Abstract
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.
Highlights
repeats in the structural toxin (RTX) are large pore-forming proteins named due to the presence of characteristic arrays of glycine and aspartate-rich nonapeptide repeats in the C-terminal half of their amino acid sequence
The impact of RTX toxins from important animal pathogens is discussed in terms of basic knowledge of the molecular mechanisms of pathogenicity, their use for development of diagnostic tools, and their role in the design and development of potential vaccines against major animal epizootics caused by RTX-producing bacterial pathogens
Actinobacillus suis and (Actinobacillus) rossii, are two minor pig pathogens that cause moderate respiratory distress in early-weaned pigs and occasional abortions but not pleuropneumonia. Both secrete significant amounts of ApxI and ApxII [90,98,99,100,101]. This reveals that the significant tissue damage and pathologies described in pigs inoculated tracheally with recombinant RTX toxins, ApxI, ApxII, and ApxIII, which resulted in porcine pleuropneumonia, are not sufficient to produce the disease under natural conditions of infection
Summary
RTX (repeats in the structural toxin) are large pore-forming proteins named due to the presence of characteristic arrays of glycine and aspartate-rich nonapeptide repeats in the C-terminal half of their amino acid sequence. Genes encoding the T1SS proteins RtxB and RtxD are generally located together with the activator and structural RTX genes on a polycistronic operon and are tightly regulated They form, with the intrinsic outer membrane protein TolC, the secretion channel (Figure 1). The N-terminal domain that contains an amphipathic segment and several hydrophobic domains (Figure 1) is essential for the hemolytic and cytotoxic activity of RTX toxins These hydrophobic domains can form α-helical structures that insert into lipid bilayers and form pores, which are responsible for membrane permeabilization [4,32,33,34]. The impact of RTX toxins from important animal pathogens is discussed in terms of basic knowledge of the molecular mechanisms of pathogenicity, their use for development of diagnostic tools, and their role in the design and development of potential vaccines against major animal epizootics caused by RTX-producing bacterial pathogens
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