Epitope vaccine: a new strategy against HIV-1

Evaluation of a multi-epitope subunit vaccine against avian leukosis virus subgroup J in chickens

The intricate sequence and antigenic variability of avian leukosis virus subgroup J (ALV-J) have led to unprecedented difficulties in the development of vaccines. Much experimental evidence demonstrates that ALV-J mutants have caused immune evasion and pose a challenge for traditional efforts to develop effective vaccines. To investigate the potential of a multi-epitope vaccination strategy to prevent chickens against ALV-J infections, a recombinant chimeric multi-epitope protein X (rCMEPX) containing both immunodominant B and T epitope concentrated domains selected from the major structural protein of ALV-J using bioinformatics approach was expressed in Escherichia coli Rosetta (DE3). Its immunogenicity and protective efficacy was studied in chickens. The results showed that rCMEPX could elicit neutralizing antibodies and cellular responses, and antibodies induced by rCMEPX could specifically recognize host cell naturally expressed ALV-J proteins, which indicated that the rCMEPX is a good immunogen. Challenge experiments showed 80% chickens that received rCMEPX were well protected against ALV-J challenge. This is the first report of a chimeric multi-epitope protein as a potential immunogen against ALV-J.

Mucosal antibodies induced by tandem repeat of 2F5 epitope block transcytosis of HIV-1

Induction of mucosal antibodies to prevent HIV infection is an important strategy for the HIV-1 prophylaxis. Here we report an epitope-vaccine based antigen that was able to elicit mucosal antibodies capable of blocking HIV-1 transcytosis. Because the ELDKWA epitope of neutralizing antibody 2F5 plays a crucial role in transcytosis, a series of immunogens that contain tandem copies of ELDKWA were prepared. Mice were immunized with these immunogens intranasally, and received intraperitoneal+intranasal boosters. The immunogens that contained more ELDKWA epitopes elicited higher level of mucosal ELDKWA-epitope specific IgAs and systemic IgGs. Although the antisera from the immunized mice exhibited mild neutralizing potency to HIV-1 isolates HXB2 and JRFL, the affinity purified mucosal ELDKWA-epitope specific antibodies could block the transcytosis of cell-free CNE3 (a primary isolate of subtype CRF01_AE) in human tight epithelial models.

Discovery of novel targets for multi-epitope vaccines: screening of HIV-1 genomes using association rule mining

Background

Studies have shown that in the genome of human immunodeficiency virus (HIV-1) regions responsible for interactions with the host's immune system, namely, cytotoxic T-lymphocyte (CTL) epitopes tend to cluster together in relatively conserved regions. On the other hand, "epitope-less" regions or regions with relatively low density of epitopes tend to be more variable. However, very little is known about relationships among epitopes from different genes, in other words, whether particular epitopes from different genes would occur together in the same viral genome. To identify CTL epitopes in different genes that co-occur in HIV genomes, association rule mining was used.

Results

Using a set of 189 best-defined HIV-1 CTL/CD8+ epitopes from 9 different protein-coding genes, as described by Frahm, Linde & Brander (2007), we examined the complete genomic sequences of 62 reference HIV sequences (including 13 subtypes and sub-subtypes with approximately 4 representative sequences for each subtype or sub-subtype, and 18 circulating recombinant forms). The results showed that despite inclusion of recombinant sequences that would be expected to break-up associations of epitopes in different genes when two different genomes are recombined, there exist particular combinations of epitopes (epitope associations) that occur repeatedly across the world-wide population of HIV-1. For example, Pol epitope LFLDGIDKA is found to be significantly associated with epitopes GHQAAMQML and FLKEKGGL from Gag and Nef, respectively, and this association rule is observed even among circulating recombinant forms.

Conclusion

We have identified CTL epitope combinations co-occurring in HIV-1 genomes including different subtypes and recombinant forms. Such co-occurrence has important implications for design of complex vaccines (multi-epitope vaccines) and/or drugs that would target multiple HIV-1 regions at once and, thus, may be expected to overcome challenges associated with viral escape.

The recombinant immunogen with high-density epitopes of ELDKWA and ELDEWA induced antibodies recognizing both epitopes on HIV-1 gp41

The high mutation rate of HIV-1 (human immunodeficiency virus-1) is a major obstacle to developing an effective vaccine. The mutation of ELDKWA-(aa669-674) to ELDEWA-epitope on HIV-1 gp41 caused the immune escape from neutralization by potent anti-HIV-1 human monoclonal antibody (mAb) 2F5. In this study, we suggested and evaluated a multi-epitope vaccine as a new strategy to develop HIV-1 vaccines. A glutathione S-transferase (GST) fusion protein (GST-K8E8) containing 8 copies of ELDKWA-and mutated ELDEWA-epitopes was constructed and used to immunize mice or rabbits. Analysis of the antisera (rAS3) induced by GST-K8E8 suggested that multi-epitope vaccine immunogen could raise antibodies in mice and rabbits against either the original ELDKWA-epitope or the mutated ELDEWA-epitope that resulted in immune escape. Briefly, ELDKWA-epitope-specific antibodies, directly purified from rAS3 by ELDKWA-epitope-peptide affinity chromatography, recognized either original gp41 protein (ELDKWA, rgp41K) or mutated gp41 (ELDEWA, rgp41E) in immunoblotting assay; in contrast, the existing ELDKWA-epitope antibodies recognized only rgp41K but not rgp41E, which were purified by ELDKWA-epitope-peptide affinity chromatography from rAS3 that were firstly completely pre-absorbed by ELDEWA-epitope-peptide affinity beads. And the same results were also observed when detecting the ELDEWA-epitope-specific antibodies in rAS3 by a means similar to the above. All the data presented here demonstrated that a high density multi-epitope vaccine could be an interesting strategy against HIV-1 mutation.

Mucosal and Systemic Adjuvant Effects of Cholera Toxin and Cry1Ac Protoxin on the Specific Antibody Response to HIV-1 C4/V3 Peptides Are Different and Depend on the Antigen Co-administered

Evidence from several sources support the assertion that cholera toxin (CT) is a potent immunogen and mucosal adjuvant; however there are also reports showing its lack of adjuvanticity against some antigens. Cry1Ac protoxin also exerts adjuvant effects in the antibody responses to proteins and polysaccharides but its adjuvanticity with regard to peptide vaccines had not been tested. To probe whether the adjuvant effects of these proteins varied depending on the antigen co-administered, we evaluated antipeptide antibody responses in serum and mucosal samples (vaginal, intestinal, and pulmonary) of mice that were immunized by intranasal or intraperitoneal routes with one of two distinct hybrid C4/V3 HIV peptides, either alone or with CT or Cry1Ac. The tested HIV 1 peptides differed in two aminoacids, T1SP10MN(A) was modified at the SP10 region by the substitution of the isoleucines 12 and 14 for cysteines and was called modified (m)T1SP10MN(A). Our data indicate that the adjuvant effects of CT and Cry1Ac are different. In addition they vary depending on the antigen co-administered and the number of antigen doses, because after three doses moderate adjuvant effects of CT and Cry1Ac on anti-peptide serum and mucosal antibody responses were observed only against the mT1SP10MN(A). In contrast, to attain significant adjuvant effects against the T1SP10MN, four doses were required. Interestingly we found that modification of the HIV peptide increases its immunogenicity without altering the principal neutralizing determinant (SP10).

Neutralization of HIV-1 primary isolate by ELDKWA-specific murine monoclonal antibodies

ELDKWA on HIV-1 gp41 is a conserved epitope recognized by one broadly neutralizing monoclonal antibody 2F5, which is a promising candidate target for vaccine design. Here we report two ELDKWA-specific monoclonal antibodies (mAbs), 18F11 and 7E10, that were screened from the splenocytes of mice immunized by recombinant GST-(ELDKWA)4 protein. In further evaluation, these mAbs exhibited appreciable neutralizing activities against HIV-1 primary isolate 92US675 (clade B) with IC50 (50% inhibition concentration) of 6.84 +/- 0.36 microg/ml and 10.66 +/- 1.69 microg/ml, respectively. Unexpectedly, neither of these two murine mAbs could neutralize laboratory-adapted strain HIV-1 IIIB (clade B). As a control, human mAb 2F5 neutralized both primary and laboratory-adapted strains. These data strongly suggest that ELDKWA-specific antibodies induced by different antigenic formats show different neutralizing activities against HIV-1, which implies another complication in the development of effective vaccines.

Predefined spacers between epitopes on a recombinant epitope-peptide impacted epitope-specific antibody response

We have developed a widely applicable method to construct epitope-peptide gene for epitope-vaccine strategy recently. In this study, we wanted to know whether the predefined spacers between epitopes on a recombinant epitope-peptide impacted the production of epitope-specific antibodies. The neutralizing epitope ELDKWA on the C-domain of HIV-1 gp41 was defined by the monoclonal antibody (mAb) 2F5 with broad neutralizing activity. We constructed three recombinant ELDKWA-epitope-peptides with different spacers between epitopes. The recombinant epitope-peptide GST-K8, GST-S8 and GST-R8 were bearing eight copies of ELDKWA-epitope with amino acid spacer GS, GSGGGGS and RS, respectively. GST-K8 and GST-S8 could induce high titer of ELDKWA-epitope-specific antibodies, much better than GST-R8. Besides, both antibodies could recognize the recombinant soluble gp41 and the transfected CHO-WT cells that stably express HIV-1 envelope glycoprotein on the cell surfaces. These experimental results indicated that the spacer GSGGGGS and GS were feasible in constructing a recombinant epitope-vaccine.

High epitope density in a single recombinant protein molecule of the extracellular domain of influenza A virus M2 protein significantly enhances protective immunity

The degree of epitope density has been shown to be a critical factor influencing the magnitude of epitope-specific responses. However, whether high epitope density in just a single protein molecule can still enhance the humoral response or, more importantly, the protective immunity, has not been determined. To test this, five glutathione-S-transferase fusion proteins bearing various numbers of copies of the M2e epitope on M2 protein of influenza virus (1, 2, 4, 8 and 16 copies) were prepared, and used to immunize mice and rabbits. Our data show clearly that M2e-specific humoral response was enhanced with increasing epitope density. By lethal challenge assay in mice, it was observed that recombinant proteins with higher M2e epitope densities resulted in higher survival rates and slower weight losses. The survival rate was directly related to the degree of epitope density in the single recombinant protein: 100% in the case of 16 M2e copies; 50% with 4 M2e epitopes; and 0% with one.

High epitope density in a single protein molecule significantly enhances antigenicity as well as immunogenicity: a novel strategy for modern vaccine development and a preliminary investigation about B?cell discrimination of monomeric proteins

Although early studies have shown a close correlation between epitope density and epitope-specific humoral immune responses, few attempts have been made to quantitatively compare the antigenic and immunogenic differences between protein molecules bearing low or high degrees of epitope density, nor have studies quantitatively investigated the mechanism of B cell discrimination of monomeric antigens. In this study, we prepared glutathione S-transferase (GST) fusion proteins bearing various copies of the M2e epitope from the influenza virus M2 protein [GST-(M2e)8, GST-(M2e)4 and GST-(M2e)1], which were used to detect and compare the real-time kinetic binding with M2e-specific mAb by surface plasma resonance. Our data show clearly that fusion proteins bearing higher M2e epitope density resulted in higher average avidity for M2e-specific mAb. Furthermore, it was observed that fusion proteins bearing high M2e epitope density could induce polyclonal antibodies (pAb) with enhanced an average affinity constant (KA) for M2e epitope peptide compared to fusion proteins bearing low epitope density. The average KA of pAb induced by GST-(M2e)8 (3.08 x 10(8) M(-1) or 9.96 x 10(8) M(-1)) was up to two orders of magnitude greater than the average KA of pAb induced by GST-(M2e)1 (2.00 x 10(6) M(-1) or 3.43 x 10(6) M(-1)). Thus, the data presented here demonstrate that high epitope density in a single protein molecule significantly enhances antigenicity and immunogenicity. These findings enrich our knowledge of how epitope density might relate to the recognition, activation and antibody production processes of epitope-specific immature B cells.

Sequence comparison between the extracellular domain of M2 protein human and avian influenza A virus provides new information for bivalent influenza vaccine design

To prevent the human and economic losses caused by human and avian influenza viruses, it is necessary to prepare safe bivalent influenza vaccines. Recent studies found that human influenza vaccines based on the extracellular domain of influenza M2 protein (M2e) induced broad-spectrum protective immunity in various antigen constructs. A prerequisite for using the M2e protein as a bivalent influenza vaccine component was to find out the sequence differences between human and non-human (avian or swine) influenza M2e proteins. Here, we completed such a comparison using 716 influenza M2e sequences available in Genbank. The results found one region on M2e protein consistent with host restriction specificities: PIRNEWGCRCN, PTRNGWECKCS and PIRNGWECRCN (aa10-20; the human, avian and swine specific M2e sequence, respectively). Interestingly, the comparison result was then validated by immunoblotting and enzyme-linked immunosorbent assay. The monoclonal antibody against the EVETPIRN sequence (aa6-13) of human M2e protein could weakly recognize avian M2e proteins bearing the EVETPTRN sequence (aa6-13) but failed to recognize avian M2e proteins bearing the EVETLTRN sequence (aa6-13). The data in this study provided useful information in the race to develop bivalent influenza vaccines against avian and human influenza A virus infection in human beings.

Design and construction of a recombinant epitope-peptide gene as a universal epitope-vaccine strategy

The multiple sequence and antigenic variability of HIV-1 has led to unprecedented difficulties in the development of effective vaccines. More and more experimental evidence suggests that HIV-1 mutants have resulted in immune evasion and pose a challenge for conventional efforts to prepare effective vaccines. We have suggested that an epitope-vaccine could offer a new strategy against HIV-1. Several candidate epitope-vaccines were prepared and systematically studied in animal experiments. Here, we have proposed a universal method for the preparation of immunogens for an epitope-vaccine by genetic engineering. With this method, copies of epitopes from one or more antigens can be conveniently combined into one recombinant immunogen, which may also help to design epitope-vaccines against other viral infections.

Recombinant protein comprising multi-neutralizing epitopes induced high titer of antibodies against Influenza A virus

In previous studies, we suggested that epitope-vaccine might be a new strategy against virus infection. Based on this hypothesis, we designed and expressed a recombinant immunogen (multi-epitope-peptide) comprising repeats of three neutralizing-epitopes (neutralizing epitopes: aa92-105, 127-133 and 183-195) of hemagglutininin (HA) of influenza virus (H3N2) in E. coli. After vaccination, the recombinant multi-epitope protein could induce a high level of antibodies with predefined multi-epitope-specificity in mice and rabbits. The epitope-specific antibodies in sera were tested using three different epitope-peptides (synthetic peptides) in ELISA assay, and the serum dilutions from 1 : 6400 to 1 : 25600 were confirmed. In western blot analysis, both the antiserum and the antibodies purified by synthetic epitope-peptide coupled sepharose columns could recognize natural HA from influenza virus particles (strain A/Wuhan/359/95 H3N2). In hemagglutination inhibition (HI) tests, these three antisera at the dilutions from 1 : 20 to 1 : 80 showed inhibitory activity. Interestingly, antisera and purified antibodies induced by the epitope-vaccine could partially inhibit plaque-formation of influenza virus (strain A/Wuhan/359/95) on MDCK cell monolayers. These results suggest that the recombinant multi-epitope vaccine can simultaneously induce multi-antiviral activities against influenza virus, which may provide a new way to develop effective vaccines against influenza virus.

Epitope-vaccine strategy against HIV-1: today and tomorrow

Vaccines play important roles in preventing infectious diseases caused by different pathogens. However, some pathogens such as HIV-1 challenge current vaccine strategy. Poor immunogenicity and the high mutation rate of HIV-1 make great difficulties in inducing potent immune responses strong enough to prevent infection via vaccination. Epitope-vaccine, which could intensively enhance predefined epitope-specific immune responses, was suggested as a new strategy against HIV-1 and HIV-1 mutation. Epitope-vaccines afford powerful approaches to elicit potent, broad and complete immune protection against not only primary homologous viral isolates but also heterologous viral mutants. Although most studies are still preliminary now, epitope-vaccine as a novel strategy against the AIDS epidemic has great developmental potential. To trigger T-cell-dependent IgG antibody responses and improve affinities of the epitope-specific antibodies, approaches such as recombinant multi-epitope-vaccination and prime-boosting vaccination were suggested. Cellular immune responses, especially CTL responses, could also be elicited and enhanced in addition to humoral immune responses. Developed epitope-vaccines activating both arms of the immune system would benefit prevention and immunotherapy not only against HIV but also other chronic infections.

Candidate peptide vaccine induced protection against classical swine fever virus

Former investigations demonstrated that the envelope glycoprotein E2 could protect pigs from classical swine fever virus (CSFV). Based on these findings, we prepared synthetic peptide vaccine using E2 N-terminal antigenic units B/C and hoped to induce protective activity against lethal challenge of virulent CSFV strain Shimen. Five overlapped peptides sequence-covering amino acids 693-777 on E2 of Shimen were synthesized and then conjugated with bovine serum albumin (BSA), respectively. In the vaccination course, the candidate peptide vaccines in combination (multi-peptide vaccine (MPV)) were applied for immunization of pigs (n=10) and induced strong antibody response against CSFV. It is subsequently demonstrated that this peptide vaccine could provide immunized pigs complete protection against lethal CSFV challenge as C-strain does, while all non-immunized pigs in negative control group manifested obvious typical symptoms and died during the second and third weeks after viral challenge. In order to confirm the neutralizing activity of the polyclonal antibodies induced by MPV, neutralization assay were carried out on rabbits. The live C-strain alone could ordinarily induce typical fever on rabbits. The typical fever of rabbits induced by the live C-strain could be inhibited by pre-incubation with the anti-sera (dilution 1:4 and 1:16) induced by MPV, but not inhibited by pre-incubation with the same anti-sera from which the antibodies against five peptides were removed by peptide-specific affinity chromatography, which indicates that these peptide-specific antibodies in the anti-sera induced by MPV provided protective activity against CSFV. Our finding provides a new way to develop marker vaccine against CSFV.

A predefined epitope-specific monoclonal antibody recognizes ELDEWA-epitope just presenting on gp41 of HIV-1 O clade

Diverse variation of HIV-1 is a grave challenge for prevention of viral infection and immunotherapy. Monoclonal antibody (mAb) 2F5 recognizing an epitope ELDKWA (aa669-674) on HIV-1 envelope protein gp41 showed broad neutralizing activity against a lot of HIV-1 strains including primary isolates. However, viral mutation from ELDKWA to ELDEWA resulted in viral evasion from neutralization by mAb 2F5. Using ELDEWA-epitope-peptide (C-GFLDEWAGELDEWA) conjugated with carrier protein keyhole limpet hemocyanin (KLH), a mAb 14D9 (IgGl) was prepared and identified as the mAb with predefined ELDEWA-epitope specificity. The mAb 14D9 recognized the ELDEWA epitope, but not other three epitopes (ELDKWA, ELNKWA and ELEKWA). In comparison, mAb 2F5 could recognize only ELDKWA, but not three neutralization-resistant epitopes (ELDEWA, ELNKWA and ELEKWA). Interestingly, we searched several authoritative HIV sequence databases (http://hiv-web.lanl.gov) and found out that nearly all the viral isolates bearing the ELDEWA epitope belong to the O clade, the only exceptional viral isolate bearing the epitope ELDEWA has been demonstrated to be an intergroup M/O recombinant, which suggests that the ELDEWA-epitope on gp41 represents a specific epitope-marker of HIV-1 O clade. To confirm whether the mAb 14D9 recognizes gp41 of HIV-1 O clade, the rsgp41(IIIB), bearing ELDKWA-epitope was site-directed-mutated to the rsgp41 bearing ELDEWA-epitope. The mAb 14D9 could bind to rsgp41 bearing ELDEWA-epitope in immunoblotting analysis, did not bind to rsgp41 bearing ELDKWA-epitope. These experimental results suggest that the mAb 14D9 with predefined ELDEWA-epitope specificity may be applied to HIV-1 O clade identification.

Neutralizing antibodies mechanism of neutralization and protective activity against HIV-1

The role of the humoral immune response in prevention against HIV-1 infection is still incompletely understood. However, neutralizing antibodies to certain epitopes on HIV-1 envelope glycoproteins inhibit HIV-1 infection in vitro and in vivo. Passive administration of these antibodies by themselves or in combination completely protected hu-PBL-SCID mice or macaques from intravenous, vaginal, as well as maternal-fetal mucosal transmission. All these studies provide direct experimental evidence that neutralizing antibodies are potent enough to prevent HIV infection, and strongly suggest that neutralizing-antibody-based vaccines could provide effective protection against HIV-1, despite the potent action of CTLs. Some neutralizing epitopes have been defined in vitro and in vivo. Unfortunately, none of the neutralizing-antibody-based candidate vaccines has been demonstrated to induce enough protective activity. Weak antigenicity and immunogenicity of neutralizing epitopes on native or recombinant proteins and other factors made it difficult to induce neutralizing-epitope-specific antibody responses in vivo enough to prevent against primary isolates. Recent studies indicated that HIV-1 variations resulted in escape from neutralization or the CTL responses, which may be the principal challenge for HIV-1 prevention. Epitope vaccine as a new strategy activating both arms of the immune system, namely, using the "principal neutralizing epitopes" and the CTL epitopes in combination, should provide new hope for developing an effective vaccine to halt the HIV-1 epidemic.

Recombinant multi-epitope vaccine induce predefined epitope-specific antibodies against HIV-1

Monoclonal antibody 2F5 recognizing ELDKWA-epitope on HIV-1 gp41 has significant neutralization potency against 90% of the investigated viruses of African, Asia, American and European strains, but antibodies responses to ELDKWA-epitope in HIV-1 infected individuals were very low. Based on the epitope-vaccine strategy suggested by us, a recombinant glutathione S-transferase (GST) fusion protein (GST-MELDKWAGELDKWAGELDKWAVDIGPGRAFYGPGRAFYGPGRAFY) as vaccine antigen containing three repeats of neutralizing epitope ELDKWA on gp41 and GPGRAFY on gp120 was designed and expressed in Escherichia coli. After vaccination course, the recombinant multi-epitope vaccine could induce high levels of predefined multi-epitope-specific antibodies in mice. These antibodies in sera could bind to both neutralizing epitopes on gp41 peptide, V3 loop peptide and recombinant soluble gp41 (aa539-684) in ELISA assay (antisera dilution: 1:1,600-25,600), while normal sera did not. Moreover, these antibodies in sera could recognize the CHO-WT cells which expressed HIV-1 envelope glycoprotein on the cell surfaces, indicating that the predefined epitope-specific antibodies could recognize natural envelope protein of HIV-1 though these antibodies were induced by recombinant multi-epitope-vaccine. These experimental results suggested a possible way to develop recombinant multi-epitope vaccine inducing multi-antiviral activities against HIV-1.

Sequence variation and consensus sequence of V3 loop on HIV-1 gp120

Mutation in the V3 loop of HIV-1 gp120 could affect syncytium formation, virus infectivity and neutralization. To acquire more information of the V3 loop mutation, we analyzed amino acid sequences of the V3 loop of 24504 isolates from most HIV-1 clades (including A, B, C, D, E, F, G and H clades). The consensus sequence of the V3 loop of each subtype with the highest frequency emerging on each position is constituted and the conservation of each amino acid in this region is also calculated. Exploring the restricted mutation of the V3 region could help to understand mechanism of HIV entry and to develop new strategy against HIV-1.

A candidate vaccine against influenza virus intensively improved the immunogenicity of a neutralizing epitope

BACKGROUND

The hemagglutinin (HA) of influenza viruses is one of the major targets of the humoral response. The role of serum antibody to HA in the protection against infection has been demonstrated by long-standing observation. In previous studies, we suggested that an epitope vaccine might be a new strategy against the virus.

METHODS

HA sequences of 491 H3 subtype strains from the influenza sequence database were compared and analyzed. To acquire information on the immunogenicity of the F3 epitope, F3-epitope-specific antibody levels in 81 patient sera infected with influenza virus were tested by ELISA. Based on the theory of the epitope vaccine, we designed an epitope peptide F3 (C-KAYSNCYPYDVPDY-G-KAYSNCYPYDVPDY), which contains the repeated F3 epitope KAYSNCYPYDVPDY (aa92-105) on HA (H3N2). The specificity and the titer of the antibodies induced by the epitope vaccine were determined by ELISA. The neutralizing activities of these anti-F3 antibodies were shown by inhibiting influenza virus infection of MDCK cells.

RESULTS AND CONCLUSION

Comparison of HA sequences of 491 H3 subtype strains indicates that this epitope is highly conservative. Analysis of the sera from influenza virus-infected patients revealed a very low level of F3 epitope-specific antibodies, suggesting the poor immunogenicity of the F3 epitope on influenza virus. The epitope vaccine based on the F3 epitope induced high levels of F3 epitope-specific antibodies recognizing the epitope peptide F3 (antibody titer in antisera up to 1:25,600). Besides, the antisera could also recognize the natural HA in Western blotting. Interestingly, these antisera induced by the epitope vaccine could inhibit infection of MDCK cells by influenza virus (strain A/Wuhan/359/95) in the neutralization assay. These results suggest that the epitope vaccine can intensively increase the immunogenicity of neutralizing epitopes and may provide a new way to develop an effective vaccine against influenza virus.

Immunogenicity and specificity of the candidate multi-epitope-vaccines against HIV-1

The failure of some candidate HIV-1 vaccines may result from inducing very weak neutralization activity against representative primary viral isolates. Based on our hypothesis that epitope-vaccine may be a new strategy to induce high levels of neutralizing antibodies against HIV-1, we designed two candidate multi-epitope-vaccines, EP1 [C-G-(ELDKWA-GPGRAFY)2-K] and EP2 (CG-GPGRAFY-G-ELDKWA-G-RILAVERYLKD), containing three neutralizing epitopes (GPGRAFY, ELDKWA and RILAVERYLKD) on HIV-1 envelope protein, and expected them to induce epitope-specific antibodies of predefined epitope-specificity. The two peptides were conjugated to carrier protein bovine serum albumin (BSA) and used for immunization of rabbits. Proteins were purified from the rabbit sera induced by both candidate multi-epitope-vaccines (EP1-BSA and EP2-BSA) through affinity chromatography with epitope-peptide-conjugated sepharose-column, and identified as antibodies in silver-staining and immunoblotting. These antibodies were demonstrated to recognize three neutralizing epitopes on peptides and the recombinant gp41 in ELISA-assay and immunoblotting. These results indicated that both candidate multi-epitope-vaccines could induce high levels of antibodies of predefined epitope-specificity which recognized a few of neutralizing epitopes on peptides and protein, providing experimental evidence for the new strategy to develop an effective neutralizing-antibody-based multi-epitope-vaccine against HIV-1.

HIV epitope-peptides in aluminum adjuvant induced high levels of epitope-specific antibodies

Some neutralizing epitopes on HIV-1 envelope proteins were shown to induce antibodies that could effectively inhibit the infection of different HIV-1 strains in vitro. But only very low levels of antibodies to these epitopes were determined in the HIV-1 infected individuals. In this study, the aluminum (alum) adjuvant to increase the immunogenicity of the neutralizing epitopes was used. Three epitope-peptides [C-(ELDKWAG)4, C-(RILAVERYLKD-G)2 and C-(GPGRAFY)2], which contain three epitopes (ELDKWA, RILAVERYLKD, GPGRAFY) from the HIV-1 Env proteins, were synthesized and conjugated to carrier protein keyhole limpet hemocyanin (KLH). The epitope-vaccines C-(ELDKWAG)4-KLH and C-(RILAVERYLKD-G)2-KLH in alum induced high levels of epitope-specific antibodies recognizing the epitopes from epitope-peptides C-(ELDKWAG)4 and C-(RILAVERYLKD-G)2, as well as the gp41 C-domain peptides P2 [C-TSLIHSLIEESQNQQEKNEQELLELDKWA (aa 646-674)] and P1 [LQARILAVERYLKDQQL (aa 583-599)] and the recombinant soluble gp41 (rsgp41) bearing both epitopes (antibody titer in rabbit sera was 1:12800-25,600 dilution). Immunoblotting analysis demonstrated that the antibodies in both antisera bound to rsgp41, indicating that both antibodies recognized the natural epitopes on rsgp41 protein. The epitope-vaccines C-(GPGRAFY)2-KLH induced moderate GPGRAFY epitope-specific antibody response with a titer of 1:6,400. In contrast, as it was demonstrated in previous studies, the immunization with rgp160 induced weak antibody response to these three epitopes (titer of 1:400-1600). This suggests that epitope-peptides conjugated to KLH when infected with alum significantly increases immunogenicity of gp41 neutralizing epitopes providing a hope for the development of an HIV-1 vaccine.

HIV-1 gp41 and type I interferon: sequence homology and biological as well as clinical implications

HIV-1 gp41-like human type I interferon (IFN) could inhibit lymphocyte proliferation and up-modulate MHC class I and II and ICAM-1 molecule expression. Sequence comparison indicates that a similar epitope RILAV-YLKD exists between N-domain of gp41 and two regions in IFN-alpha(aa29-35 and 113-129), IFN-beta (aa31-37 and 125-138) and IFN-omega (aa29-35 and 123-136), which was shown to form IFN-alpha/beta-receptor binding site. Weak sequence similarity was also found to exist in both regions on gp41 and type I IFN of murine and bovine. Experimental studies indicated that a common immunological epitope exists between gp41 and IFN-alpha and -beta. Antibodies against human IFN-alpha and -beta recognized the common immunological epitope and inhibited gp41-binding to the potential cellular receptor protein p45. Moreover, the polyclonal antibody to IFN-beta completely inhibited gp41-binding to human T, B cells and monocytic cells, while IFN-alpha could only inhibit this binding incompletely. It was interestingly observed that human IFN-beta after preincubating with cells could incompletely inhibit the binding of gp41 to human B cells and monocytic cells, and very weakly inhibit the binding to human T cells, indicating that the receptor for IFN-beta-binding may be involved in gp41 binding. This potential relationship may be based on the amino acid sequence homology in the receptor binding region between gp41 and IFN-beta. It was observed that the increased levels of antibodies against human IFN-alpha and -beta exist in HIV-1-infected individuals and are associated with the common epitope on gp41. Besides, several studies provided experimental evidence that the common immunological epitope could induce protective activity against HIV-1. The IFN-alpha-based vaccine has showed a significant reduction of disease progression in IFN-alpha-vaccine-treated HIV-infected patients. Recent experimental evidence indicates that gp41 and IFN-beta were involved in downregulation of CCR5 expression and induction of cell activation or signal transduction. Whether it may be performed by a similar mechanism is still to be investigated.

N- and C-domains of HIV-1 gp41: mutation, structure and functions

Recent studies demonstrated that the N- and C-domains of HIV-1 gp41 is involved in virus-mediated membrane fusion resulting in HIV-entry into the target cells. Up to now, viral mutation baffled many scientists to develop effective vaccines and drugs against HIV-1. To acquire more information of mutation of gp41 and to reveal the relationship of structure and function of the N- and C-domains, we compared and analyzed amino acid sequences of the gp41 ectodomain (aa 512-681) of 862 isolates from most HIV-1 clades (including A, B, C, D, E, F, G, H, I, J and O clades). A consensus sequence of the ectodomain with the highest frequency emerging on each position is constituted. The fusion domain and the N-domain belong to the most conserved regions in gp41, and most variable residues assemble partial to the C terminal of gp41. The hydrophobicity of each position is also calculated. The a and d positions in the N-domain for maintaining stabilization of the trimeric coiled coil interactions are highly conservative, and the e and g positions in the C-domain to retain the interaction show also highly conservative. The strange high conservation of the c residues may have an implication in the coiled coil structure. The highly conserved residues form the lining of the hydrophobic cavity and the deep cavity is an ideal target for small molecular inhibitors. On the C-terminal of the C-domain there is a highly conserved segment GIVQQQ. They are intimately involved in forming the three interfaces between neighboring helices. The function of the N- and C-domains, such as binding to the potential cellular receptor and inducing protective activities, are also discussed. These studies on the mutation, structure and functions of the N- and C-domains suggested that both domains become a new focus to develop effective vaccine and antiviral drugs in the new strategies.

ELNKWA-epitope specific antibodies induced by epitope-vaccine recognize ELDKWA- and other two neutralizing-resistant mutated epitopes on HIV-1 gp41

Based on the fact that monoclonal antibody (mAb) 2F5 recognizing ELDKWA-epitope on HIV-1 gp41 separately or in combination with other mAbs showed potent neutralizing activity to a wide range of primary HIV-1 isolates in vivo and in vitro, but this epitope undergoes restricted mutation. ELNKWA is a neutralizing-resistant mutated epitope. We induced ELNKWA-epitope-specific polyclonal and monoclonal antibodies and studied the interaction of the antibodies with ELDKWA-epitope and other two neutralizing-resistant mutated epitopes. The candidate ELNKWA-epitope-vaccine induced a high level of antibodies to the ELNKWA-epitope-peptide. The ELNKWA-epitope-specific polyclonal antibodies bound not only the ELNKWA-, but also ELDKWA-, ELEKWA- and ELDEWA-epitope-peptides in ELISA-assay. Moreover, the antibodies also recognized four C-domain-peptides (P5, P6, P7, P8) which contain these four epitopes, respectively. Interestingly, an ELNKWA-epitope-specific monoclonal antibody (TH-Ab1) induced by the candidate ELNKWA-epitope-vaccine could also recognize the four C-domain-peptides containing ELNKWA-, ELDKWA-, ELEKWA- and ELDEWK-epitopes. These results indicate that the candidate ELNKWA-epitope-vaccine could induce high levels of antibodies, which recognize the neutralizing epitope ELDKWA and three neutralizing-resistant mutated epitopes, suggesting that the candidate ELNKWA-epitope-vaccine may help to overcome the problem of viral escape from neutralization through mutation at D or K position, and may be developed as an effective vaccine with a broad neutralizing activity against HIV-1.

Candidate multi-epitope vaccines in aluminium adjuvant induce high levels of antibodies with predefined multi-epitope specificity against HIV-1

Some neutralizing epitopes on HIV-1 envelope proteins were identified to induce antibodies which could effectively inhibit the infection of different strains in vitro. But only very low levels of these antibodies were determined in the HIV-1 infected individuals. To increase the levels of protective antibodies in vivo, we suggested multi-epitope vaccine as a new strategy to induce high level of neutralization antibodies with predefined multi-epitope specificity. A synthesized epitope peptide MP (CG-GPGRAFY-G-ELDKWA-G-RILAVERYLKD) containing three neutralizing epitopes (GPGRAFY, ELDKWA, RILAVERYLKD) was conjugated to carrier protein KLH, and then used for immunization in mouse together with aluminium adjuvant or Freund's adjuvant (FA). The candidate MP-KLH multi-epitope vaccine in aluminium adjuvant could induce antibody response very strongly to the epitope peptide C-(RILAVERYLKD-G)2 and the immunosuppressive peptide (P1) (LQARILAVERYLKDQQL) (antibody titer: 1:51200), strongly to the epitope peptide C-(ELDKWA-G)4 and the C-domain peptide (P2) (1:12800), and moderately to the epitope peptide C-(GPGRAFY)4 and the V3 loop peptide (1:1600). The immunoblotting analysis demonstrated that the antibodies in sera could recognize P1, P2, V3 loop peptides and rsgp41 (aa 539-684). These results are similar with that in the case of PI-BSA in FA, and suggest that the multi-epitope vaccine in aluminium could induce high levels of antibodies of predefined multi-epitope specificity, which provides experimental evidence for the new strategy to develop an effective neutralizing antibody-based multi-epitope vaccine against HIV-1.

Induction of high levels of epitope-specific antibodies by epitope/peptide candidate vaccines against human immunodeficiency virus type-1 (HIV-1)

To test the immunogenicity of GPGRAFY-epitope-based candidate vaccines, a peptide with four repetitive GPGRAFY epitopes, V3-P1 [C-(GPGRAFY)4], and a peptide (PND) of the principal neutralizing domain (V3 loop: amino acid 301-328: C-TRPNNNTRKSIRIQRGPGRAFYTIGKI) on gp120 were synthesized and covalently coupled to a carrier protein BSA. Immunization of BALB/c mice and New Zealand White Rabbits with these conjugate vaccines engendered strong antibody responses against the PND (mouse serum titer by 1:12,800-25,600; rabbit serum titer by 1:6,400-12,800). Interestingly, the V3-P1-BSA conjugates and the PND-BSA conjugates could induce high levels of GPGRAFY-epitope-specific antibodies in the mice and rabbits (mouse serum titer by 1:25,600; rabbit serum titer by 1:12,800-25,600), while a recombinant gp160 subunit vaccine induced a low level of GPGRAFY-epitope-specific antibodies (serum titer by 1:400-1,600 in mice and rabbits). To confirm the above results, GPGRAFY-epitope-specific antibodies were isolated from rabbit sera induced by V3-P1-BSA, PND-BSA conjugates and rgp160 vaccine. In fact, 23-38 and 13-22 microg epitope-specific antibodies per milliliter serum were isolated from rabbit sera induced by V3-P1-BSA and PND-BSA conjugate, respectively, while 1.34 microg epitope-specific antibodies per milliliter serum were identified in rabbit serum induced by rgp160 vaccine. In the control group, only 0.069 microg proteins per milliliter serum were found in pooled pre-immune serum (normal serum). These results from mouse and rabbit experiments indicate that epitope and peptide vaccines both induce high levels of GPGRAFY-epitope-specific antibodies in comparison with rgp160 subunit vaccine, suggesting that epitope/peptide vaccines may be a new strategy to induce protective activity.

Induction of high levels of antibodies recognizing the neutralizing epitope ELDKWA and the D- or K-position-mutated epitopes by candidate epitope vaccines against HIV-1

Monoclonal antibody 2F5 recognizing the ELDKWA epitope on HIV-1 gp41 has a significant neutralization potency against 90% of the investigated viruses of African, Asian, American, and European strains, but the antibody responses to the epitope 2F5 in HIV-1-infected individuals were very low. We attempted to induce high levels of epitope-specific antibodies to ELDKWA and its three mutated epitopes by candidate epitope vaccines. The four candidate epitope vaccines all induced strong antibody responses at dilutions from about 1:6,400 to 1:25,600. We tested the cross-reactions between these antisera and four epitope peptides. The ELDKWA-specific antisera showed strong cross-reactivity with three neutralizing-resistant mutated epitopes which contain changes in the D or K positions of the epitope sequence. Virus variants containing these changes could escape neutralization by monoclonal antibody 2F5. In immunoblotting analysis, the ELDKWA, ELDEWA, and ELEKWA epitope specific antibodies all recognized rsgp41 which confirms that the antibodies against both mutated epitopes, ELDEWA and ELEKWA, could cross-react with the native epitope on rsgp41. Although it is not clear whether the polyclonal antibodies induced by the ELDKWA epitope vaccine could neutralize the mutated viruses containing these mutated epitopes, it is conceivable that epitope vaccines based on mutated epitopes could induce strong antibody responses with predefined epitope specificity to neutralize mutated viruse containing the mutated epitope. An epitope vaccine, using different epitopes including mutated epitopes, could provide a new concept for developing a new vaccine against HIV-1.

Induction of monoclonal antibody with predefined ELNKWA epitope specificity by epitope vaccine

Since the hybridoma technique to produce monoclonal antibodies (MAbs) was discovered, thousands of MAbs with predefined protein specificity have been produced, and a natural or recombinant protein as antigen is necessary for inducing MAbs in the conventional hybridoma technique. To induce epitope-specific MAbs, we suggest an epitope vaccine as a new technique to induce MAbs with predefined epitope specificity. ELDKWA was identified as an important neutralizing epitope on HIV-1 gp41. The MAb 2F5, recognizing ELDKWA epitope, has shown broad neutralizing activity to many HIV strains, including primary isolates, but the mutant in ELNKWA epitope results in escape 2F5-based neutralization. To produce MAbs recognizing this mutated epitope for consideration of passive immunotherapy against the mutant bearing the ELNKWA epitope, MAbs with predefined ELNKWA epitope specificity were induced by synthetic epitope-peptide instead of a natural or recombinant gp41 bearing this epitope. Three MAbs were identified to recognize ELNKWA epitope on the synthetic epitope-peptide, and interestingly could bind the recombinant gp41 with ELDKWA epitope in an ELISA assay and immunoblotting analysis.

Induction of high level of specific antibody response to the neutralizing epitope ELDKWA on HIV-1 gp41 by peptide-vaccine

The monoclonal antibody 2F5 recognizing the neutralizing epitope ELDKWA on the C-domain could neutralize 90% of the investigated HIV-1 isolates. Low levels of ELDKWA-epitope-specific antibodies were observed in HIV-1-infected individuals. To induce high levels of antibodies to ELDKW-epitope, C-domain peptide (P2) was conjugated with a carrier peptide (KGGG)(7)-K (K/G). P2-K/G-conjugate induced high level of antibodies in mice by titer 1:25,600 to ELDKWA-epitope. P2-K/G-BSA-conjugate induced antibody response to ELDKWA-epitope (1:320-6400) in mice. The ELDKWA-epitope-specific antibodies of 19.8 and 34.6 microg/per milliliter serum were isolated from two rabbit antiserums (1:25,600). The levels of ELDKWA-epitope-specific antibodies induced in rabbits were greater than 1 microg/ml, a level considered to confer long-term protection. These results demonstrate the potential role of the C-domain peptide of gp41 to develop an effective ELDKWA-based epitope/peptide-vaccine against HIV-1.