Porin of Shigella dysenteriae enhances Toll-like receptors 2 and 6 of mouse peritoneal B-2 cells and induces the expression of immunoglobulin M, immunoglobulin G2a and immunoglobulin A

Protein-Based Adjuvants for Vaccines as Immunomodulators of the Innate and Adaptive Immune Response: Current Knowledge, Challenges, and Future Opportunities

New-generation vaccines, formulated with subunits or nucleic acids, are less immunogenic than classical vaccines formulated with live-attenuated or inactivated pathogens. This difference has led to an intensified search for additional potent vaccine adjuvants that meet safety and efficacy criteria and confer long-term protection. This review provides an overview of protein-based adjuvants (PBAs) obtained from different organisms, including bacteria, mollusks, plants, and humans. Notably, despite structural differences, all PBAs show significant immunostimulatory properties, eliciting B-cell- and T-cell-mediated immune responses to administered antigens, providing advantages over many currently adopted adjuvant approaches. Furthermore, PBAs are natural biocompatible and biodegradable substances that induce minimal reactogenicity and toxicity and interact with innate immune receptors, enhancing their endocytosis and modulating subsequent adaptive immune responses. We propose that PBAs can contribute to the development of vaccines against complex pathogens, including intracellular pathogens such as Mycobacterium tuberculosis, those with complex life cycles such as Plasmodium falciparum, those that induce host immune dysfunction such as HIV, those that target immunocompromised individuals such as fungi, those with a latent disease phase such as Herpes, those that are antigenically variable such as SARS-CoV-2 and those that undergo continuous evolution, to reduce the likelihood of outbreaks.

Modulation of host cellular responses by gram-negative bacterial porins

The outer membrane of a gram-negative bacteria encapsulates the plasma membrane thereby protecting it from the harsh external environment. This membrane acts as a sieving barrier due to the presence of special membrane-spanning proteins called "porins." These porins are β-barrel channel proteins that allow the passive transport of hydrophilic molecules and are impermeable to large and charged molecules. Many porins form trimers in the outer membrane. They are abundantly present on the bacterial surface and therefore play various significant roles in the host-bacteria interactions. These include the roles of porins in the adhesion and virulence mechanisms necessary for the pathogenesis, along with providing resistance to the bacteria against the antimicrobial substances. They also act as the receptors for phage and complement proteins and are involved in modulating the host cellular responses. In addition, the potential use of porins as adjuvants, vaccine candidates, therapeutic targets, and biomarkers is now being exploited. In this review, we focus briefly on the structure of the porins along with their important functions and roles in the host-bacteria interactions.

Do Engineered Nanomaterials Affect Immune Responses by Interacting With Gut Microbiota?

Engineered nanomaterials (ENMs) have been widely exploited in several industrial domains as well as our daily life, raising concern over their potential adverse effects. While in general ENMs do not seem to have detrimental effects on immunity or induce severe inflammation, their indirect effects on immunity are less known. In particular, since the gut microbiota has been tightly associated with human health and immunity, it is possible that ingested ENMs could affect intestinal immunity indirectly by modulating the microbial community composition and functions. In this perspective, we provide a few pieces of evidence and discuss a possible link connecting ENM exposure, gut microbiota and host immune response. Some experimental works suggest that excessive exposure to ENMs could reshape the gut microbiota, thereby modulating the epithelium integrity and the inflammatory state in the intestine. Within such microenvironment, numerous microbiota-derived components, including but not limited to SCFAs and LPS, may serve as important effectors responsible of the ENM effect on intestinal immunity. Therefore, the gut microbiota is implicated as a crucial regulator of the intestinal immunity upon ENM exposure. This calls for including gut microbiota analysis within future work to assess ENM biocompatibility and immunosafety. This also calls for refinement of future studies that should be designed more elaborately and realistically to mimic the human exposure situation.

Bacterial Protein Toll-Like-Receptor Agonists: A Novel Perspective on Vaccine Adjuvants

Adjuvants have been used in vaccines for over a century, however, the search for safe and effective vaccine adjuvants continues. In recent decades toll-like-receptor (TLR) agonists have been investigated as potential vaccine adjuvants. In this regard, the majority of the currently investigated TLR agonists are non-protein microbial components such as lipopolysaccharides, oligonucleotides, and lipopeptides. On the other hand, a growing number of studies reveal that TLR signaling and immune responses can be activated by numerous bacterial proteins. However, their potential roles as adjuvants have been somewhat overlooked. Herein, we discuss several such bacterial proteins which exhibit adjuvant properties, including the activation of TLR signaling, antigen presenting cell maturation, pro-inflammatory cytokine production and adaptive immune response. The protein nature of these TLR agonists presents several unique features not shared by non-protein TLR agonists. These properties include the amenability for modifying the structure and function as necessary for optimal immunogenicity and minimal toxicity. Protein adjuvants can be genetically fused to protein antigens which ensure the co-delivery of adjuvant-antigen not only into the same cell but also in the same endocytic cargo, leading to more effective activation of innate and adaptive immune response.

Genomic Region Containing Toll-Like Receptor Genes Has a Major Impact on Total IgM Antibodies Including KLH-Binding IgM Natural Antibodies in Chickens

Natural antibodies (NAb) are antigen binding antibodies present in individuals without a previous exposure to this antigen. Keyhole limpet hemocyanin (KLH)-binding NAb levels were previously associated with survival in chickens. This suggests that selective breeding for KLH-binding NAb may increase survival by means of improved general disease resistance. Genome-wide association studies (GWAS) were performed to identify genes underlying genetic variation in NAb levels. The studied population consisted of 1,628 adolescent layer chickens with observations for titers of KLH-binding NAb of the isotypes IgM, IgA, IgG, the total KLH-binding (IgT) NAb titers, total antibody concentrations of the isotypes IgM, IgA, IgG, and the total antibodies concentration in plasma. GWAS were performed using 57,636 single-nucleotide polymorphisms (SNP). One chromosomal region on chromosome 4 was associated with KLH-binding IgT NAb, and total IgM concentration, and especially with KLH-binding IgM NAb. The region of interest was fine mapped by imputing the region of the study population to whole genome sequence, and subsequently performing an association study using the imputed sequence variants. 16 candidate genes were identified, of which FAM114A1, Toll-like receptor 1 family member B (TLR1B), TLR1A, Krüppel-like factor 3 (KLF3) showed the strongest associations. SNP located in coding regions of the candidate genes were checked for predicted changes in protein functioning. One SNP (at 69,965,939 base pairs) received the maximum impact score from two independent prediction tools, which makes this SNP the most likely causal variant. This SNP is located in TLR1A, which suggests a fundamental role of TLR1A on regulation of IgM levels (i.e., KLH-binding IgM NAb, and total IgM concentration), or B cells biology, or both. This study contributes to increased understanding of (genetic) regulation of KLH-binding NAb levels, and total antibody concentrations.

Breaking Hepatitis B Virus Tolerance and Inducing Protective Immunity Based on Mimicking T Cell-Independent Antigen

There are over 350 million chronic carriers of hepatitis B virus (HBV) in the world, of whom about a third eventually develop severe HBV-related complications. HBV contributes to liver cirrhosis and hepatocellular carcinoma development. Remarkable progress has been made in selective inhibition of HBV replication by nucleoside analogs. However, how to generate protective antibody of HBsAb in HBV-infected patients after HBV-DNA becomes negative still remains a challenge for scientists. In this study, we show that OmpC-HBsAg 'a' epitope chimeric protein vaccine can break HBV tolerance and induce protective immunity in HBV transgenic mice based on mimicking T cell-independent antigen to bypass T cells from the adaptive immune system. The antibodies induced by the vaccine have the ability to prevent HBV virion infection of human hepatocytes.

Pathogen-associated porin turns IL-10 competent B-1a cells toward proinflammatory cytokine response

Shigellosis is a major problem in the developing countries causing mortality and morbidity particularly among the children. Shigella spp. harbours the epithelial cells of the human colon to infect the host and spread the disease. We analyzed the response of B-1a cells, the major component of the mucosal immune system to porin of Shigella dysenteriae type 1. We show that porin while proliferating B-1a cells, deplete Siglec-G, the inhibitory molecule present on B-1a cells. Adjuvanticity of porin has been shown to govern innate signaling for promoting host adaptive immune response. Up-regulation of CD69 and CD40 denotes activation of the cells parallel to abrogation of Siglec-G. As a result of cell activation, porin stimulated the inflammatory cytokines of CD5⁺ B-1a cells, otherwise rich in IL-10. The work shows B-1a cell responses promote the immunopotentiating activity of porin.

Bacterial ligand stimulates TLR2-dependent chemokines of colon cell

Shigella spp. are known to penetrate the colonic epithelial cells causing shigellosis, which results in production of convalescent antibodies against porin, the surface exposed major outer membrane protein. Porin has been categorized as primarily TLR2-ligand and here we validated its signaling procedure in colonic INT-407 cells simulating the host scenario. Porin up-regulated TLR2 and -6 followed by TLR2·MYD88 complex formation suggesting direct involvement of MYD88 for downstream signaling. Translocation of NF-κB p65 and p50 subunits on to the nucleus indicates involvement of the transcription factor in signaling. Porin-induced TLR signaling specifically stimulated the pro-inflammatory chemokine panel comprising of MIP-1α, MCP-1 and IL-8. Inhibition studies of TLR2 and NF-κB led to abrogation of the pro-inflammatory chemokine response, showing TLR-dependent signaling through NF-κB regulate gut activity. This work elucidates TLR2 not only scans pathogen-associated molecule but also has a direct role in maneuvering colon cell response.

Listeria monocytogenes-derived listeriolysin O has pathogen-associated molecular pattern-like properties independent of its hemolytic ability

There is a constant need for improved adjuvants to augment the induction of immune responses against tumor-associated antigens (TAA) during immunotherapy. Previous studies have established that listeriolysin O (LLO), a cholesterol-dependent cytolysin derived from Listeria monocytogenes, exhibits multifaceted effects to boost the stimulation of immune responses to a variety of antigens. However, the direct ability of LLO as an adjuvant and whether it acts as a pathogen-associated molecular pattern (PAMP) have not been demonstrated. In this paper, we show that a detoxified, nonhemolytic form of LLO (dtLLO) is an effective adjuvant in tumor immunotherapy and may activate innate and cellular immune responses by acting as a PAMP. Our investigation of the adjuvant activity demonstrates that dtLLO, either fused to or administered as a mixture with a human papillomavirus type 16 (HPV-16) E7 recombinant protein, can augment antitumor immune responses and facilitate tumor eradication. Further mechanistic studies using bone marrow-derived dendritic cells suggest that dtLLO acts as a PAMP by stimulating production of proinflammatory cytokines and inducing maturation of antigen-presenting cells (APC). We propose that dtLLO is an effective adjuvant for tumor immunotherapy, and likely for other therapeutic settings.

The FomA porin from Fusobacterium nucleatum is a Toll-like receptor 2 agonist with immune adjuvant activity

Many bacterial components selectively activate immune and nonhematopoietic target cells via Toll-like receptor (TLR) signaling; modulation of such host responses defines the immune adjuvant properties of these bacterial products. For example, the outer membrane protein porins from Neisseria, Salmonella, and Shigella are known TLR2 agonists with established systemic and mucosal immune adjuvanticity. Early work indicated that the FomA porin from Fusobacterium nucleatum has immune adjuvant activity in mice. Using a purified recombinant FomA, we have verified its immune stimulatory properties and have defined a role for TLR2 signaling in its in vitro and in vivo activity. FomA induces interleukin 8 (IL-8) secretion and NF-κB-dependent luciferase activity in HEK cells expressing TLR2, IL-6 secretion, and cell surface upregulation of CD86 and major histocompatibility complex (MHC) II in primary B cells from wild-type mice, but it fails to activate cells from TLR2 knockout mice. Accordingly, the immune adjuvant activity of FomA is also TLR2 dependent. In a mouse model of immunization with ovalbumin (OVA), FomA induces enhanced production of OVA-specific IgM and IgG, including IgG1 and IgG2b antibodies, as well as enhanced secretion of IL-10 and IL-6, consistent with a Th2-type adjuvant effect. We also observe a moderate production of anti-FomA antibodies, suggesting that FomA is also immunogenic, a quality that is also TLR2 dependent. Therefore, modulation of host immune responses by FomA may be effective for targeting general host immunity not only to pathogens (as a novel TLR2 adjuvant) but also to F. nucleatum itself (as an antigen), expanding its use as a self-adjuvanted antigen in an immunization strategy against polymicrobial infections, including those by F. nucleatum.

Hemolysin induces Toll-like receptor (TLR)-independent apoptosis and multiple TLR-associated parallel activation of macrophages

Vibrio cholerae hemolysin (HlyA) displays bipartite property while supervising macrophages (MΦ). The pore-forming toxin causes profound apoptosis within 3 h of exposure and in parallel supports activation of the defying MΦ. HlyA-induced apoptosis of MΦ remains steady for 24 h, is Toll-like receptor (TLR)-independent, and is driven by caspase-9 and caspase-7, thus involving the mitochondrial or intrinsic pathway. Cell activation is carried forward by time dependent up-regulation of varying TLRs. The promiscuous TLR association of HlyA prompted investigation, which revealed the β-prism lectin domain of HlyA simulated TLR4 up-regulation by jacalin, a plant lectin homologue besides expressing CD86 and type I cytokines TNF-α and IL-12. However, HlyA cytolytic protein domain up-regulated TLR2, which controlled CD40 for continuity of cell activation. Expression of TOLLIP before TLR2 and TLR6 abrogated TLR4, CD40, and CD86. We show that the transient expression of TOLLIP leading to curbing of activation-associated capabilities is a plausible feedback mechanism of MΦ to deploy TLR2 and prolong activation involving CD40 to encounter the HlyA cytolysin domain.

Innate immune function of the neisserial porins and the relationship to vaccine adjuvant activity

Neisseria meningitidis is a Gram-negative pathogenic bacteria responsible for bacterial meningitis and septicemia. Porins are the most represented outer membrane proteins in the pathogenic Neisseria species, functioning as pores for the exchange of ions, and are characterized by a trimeric beta-barrel structure. Neisserial porins have been shown to act as adjuvants in the immune response via activation of B cells and other antigen-presenting cells. Their effect on the immune response is mediated by upregulation of the costimulatory molecule B7-2 (CD86) on the surface of antigen-presenting cells, an effect that is dependent on Toll-like receptor (TLR)2 and MyD88, through a cascade of signal transduction events mediated by direct binding of the porin to the TLR2-TLR1 heterodimer. This article summarizes work carried out investigating the mechanisms of the immune stimulating capacity of the neisserial porins (specifically meningococcal PorB), emphasizing cellular events involved in antigen-presenting cell activation and induction of expression of cell surface molecules involved in the immune response.

The contribution of Toll-like receptor 2 to the innate recognition of a Leishmania infantum silent information regulator 2 protein

We have characterized a Leishmania protein belonging to the silent information regulator 2 (SIR2) family [SIR2 related protein 1 (SIR2RP1)] that might play an immunoregulatory role during infection through its capacity to trigger B-cell effector functions. We report here that SIR2RP1 leads to the proliferation of activated B cells, causing increased expression of major histocompatibility complex (MHC) II and the costimulatory molecules CD40 and CD86, which are critical ligands for T-cell cross-talk during the development of adaptive immune responses. In contrast, B cells isolated from Toll-like receptor 2 (TLR2) knockout mice were unable to respond to the SIR2RP1 stimulus. Similarly, SIR2RP1 induced the maturation of dendritic cells (DCs) in a TLR2-dependent manner with the secretion of pro-inflammatory cytokines [interleukin (IL)-12 and tumour necrosis factor (TNF)-alpha] and enhanced the costimulatory properties of DCs. Nevertheless, immunization assays demonstrated that TLR2-deficient mice were able to mount a specific humoral response to SIR2RP1. Interestingly, further investigations showed that macrophages were activated by SIR2RP1 even in the absence of TLR2. Therefore, a different type of interplay between SIR2RP1 and the major antigen-presenting cells in vivo could explain the immune response observed in TLR2-deficient mice. Together, these results demonstrate that TLR2 signalling contributes to SIR2RP1 recognition by innate immune host cells.

Toll-like receptors in liver ischemia reperfusion injury: a novel target for therapeutic modulation?

BACKGROUND

There is increasing evidence that Toll-like receptors (TLRs) sense host tissue damage by engaging with endogenous ligands. TLRs are considered to be involved in many primarily non-immune-related diseases. Hepatic ischemia reperfusion injury (IRI) represents one of these disorders.

OBJECTIVE

To present the latest findings supporting the involvement of TLRs in liver IRI and to explore their role as potential targets for therapeutic intervention.

METHODS

A review of the literature summarizing the latest advances in TLR signaling, the role of TLRs in each hepatic cell population and the involvement of TLRs in the pathophysiology of hepatic IRI. The potential role of TLR-targeting treatment strategies in liver IRI is discussed.

CONCLUSIONS

Recent experimental evidence suggests that TLR activation on Kupffer cells provides the triggering signal for pro-inflammatory responses that lead to liver IRI. Modulating TLR signaling could have a beneficial effect in patients with liver IRI.

TLR2 and TLR4 signaling shapes specific antibody responses to Salmonella typhi antigens

TLR directly induce innate immune responses by sensing a variety of microbial components and are critical for the fine-tuning of subsequent adaptive immune responses. However, their impact and mechanism of action on antibody responses against bacterial antigens are not yet fully understood. Salmonella enterica serovar Typhi (S. typhi) porins have been characterized as inducers of long-lasting specific antibody responses in mice. In this report, we show that immunization of TLR4-deficient (TLR4(-/-)), myeloid differentiating gene 88-deficient and Toll/IL-R domain-containing adaptor-inducing IFN-beta-deficient mice with S. typhi porins led to significantly reduced B-cell responses. TLR2(-/-) mice, as well, showed reduced IgG titers with a more pronounced impairment in the production of IgG3 anti-porins antibodies. Adoptive transfer of TLR2(-/-)- or TLR4(-/-)-B cells into B-cell-deficient mice revealed a direct effect of TLR4 on B cells for the primary IgM response, whereas stimulation of B cells via TLR2 was important for IgG production. Furthermore, S. typhi porins were found to efficiently elicit maturation of CD11c(+) conventional DC. Taken together, S. typhi porins represent not only a suitable B-cell antigen for vaccination, but exhibit potent TLR-dependent stimulatory functions on B cells and DC, which help to further enhance and shape the antibody response.

Toll-like receptor modulation: a novel therapeutic strategy in cardiovascular disease?

BACKGROUND

Toll-like receptors (TLRs) have been recently recognised as primary receptors in the innate immune system. Apart from initiating a prompt immune response against invading pathogens, TLRs are also considered to be an important link between innate immunity, inflammation and a variety of clinical disorders, including cardiovascular diseases. TLR signalling manipulation with novel drugs could offer important opportunities for cardiovascular disease modification.

OBJECTIVE

To present the latest knowledge supporting the involvement of TLRs in the pathogenesis and progress of cardiovascular diseases and explore the role of TLRs as potential targets for therapeutic intervention in cardiovascular territory.

METHODS

A review of the literature documenting implication of TLR signalling in cardiovascular disorders. Current progress in TLR-targeting drug development and the potential role of such a treatment strategy in cardiovascular disorders are discussed.

CONCLUSIONS

A growing body of evidence supports a role for TLRs in cardiovascular disease initiation and progression. Altering TLR signalling with novel drugs could be a beneficial therapeutic strategy for patients with cardiovascular disorders.

Role of protein tyrosine kinase and Erk1/2 activities in the Toll-like receptor 2-induced cellular activation of murine B cells by neisserial porin

Neisserial porins are potent immune adjuvants and have been demonstrated to stimulate and induce the activation of human and murine B lymphocytes. Their immunopotentiating ability is due largely to the upregulation of the surface expression of the costimulatory ligand CD86 (B7-2) on B cells and other antigen-presenting cells. Porin-induced activation is dependent on the innate immune pattern recognition receptor Toll-like receptor 2 (TLR2). These data have led us to investigate the signal transduction events induced by PorB from Neisseria meningitidis and then, using inhibitors of these pathways, to establish the mechanism by which this bacterial major outer membrane protein induces CD86 upregulation and the proliferation of murine B cells. PorB was able to induce (i) protein tyrosine kinase (PTK) activity, (ii) the phosphorylation of Erk1 and Erk2, and (iii) IkappaB-alpha phosphorylation, leading to NF-kappaB nuclear translocation in B cells in a TLR2-dependent manner. PorB-induced NF-kappaB nuclear translocation was not dependent on either PTK or Erk1/2 activities. However, B-cell proliferation and the induction of increased surface expression of CD86 by PorB were dependent on PTK activity and not Erk1/2 activation. In conclusion, PorB acts through TLR2 as a B-cell mitogen, triggering tyrosine phosphorylation of various cellular proteins that are involved in proliferation and CD86 expression, as well as the phosphorylation of Erk1/2, which is not necessary for CD86 upregulation or the proliferation of B cells.

Role of the innate immune system in host defence against bacterial infections: focus on the Toll-like receptors

The innate immunity plays a critical role in host protection against pathogens and it relies amongst others on pattern recognition receptors such as the Toll-like receptors (TLRs) and the nucleotide-binding oligomerization domains proteins (NOD-like receptors, NLRs) to alert the immune system of the presence of invading bacteria. Since their recent discovery less than a decade ago, both TLRs and NLRs have been shown to be crucial in host protection against microbial infections but also in homeostasis of the colonizing microflora. They recognize specific microbial ligands and with the use of distinct adaptor molecules, they activate different signalling pathways that in turns trigger subsequent inflammatory and immune responses that allows a immediate response towards bacterial infections and the initiation of the long-lasting adaptive immunity. In this review, we will focus on the role of the TLRs against bacterial infections in humans in contrast to mice that have been used extensively in experimental models of infections and discuss their role in controlling normal flora or nonpathogenic bacteria. We also highlight how bacteria can evade recognition by TLRs.

Non-lipooligosaccharide-mediated signalling via Toll-like receptor 4 causes fatal meningococcal sepsis in a mouse model

Meningococcal lipooligosaccharide (LOS) is a major inflammatory mediator of fulminant meningococcal sepsis and meningitis with disease severity correlating with circulating concentrations of LOS and proinflammatory cytokines. In this study we show that the proinflammatory response to live meningococci in a mouse model of sepsis involves TLR4, but can develop independently of the expression of LOS. This is supported by data showing that in vivo an isogenic LOS-deficient strain, lpxA, induced equivalent disease severity and similar proinflammatory responses as the serogroup C wild-type parent strain FAM20. This response was abolished in TLR4-/- mice, and neither the wild-type strain of meningococci nor the LOS-deficient mutant was able to cause fatal sepsis in these mice. Mouse survival correlated with low levels of cytokines and chemokines, the chemotactic complement factor C5a and neutrophil levels in blood at 24 h post infection. These data suggest that during meningococcal sepsis the recognition of one or more unidentified non-LOS component(s) by TLR4 is important in stimulating proinflammatory responses, and that fatality associated with meningococcal sepsis in mice is induced by the proinflammatory host response.

Meningococcal porin PorB binds to TLR2 and requires TLR1 for signaling

TLR2 plays a key role in the initiation of the cellular innate immune responses by a wide range of bacterial products. TLRs signaling, including TLR2 and its coreceptors TLR1 and TLR6, is mediated by a number of specific ligands. Although many of the TLR-mediated cell signaling pathways have been elucidated in the past few years, the molecular mechanisms that lead to cell activation are still poorly understood. In this study, we investigate the interaction of PorB from Neisseria meningitidis with TLR2 and describe the direct binding of a bacterial protein to TLR2 for the first time. Using labeled PorB, we demonstrate its binding to TLR2 both in its soluble form in vitro, and when it is over-expressed on the surface of human embryonic kidney 293 cells. We also show that TLR2-mediated binding of PorB is directly related to cellular activation. In addition, using 293 cells expressing the chimeric TLR2/TLR1 and TLR2/TLR6 complexes, we report the selectivity of PorB binding to the TLR2/TLR1 heterodimer, which is required for initiating signaling in transfected 293 cells and in murine B cells. Together, these data provide new evidence that TLR2 recognizes PorB through direct binding, and that PorB-induced cell activation is mediated by a TLR2/TLR1 complex.

Salmonella porins induce a sustained, lifelong specific bactericidal antibody memory response

We examined the ability of porins from Salmonella enterica serovar typhi to induce a long-term antibody response in BALB/c mice. These porins triggered a strong lifelong production of immunoglobulin G (IgG) antibody in the absence of exogenous adjuvant. Analysis of the IgG subclasses produced during this antibody response revealed the presence of the subclasses IgG2b, IgG1, IgG2a and weak IgG3. Despite the high homology of porins, the long-lasting anti-S. typhi porin sera did not cross-react with S. typhimurium. Notably, the antiporin sera showed a sustained lifelong bactericidal-binding activity to the wild-type S. typhi strain, whereas porin-specific antibody titres measured by enzyme-linked immunosorbent assay (ELISA) decreased with time. Because our porin preparations contained the outer membrane proteins C and F (OmpC and OmpF), we evaluated the individual contribution of each porin to the long-lasting antibody response. OmpC and OmpF induced long-lasting antibody titres, measured by ELISA, which were sustained for 300 days. In contrast, although OmpC induced sustained high bactericidal antibody titres for 300 days, postimmunization, the bactericidal antibody titre induced by OmpF was not detected at day 180. These results indicate that OmpC is the main protein responsible for the antibody-mediated memory bactericidal response induced by porins. Taken together, our results show that porins are strong immunogens that confer lifelong specific bactericidal antibody responses in the absence of added adjuvant.