Leukocyte cytotoxicity in a persistent virus infection: presence of direct cytotoxicity but absence of antibody-dependent cellular cytotoxicity in horses infected with equine infectious anemia virus

Fusobacterium equinum possesses a leukotoxin gene and exhibits leukotoxin activity

Fusobacterium equinum, a gram negative, rod-shaped and an obligate anaerobic bacterium is a newly described species. The organism is associated with necrotic infections of the respiratory tract in horses that include necrotizing pneumonia, pleuritis and paraoral infections. The species is closely related to F. necrophorum that causes liver abscesses in cattle and sheep, calf-diphtheria in cattle, and foot-rot in sheep and cattle. Leukotoxin, an exotoxin, is an important virulence factor in bovine strains of F. necrophorum. Our objective was to examine strains (n=10) of F. equinum for leukotoxin (lktA) gene and its toxic effects on equine leukocytes. Southern hybridization and partial DNA sequencing revealed that all the 10 strains had the lktA gene with greater similarities to F. necrophorum subsp. necrophorum. The secreted leukotoxin was detected in the culture supernatant and its biological activity was determined by viability assays with equine polymorphonuclear cells (PMNs) using flow cytometry. While culture supernatants of four strains (E1, E7, E9, and E10) were highly toxic to equine PMNs; strain E5 was moderately toxic and the remaining strains (E2, E3, E4, E6, and E8) were only mildly toxic. Our data indicated that F. equinum isolates had lktA gene and its product was toxic to equine leukocytes. Therefore, leukotoxin may be an important virulence factor in F. equinum infections.

Equine infectious anemia virus envelope evolution in vivo during persistent infection progressively increases resistance to in vitro serum antibody neutralization as a dominant phenotype

Equine infectious anemia virus (EIAV) infection of horses is characterized by well-defined waves of viremia associated with the sequential evolution of distinct viral populations displaying extensive envelope gp90 variation; however, a correlation of in vivo envelope evolution with in vitro serum neutralization phenotype remains undefined. Therefore, the goal of the present study was to utilize a previously defined panel of natural variant EIAV envelope isolates from sequential febrile episodes to characterize the effects of envelope variation during persistent infection on viral neutralization phenotypes and to define the determinants of EIAV envelope neutralization specificity. To assess the neutralization phenotypes of the sequential EIAV envelope variants, we determined the sensitivity of five variant envelopes to neutralization by a longitudinal panel of immune serum from the source infected pony. The results indicated that the evolution of the EIAV envelope sequences observed during sequential febrile episodes produced an increasingly neutralization-resistant phenotype. To further define the envelope determinants of EIAV neutralization specificity, we examined the neutralization properties of a panel of chimeric envelope constructs derived from reciprocal envelope domain exchanges between selected neutralization-sensitive and neutralization-resistant envelope variants. These results indicated that the EIAV gp90 V3 and V4 domains individually conferred serum neutralization resistance while other envelope segments in addition to V3 and V4 were evidently required for conferring total serum neutralization sensitivity. These data clearly demonstrate for the first time the influence of sequential gp90 variation during persistent infection in increasing envelope neutralization resistance, identify the gp90 V3 and V4 domains as the principal determinants of antibody neutralization resistance, and indicate distinct complex cooperative envelope domain interactions in defining sensitivity to serum antibody neutralization.

Frequency of memory cytotoxic T lymphocytes to equine infectious anemia virus proteins in blood from carrier horses

Horses with equine infectious anemia virus (EIAV) have episodes of viremia and disease; however, most eventually become inapparent carriers. A possible mechanism of control is cytotoxic T lymphocytes (CTL). To evaluate CTL in inapparent carriers with low viral loads, peripheral blood mononuclear cells (PBMC) were stimulated in vitro with autologous EIAV-infected PBMC and human IL-2 to detect memory CTL (CTLm). In initial studies, three carriers had CTLm and one of these had low-level effector CTL (CTLe). The CTLm were restricted by equine lymphocyte alloantigen-A (ELA-A) locus encoded MHC class I molecules on autologous equine kidney (EK) target cells. In addition, EK cells did not express MHC class II molecules. The CTLm frequency in PBMC from five inapparent carriers infected for 22 to 50 months was determined by limiting dilution analysis. PBMC were diluted, stimulated, and tested on EK cell targets infected with EIAV and recombinant vaccinia viruses expressing EIAV Env or Gag/Pr proteins. All five carriers had CTLm to EIAV-infected targets, while four had CTLm to targets expressing Env and four had CTLm to targets expressing Gag/Pr proteins. The CTLm frequency range was 60 to 468 per 10(6) PBMC to EIAV-infected targets, 4 to 286 to Env-expressing targets, and 25 to 190 to Gag/Pr-expressing targets. These results should facilitate the identification of epitopes recognized by predominant CTLm from horses controlling a lentivirus infection.

Maturation of the cellular and humoral immune responses to persistent infection in horses by equine infectious anemia virus is a complex and lengthy process

Equine infectious anemia virus (EIAV) provides a natural model system by which immunological control of lentivirus infections may be studied. To date, no detailed study addressing in parallel both the humoral and cellular immune responses induced in horses upon infection by EIAV has been conducted. Therefore, we initiated the first comprehensive characterization of the cellular and humoral immune responses during clinical progression from chronic disease to inapparent stages of EIAV infection. Using new analyses of antibody avidity and antibody epitope conformation dependence that had not been previously employed in this system, we observed that the humoral immune response to EIAV required a 6- to 8-month period in which to fully mature. During this time frame, EIAV-specific antibody evolved gradually from a population characterized by low-avidity, nonneutralizing, and predominantly linear epitope specificity to an antibody population with an avidity of moderate to high levels, neutralizing activity, and predominantly conformational epitope specificity. Analyses of the cell-mediated immune response to EIAV revealed CD4+ and CD8+ major histocompatibility complex-restricted, EIAV-specific cytotoxic T-lymphocyte (CTL) activity apparent within 3 to 4 weeks postinfection, temporally correlating with the resolution of the primary viremia. After resolution of the initial viremia, EIAV-specific CTL activity differed greatly among the experimentally infected ponies, with some animals having readily detectable CTL activity while others had little measurable CTL activity. Thus, in contrast to the initial viremia, it appeared that no single immune parameter correlated with the resolution of further viremic episodes. Instead, immune control of EIAV infection during the clinically inapparent stage of infection appears to rely on a complex combination of immune system mechanisms to suppress viral replication that effectively functions only after the immune system has evolved to a fully mature state 6 to 8 months postinfection. These findings strongly imply the necessity for candidate EIAV and other lentivirus vaccines to achieve this immune system maturation for efficacious immunological control of lentivirus challenge.

Control of equine infectious anemia virus is not dependent on ADCC mediating antibodies

Horses infected with equine infectious anemia virus (EIAV) have recurrent episodes of viremia which are eventually controlled, but the immune mechanisms have not been identified. Antibodies were detected to the surface of EIAV-infected cells within 1 month postinfection and remained for at least 3.5 years postinfection. These antibodies recognized cell surface-exposed envelope (Env) glycoproteins, but could not mediate antibody dependent cellular cytotoxicity (ADCC) using EIAV-WSU5-infected equine kidney (EK) cells as targets and peripheral blood mononuclear cells (PBMC) or polymorphonuclear cells (PMN) as effector cells. Furthermore, purified IgG antibodies from horses infected with either EIAV-WSU5 or EIAV-Wyo did not mediate ADCC of infected target cells. Armed effector cells could not be detected in infected horse blood nor could effector cells be prearmed by incubation with serum antibodies to cell surface antigens. The use of EIAV-WSU5-infected equine macrophages as target cells did not result in ADCC. In contrast, serum antibody from EHV-1 vaccinated horses and PBMC or PMN as effector cells caused ADCC of EHV-1-infected EK cells. These results indicate that ADCC is not involved in the control of EIAV in carrier horses.

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.

Clinical signs and humoral immune response in horses following equine herpesvirus type-1 infection and their susceptibility to equine herpesvirus type-4 challenge

A group of three horses was experimentally infected with equine herpesvirus type 1 (EHV-1) and showed clinical signs characterised by a biphasic febrile response, leucopenia and cell associated viraemia accompanied by virus shedding from the nasopharynx. A second exposure to the virus 18 days later resulted in the isolation of virus from the nasopharynx of one horse. This and a further group of three EHV-1 seropositive horses were subsequently infected with equine herpesvirus type 4 (EHV-4) 147 days after the initial EHV-1 infection and virus was shed from the nasopharynx in the absence of clinical disease. Following the first EHV-1 infection, virus specific immunoglobulin M (IgM) was present by day 5 and remained high until the second exposure at day 18 at which point levels decreased. In contrast, EHV-1 specific IgG, detected at day 6 peaked at day 18, after which time levels remained high. Virus neutralising antibodies and antibodies able to mediate antibody-dependent cellular cytotoxicity were present by day 10. The immune response to EHV-1 is discussed with reference to the disease.

Qualitative analyses of cellular immune functions in equine infectious anemia show homology with AIDS

Equine infectious anemia (EIA) is a disease caused by a lymphocytotropic lentivirus which belongs to the same subfamily as HIV. Because of the very close relationship of their predicted gag and pol gene products and similarities in clinical manifestations of the disease, EIA served as a model to study immunological events involved in the host defence against lymphocytotropic viral infections. The existence of antibody dependent cell-mediated cytotoxicity against autologous EIA virus infected lymphoblasts was demonstrated in vitro at the beginning of an acute attack of the disease. Cytotoxic activity was not found or was very low during chronic infection. Reactivity of peripheral blood lymphocytes to Protein A and allogeneic cells in vitro was significantly suppressed in the presence of an acute serum, while the reactivity to PHA was at the normal level. These results suggest that cellular immune mechanism(s) are also involved in removal of EIA virus infected cells as has been reported recently in HIV-1 infected individuals and that EIAV and HIV immunopathogenesis show similarities in affecting the immune response of the host.

T and B lymphocytes in horses persistently infected with equine infectious anaemia virus

The percentage of T and B lymphocytes in the peripheral blood of horses chronically infected with equine infectious anaemia (EIA) virus was determined and the results were compared with the percentage of these cells in healthy uninfected horses. Cells with membrane receptors for sheep erythrocytes (T and active T lymphocytes) were determined by E and A rosette techniques, while cells with receptors for the C3b component of complement and those with receptors for mouse erythrocytes (B lymphocytes), were determined by the EAC rosette method. The percentage of Fe positive cells was assayed by the EA rosette test. The majority of peripheral blood lymphocytes (PBL) from both uninfected and EIA-infected horses formed rosettes of each kind with only three erythrocytes indicating a low density of the corresponding receptors on the cell membrane under the condition of the assays used. The percentage of T lymphocytes in the peripheral blood of diseased horses (52.4 +/- 1.6%), as detected by E rosettes, was significantly (p less than 0.01) higher than in control animals (42.4 +/- 3.5%). In clinically healthy horses 8.9 +/- 1.1% of PBL were identified by A rosettes as active T cells, whereas animals with a chronic form of EIA had a much lower (p less than 0.001) percentage of these cells (4.7 +/- 0.7%). In the B lymphocyte subpopulations the percentages of cells bearing Fc and C3b receptors were markedly elevated (p less than 0.001) in EIA-infected horses (24.7 +/- 0.8% and 42.8 +/- 2.2% respectively) as compared to uninfected animals (15.1 +/- 1.4% and 29.6 +/- 1.2% respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Cell-mediated cytotoxicity of bovine mononuclear cells to IBRV-infected cells: dependence on Sephadex G-10 adherent cells

Following intranasal inoculation of cattle with infectious bovine rhinotracheitis virus (IBRV) mononuclear cells that produced a genetically unrestricted cytotoxic response against IBRV-infected, but not against uninfected cells, were present in peripheral blood. Cytotoxicity was detected between 6 and 14 days after primary infection in a 20 h, but not in a 5 h, 51Cr-release assay. Cytotoxic activity was present in peripheral blood mononuclear cells from infected and subsequently hyperimmunized cattle for a considerably longer time. Neither natural cytotoxicity, antibody-dependent cell cytotoxicity, nor antibody produced during the assay was responsible for the cytotoxicity. However, cytotoxicity was dependent upon an adherent mononuclear cell that was partially removed by passage over nylon wool and completely removed by passage over Sephadex G-10.

Antibody-dependent cell-mediated cytotoxicity in sheep

The ability of sheep leukocytes to mediate antibody — dependent cell-mediated cytotoxicity (ADCC) and that of sheep serum IgG₁ and IgG₂ to induce ADCC were investigated. Partial characterization of effector cells was attempted. These investigations revealed that ADCC occurs in sneep. With chicken erythrocytes (CRBC) as the target cells, polymorphonu-cleated cells (PMN), and monocytes, were the most effective leukocytes. Ovine peripheral blood lymphocytes (PBL) also mediated ADCC, and within the PBL population, T-cells were capable of mediating ADCC. The T-cells were obtained by nylon wool fractionation and selective agglutination by peanut agglutinin (PNA) and Helix pomatia agglutinin (HPA). Both nylon wool adherent and non-adherent fractions were active in ADCC, although the former were more active, implying heterogeneity in nylon wool adherence among ovine K-cells. Depletion of B (SIg⁺) cells did not affect ADCC activity of the remaining cells. Depletion of Fc⁺ cells markedly reduced cytotoxic activity of PBL. Both sheep IgG₁ and IgG₂ anti-CRBC immunoglobulins were able to induce ADCC.

Antigenic stimulation of T lymphocytes in chronic nononcogenic retrovirus infection: equine infectious anemia

Equine infectious anemia is a chronic disease of horses caused by a nononcogenic retrovirus. Studies were undertaken to determine the types of cells involved in the in vitro lymphoproliferative response to viral antigens and the dynamics of this reaction. It was observed that reactive lymphocytes were present at unpredictable times in the peripheral blood of infected horses. This reaction was shown to be specific for the interaction of equine infectious anemia virus and T lymphocytes. Enriched B-lymphocyte populations did not divide when exposed to equine infectious anemia virus. Macrophages were depleted from the reaction by two methods: adherence to Sephadex and a combination of binding to Sephadex and adherence to complement-coated erythrocytes. Both methods reduced the number of monocytes, but only the combination of Sephadex and complement-coated cells removed the accessory cells needed for lymphocyte proliferation. We conclude that during the chronic stages of equine infectious anemia the number of antigen-reactive T lymphocytes fluctuates within the peripheral blood and that these cells require a complement-binding cell for reaction. The relationship of these cells to the lymphoproliferative stages of this disease is discussed.