Fine specificity of equine infectious anaemia virus gp90-specific antibodies associated with protective and enhancing immune responses in experimentally infected and immunized ponies

Lessons in AIDS vaccine development learned from studies of equine infectious, anemia virus infection and immunity

Equine infectious anemia (EIA), identified in 1843 [1] as an infectious disease of horses and as a viral infection in 1904, remains a concern in veterinary medicine today. Equine infectious anemia virus (EIAV) has served as an animal model of HIV-1/AIDS research since the original identification of HIV. Similar to other lentiviruses, EIAV has a high propensity for genomic sequence and antigenic variation, principally in its envelope (Env) proteins. However, EIAV possesses a unique and dynamic disease presentation that has facilitated comprehensive analyses of the interactions between the evolving virus population, progressive host immune responses, and the definition of viral and host correlates of immune control and vaccine efficacy. Summarized here are key findings in EIAV that have provided important lessons toward understanding long term immune control of lentivirus infections and the parameters for development of an enduring broadly protective AIDS vaccine.

Development of a high throughput, semi-automated, infectious center cell-based ELISA for equine infectious anemia virus

A faster semi-automated 96-well microtiter plate assay to determine viral infectivity titers, or viral focal units (vfu), of equine infectious anemia virus (EIAV) stocks is described. Optimization of the existing method modernizes a classic virological technique for viral titer determination by quantitating EIAV in experimentally infected cells via a cell-based ELISA. To allow for automation, multiple parameters of the current assay procedures were modified resulting in an assay that required only one quarter the original amount of virus and/or serum for infectivity or neutralization assays, respectively. Equivalent reductions in the required volumes of tissue culture, cell processing, and protein detection reagents were also achieved. Additionally, the new assay decreased the time required from start to finish from 10 days to 6 days (viral titer) or 7 days (viral neutralization), while increasing the number of samples that can be processed concurrently by transition to a 96-well microtiter plate format and by automated counting.

Small ruminant lentiviruses: immunopathogenesis of visna-maedi and caprine arthritis and encephalitis virus

The small ruminant lentiviruses include the prototype for the genus, visna-maedi virus (VMV) as well as caprine arthritis encephalitis virus (CAEV). Infection of sheep or goats with these viruses causes slow, progressive, inflammatory pathology in many tissues, but the most common clinical signs result from pathology in the lung, mammary gland, central nervous system and joints. This review examines replication, immunity to and pathogenesis of these viruses and highlights major differences from and similarities to some of the other lentiviruses.

Amino acid mutations in the env gp90 protein that modify N-linked glycosylation of the Chinese EIAV vaccine strain enhance resistance to neutralizing antibodies

The Chinese EIAV vaccine is an attenuated live-virus vaccine obtained by serial passage of a virulent horse isolate (EIAV(L)) in donkeys (EIAV(D)), and subsequently in donkey cells in vitro. In this study, we compare the env gene of the original horse virulent virus (EIAV(L)) with attenuated strains serially passaged in donkey MDM (EIAV(DLV)), and donkey dermal cells (EIAV(FDDV)). Genetic comparisons among parental and attenuated strains found that vaccine strains contained amino acid substitutions/deletions in gp90 that resulted in a loss of three potential N-linked glycosylation sites, designated g5, g9, and g10. To investigate the biological significance of these changes, reverse-mutated viruses were constructed in the backbone of the EIAV(FDDV) infectious molecular clone (pLGFD3). The resulting virus stocks were characterized for replication efficiency in donkey dermal cells and donkey MDM, and were tested for sensitivity to neutralization using sera from two ponies experimentally infected with EIAV(FDDV). The results clearly show that these mutations generated by site-directed mutagenesis resulted in cloned viruses with enhanced resistance to serum-neutralizing antibodies that were also able to recognize parental viruses. The results of this study indicate that these mutations play an important role in the attenuation of the EIAV vaccine strains.

An EIAV field isolate reveals much higher levels of subtype variability than currently reported for the equine lentivirus family

Background

Equine infectious anemia virus (EIAV), a lentivirus that infects horses, has been utilized as an animal model for the study of HIV. Furthermore, the disease associated with the equine lentivirus poses a significant challenge to veterinary medicine around the world. As with all lentiviruses, EIAV has been shown to have a high propensity for genomic sequence and antigenic variation, especially in its envelope (Env) proteins. Recent studies have demonstrated Env variation to be a major determinant of vaccine efficacy, emphasizing the importance of defining natural variation among field isolates of EIAV. To date, however, published EIAV sequences have been reported only for cell-adapted strains of virus, predominantly derived from a single primary virus isolate, EIAV_Wyoming (EIAV_WY).

Results

We present here the first characterization of the Env protein of a natural primary isolate from Pennsylvania (EIAV_PA) since the widely utilized and referenced EIAV_WY strain. The data demonstrated that the level of EIAV_PA Env amino acid sequence variation, approximately 40% as compared to EIAV_WY, is much greater than current perceptions or published reports of natural EIAV variation between field isolates. This variation did not appear to give rise to changes in the predicted secondary structure of the proteins. While the EIAV_PA Env was serologically cross reactive with the Env proteins of the cell-adapted reference strain, EIAV_PV (derivative of EIAV_WY), the two variant Envs were shown to lack any cross neutralization by immune serum from horses infected with the respective virus strains.

Conclusion

Taking into account the significance of serum neutralization to universal vaccine efficacy, these findings are crucial considerations towards successful EIAV vaccine development and the potential inclusion of field isolate Envs in vaccine candidates.

Specificity of serum neutralizing antibodies induced by transient immune suppression of inapparent carrier ponies infected with a neutralization-resistant equine infectious anemia virus envelope strain

It has been previously reported that transient corticosteroid immune suppression of ponies experimentally infected with a highly neutralization resistant envelope variant of equine infectious anemia virus (EIAV), designated EIAVΔPND, resulted in the appearance of type-specific serum antibodies to the infecting EIAVΔPNDvirus. The current study was designed to determine if this induction of serum neutralizing antibodies was associated with changes in the specificity of envelope determinants targeted by serum antibodies or caused by changes in the nature of the antibodies targeted to previously defined surface envelope gp90 V3 and V4 neutralization determinants. To address this question, the envelope determinants of neutralization by post-immune suppression serum were mapped. The results demonstrated that the neutralization sensitivity to post-immune suppression serum antibodies mapped specifically to the surface envelope gp90 V3 and V4 domains, individually or in combination. Thus, these data indicate that the development of serum neutralizing antibodies to the resistant EIAVΔPNDwas due to an enhancement of host antibody responses caused by transient immune suppression and the associated increase in virus replication.

Multiple RNA splicing and the presence of cryptic RNA splice donor and acceptor sites may contribute to low expression levels and poor immunogenicity of potential DNA vaccines containing the env gene of equine infectious anemia virus (EIAV)

The env gene is an excellent candidate for inclusion in any DNA-based vaccine approach against equine infectious anemia virus (EIAV). Unfortunately, this gene is subjected to mutational pressure in E. coli resulting in the introduction of stop codons at the 5' terminus unless it is molecularly cloned using very-low-copy-number plasmid vectors. To overcome this problem, a mammalian expression vector was constructed based on the low-copy-number pLG338-30 plasmid. This permitted the production of full-length EIAV env gene clones (plcnCMVenv) from which low-level expression of the viral surface unit glycoprotein (gp90) was detected following transfection into COS-1 cells. Although this suggested the nuclear export of complete env mRNA moieties at least two additional polypeptides of 29 and 20kDa (probably Rev) were produced by alternative splicing events as demonstrated by the fact that their synthesis was prevented by mutational inactivation of EIAV env splice donor 3 (SD3) site. The plcnCMVenv did not stimulate immune responses in mice or in horses, whereas an env construct containing an inactivated SD3 site (plcnCMVDeltaSD3) did induce weak humoral responses against gp90 in mice. This poor immunogenicty in vivo was probably not related to the inherent antigenicity of the proteins encoded by these constructs but to some fundamental properties of EIAV env gene expression. Attempts to modify one of these properties by mutational inactivation of known viral RNA splice sites resulted in activation of previously unidentified cryptic SD and slice acceptor sites.

B-cell epitopes of the envelope glycoprotein of caprine arthritis-encephalitis virus and antibody response in infected goats

Goats infected with caprine arthritis-encephalitis virus (CAEV) develop high titres of antibodies to Env. Not only is no consistent neutralizing response found but anti-Env antibodies have even been associated with disease in infected goats. To identify the continuous antigenic determinants involved in this atypical anti-Env response, we mapped CAEV-CO Env by screening an epitope expression library with infected goat sera. In addition to the four previously described epitopes, seven novel antigenic sites were identified, of which five were located on the surface (SU) and two in the transmembrane (TM) subunits of Env. The SU antibody-binding domains located in the variable regions of the C-terminal part of the molecule (SU3 to SU5) showed the strongest reactivity and induced a rapid seroconversion in six experimentally infected goats. However, the response to these immunodominant epitopes did not appear to be associated with any neutralizing activity. The pattern of serum reactivity of naturally infected goats with these epitopes was restricted, suggesting a type-specific reaction. Interestingly, the reactivity of peptides representing SU5 sequences derived from CAEV field isolates varied with the geographical and/or breeding origin of the animals. This suggests that peptides corresponding to the immunodominant SU epitopes may well be useful in the serotyping of CAEV isolates. Furthermore, the identification of the CAEV Env epitopes will permit us to functionally dissect the antibody response and to address the role of anti-Env antibodies either in the protection from or in the pathogenesis of CAEV infection.

Evaluation of antibody parameters as potential correlates of protection or enhancement by experimental vaccines to equine infectious anemia virus

We previously demonstrated in trials of a variety of experimental vaccines to equine infectious anemia virus (EIAV) a remarkable spectrum of efficacy ranging from sterilizing protection to severe enhancement of virus replication and disease, depending on the immunization strategy used. This range of vaccine efficacy observed in vivo offers a unique opportunity for evaluating potential in vitro immune correlates of protection and enhancement. We describe here a comprehensive analysis and comparison of EIAV envelope-specific antibody responses elicited by attenuated, inactivated whole virus and envelope subunit vaccines to EIAV, and we evaluate the potential of in vitro antibody assays as correlates of protection or enhancement. Thus vaccine-induced serum antibody responses in experimentally immunized ponies at the day of challenge were assayed using a panel of quantitative, qualitative, and functional in vitro assays, including end-point titer of total and isotypic IgG, serum antibody avidity, conformational dependence, and serum neutralization. The results of these studies revealed substantial differences in the EIAV envelope-specific antibody responses elicited by the different vaccines, indicating the importance of envelope glycoprotein antigen presentation in determining the specificity of vaccine immunity. Although no single in vitro parameter provided a statistically significant correlate of protection or enhancement, the use of multiple parameters (titer, avidity index, and conformation ratio) could be used as a reliable correlate of vaccine protection and that the level of vaccine protection was closely associated with the development of mature antibody responses. These studies demonstrate the importance of using multiple antibody assays to evaluate lentiviral vaccine responses and emphasize the need for the development of new in vitro antibody assays that may provide more insight into vaccine protection and enhancement.

Inactivation of a common epitope responsible for the induction of antibody-dependent enhancement of HIV

BACKGROUND

The primary antigenic domain responsible for complement-mediated antibody-dependent enhancement (C'-ADE) of HIV and simian immunodeficiency virus resides in the principal immunodominant sequence of the transmembrane protein.

OBJECTIVE

To identify whether there are amino-acid residues common to the epitopes of the known enhancing human monoclonal antibodies (MAb), and to provide a structural model for this functional region present on the HIV envelope. Since our model predicts that this region is involved in the association of gp120 with gp41, this association was monitored for each mutant.

DESIGN

The binding of enhancing human MAb to point and deletion mutations within the enhancing domain was analyzed by two methods. The first analyzed binding to mutants expressed in COS cells: the second quantified the binding of four enhancing human MAb to each mutant gp160 versus wild-type control by enzyme-linked immunosorbent assay (ELISA).

METHODS

Site-directed mutagenesis was used to produce specific deletions and point mutants, which were expressed in COS cells. Binding of MAb 50-69 and V3-loop MAb 5F7 were visualized in the wild-type and each of the mutant constructs by immunohistochemistry. Quantitative evaluation of enhancing human MAb binding to each mutant versus wild-type was performed by ELISA. A model for the enhancing domain and its relationship to gp120 association with gp41 was provided by molecular dynamics and ligand docking methods.

RESULTS

All available enhancing human MAb known to bind to the principal immunodominant region of gp41 were unable to bind to deletions involving the disulfide loop, which in our molecular model provided the primary association site between gp120 and gp41. Point mutations in the loop blocked this association, but had a quantitatively smaller effect on the binding of the enhancing human MAb. A conservative W596Y mutation completely blocked the binding of all human MAb, but had no effect on gp120-gp41 association.

CONCLUSIONS

A variety of mutations within the primary C'-ADE domain inhibit binding of enhancing human MAb as well as blocking the association of gp120 and gp41. A conservative W596Y mutation blocks binding of all enhancing human MAb with retention of gp120-gp41 association. These data are important to the design of vaccines in which the primary enhancing epitope is disarmed to prevent the subsequent induction of an amnestic response that could lead to viral enhancement of infection. The retention of the gp120-gp41 association is postulated to yield an immunogen similar to natural infection for both subunit and genetic vaccines.