Equine monocyte-derived macrophage cultures and their applications for infectivity and neutralization studies of equine infectious anemia virus

The Horse as a Model for the Study of Cutaneous Wound Healing

Significance: Cutaneous wounds are a major problem in both human and equine medicine. The economic cost of treating skin wounds and related complications in humans and horses is high, and in both species, particular types of chronic wounds do not respond well to current therapies, leading to suffering and morbidity. Recent Advances: Conventional methods for the treatment of cutaneous wounds are generic and have not changed significantly in decades. However, as more is learned about the mechanisms involved in normal skin wound healing, and how failure of these processes leads to chronic nonhealing wounds, novel therapies targeting the specific pathologies of hard-to-heal wounds are being developed and evaluated. Critical Issues: Physiologically relevant animal models are needed to (1) study the mechanisms involved in normal and impaired skin wound healing and (2) test newly developed therapies. Future Directions: Similarities in normal wound healing in humans and horses, and the natural development of distinct types of hard-to-heal chronic wounds in both species, make the horse a physiologically relevant model for the study of mechanisms involved in wound repair. Horses are also well-suited models to test novel therapies. In addition, studies in horses have the potential to benefit veterinary, as well as human medicine.

The equine mononuclear phagocyte system: The relevance of the horse as a model for understanding human innate immunity

The mononuclear phagocyte system (MPS) is a family of cells of related function that includes bone marrow progenitors, blood monocytes and resident tissue macrophages. Macrophages are effector cells in both innate and acquired immunity. They are a major resident cell population in every organ and their numbers increase in response to proinflammatory stimuli. Their function is highly regulated by a wide range of agonists, including lymphokines, cytokines and products of microorganisms. Macrophage biology has been studied most extensively in mice, yet direct comparisons of rodent and human macrophages have revealed many functional differences. In this review, we provide an overview of the equine MPS, describing the variation in the function and phenotype of macrophages depending on their location and the similarities and differences between the rodent, human and equine immune response. We discuss the use of the horse as a large animal model in which to study macrophage biology and pathological processes shared with humans. Finally, following the recent update to the horse genome, facilitating further comparative analysis of regulated gene expression between the species, we highlight the importance of future transcriptomic macrophage studies in the horse, the findings of which may also be applicable to human as well as veterinary research.

Attenuation of Equine Lentivirus Alters Mitochondrial Protein Expression Profile from Inflammation to Apoptosis

Equine infectious anemia virus (EIAV) is an equine lentivirus similar to HIV-1, targets host immune cells, and causes a life-long infection in horses. The Chinese live EIAV vaccine is attenuated from long-term passaging of a highly virulent strain in vitro The parent pathogenic strain (EIAV_DLV34) induces a host inflammatory storm to cause severe pathological injury of animals. However, the vaccine strain (EIAV_DLV121) induces a high level of apoptosis to eliminate infected cells. To investigate how these processes are regulated, we performed a comparative proteomics analysis and functional study in equine monocyte-derived macrophages (eMDMs) and found that the divergent mitochondrial protein expression profiles caused by EIAV strains with different virulence led to disparate mitochondrial function, morphology, and metabolism. This in turn promoted the distinct transformation of macrophage inflammatory polarization and intrinsic apoptosis. In EIAV_DLV34-infected cells, a high level of glycolysis and increased mitochondrial fragmentation were induced, resulting in the M1-polarized proinflammatory-type transformation of macrophages and the subsequent production of a strong inflammatory response. Following infection with EIAV_DLV121, the infected cells were transformed into M2-polarized anti-inflammatory macrophages by inhibition of glycolysis. In this case, a decrease in the mitochondrial membrane potential and impairment of the electron transport chain led to increased levels of apoptosis and reactive oxygen species. These results correlated with viral pathogenicity loss and may help provide an understanding of the key mechanism of lentiviral attenuation.IMPORTANCE Following viral infection, the working pattern and function of the cell can be transformed through the impact on mitochondria. It still unknown how the mitochondrial response changes in cells infected with viruses in the process of virulence attenuation. EIAV_DLV121 is the only effective lentiviral vaccine for large-scale use in the world. EIAV_DLV34 is the parent pathogenic strain. Unlike EIAV_DLV34-induced inflammation storms, EIAV_DLV121 can induce high levels of apoptosis. For the first time, we found that, after the mitochondrial protein expression profile is altered, EIAV_DLV34-infected cells are transformed into M1-polarized-type macrophages and cause inflammatory injury and that the intrinsic apoptosis pathway is activated in EIAV_DLV121-infected cells. These studies shed light on how the mitochondrial protein expression profile changes between cells infected by pathogenic lentivirus strains and cells infected by attenuated lentivirus strains to drive different cellular responses, especially from inflammation to apoptosis.

Macrophage effector responses of horses are influenced by expression of CD154

Reactive intermediates contribute to innate immunity by providing phagocytes with a mechanism of defense against bacteria, viruses and parasites. To better characterize the role of CD154 in the production of reactive intermediates, we cloned and expressed recombinant equine CD154 (reqCD154) in Chinese Hamster Ovary (CHO). In co-culture experiments, CHO cells ectopically expressing reqCD154 elicited superoxide production in monocyte-derived macrophages (MDM). Collectively, our results indicate that regulation of CD154 expression plays a role in innate host defenses.

[Mouse embryonic stem cells - a new cellular system for studying the equine infectious anemia virus in vitro and in vivo]

The complexity of the pathogenesis and insufficient knowledge about the slow retroviral infections, which include equine infectious anemia, necessitates finding an adequate laboratory model for the study of the infection process and immunogenesis to create means of prevention and treatment of diseases. Data about strains and cellular tropism of the virus are discussed. It was shown that mouse embryonic stem cells (ESCS) exhibited unique properties and characteristics. In contrast to fibroblasts and other cell types, these cells can be considered as a new cell system for studying EIAV in vitro and in vivo. Under differentiation-inducing conditions they are able to reproduce in vitro embryogenesis cells and form cells of three germ layers. Differentiation of mouse ESCs in the direction of hematopoiesis could contribute new knowledge and understanding of viral tropism EIAV in vitro. ESC can be returned back to the early pre-implantation embryo. Once in the germ cell environment, they participate in the formation of tissues and organs of the developing fetus. Thus, the adaptation of the mouse ESC to the equine EIAV through genetic transformation makes it possible to get closer to the creation of a laboratory model for the study of the in vivo immune response in the lentiviral infection.

Proteomic alteration of equine monocyte-derived macrophages infected with equine infectious anemia virus

Similar to the well-studied viruses human immunodeficiency virus (HIV)-1 and simian immunodeficiency virus (SIV), equine infectious anemia virus (EIAV) is another member of the Lentivirus genus in the family Retroviridae. Previous studies revealed that interactions between EIAV and the host resulted in viral evolution in pathogenicity and immunogenicity, as well as adaptation to the host. Proteomic analysis has been performed to examine changes in protein expression and/or modification in host cells infected with viruses and has revealed useful information for virus-host interactions. In this study, altered protein expression in equine monocyte-derived macrophages (eMDMs, the principle target cell of EIAV in vivo) infected with the EIAV pathogenic strain EIAV(DLV34) (DLV34) was examined using 2D-LC-MS/MS coupled with the iTRAQ labeling technique. The expression levels of 210 cellular proteins were identified to be significantly upregulated or downregulated by infection with DLV34. Alterations in protein expression were confirmed by examining the mRNA levels of eight selected proteins using quantitative real-time reverse-transcription PCR, and by verifying the levels of ten selected proteins using parallel reaction monitoring (PRM). Further analysis of GO and Kyoto Encyclopedia of Genes and Genomes (KEGG)-Pathway enrichment demonstrated that these differentially expressed proteins are primarily related to the biological processes of oxidative phosphorylation, protein folding, RNA splicing, and ubiquitylation. Our results can facilitate a better understanding of the host response to EIAV infection and the cellular processes required for EIAV replication and pathogenesis.

Effects of age and macrophage lineage on intracellular survival and cytokine induction after infection with Rhodococcus equi

Rhodococcus equi, a facultative intracellular pathogen of macrophages, causes life-threatening pneumonia in foals and in people with underlying immune deficiencies. As a basis for this study, we hypothesized that macrophage lineage and age would affect intracellular survival of R. equi and cytokine induction after infection. Monocyte-derived and bronchoalveolar macrophages from 10 adult horses and from 10 foals (sampled at 1-3 days, 2 weeks, 1 month, 3 months, and 5 months of age) were infected ex vivo with virulent R. equi. Intracellular R. equi were quantified and mRNA expression of IL-1β, IL-4, IL-6, IL-8, IL-10, IL-12 p40, IL-18, IFN-γ, and TNF-α was measured. Intracellular replication of R. equi was significantly (P<0.001) greater in bronchoalveolar than in monocyte-derived macrophages, regardless of age. Regardless of the macrophage lineage, replication of R. equi was significantly (P=0.002) higher in 3-month-old foals than in 3-day old foals, 2-week-old foals, 1-month-old foals, and adult horses. Expression of IL-4 mRNA was significantly higher in monocyte-derived macrophages whereas expression of IL-6, IL-18, and TNF-α was significantly higher in bronchoalveolar macrophages. Induction of IL-1β, IL-10, IL-12 p40, and IL-8 mRNA in bronchoalveolar macrophages of 1-3-day old foals was significantly higher than in older foals or adult horses. Preferential intracellular survival of R. equi in bronchoalveolar macrophages of juvenile horses may play a role in the pulmonary tropism of the pathogen and in the window of age susceptibility to infection.

An attenuated EIAV strain and its molecular clone strain differentially induce the expression of Toll-like receptors and type-I interferons in equine monocyte-derived macrophages

Activations of endosomal TLRs include TLR3, TLR7/8, and TLR9 stimulates the production of cytokines, such as type I interferons (IFNs), and therefore involves in virus-host interactions. In the present study, two equine anemia virus (EIAV) strains EIAVFDDV13 and EIAVFDDV3-8, which showed different induction on protective immunity, were compared regarding their ability to regulate the expression of endosomal TLRs, as well as type I IFNs, after infection of equine monocyte-derived macrophages (eMDMs). Our results showed that EIAVFDDV13 dramatically up-regulated the expression of TLR3 and IFNβ and less robustly up-regulated the expression of TRL9 and IFNα1, whereas EIAVFDDV3-8 induced significantly lower expression of type I IFN mRNA and protein and more strongly down-regulated the expression of TLR7 and TLR8. In addition, no significant differences in cell apoptosis were observed between these two strains. Given that the genomic variation of EIAVFDDV13 is considerably higher than that of molecular clone EIAVFDDV3-8, our results suggest that stronger TLR3 activation and increased INFβ production induced by the multi-species strain are associated with an effective vaccine-elicited protective immune response.

Envelope determinants of equine lentiviral vaccine protection

Lentiviral envelope (Env) antigenic variation and associated immune evasion present major obstacles to vaccine development. The concept that Env is a critical determinant for vaccine efficacy is well accepted, however defined correlates of protection associated with Env variation have yet to be determined. We reported an attenuated equine infectious anemia virus (EIAV) vaccine study that directly examined the effect of lentiviral Env sequence variation on vaccine efficacy. The study identified a significant, inverse, linear correlation between vaccine efficacy and increasing divergence of the challenge virus Env gp90 protein compared to the vaccine virus gp90. The report demonstrated approximately 100% protection of immunized ponies from disease after challenge by virus with a homologous gp90 (EV0), and roughly 40% protection against challenge by virus (EV13) with a gp90 13% divergent from the vaccine strain. In the current study we examine whether the protection observed when challenging with the EV0 strain could be conferred to animals via chimeric challenge viruses between the EV0 and EV13 strains, allowing for mapping of protection to specific Env sequences. Viruses containing the EV13 proviral backbone and selected domains of the EV0 gp90 were constructed and in vitro and in vivo infectivity examined. Vaccine efficacy studies indicated that homology between the vaccine strain gp90 and the N-terminus of the challenge strain gp90 was capable of inducing immunity that resulted in significantly lower levels of post-challenge virus and significantly delayed the onset of disease. However, a homologous N-terminal region alone inserted in the EV13 backbone could not impart the 100% protection observed with the EV0 strain. Data presented here denote the complicated and potentially contradictory relationship between in vitro virulence and in vivo pathogenicity. The study highlights the importance of structural conformation for immunogens and emphasizes the need for antibody binding, not neutralizing, assays that correlate with vaccine protection.

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.

Effects of two commercially available immunostimulants on leukocyte function of foals following ex vivo exposure to Rhodococcus equi

The objective of this study was to determine the effect of immunostimulants on neutrophil, macrophage, and lymphocyte function following ex vivo exposure to Rhodococcus equi. Eighteen foals were randomly assigned to one of 3 treatment groups. Treatment consisted of inactivated Propionibacterium acnes (PA), inactivated parapoxvirus ovis (PPVO), or saline (control) administered on days 0 (7 days of age), 2, and 8. Bronchoalveolar lavage (BAL) fluid and blood were collected on days 0 (baseline), 12, 24 and 36. Intracellular replication of R. equi in macrophages, cytokine induction by R. equi-infected macrophages, phagocytic and oxidative burst activity of neutrophils, lymphoproliferative responses, and cytokine induction of proliferating lymphocytes were measured. Neutrophils from foals treated with PPVO had significantly greater ability to phagocytize R. equi and undergo oxidative burst on day 12 and day 24 compared to baseline values. On day 24, foals treated with PPVO had significantly greater phagocytosis and oxidative burst than foals treated with PA. Treatment with PA resulted in significantly less intracellular proliferation of R. equi within monocyte-derived macrophages on day 12 compared to control foals. The ability of R. equi to replicate in BAL macrophages decreased significantly (P=0.005) with time with lower replication in BAL macrophages of older foals compared to younger foals, regardless of treatment. On day 12, TNF-α induction in monocyte-derived macrophages and IL-12 p40 induction in BAL macrophages infected with R. equi was significantly higher in foals treated with PPVO than in controls. Lymphoproliferative responses and IFN-γ induction were not significantly different between groups.

Canine influenza virus replicates in alveolar macrophages and induces TNF-alpha

Canine influenza virus (CIV) is a recently emergent pathogen of dogs that has caused highly contagious respiratory disease in racing Greyhounds, pet dogs, and shelter animals. Initial characterizations of CIV-induced respiratory disease suggested alveolar macrophages may be susceptible to virus infection. To investigate the role of the alveolar macrophage in the pathogenesis of CIV infection, primary alveolar macrophages were inoculated with CIV and studied from 0 to 48 hours later. Virus titers in alveolar macrophage culture supernatants increased significantly (P < .05, n = 7) from 3 to 24 hours following virus inoculation. Virus matrix gene expression was significantly increased (P < .05, n = 14) at 3, 6, and 12 hours after inoculation, peaking at 6,445-fold the level of RNA detectable immediately following inoculation. Virus-inoculated macrophages demonstrated significantly (P < .05, n = 5) decreased viability (30% trypan blue positive) by 12 hours after inoculation compared with mock-inoculated cells (5% trypan blue positive). By 12 hours after inoculation, tumor necrosis factor-alpha (TNF-alpha) and interleukin-10 (IL-10) mRNA levels were significantly (P < .05, n = 11) increased over those immediately following inoculation. Only TNF-alpha protein levels were significantly increased (P < .05, n = 11) at 12 hours after inoculation. In conclusion, the results indicate that CIV replicates in canine alveolar macrophages and induces TNF-alpha expression and cell death.

Foal monocyte-derived dendritic cells become activated upon Rhodococcus equi infection

Susceptibility of foals to Rhodococcus equi pneumonia is exclusive to the first few months of life. The objective of this study was to investigate the immediate immunologic response of foal and adult horse antigen-presenting cells (APCs) upon infection with R. equi. We measured the activation of the antigen-presenting major histocompatibility complex (MHC) class II molecule, costimulatory molecules CD40 and CD86, the cytokine interleukin-12 (IL-12), and the transcriptional factor interferon regulatory factor 1 (IRF-1) in monocyte-derived macrophages (mMOs) and dendritic cells (mDCs) of adult horses and foals of different ages (from birth to 3 months of age) infected with virulent R. equi or its avirulent, plasmid-cured derivative. Infection with virulent or avirulent R. equi induced (P <or= 0.01) the expression of IL-12p35 and IL-12p40 mRNAs in foal mMOs and mDCs at different ages. This response was likely mediated by the higher (P=0.008) expression of IRF-1 in foal mDCs at birth than in adult horse mDCs. R. equi infection promoted comparable expression of costimulatory molecules CD86 and CD40 in foal and adult horse cells. The cytokine and costimulatory response by foal mDCs was not accompanied by robust MHC class II molecule expression. These data suggest that foal APCs detect the presence of R. equi and respond with the expression of the Th1-inducing cytokine IL-12. Nevertheless, there seems to be a limitation to MHC class II molecule expression which we hypothesize may compromise the efficient priming of naïve effector cells in early life.

Characterization of an equine macrophage cell line: application to studies of EIAV infection

EIAV is a monocyte/macrophage tropic virus. To date, even though EIAV has been under investigation for numerous years, very few details have been elucidated about EIAV/macrophage interactions. This is largely due to the absence of an equine macrophage cell line that would support viral replication. Herein we describe the spontaneous immortalization and generation of a clonal equine macrophage-like (EML) cell line with the functional and immunophenotype characteristics of differentiated equine monocyte derived macrophage(s) (eMDM(s)). These cells possess strong non-specific esterase (NSE) activity, are able to phagocytose fluorescent bioparticles, and produce nitrites in response to LPS. The EML-3C cell line expresses the EIAV receptor for cellular entry (ELR1) and supports replication of the virulent EIAV(PV) biological clone. Thus, EML-3C cells provide a useful cell line possessing equine macrophage related properties for the growth and study of EIAV infection as well as of other equine macrophage tropic viruses.

Equine infectious anemia virus resists the antiretroviral activity of equine APOBEC3 proteins through a packaging-independent mechanism

Equine infectious anemia virus (EIAV), uniquely among lentiviruses, does not encode a vif gene product. Other lentiviruses, including human immunodeficiency virus type 1 (HIV-1), use Vif to neutralize members of the APOBEC3 (A3) family of intrinsic immunity factors that would otherwise inhibit viral infectivity. This suggests either that equine cells infected by EIAV in vivo do not express active A3 proteins or that EIAV has developed a novel mechanism to avoid inhibition by equine A3 (eA3). Here, we demonstrate that horses encode six distinct A3 proteins, four of which contain a single copy of the cytidine deaminase (CDA) consensus active site and two of which contain two CDA motifs. This represents a level of complexity previously seen only in primates. Phylogenetic analysis of equine single-CDA A3 proteins revealed two proteins related to human A3A (hA3A), one related to hA3C, and one related to hA3H. Both equine double-CDA proteins are similar to hA3F and were named eA3F1 and eA3F2. Analysis of eA3F1 and eA3F2 expression in vivo shows that the mRNAs encoding these proteins are widely expressed, including in cells that are natural EIAV targets. Both eA3F1 and eA3F2 inhibit retrotransposon mobility, while eA3F1 is a potent inhibitor of a Vif-deficient HIV-1 mutant and induces extensive editing of HIV-1 reverse transcripts. However, both eA3F1 and eA3F2 are weak inhibitors of EIAV. Surprisingly, eA3F1 and eA3F2 were packaged into EIAV and HIV-1 virions as effectively as hA3G, although only the latter inhibited EIAV infectivity. Moreover, all three proteins bound both the HIV-1 and EIAV nucleocapsid protein specifically in vitro. It therefore appears that EIAV has evolved a novel mechanism to specifically neutralize the biological activities of the cognate eA3F1 and eA3F2 proteins at a step subsequent to virion incorporation.

Activation of peripheral blood monocytes results in more robust production of IL-10 in neonatal foals compared to adult horses

Foals are particularly vulnerable to infection by Rhodococcus equi during the first 2 weeks of life whereas mature horses are not. While an innate immunodeficiency likely accounts for this clinically relevant vulnerability, the factors that contribute to infection by R. equi have not been fully elucidated. In this study, we demonstrate that cells of the monocyte lineage, including monocytes, macrophages, and dendritic cells, that have been activated with LPS and IFN-gamma, respond with a statistically significant, greater amount of cytokine mRNA production of IL-10, IL-12p35, and IL-12p40 than unstimulated control cells. Interestingly, activation of neonatal cells resulted in a twofold log increase in baseline cytokine mRNA expression of IL-10 compared with adult cells. In contrast, no significant differences in mean cytokine mRNA expression of IL-12p35 and IL-12p40 were detected, suggesting that the defect in chromosomal remodeling that prevents IL-12p35 gene transcription as a cause for decreased IL-12 synthesis in human neonates is not a likely occurrence in equine neonates. Collectively, these differences indicate that in vivo activation of equine cells of the monocyte lineage may result in different autocrine and paracrine cellular responses that vary according to age, with potential impact on regulation of adaptive and innate immune responses.

Validation of quantitative polymerase chain reaction assays for measuring cytokine expression in equine macrophages

The study of the equine immune system and inflammatory responses, by measuring cytokine expression, can provide important insight into disease pathogenesis in the horse. A set of quantitative real-time polymerase chain reaction (QPCR) assays for the equine cytokines IL-1alpha, IL-1beta, IL-6, IL-8 and TNF-alpha were validated using QPCR primers and probes which were generated for the equine IL-1alpha, IL-1beta, IL-6, IL-8, TNF-alpha and 18S genes. Amplification efficiency, intra-assay and inter-assay variation were determined using 10-fold dilutions of plasmid for each gene. Under these conditions the amplification efficiencies of the primers and probes ranged from 99% to 101%. The mean coefficient of variation (CV) across five sets of plasmid DNA for both intra-assay and inter-assay variation was 0.63% (range 0.2% to 1.8%). Amplification efficiency was also determined using 2-fold dilutions of cDNA and under these conditions amplification efficiency ranged from 83% to 95%. The specificity of amplification was confirmed by DNA sequencing of reaction products. The QPCR assays were also evaluated using three sets of cDNA from equine monocyte derived macrophages (EMDM) stimulated for 1 h with lipopolysaccharide (LPS). The general trend was the same for all three samples with IL-1alpha showing the greatest induction and IL-6 the lowest induction. The range of cytokine induction was greater than has previously been reported with values ranging from 12-fold to 30,000-fold. We present a set of QPCR primers and probes that are suitable for quantitation of expression of a set of equine cytokines. The primers and probes have been rigorously analyzed, and we demonstrate that they are specific for the desired genes, have a high amplification efficiency and the assays are highly reproducible.

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.

Immune suppression of challenged vaccinates as a rigorous assessment of sterile protection by lentiviral vaccines

We previously reported that an experimental live-attenuated equine infectious anemia virus (EIAV) vaccine, containing a mutated S2 accessory gene, provided protection from disease and detectable infection after virulent virus (EIAV(PV)) challenge [Li F, Craigo JK, Howe L, Steckbeck JD, Cook S, Issel C, et al. A live-attenuated equine infectious anemia virus proviral vaccine with a modified S2 gene provides protection from detectable infection by intravenous virulent virus challenge of experimentally inoculated horses. J Virol 2003;77(13):7244-53; Craigo JK, Li F, Steckbeck JD, Durkin S, Howe L, Cook SJ, et al. Discerning an effective balance between equine infectious anemia virus attenuation and vaccine efficacy. J Virol 2005;79(5):2666-77]. To determine if attenuated EIAV vaccines actually prevent persistent infection by challenge virus, we employed a 14-day dexamethasone treatment of vaccinated horses post-challenge to suppress host immunity and amplify replication levels of any infecting EIAV. At 2 months post-challenge the horses were all protected from virulent-virus challenge, evidenced by a lack of EIA signs and detectable challenge plasma viral RNA. Upon immune suppression, 6/12 horses displayed clinical EIA. Post-immune suppression characterizations demonstrated that the attenuated vaccine evidently prevented detectable challenge virus infection in 50% of horses. These data highlight the utility of post-challenge immune suppression for evaluating persistent viral vaccine protective efficacy.

Genetic evidence for the existence of interleukin-23 and for variation in the interleukin-12 and interleukin-12 receptor genes in the horse

Immune loci, characterized by features reflecting their role in defense reactions and consequently related to evolutionary mechanisms, including polymorphisms or association with disease are suitable candidates for comparative analysis. Interleukin-12 and related cytokines are key molecules regulating natural and specific immune responses. In this study, we analyzed four horse IL12-related genes: IL23p19, IL12Rbeta2, IL12p40, and IL12p35. Genomic nucleotide sequence of the horse IL23 p19 sub-unit encoding gene was determined. The horse IL23p19 gene consists of four exons; its total mRNA length is 1004 bp, with a coding region of 579 bp. The predicted amino acid sequence of the horse IL23p19 sub-unit showed 88.0% sequence identity with the human sequence. A partial genomic sequence highly homologous to human IL12Rbeta2 suggesting existence of this gene in the horse was retrieved. Single nucleotide polymorphisms (SNPs) were identified in all four genes analyzed. PCR-RFLP genotyping was developed for selected SNPs. Inter-breed differences in allele and genotype frequencies were observed in IL12p35 SNP 242. The results showed that horse IL12-related genes are comparable to their counterparts in other mammalian species in terms of their structure and their genetic variation.

Combined amino acid mutations occurring in the envelope closely correlate with pathogenicity of EIAV

The Chinese equine infectious anemia virus (EIAV) donkey-leukocyte attenuated vaccine (DLV) provides a unique natural model system to study the attenuation mechanism and immunological control of lentivirus replication. Critical consensus mutations were identified between virulent Chinese EIAV strains and vaccine strains. Based on a full-length infectious clone of EIAV vaccine strain pLGFD3, two molecular clones, mFD5-4-7 and mFD7-2-11, were successfully constructed, in which 4 and 6 critical consensus mutations in the env gene of the vaccine strain were point-mutated to the wild-type sequence, respectively by an overlap PCR mutagenesis strategy. The infectivity, virulence, and pathogenesis of the constructed clones were investigated in vitro using a reverse transcriptase assay, an indirect immunofluorescence assay, observation of cytopathogenic effect, and virion observation as well as in vivo by inoculation of animals with the resulting infectious clones. The pathogenic symptoms in horses inoculated with mFD7-2-11 were more severe than those inoculated with mFD5-4-7, whereas no pathogenic symptoms were detected in animals inoculated with their parental clone pLGFD3 strain. The results indicate that the consensus mutation residues of the env region involved in this study play significant roles in the virulence and pathogenicity of EIAV. This will contribute to the elucidation of the attenuating and protective mechanisms of the Chinese EIAV vaccine.

Receptor-mediated entry by equine infectious anemia virus utilizes a pH-dependent endocytic pathway

Previous studies of human and nonhuman primate lentiviral entry mechanisms indicate a predominant use of pH-independent pathways, although more recent studies of human immunodeficiency virus type 1 entry appear to reveal the use of a low-pH-dependent entry pathway in certain target cells. To expand the characterization of the specificity of lentiviral entry mechanisms, we have in the current study examined the entry pathway of equine infectious anemia virus (EIAV) during infection of its natural target, equine macrophages, permissive equine fibroblastic cell lines, and an engineered mouse cell line expressing the recently defined equine lentivirus receptor-1. The specificity of EIAV entry into these various cells was determined by assaying the effects of specific drug treatments on the level of virus entry as measured by quantitative real-time PCR assay of early reverse transcripts or by measurements of virion production. The results of these studies demonstrated that EIAV entry into all cell types was substantially inhibited in a dose-dependent manner by treatment with the vacuolar H+-ATPase inhibitors concanamycin A and bafilomycin A1 or the lysosomotropic weak base ammonium chloride. In contrast, treatments with sucrose to block clathrin-mediated endocytosis or with chloroquine to block organelle acidification failed to inhibit EIAV entry into the same target cells. The observed inhibition of EIAV entry was shown not to be related to cytotoxicity. Taken together, these experiments reveal for the first time that EIAV receptor-mediated entry into target cells is via a low-pH-dependent endocytic pathway.

Replication of West Nile virus in equine peripheral blood mononuclear cells

A cell model of primary monocytes and other mononuclear cells isolated from equine blood was used to study the kinetics of West Nile virus (WNV) replication in a natural host. West Nile virus has emerged on the North American continent as a significant cause of morbidity and mortality in a wide range of avian and mammalian species. While other flaviviruses are known to infect monocytes and lymphocytes, the ability of WNV to productively replicate in specific immune cells of peripheral blood has not been assessed. In this study, enriched populations of monocytes and lymphocytes as well as purified monocytes, CD4+, CD8+ and B lymphocytes were obtained from equine blood. Productive WNV replication was demonstrated by viral growth curves, quantitative RT-PCR for WNV RNA, and indirect immunofluorescence detection of a non-structural WNV protein. Enriched and purified monocytes consistently supported productive viral replication in blood from nine of nine horses tested while a minor subset of CD4+ lymphocytes supported productive replication in cells from three of the nine horses tested. Peak viral titers of 3.2-6.6 log10 PFU/ml were reached at 6 days post-inoculation (p.i.) and titers were maintained through 10-15 days p.i. Activation of monocytes with bacterial lipopolysaccharide, which resulted in activation of nuclear transcription factor kappaB (NF-kappaB) plus elevation of nitric oxide and type I interferon levels, reduced or eliminated WNV replication. These results suggest that immune cells of the peripheral blood may serve as target cells for initial replication of WNV and may play a role in subsequent viral dissemination. Furthermore, primary equine immune cell cultures represent a potentially useful model of a natural WNV host when testing compounds such as antivirals for use in WNV treatment.

Amino acid mutations of the infectious clone from Chinese EIAV attenuated vaccine resulted in reversion of virulence

The Chinese equine infectious anemia virus (EIAV) donkey-leukocyte attenuated vaccine (DLV) provides a unique natural model system by which attenuated mechanism and immunological control of lentivirus replication may be studied. We analyzed the critical consensus mutations that occurred during the viral passages in vitro and in vivo for vaccine's preparation. Based on the full-length infectious clone pLGFD3 (EIAV vaccine background) and according to mutations displayed during viral attenuation, we successfully constructed an infectious clones pLG5-3-l in which gag and env genes were point-mutated by overlap PCR mutagenesis strategy. pLG5-3-l was proved to have the ability of effective replication in vitro cells culture systems by Reverse Transcriptase Assay and virion observation under electron microscopy. Results of the in vivo experiments indicated that marked differences occurred between the mutated virus and their parental virus in clinical manifestation and plasma viral replication during 6-month observation period. In contrast to asymptom of animals infected with pLGFD3-V, the mutated virus (pLG5-3-l-V) developed typical clinical progression in the corresponding experimentally infected animals. The results of the distinct differences in clinical profiles and viral dynamics before and after mutation of EIAV infectious clone will help to understand the protective mechanism of Chinese EIAV vaccine and shed light on novel HIV vaccine design.

A tumor necrosis factor receptor family protein serves as a cellular receptor for the macrophage-tropic equine lentivirus

Characterization of cellular receptors for human, simian, and feline immunodeficiency viruses that are tropic for lymphocytes and macrophages have revealed a common theme of a sequential binding of viral envelope proteins with two coreceptors to mediate virus infection of target cells. In contrast to these dual tropic immunodeficiency viruses, the ungulate lentiviruses, including equine infectious anemia virus (EIAV), exclusively infect cells of the monocyte-macrophage lineage to cause progressive degenerative diseases without clinical immunodeficiency. EIAV causes a uniquely dynamic disease that is characterized by recurrent disease episodes including fever, diarrhea, lethargy, anemia, and thrombocytopenia. Although EIAV provides an important animal model for lentivirus disease resulting from macrophage infection, to date there has been no definition of the specific cellular receptor(s) used by the equine lentivirus to infect target cells. In the current study, we have identified and cloned a functional receptor for EIAV, designated equine lentivirus receptor-1 (ELR1), related to the family of TNF receptor (TNFR) proteins. ELR1 was shown to be expressed in various equine cells permissive for EIAV replication in vitro, including monocytes and macrophages. In contrast, EIAV-resistant human, murine, and simian cells were negative for ELR1 expression but became susceptible to virus infection when transduced with a recombinant murine retrovirus expressing the ELR1. Thus, these results identify a specific functional receptor for a macrophagetropic lentivirus and indicate that infection by EIAV may be mediated by a single receptor, in contrast to coreceptors used by the lymphotropic immunodeficiency lentiviruses.

Rhodococcus equi-specific cytotoxic T lymphocytes in immune horses and development in asymptomatic foals

Rhodococcus equi is an important cause of pneumonia in young horses; however, adult horses are immune due to their ability to mount protective recall responses. In this study, the hypothesis that R. equi-specific cytotoxic T lymphocytes (CTL) are present in the lung of immune horses was tested. Bronchoalveolar lavage (BAL)-derived pulmonary T lymphocytes stimulated with R. equi lysed infected alveolar macrophages and peripheral blood adherent cells (PBAC). As with CTL obtained from the blood, killing of R. equi-infected targets by pulmonary effectors was not restricted by equine lymphocyte alloantigen-A (ELA-A; classical major histocompatibility complex class I), suggesting a novel or nonclassical method of antigen presentation. To determine whether or not CTL activity coincided with the age-associated susceptibility to rhodococcal pneumonia, CTL were evaluated in foals. R. equi-stimulated peripheral blood mononuclear cells (PBMC) from 3-week-old foals were unable to lyse either autologous perinatal or mismatched adult PBAC targets. The defect was not with the perinatal targets, as adult CTL effectors efficiently killed infected targets from 3-week-old foals. In contrast, significant CTL activity was present in three of five foals at 6 weeks of age, and significant specific lysis was induced by PBMC from all foals at 8 weeks of age. As with adults, lysis was ELA-A unrestricted. Two previously described monoclonal antibodies, BCD1b3 and CD1F2/1B12.1, were used to examine the expression of CD1, a nonclassical antigen-presenting molecule, on CTL targets. These antibodies cross-reacted with both foal and adult PBAC. However, neither antibody bound alveolar macrophages, suggesting that the R. equi-specific, major histocompatibility complex-unrestricted lysis is not restricted by a surface molecule identified by these antibodies.

Rhodococcus equi-infected macrophages are recognized and killed by CD8+ T lymphocytes in a major histocompatibility complex class I-unrestricted fashion

The goal of this research was to examine the role of cytotoxic T lymphocytes (CTL) in the control of Rhodococcus equi and specifically to determine if R. equi-specific CD8+ CTL occurred in the blood of immune horses. Equine peripheral blood mononuclear cells stimulated with antigen-presenting cells either infected with R. equi or exposed to soluble R. equi antigen lysed R. equi-infected target cells. Lysis was decreased to background by depletion of either CD2+ or CD3+ cells, indicating that the effector cell had a T-lymphocyte, but not NK cell, phenotype. Stimulation induced an increased percentage of CD8+ T cells in the effector population, and depletion of CD8+ T cells resulted in significantly decreased lysis of infected targets. Killing of R. equi-infected macrophages by effector cells was equally effective against autologous and equine leukocyte antigen A (classical major histocompatibility complex [MHC] class I) mismatched targets. To evaluate potential target antigens, target cells were infected with either virulent (80.6-kb plasmid-containing) or avirulent (plasmid-cured) R. equi. The degree of lysis was not altered by the presence of the plasmid, providing evidence that the virulence plasmid, which is required for survival within macrophages, was not necessary for recognition and killing of R. equi-infected cells. These data indicate that immunocompetent adult horses develop R. equi-specific CD8+ CTL, which may play a role in immunity to R. equi. The apparent lack of restriction via classical MHC class I molecules suggests a novel or nonclassical method of antigen processing and presentation, such as presentation by CD1 or other nonclassical MHC molecules.

Differential effects of actin cytoskeleton dynamics on equine infectious anemia virus particle production

Retrovirus assembly and budding involve a highly dynamic and concerted interaction of viral and cellular proteins. Previous studies have shown that retroviral Gag proteins interact with actin filaments, but the significance of these interactions remains to be defined. Using equine infectious anemia virus (EIAV), we now demonstrate differential effects of cellular actin dynamics at distinct stages of retrovirus assembly and budding. First, virion production was reduced when EIAV-infected cells were treated with phallacidin, a cell-permeable reagent that stabilizes actin filaments by slowing down their depolymerization. Confocal microscopy confirmed that the inhibition of EIAV production correlated temporally over several days with the incorporation dynamics of phallacidin into the actin cytoskeleton. Although the overall structure of the actin cytoskeleton and expression of viral protein appeared to be unaffected, phallacidin treatment dramatically reduced the amount of full-length Gag protein associated with the actin cytoskeleton. These data suggest that an association of full-length Gag proteins with de novo actin filaments might contribute to Gag assembly and budding. On the other hand, virion production was enhanced when EIAV-infected cells were incubated briefly (2 h) with the actin-depolymerizing drugs cytochalasin D and latrunculin B. Interestingly, the enhanced virion production induced by cytochalasin D required a functional late (L) domain, either the EIAV YPDL L-domain or the proline-rich L domains derived from human immunodeficiency virus type 1 or Rous sarcoma virus, respectively. Thus, depolymerization of actin filaments may be a common function mediated by retrovirus L domains during late stages of viral budding. Taken together, these observations indicate that dynamic actin polymerization and depolymerization may be associated with different stages of viral production.

Enhancement of equine infectious anemia virus virulence by identification and removal of suboptimal nucleotides

Pathogenicity was reportedly restored to an avirulent molecular clone of equine infectious anemia virus (EIAV) by substitution of 3' sequences from the pathogenic variant strain (EIAV(PV)). However, the incidence of disease in horses/ponies was found to be significantly lower (P = 0.016) with the chimeric clone (EIAV(UK)) than with EIAV(PV). This was attributable to 3' rather than 5' regions of the proviral genome, where EIAV(UK) differs from the consensus EIAV(PV) sequence by having a 68-bp duplication in the 3' LTR and arginine (R(103)) rather than tryptophan (W(103)) at position 103 in the second exon of rev. In EIAV(UK) recipients the duplication was rapidly eliminated and R(103) replaced by W(103) in the viral population. Furthermore, removal of the 3' variant sequences from EIAV(UK) (EIAV(UK3)) resulted in an equivalent (P = 0.013) disease potential in Equus caballus to EIAV(PV). The 68-bp duplication and/or R(103) may limit peak viral RNA accumulation during acute infection.

Equine infectious anemia in mules: virus isolation and pathogenicity studies

There appears to be a lack of information concerning responses of mules to natural infection or experimental inoculation with equine infectious anemia virus (EIAV). In the present study EIAV was isolated from mules, for the first time, and its pathogenicity in naturally infected and experimentally inoculated animals was investigated. Two naturally infected (A and B) and three EIAV free mules (C, D and E) were used for this purpose. Mule A developed clinical signs, whereas mule B remained asymptomatic until the end of the study. Mules C and D were each inoculated with 10ml of blood from mule A and developed signs of the disease; they were euthanatized or died at day 22 and 25 post-inoculation, respectively. Mule E served as a negative control. The virus was isolated from the plasma samples of mules with clinical signs of the disease (A, C and D), but not from the asymptomatic mule B. Both proviral DNA and viral RNA were amplified from blood and tissues of the infected animals by nested polymerase chain reaction (nPCR). Antibodies were not detected in the two experimentally infected mules until their natural death or euthanasia. Clinicopathological and laboratory findings showed that, in mules, EIAV produced clinical signs similar to those observed in horses and ponies. Nested PCR proved to be a rapid, sensitive and specific diagnostic method for the detection of EIAV, regardless of the disease stage.

Simultaneous quantitation of equine cytokine mRNAs using a multi-probe ribonuclease protection assay

A rapid multi-probe ribonuclease protection assay (RPA) was developed to quantitate equine-specific cytokine mRNA levels in activated equine monocyte-derived macrophages (EMDM) and equine peripheral blood mononuclear cells (EPBMC). Eleven template plasmids specific to 10 equine cytokine genes and the beta-actin gene were generated from which radiolabeled anti-sense RNA probes were produced. The multi-probe RPA simultaneously quantitated mRNA levels of equine IL-1alpha, IL-1beta, IL-6, IL-8, IL-10, IL-12 p35, IL-12 p40, IFN-gamma, TGF-1beta and TNF-alpha in EPBMC and EMDM with coefficients of variation as low as 0.03-0.08 (3-8%) when normalized to beta-actin expression. This sensitive and rapid assay provides a valuable tool for studies of equine immune responses.

Functional replacement and positional dependence of homologous and heterologous L domains in equine infectious anemia virus replication

We have previously demonstrated by Gag polyprotein budding assays that the Gag p9 protein of equine infectious anemia virus (EIAV) utilizes a unique YPDL motif as a late assembly domain (L domain) to facilitate release of the budding virus particle from the host cell plasma membrane (B. A. Puffer, L. J. Parent, J. W. Wills, and R. C. Montelaro, J. Virol. 71:6541-6546, 1997). To characterize in more detail the role of the YPDL L domain in the EIAV life cycle, we have examined the replication properties of a series of EIAV proviral mutants in which the parental YPDL L domain was replaced by a human immunodeficiency virus type 1 (HIV-1) PTAP or Rous sarcoma virus (RSV) PPPY L domain in the p9 protein or by proviruses in which the parental YPDL or HIV-1 PTAP L domain was inserted in the viral matrix protein. The replication properties of these L-domain variants were examined with respect to Gag protein expression and processing, virus particle production, and virus infectivity. The data from these experiments indicate that (i) the YPDL L domain of p9 is required for replication competence (assembly and infectivity) in equine cell cultures, including the natural target equine macrophages; (ii) all of the functions of the YPDL L domain in the EIAV life cycle can be replaced by replacement of the parental YPDL sequence in p9 with the PTAP L-domain segment of HIV-1 p6 or the PPPY L domain of RSV p2b; and (iii) the assembly, but not infectivity, functions of the EIAV proviral YPDL substitution mutants can be partially rescued by inclusions of YPDL and PTAP L-domain sequences in the C-terminal region of the EIAV MA protein. Taken together, these data demonstrate that the EIAV YPDL L domain mediates distinct functions in viral budding and infectivity and that the HIV-1 PTAP and RSV PPPY L domains can effectively facilitate these dual replication functions in the context of the p9 protein. In light of the fact that YPDL, PTAP, and PPPY domains evidently have distinct characteristic binding specificities, these observations may indicate different portals into common cellular processes that mediate EIAV budding and infectivity, respectively.

Equine infectious anemia virus genomic evolution in progressor and nonprogressor ponies

A primary mechanism of lentivirus persistence is the ability of these viruses to evolve in response to biological and immunological selective pressures with a remarkable array of genetic and antigenic variations that constitute a perpetual natural experiment in genetic engineering. A widely accepted paradigm of lentivirus evolution is that the rate of genetic variation is correlated directly with the levels of virus replication: the greater the viral replication, the more opportunities that exist for genetic modifications and selection of viral variants. To test this hypothesis directly, we examined the patterns of equine infectious anemia virus (EIAV) envelope variation during a 2.5-year period in experimentally infected ponies that differed markedly in clinical progression and in steady-state levels of viral replication as indicated by plasma virus genomic RNA assays. The results of these comprehensive studies revealed for the first time similar extents of envelope gp90 variation in persistently infected ponies regardless of the number of disease cycles (one to six) and viremia during chronic disease. The extent of envelope variation was also independent of the apparent steady-state levels of virus replication during long-term asymptomatic infection, varying from undetectable to 10(5) genomic RNA copies per ml of plasma. In addition, the data confirmed the evolution of distinct virus populations (genomic quasispecies) associated with sequential febrile episodes during acute and chronic EIA and demonstrated for the first time ongoing envelope variation during long-term asymptomatic infections. Finally, comparison of the rates of evolution of the previously defined EIAV gp90 variable domains demonstrated distinct differences in the rates of nucleotide and amino acid sequence variation, presumably reflecting differences in the ability of different envelope domains to respond to immune or other biological selection pressures. Thus, these data suggest that EIAV variation can be associated predominantly with ongoing low levels of virus replication and selection in target tissues, even in the absence of substantial levels of plasma viremia, and that envelope variation continues during all stages of persistent infection as the virus successfully avoids clearance by host defense mechanisms.

Detection of horses infected naturally with equine infectious anemia virus by nested polymerase chain reaction

A nested polymerase chain reaction (PCR) amplifying a region of the gag gene of equine infectious anemia virus (EIAV) was developed for the rapid and direct detection of proviral DNA from the peripheral blood of naturally infected horses and was compared with the Coggins test. DNA prepared from white blood cells of 122 field horses from 15 stables with reported cases of EIAV and one seronegative stable were analysed. Amplifications of expected size fragments were obtained by nested PCR for 88 horses using two different sets of primers targeting the gag region. The specificity of the amplified products was confirmed by hybridization using a digoxigenin-labeled probe. Gag-nested PCR-restriction fragment length polymorphism analysis distinguished two different subtypes of gag gene, A and B. Subtype A was found to be the most prevalent among the infected horses that were tested. The PCR-gag amplified sequence of subtype A shared 84.6% nucleotide and 93% deduced amino acid sequence identities with the prototype Wyoming strain whereas subtype B sequence was almost 100% identical to the prototype. Sequence analysis of gag subtype A suggests the presence of a novel EIAV variant among infected horses in Canada. The nested PCR assay developed in the present study detected more EIAV positive animals and was found as specific as the agar gel immunodiffusion (Coggins) assay and offers great potential a diagnostic test for the detection of EIAV infections in field horses.

Tissue sites of persistent infection and active replication of equine infectious anemia virus during acute disease and asymptomatic infection in experimentally infected equids

Equine infectious anemia virus (EIAV) infection of horses is characterized by recurring cycles of disease and viremia that typically progress to an inapparent infection in which clinical symptoms are absent as host immune responses maintain control of virus replication indefinitely. The dynamics of EIAV viremia and its association with disease cycles have been well characterized, but there has been to date no comprehensive quantitative analyses of the specific tissue sites of EIAV infection and replication in experimentally infected equids during acute disease episodes and during asymptomatic infections in long-term inapparent carriers. To characterize the in vivo site(s) of viral infection and replication, we developed a quantitative competitive PCR assay capable of detecting 10 copies of viral DNA and a quantitative competitive reverse transcription-PCR assay with a sensitivity of about 30 copies of viral singly spliced mRNA. Animals were experimentally infected with one of two reference viruses: the animal-passaged field isolate designated EIAV(Wyo) and the virulent cell-adapted strain designated EIAV(PV). Tissues and blood cells were isolated during the initial acute disease or from asymptomatic animals and analyzed for viral DNA and RNA levels by the respective quantitative assays. The results of these experiments demonstrated that the appearance of clinical symptoms in experimentally infected equids coincided with rapid widespread seeding of viral infection and replication in a variety of tissues. During acute disease, the predominant cellular site of viral infection and replication was the spleen, which typically accounted for over 90% of the cellular viral burden. In asymptomatic animals, viral DNA and RNA persisted in virtually all tissues tested, but at extremely low levels, a finding indicative of tight but incomplete immune control of EIAV replication. During all disease states, peripheral blood mononuclear cells (PBMC) were found to harbor less than 1% of the cellular viral burden. These quantitative studies demonstrate that tissues, rather than PBMC, constitute the predominant sites of virus replication during acute disease in infected equids and serve as resilient reservoirs of virus infection, even in the presence of highly effective immune responses that maintain a stringent control of virus replication in long-term inapparent carriers. Thus, these observations with EIAV, a predominantly macrophage-tropic lentivirus, highlight the role of tissues in sequestering lentiviral infections from host immune surveillance.

The S2 gene of equine infectious anemia virus is a highly conserved determinant of viral replication and virulence properties in experimentally infected ponies

Equine infectious anemia virus (EIAV) is genetically one of the simplest lentiviruses in that the viral genome encodes only three accessory genes, tat, rev, and S2. Although serological analyses demonstrate the expression of the S2 protein in persistently infected horses, the role of this viral gene remains undefined. We recently reported that the S2 gene is not essential for EIAV replication in primary equine macrophages, as EIAV mutants lacking the S2 gene replicate to levels similar to those of the parental virus (F. Li, B. A. Puffer, and R. C. Montelaro, J. Virol. 72:8344-8348, 1998). We now describe in vivo studies that examine the evolution and role of the S2 gene in ponies experimentally infected with EIAV. The results of these studies reveal for the first time that the S2 gene is highly conserved during persistent infection and that deletion of the S2 gene reduces viral virulence and virus replication levels compared to those of the parental virus containing a functional S2 gene. These data indicate that the EIAV S2 gene is in fact an important determinant of viral replication and pathogenic properties in vivo, despite the evident lack of S2 influence on viral replication levels in vitro. Thus, these observations suggest in vivo functions of EIAV S2 that are not adequately reflected in simple infections of cultured cells, including natural target macrophages.

Functional characterization of equine dendritic cells propagated ex vivo using recombinant human GM-CSF and recombinant equine IL-4

Naive T cells can be activated both in vivo and in vitro by specialized antigen presenting cells, dendritic cells (DC), with potent antigen-specific, immunostimulatory activity. Indeed, DC can provide an extremely powerful and important immunological tool by which to potentiate the immune response for specific recognition of foreign antigens. Until recently, the direct isolation of DC from PBMC required laborious procedures with extremely poor yields (<0.1%). Methods have been developed for the human, lower primate, and murine model systems to propagate large numbers of DC from PBMC or bone marrow ex vivo with various cytokines. However, all other model systems, including equine, still require the laborious isolation procedures to obtain DC. In this study, we have adapted the methods developed for the human system to generate large numbers of equine DC from PBMC precursors using recombinant human GM-CSF and recombinant equine IL-4. Our report is the first documentation of ex vivo generated DC from PBMC in a domesticated animal model system. Equine DC derived from PBMC were rigorously characterized by analyzing morphological, phenotypic, and functional properties and were determined to have similar attributes as DC generated from human PBMC. Equine DC appeared stellate with large projectiles and veils and had cell surface antigens at similar levels as those defined on human and murine DC. Furthermore, functional attributes of the DC included rapidly capturing antigens by pinocytosis, receptor-mediated endocytosis, and phagocytosis, activating naive T cells in a mixed leukocyte reaction to a much greater extent than macrophage or lymphoblasts, presenting soluble and particulate antigen 10-100 fold more effectively to T cells on a per cell basis than macrophage or lymphoblasts, and presenting soluble and particulate antigen to both CD4+ and CD8+ T cells. Taken together, our study provides a framework by which equine DC can now be readily produced from PBMC precursors and presents an impetus for and model by which DC can be simply generated in other animal model systems.

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.

In vitro antibody-dependent enhancement assays are insensitive indicators of in vivo vaccine enhancement of equine infectious anemia virus

We have previously demonstrated a high propensity for enhancement of virus replication and disease resulting from experimental immunization of ponies with a baculovirus recombinant envelope (rgp90) vaccine from equine infectious anemia virus (EIAV). The current studies were undertaken to examine the correlation between the observed in vivo vaccine enhancement and in vitro assays for antibody-dependent enhancement (ADE) of EIAV replication. Toward this goal an optimized EIAV in vitro enhancement assay was developed using primary equine macrophage cells and used to evaluate the enhancement properties of immune serum taken from rgp90 immunized ponies that displayed various levels of vaccine enhancement after experimental challenge with EIAV. For comparison, we analyzed in parallel immune serum samples from a group of ponies immunized with a viral envelope subunit vaccine (LL-gp) that produced sterile protection from EIAV challenge. The results of these assays demonstrated that the rgp90 immune serum had a greater propensity for in vitro enhancement of EIAV replication than serum from the protected LL-gp immunized ponies; in vitro enhancement levels for the rgp90 immune sera averaged about 1.5, with a maximum enhancement value of about 2.0. While distinguishing between immune serum produced by the rgp90 and LL-gp immunizations, the in vitro enhancement assay failed to reliably correlate with the severity of in vivo enhancement observed among the rgp90 vaccine recipients. Vaccinated ponies that experienced moderate to no disease signs displayed levels of in vitro enhancement similar to those of ponies that experienced severe and fatal enhancement of disease after viral challenge. The observed in vitro enhancement was demonstrated to be dependent on serum immunoglobulin, but independent of complement. These studies demonstrate in the EIAV system that in vitro ADE assays appear to be relatively insensitive indicators of the severity of in vivo enhancement and that relatively low levels of in vitro ADE can be associated with severe to fatal enhancement of virus replication and disease in vivo. These observations suggest that relatively low levels of serum ADE observed in other lentivirus systems, including HIV-1, may have more profound effects on in vivo virus replication and disease than previously recognized.