Protective immunogenicity of two synthetic peptides selected from the amino acid sequence of Bordetella pertussis toxin subunit S1

Assembly of Immunogenic Protein Particles toward Advanced Synthetic Vaccines

Immunogenic carrier proteins such as the non-toxic diphtheria toxin variant, cross-reacting material 197 (CRM197), are widely used in subunit vaccine formulations to boost immunogenicity of chemically conjugated antigens. Conjugate vaccines are inherently expensive due to laborious manufacturing steps. Here, this work develops a particulate vaccine platform based on using engineered Escherichia coli to assemble CRM197-antigen fusion proteins into discrete submicron-sized particles. This approach enables precise loading of diverse antigens and epitopes enhancing their immunogenicity. A cost-effective, high-yield, and scalable biomanufacturing process is developed. Purified particulate CRM197-antigen vaccines are ambient-temperature stable. CRM197 particles incorporating pathogen-specific antigens or epitopes from SARS-CoV-2, Streptococcus pyogenes (group A), and Mycobacterium tuberculosis induced cell-mediated and humoral immune responses mediating protective immunity in respective animal models of infection. The CRM197 particle vaccine platform is versatile, enabling co-delivery of selected antigens/epitopes together with immunogenic CRM197 as discrete stable particles avoiding laborious manufacture of soluble CRM197 and antigen followed by chemical conjugation.

Antibodies induced by oral immunization of mice with a recombinant protein produced in tobacco plants harboring Bordetella pertussis epitopes

Bordetella pertusis causes whooping cough or pertussis, disease that has not been eradicated and is reemerging despite the availability and massive application for decades of vaccines, such as Boostrix® which is an acellular vaccine harboring two regions of S1 subunit of the pertussis toxin, one region of filamentous hemagglutinin and one region of pertactin. In 2008, the World Health Organization estimated 16 million new cases and 95% occurred in developing countries with 195,000 children's deaths. We attempt to improve the vaccine against whooping cough and reduce its production costs by obtaining plants and bacteria expressing a heterologous protein harboring pertactin, pertussis toxin, and filamentous hemagglutinin epitopes from B. pertussis and assessing its immunogenicity after oral administration to mice. First, we designed a synthetic gene that encodes a multiepitope, then it was cloned into a vector for transient transformation by infiltration of tobacco plants with low amounts of nicotine; the codon bias-optimized construct was also cloned into an Escherichia coli expression vector. Recombinant proteins from E. coli cells (PTF) and tobacco leaves (PTF-M3') were purified by nickel affinity with a yield of 0.740 mg of recombinant protein per g dry weight. Purified recombinant proteins were administered orally to groups of Balb/c mice using the Boostrix® vaccine and vehicle (PBS) as positive and negative controls, respectively. A higher mucosal and systemic antibody responses were obtained in mice receiving the PTF and PTF-M3' proteins than Boostrix® or PBS. These findings prove the concept that oral administration of multiepitope recombinant proteins expressed in plants may be a potential edible vaccine.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11240-021-02107-1.

Cloning and Expression of S1 Subunit of Pertussis Toxin in Escherichia coli

Bordetella pertussis is a gram negative bacterium that causes respiratory tract infection in human (whooping cough). Pertussis toxin (PT) is the main component of current acellular pertussis vaccine and the S1 (subunit1) is the main immunogenic part of it. Thus, S1 has been the target of many studies as a potent candidate of acellular vaccine against Bordetella pertussis, lacking the side effects of whole cell based ones. S1 gene was amplified and inserted in three expression vectors including pET-14b, pET-22b(+) and pAED4. The possibility and level of expression of these constructs were investigated in BL21 (DE3) strain of Escherichia coli (E.coli) as expression host. The highest expression was in pET-22b(+)-S1. Best expression achieved 6 hr post induction with 0.2 mM IPTG in LB broth containing ampicillin, at 30°C with shaking (250 rpm). Recombinant S1 protein was observed in two distinct separated proteins with 28 and 31 kDa estimated molecular weight. In spite of toxicity of PT and S1 in the E.coli, considerable amount of S1 was expressed in E.coli. Two rS1 bands were detected on SDS-PAGE. Both were confirmed as S1 in western blot with specific monoclonal and polyclonal antibodies against pertussis toxin. Appearance of two distinct bands could be the result of leader peptidase activity or nonspecific peptidase from E.coli on recombinant S1. As the recombinant S1 is a suitable antigen for studies as a candidate acellular vaccine or development of ELISA for detection of Bordetella pertussis, further studies are underway.

Pentapeptide commonality between Corynebacterium diphtheriae toxin and the Homo sapiens proteome

Cross-reactivity may affect diagnostic tests and cause harmful autoimmune reactions following immunotherapy. To predict potential cross-reactivity and search for safe immunotherapeutic approaches, we analyzed sequence identity between microbial antigens and the human proteome. Using diphtheria toxin (DT) as a model, we examined its patterns of identity with human proteins at the pentapeptide level. DT shares 503 pentapeptides with the human proteome, while only 31 pentapeptides are unique to the toxin. DT pentapeptide identity involves multiple/repeated matches in human proteins (a total of 4966 occurrences). Human proteins containing bacterial peptide matches include antigens linked to fundamental cellular functions, such as cell cycle control, proliferation, development and differentiation. The data presented in this article offer a rational basis for designing peptide-based vaccines that specifically target DT and thus eliminate the potential risk of cross-reactivity with human proteins. More generally, this study proposes a methodological approach for avoiding cross-reactivity in immune reactions.

Proposing low-similarity peptide vaccines against Mycobacterium tuberculosis

Using the currently available proteome databases and based on the concept that a rare sequence is a potential epitope, epitopic sequences derived from Mycobacterium tuberculosis were examined for similarity score to the proteins of the host in which the epitopes were defined. We found that: (i) most of the bacterial linear determinants had peptide fragment(s) that were rarely found in the host proteins and (ii) the relationship between low similarity and epitope definition appears potentially applicable to T-cell determinants. The data confirmed the hypothesis that low-sequence similarity shapes or determines the epitope definition at the molecular level and provides a potential tool for designing new approaches to prevent, diagnose, and treat tuberculosis and other infectious diseases.

Pathogen proteins eliciting antibodies do not share epitopes with host proteins: a bioinformatics approach

The best way to prevent diseases caused by pathogens is by the use of vaccines. The advent of genomics enables genome-wide searches of new vaccine candidates, called reverse vaccinology. The most common strategy to apply reverse vaccinology is by designing subunit recombinant vaccines, which usually generate an humoral immune response due to B-cell epitopes in proteins. A major problem for this strategy is the identification of protective immunogenic proteins from the surfome of the pathogen. Epitope mimicry may lead to auto-immune phenomena related to several human diseases. A sequence-based computational analysis has been carried out applying the BLASTP algorithm. Therefore, two huge databases have been created, one with the most complete and current linear B-cell epitopes, and the other one with the surface-protein sequences of the main human respiratory bacterial pathogens. We found that none of the 7353 linear B-cell epitopes analysed shares any sequence identity region with human proteins capable of generating antibodies, and that only 1% of the 2175 exposed proteins analysed contain a stretch of shared sequence with the human proteome. These findings suggest the existence of a mechanism to avoid autoimmunity. We also propose a strategy for corroborating or warning about the viability of a protein linear B-cell epitope as a putative vaccine candidate in a reverse vaccinology study; so, epitopes without any sequence identity with human proteins should be very good vaccine candidates, and the other way around.

DNA vaccine encoding pertussis toxin S1 subunit induces protection against Bordetella pertussis in mice

Pertussis toxin (PT) is the major virulence factor of Bordetella pertussis, and detoxified PT is a crucial antigen of acellular pertussis vaccine. Here, plasmid DNA expressing the pertussis toxin S1 subunit (pcDNA/S1) of B. pertussis was evaluated for immunogenicity and for the ability to induce protection against PT challenge or B. pertussis infection in mice. The gene gun delivery of pcDNA/S1, performed by inserting the S1 gene into a mammalian expression vector, successfully induced anti-PT IgG antibody production. Immunization of mice with pcDNA/S1 significantly inhibited leukocytosis-promoting activity caused by PT or B. pertussis. In addition, pcDNA/S1 induced significant protection against intracerebral challenge with a lethal dose of B. pertussis. The results of the present study demonstrated that a DNA vaccine encoding the PT-S1 subunit induced protection against B. pertussis infection in mice. Thus, this vaccine preparation is potentially applicable for the production of novel vaccines against B. pertussis infection.

Preparation of pneumococcal capsular polysaccharide-protein conjugate vaccines utilizing new fragmentation and conjugation technologies

There is a global urgent need for a new efficient and inexpensive vaccine to combat pneumococcal disease, which should also be affordable in developing countries. In view of this need a simple low-cost technique to prepare such a vaccine was developed. The preparation of serotype 14 and 23F pneumococcal capsular polysaccharide (PnPS)-protein conjugates to be included in a forthcoming multivalent PnPS conjugate vaccine is described. Commercial lots of PnPSs produced according to Good Manufacturing Practice from Streptococcus pneumoniae serotype 14 (PS14) and 23F (PS23F) were partially depolymerized by sonication or irradiation in an electron beam accelerator. The PnPS fragments were conjugated to tetanus toxoid (TT) using a recently developed conjugation chemistry. The application of these new simple, efficient and inexpensive fragmentation and conjugation technologies allowed the synthesis of several PnPS-protein conjugates containing PnPS fragments of preselected sizes and differing in the degree of substitution. The PS14TT and PS23FTT conjugate vaccine candidates were characterized chemically and their immunogenicity was evaluated in rabbits and mice. All PnPS conjugate vaccines, unlike the corresponding plain polysaccharides, produced high IgG titres in both animal species. The PS14TT conjugates tended to be more immunogenic than the PS23FTT conjugates. The immune response to the PS14TT conjugates, but not to the PS23FTT conjugates, was related to the size of the conjugated polysaccharide hapten. Both types of conjugates elicited strong booster effects upon secondary immunizations, resulting in high IgG1, IgG2a and IgG2b titres.

Induction of CD4(+) and CD8(+) Bordetella pertussis toxin subunit S1 specific T cells by immunization with synthetic peptides

In this study two synthetic peptides from the Bordetella pertussis toxin subunit S1 were conjugated to human anti-idiotypic antibodies and used as an immunogen in cancer patients to induce immunity. The aims of the present report are to explain why no carrier or adjuvant effect of the conjugated pertussis peptides could be established regarding induction of responses against the anti-idiotype and to explore the type and quality of induced anti-pertussis immune responses. The lack of carrier and adjuvant effect of the peptides might be related to the fact that the anti-idiotypic antibodies by themselves include helper epitopes and that none of the patients had a detectable T cell response against any of the selected peptides before immunization, which might be a requirement for an adjuvant effect. However, three of four immunized patients mounted a humoral as well as cellular response against the pertussis peptides used. The induced T cell immunity was restricted to one of the two peptides in responding patients. Established T cell lines and MHC blocking studies indicated that the T cell epitopes of the two peptides had a different MHC restriction. The type of T cell response induced seemed to govern the humoral response. The only durable antibody response was accompanied by the presence of a CD4(+) T cell response against the same peptide. Immunization with an anti-idiotype conjugated to synthetic peptides might thus induce both a B and a T cell response against the peptides and the type of induced T cells (CD4 or CD8) governs the quality of the humoral response. Moreover, the possibility of boosting or inducing a response against the antigen from which the peptide sequences were deduced also seemed feasible.

Unique chemical reactivity of His-21 of CRM-197, a mutated diphtheria toxin

CRM-197 is a mutated diphtheria toxin (63000 Da) widely used as a carrier protein of conjugated vaccines. Among the 14 histidines of CRM-197, His-21 was found to be modified selectively with iodoacetamide based reagents. This finding suggests a simplified method for the preparation of conjugate vaccines crosslinked to CRM-197. A bifunctional iodoacetamide, N,N'-(2-hydroxy-1,3-propanediyl)-bis-12-iodoacetamidel (I-CH2-CONH-CH2-CH(OH)-CH2-NHCO-CH2-I) (HPBIA), was synthesized and allowed to react with CRM-197. In the alkaline buffer of pH 8.0-8.4, HPBIA was shown to react and intra-bridge His-21 and Lys-24 of CRM-197 sequentially. At lower pH (7.1-7.5) in the phosphate buffer, the reactivity of Lys-24 toward HPBIA was suppressed drastically. Under these conditions, His-21 could be specifically labeled with HPBIA. Initial experiments have demonstrated that HPBIA modified CRM-197 is able to crosslink to a cysteine-containing peptide. These results offer a potential route for improving the homogeneity of CRM-197 based protein-peptide as well as protein-polysaccharide conjugates.

Human anti-idiotypic antibodies induced a humoral and cellular immune response against a colorectal carcinoma-associated antigen in patients

Induction of immunity against antigens expressed on tumor cells might prevent or delay recurrence of the disease. Six patients operated on for colorectal carcinoma were immunized with human monoclonal anti-idiotypic antibodies (h-Ab2) against the mouse 17-1A anti-colon carcinoma antibody, mimicking a nominal antigen (GA733-2). All patients developed a long-lasting T-cell immunity against the extracellular domain of GA733-2 (GA733-2E) (produced in a baculovirus system) and h-Ab2. This was shown in vitro by specific cell proliferation (DNA-synthesis) assay as well as by interleukin 2 and interferon gamma production and in vivo by the delayed-type hypersensitivity reaction. Five patients mounted a specific humoral response (IgG) against the tumor antigen GA733-2E (ELISA) and tumor cells expressing GA733-2. Epitope mapping using 23 overlapping peptides of GA733-2E revealed that the B-cell epitope was localized close to the N terminus of GA733-2. Binding of the antibodies to the tumor antigen and to one 18-aa peptide was inhibited by h-Ab2, indicating that the antibodies were able to bind to the antigen as well as to h-Ab2. The results suggest that our h-Ab2 might be able to induce an anti-tumor immunity which may control the growth of tumor cells in vivo.

Neutralizing antibodies and immunoprotection against pertussis and tetanus obtained by use of a recombinant pertussis toxin-tetanus toxin fusion protein

The currently available diphtheria-tetanus-whole-cell pertussis (DTP) vaccines are associated with a variety of problems, including undesirable side effects and inconsistent efficacy. These problems are probably related to the poor definition of such vaccines, especially with respect to the whole-cell component against pertussis. Ideal vaccines should include only immunoprotective antigens with no toxin activity. As an initial step towards obtaining a well-defined and simplified DTP vaccine, a pertussis toxin-tetanus toxin chimeric protein was constructed. A soluble form of the pertussis toxin S1 subunit was fused to the protective fragment C of tetanus toxin, and the recombinant hybrid protein was produced in Escherichia coli. The 75-kDa fusion protein (p75) was overexpressed as a soluble molecule and purified to near homogeneity by two consecutive chromatographic steps. Purified p75 retained its ability to bind to ganglioside GT1b, the receptor for tetanus toxin, and to be recognized by protective and neutralizing anti-pertussis toxin antibodies specific for conformational epitopes. When administered to mice, the hybrid protein was found to be nontoxic but immunogenic. In addition, it was capable of inducing strong protection against tetanus and some protection against pertussis, as well as eliciting a pertussis toxin-neutralizing antibody response. Although the levels of anti-pertussis toxin antibodies were rather low, neutralizing titers of the immunized mice correlated well with anti-pertussis toxin titers, indicating that protective epitopes are conserved in the recombinant protein.

Identification of B-cell epitopes on the S4 subunit of pertussis toxin

The main purpose of the present study was to identify B-cell epitopes on the S4 subunit of pertussis toxin (PT) by the synthetic peptide approach. Two strategies were followed: (i) screening of two series of overlapping peptides (12- and 25-residue peptides) covering the entire S4 sequence by a panel of murine monoclonal anti-PT antibodies and various polyclonal anti-PT antisera in an enzyme-linked immunosorbent assay (ELISA), and (ii) analysis of the S4 amino acid sequence by a predictive algorithm followed by synthesis and immunization of mice with the predicted peptides coupled to diphtheria toxoid. The anti-peptide conjugate antisera were tested in an ELISA for cross-reactivity with native PT, B oligomer, and S4. Screening of the free peptides in an ELISA by the PT antisera indicated the presence of six B-cell epitope-containing domains covered by residues 18 to 32, 33 to 46, 39 to 52, 51 to 65, 71 to 84, and 91 to 106. None of the peptides, however, were recognized by the monoclonal anti-PT antibodies in an ELISA. Immunization with six computer-predicted peptides (B1 to B6) and three potential T-cell epitopes (T1 to T3) gave rise to very high antibody responses towards the homologous conjugates. With the exception of the anti-T1/diphtheria toxoid antisera, all anti-peptide conjugate antisera cross-reacted with PT in an ELISA at different levels. None of these anti-peptide conjugate antisera, however, showed any PT-neutralizing effect as measured by the Chinese hamster ovary cell assay and the leukocytosis-promoting activity test. The results of the present study suggest that discontinuous epitopes are predominant in the S4 subunit of native PT.

Identification of murine T-cell epitopes on the S4 subunit of pertussis toxin

The aim of the present study was to identify murine T-cell epitopes on pertussis toxin subunit S4. Six mouse strains with five different haplotypes at the H-2 locus were immunized with the pertussis toxin B oligomer. Lymph node lymphocytes were isolated and stimulated in an in vitro proliferation assay with pertussis toxin components and 11 overlapping synthetic peptides synthesized on the basis of the primary sequence of S4. In vitro proliferative responses to the synthetic peptides revealed the presence of four distinct murine T-cell epitopes on subunit S4. The recognition of the peptides was major histocompatibility complex restricted. Immunizing four of the six mouse strains with the synthetic peptides showed that the peptides which were demonstrated to contain T-cell epitopes following immunization with the B oligomer were able to induce proliferative responses to detoxified pertussis toxin and pertussis toxin components containing subunit S4. One of the identified murine T-cell epitopes corresponded to one of the major human T-cell epitopes previously identified on subunit S4. It is hoped that this murine model system will facilitate the development of a synthetic immunogen mimicking the protective properties of pertussis toxin.

Identification of T- and B-cell epitopes of the S2 and S3 subunits of pertussis toxin by use of synthetic peptides

To design an optimized synthetic vaccine against whooping cough, we have studied the biological and immunological properties of three peptides of the S2 subunit and nine overlapping synthetic peptides covering the entire sequence of the S3 subunit of pertussis toxin (PT). Synthetic peptides corresponding to sequences 18 to 41, 78 to 108, 134 to 154, and 149 to 176 of S3 were found to be consistently capable of stimulating the proliferation of PT-specific T-cell lines primed with pertussis toxoid in both BALB/c and A/J strains of mice. All synthetic peptides were recognized by rabbit antisera raised against PT or pertussis toxoid. Both S2 and S3 peptide-keyhole limpet hemocyanin (KLH) conjugates in the presence of complete Freund's adjuvant induced peptide-specific antibody responses in rabbits, and the antisera raised against S2(1-23), S3(18-41), S3(37-64), and S3(149-176) peptide-KLH conjugates cross-reacted with both subunits in the immunoblots. All antisera except those against S2(123-154) and S3(103-127) reacted with native PT in an enzyme-linked immunosorbent assay (ELISA) with PT directly coated onto microtiter wells. In contrast, antisera raised against S2(123-154), S3(1-23), S3(18-41), S3(37-64), S3(60-87), and S3(103-127) peptide-KLH conjugates recognized native PT in a fetuin-PT capture ELISA. S2(78-98), S3(1-23), and S3(149-176) peptide-KLH conjugates elicited good PT-neutralizing antibody responses as judged by the antitoxin CHO cell assay. Identification of these B-cell neutralization epitopes and T-cell immunodominant determinants represents a first step towards the rational design of a synthetic vaccine against whooping cough.

Identification of human T-cell epitopes on the S4 subunit of pertussis toxin

Ten adult humans were vaccinated with the Japanese acellular pertussis vaccine JNIH-3, containing detoxified pertussis toxin (PT), formaldehyde, and filamentous hemagglutinin. The vaccination induced a specific antibody response to PT and filamentous hemagglutinin, and a Western blot (immunoblot) analysis of the antibody response to PT revealed antibodies to PT subunits S1, S2, S3, S4 and S5. The response of peripheral lymphocytes to PT was assessed in an in vitro proliferation assay. A proliferative response to detoxified PT and PT dimers S2-S4 and S3-S4 was found, and it was further demonstrated that the proliferative response to detoxified PT and dimer S2-S4 was mediated by T cells of the CD4+ phenotype. The specificity of the proliferative response to subunit S4 was analyzed with a range of synthetic peptides synthesized on the basis of the primary sequence of subunit S4. The proliferative response to the peptides revealed two major and one minor T-cell epitope located in the NH2-terminal end of subunit S4.