Transcytosis of F4 fimbriae by villous and dome epithelia in F4-receptor positive pigs supports importance of receptor-dependent endocytosis in oral immunization strategies

Oral lymphatic delivery of alpha-galactosylceramide and ovalbumin evokes anti-cancer immunization

We developed an orally delivered nanoemulsion that induces cancer immunization. It consists of tumor antigen-loaded nano-vesicles carrying the potent invariant natural killer T-cell (iNKT) activator α-galactosylceramide (α-GalCer), to trigger cancer immunity by effectively activating both innate and adaptive immunity. It was validated that adding bile salts to the system boosted intestinal lymphatic transport as well as the oral bioavailability of ovalbumin (OVA) via the chylomicron pathway. To increase intestinal permeability further and amplify the antitumor responses, an ionic complex of cationic lipid 1,2-dioleyl-3-trimethylammonium propane (DTP) with sodium deoxycholate (DA) (DDP) and α-GalCer were anchored onto the outer oil layer to form OVA-NE#3. As expected, OVA-NE#3 exhibited tremendously improved intestinal cell permeability as well as enhanced delivery to mesenteric lymph nodes (MLNs). Subsequent activation of dendritic cells and iNKTs, in MLNs were also observed. Tumor growth in OVA-expressing mice with melanoma was more strongly suppressed (by 71%) after oral administration of OVA-NE#3 than in untreated controls, confirming the strong immune response induced by the system. The serum levels of OVA-specific IgG1 and IgG2a were 3.52- and 6.14-fold higher than in controls. Treating OVA-NE#3 increased the numbers of tumor-infiltrating lymphocytes, including cytotoxic T-cell and M1-like macrophage. Antigen- and α-GalCer-associated enrichment of dendritic cells and iNKTs in tumor tissues also increased after OVA-NE#3 treatment. These observations indicate that our system induces both cellular and humoral immunity by targeting the oral lymphatic system. It may offer a promising oral anti-cancer vaccination strategy that involves the induction of systemic anti-cancer immunization.

Antibody-Mediated Targeting of Antigens to Intestinal Aminopeptidase N Elicits Gut IgA Responses in Pigs

Many pathogens enter the host via the gut, causing disease in animals and humans. A robust intestinal immune response is necessary to protect the host from these gut pathogens. Despite being best suited for eliciting intestinal immunity, oral vaccination remains a challenge due to the gastrointestinal environment, a poor uptake of vaccine antigens by the intestinal epithelium and the tolerogenic environment pervading the gut. To improve uptake, efforts have focused on targeting antigens towards the gut mucosa. An interesting target is aminopeptidase N (APN), a conserved membrane protein present on small intestinal epithelial cells shown to mediate epithelial transcytosis. Here, we aimed to further optimize this oral vaccination strategy in a large animal model. Porcine APN-specific monoclonal antibodies were generated and the most promising candidate in terms of epithelial transcytosis was selected to generate antibody fusion constructs, comprising a murine IgG1 or porcine IgA backbone and a low immunogenic antigen: the F18-fimbriated E. coli tip adhesin FedF. Upon oral delivery of these recombinant antibodies in piglets, both mucosal and systemic immune responses were elicited. The presence of the FedF antigen however appeared to reduce these immune responses. Further analysis showed that F18 fimbriae were able to disrupt the antigen presenting capacity of intestinal antigen presenting cells, implying potential tolerogenic effects of FedF. Altogether, these findings show that targeted delivery of molecules to epithelial aminopeptidase N results in their transcytosis and delivery to the gut immune systems. The results provide a solid foundation for the development of oral subunit vaccines to protect against gut pathogens.

Molecular determinants of surface colonisation in diarrhoeagenic Escherichia coli (DEC): from bacterial adhesion to biofilm formation

Escherichia coli is primarily known as a commensal colonising the gastrointestinal tract of infants very early in life but some strains being responsible for diarrhoea, which can be especially severe in young children. Intestinal pathogenic E. coli include six pathotypes of diarrhoeagenic E. coli (DEC), namely, the (i) enterotoxigenic E. coli, (ii) enteroaggregative E. coli, (iii) enteropathogenic E. coli, (iv) enterohemorragic E. coli, (v) enteroinvasive E. coli and (vi) diffusely adherent E. coli. Prior to human infection, DEC can be found in natural environments, animal reservoirs, food processing environments and contaminated food matrices. From an ecophysiological point of view, DEC thus deal with very different biotopes and biocoenoses all along the food chain. In this context, this review focuses on the wide range of surface molecular determinants acting as surface colonisation factors (SCFs) in DEC. In the first instance, SCFs can be broadly discriminated into (i) extracellular polysaccharides, (ii) extracellular DNA and (iii) surface proteins. Surface proteins constitute the most diverse group of SCFs broadly discriminated into (i) monomeric SCFs, such as autotransporter (AT) adhesins, inverted ATs, heat-resistant agglutinins or some moonlighting proteins, (ii) oligomeric SCFs, namely, the trimeric ATs and (iii) supramolecular SCFs, including flagella and numerous pili, e.g. the injectisome, type 4 pili, curli chaperone-usher pili or conjugative pili. This review also details the gene regulatory network of these numerous SCFs at the various stages as it occurs from pre-transcriptional to post-translocational levels, which remains to be fully elucidated in many cases.

Advances in Oral Subunit Vaccine Design

Many pathogens invade the host at the intestinal surface. To protect against these enteropathogens, the induction of intestinal secretory IgA (SIgA) responses is paramount. While systemic vaccination provides strong systemic immune responses, oral vaccination is the most efficient way to trigger protective SIgA responses. However, the development of oral vaccines, especially oral subunit vaccines, is challenging due to mechanisms inherent to the gut. Oral vaccines need to survive the harsh environment in the gastrointestinal tract, characterized by low pH and intestinal proteases and need to reach the gut-associated lymphoid tissues, which are protected by chemical and physical barriers that prevent efficient uptake. Furthermore, they need to surmount default tolerogenic responses present in the gut, resulting in suppression of immunity or tolerance. Several strategies have been developed to tackle these hurdles, such as delivery systems that protect vaccine antigens from degradation, strong mucosal adjuvants that induce robust immune responses and targeting approaches that aim to selectively deliver vaccine antigens towards specific immune cell populations. In this review, we discuss recent advances in oral vaccine design to enable the induction of robust gut immunity and highlight that the development of next generation oral subunit vaccines will require approaches that combines these solutions.

Evaluating single-domain antibodies as carriers for targeted vaccine delivery to the small intestinal epithelium

Targeting a vaccine to the mucosal surface has recently been recognized as a promising approach to efficiently induce mucosal immune responses against enteric pathogens. However, poor uptake and inefficient transport of orally delivered subunit vaccines across the intestinal epithelium combined with weak immune responses still present important bottlenecks for mucosal vaccination. A possible strategy suggested to surmount these hurdles is to target the selected antigen to transcytotic receptors, such as aminopeptidase N (APN) present on enterocytes and antigen-presenting cells (APCs). Therefore, we aimed to identify potent and selective VHHs against porcine aminopeptidase N (pAPN), that were fused to the fragment crystallizable (Fc) domain of the murine IgG2a, resulting in dimeric VHH-MG fusions. Out of a library of 30 VHH-MG fusion candidates, two fusions displaying the best binding on pAPN-expressing cells were selected and showed in vivo internalization across the porcine gut epithelium. One of these fusions triggered systemic and intestinal IgA responses upon oral administration. Our results demonstrate the potential of bivalent VHH-MG fusions as delivery vehicles for vaccine antigens. VHH-mediated targeting of antigens to APN to generate protective immunity at the mucosal surface remains to be further validated.

Molecular networks affected by neonatal microbial colonization in porcine jejunum, luminally perfused with enterotoxigenic Escherichia coli, F4ac fimbria or Lactobacillus amylovorus

The development of an early complex gut microbiota may play an important role in the protection against intestinal dysbiosis later in life. The significance of the developed microbiota for gut barrier functionality upon interaction with pathogenic or beneficial bacteria is largely unknown. The transcriptome of differently perfused jejunal loops of 12 caesarian-derived pigs, neonatally associated with microbiota of different complexity, was studied. Piglets received pasteurized sow colostrum at birth (d0), a starter microbiota (Lactobacillus amylovorus (LAM), Clostridium glycolicum, and Parabacteroides) on d1-d3, and a placebo inoculant (simple association, SA) or an inoculant consisting of sow’s diluted feces (complex association, CA) on d3-d4. On d 26–37, jejunal loops were perfused for 8 h with either enterotoxigenic Escherichia coli F4 (ETEC), purified F4 fimbriae, LAM or saline control (CTRL). Gene expression of each intestinal loop was analyzed by Affymetrix Porcine Gene 1.1_ST array strips. Gene Set Enrichment Analysis was performed on expression values. Compared to CTRL, 184 and 74; 2 and 139; 2 and 48 gene sets, were up- and down-regulated by ETEC, F4 and LAM, respectively. ETEC up-regulated networks related to inflammatory and immune responses, RNA processing, and mitosis. There was a limited overlap in up-regulated gene sets between ETEC and F4 fimbriae. LAM down-regulated genes related to inflammatory and immune responses, as well as to cellular compound metabolism. In CA pigs, 57 gene sets were up-regulated by CA, while 73 were down-regulated compared to SA. CA up-regulated gene sets related to lymphocyte modulation and to cellular defense in all loop perfusions. In CA pigs, compared to SA pigs, genes for chemokine and cytokine activity and for response to external stimuli were down-regulated in ETEC-perfused loops and up-regulated in CTRL. The results highlight the importance of the nature of neonatal microbial colonization in the response to microbial stimuli later in life.

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.

Plant-based solutions for veterinary immunotherapeutics and prophylactics

An alarming increase in emergence of antibiotic resistance among pathogens worldwide has become a serious threat to our ability to treat infectious diseases according to the World Health Organization. Extensive use of antibiotics by livestock producers promotes the spread of new resistant strains, some of zoonotic concern, which increases food-borne illness in humans and causes significant economic burden on healthcare systems. Furthermore, consumer preferences for meat/poultry/fish produced without the use of antibiotics shape today's market demand. So, it is viewed as inevitable by the One Health Initiative that humans need to reduce the use of antibiotics and turn to alternative, improved means to control disease: vaccination and prophylactics. Besides the intense research focused on novel therapeutic molecules, both these strategies rely heavily on the availability of cost-effective, efficient and scalable production platforms which will allow large-volume manufacturing for vaccines, antibodies and other biopharmaceuticals. Within this context, plant-based platforms for production of recombinant therapeutic proteins offer significant advantages over conventional expression systems, including lack of animal pathogens, low production costs, fast turnaround and response times and rapid, nearly-unlimited scalability. Also, because dried leaves and seeds can be stored at room temperature for lengthy periods without loss of recombinant proteins, plant expression systems have the potential to offer lucrative benefits from the development of edible vaccines and prophylactics, as these would not require "cold chain" storage and transportation, and could be administered in mass volumes with minimal processing. Several biotechnology companies currently have developed and adopted plant-based platforms for commercial production of recombinant protein therapeutics. In this manuscript, we outline the challenges in the process of livestock immunization as well as the current plant biotechnology developments aimed to address these challenges.

Effect of added dietary threonine on growth performance, health, immunity and gastrointestinal function of weaning pigs with differing genetic susceptibility to Escherichia coli infection and challenged with E. coli K88ac

Threonine (Thr) is important for mucin and immunoglobulin production. We studied the effect of added dietary Thr on growth performance, health, immunity and gastrointestinal function of weaning pigs with differing genetic susceptibility to E. coli K88ac (ETEC) infection and challenged with ETEC. Forty-eight 24-day-old weaned pigs were divided into two groups by their ETEC susceptibility using mucin 4 (MUC4) gene as a marker (2 MUC4(-/-) , not-susceptible, and 2 MUC4(+/+) , susceptible, pigs per litter). Within genotype, pigs were fed two different diets: 8.5 (LThr) or 9.0 (HThr) g Thr/kg. Pigs were orally challenged on day 7 after weaning and slaughtered on day 12 or 13 after weaning. Before ETEC challenge, HThr pigs ate more (p < 0.05). The diet did not affect post-challenge growth, but HThr tended to increase post-challenge feed efficiency (p = 0.087) and overall growth (p = 0.087) and feed efficiency (p = 0.055). Before challenge, HThr pigs excreted less E. coli (p < 0.05), while after challenge, diet did not affect the number of days with diarrhoea and ETEC excretion. MUC4(+/+) pigs responded to the challenge with more diarrhoea, ETEC excretion and anti-K88 IgA in blood and jejunal secretion (p < 0.001). HThr pigs had a higher increase of anti-K88 IgA values in jejunal secretion (p = 0.089) and in blood (p = 0.089, in MUC4(+/+) pigs only). Thr did not affect total IgA and IgM values, morphometry of jejunum, goblet cells count in colon, total mucin from jejunum and colon, but varied jejunal goblet cells counts (p < 0.05). In the first two post-weaning weeks, 8.5 g Thr/kg diet may be not sufficient to optimize initial feed intake, overall feed efficiency and intestinal IgA secretion and to control the gut microbiota in the first post-weaning week, irrespective of the pig genetic susceptibility to ETEC infection.

Targeted nanoparticles with novel non-peptidic ligands for oral delivery

Orally administered targeted nanoparticles have a large number of potential biomedical applications and display several putative advantages for oral drug delivery, such as the protection of fragile drugs or modification of drug pharmacokinetics. These advantages notwithstanding, oral drug delivery by nanoparticles remains challenging. The optimization of particle size and surface properties and targeting by ligand grafting have been shown to enhance nanoparticle transport across the intestinal epithelium. Here, different grafting strategies for non-peptidic ligands, e.g., peptidomimetics, lectin mimetics, sugars and vitamins, that are stable in the gastrointestinal tract are discussed. We demonstrate that the grafting of these non-peptidic ligands allows nanoparticles to be targeted to M cells, enterocytes, immune cells or L cells. We show that these grafted nanoparticles could be promising vehicles for oral vaccination by targeting M cells or for the delivery of therapeutic proteins. We suggest that targeting L cells could be useful for the treatment of type 2 diabetes or obesity.

Targeting aminopeptidase N, a newly identified receptor for F4ac fimbriae, enhances the intestinal mucosal immune response

Enterotoxigenic Escherichia coli (ETEC) are a major cause of diarrhea in human and animal. In piglets, ETEC having F4 fimbriae (F4(+) ETEC) induce severe diarrhea, dependent on the presence of receptors for F4 (F4R). In this study, porcine aminopeptidase N (pAPN) was identified as an F4R by comparative proteomic analysis of brush border proteins of F4R(+) and F4R(-) pigs and by adherence/internalization experiments on pAPN-transfected cells. Binding of F4 fimbriae to pAPN depended on sialic acid containing carbohydrate moieties, and resulted in clathrin-mediated endocytosis of the fimbriae. Endocytosis via pAPN was not restricted to F4 fimbriae, but was also observed for anti-pAPN antibodies. Both F4 fimbriae- and pAPN-specific antibodies were taken up in vivo by porcine enterocytes and induced subsequently a rapid immunoglobulin A and G response. In conclusion, we identified pAPN as an endocytotic receptor for F4 fimbriae and highlight the opportunity to target vaccine antigens to this epithelial receptor.

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.

Clathrin-mediated endocytosis and transcytosis of enterotoxigenic Escherichia coli F4 fimbriae in porcine intestinal epithelial cells

Enterotoxigenic Escherichia coli (ETEC) cause severe diarrhea in neonatal and recently weaned piglets. Previously, we demonstrated that oral immunization of F4 receptor positive piglets with purified F4 fimbriae induces a protective F4-specific intestinal immune response. However, in F4 receptor negative animals no F4-specific immune response can be elicited, indicating that the induction of an F4-specific mucosal immune response upon oral immunisation is receptor-dependent. Although F4 fimbriae undergo transcytosis across the intestinal epithelium in vivo, the endocytosis pathways used remain unknown. In the present study, we characterized the internalization of F4 fimbriae in the porcine intestinal epithelial cell line IPEC-J2. The results in the present study demonstrate that F4 fimbriae are internalized through a clathrin-dependent pathway. Furthermore, our results suggest that F4 fimbriae are transcytosed across differentiated IPEC-J2 cells. This receptor-dependent transcytosis of F4 fimbriae may explain the immunogenicity of these fimbriae upon oral administration in vivo.