Rapid emergence of novel antigenic and genetic variants of equine infectious anemia virus during persistent infection

Survivors Remorse: antibody-mediated protection against HIV-1

It is clear that antibodies can play a pivotal role in preventing the transmission of HIV-1 and large efforts to identify an effective antibody-based vaccine to quell the epidemic. Shortly after HIV-1 was discovered as the cause of AIDS, the search for epitopes recognized by neutralizing antibodies became the driving strategy for an antibody-based vaccine. Neutralization escape variants were discovered shortly thereafter, and, after almost three decades of investigation, it is now known that autologous neutralizing antibody responses and their selection of neutralization resistant HIV-1 variants can lead to broadly neutralizing antibodies in some infected individuals. This observation drives an intensive effort to identify a vaccine to elicit broadly neutralizing antibodies. In contrast, there has been less systematic study of antibody specificities that must rely mainly or exclusively on other protective mechanisms, although non-human primate (NHP) studies as well as the RV144 vaccine trial indicate that non-neutralizing antibodies can contribute to protection. Here we propose a novel strategy to identify new epitope targets recognized by these antibodies for which viral escape is unlikely or impossible.

Identification and characterization of a common B-cell epitope on EIAV capsid proteins

The equine infectious anemia virus (EIAV) capsid protein (p26) is one of the major immunogenic proteins during EIAV infection and is widely used for the detection of EIAV antibodies in horses. However, few reports have described the use of EIAV-specific monoclonal antibodies (MAbs) in etiological and immunological detection. Previously, we developed an antigen capture enzyme-linked immunosorbent assay (AC-ELISA) for the quantification of the EIAV p26 protein level. However, the epitopes recognized by the MAbs were not identified, and the utilization of the MAbs needs to be evaluated. In this study, we characterized two monoclonal antibodies (9H8 and 1G11 MAbs) against EIAV p26. Two B-cell epitopes are located in amino acid residues, ⁷³NLDKIAEE⁸¹ (HE) and ¹⁹⁹KNAMRHLRPEDTLEEKMYAC²¹⁸ (GE) for the 9H8 and 1G11 MAbs, respectively. The 1G11 epitope (GE) varied among viruses isolated worldwide but can be recognized by anti-EIAV sera from different regions, including China, the USA, and Argentina. Meanwhile, 1G11 MAb could react with the mutants of almost all the EIAV strains. Furthermore, we found that the histidine at position 204 (H204), leucine at position 205 (L205), and aspartic acid at position 209 (D209) of EIAV p26 individually played pivotal roles in binding with the 1G11 MAb. Our results revealed that the GE peptide might be a common B-cell binding epitope of EIAV antibodies. This is also the first report to identify a broad-spectrum monoclonal antibody (1G11) against p26 of EIAV. These findings may provide a useful basis for the development of new diagnostic assays for EIAV.

Equine infectious anemia in 2014: live with it or eradicate it?

In the absence of an effective vaccine, the success of the test and removal approach for the control of equine infectious anemia (EIA) cannot be overstated, at least in those areas where testing has been traditionally routine. This article addresses 4 main aspects: what has been learned about EIA virus, host control of its replication, and inapparent carriers; international status regarding the control of EIA; diagnostic and laboratory investigation; and reducing the spread of blood-borne infections by veterinarians. An attempt is made to put these issues into practical contemporary perspectives for the equine practitioner.

Development, evaluation, and laboratory validation of immunoassays for the diagnosis of equine infectious anemia (EIA) using recombinant protein produced from a synthetic p26 gene of EIA virus

Equine infectious anemia (EIA)-a retroviral disease caused by equine infectious anemia virus (EIAV)-is a chronic, debilitating disease of horses, mules, and donkeys. EIAV infection has been reported worldwide and is recognized as pathogen of significant economic importance to the horse industry. This disease falls under regulatory control program in many countries including India. Control of EIA is based on identification of inapparent carriers by detection of antibodies to EIAV in serologic tests and "Stamping Out" policy. The current internationally accepted test for diagnosis of EIA is the agar gel immune-diffusion test (AGID), which detects antibodies to the major gag gene (p26) product. The objective of this study was to develop recombinant p26 based in-house immunoassays [enzyme linked immunosorbent assays (ELISA), and AGID] for EIA diagnosis. The synthetic p26 gene of EIAV was expressed in Escherichia coli and diagnostic potential of recombinant p26 protein were evaluated in ELISA and AGID on 7,150 and 1,200 equine serum samples, respectively, and compared with commercial standard AGID kit. The relative sensitivity and specificity of the newly developed ELISA were 100 and 98.6 %, respectively. Whereas, relative sensitivity and specificity of the newly developed AGID were in complete agreement in respect to commercial AGID kit. Here, we have reported the validation of an ELISA and AGID on large number of equine serum samples using recombinant p26 protein produced from synthetic gene which does not require handling of pathogenic EIAV. Since the indigenously developed reagents would be economical than commercial diagnostic kit, the rp26 based-immunoassays could be adopted for the sero-diagnosis and control of EIA in India.

N-Glycans on the Nipah virus attachment glycoprotein modulate fusion and viral entry as they protect against antibody neutralization

Nipah virus (NiV) is the deadliest known paramyxovirus. Membrane fusion is essential for NiV entry into host cells and for the virus' pathological induction of cell-cell fusion (syncytia). The mechanism by which the attachment glycoprotein (G), upon binding to the cell receptors ephrinB2 or ephrinB3, triggers the fusion glycoprotein (F) to execute membrane fusion is largely unknown. N-glycans on paramyxovirus glycoproteins are generally required for proper protein conformational integrity, transport, and sometimes biological functions. We made conservative mutations (Asn to Gln) at the seven potential N-glycosylation sites in the NiV G ectodomain (G1 to G7) individually or in combination. Six of the seven N-glycosylation sites were found to be glycosylated. Moreover, pseudotyped virions carrying these N-glycan mutants had increased antibody neutralization sensitivities. Interestingly, our results revealed hyperfusogenic and hypofusogenic phenotypes for mutants that bound ephrinB2 at wild-type levels, and the mutant's cell-cell fusion phenotypes generally correlated to viral entry levels. In addition, when removing multiple N-glycans simultaneously, we observed synergistic or dominant-negative membrane fusion phenotypes. Interestingly, our data indicated that 4- to 6-fold increases in fusogenicity resulted from multiple mechanisms, including but not restricted to the increase of F triggering. Altogether, our results suggest that NiV-G N-glycans play a role in shielding virions against antibody neutralization, while modulating cell-cell fusion and viral entry via multiple mechanisms.

Molecular detection, epidemiology, and genetic characterization of novel European field isolates of equine infectious anemia virus

The application of molecular diagnostic techniques along with nucleotide sequence determination to permit contemporary phylogenetic analysis of European field isolates of equine infectious anemia virus (EIAV) has not been widely reported. As a result, of extensive testing instigated following the 2006 outbreak of equine infectious anemia in Italy, 24 farms with a history of exposure to this disease were included in this study. New PCR-based methods were developed, which, especially in the case of DNA preparations from peripheral blood cells, showed excellent correlation with OIE-approved agar gel immunodiffusion (AGID) tests for identifying EIAV-infected animals. In contrast, the OIE-recommended oligonucleotide primers for EIAV failed to react with any of the Italian isolates. Similar results were also obtained with samples from four Romanian farms. In addition, for the first time complete characterization of gag genes from five Italian isolates and one Romanian isolate has been achieved, along with acquisition of extensive sequence information (86% of the total gag gene) from four additional EIAV isolates (one Italian and three Romanian). Furthermore, in another 23 cases we accomplished partial characterization of gag gene sequences in the region encoding the viral matrix protein. Analysis of this information suggested that most Italian isolates were geographically restricted, somewhat reminiscent of the "clades" described for human immunodeficiency virus type 1 (HIV-1). Collectively this represents the most comprehensive genetic study of European EIAV isolates conducted to date.

Mutant spectra in virus behavior

RNA viruses replicate as complex mutant spectra, also termed ‘mutant clouds’, known as viral quasispecies. While this is a widely observed viral population structure, it is less known that a number of biologically relevant features of this important group of viral pathogens depend on (or are strongly influenced by) the complexity and composition of mutant spectra. Among them, fitness increase or decrease depending on intrapopulation complementation or interference, selection triggered by memory genomes, pathogenic potential of viruses, disease evolution and the response to antiviral treatments. Quasispecies represent the recognition of complex behavior in viruses, and it is an oversimplification to equate such a population structure with the classic polymorphism of population biology. Darwinian principles acting on genome collectivities that replicate with high error rates provide a unique population structure prone to flexible and largely unpredictable behavior.

The ever-expanding diversity of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) virus appeared 20 years ago as the cause of a new disease in swine. Today PRRS is the most significant swine disease worldwide in spite of intensive immunological interventions. The virus showed remarkable genetic variation with two geographically distinct genotypes at the time of its discovery, indicating the possibility of prolonged evolutionary divergence prior to its appearance as a swine pathogen. Since then, both type 1 and type 2 have spread geographically, radiated genetically, and acquired new phenotypic characteristics, especially increased virulence. Here, we explore various hypotheses that might account for rapid expansion and diversification of PRRSV, including mechanisms specific to PRRSV and other arteriviruses, cellular modification processes, and immunological selection. Phylogenetic analysis of PRRSV has provided a broadly applicable means to relate diverse isolates, but it does not explain biological variation in virulence or immunological cross-protection. We present other methods of classification and review their limitations. Major questions about PRRSV remain unanswered despite intensive investigation, suggesting that the interaction of PRRSV with pigs involves novel biological processes that may be relevant to other RNA virus and host interactions.

Selection of a rare neutralization-resistant variant following passive transfer of convalescent immune plasma in equine infectious anemia virus-challenged SCID horses

Vaccines preventing HIV-1 infection will likely elicit antibodies that neutralize diverse strains. However, the capacity for lentiviruses to escape broadly neutralizing antibodies (NAbs) is not completely understood, nor is it known whether NAbs alone can control heterologous infection. Here, we determined that convalescent immune plasma from a horse persistently infected with equine infectious anemia virus (EIAV) neutralized homologous virus and several envelope variants containing heterologous principal neutralizing domains (PND). Plasma was infused into young horses (foals) affected with severe combined immunodeficiency (SCID), followed by challenge with a homologous EIAV stock. Treated SCID foals were protected against clinical disease, with complete prevention of infection occurring in one foal. In three SCID foals, a novel neutralization-resistant variant arose that was found to preexist at a low frequency in the challenge inoculum. In contrast, SCID foals infused with nonimmune plasma developed acute disease associated with high levels of the predominant challenge virus. Following transfer to an immunocompetent horse, the neutralization-resistant variant induced a single febrile episode and was subsequently controlled in the absence of type-specific NAb. Long-term control was associated with the presence of cytotoxic T lymphocytes (CTL). Our results demonstrate that immune plasma with neutralizing activity against heterologous PND variants can prevent lentivirus infection and clinical disease in the complete absence of T cells. Importantly, however, rare neutralization-resistant envelope variants can replicate in vivo under relatively broad selection pressure, highlighting the need for protective lentivirus vaccines to elicit NAb responses with increased breadth and potency and/or CTL that target conserved epitopes.

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.

Systematic epitope analysis of the p26 EIAV core protein

The major core protein of equine infectious anemia virus (EIAV), p26, is one of the primary immunogenic structural proteins during a persistent infection of horses and is highly conserved among antigenically variants of viral isolates. In order to investigate its immune profile in more detail for a better diagnostic, an epitope mapping was carried out by means of two libraries of overlapping peptide fragments prepared by simultaneous and parallel SPPS on derivatized cellulose membranes (SPOT synthesis). Polyclonal equine sera from infected horses were used for the biological assay. Particularly two promising continuous epitopes (NAMRHL and MYACRD) were localized on the C-terminal extreme of p26, region 194-222. A cyclic synthetic fragment of 29 amino acid residues containing the identified epitopes was designed and studied. A significant conformational change towards a helical structure was observed when the peptide was cyclized by a bridge between Cys198 and Cys218. This observation correlated with an improvement of its ability to be recognized by specific antibodies in an EIA (Enzyme-linked Immunosorbent assay). These results suggest that the conformationally restricted synthetic antigen adequately mimics the native structure of this region of p26 core protein.

Serodiagnosis of equine infectious anemia by agar gel immunodiffusion and ELISA using a recombinant p26 viral protein expressed in Escherichia coli as antigen

We used a p26 recombinant protein (p26r) from equine infectious-anemia virus (EIAV) expressed in Escherichia coli as antigen to standardize an agar-gel immunodiffusion (AGIDp26r) test and an indirect ELISA (ELISAp26r) for the detection of antibodies against EIAV in 720 equine sera from Brazil. We evaluated the tests's relative diagnostic sensitivities (relSe) and relative diagnostic specificities (relSp) against a commercial AGID kit (Idexx, USA). We used three sera panels: panel A--196 AGID-negative sera from an AIE non-endemic controlled area; panel B--194 AGID-negative sera from an AIE endemic area and panel C--330 AGID-positive sera from an AIE endemic area. ELISAp26r cut-off value was defined with TG-ROC using sera from panels A and C. AGIDp26r showed an agreement of 100% with the commercial kit. When applied to sera from panels A and C, ELISAp26r showed an agreement of 100% with the kit, but, although relSe was 100% for panel C, the ELISAp26r had relSp of 93.3%.

Immune selection of equine infectious anemia virus env variants during the long-term inapparent stage of disease

The principal neutralizing domain (PND) of equine infectious anemia virus (EIAV) is located in the V3 region of SU. Genetic variation in the PND is considered to play an important role in immune escape and EIAV persistence; however, few studies have characterized genetic variation in SU during the inapparent stage of disease. To better understand the mechanisms of virus persistence, we undertook a longitudinal study of SU variation in a pony experimentally inoculated with the virulent EIAV(Wyo). Viral RNA isolated from the inoculum and from sequential sera samples was amplified by RT-PCR, cloned, and individual clones were sequenced. Of the 147 SU clones obtained, we identified 71 distinct V3 variants that partitioned into five major non-overlapping groups, designated PND-1 to PND-5, which segregated with specific stages of clinical disease. Genotypes representative of each group were inserted into an infectious molecular clone, and chimeric viruses were tested for susceptibility to neutralization by autologous sera from successive times post-infection. Overall, there was a trend for increasing resistance to neutralizing antibody during disease progression. The PND genotype associated with recrudescence late in infection was resistant to both type-specific and broadly neutralizing antibody, and displayed a reduced replication phenotype in vitro. These findings indicate that neutralizing antibody exerts selective pressure throughout infection and suggest that viral strategies of immune evasion and persistence change in the face of an evolving and maturing host immune response.

N-glycans on Nipah virus fusion protein protect against neutralization but reduce membrane fusion and viral entry

Nipah virus (NiV) is a deadly emerging paramyxovirus. The NiV attachment (NiV-G) and fusion (NiV-F) envelope glycoproteins mediate both syncytium formation and viral entry. Specific N-glycans on paramyxovirus fusion proteins are generally required for proper conformational integrity and biological function. However, removal of individual N-glycans on NiV-F had little negative effect on processing or fusogenicity and has even resulted in slightly increased fusogenicity. Here, we report that in both syncytium formation and viral entry assays, removal of multiple N-glycans on NiV-F resulted in marked increases in fusogenicity (>5-fold) but also resulted in increased sensitivity to neutralization by NiV-F-specific antisera. The mechanism underlying the hyperfusogenicity of these NiV-F N-glycan mutants is likely due to more-robust six-helix bundle formation, as these mutants showed increased fusion kinetics and were more resistant to neutralization by a fusion-inhibitory reagent based on the C-terminal heptad repeat region of NiV-F. Finally, we demonstrate that the fusogenicities of the NiV-F N-glycan mutants were inversely correlated with the relative avidities of NiV-F's interactions with NiV-G, providing support for the attachment protein "displacement" model of paramyxovirus fusion. Our results indicate that N-glycans on NiV-F protect NiV from antibody neutralization, suggest that this "shielding" role comes together with limiting cell-cell fusion and viral entry efficiencies, and point to the mechanisms underlying the hyperfusogenicity of these N-glycan mutants. These features underscore the varied roles that N-glycans on NiV-F play in the pathobiology of NiV entry but also shed light on the general mechanisms of paramyxovirus fusion with host cells.

The S2 accessory gene of equine infectious anemia virus is essential for expression of disease in ponies

Equine infectious anemia virus (EIAV) is a macrophage-tropic lentivirus that persistently infects horses and causes a disease that is characterized by periodic episodes of fever, thrombocytopenia, and viremia. EIAV encodes only four regulatory/accessory genes, (tat, rev, ttm, and S2) and is the least genetically complex of all known lentiviruses. We sought to determine the role of the EIAV S2 accessory gene of EIAV by introducing mutations that would prevent S2 expression on the p19/wenv17 infectious molecular clone. Virus derived from the p19/wenv17 molecular clone is highly virulent and routinely fatal when given in high doses (J. Virol. 72 (1998) 483). In contrast, an S2 deletion mutant on the p19/wenv17 background is unable to induce acute disease and plasma virus loads were reduced by 2.5 to 4.0 logs at 15 days post-infection. The S2 deleted virus failed to produce any detectable clinical signs during a 5-month observation period. These results demonstrate that S2 gene expression is essential for disease expression of EIAV.

Early detection of dominant Env-specific and subdominant Gag-specific CD8+ lymphocytes in equine infectious anemia virus-infected horses using major histocompatibility complex class I/peptide tetrameric complexes

Cytotoxic T lymphocytes (CTL) are critical for control of lentiviruses, including equine infectious anemia virus (EIAV). Measurement of equine CTL responses has relied on chromium-release assays, which do not allow accurate quantitation. Recently, the equine MHC class I molecule 7-6, associated with the ELA-A1 haplotype, was shown to present both the Gag-GW12 and Env-RW12 EIAV CTL epitopes. In this study, 7-6/Gag-GW12 and 7-6/Env-RW12 MHC class I/peptide tetrameric complexes were constructed and used to analyze Gag-GW12- and Env-RW12-specific CTL responses in two EIAV-infected horses (A2164 and A2171). Gag-GW12 and Env-RW12 tetramer-positive CD8+ cells were identified in nonstimulated peripheral blood mononuclear cells as early as 14 days post-EIAV inoculation, and frequencies of tetramer-positive cells ranged from 0.4% to 6.7% of nonstimulated peripheral blood CD8+ cells during the 127-day study period. Although both horses terminated the initial viremic peak, only horse A2171 effectively controlled viral load. Neutralizing antibody was present during the initial control of viral load in both horses, but the ability to maintain control correlated with Gag-GW12-specific CD8+ cells in A2171. Despite Env-RW12 dominance, Env-RW12 escape viral variants were identified in both horses and there was no correlation between Env-RW12-specific CD8+ cells and control of viral load. Although Gag-GW12 CTL escape did not occur, a Gag-GW12 epitope variant arose in A2164 that was recognized less efficiently than the original epitope. These data indicate that tetramers are useful for identification and quantitation of CTL responses in horses, and suggest that the observed control of EIAV replication and clinical disease was associated with sustained CTL recognition of Gag-specific epitopes.

Molecular analysis of the proviral DNA of equine infectious anemia virus in mules in Greece

Molecular analysis of the regulatory and structurally important genetic segments of equine infectious anemia virus (EIAV) in mules is presented. We have previously reported clinicopathological and laboratory findings in mules infected with EIAV, both naturally and after experimental inoculation. In this study the fragment coding for integrase, gp90, tat and the fusion domain of gp45 of the proviral genome from these animals was sequenced and compared with one another and with that of EIAV strains already published in the literature. Significant variations were observed mainly in the sequences of the gp90 surface protein. In the two wild type sequences, there were substitutions in the V5 hypervariable domain of this protein. In the sequences of the experimentally inoculated animals and the donor strain, variations were due to insertions/duplications in the V3 principal neutralizing domain (PND) and substitutions in the V5 hypervariable domain. Finally, when compared with the already published strains, the wild type sequences had single amino acid substitutions across the whole protein and multiple substitutions in the V4-V6 variable domains. In general, the two Greek wild type sequences were closer to two of the American strains (WSU5 and Massachusetts), than to the two Japanese (V26 and V70) or the third American strain (Wyoming_wi) used in this study.

Development of recombinant capsid antigen/transmembrane epitope fusion proteins for serological diagnosis of animal lentivirus infections

Among animal lentiviruses, Feline immunodeficiency virus (FIV), Equine infectious anaemia virus (EIAV) and Small ruminant lentiviruses (SRLV) are important pathogens associated with a variety of clinical pictures including immunodeficiency, anaemia, arthritis, pneumonia. The detection of viral antibody response represents a practical diagnostic approach in all lentivirus infections since they remain detectable long life. Capsid antigen (CA) is the major viral core protein and specific antibodies against this antigen are usually first recognised in infected sheep, goat and horse, remaining detectable for long period. Transmembrane (TM) domain of envelope glycoprotein contains a well conserved motif known to form an immunodominant epitope in several lentiviruses. In this study a simple strategy was developed to express the entire CA and the TM epitope in a single fusion protein from equine, feline and small ruminant lentiviruses in prokaryotic system and evaluated the diagnostic utility of a purified preparation in an indirect ELISA for each of the three infections. Results demonstrate that, for FIV and SRLV infections, the combination of CA and TM fractions increases the sensitivity of diagnostic tests based only on CA. The corresponding CA/TM antigen from EIAV showed excellent agreement with Coggins test.

Adaptive immunity is the primary force driving selection of equine infectious anemia virus envelope SU variants during acute infection

Equine infectious anemia virus (EIAV) is a lentivirus that causes persistent infection in horses. The appearance of antigenically distinct viral variants during recurrent viremic episodes is thought to be due to adaptive immune selection pressure. To test this hypothesis, we evaluated envelope SU cloned sequences from five severe combined immunodeficient (SCID) foals infected with EIAV. Within the SU hypervariable V3 region, 8.5% of the clones had amino acid changes, and 6.4% had amino acid changes within the known cytotoxic T lymphocyte (CTL) epitope Env-RW12. Of all the SU clones, only 3.1% had amino acid changes affecting potential N-linked glycosylation sites. In contrast, a much higher degree of variation was evident in SU sequences obtained from four EIAV-infected immunocompetent foals. Within V3, 68.8% of the clones contained amino acid changes, and 50% of the clones had amino acid changes within the Env-RW12 CTL epitope. Notably, 31.9% of the clones had amino acid changes affecting one or more glycosylation sites. Marked amino acid variation occurred in cloned SU sequences from an immune-reconstituted EIAV-infected SCID foal. Of these clones, 100% had amino acid changes within V3, 100% had amino acid changes within Env-RW12, and 97.5% had amino acid changes affecting glycosylation sites. Analysis of synonymous and nonsynonymous nucleotide substitutions revealed statistically significant differences between SCID and immunocompetent foals and between SCID foals and the reconstituted SCID foal. Interestingly, amino acid selection at one site occurred independently of adaptive immune status. Not only do these data indicate that adaptive immunity primarily drives the selection of EIAV SU variants, but also they demonstrate that other selective forces exist during acute infection.

A longitudinal assessment of autologous neutralizing antibodies in children perinatally infected with human immunodeficiency virus type 1

The evolution of autologous neutralizing antibodies to sequential human immunodeficiency virus type 1 (HIV-1) isolates was studied in a population of 16 children who were perinatally infected with human immunodeficiency virus type 1. The cohort included seven children with rapid disease progression (RP) and nine who had nonrapid disease progression (NRP). Four of the NRP after 6 months of age harbored viruses that could be neutralized by antibodies found in autologous contemporaneous plasma (titers up to 1:640) while the majority of longitudinally collected viruses from five NRP were resistant to neutralization with contemporaneous plasma. Because of their shorter survival, only five of the RP had studies after 6 months of age; three of the five had neutralizing antibodies to contemporaneous virus isolates and the highest titers were 1:20. The highest titers in RP (up to 1:160) occurred in specimens obtained prior to 6 months of age but these were most likely of maternal origin. Most isolates that were not neutralized by contemporaneous plasma could be neutralized using noncontemporaneous plasma obtained months to years after the virus isolates. These autologous noncontemporaneous neutralizing antibodies persisted for years, had titers that were higher to viruses isolated at younger ages, and were generally more potent in children with NRP than RP. Demonstration of neutralizing antibodies to viruses previously resistant to neutralization by contemporaneous plasma suggests a continuous evolution of virus variants in vivo that are able to escape the effect of neutralizing antibodies.

Viral persistence: HIV's strategies of immune system evasion

In contrast to most animal viruses, infection with the human and simian immunodeficiency viruses results in prolonged, continuous viral replication in the infected host. Remarkably, viral persistence is not thwarted by the presence of apparently vigorous, virus-specific immune responses. Several factors are thought to contribute to persistent viral replication, most notably the destruction of virus-specific T helper cells, the emergence of antigenic escape variants, and the expression of an envelope complex that structurally minimizes antibody access to conserved epitopes. Not as well understood, though potentially important, is the ability of at least one viral encoded protein (Nef) to prevent presentation of viral antigens in the context of major histocompatibility complex. The future success of antiviral therapies and vaccination strategies may depend largely on understanding how and to what degree each of these factors (and presumably others) contributes to immune evasion.

Genetic and biological variation in equine infectious anemia virus Rev correlates with variable stages of clinical disease in an experimentally infected pony

Genetic and biological variation in the regulatory protein Rev of equine infectious anemia virus (EIAV) were examined throughout a clinically dynamic disease course of an experimentally infected pony. Following infection with the virulent EIAV(Wyo), the pony underwent a variable disease course, including an acute fever episode at 12 days postinfection (DPI), multiple recurrent fever episodes until 135 DPI, a prolonged subclinical period, and two late fever episodes. Viral RNA was isolated from the inoculum and sequential sera samples, and the rev exon 2/gp45 overlapping ORFs were amplified, cloned, and sequenced. Novel variants were found throughout infection, and genetic analyses indicated that both the Rev and gp45 ORFs were under selective pressure. The Rev variant predominant in the inoculum, R1, remained predominant during the early periods following infection (until 35 DPI); however, R1 was replaced by new predominant variants during the recurrent fever period (67-135 DPI). R1 reemerged as the predominant variant during the afebrile period, but a new predominant variant, R93, was associated with the late fever episodes. Rev variants predominant during recurrent febrile and late-febrile periods had significantly higher Rev-mediated nuclear export activity than the variants predominant during the acute and afebrile periods. Statistical correlation was found between Rev activity and different stages of clinical disease. Together, these results suggest that genetic and biological variation in rev may be a contributing factor in EIAV disease progression.

Immune responses and viral replication in long-term inapparent carrier ponies inoculated with equine infectious anemia virus

Persistent infection of equids by equine infectious anemia virus (EIAV) is typically characterized by a progression during the first year postinfection from chronic disease with recurring disease cycles to a long-term asymptomatic infection that is maintained indefinitely. The goal of the current study was to perform a comprehensive longitudinal analysis of the course of virus infection and development of host immunity in experimentally infected horses as they progressed from chronic disease to long-term inapparent carriage. We previously described the evolution of EIAV genomic quasispecies (C. Leroux, C. J. Issel, and R. C. Montelaro, J. Virol. 71:9627-9639, 1997) and host immune responses (S. A. Hammond, S. J. Cook, D. L. Lichtenstein, C. J. Issel, and R. C. Montelaro, J. Virol. 71:3840-3852, 1997) in four experimentally infected ponies during sequential disease episodes associated with chronic disease during the first 10 months postinfection. In the current study, we extended the studies of these experimentally infected ponies to 3 years postinfection to characterize the levels of virus replication and development of host immune responses associated with the progression from chronic disease to long-term inapparent infection. The results of these studies revealed over a 10(3)-fold difference in the steady-state levels of plasma viral RNA detected during long-term inapparent infection that correlated with the severity of chronic disease, indicating different levels of control of virus replication during long-term inapparent infections. Detailed analyses of antibody and cellular immune responses in all four ponies over the 3-year course of infection revealed a similar evolution during the first year postinfection of robust humoral and cellular immunity that then remained relatively constant during long-term inapparent infection. These observations indicate that immune parameters that have previously been correlated with EIAV vaccine protection fail to provide reliable immune correlates of control of virus replication or clinical outcome in experimental infections. Thus, these data emphasize the differences between immunity to virus exposure and immune control of an established viral infection and further emphasize the need to develop and evaluate novel immunoassays to define reliable immune correlates to vaccine and infection immunity, respectively.

Effects of long terminal repeat sequence variation on equine infectious anemia virus replication in vitro and in vivo

The long terminal repeat (LTR) is reported to be one of the most variable portions of the equine infectious anemia virus (EIAV) genome. To date, however, no information is available on the effects of observed sequence variations on viral replication properties, despite a widespread assumption of the biological importance of EIAV LTR variation. EIAV LTR sequence variability is confined mostly to a small portion of the enhancer within the U3 segment of the LTR. Analysis of published EIAV LTR sequences revealed six different types of LTR based on the pattern of putative transcription factor motifs within the variable region of the enhancer. To test directly the significance of LTR variation, the in vitro and in vivo replication properties of two variant LTR species were investigated using two isogenic viruses, EIAV(19-2) and EIAV(19-2-6A), differing only within the enhancer region. The results of these studies demonstrated that the two variants replicated with similar kinetics and to equal levels in cultured equine fibroblasts or in equine macrophage, the natural target cell of EIAV, even after prolonged serial passage in the latter cell type. Furthermore, EIAV(19-2) and EIAV(19-2-6A) variants demonstrated similar replication levels in experimentally infected ponies. However, ponies infected with EIAV(19-2-6A) exhibited a rapid switch in the prevalent LTR type, such that by 112 days postinfection, no original-LTR-type viruses were evident. This specific and rapid shift in LTR quasispecies indicates an in vivo selection that is not reflected in simple in vitro replication rates, suggesting undefined selection pressures in vivo that drive LTR variation during persistent EIAV infection.

Application of equine infectious anemia virus core proteins produced in a baculovirus expression system to serological diagnosis

Equine infectious anemia virus (EIAV) core proteins were obtained from a baculovirus expression system. Recombinant baculoviruses (rBVs) highly expressed the Gag precursor and p26 antigens in an rBV-infected Sf21 cell culture supernatant. Enzyme-linked immunosorbent assay (ELISA) and agar gel immunodiffusion (AGID) were conducted using the expressed proteins to detect antibodies from experimentally infected horses. The expressed antigens showed low background levels, high specificity and sensitivity in ELISA and AGID. The results of the serological tests using the expressed antigens were identical to those using a manufactured trial antigen. rBVs containing gag and p26 genes were found to express high quality and large quantities of Gag and p26 antigens, respectively. The antigens were quite useful for detecting anti-EIAV antibodies from virus-infected horses.

Tracking members of the simian immunodeficiency virus deltaB670 quasispecies population in vivo at single-cell resolution

Genetically distinct lentiviruses constitute a quasispecies population that can evolve in response to selective forces. To move beyond characterization of the population as a whole to the behavior of individual members, we devised an in situ hybridization approach that uses genotype-specific probes. We used probes that detect simian immunodeficiency viruses (SIV) that differ in sequence in the V1 region of the surface envelope glycoprotein (env) gene to investigate the replication and cellular tropisms of four viral variants in the tissues of infected rhesus macaques. We found that the V1 genotypic variants replicated in spatially defined patterns and to different extents at each anatomic site. The two variants that replicated most extensively in animals with AIDS were detected in both macrophages and T lymphocytes in tissues. By extension of this approach, it will be possible to investigate the role of individual lentiviruses in a quasispecies in pathogenesis and to evaluate the effects of antiviral or immunotherapeutic treatment on select members of a quasispecies.

Genetic variation of envelope gp90 gene of equine infectious anemia virus isolated from an experimentally infected horse

Six strains of equine infectious anemia virus (EIAV) were recovered from febrile and non-febrile stages of a horse experimentally infected with the P337-V70 strain given once to a horse. The env gp90 genes of the isolates, the P337-V70 and P337-V26, avirulent virus derived from the P337-V70 strain, were sequenced. A comparison of the gp90 gene sequences revealed that amino acid variations among the viruses tested showed as high as 8.2 to 11.5%. In addition, the comparison also indicated that the isolates that recovered from the non-febrile stage were contained in nucleotide insertions in the principal neutralizing domain (PND) region. The insertions were arranged regularly with smaller segments. The nucleotide sequence of the P337-V26 gp90 gene was found to contain a six-nucleotides insertion and seven nucleotide substitutions outside the PND region, when compared with that of the P337-V70 strain.

Novel and dynamic evolution of equine infectious anemia virus genomic quasispecies associated with sequential disease cycles in an experimentally infected pony

We have investigated the genetic evolution of three functionally distinct regions of the equine infectious anemia virus (EIAV) genome (env, rev, and long terminal repeat) during recurring febrile episodes in a pony experimentally infected with a well-characterized reference biological clone designated EIAV(PV). Viral populations present in the plasma of an EIAV(PV)-infected pony during sequential febrile episodes (18, 34, 80, 106, and 337 days postinfection) were amplified from viral RNA, analyzed, and compared to the inoculated strain. The comparison of the viral quasispecies showed that the inoculated EIAV(PV) quasispecies were all represented during the first febrile episode, but entirely replaced at the time of the second febrile episode, and that new predominant quasispecies were associated with each subsequent cycle of disease. One of the more surprising results was the in vivo generation of large deletion (up to 15 amino acids) in the principal neutralizing domain (PND) of gp90 during the third febrile episode. This deletion did not alter the competence for in vitro replication as shown by the analysis of a env chimeric clone with a partially deleted PND and did not altered the fitness of the virus in vivo, since this partially deleted envelope became the major population during the fourth febrile episode. Finally, we showed that the amino acid mutations were not randomly distributed but delineated eight variables regions, V1 to V8, with V3 containing the PND region. These studies provide the first detailed description of the evolution of EIAV genomic quasispecies during persistent infection and reveal new insights into the genetics and potential mechanisms of lentivirus genomic variation.

Suppression of Megakaryocyte Colony Growth by Plasma From Foals Infected With Equine Infectious Anemia Virus

Foals infected with equine infectious anemia virus become thrombocytopenic 7 to 20 days after virus inoculation, and within a few days following the onset of detectable viremia. The thrombocytopenia is associated with suppression of platelet production. Possible mediators of suppression of thrombopoiesis include tumor necrosis factor-α (TNF-α) and transforming growth factor-β (TGF-β), cytokines that are released during inflammation. To assess effects of plasma or serum from infected foals on megakaryocyte (MK) growth and maturation in vitro, equine low-density bone marrow cells were cultured for clonogenic and ploidy assays. Neutralizing antibodies to TNF-α and TGF-β were added to cultures to determine the contribution of these cytokines to suppression of thrombopoiesis. Plasma from the immediately pre-thrombocytopenia (Pre-Tp) period significantly reduced MK colony numbers. This suppression was partially reversed upon antibody neutralization of plasma TNF-α, TGF-β, or both. There were no differences in ploidy distribution of MK grown in the presence of preinfection serum compared with those grown in the presence of Pre-Tp serum. These results indicate that TNF-α and TGF-β may contribute to suppression of MK proliferation and represent likely factors in the pathogenesis of thrombocytopenia.

Suppression of Megakaryocyte Colony Growth by Plasma From Foals Infected With Equine Infectious Anemia Virus

Foals infected with equine infectious anemia virus become thrombocytopenic 7 to 20 days after virus inoculation, and within a few days following the onset of detectable viremia. The thrombocytopenia is associated with suppression of platelet production. Possible mediators of suppression of thrombopoiesis include tumor necrosis factor-α (TNF-α) and transforming growth factor-β (TGF-β), cytokines that are released during inflammation. To assess effects of plasma or serum from infected foals on megakaryocyte (MK) growth and maturation in vitro, equine low-density bone marrow cells were cultured for clonogenic and ploidy assays. Neutralizing antibodies to TNF-α and TGF-β were added to cultures to determine the contribution of these cytokines to suppression of thrombopoiesis. Plasma from the immediately pre-thrombocytopenia (Pre-Tp) period significantly reduced MK colony numbers. This suppression was partially reversed upon antibody neutralization of plasma TNF-α, TGF-β, or both. There were no differences in ploidy distribution of MK grown in the presence of preinfection serum compared with those grown in the presence of Pre-Tp serum. These results indicate that TNF-α and TGF-β may contribute to suppression of MK proliferation and represent likely factors in the pathogenesis of thrombocytopenia.

Virus load and antigenic diversity

In this paper, we analyse mathematical models for the interaction between virus replication and immune responses. We show that the immune system can provide selection pressure for or against viral diversity. The paper provides new insights into the relationship between virus load (=the abundance of virus in an infected individual) and antigenic diversity. Antigenic variation can increase virus load during infections, but the correlation between load and diversity in comparisons among different infected individuals can be positive or negative, depending on whether individuals differ in their cross-reactive or strain-specific immune responses. We derive two models: our first model applies to any replicating parasite that can escape from immune responses; our second model includes immune function impairment, and specifically describes infections with the human immunodeficiency virus (HIV).

In vivo dynamics of equine infectious anemia viruses emerging during febrile episodes: insertions/duplications at the principal neutralizing domain

Equine infectious anemia virus (EIAV) is a good model for studying mechanisms generating escaped retrovirus variants. We previously sequenced the entire gp90-encoding region of 22 cDNA clones obtained from five antigenically distinct isolates (F1V to F5V) recovered during febrile episodes in horse 493 experimentally infected with the Japanese virulent EIAV strain V70. The results showed that the mutations occurred in the principal neutralizing domain (PND) by insertions/duplications. In this study, we further characterized the PND of virus isolates sequentially recovered during 22 febrile episodes in seven horses newly infected with V70 or one of the V70-derived variants. Sequencing of 70 cDNA clones derived from the 22 episodes confirmed the generation of various new viral quasispecies with insertions/duplications in the PND. Although the insertion/duplication sequences in a total of 92 cDNA clones were extensively heterogeneous, we hypothesized that all the insertions/duplications occurred during reverse transcription from viral genomic RNA to minus strand DNA. The insertion/duplication regions were derived from a part of the PND sequence, which consisted of five small units. These small units, some with various substitutions and/or deletions, were also generated, especially in regions with insertions/duplications. Of particular note was that all these virus variants, except for two cDNA variants, were generated by essentially four different duplication pathways. Thus, these results extend the significance of insertions/duplications in the PND to the novel generation of EIAV in vivo during febrile episodes.

Cloning, expression, purification, and characterization of the major core protein (p26) from equine infectious anemia virus

The gene coding for the major core protein (p26) of the lentivirus equine infectious anemia virus (EIAV) was cloned from EIAV infected serum, expressed in E. coli, and the resultant protein purified to electrophoretic homogeneity. The protein was expressed in a soluble form and was purified by conventional protein separation methods. When analyzed by SDS-PAGE, under both reducing and non-reducing conditions, the purified protein migrated as a 26 kDa monomer. Recombinant p26 (rp26), therefore, does not contain any intermolecular disulfide bond. Gel filtration chromatography also indicated that the protein occurs as a monomer in solution. Labeling of free sulphydryl groups with [1-14C]iodoacetamide suggests that none of the three cysteine residues of rp26 is involved in intramolecular disulfide bonds. The circular dichroism spectrum of rp26 was consistent with the following assignment of secondary structure elements: 51% a-helix, 15% beta-turn, and 34% aperiodic. Fluorescencespectroscopy revealed that the three tryptophan residues in rp26 occupy two different environments. These data support the conclusion that the recombinant protein is folded into an ordered and probably native conformation. Immunoblotting and enzyme immunoassay with EIAV infected sera demonstrated that recombinant p26 protein may be useful for diagnostic purposes.

Phorbol ester stimulation of equine macrophage cultures alters expression of equine infectious anemia virus

Equine infectious anemia virus (EIAV) is a lentivirus that replicates predominantly in mature tissue macrophages. Viral expression is strongly influenced by the state of differentiation of the host cell. While blood monocytes can be infected, viral transcription is limited until the cell differentiates into a mature macrophage. Activation of mature macrophages infected with EIAV might also alter viral expression, presumably through binding of cellular transcription factors to viral nucleic acid sequences within the long terminal repeat (LTR). Using DNA amplification techniques, we compared LTR sequences of U.S. field strains of EIAV to sequences of a laboratory adapted strain of the virus. All field strain sequences were more closely related to Wyoming strain than to the Malmquist laboratory adapted strain or a previously sequenced infectious molecular clone of EIAV. Primary equine monocyte-derived macrophage cultures were infected with virulent and avirulent strains of EIAV and the effects of macrophage stimulation on EIAV expression were determined. Stimulation of macrophages with phorbol ester activated the cells to secrete tumor necrosis factor alpha (TNF alpha). This activation signal also resulted in a significant downregulation of viral expression as determined by supernatant reverse transcriptase activity. This effect occurred independent of the virulence of the virus strain used or the nucleic acid sequence of the viral LTR. This may represent an adaptive response of EIAV to evade the host immune response and establish a persistent infection.

Genomic quasispecies associated with the initiation of infection and disease in ponies experimentally infected with equine infectious anemia virus

Equine infectious anemia virus (EIAV) provides a uniquely dynamic system in which to study the mechanism and role of genomic variation in lentiviral persistence and pathogenesis. We have used a Shetland pony model of infection to investigate the association of specific long terminal repeat (LTR) and env gene genomic sequences with the initiation of infection and the onset of disease. We analyzed viral RNA isolated from a pathogenic stock of virus (EIAV PV) and from plasma taken during the first disease episode from two ponies infected with EIAV PV. Overall sequence variation within gp90 was low in EIAV PV and only slightly higher in plasma virus samples isolated from ponies during the first disease episode. However, a high proportion of mutations were localized to the principal neutralizing domain in EIAV PV and to the principal neutralizing domain and the gp90 hypervariable region in the two pony-derived samples. The rate of fixation of mutations was analyzed and determined to be approximately 4 x 10(-2) mutations per site per year. Sequence diversity within the U3 region of the LTR was extremely low, which suggested that the previously reported hypervariability of this region may be a consequence of selection for replication of EIAV in different host cells. The predominant EIAV PV env and LTR sequences were used to construct chimeric viruses so that the contribution of these sequences to viral pathogenicity could be examined. The chimeras replicated in cultured equine monocytes to the same extent as the parental nonpathogenic virus and did not cause disease in Shetland ponies by 120 days postinfection, suggesting that the EIAV genomic determinants of pathogenesis are complex.

Equine infectious anemia virus replication is upregulated during differentiation of blood monocytes from acutely infected horses

Equine infectious anemia virus is a lentivirus that replicates in mature tissue macrophages of horses. Ponies were infected with equine infectious anemia virus. During febrile episodes, proviral DNA was detectable, but viral mRNA was not detectable. As cultured blood monocytes from these ponies differentiated into macrophages, viral expression was upregulated. In situ hybridization confirmed that viral transcription occurred in mature macrophages.

Enhanced sensitivity to neutralizing antibodies in a variant of equine infectious anemia virus is linked to amino acid substitutions in the surface unit envelope glycoprotein

Serial passage of the prototype (PR) cell-adapted Wyoming strain of equine infectious anemia virus (EIAV) in fetal donkey dermal (FDD) rather than fetal horse (designated fetal equine kidney [FEK]) cell cultures resulted in the generation of a variant virus strain which produced accelerated cytopathic effects in FDD cells and was 100- to 1,000-fold more sensitive to neutralizing antibodies than its parent. This neutralization-sensitive variant was designated the FDD strain. Although there were differences in glycosylation between the PR and FDD strains, passage of the FDD virus in FEK cells did not reduce its sensitivity to neutralizing antibody. Nucleotide sequencing of the region encoding the surface unit (SU) protein from the FDD strain revealed nine amino acid substitutions compared with the PR strain. Two of these substitutions resulted in changes in the polarity of charge, four caused the introduction of a charged residue, and three had no net change in charge. Nucleotide sequence analysis was extended to the region of the FDD virus genome encoding the extracellular domain of the transmembrane envelope glycoprotein (TM). Unlike the situation with the FDD virus coding region, there were minor variations in nucleotide sequence between individual molecular clones containing this region of the TM gene. Although each clone contained three nucleotide substitutions compared with the PR strain, only one of these was common to all, and this did not affect the amino acid content. Of the remaining two nucleotide substitutions, only one resulted in an amino acid change, and in each case, this change appeared to be conservative. To determine if amino acid substitutions in the SU protein of FDD cell-grown viruses were responsible for the enhanced sensitivity to neutralizing antibodies, chimeric viruses were constructed by using an infectious molecular clone of EIAV. These chimeric viruses contained all of the amino acid substitutions found in the FDD virus strain and were significantly more sensitive to neutralizing antibodies than viruses from the parental (PR) molecular clone. These results demonstrated that sensitivity to neutralizing antibodies in EIAV can be conferred by amino acid residues in the SU protein. However, such amino acid substitutions were not sufficient to enhance cytopathogenicity, as the chimeric viruses did not cause excessive degenererative effects in FDD cells, as was observed with the parental FDD virus strain.

A Viral Mechanism in Acute Exacerbations of Chronic Type B Hepatitis: Hepatitis B Virus Reinfection and Subsequent Reactivation of Two Viral Strains

Chronic hepatitis B virus (HBV) infections are frequently associated with exacerbations of hepatitis of which the majority are due to reactivation of viral activity. Variation in a viral genome during persistent infection has been shown to be a possible cause for reactivation. In this study, we have found another possible mechanism. HBV in a patient with repeated exacerbations was isolated at six different times during follow-up and was characterized by polymerase chain reaction and DNA sequencing. The first episode of exacerbation was accompanied with increased replication of an HBV strain. The second episode, however, was associated with the sudden appearance of an HBV strain that displayed enough sequence variations to warrant the designation as a separate strain. The results suggested a reinfection event by another independent HBV. Subsequent exacerbations were then related to coactivation of both viral strains. These observations provide significant information toward understanding the acute exacerbations of chronic type B hepatitis. Copyright 2001 S. Karger AG, Basel

Molecular and biological characterization of a newly isolated Anaplasma marginale strain

Anaplasma marginale, a rickettsial hemoparasite of cattle and other ruminants, results in significant economic losses worldwide. Distinct strains of A. marginale have been identified based on differences in tick transmissibility, molecular size of surface proteins and DNA restriction fragments, and reactivity to a panel of monoclonal antibodies. These different strains vary considerably in their virulence, antigenic composition, and ability to protect against heterologous challenge. In this paper, we report on the molecular characterization of a newly isolated strain of A. marginale, designated St. Maries, recovered from an acutely infected cow in northern Idaho. Dermacentor andersoni ticks taken from the infected animal were tested for infection by RNA probe analysis. The infection rate of male ticks (as determined by midgut infection) was 100%, and the infection rate of female ticks was 83%. Infected male ticks were able to transmit the St. Maries strain to a susceptible calf. The high infection rate in male ticks may be particularly relevant, given that male ticks are believed to be epidemiologically important in transmission of A. marginale because of their intermittent feeding behavior, which promotes interhost transfer. The newly isolated strain differs from other US strains, including strains previously isolated in Idaho and Washington, based on reactivity to a panel of monoclonal antibodies and restriction fragment length polymorphisms. These results imply that antigenically distinct strains of A. marginale may arise within the same region.(ABSTRACT TRUNCATED AT 250 WORDS)

Major histocompatibility complex-restricted CD8+ cytotoxic T lymphocytes from horses with equine infectious anemia virus recognize Env and Gag/PR proteins

Cytotoxic T lymphocytes (CTL) can control some viral infections and may be important in the control of lentiviruses, including human immunodeficiency virus type 1. Since there is limited evidence for an in vivo role of CTL in control of lentiviruses, dissection of immune mechanisms in animal lentiviral infections may provide needed information. Horses infected with equine infectious anemia virus (EIAV) a lentivirus, have acute plasma viremia which is terminated in immunocompetent horses. Viremic episodes may recur, but most horses ultimately control infection and become asymptomatic carriers. To begin dissection of the immune mechanisms involved in EIAV control, peripheral blood mononuclear cells (PBMC) from infected horses were evaluated for CTL to EIAV-infected cells. By using noninfected and EIAV-infected autologous equine kidney (EK) cells in 51Cr-release assays, EIAV-specific cytotoxic activity was detected in unstimulated PBMC from three infected horses. The EIAV-specific cytotoxic activity was major histocompatibility complex (MHC) restricted, as determined by assaying EIAV-infected heterologous EK targets, and was mediated by CD8+ T lymphocytes, as determined by depleting these cells by a panning procedure with an anti-CD8 monoclonal antibody. MHC-restricted CD8+ CTL in unstimulated PBMC from infected horses caused significant specific lysis of autologous EK cells infected with recombinant vaccinia viruses expressing EIAV genes, either env or gag plus 5' pol. The EIAV-specific MHC-restricted CD8+ CTL were detected in two EIAV-infected horses within a few days after plasma viremia occurred and were present after viremia was terminated. The detection of these immune effector cells in EIAV-infected horses permits further studies to determine their in vivo role.