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IL-2 and equine chorionic gonadotrophin (eCG) initiated reactivation of equid herpesvirus-1 (EHV-1) from venous lymphocytes at a frequency of 1/10 5. Indirect immunofluoresence showed that > 80% of virus-positive leukocytes were CD5 /CD8 with the remaining 20% being CD5 /CD8 /CD4 . Cocultivation demonstrated that the reactivated virus was infectious. In addition, virus was reactivated in vitro from leukocytes of > 70% of horses by the mitogens phytohaemagglutinin (PHA) and poke- weed mitogen (PWM). Transfer of supernatants showed that IL-2 and eCG acted indirectly by causing the release of other mediators from adherent cells; these mediators then reactivated EHV-1 from T cells. Blocking experiments with anti-IL-2 showed that PWM and PHA acted via IL-2 but that eCG did not. This is the first clear definition of the lymphoid cells that harbour latent EHV-1 in vivo and correlates with current RT-PCR and in situ hybridization of latency-associated transcripts in lymphocytes. This method of reactivation in vitro can be used to detect horses carrying latent EHV-1 in vivo and also has the potential to dissect the sequence of events involved in reactivation in vitro.
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