Helix stabilization in the C-terminal peptide of chicken riboflavin carrier protein enhances immunogenicity and prolongs contraceptive potential as an epitope-based vaccine in female rats

Prediction of Variable-Length B-Cell Epitopes for Antipeptide Paratopes Using the Program HAPTIC

BACKGROUND

B-cell epitope prediction for antipeptide antibody responses enables peptide-based vaccine design and related translational applications. This entails estimating epitopeparatope binding free-energy changes from antigen sequence; but attempts to do so assuming uniform epitope length (e.g., of hexapeptide sequences, each spanning a typical paratope diameter when fully extended) have neglected empirically established variation in epitope length.

OBJECTIVE

This work aimed to develop a sequence-based physicochemical approach to variablelength B-cell epitope prediction for antipeptide paratopes recognizing flexibly disordered targets.

METHODS

Said approach was developed by analogy between epitope-paratope binding and protein folding modeled as polymer collapse, treating paratope structure implicitly. Epitope-paratope binding was thus conceptually resolved into processes of epitope compaction, collapse and contact, with epitope collapse presenting the main entropic barrier limiting epitope length among nonpolyproline sequences. The resulting algorithm was implemented as a computer program, namely the Heuristic Affinity Prediction Tool for Immune Complexes (HAPTIC), which is freely accessible via an online interface (http://badong.freeshell.org/haptic.htm). This was used in conjunction with published data on representative known peptide immunogens.

RESULTS

HAPTIC predicted immunodominant epitope sequences with lengths limited by penalties for both compaction and collapse, consistent with known paratope-bound structures of flexibly disordered epitopes. In most cases, the predicted association constant was greater than its experimentally determined counterpart but below the predicted upper bound for affinity maturation in vivo.

CONCLUSION

HAPTIC provides a physicochemically plausible means for estimating the affinity of antipeptide paratopes for sterically accessible and flexibly disordered peptidic antigen sequences by explicitly considering candidate B-cell epitopes of variable length.

On the meaning of affinity limits in B-cell epitope prediction for antipeptide antibody-mediated immunity

B-cell epitope prediction aims to aid the design of peptide-based immunogens (e.g., vaccines) for eliciting antipeptide antibodies that protect against disease, but such antibodies fail to confer protection and even promote disease if they bind with low affinity. Hence, the Immune Epitope Database (IEDB) was searched to obtain published thermodynamic and kinetic data on binding interactions of antipeptide antibodies. The data suggest that the affinity of the antibodies for their immunizing peptides appears to be limited in a manner consistent with previously proposed kinetic constraints on affinity maturation in vivo and that cross-reaction of the antibodies with proteins tends to occur with lower affinity than the corresponding reaction of the antibodies with their immunizing peptides. These observations better inform B-cell epitope prediction to avoid overestimating the affinity for both active and passive immunization; whereas active immunization is subject to limitations of affinity maturation in vivo and of the capacity to accumulate endogenous antibodies, passive immunization may transcend such limitations, possibly with the aid of artificial affinity-selection processes and of protein engineering. Additionally, protein disorder warrants further investigation as a possible supplementary criterion for B-cell epitope prediction, where such disorder obviates thermodynamically unfavorable protein structural adjustments in cross-reactions between antipeptide antibodies and proteins.

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.

Epitope mapping and evaluation of specificity of T-helper sites in four major antigenic peptides of chicken riboflavin carrier protein in outbred rats

This paper reviews our studies on synthetic peptides spanning the major antigenic determinants of the chicken riboflavin carrier protein (RCP; 219 AA). These determinants are composed of residues 4-24 (YGC), 64-83 (CED), 130-147 (GEN), and 200-219 (HAC) and function as minivaccines in terms of eliciting anti-peptide antibodies which recognize the native protein and are particularly promising contraceptive vaccine candidates. We have used 15-residue synthetic peptides to define short sequences involved in interaction with antibody and with T-cells. We have mapped the boundaries of T-cell epitopes of these peptides in outbred rats by immunizing the animals with each peptide and assaying the popliteal lymph node cell proliferation against a series of overlapping synthetic 15-mers covering the entire length of the individual peptides. The peptides YGC, GEN, and HAC harboured a single T-cell epitope each whereas the peptide CED exhibited bimodal response possessing two epitopes, one at N-terminus and the other at the C-terminus. These studies provide insight into the way in which an immunogen is viewed by the immune system. In addition, preferential T-cell helper function for B cells recognizing unique determinants on the same molecule was demonstrated. This information helps in exploiting synthetic peptides in the construction of designer immunogens which have potential as candidate vaccines.

Status of immunodiagnosis and immunocontraceptive vaccines in India

The article focuses on the Indian initiative of making kits for diagnosis of various infectious and non-infectious diseases as well as reproductive hormones and hormones in various other endocrine disorders. Indigenous diagnostic kits for the detection of various infections such as filariasis, typhoid, amebiasis, Japanese encephalitis, hepatitis, HIV, dengue, leishmaniasis, malaria, rabies, toxoplasmosis, rotavirus, and group A streptococci have been developed. Agreements to transfer the know-how of some of these leads to industries have been signed. The know-how of enzyme-linked immunosorbent assay (ELISA) for detection of hepatitis C has been successfully transferred to industry and is being commercially produced. For detection of HIV-1 and HIV-2, indigenous diagnostic kits based on three different formats, namely ELISA, Western blot and rapid test have been developed and are being commercially produced by Indian industries. The factors influencing the successful transfer of laboratory-scale diagnostic assays from academia to industry and their commercial exploitation have been discussed. Indian scientists have made seminal contributions in exploring the possibility to develop an effective and safe contraceptive vaccine to control the increasing human population of India. Achieving contraception by means of vaccine is a novel approach, which entails generation of a specific antibody response against antigens critically involved in the process of mammalian reproduction. In India, three major programs on contraceptive vaccines based on the beta-subunit of human chorionic gonadotrophin ((beta)hCG) for women, ovine follicle stimulating hormone (oFSH) for men, and riboflavin carrier protein for both males and females have been initiated. The work at the National Institute of Immunology, New Delhi on contraceptive vaccine for women, based on (beta)hCG, has demonstrated, for the first time, that it is feasible to regulate fertility by such an approach. Basic research being carried out to achieve immunocontraception by interfering at sperm-oocyte interaction level has been briefly discussed. These developments are still at the research stage. In addition to advances in the area of contraceptive vaccines, a non-steroidal contraceptive oral pill has been developed by Central Drug Research Institute, Lucknow, commercially produced by two Indian pharmaceutical companies and has been incorporated in the National Family Welfare Program. Another interesting approach for fertility regulation in male has been developed in India, which involves vas occlusion with styrene maleic anhydride (SMA) and is currently undergoing clinical trials in human subjects.

Mutational effects on protein folding stability and antigenicity: the case of streptococcal pyrogenic exotoxin A

The influence of mutationally induced changes in protein folding on development of effective neutralizing antibodies during vaccination remains largely unexplored. In this study, we probed how mutational substitutions of streptococcal pyrogenic exotoxin A (SPEA), a model bacterial superantigen, affect native conformational stability and antigenicity. Stability changes for the toxin variants were determined using circular dichroism and fluorescence measurements, and scanning calorimetry. Self-association was assayed by dynamic light scattering. Inactivated SPEA proteins containing particular combinations of mutations elicited antibodies in HLA-DQ8 transgenic mice that neutralized SPEA superantigenicity in vitro, and protected animals from lethal toxin challenge. However, a highly destabilized cysteine-free mutant of SPEA did not provide effective immunity, nor did an irreversibly denatured version of an otherwise effective mutant protein. These results suggest that protein conformation plays a significant role in generating effective neutralizing antibodies to this toxin, and may be an important factor to consider in vaccine design.

Expression, purification, and characterization of minimized chicken riboflavin carrier protein from a synthetic gene in Escherichia coli

Minimized proteins have long been used to elicit immune response to particular regions of a protein antigen. Most efforts to derive minimized proteins have employed synthetic peptide fragments. Here we describe molecular cloning and production of a minimized chicken riboflavin carrier protein (mini-RCP) sequence that harbours all the four neutralizing epitopes but lacks the sequences that otherwise elicit undesirable antibodies. The gene encoding mini-RCP is engineered by contiguous alignment of nucleotide sequences coding for selected epitopes of chicken RCP separated by leucyl alanine residues. The gene has been constructed from eight oligonucleotides by employing overlapping PCR strategy and expressed in Escherichia coli, using the T7 promoter system. The recombinant protein could be purified to homogeneity by a single step Ni2+ affinity chromatography. Western blot experiments using epitope specific antisera confirm that the corresponding linear amino acid sequences are available for immunorecognition in the engineered protein. This methodology enables continuous production and purification in bulk amounts of the minimized RCP as a source of candidate immunocontraceptive vaccine in mammals.