In vitro reactivation of latent equid herpesvirus-1 from CD5(+)/CD8(+) leukocytes indirectly by IL-2 or chorionic gonadotrophin

IL-2 and equine chorionic gonadotrophin (eCG) initiated reactivation of equ id herpesvirus-l (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(-). Cocu ltivation demonstrated that the reactivated virus was infectious. In additi on, virus was reactivated in vitro from leukocytes of > 70% of horses by th e mitogens phytohaemagglutinin (PHA) and pokeweed mitogen (PWM). Transfer o f supernatants showed that IL-2 and eCG acted indirectly by causing the rel ease of other mediators from adherent cells; these mediators then reactivat ed 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 defini tion of the lymphoid cells that harbour latent EHV-1 in vivo and correlates with current RT-PCR and in situ hybridization of latency-associated transc ripts in lymphocytes. This method of reactivation in vitro can be used to d etect horses carrying latent EHV-1 in vivo and also has the potential to di ssect the sequence of events involved in reactivation in vitro.