Oral immunization of piglets with recombinant F4 fimbrial adhesin FaeG monomers induces a mucosal and systemic F4-specific immune response

The immune response of pregnant sow after vaccination with crude fimbriae (F4) extracts vaccine and immunoprotection of nursery pig against pathogenic E. coli (F4+ETEC)

BACKGROUND

Neonatal and post-weaning diarrhea is a concern disease caused by enterotoxigenic Escherichia coli fimbriae F4 (F4+ETEC) in pig farms. Diarrhea outbreaks are often severe and costly due to the high prevalence and spread of the disease within the same herd. Vaccine is one of strategic solution in protecting pig against F4+ETEC infection in particular pig farm. In present study, we conducted two trials of vaccination with crude F4 fimbriae extract vaccine in pregnant sow and nursery pigs.

METHODS

In experiment 1 (20 sows; non-vaccinated control, n=10), we vaccinated pregnant sows (n=10) twice at 4 wk and 2 wk before farrowing and evaluated impact of vaccination on maternal immunity. The sow serum and colostrum were collected before vaccination, 2 and 4 weeks after vaccination, 6 hours after farrowing, respectively, and the piglet's serum from both groups (2 piglet/sow, 10 piglets from each group) were also collected on 3 days old to measure F4 specific IgG, F4 specific IgA using in house ELISA kit. In experiment 2, to optimize doses and dosage of candidate vaccine in piglets, 18 piglets (3 piglets/group) were allocated into five immunized groups and one control group (unimmunized group), we immunized piglets twice at 4 and 6 weeks old with difference doses (i.e., 0, 50, 100, 150, 200 µg), and for a dose 150 µg, we immunized with two dosages at 1 ml and 2 ml. Piglets were challenged with a 3 ml dose of 3 × 109 CFU/ml bacterial culture of enterotoxigenic Escherichia coli (F4+ETEC) in order to evaluate the efficacy of vaccine. After challenging, the clinical sign of the piglets was daily observed and the rectal swab was performed every day for investigation of the fecal shedding of Escherichia coli (F4+ETEC) by using PCR technique. Serum were collected before, 2 and 4 weeks after vaccination and 1 week after challenge to measure F4 specific IgG, F4 specific IgA using in house ELISA kit and cytokines levels (i.e., IL-1 beta, IL-6, IL-8 and TNF alpha) before and 1 week after challenge using commercial ELISA kit.

RESULTS

The levels of antibody results showed that in experiment 1, the anti-F4 antibody levels both F4 specific IgG and F4 specific IgA in serum and colostrum of vaccinated sow increased significantly after vaccination. The piglets of immunized sows have antibody level both F4 specific IgG and F4 specific IgA in their serum higher than those piglets of unimmunized sows significantly (p < 0.01). In experiment 2, irrespective of different doses and dosage, there is no difference in term of F4 specific IgG and F4 specific IgA levels among immunized groups. However, all of vaccinated piglets showed F4 specific IgG and F4 specific IgA levels higher and the elimination of Escherichia coli (F4+ETEC) in feces post challenge faster (< 3 days) than unvaccinated group (> 5 days). For cytokines levels, a higher level of IL-1 beta, IL-6, IL-8 and TNF alpha at 1 week after challenge in vaccinated groups was found when compared with the levels in non-vaccinated group.

CONCLUSIONS

Our results suggest that crude F4 fimbriae extract autogenous vaccine is a candidate vaccine for protecting piglets against diarrhea disease caused by enterotoxigenic Escherichia coli (F4+ETEC) and vaccination the pregnant sow twice before farrowing is one of strategies to provide maternal derived antibody to the newborn piglets for against enterotoxigenic Escherichia coli (F4+ETEC) during early life.

A Virus-like Particle-Based F4 Enterotoxigenic Escherichia coli Vaccine Is Inhibited by Maternally Derived Antibodies in Piglets but Generates Robust Responses in Sows

F4-positive enterotoxigenic Escherichia coli is associated with diarrhea and poor growth outcomes in neonatal and newly weaned piglets and is thus a major economic and welfare burden in the swine industry. Vaccination of sows with F4 fimbriae protects against the neonatal disease via passive transfer of maternal immunity. However, this strategy does not protect against infection post-weaning. Consequently, prevention and treatment methods in weaner pigs heavily rely on the use of antimicrobials. Therefore, in order to reduce antimicrobial consumption, more effective prophylactic alternatives are needed. In this study, we describe the development of a capsid virus-like particle (cVLP)-based vaccine targeting the major F4 fimbriae subunit and adhesion molecule, FaeG, and evaluate its immunogenicity in mice, piglets, and sows. cVLP-display significantly increased systemic and mucosal antibody responses towards the recombinant FaeG antigen in mice models. However, in piglets, the presence of anti-F4 maternally derived antibodies severely inhibited the induction of active humoral responses towards the FaeG antigen. This inhibition could not be overcome, even with the enhanced immunogenicity achieved via cVLP display. However, in sows, intramuscular vaccination with the FaeG.cVLP vaccine was able to generate robust IgG and IgA responses that were comparable with a commercial fimbriae-based vaccine, and which were effectively transferred to piglets via colostrum intake. These results demonstrate that cVLP display has the potential to improve the systemic humoral responses elicited against low-immunogenic antigens in pigs; however, this effect is dependent on the use of antigens, which are not the targets of pre-existing maternal immunity.

Swine enteric colibacillosis: Current treatment avenues and future directions

Enteric colibacillosis is a common disease in nursing and weanling pigs. It is caused by the colonization of the small intestine by enterotoxigenic strains of Escherichia coli (ETEC) that make use of specific fimbria or pili to adhere to the absorptive epithelial cells of the jejunum and ileum. Once attached, and when both the immunological systems and the gut microbiota are poorly developed, ETEC produce one or more enterotoxins that can have local and, further on, systemic effects. These enterotoxins cause fluid and electrolytes to be secreted into the intestinal lumen of animals, which results in diarrhea, dehydration, and acidosis. From the diversity of control strategies, antibiotics and zinc oxide are the ones that have contributed more significantly to mitigating post-weaning diarrhea (PWD) economic losses. However, concerns about antibiotic resistance determined the restriction on the use of critically important antimicrobials in food-producing animals and the prohibition of their use as growth promoters. As such, it is important now to begin the transition from these preventive/control measures to other, more sustainable, approaches. This review provides a quick synopsis of the currently approved and available therapies for PWD treatment while presenting an overview of novel antimicrobial strategies that are being explored for the control and treatment of this infection, including, prebiotics, probiotics, synbiotics, organic acids, bacteriophages, spray-dried plasma, antibodies, phytogenic substances, antisense oligonucleotides, and aptamers.

Maternal Vaccination. Immunization of Sows during Pregnancy against ETEC Infections

The immunology of pregnancy is an evolving consequence of multiple reciprocal interactions between the maternal and the fetal-placental systems. The immune response must warrant the pregnancy outcome (including tolerance to paternal antigens), but at the same time, efficiently respond to pathogenic challenges. Enterotoxigenic Escherichia coli (ETEC) strains are a major cause of illness and death in neonatal and recently weaned pigs. This review aims to give an overview of the current rationale on the maternal vaccination strategies for the protection of the newborn pig against ETEC. Newborn piglets are immunodeficient and naturally dependent on the maternal immunity transferred by colostrum for protection—a maternal immunity that can be obtained by vaccinating the sow during pregnancy. Our current knowledge of the interactions between the pathogen strategies, virulence factors, and the host immune system is aiding the better design of vaccination strategies in this particular and challenging host status. Challenges include the need for better induction of immunity at the mucosal level with the appropriate use of adjuvants, able to induce the most appropriate and long-lasting protective immune response. These include nanoparticle-based adjuvants for oral immunization. Experiences can be extrapolated to other species, including humans.

Intestinal challenge with enterotoxigenic Escherichia coli in pigs, and nutritional intervention to prevent postweaning diarrhea

Gut health of nursery pigs immediately after weaning is tightly associated with their growth performance and economic values. Postweaning diarrhea (PWD) is one of the major concerns related to gut health of nursery pigs which often is caused by infections of enterotoxigenic Escherichia coli (ETEC), mainly including F4 (K88)+ and F18+E. coli. The main virulence factors of ETEC are adhesins (fimbriae or pili) and enterotoxins. The common types of fimbriae on ETEC from PWD pigs are F18+ and F4+. Typically, PWD in pigs is associated with both F18+ and F4+ ETEC infections whereas pre-weaning diarrhea in pigs is associated with F4+ ETEC infection. Enterotoxins including heat-labile enterotoxins (LT) and heat-stable peptide toxins (ST) are associated with causing diarrhea in pigs. At least 109 to 1010 ETEC are required to induce diarrhea in nursery pigs typically lasting 1 to 5 days after ETEC infection. Antibiotics used to be the most effective way to prevent PWD, however, with the increased bacterial resistance to antibiotics, alternatives to the use of antibiotics are urgently needed to prevent PWD. Immunopropylaxis and nutritional intervention of antimicrobial minerals (such as zinc oxide and copper sulfate), organic acids, functional feedstuffs (such as blood plasma and egg yolk antibodies), direct fed microbials, phytobiotics, and bacteriophage can potentially prevent PWD associated with ETEC. Some other feed additives such as nucleotides, feed enzymes, prebiotic oligosaccharides, and clay minerals can enhance intestinal health and thus indirectly help with preventing PWD. Numerous papers show that nutritional intervention using selected feed additives can effectively prevent PWD.

A food-grade fimbrial adhesin FaeG expression system inLactococcus lactisandLactobacillus casei

Enterotoxigenic Escherichia coli (ETEC) infection is the major cause of diarrhea in neonatal piglets. The fimbriae as colonizing factor in the pathogenesis of ETEC constitute a primary target for vaccination against ETEC. Lactic acid bacteria (LAB) are attractive tools to deliver antigens at the mucosal level. With the safety of genetically modified LAB in mind, a food-grade secretion vector (pALRc or pALRb) was constructed with DNA entirely from LAB, including the replicon, promoter, signal peptide, and selection marker alanine racemase gene (alr). To evaluate the feasibility of the system, the nuclease gene (nuc) from Staphylococcus aureus was used as a reporter to be expressed in both Lactococcus lactis and Lactobacillus casei. Subsequently, the extracellular secretion of the fimbrial adhesin FaeG of ETEC was confirmed by Western blot analysis. These results showed that this food-grade expression system has potential as the delivery vehicle for the safe use of genetically modified LAB for the development of vaccines against ETEC infection.

Receptor for the F4 fimbriae of enterotoxigenic Escherichia coli (ETEC)

Infection with F4(+) enterotoxigenic Escherichia coli (ETEC) responsible for diarrhea in neonatal and post-weaned piglets leads to great economic losses in the swine industry. These pathogenic bacteria express either of three fimbrial variants F4ab, F4ac, and F4ad, which have long been known for their importance in host infection and initiating protective immune responses. The initial step in infection for the bacterium is to adhere to host enterocytes through fimbriae-mediated recognition of receptors on the host cell surface. A number of receptors for ETEC F4 have now been described and characterized, but their functions are still poorly understood. The current review summarizes the latest research addressing the characteristics of F4 fimbriae receptors and the interactions of F4 fimbriae and their receptors on host cells. These include observations that as follows: (1) FaeG mediates the binding activities of F4 and is an essential component of the F4 fimbriae, (2) the F4 fimbrial receptor gene is located in a region of chromosome 13, (3) the biochemical properties of F4 fimbrial receptors that form the binding site of the bacterium are now recognized, and (4) specific receptors confer susceptibility/resistance to ETEC F4 infection in pigs. Characterizing the host-pathogen interaction will be crucial to understand the pathogenicity of the bacteria, provide insights into receptor activation of the innate immune system, and develop therapeutic strategies to prevent this illness.

F4+ enterotoxigenic Escherichia coli (ETEC) adhesion mediated by the major fimbrial subunit FaeG

The FaeG subunit is the major constituent of F4(+) fimbriae, associated with glycoprotein and/or glycolipid receptor recognition and majorly contributes to the pathogen attachment to the host cells. To investigate the key factor involved in the fimbrial binding of F4(+) Escherichia coli, both the recombinant E. coli SE5000 strains carrying the fae operon gene clusters that express the different types of fimbriae in vitro, named as rF4ab, rF4ac, and rF4ad, respectively, corresponding to the fimbrial types F4ab, F4ac, and F4ad, and the three isogenic in-frame faeG gene deletion mutants were constructed. The adhesion assays and adhesion inhibition assays showed that ΔfaeG mutants had a significant reduction in the binding to porcine brush border as well as the intestinal epithelial cell lines, while the complemented strain ΔfaeG/pfaeG restored the adhesion function. The recombinant bacterial strains rF4ab, rF4ac, and rF4ad have the same binding property as wild-type F4(+) E. coli strains do and improvement in terms of binding to porcine brush border and the intestinal epithelial cells, and the adherence was blocked by the monoclonal antibody anti-F4 fimbriae. These data demonstrate that the fimbrial binding of F4(+) E. coli is directly mediated by the major FaeG subunit.

Cloning and Expression of Genes Encoding F107-C and K88-1NT Fimbrial Proteins of Enterotoxigenic Escherichia coli from Piglets

We cloned two genes coding F107-C and K88-1NT fimbrial subunits from strains E. coli C and 1NT isolated from Thua Thien Hue province, Vietnam. The mature peptide of faeG gene from strain E. coli 1NT (called faeG-1NT) is 100 % similarity with faeG gene, while the CDS of fedA gene from strain C (called fedA-C) has a similarity of 97 % with the fedA gene. Expression of the faeG-1NT and fedA-C genes in E. coli BL21 Star™ (DE3) produced proteins of ~31 and 22 kDa, respectively. The effect of IPTG concentration on the K88-1NT and F107-C fimbriae production was investigated. The results showed that 0.5 mM IPTG is suitable for higher expression of K88-1NT subunit, while 0.75 mM IPTG strongly stimulated expression of F107-C subunit. The optimal induction time for expression was also examined. Generally, highest expression of K88-1NT subunit occurred after 6 h of induction, while that of F107-C subunit is after 14 h.

Subcutaneous or oral immunization of mice with Lactococcus lactis expressing F4 fimbrial adhesin FaeG

Enterotoxigenic Escherichia coli (ETEC) is one of the most common causes of diarrhea in neonatal and postweaning piglets. Fimbrial adhesion of ETEC has been considered an important colonization factor with antigenicity. To safely and effectively deliver the F4 (K88) fimbrial adhesin FaeG to the immune system, we have previously constructed the secretory expression vector pNZ8112-faeG, and FaeG was produced in cytoplasmic form in Lactococcus lactis. In this work, BALB/c mice were immunized with recombinant L. lactis to further determine the immunogenicity of recombinant FaeG (rFaeG) via the subcutaneous or oral route. Subcutaneous immunization in mice with recombinant L. lactis induced a significant increase in the F4-specific serum IgG titer and the number of antibody-secreting cells (ASCs) in the spleen. Oral immunization of mice with recombinant L. lactis induced mucosal and systemic F4-specific immune responses and increased the number of ASCs in the spleen, mesenteric lymph nodes and Peyer's patches. High-dose (2.8 × 10(11) CFU) recombinant strains and adjuvant cholera toxin B subunit enhanced specific mucosal immune responses. The results suggest the feasibility of delivering rFaeG expressed in L. lactis to the immune system in order to induce an F4-specific immune response.

Production of a subunit vaccine candidate against porcine post-weaning diarrhea in high-biomass transplastomic tobacco

Post-weaning diarrhea (PWD) in piglets is a major problem in piggeries worldwide and results in severe economic losses. Infection with Enterotoxigenic Escherichia coli (ETEC) is the key culprit for the PWD disease. F4 fimbriae of ETEC are highly stable proteinaceous polymers, mainly composed of the major structural subunit FaeG, with a capacity to evoke mucosal immune responses, thus demonstrating a potential to act as an oral vaccine against ETEC-induced porcine PWD. In this study we used a transplastomic approach in tobacco to produce a recombinant variant of the FaeG protein, rFaeG(ntd/dsc), engineered for expression as a stable monomer by N-terminal deletion and donor strand-complementation (ntd/dsc). The generated transplastomic tobacco plants accumulated up to 2.0 g rFaeG(ntd/dsc) per 1 kg fresh leaf tissue (more than 1% of dry leaf tissue) and showed normal phenotype indistinguishable from wild type untransformed plants. We determined that chloroplast-produced rFaeG(ntd/dsc) protein retained the key properties of an oral vaccine, i.e. binding to porcine intestinal F4 receptors (F4R), and inhibition of the F4-possessing (F4+) ETEC attachment to F4R. Additionally, the plant biomass matrix was shown to delay degradation of the chloroplast-produced rFaeG(ntd/dsc) in gastrointestinal conditions, demonstrating a potential to function as a shelter-vehicle for vaccine delivery. These results suggest that transplastomic plants expressing the rFaeG(ntd/dsc) protein could be used for production and, possibly, delivery of an oral vaccine against porcine F4+ ETEC infections. Our findings therefore present a feasible approach for developing an oral vaccination strategy against porcine PWD.

Erythrocyte and porcine intestinal glycosphingolipids recognized by F4 fimbriae of enterotoxigenic Escherichia coli

Enterotoxigenic F4-fimbriated Escherichia coli is associated with diarrheal disease in neonatal and postweaning pigs. The F4 fimbriae mediate attachment of the bacteria to the pig intestinal epithelium, enabling an efficient delivery of diarrhea-inducing enterotoxins to the target epithelial cells. There are three variants of F4 fimbriae designated F4ab, F4ac and F4ad, respectively, having different antigenic and adhesive properties. In the present study, the binding of isolated F4ab, F4ac and F4ad fimbriae, and F4ab/ac/ad-fimbriated E. coli, to glycosphingolipids from erythrocytes and from porcine small intestinal epithelium was examined, in order to get a comprehensive view of the F4-binding glycosphingolipids involved in F4-mediated hemagglutination and adhesion to the epithelial cells of porcine intestine. Specific interactions between the F4ab, F4ac and F4ad fimbriae and both acid and non-acid glycosphingolipids were obtained, and after isolation of binding-active glycosphingolipids and characterization by mass spectrometry and proton NMR, distinct carbohydrate binding patterns were defined for each fimbrial subtype. Two novel glycosphingolipids were isolated from chicken erythrocytes, and characterized as GalNAcα3GalNAcß3Galß4Glcß1Cer and GalNAcα3GalNAcß3Galß4GlcNAcß3Galß4Glcß1Cer. These two compounds, and lactosylceramide (Galß4Glcß1Cer) with phytosphingosine and hydroxy fatty acid, were recognized by all three variants of F4 fimbriae. No binding of the F4ad fimbriae or F4ad-fimbriated E. coli to the porcine intestinal glycosphingolipids occurred. However, for F4ab and F4ac two distinct binding patterns were observed. The F4ac fimbriae and the F4ac-expressing E. coli selectively bound to galactosylceramide (Galß1Cer) with sphingosine and hydroxy 24:0 fatty acid, while the porcine intestinal glycosphingolipids recognized by F4ab fimbriae and the F4ab-fimbriated bacteria were characterized as galactosylceramide, sulfatide (SO(3)-3Galß1Cer), sulf-lactosylceramide (SO(3)-3Galß4Glcß1Cer), and globotriaosylceramide (Galα4Galß4Glcß1Cer) with phytosphingosine and hydroxy 24:0 fatty acid. Finally, the F4ad fimbriae and the F4ad-fimbriated E. coli, but not the F4ab or F4ac subtypes, bound to reference gangliotriaosylceramide (GalNAcß4Galß4Glcß1Cer), gangliotetraosylceramide (Galß3GalNAcß4Galß4Glcß1Cer), isoglobotriaosylceramide (Galα3Galß4Glcß1Cer), and neolactotetraosylceramide (Galß4GlcNAcß3Galß4Glcß1Cer).

A tripartite fusion, FaeG-FedF-LT(192)A2:B, of enterotoxigenic Escherichia coli (ETEC) elicits antibodies that neutralize cholera toxin, inhibit adherence of K88 (F4) and F18 fimbriae, and protect pigs against K88ac/heat-labile toxin infection

Enterotoxigenic Escherichia coli (ETEC) strains expressing K88 (F4) or F18 fimbriae and heat-labile (LT) and/or heat-stable (ST) toxins are the major cause of diarrhea in young pigs. Effective vaccines inducing antiadhesin (anti-K88 and anti-F18) and antitoxin (anti-LT and anti-ST) immunity would provide broad protection to young pigs against ETEC. In this study, we genetically fused nucleotides coding for peptides from K88ac major subunit FaeG, F18 minor subunit FedF, and LT toxoid (LT(192)) A2 and B subunits for a tripartite adhesin-adhesin-toxoid fusion (FaeG-FedF-LT(192)A2:B). This fusion was used for immunizations in mice and pigs to assess the induction of antiadhesin and antitoxin antibodies. In addition, protection by the elicited antiadhesin and antitoxin antibodies against a porcine ETEC strain was evaluated in a gnotobiotic piglet challenge model. The data showed that this FaeG-FedF-LT(192)A2:B fusion elicited anti-K88, anti-F18, and anti-LT antibodies in immunized mice and pigs. In addition, the anti-porcine antibodies elicited neutralized cholera toxin and inhibited adherence against both K88 and F18 fimbriae. Moreover, immunized piglets were protected when challenged with ETEC strain 30302 (K88ac/LT/STb) and did not develop clinical disease. In contrast, all control nonvaccinated piglets developed severe diarrhea and dehydration after being challenged with the same ETEC strain. This study clearly demonstrated that this FaeG-FedF-LT(192)A2:B fusion antigen elicited antibodies that neutralized LT toxin and inhibited the adherence of K88 and F18 fimbrial E. coli strains and that this fusion could serve as an antigen for vaccines against porcine ETEC diarrhea. In addition, the adhesin-toxoid fusion approach used in this study may provide important information for developing effective vaccines against human ETEC diarrhea.

Enterotoxigenic Escherichia coli (K88) induce proinflammatory responses in porcine intestinal epithelial cells

Infections with F4(+) enterotoxigenic Escherichia coli (ETEC) causes severe diarrhoea in piglets, resulting in morbidity and mortality. F4 fimbriae are the key virulence factors mediating the attachment of F4(+) ETEC to the intestinal epithelium. Intestinal epithelial cells (IEC) are recently being recognized as important regulators of the intestinal immune system through the secretion of cytokines, however, data on how F4(+) ETEC affect this cytokine secretion are scarce. By using ETEC strains expressing either polymeric, monomeric or F4 fimbriae with a reduced polymeric stability, we demonstrated that polymeric fimbriae are essential for adhesion to porcine IEC and the secretion of IL-6 and IL-8 by IEC. Remarkably, this cytokine secretion was not abrogated following stimulation with an F4-negative strain. Since this strain expresses flagellin, TLR5 mediated signalling could be involved. Indeed, porcine IEC express TLR5 and purified flagellin induced IL-6 and IL-8 secretion, indicating that, as for other pathogens, flagellin is the dominant virulence factor involved in the induction of proinflammatory responses in IEC. These results indicate a potential mucosal adjuvant capacity of ETEC-derived flagellin and may improve rational vaccine design against F4(+) ETEC infections.

Immunogenicity of recombinant F4 (K88) fimbrial adhesin FaeG expressed in tobacco chloroplast

To test the possibility of producing the novel vaccine in plants against diarrhea normally found in neonatal and newly weaned piglets, the faeG gene, encoding a major F4ac fimbrial subunit protein, was introduced into the tobacco chloroplast genome. After two rounds of selection under spectinomycin, we obtained the transgenic plants nearly homoplasmic. RNA gel blot analysis indicated that faeG and the antibiotic selective gene aminoglycoside 3' adenylyltransferase (aadA) were highly transcribed as a dicistron, while the translational level of recombinant FaeG in transplastomic tobacco was about 0.15% of total soluble protein. The immunogenicity of recombinant FaeG produced in tobacco chloroplasts was confirmed by the observation that FaeG-specific antibodies were elicited in mice immunized with total soluble protein of transgenic plants, as well as the result that mouse sera stimulated by chloroplast-derived recombinant FaeG could neutralize F4ac enterotoxigenic Escherichia coli (ETEC) in vivo. This study provides a new alternative for producing the ETEC vaccine using the chloroplast expression system.

Immune responses elicited in mice with recombinant Lactococcus lactis expressing F4 fimbrial adhesin FaeG by oral immunization

Enterotoxigenic Escherichia coli (ETEC) is a major pathogenic agent causing piglet diarrhea. The major subunit and adhesin FaeG of F4(+) ETEC is an important virulence factor with strong immunogenicity. To determine whether Lactococcus lactis (L. lactis) could effectively deliver FaeG to the mucosal immune system, recombinant L. lactis expressing FaeG was constructed, and immune responses in mice following oral route delivery of recombinant L. lactis were explored. The production of FaeG expressed in L. lactis was up to approximately 10% of soluble whole-cell proteins, and recombinant FaeG (rFaeG) possessed good immunoreactivity by Western blot analysis. Oral immunization with recombinant L. lactis expressing FaeG induced F4-specific mucosal and systemic immune responses in the mice. In addition, high dose recombinant L. lactis or co-administration of high dose recombinant L. lactis with CTB enhanced the immune responses. These results suggested that L. lactis expressing FaeG was a promising candidate vaccine against ETEC.

Histomorphometric evaluation of intestinal cellular immune responses in pigs immunized with live oral F4ac+ non-enterotoxigenic E. coli vaccine against postweaning colibacillosis

Enterotoxigenic Escherichia coli (ETEC) infection is the most common type of porcine postweaning colibacillosis (PWC). Among fimbriae of porcine ETEC strains the best studied family of fimbriae are the members of F4 adhesins, existing in at least three variants: ab, ac, ad. Active immunization against porcine PWC is difficult due to: i) ETEC strains are only one of the essential predisposing factors, ii) the success of vaccinal antigen uptake depends on the presence of enterocyte receptors for F4 adhesins, iii) the intestinal immune system may react with tolerance or hypersensitivity to the same antigens depending on the dose and form of the vaccinal immunogen, and iv) kinetics of the specific immune responses may be different in the case of F4 (earlier) and the other ETEC adhesins, particularly F18 (later). The aim of this study was to test the effectiveness of a live attenuated F4ac+ non-ETEC vaccine against porcine PWC by analyzing quantitative differences in the small intestinal lymphoid and myeloid cell subsets of immunized (with or without levamisole given as an adjuvant) vs control non-immunized pigs. Four week-old pigs were intragastrically immunized with a vaccine candidate F4ac+ non-ETEC strain 2407 at day 0, challenged 7 days later with a virulent F4ac+ strain ETEC 11-800/1/94, euthanatized at day 13 and sampled for immunohistology. Non-immunized pigs received saline at day 0 and were processed as the principals. Immuno-phenotypes of lymphoid and myeloid cell subsets were demonstrated within jejunal and ileal mucosa by immunohistochemical avidin-biotin complex method and corresponding morphometric data were analyzed using software program Lucia G for digital image analyses. Monoclonal antibodies reactive with surface molecules on porcine immune cells such as CD3, CD45RA, CD45RC, CD21 and SWC3 enabled clear insight into distribution patterns and amount of these cells within the gut-associated lymphoid tissues (GALT) examined. The numbers of jejunal and ileal cell subsets tested were significantly increased (at P<0.5 or lower) in both principal groups (vaccinated or levamisole primed-vaccinated) of pigs, compared to those recorded in the control non-vaccinated pigs. Based on the histomorphometric quantification of porcine intestinal immune cells from the GALT compartments tested, it is possible to differentiate the responses of pigs immunized by an experimental mucosal vaccine from those of non-immunized pigs.

Targeting of Escherichia coli F4 fimbriae to Fcgamma receptors enhances the maturation of porcine dendritic cells

F4(+) enterotoxigenic Escherichia coli (ETEC) infections are an important cause of postweaning diarrhoea in piglets and an oral immunization of piglets with purified F4 fimbriae protects them from a subsequent F4(+) ETEC infection. However, oral immunization of suckling piglets is hampered due to the immature status of their immune system. Targeting of antigens to Fcgamma receptors (FcgammaR) on human and murine dendritic cells (DC) has been shown to enhance DC maturation and both humoral and cellular immune responses. To investigate the effect of F4 fimbriae incorporated in immune complexes (F4-IC) on porcine DC, we used porcine monocytic-derived DC (MoDC) as a model system. The results in this study demonstrate that FcgammaRI, II and III mRNA is expressed by porcine MoDC. Furthermore, we show that FcgammaRII and III are expressed on the cell surface and that F4-IC are internalized by MoDC via FcgammaR. This FcgammaR ligation induced a significantly enhanced expression of Major Histocompatibility complex (MHCII) class II and the costimulatory molecules CD80/86 and CD40 by MoDC compared with immature MoDC. Furthermore, the phagocytic capacity of F4-IC stimulated MoDC was reduced as evidenced by a reduced uptake of DQ-ovalbumin and FITC-dextran. In an allogenic and autologous mixed lymphocyte reaction, these F4-IC-activated MoDC showed an improved T cell stimulatory capacity in comparison with immature MoDC. The F4-IC induced DC maturation correlated with significant higher expression levels of several pro-inflammatory cytokines such as interleukine (IL) 1beta, IL-6 and Tumor necrosis factor alpha, the chemokine IL-8 and IL-12p40 in comparison with immature MoDC. Altogether, these results clearly demonstrate that FcgammaR engagement enhances the maturation of porcine MoDC, which may suggest that antigen targeting to FcgammaR on DC could improve vaccine design against infections.

The food contaminant fumonisin B(1) reduces the maturation of porcine CD11R1(+) intestinal antigen presenting cells and antigen-specific immune responses, leading to a prolonged intestinal ETEC infection

Consumption of food or feed contaminated with fumonisin B₁ (FB₁), a mycotoxin produced by Fusarium verticillioides, can lead to disease in humans and animals. The present study was conducted to examine the effect of FB₁ intake on the intestinal immune system. Piglets were used as a target and as a model species for humans since their gastro-intestinal tract is very similar. The animals were orally exposed to a low dose of FB₁ (1 mg/kg body weight FB₁) for 10 days which did not result in clinical signs. However, when compared to non-exposed animals, FB₁-exposed animals showed a longer shedding of F4⁺ enterotoxigenic Escherichia coli (ETEC) following infection and a lower induction of the antigen-specific immune response following oral immunization. Further analyses to elucidate the mechanisms behind these observations revealed a reduced intestinal expression of IL-12p40, an impaired function of intestinal antigen presenting cells (APC), with decreased upregulation of Major Histocompatibility Complex Class II molecule (MHC-II) and reduced T cell stimulatory capacity upon stimulation. Taken together, these results indicate an FB₁-mediated reduction of in vivo APC maturation.

The excretion of F18+ E. coli is reduced after oral immunisation of pigs with a FedF and F4 fimbriae conjugate

Currently, no vaccines are available for edema disease and post-weaning diarrhoea (PWD) in pigs. In the present study, a subunit vaccine containing the F18 fimbrial adhesin FedF was studied. Hereto, recombinant FedF was produced as a fusion protein with maltose-binding protein. Even though the produced MBPFedF was shown to attach in vitro to enterocytes, almost no FedF-specific immune response could be detected after oral administration to piglets. The delivery of FedF to the intestinal mucosa was improved by conjugating the MBPFedF to F4 fimbriae. Indeed, this conjugation induced a systemic and local FedF-specific immune response and led to a reduction in excretion after infection with F18+ E. coli. Although complete protection was not observed, the conjugation between FedF and F4 fimbriae can be considered as a first step towards the development of a combined vaccine against F4+ and F18+ E. coli infections.

Effect of beta-glucans on an ETEC infection in piglets

The effect of orally administered beta-glucans in protecting pigs against an ETEC infection after weaning was analysed in this study. Three beta-glucans that differed in origin (Saccharomyces cerevisiae (MCG (Macrogard) and G2) or Sclerotium rolfsii (G3)) and/or extraction procedure were tested. Pigs fed for 2 weeks after weaning with these glucans were less susceptible to an F4+ ETEC infection in comparison with the control group. This was evidenced by a reduction in the faecal excretion of F4+ Escherichia coli as well as a reduced F4-specific serum antibody response. This decrease in faecal excretion was statistically significant for pigs fed with the MCG glucan in a first experiment and with the G3 glucan in a second experiment; diarrhoea was milder in the glucan-supplemented groups and was significantly reduced in the MCG-supplemented group. Furthermore, a lower amount of F4-specific IgM antibody-secreting cells (ASC) was found in the lymphoid tissues of pigs fed with G2 or G3 glucans in comparison with the control pig, as well as lower F4-specific IgA ASC in G3-fed pigs in comparison with the control pig. This study showed that beta-glucans can protect against an ETEC infection. Both MCG from S. cerevisiae and G3 from S. rolfsii, resulted in significant effects. To our knowledge, this is the first in vivo study, in which the use of beta-glucans as feed ingredient for just-weaned piglets was tested for their protective effects against ETEC infection.

Mucosal immunization of piglets with purified F18 fimbriae does not protect against F18+ Escherichia coli infection

Post-weaning diarrhoea and oedema disease in weaned piglets are caused by infection with F4+ or F18+ Escherichia coli strains. There is no commercial vaccine available, but it is shown that oral immunization of weaned piglets with purified F4 fimbriae induces a protective mucosal immune response. In the present study, piglets were orally and nasally immunized with purified F18 fimbriae in the presence of the mucosal adjuvant LT(R192G) or CTA1-DD, respectively. This immunization could not lead to protection against F18+ E. coli infection. The induced F18-specific immune response was directed towards the major subunit FedA and weakly towards the adhesive subunit FedF. The results of these experiments demonstrate that it is difficult to induce protective immunity against F18+ E. coli using the whole fimbriae due to the low response against the adhesin.

Chloroplasts assemble the major subunit FaeG of Escherichia coli F4 (K88) fimbriae to strand-swapped dimers

F4 fimbriae encoded by the fae operon are the major colonization factors associated with porcine neonatal and postweaning diarrhoea caused by enterotoxigenic Escherichia coli (ETEC). Via the chaperone/usher pathway, the F4 fimbriae are assembled as long polymers of the major subunit FaeG, which also possesses the adhesive properties of the fimbriae. Intrinsically, the incomplete fold of fimbrial subunits renders them unstable and susceptible to aggregation and/or proteolytic degradation in the absence of a specific periplasmic chaperone. In order to test the possibility of producing FaeG in plants, FaeG expression was studied in transgenic tobacco plants. FaeG was directed to different subcellular compartments by specific targeting signals. Targeting of FaeG to the chloroplast results in much higher yields than FaeG targeting to the endoplasmic reticulum or the apoplast. Two chloroplast-targeted FaeG variants were purified from tobacco plants and crystallized. The crystal structures show that chloroplasts circumvent the absence of the fimbrial assembly machinery by assembling FaeG into strand-swapped dimers. Furthermore, the structures reveal how FaeG combines the structural requirements of a major fimbrial subunit with its adhesive role by grafting an additional domain on its Ig-like core.

Glycosylated F4 (K88) fimbrial adhesin FaeG expressed in barley endosperm induces ETEC-neutralizing antibodies in mice

The F4-positive enterotoxigenic Escherichia coli (ETEC) strains are a frequent cause of porcine post-weaning diarrhea. Orally administered F4 fimbriae or FaeG, the major subunit and adhesin of F4, induce a protective mucosal immune response in F4 receptor-positive piglets. Feed plants carrying immunogenic subunit proteins can offer great advantages for oral vaccination of domestic animals. Here, we describe high-level endosperm-specific production (1% of total soluble proteins) of FaeG in the crop plant barley. The endoplasmic reticulum-targeted recombinant endospermic FaeG (erFaeG) was shown to be heterogeneously glycosylated. The erFaeG showed resistance at digestive conditions simulating piglet gastric fluid. Glycosylation did not abolish the immunogenic character of the FaeG protein, since erFaeG was able to induce F4 fimbria-specific antibodies in mice. Biological activity of these anti-F4 antibodies was demonstrated in vitro by blocking the attachment of the F4+ ETEC to the F4 receptors present on porcine intestinal enterocytes.

F4 (K88) fimbrial adhesin FaeG expressed in alfalfa reduces F4+ enterotoxigenic Escherichia coli excretion in weaned piglets

Transgenic plants are attractive bioreactors to large-scale production of recombinant proteins because of their relatively low cost. This study reports for the first time the use of transgenic plants to reduce enterotoxigenic Escherichia coli (ETEC) excretion in its natural host species. The DNA sequence encoding the major subunit and adhesin FaeG of F4+ ETEC was transformed into edible alfalfa plants. Targeting of FaeG production to chloroplasts led to FaeG levels of up to 1% of the total soluble protein fraction of the transgenic alfalfa. Recombinant plant-produced FaeG (pFaeG) remained stable for 2 years when the plant material was dried and stored at room temperature. Intragastric immunization of piglets with pFaeG induced a weak F4-specific humoral response. Co-administration of pFaeG and the mucosal adjuvant cholera toxin (CT) enhanced the immune response against FaeG, reflected a better induction of an F4-specific immune response. In addition, the intragastric co-administration of CT with pFaeG significantly reduced F4+ E. coli excretion following F4+ ETEC challenge as compared with pigs that had received nontransgenic plant material. In conclusion, transgenic plants producing the FaeG subunit protein could be used for production and delivery of oral vaccines against F4+ ETEC infections.

Fimbriae of enterotoxigenic Escherichia coli function as a mucosal carrier for a coupled heterologous antigen

Receptor-mediated uptake of orally administered antigen can lead to an antigen-specific immune response, whereas oral administration of most other non-replicating soluble antigens results in the induction of oral tolerance. In the present study, it is shown that fimbriae purified from an F4(K88)(+) enterotoxigenic Escherichia coli strain can function as a mucosal carrier molecule for the model antigen human serum albumin (HSA). Glutaraldehyde-coupled F4/HSA conjugates were able to bind F4 receptor positive (F4R(+)) enterocytes, but not to F4R(-) enterocytes. Moreover, oral immunization of F4R(+) pigs with F4/HSA conjugates induced a HSA-specific immune response, whereas oral immunization with HSA/HSA conjugates did not. This mucosal carrier function of F4 fimbriae was improved following oral co-administration of the F4/HSA conjugates with the mucosal adjuvant cholera toxin (CT) to F4R(+) pigs, since both humoral and cellular HSA-specific responses were significantly increased. In comparison with F4R(+) pigs, the HSA-specific response was reduced following oral F4/HSA+CT immunization of F4R(-) pigs. This indicates that F4 fimbriae as mucosal carrier and CT as adjuvant synergistically improve the induction of a HSA-specific immune response following oral immunization of pigs. These results could open new perspectives in the development of vaccines against enteropathogens.

Cholera toxin improves the F4(K88)-specific immune response following oral immunization of pigs with recombinant FaeG

Oral immunization of both humans and animals with non-replicating soluble antigens often results in the induction of oral tolerance. However, receptor-dependent uptake of orally administered soluble antigens can lead to the induction of an antigen-specific immune response. Indeed, oral immunization of pigs with recombinant FaeG (rFaeG), the adhesin of the F4(K88) fimbriae of enterotoxigenic Escherichia coli (ETEC), induces an F4-specific humoral and cellular immune response. This response is accompanied with a reduction in the excretion of F4(+)E. coli following challenge. To improve the immune response against F4, rFaeG was orally co-administered with the mucosal adjuvant cholera toxin (CT). Oral immunization of pigs with rFaeG and CT significantly improved the induction of an F4-specific humoral and cellular immune response and also significantly reduced the faecal F4(+)E. coli excretion following F4(+) ETEC challenge as compared to rFaeG-immunized pigs. Therefore, the present study demonstrates that CT can act in pigs as a mucosal adjuvant for antigens that bind to the intestinal epithelium by a CT-receptor-independent mechanism.

F4 fimbriae expressed by porcine enterotoxigenic Escherichia coli, an example of an eccentric fimbrial system?

An overwhelming number of infectious diseases in both humans and animals are initiated by bacterial adhesion to carbohydrate structures on a mucosal surface. Most bacterial pathogens mediate this adhesion by fimbriae or pili which contain an adhesive lectin subunit. The importance of fimbriae as virulence factors led to research elucidating the regulation of fimbrial expression and their molecular assembly process. This review provides an overview of the current knowledge of induction, expression and assembly of F4 (K88) fimbriae and discusses its unique as well as its identical characteristics compared to other intensively studied fimbriae or pili expressed by Escherichia coli.