RGD site is deprived of binding capacity to LSM 192, and LppT containing RGE rather than RGD shows reduced binding. Synthetic nonapeptides derived from LppT containing RGD competitively inhibit binding of LSM 192 cells to LppT-coated slides, whereas nonapeptides containing RAD rather than RGD do not inhibit. RGD-containing, LppT-derived nonapeptides are able to directly inhibit binding of M. conjunctivae to LSM 192 cells by competitive inhibition, whereas the analogous nonapeptide containing RAD rather than RGD or the fibronectin-derived RGD hexapeptide has no inhibitory effect. These results reveal LppT as the first candidate of a RGD lectin in Mycoplasma species that is assumed to bind to beta integrins.

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.

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.

The wild boar (Sus scrofa) lymphocyte function-associated antigen-1 (CD11a/CD18) receptor: cDNA sequencing, structure analysis and comparison with homologues

Background

The most predominant beta2-integrin lymphocyte function-associated antigen-1 (LFA-1, CD11a/CD18, alphaLbeta2), expressed on all leukocytes, is essential for many adhesive functions of the immune system. Interestingly, RTX toxin-producing bacteria specifically target this leukocyte beta2-integrin which exacerbates lesions and disease development.

Results

This study reports the sequencing of the wild boar beta2-integrin CD11a and CD18 cDNAs. Predicted CD11a and CD18 subunits share all the main structural characteristics of their mammalian homologues, with a larger interspecies conservation for the CD18 than the CD11a. Besides these strong overall similarities, wild boar and domestic pig LFA-1 differ by 2 (CD18) and 1 or 3 (CD11a) substitutions, of which one is located in the crucial I-domain (CD11a, E168D).

Conclusion

As most wild boars are seropositive to the RTX toxin-producing bacterium Actinobacillus pleuropneumoniae and because they have sustained continuous natural selection, future studies addressing the functional impact of these polymorphisms could bring interesting new information on the physiopathology of Actinobacillus pleuropneumoniae-associated pneumonia in domestic pigs.

Integrin-EGF-3 domain of bovine CD18 is critical for Mannheimia haemolytica leukotoxin species-specific susceptibility

Mannheimia haemolytica leukotoxin (LktA) is the primary virulence factor contributing to the pathogenesis of lung injury in bovine pneumonic pasteurellosis. Results from the authors' previous studies demonstrated that the site required for LktA binding leading to susceptibility to its biological effects resides within amino acid residues 500-600 of the extracellular region of bovine CD18. Experiments were designed to identify a much smaller functional domain within this 100 amino acid region of bovine CD18 that is critically required for species-specific susceptibility to LktA effects. Chimeric bovine X human CD18 with different integrin epidermal growth factor(I-EGF) like domains switched between bovine and human CD18 were generated and coexpressed with bovine CD11a in the human K562 cell line. The resulting chimeric transductants were tested for susceptibility to LktA-induced effects. The results demonstrate unequivocally that the I-EGF-3 domain of bovine CD18 (amino acid residues 541-581) is critical for conferring species-specific susceptibility to M. haemolytica leukotoxin.

Cloning and comparison of bighorn sheep CD18 with that of domestic sheep, goats, cattle, humans and mice

Previously, we have shown that CD18, the beta-subunit of beta(2)-integrins, serves as a receptor for leukotoxin (Lkt) secreted by Mannheimia (Pasteurella) haemolytica on bovine leukocytes. Anti-CD18 monoclonal antibodies (mAbs) inhibit Lkt-induced cytolysis of bighorn sheep (Ovis canadensis) leukocytes suggesting that CD18 may serve as a receptor for Lkt on the leukocytes of this species as well. Confirmation of bighorn sheep CD18 as a receptor for Lkt, and elucidation of the enhanced Lkt-susceptibility of bighorn sheep polymorphonuclear leukocytes (PMNs), necessitates the cloning and sequencing of cDNA encoding bighorn sheep CD18. Hence, in this study we cloned and sequenced the cDNA encoding CD18 of bighorn sheep, and compared with that of other animal species. The cDNA of bighorn sheep CD18 has an open reading frame (ORF) of 2310bp. CD18 sequences obtained individually from peripheral blood mononuclear cells (PBMCs) and PMNs were identical to each other. Comparison of the deduced 770-amino acid sequence of CD18 of bighorn sheep with that of domestic sheep, goats, cattle, humans and mice revealed 99, 98, 95, 82 and 80% identity, respectively. Availability of cloned bighorn sheep CD18 cDNA should allow the molecular characterization of M. haemolytica Lkt-receptor interactions in bighorn sheep and other ruminants that are susceptible to this disease.

Molecular characterisation of the caprine (Capra hircus) lymphocyte function-associated antigen-1 alpha subunit-encoding cDNA

Background

Lymphocyte function-associated antigen-1 (LFA-1, CD11a/CD18, alpha L beta 2) is required for many cellular adhesive interactions during the immune response.

Results

The Capra hircus CD11a-encoding cDNA was sequenced and compared with its human, murine, rat, bovine and ovine counterparts. Despite some focal differences, it shares all the main characteristics of its known mammalian homologues.

Conclusion

Therefore, along with the caprine CD18-encoding cDNA, which has been available for a few months, the sequence data revealed here will allow the Capra hircus LFA-1 expression in vitro as a tool to explore the specificities of inflammation in the caprine species.

Mapping of the binding site for Mannheimia haemolytica leukotoxin within bovine CD18

To map the site involved in Mannheimia haemolytica leukotoxin (LktA) binding and biological activity within bovine CD18, bovine x human CD18 chimeric constructs were generated and coexpressed with bovine CD11a in K562 cells. Studies with the chimeric leukocyte function-associated antigen 1 transductants demonstrate that the site required for LktA binding and biological effects resides within amino acid residues 500 and 600 of the extracellular region of bovine CD18.