FliC's Hypervariable D3 Domain Is Required for Robust Anti-Flagellin Primary Antibody Responses

Differential Adjuvant Activity by Flagellins from Escherichia coli, Salmonella enterica Serotype Typhimurium, and Pseudomonas aeruginosa

(1) Background: The adjuvant properties of flagellin from various bacterial species have been extensively studied; however, a systematic comparison of the immunoadjuvant effects of flagellins from different bacterial species is lacking. This study aims to analyze the amino acid sequences and structural features of flagellins from Escherichia coli (FliCE.C), Salmonella enterica serotype Typhimurium (FliCS.T), and Pseudomonas aeruginosa (FliCP.A), and to evaluate their adjuvant activities in terms of Toll-like receptor 5 (TLR5) activation, antibody production, and cytokine responses in a murine model. (2) Methods: Bioinformatics analysis was conducted to compare the amino acid sequences and structural domains (D0, D1, D2, and D3) of flagellins from the three bacterial species. PyMol atomic models were used to confirm structural differences. Toll-like receptor 5 (TLR5) activation assays were performed to measure IL-8 and TNF-α production in vitro. The IgG antibody titers against the model antigen FaeG and cytokine responses, including IL-4 and TNF-α secretion were evaluated in a murine model. (3) Results: Bioinformatics analysis revealed that the D0 and D1 domains are highly conserved, whereas the D2 and D3 domains exhibit significant variability across the three species. Structural analysis via PyMol confirmed these differences, particularly in the D2 and D3 domains. TLR5 activation assays showed that FliCS.T and FliCP.A induced higher levels of IL-8 and TNF-α production compared to FliCE.C, indicating species-specific variations in TLR5 activation. In the murine model, FliCS.T as an adjuvant produced higher antibody titers against FaeG and increased IL-4 secretion in splenocytes compared to FliCE.C and FliCP.A. FliCP.A induced higher TNF-α expression than FliCS.T and FliCE.C, suggesting FliCS.T and FliCP.A are more effective at inducing T-cell responses. (4) Conclusions: This study highlights the potential of FliCS.T and FliCP.A as potent vaccine adjuvants. The results provide insights into the structure-function relationships of these flagellins and support their application in enhancing immune responses against diverse pathogens.

Double-layered protein nanoparticles conjugated with truncated flagellin induce improved mucosal and systemic immune responses in mice

Influenza viral infection poses a severe risk to global public health. Considering the suboptimal protection provided by current influenza vaccines against circulating influenza A viruses, it is imperative to develop novel vaccine formulations to combat respiratory infections. Here, we report the development of an intranasally-administered, self-adjuvanted double-layered protein nanoparticle consisting of influenza nucleoprotein (NP) cores coated with hemagglutinin (HA) and a truncated form of bacterial flagellin (tFliC). Intranasal vaccination of these nanoparticles notably amplified both antigen-specific humoral and cellular immune responses in the systematic compartments. Elevated antigen-specific IgA and IgG levels in mucosal washes, along with increased lung-resident memory B cell populations, were observed in the respiratory system of the immunized mice. Furthermore, intranasal vaccination of tFliC-adjuvanted nanoparticles enhanced survival rates against homologous and heterologous H3N2 viral challenges. Intriguingly, mucosal slow delivery of the prime dose (by splitting the dose into 5 applications over 8 days) significantly enhanced germinal center reactions and effector T-cell populations in lung draining lymph nodes, therefore promoting the protective efficacy against heterologous influenza viral challenges compared to single-prime immunization. These findings highlight the potential of intranasal immunization with tFliC-adjuvanted protein nanoparticles to bolster mucosal and systemic immune responses, with a slow-delivery strategy offering a promising approach for combating influenza epidemics.

FlgM is required to evade NLRC4-mediated host protection against flagellated Salmonella

Salmonella enterica serovar Typhimurium is a leading cause of gastroenteritis worldwide and a deadly pathogen in children, immunocompromised patients, and the elderly. Salmonella induces innate immune responses through the NLRC4 inflammasome, which has been demonstrated to have distinct roles during systemic and mucosal detections of flagellin and non-flagellin molecules. We hypothesized that NLRC4 recognition of Salmonella flagellin is the dominant protective pathway during infection. To test this hypothesis, we used wild-type, flagellin-deficient, and flagellin-overproducing Salmonella to establish the role of flagellin in mediating NLRC4-dependent host resistance during systemic and mucosal infections in mice. We observed that during the systemic phase of infection, Salmonella efficiently evades NLRC4-mediated innate immunity. During mucosal Salmonella infection, flagellin recognition by the NLRC4 inflammasome pathway is the dominant mediator of protective innate immunity. Deletion of flgM results in constitutive expression of flagellin and severely limits systemic and mucosal Salmonella infections in an NLRC4 inflammasome-dependent manner. These data establish that recognition of Salmonella's flagellin by the NLRC4 inflammasome during mucosal infection is the dominant innate protective pathway for host resistance against the enteric pathogen and that FlgM-mediated evasion of the NLRC4 inflammasome enhances virulence and intestinal tissue destruction.

An unbroken network of interactions connecting flagellin domains is required for motility in viscous environments

In its simplest form, bacterial flagellar filaments are composed of flagellin proteins with just two helical inner domains, which together comprise the filament core. Although this minimal filament is sufficient to provide motility in many flagellated bacteria, most bacteria produce flagella composed of flagellin proteins with one or more outer domains arranged in a variety of supramolecular architectures radiating from the inner core. Flagellin outer domains are known to be involved in adhesion, proteolysis and immune evasion but have not been thought to be required for motility. Here we show that in the Pseudomonas aeruginosa PAO1 strain, a bacterium that forms a ridged filament with a dimerization of its flagellin outer domains, motility is categorically dependent on these flagellin outer domains. Moreover, a comprehensive network of intermolecular interactions connecting the inner domains to the outer domains, the outer domains to one another, and the outer domains back to the inner domain filament core, is required for motility. This inter-domain connectivity confers PAO1 flagella with increased stability, essential for its motility in viscous environments. Additionally, we find that such ridged flagellar filaments are not unique to Pseudomonas but are, instead, present throughout diverse bacterial phyla.

Deimmunization of flagellin adjuvant for clinical application

Flagellin is the cognate ligand for host pattern recognition receptors, toll-like receptor 5 (TLR5) in the cell surface, and NAIP5/NLRC4 inflammasome in the cytosol. TLR5-binding domain is located in D1 domain, where crucial amino acid sequences are conserved among diverse bacteria. The highly conserved C-terminal 35 amino acids of flagellin were proved to be responsible for the inflammasome activation by binding to NAIP5. D2/D3 domains, located in the central region and exposed to the outside surface of flagellar filament, are heterogeneous across bacterial species and highly immunogenic. Taking advantage of TLR5- and NLRC4-stimulating activities, flagellin has been actively developed as a vaccine adjuvant and immunotherapeutic. Because of its immunogenicity, there exist worries concerning diminished efficacy and possible reactogenicity after repeated administration. Deimmunization of flagellin derivatives while preserving the TLR5/NLRC4-mediated immunomodulatory activity should be the most reasonable option for clinical application. This review describes strategies and current achievements in flagellin deimmunization.

Involvement of Flagellin in Kin Recognition between Bacillus velezensis Strains

Kin discrimination in nature is an effective way for bacteria to stabilize population cooperation and maintain progeny benefits. However, so far, the research on kin discrimination for Bacillus still has concentrated on "attack and defense" between cells and diffusion-dependent molecular signals of quorum sensing, kin recognition in Bacillus, however, has not been reported. To determine whether flagellar is involve in the kin recognition of Bacillus, we constructed Bacillus velezensis SQR9 assembled with flagellin of its kin and non-kin strains, and performed a swarm boundary assay with SQR9, then analyzed sequence variation of flagellin and other flagellar structural proteins in B. velezensis genus. Our results showed that SQR9 assembled with flagellin of non-kin strains was more likely to form a border phenotype with wild-type strain SQR9 in swarm assay than that of kin strains, and that non-kin strains had greater variation in flagellin than kin strains. In B. velezensis, these variations in flagellin were prevalent and had evolved significantly faster than other flagellar structural proteins. Therefore, we proposed that flagellin is an effective tool partly involved in the kin recognition of B. velezensis strains. IMPORTANCE Kin selection plays an important role in stabilizing population cooperation and maintaining the progeny benefits for bacteria in nature. However, to date, the role of flagellin in kin recognition in Bacillus has not been reported. By using rhizospheric Bacillus velezensis SQR9, we accomplished flagellin region interchange among its related strains, and show that flagellin acts as a mediator to distinguish kin from non-kin in B. velezensis. We demonstrated the polymorphism of flagellin in B. velezensis through alignment analysis of flagellin protein sequences. Therefore, it was proposed that flagellin was likely to be an effective tool for mediating kin recognition in B. velezensis.

Flagellin Improves the Immune Response of an Infectious Bursal Disease Virus (IBDV) Subunit Vaccine

Flagellin activates the immune system through Toll-like receptor 5 (TLR5) and can work as an adjuvant for subunit vaccines. In this study, we tested the adjuvancy of two different N-terminal fragments of flagellin, (1) FliC99, residues 1-99, and (2) FliC176, residues 1-176, to incorporate larger areas of the hotspot region for potentially higher levels of TLR5 activation and immune response. A truncated version of the VP2 protein (name tVP2, residues 199-356) of the Infectious bursal disease virus (IBDV) was genetically linked to the flagellin constructs, and the immune response was evaluated in chickens. Results showed that both chimeric antigen-adjuvant constructs increased humoral (total IgG titers), cellular and cytokine immune response (IL-4, IFN-γ). The resulting antibody also successfully neutralized IBDV. We conclude that the N-terminus of flagellin can act as an immune activator to enhance vaccine efficacy.

Fighting Antibiotic-Resistant Bacteria : Promising Strategies Orchestrated by Molecularly Imprinted Polymers

Infections caused by antibiotic-resistant bacteria are difficult and sometimes impossible to treat, making them one of the major public health problems of our time. We highlight how one unique material , molecularly imprinted polymers (MIPs), can orchestrate several strategies to fight this major societal issue. MIPs are tailor-made biomimetic supramolecular receptors that recognize and bind target molecules with a high affinity and selectivity, comparable to those of antibodies. While research on MIPs for combatting cancer has been constantly flourishing, comprehensive work on their involvement in combatting resistant superbugs has been rather scarce. This review aims at filling this gap. We will describe what are the causes of bacterial resistance and at which level MIPs can deploy their weapons. MIPs' targets can be biofilm constituents, quorum sensing messengers, bacterial surface proteins and antibiotic-deactivating enzymes, among others. We will conclude on the current challenges and future developments in this field.

Deimmunization of flagellin for repeated administration as a vaccine adjuvant

Flagellin, a protein-based Toll-like receptor agonist, is a versatile adjuvant applicable to wide spectrum of vaccines and immunotherapies. Given reiterated treatments of immunogenic biopharmaceuticals should lead to antibody responses precluding repeated administration, the development of flagellin not inducing specific antibodies would greatly expand the chances of clinical applications. Here we computationally identified immunogenic regions in Vibrio vulnificus flagellin B and deimmunized by simply removing a B cell epitope region. The recombinant deimmunized FlaB (dFlaB) maintains stable TLR5-stimulating activity. Multiple immunization of dFlaB does not induce FlaB-specific B cell responses in mice. Intranasally co-administered dFlaB with influenza vaccine enhanced strong Ag-specific immune responses in both systemic and mucosal compartments devoid of FlaB-specific Ab production. Notably, dFlaB showed better protective immune responses against lethal viral challenge compared with wild type FlaB. The deimmunizing B cell epitope deletion did not compromise stability and adjuvanticity, while suppressing unwanted antibody responses that may negatively affected vaccine antigen-directed immune responses in repeated vaccinations. We explain the underlying mechanism of deimmunization by employing molecular dynamics analysis.

Bacterial Flagellar Filament: A Supramolecular Multifunctional Nanostructure

The bacterial flagellum is a complex and dynamic nanomachine that propels bacteria through liquids. It consists of a basal body, a hook, and a long filament. The flagellar filament is composed of thousands of copies of the protein flagellin (FliC) arranged helically and ending with a filament cap composed of an oligomer of the protein FliD. The overall structure of the filament core is preserved across bacterial species, while the outer domains exhibit high variability, and in some cases are even completely absent. Flagellar assembly is a complex and energetically costly process triggered by environmental stimuli and, accordingly, highly regulated on transcriptional, translational and post-translational levels. Apart from its role in locomotion, the filament is critically important in several other aspects of bacterial survival, reproduction and pathogenicity, such as adhesion to surfaces, secretion of virulence factors and formation of biofilms. Additionally, due to its ability to provoke potent immune responses, flagellins have a role as adjuvants in vaccine development. In this review, we summarize the latest knowledge on the structure of flagellins, capping proteins and filaments, as well as their regulation and role during the colonization and infection of the host.