Antibodies Inhibiting the Type III Secretion System of Gram-Negative Pathogenic Bacteria

Pathogenic bacteria are a global health threat, with over 2 million infections caused by Gram-negative bacteria every year in the United States. This problem is exacerbated by the increase in resistance to common antibiotics that are routinely used to treat these infections, creating an urgent need for innovative ways to treat and prevent virulence caused by these pathogens. Many Gram-negative pathogenic bacteria use a type III secretion system (T3SS) to inject toxins and other effector proteins directly into host cells. The T3SS has become a popular anti-virulence target because it is required for pathogenesis and knockouts have attenuated virulence. It is also not required for survival, which should result in less selective pressure for resistance formation against T3SS inhibitors. In this review, we will highlight selected examples of direct antibody immunizations and the use of antibodies in immunotherapy treatments that target the bacterial T3SS. These examples include antibodies targeting the T3SS of Pseudomonas aeruginosa, Yersinia pestis, Escherichia coli, Salmonella enterica, Shigella spp., and Chlamydia trachomatis.

Salmonella Pathogenicity Island 1 (SPI-1) and Its Complex Regulatory Network

Salmonella species can infect a diverse range of birds, reptiles, and mammals, including humans. The type III protein secretion system (T3SS) encoded by Salmonella pathogenicity island 1 (SPI-1) delivers effector proteins required for intestinal invasion and the production of enteritis. The T3SS is regarded as the most important virulence factor of Salmonella. SPI-1 encodes transcription factors that regulate the expression of some virulence factors of Salmonella, while other transcription factors encoded outside SPI-1 participate in the expression of SPI-1-encoded genes. SPI-1 genes are responsible for the invasion of host cells, regulation of the host immune response, e.g., the host inflammatory response, immune cell recruitment and apoptosis, and biofilm formation. The regulatory network of SPI-1 is very complex and crucial. Here, we review the function, effectors, and regulation of SPI-1 genes and their contribution to the pathogenicity of Salmonella.

Baicalin protects mice against Salmonella typhimurium infection via the modulation of both bacterial virulence and host response

BACKGROUND

The worsening problems of antibiotic resistance prompt the need for alternative strategies. Baicalin, which is isolated from Scutellaria baicalensisi, has been demonstrated to exhibit anti-inflammatory, anti-virulence and antimicrobial effects. Salmonella typhimurium is an important foodborne pathogenic bacteriaum that causes gastrointestinal disease in humans and many animals.

PURPOSE

The aim of this study was to investigate the effects of baicalin on S. typhimurium infection in mice and its possible mechanism in vitro.

STUDY DESIGN

To evaluate the effect of baicalin in vivo, mice were orally administered of baicalin, and then were infected by an intragastric administration of S. typhimurium. The minimal inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) of baicalin, baicalein, and oroxylin A against S. typhimurium were detected under the guides of the Clinical and Laboratory Standards Institute. In vitro, Caco-2 cells were infected with S. typhimurium in the presence or absence of baicalin, baicalein, and oroxylin A at sub-MICs.

METHODS

In the in vivo experiment, the body weight loss, the serum levels of TNFα,  IL-6, and lactic dehydrogenase (LDH), the pathological changes of the caecum and the caecum bacterial burdens were examined. The MICs and MBCs of baicalin, baicalein, and oroxylin A against S. typhimurium were detected by two-fold serial dilutions. In vitro, Caco-2 cells were infected with S. typhimurium, and the invasion capacity, TNFα, nitrate, and LDH were analysed. The transcription levels of Salmonella pathogenicity island 1 virulence associated genes (sopB, sopE, sopE2) of S. typhimurium in the presence of baicalin, baicalein, and oroxylin A were detected by qRT-PCR.

RESULTS

Our results showed that baicalin significantly decreased the body weight loss, the serum levels of TNFα,  IL-6, and LDH, and the caecum bacterial burdens of mice challenged with S. typhimurium. Histological examination showed that baicalin decreased the lesion in the caecum of S. typhimurium-infected mice. MICs and MBCs of baicalin, and oroxylin A. against S. typhimurium were > 128 µg/ml. MICs and MBCs of baicalein against S. typhimurium were 64 µg/ml, and > 128 µg/ml, respectively. Pretreatment of Caco-2 cells or S. typhimurium with baicalin, baicalein, and oroxylin A significantly inhibited the invasion of Caco-2 cells by S. typhimurium in a dose-dependent manner. Sub-MICs of baicalin, baicalein, and oroxylin A also significantly decreased the levels of TNFα, nitrate, and LDH from S. typhimurium-infected Caco-2 cells. Moreover, the transcription levels of sopB, sopE, and sopE2 were significantly suppressed by baicalin, baicalein, and oroxylin A.

CONCLUSIONS

These results demonstrated that baicalin is a promising agent for the prevention of S. typhimurium infection via the modulation of both bacterial virulence and host response.

A proteomic approach to the development of DIVA ELISA distinguishing pigs infected with Salmonella Typhimurium and pigs vaccinated with a Salmonella Typhimurium-based inactivated vaccine

BACKGROUND

Salmonella enterica serovar Typhimurium is one of the most common enteropathogenic bacteria found in pigs in Europe. In our previous work, we demonstrated the protective effects in suckling piglets when their dams had been vaccinated with an S. Typhimurium-based inactivated vaccine. This study is focused on a procedure leading to serological discrimination between vaccinated and infected pigs. As we supposed, distinct environment during natural infection and in bacterial cultures used for vaccine preparation led to a slightly different spectrum of expressed S. Typhimurium proteins. The examination of porcine antibodies produced after the experimental infection with S. Typhimurium or after vaccination with S. Typhimurium-based inactivated vaccine by affinity chromatography and mass spectrometry revealed differences in antibody response applicable for serological differentiation of infected from vaccinated animals.

RESULTS

Antibodies against Salmonella SipB, SipD and SseB proteins were detected at much higher levels in post-infection sera in comparison with control and post-vaccination sera. On the other hand, proteins BamB, OppA and a fragment of FliC interacted with antibodies from post-vaccination sera with a much higher intensity than from control and post-infection sera. In addition, we constructed ELISA assays using post-infection antigen - SipB protein and post-vaccination antigen - FliC-fragment and evaluated them on a panel of individual porcine sera.

CONCLUSIONS

The analysis of antibody response of infected and vaccinated pigs by proteomic tools enabled to identify S. Typhimurium antigens useful for distinguishing infected from vaccinated animals. This approach can be utilized in other challenges where DIVA vaccine and a subsequent serological assay are required, especially when genetic modification of a vaccine strain is not desirable.

Direct neutralization of type III effector translocation by the variable region of a monoclonal antibody to Yersinia pestis LcrV

Plague is an acute infection caused by the Gram-negative bacterium Yersinia pestis. Antibodies that are protective against plague target LcrV, an essential virulence protein and component of a type III secretion system of Y. pestis. Secreted LcrV localizes to the tips of type III needles on the bacterial surface, and its function is necessary for the translocation of Yersinia outer proteins (Yops) into the cytosol of host cells infected by Y. pestis. Translocated Yops counteract macrophage functions, for example, by inhibiting phagocytosis (YopE) or inducing cytotoxicity (YopJ). Although LcrV is the best-characterized protective antigen of Y. pestis, the mechanism of protection by anti-LcrV antibodies is not fully understood. Antibodies bind to LcrV at needle tips, neutralize Yop translocation, and promote opsonophagocytosis of Y. pestis by macrophages in vitro. However, it is not clear if anti-LcrV antibodies neutralize Yop translocation directly or if they do so indirectly, by promoting opsonophagocytosis. To determine if the protective IgG1 monoclonal antibody (MAb) 7.3 is directly neutralizing, an IgG2a subclass variant, a deglycosylated variant, F(ab')2, and Fab were tested for the ability to inhibit the translocation of Yops into Y. pestis-infected macrophages in vitro. Macrophage cytotoxicity and cellular fractionation assays show that the Fc of MAb 7.3 is not required for the neutralization of YopJ or YopE translocation. In addition, the use of Fc receptor-deficient macrophages, and the use of cytochalasin D to inhibit actin polymerization, confirmed that opsonophagocytosis is not required for MAb 7.3 to neutralize translocation. These data indicate that the binding of the variable region of MAb 7.3 to LcrV is sufficient to directly neutralize Yop translocation.

Salmonella pathogenicity and host adaptation in chicken-associated serovars

Enteric pathogens such as Salmonella enterica cause significant morbidity and mortality. S. enterica serovars are a diverse group of pathogens that have evolved to survive in a wide range of environments and across multiple hosts. S. enterica serovars such as S. Typhi, S. Dublin, and S. Gallinarum have a restricted host range, in which they are typically associated with one or a few host species, while S. Enteritidis and S. Typhimurium have broad host ranges. This review examines how S. enterica has evolved through adaptation to different host environments, especially as related to the chicken host, and continues to be an important human pathogen. Several factors impact host range, and these include the acquisition of genes via horizontal gene transfer with plasmids, transposons, and phages, which can potentially expand host range, and the loss of genes or their function, which would reduce the range of hosts that the organism can infect. S. Gallinarum, with a limited host range, has a large number of pseudogenes in its genome compared to broader-host-range serovars. S. enterica serovars such as S. Kentucky and S. Heidelberg also often have plasmids that may help them colonize poultry more efficiently. The ability to colonize different hosts also involves interactions with the host's immune system and commensal organisms that are present. Thus, the factors that impact the ability of Salmonella to colonize a particular host species, such as chickens, are complex and multifactorial, involving the host, the pathogen, and extrinsic pressures. It is the interplay of these factors which leads to the differences in host ranges that we observe today.

Surface organelles assembled by secretion systems of Gram-negative bacteria: diversity in structure and function

Gram-negative bacteria express a wide variety of organelles on their cell surface. These surface structures may be the end products of secretion systems, such as the hair-like fibers assembled by the chaperone/usher (CU) and type IV pilus pathways, which generally function in adhesion to surfaces and bacterial-bacterial and bacterial-host interactions. Alternatively, the surface organelles may be integral components of the secretion machinery itself, such as the needle complex and pilus extensions formed by the type III and type IV secretion systems, which function in the delivery of bacterial effectors inside host cells. Bacterial surface structures perform functions critical for pathogenesis and have evolved to withstand forces exerted by the external environment and cope with defenses mounted by the host immune system. Given their essential roles in pathogenesis and exposed nature, bacterial surface structures also make attractive targets for therapeutic intervention. This review will describe the structure and function of surface organelles assembled by four different Gram-negative bacterial secretion systems: the CU pathway, the type IV pilus pathway, and the type III and type IV secretion systems.

Multi-Functional Characteristics of the Pseudomonas aeruginosa Type III Needle-Tip Protein, PcrV; Comparison to Orthologs in other Gram-negative Bacteria

Pseudomonas aeruginosa possesses a type III secretion system (T3SS) to intoxicate host cells and evade innate immunity. This virulence-related machinery consists of a molecular syringe and needle assembled on the bacterial surface, which allows delivery of T3 effector proteins into infected cells. To accomplish a one-step effector translocation, a tip protein is required at the top end of the T3 needle structure. Strains lacking expression of the functional tip protein fail to intoxicate host cells. P. aeruginosa encodes a T3S that is highly homologous to the proteins encoded by Yersinia spp. The needle-tip proteins of Yersinia, LcrV, and P. aeruginosa, PcrV, share 37% identity and 65% similarity. Other known tip proteins are AcrV (Aeromonas), IpaD (Shigella), SipD (Salmonella), BipD (Burkholderia), EspA (EPEC, EHEC), Bsp22 (Bordetella), with additional proteins identified from various Gram-negative species, such as Vibrio and Bordetella. The tip proteins can serve as a protective antigen or may be critical for sensing host cells and evading innate immune responses. Recognition of the host microenvironment transcriptionally activates synthesis of T3SS components. The machinery appears to be mechanically controlled by the assemblage of specific junctions within the apparatus. These junctions include the tip and base of the T3 apparatus, the needle proteins and components within the bacterial cytoplasm. The tip proteins likely have chaperone functions for translocon proteins, allowing the proper assembly of translocation channels in the host membrane and completing vectorial delivery of effector proteins into the host cytoplasm. Multi-functional features of the needle-tip proteins appear to be intricately controlled. In this review, we highlight the functional aspects and complex controls of T3 needle-tip proteins with particular emphasis on PcrV and LcrV.

Immunization of chickens with Salmonella enterica subspecies enterica serovar Enteritidis pathogenicity island-2 proteins

Several structural components of the type III secretion systems (T3SS) encoded by Salmonella pathogenicity island (SPI)-1 and SPI-2 are exposed to the host's immune system prior to/during the infection/invasion process, making them potential vaccine candidates. In this study we evaluated whether chickens vaccinated with SPI-2 T3SS components could mount a significant humoral immune response (as measured by serum IgG titres) and whether these antibodies could be transferred to progeny (as measured by egg yolk IgG titres), and whether vaccinates and progeny of vaccinates could be protected against challenge with SE. The results of our studies show that vaccinated chickens do produce high levels of SPI-2 T3SS specific serum IgG that they are able to transfer to their progeny. It was demonstrated that vaccinates and progeny of vaccinates had lower overall countable recovered Salmonella enterica subspecies enterica serovar Enteritidis (SE) per bird in most situations.