Shigella Outer Membrane Vesicles as Promising Targets for Vaccination

The Role of Bacterial Extracellular Vesicles in the Immune Response to Pathogens, and Therapeutic Opportunities

Pathogenic bacteria have several mechanisms to evade the host's immune response and achieve an efficient infection. Bacterial extracellular vesicles (EVs) are a relevant cellular communication mechanism, since they can interact with other bacterial cells and with host cells. In this review, we focus on the EVs produced by some World Health Organization (WHO) priority Gram-negative and Gram-positive pathogenic bacteria; by spore-producing bacteria; by Mycobacterium tuberculosis (a bacteria with a complex cell wall); and by Treponema pallidum (a bacteria without lipopolysaccharide). We describe the classification and the general properties of bacterial EVs, their role during bacterial infections and their effects on the host immune response. Bacterial EVs contain pathogen-associated molecular patterns that activate innate immune receptors, which leads to cytokine production and inflammation, but they also contain antigens that induce the activation of B and T cell responses. Understanding the many effects of bacterial EVs on the host's immune response can yield new insights on the pathogenesis of clinically important infections, but it can also lead to the development of EV-based diagnostic and therapeutic strategies. In addition, since EVs are efficient activators of both the innate and the adaptive immune responses, they constitute a promising platform for vaccine development.

Identification and analysis of immunoreactive proteins of Shigella flexneri in human sera and stool specimens

Background

The method currently available to diagnose shigellosis is insensitive and has many limitations. Thus, this study was designed to identify specific antigenic protein(s) among the cell surface associated proteins (SAPs) of Shigella that would be valuable in the development of an alternative diagnostic assay for shigellosis, particularly one that could be run using a stool sample rather than serum.

Methods

The SAPs of clinical isolates of S. dysenteriae, S. boydii, Shigella flexneri, and S. sonnei were extracted from an overnight culture grown at 37 °C using acidified-glycine extraction methods. Protein profiles were observed by SDS-PAGE. To determine if antibodies specific to certain Shigella SAPs were present in both sera and stool suspensions, Western blot analysis was used to detect the presence of IgA, IgG, and IgM.

Results

Immunoblot analysis revealed that sera from patients infected with S. flexneri recognized 31 proteins. These SAP antigens are recognized by the host humoral response during Shigella infection. Specific antibodies against these antigens were also observed in intestinal secretions of shigellosis patients. Of these 31 S. flexneri proteins, the 35 kDa protein specifically reacted against IgA present in patients' stool suspensions. Further study illustrated the immunoreactivity of this protein in S. dysenteriae, S. boydii, and S. sonnei. This is the first report that demonstrates the presence of immunoreactive Shigella SAPs in stool suspensions. The SAPSs could be very useful in developing a simple and rapid serodiagnostic assay for shigellosis directly from stool specimens.

Shigella Vaccines: The Continuing Unmet Challenge

Shigellosis is a severe gastrointestinal disease that annually affects approximately 270 million individuals globally. It has particularly high morbidity and mortality in low-income regions; however, it is not confined to these regions and occurs in high-income nations when conditions allow. The ill effects of shigellosis are at their highest in children ages 2 to 5, with survivors often exhibiting impaired growth due to infection-induced malnutrition. The escalating threat of antibiotic resistance further amplifies shigellosis as a serious public health concern. This review explores Shigella pathology, with a primary focus on the status of Shigella vaccine candidates. These candidates include killed whole-cells, live attenuated organisms, LPS-based, and subunit vaccines. The strengths and weaknesses of each vaccination strategy are considered. The discussion includes potential Shigella immunogens, such as LPS, conserved T3SS proteins, outer membrane proteins, diverse animal models used in Shigella vaccine research, and innovative vaccine development approaches. Additionally, this review addresses ongoing challenges that necessitate action toward advancing effective Shigella prevention and control measures.

Refining Immunogenicity through Intradermal Delivery of Outer Membrane Vesicles against Shigella flexneri in Mice

Shigellosis remains a global health concern, especially in low- and middle-income countries. Despite improvements in sanitation, the absence of a licensed vaccine for human use has prompted global health organizations to support the development of a safe and effective multivalent vaccine that is cost-effective and accessible for limited-resource regions. Outer Membrane Vesicles (OMVs) have emerged in recent years as an alternative to live attenuated or whole-inactivated vaccines due to their immunogenicity and self-adjuvating properties. Previous works have demonstrated the safety and protective capacity of OMVs against Shigella flexneri infection in mouse models when administered through mucosal or intradermal routes. However, some immunological properties, such as the cellular response or cross-protection among different Shigella strains, remained unexplored. In this study, we demonstrate that intradermal immunization of OMVs with needle-free devices recruits a high number of immune cells in the dermis, leading to a robust cellular response marked by antigen-specific cytokine release and activation of effector CD4 T cells. Additionally, functional antibodies are generated, neutralizing various Shigella serotypes, suggesting cross-protective capacity. These findings highlight the potential of OMVs as a promising vaccine platform against shigellosis and support intradermal administration as a simple and painless vaccination strategy to address this health challenge.

Immunogenicity of Escherichia coli Outer Membrane Vesicles: Elucidation of Humoral Responses against OMV-Associated Antigens

Outer membrane vesicles (OMVs) produced by Gram-negative bacteria have emerged as a novel and flexible vaccine platform. OMVs can be decorated with foreign antigens and carry potent immunostimulatory components. Therefore, after their purification from the culture supernatant, they are ready to be formulated for vaccine use. It has been extensively demonstrated that immunization with engineered OMVs can elicit excellent antibody responses against the heterologous antigens. However, the definition of the conditions necessary to reach the optimal antibody titers still needs to be investigated. Here, we defined the protein concentrations required to induce antigen-specific antibodies, and the amount of antigen and OMVs necessary and sufficient to elicit saturating levels of antigen-specific antibodies. Since not all antigens can be expressed in OMVs, we also investigated the effectiveness of vaccines in which OMVs and purified antigens are mixed together without using any procedure for their physical association. Our data show that in most of the cases OMV-antigen mixtures are very effective in eliciting antigen-specific antibodies. This is probably due to the capacity of OMVs to "absorb" antigens, establishing sufficiently stable interactions that allow antigen-OMV co-presentation to the same antigen presenting cell. In those cases when antigen-OMV interaction is not sufficiently stable, the addition of alum to the formulation guarantees the elicitation of high titers of antigen-specific antibodies.

Coping Strategies for Pertussis Resurgence

Pertussis (whooping cough) is a respiratory disease caused primarily by Bordetella pertussis, a Gram-negative bacteria. Pertussis is a relatively contagious infectious disease in people of all ages, mainly affecting newborns and infants under 2 months of age. Pertussis is undergoing a resurgence despite decades of high rates of vaccination. To better cope with the challenge of pertussis resurgence, we evaluated its possible causes and potential countermeasures in the narrative review. Expanded vaccination coverage, optimized vaccination strategies, and the development of a new pertussis vaccine may contribute to the control of pertussis.

Bacterial Membrane Mimetics: From Biosensing to Disease Prevention and Treatment

Plasma membrane mimetics can potentially play a vital role in drug discovery and immunotherapy owing to the versatility to assemble facilely cellular membranes on surfaces and/or nanoparticles, allowing for direct assessment of drug/membrane interactions. Recently, bacterial membranes (BMs) have found widespread applications in biomedical research as antibiotic resistance is on the rise, and bacteria-associated infections have become one of the major causes of death worldwide. Over the last decade, BM research has greatly benefited from parallel advancements in nanotechnology and bioelectronics, resulting in multifaceted systems for a variety of sensing and drug discovery applications. As such, BMs coated on electroactive surfaces are a particularly promising label-free platform to investigate interfacial phenomena, as well as interactions with drugs at the first point of contact: the bacterial membrane. Another common approach suggests the use of lipid-coated nanoparticles as a drug carrier system for therapies for infectious diseases and cancer. Herein, we discuss emerging platforms that make use of BMs for biosensing, bioimaging, drug delivery/discovery, and immunotherapy, focusing on bacterial infections and cancer. Further, we detail the synthesis and characteristics of BMs, followed by various models for utilizing them in biomedical applications. The key research areas required to augment the characteristics of bacterial membranes to facilitate wider applicability are also touched upon. Overall, this review provides an interdisciplinary approach to exploit the potential of BMs and current emerging technologies to generate novel solutions to unmet clinical needs.

Flavonoids and Phenolic Acids from Aerial Part of Ajuga integrifolia (Buch.-Ham. Ex D. Don): Anti-Shigellosis Activity and In Silico Molecular Docking Studies

Shigellosis is one of the major causes of death in children worldwide. Flavonoids and phenolic acids are expected to demonstrate anti-shigellosis activity and anti-diarrheal properties. The aerial part of A. integrifolia is commonly used against diarrhea. This study aimed to identify flavonoids and phenolic acids responsible for this therapeutic purpose. Antioxidant activity, total phenol content, and total flavonoid content were determined. The antibacterial activity of the aerial part against Shigella spp. was also tested using the agar well diffusion method. HPLC analysis was performed using UHPLC-DAD for different extracts of the aerial part. Autodock Vina in the PyRx platform was used to screen responsible components. Ciprofloxacin was used as a reference drug. An enzyme taking part in pyrimidine biosynthesis was used as a target protein. Molecular docking results were visualized using Discovery Studio and LigPlot1.4.5 software. Antioxidant activity, total phenol content, and total flavonoid content are more significant for the aerial part of A. integrifolia. From HPLC analysis, the presence of the flavonoids, quercetin, myricetin, and rutin and the phenolic acids gallic acid, chlorogenic acid, and syringic acid were identified from the aerial part of A. integrifolia. Regarding the antibacterial activity, the aerial part shows considerable activity against Shigella spp. Binding energies, RMSD and Ki values, interaction type, and distance are considered to identify the components most likely responsible for the therapeutic effects and observed activity. Antioxidant activity, total phenol content, and total flavonoid content of the aerial part are in line with anti-shigellosis activity. The top five components that are most likely potentially responsible for therapeutic purposes and anti-shigellosis activity are chlorogenic acid, rutin, dihydroquercetin, dihydromyricetin, and kaempferol.