Maintenance of Marek's disease herpesvirus latency in vitro by a factor found in conditioned medium

Chicken spleen cells latently infected with Marek's disease virus were cultured with and without conditioned medium (CM) obtained from concanavalin A-stimulated chicken spleen cell cultures. The expression of viral internal antigen(s) (VIA), which is usually associated with cultivation, was prevented or markedly reduced by the CM. This effect required the continued presence of CM, since its removal after 48 h resulted in the subsequent appearance of VIA. Although CM contains both gamma interferon (IFN-gamma) and interleukin 2, our studies suggest that the 'latency-maintaining activity' (LMF) may not be associated with either of these products of stimulated lymphocytes. However, IFN-gamma may also have had some suppressive effect. LMF appears to have an Mr greater than 10,000 and to be inactivated by heating to 90 degrees C for 5 min.