Pre-infection frequencies of equine herpesvirus-1 specific, cytotoxic T lymphocytes correlate with protection against abortion following experimental infection of pregnant mares.
Abstract: In general, vaccines containing inactivated equine herpesvirus-1 (EHV-1) fail to prevent abortion in pregnant mares following infection with a virulent strain of EHV-1. We have tested the hypothesis that resistance to EHV-1-induced abortion in pregnant mares is associated with high frequencies of EHV-1 specific, major histocompatibility complex (MHC) class I-restricted, cytotoxic T lymphocytes (CTL) in the circulation. To test this theory, three groups of pregnant mares were assembled with varying backgrounds of infection or vaccination in an attempt to mimic the immune status of the general population. Group 1 mares (n=9) were untreated controls selected at random. Group 2 mares (n=5) were vaccinated three times intramuscularly with inactivated EHV-1. Group 3 mares (n=3) had been infected with EHV-1 on four previous occasions. The frequency of CTL in blood leucocytes was measured by limiting dilution analysis at three time points; at the beginning of pregnancy (approximately 28 weeks before infection) in the Group 2 and Group 3 mares (4-7 weeks of gestation) (Group 1 was unavailable for sampling) and then 2 weeks before (30-40 weeks of gestation) and 3 weeks after experimental infection in all the mares. Serum samples were collected to monitor complement fixing (CF) antibody titres. Mares in all three groups were infected experimentally with EHV-1 strain Ab4/8 by the intranasal route after which they were monitored clinically to determine the outcome of pregnancy and samples were collected to determine the duration of nasopharyngeal shedding and cell-associated viraemia. The untreated control mares showed low pre-infection CTL. After experimental infection, they all seroconverted, aborted and demonstrated expected clinical and virological signs. Some vaccinated mares (3/5) had elevated titres of CF antibody prior to their first vaccination. All the vaccinated mares seroconverted after vaccination and exhibited higher CTL frequencies than controls before infection. Four of the five foaled normally. The multiply infected mares had low CF antibody titres prior to infection and showed neither seroconversion nor clinical or virological signs after infection. All multiply infected mares exhibited high frequencies of CTL before infection and they all foaled normally. The CTL frequencies observed differed significantly from the expected frequencies in the control and multiply infected groups at 2 weeks pre-infection (P=0.034) and between the foaling and aborting mares at 2 weeks pre-infection (P=0.005) and 3 weeks post-infection (P=0.015). The results show a positive correlation between the number of virus-specific CTL in the peripheral blood of pregnant mares and their protection against abortion induced by EHV-1 infection. Therefore, as indicated by this study, rational approaches to the development of new vaccines for EHV-1 should stimulate cytotoxic immune responses and develop virus-specific CTL as pre-requisites for protection against abortion.
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The research investigates whether the frequency of specific cytotoxic T lymphocytes (CTL) in the bloodstream of pregnant horses links to resistance against abortion triggered by the equine herpesvirus-1 (EHV-1). The study findings support a positive correlation, suggesting that future vaccination strategies for EHV-1 should aim to stimulate such immune responses.
Research Methods
The researchers assembled three groups of pregnant mares with different levels of past infection or vaccination to reflect the varied immune status in general populations.
Mares in Group 1 were untreated controls chosen randomly (9 mares); those in Group 2 had been inoculated three times using inactivated EHV-1 (5 mares); while the mares in Group 3 had been infected with EHV-1 on four previous occasions (3 mares).
The measurement of CTL frequencies in circulating leukocytes was made using limiting dilution analysis at three different time points.
Blood and serum samples were collected to monitor antibody titres and the frequency of CTL. The groups were then subjected to an experimental EHV-1 infection.
Post-infection, the pregnancy outcome, duration of nasopharyngeal shedding, and cell-associated viremia were assessed.
Key Findings
All untreated control mares showed low CTL before infection and, after experimental infection, they all aborted and displayed expected symptoms.
Some of the vaccinated mares already had elevated antibody levels before the vaccination. Post-vaccination, they all developed new antibodies (seroconversion) and displayed higher CTL frequencies than control mares before infection.
Mares with multiple past EHV-1 infections had low antibody levels prior to the experiment but showed neither antibody development nor clinical symptoms after infection. They all had a high frequency of CTL before infection and foaled normally.
Significant differences were observed in CTL frequencies in the control and multiply infected mares before infection and between foaling and aborting mares before and after infection.
Implications and Conclusions
The research results highlight a positive link between the quantities of virus-specific CTL present in the blood of pregnant mares and their protection against abortion caused by EHV-1 infection.
This suggests that efficacious strategies for developing new vaccines for EHV-1 should stimulate cytotoxic immune responses and generate virus-specific CTL as prerequisites for offering protection against abortion.
Cite This Article
APA
Kydd JH, Wattrang E, Hannant D.
(2003).
Pre-infection frequencies of equine herpesvirus-1 specific, cytotoxic T lymphocytes correlate with protection against abortion following experimental infection of pregnant mares.
Vet Immunol Immunopathol, 96(3-4), 207-217.
https://doi.org/10.1016/j.vetimm.2003.08.004
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