Genetic fusion protein 3×STa-ovalbumin is an effective coating antigen in ELISA to titrate anti-STa antibodies

Melatonin as Immune Potentiator for Enhancing Subunit Vaccine Efficacy against Bovine Viral Diarrhea Virus

Bovine viral diarrhea virus (BVDV) is a pathogen associated with substantial economic losses in the dairy cattle industry. Currently, there are no effective vaccines against BVDV. Melatonin (MT) has been shown to have anti-inflammatory and anti-viral properties, and the use of MF59 in vaccines significantly enhances vaccine efficiency. Here, MT and MF59 were added into the E^rns-LTB vaccine. Subsequently, their inhibitory activity on the NF-κB signaling pathway in Mardin-Darby Bovine Kidney cells and the hippocampus was assessed using western blot and quantitative reverse transcription PCR. The findings revealed that MT in the E^rns-LTB vaccine decreases the phosphorylation of p65 proteins caused by BVDV infection. In addition, MT decreased the mRNA levels of IL-1β and IL-6 in vitro, but increased the production of IFN-α, IFN-β, Mx1 in vitro, brain-derived neurotrophic factor, cyclic amp response element-binding protein, and the stem cell factor in vivo. Furthermore, treatment with E^rns-LTB + MF59 + MT stimulated the production of T lymphocytes, alleviated pathological damage, decreased expressions of BVDV antigen, and tight junction proteins in mice. These findings imply that MT has potential for use in the E^rns-LTB vaccine to inhibit BVDV infection and regulate the immune responses of T-cells by inhibiting the NF-κB signaling pathway.

Immunogenicity characterization of genetically fused or chemically conjugated heat-stable toxin toxoids of enterotoxigenic Escherichia coli in mice and pigs

Enterotoxigenic Escherichia coli (ETEC) producing type Ib heat-stable toxin (STa) are a main cause of children's diarrhea and travelers' diarrhea, thus STa needs to be targeted in ETEC vaccine development. However, because this 19-amino acid STa is poorly immunogenic, attempts to genetically fuse or chemically couple it to carrier proteins have been made to enhance STa immunogenicity. In this study, we selected one genetic fusion and one chemical conjugate to comparatively evaluate STa immunogenicity. The genetic fusion is 3xSTaN12S-mnLTR192G/L211A carrying three toxoid (STaN12S) genetically fused to a double mutant LT monomer (mnLTR192G/L211A); the chemical conjugate is BSA-STaA14T, which has toxoid STaA14T chemically coupled to bovine serum albumin (BSA). We immunized mice with the STa toxoid fusion and chemical conjugates, and examined antibody responses. Furthermore, we immunized pigs and evaluated derived antibodies for efficacy to passively provide protection against ETEC diarrhea using a piglet model. Data showed that mice subcutaneously immunized with BSA-STaA14T or 3xSTaN12S-mnLTR192G/L211A developed a strong anti-STa antibody, and the induced antibodies exhibited equivalent toxin-neutralizing activities. Pigs immunized with 3xSTaN12S-mnLTR192G/L211A or BSA-STaA14T developed similar levels of anti-STa antibodies; piglets with passively acquired antibodies induced by the genetic fusion appeared better protected against STa + ETEC. Results from the current study indicate that the fusion and conjugate approaches are viable options for facilitating STa immunogenicity and developing ETEC vaccines.

Review of Newly Identified Functions Associated With the Heat-Labile Toxin of Enterotoxigenic Escherichia coli

Heat-labile toxin (LT) is a well-characterized powerful enterotoxin produced by enterotoxigenic Escherichia coli (ETEC). This toxin is known to contribute to diarrhea in young children in developing countries, international travelers, as well as many different species of young animals. Interestingly, it has also been revealed that LT is involved in other activities in addition to its role in enterotoxicity. Recent studies have indicated that LT toxin enhances enteric pathogen adherence and subsequent intestinal colonization. LT has also been shown to act as a powerful adjuvant capable of upregulating vaccine antigenicity; it also serves as a protein or antigenic peptide display platform for new vaccine development, and can be used as a naturally derived cell targeting and protein delivery tool. This review summarizes the epidemiology, secretion, delivery, and mechanisms of action of LT, while also highlighting new functions revealed by recent studies.

Mapping the Neutralizing Epitopes of Enterotoxigenic Escherichia coli K88 (F4) Fimbrial Adhesin and Major Subunit FaeG

Enterotoxigenic Escherichia coli (ETEC) strains that produce immunologically heterogeneous fimbriae and enterotoxins are the primary cause of neonatal diarrhea and postweaning diarrhea in young pigs. A multivalent vaccine inducing protective immunity against ideally all ETEC fimbriae and enterotoxins could be effective against diarrhea in young pigs. However, developing a vaccine to broadly protect against various ETEC virulence determinants has proven challenging. Recently developed structure- and epitope-based multiepitope fusion antigen (MEFA) technology that presents neutralizing epitopes of various virulence determinants at a backbone immunogen and that mimics epitope native immunogenicity suggests the feasibility of developing multivalent vaccines. With neutralizing epitopes from ETEC fimbria F18 and enterotoxins being identified, it becomes urgent to identify protective epitopes of K88 (F4) fimbriae, which play a major role in pig neonatal and postweaning diarrhea. In this study, we identified B-cell immunodominant epitopes in silico from the K88ac fimbrial major subunit (also adhesin) FaeG and embedded each epitope in a heterogeneous carrier for epitope fusions. We then immunized mice with each epitope fusion protein and examined epitope antigenicity and also neutralizing activities of epitope-induced antibodies. Data showed that while all nine FaeG epitope fusions induced antibodies to K88ac fimbria, anti-K88 IgG antibodies derived from epitopes MTGDFNGSVD (ep1), LNDLTNGGTK (ep2), GRTKEAFATP (ep3), ELRKPDGGTN (ep4), PMKNAGGTKVGAVKVN (ep5), and RENMEYTDGT (ep8) significantly inhibited adherence of K88ac fimbrial bacteria to porcine intestinal cell line IPEC-J2, indicating that these peptides were the neutralizing epitopes of K88ac fimbrial major subunit FaeG and suggesting the future application of FaeG epitopes in ETEC vaccine development.IMPORTANCE Enterotoxigenic Escherichia coli (ETEC) strains producing K88ac fimbriae and enterotoxins are a major cause of porcine neonatal diarrhea and postweaning diarrhea in the United States. Currently, there is no vaccine to induce broadly protective antiadhesin and antitoxin immunity against ETEC-associated diarrhea. To develop a broadly effective ETEC vaccine, we need to target the most important if not all ETEC virulence determinants. While conventional vaccinology approaches encounter difficulties at integrating or including heterogeneous ETEC fimbria and toxin antigens into a vaccine product, multiepitope fusion antigen (MEFA) structural vaccinology provides a new platform to combine neutralizing antigenic elements or epitopes from various heterogeneous virulence factors for broad immunity and protection. Identification of the neutralizing epitopes of K88ac fimbria from this study added the last antigens to an MEFA-based multivalent vaccine against ETEC-associated diarrhea in pigs. An effective vaccine against pig diarrhea can significantly improve swine health and well-being and reduce economic losses to the swine industry worldwide.

Mapping the neutralizing epitopes of F18 fimbrial adhesin subunit FedF of enterotoxigenic Escherichia coli (ETEC)

K88 and F18 fimbrial enterotoxigenic Escherichia coli (ETEC) are the major causes of post-weaning diarrhea (PWD) in pigs. A vaccine that induces broad immunity to prevent K88 and F18 fimbrial ETEC bacterial attachment and colonization in pig small intestines and to neutralize enterotoxin enterotoxicity would be effective for PWD. Structure-based multiepitope-fusion-antigen (MEFA) technology using a backbone immunogen to present neutralizing epitopes of representing virulence factors capacitates development of broadly protective ETEC vaccines. Neutralizing epitopes have been identified from K88 fimbrial adhesin (FaeG) and enterotoxins but not F18 fimbrial adhesin. In this study, we in silico identified immunodominant epitopes from F18ac fimbrial subunit FedF which plays a critical role in F18 fimbrial adherence, genetically fused each epitope to a carrier, examined immunogenicity of each epitope fusion, and determined epitope-derived antibodies neutralizing activities against F18 fimbrial adherence. Data showed that seven immune-dominant epitopes were identified from FedF subunit. Fused to heterologous human ETEC adhesin subunit CfaB, epitope fusions induced anti-F18 antibodies in subcutaneously immunized mice. Moreover, antibodies derived from each fusion significantly blocked adherence of a F18-fimbrial E. coli bacteria to pig intestinal cell line IPEC-J2. While all seven epitopes exhibited neutralizing activity, results from this study identified FedF epitopes #3 (IPSSSGTLTCQAGT) and #7 (QPDATGSWYD) the most effective for antibodies against F18 fimbrial adherence, and suggested their future application in PWD vaccine development.

Significance of Enterotoxigenic Escherichia coli (ETEC) Heat-Labile Toxin (LT) Enzymatic Subunit Epitopes in LT Enterotoxicity and Immunogenicity

Enterotoxigenic Escherichia coli (ETEC) strains producing heat-labile toxin (LT) and/or heat-stable toxin (STa) are a top cause of children's diarrhea and travelers' diarrhea. Holotoxin-structured GM1-binding LT is a strong immunogen and an effective adjuvant, and can serve a carrier or a platform for multivalent vaccine development. However, the significance of peptide domains or epitopes of LT particularly enzymatic LTA subunit in association with LT enterotoxicity and immunogenicity has not been characterized. In this study, we identified B-cell epitopes in silico from LTA subunit and examined epitopes for immunogenicity and association with LT enterotoxicity. Epitopes identified from LTA subunit were individually fused to a modified chicken ovalbumin carrier protein, and each epitope-ovalbumin fusion was used to immunize mice. Data showed all 11 LTA epitopes were immunogenic; epitope 7 (105SPHPYEQEVSA115) induced greater titers of anti-LT antibodies which neutralized LT enterotoxicity more effectively. To examine these epitopes for the significance in LT enterotoxicity, we constructed LT mutants by substituting each of 10 epitopes at the toxic A1 domain of LTA subunit with a foreign epitope and examined LT mutants for enterotoxicity and GM1-binding activity. Data showed that LT mutants exhibited no enterotoxicity but retained GM1-binding activity. The results from this study indicated that while not all immunodominant LTA epitopes were neutralizing, LT mutants with an individual epitope substituted lost enterotoxicity but retained GM1-binding activity. These results provided additional information to understand LT immunogenicity and enterotoxicity and suggested the potential application of LT platform for multivalent vaccines against ETEC diarrhea and other diseases.IMPORTANCE No vaccine is licensed for enterotoxigenic Escherichia coli (ETEC) strains, which remain a leading cause of diarrhea in children from developing countries and international travelers. GM1-binding heat-labile toxin (LT) which is a key virulence factor of ETEC diarrhea is a strong vaccine antigen and a self-adjuvant. LT can also serve a backbone or platform for MEFA (multiepitope fusion antigen), a newly developed structural vaccinology technology, to present heterogeneous epitopes (by replacing LT epitopes) and to mimic epitope antigenicity for development of broadly protective vaccines. Data from this study identified neutralizing LT epitopes and demonstrated that substitution of LT epitopes eliminated LT enterotoxicity without altering GM1-binding activity, suggesting LT is potentially a versatile MEFA platform to present heterogeneous epitopes for multivalent vaccines against ETEC and other pathogens.