Further characterization of marek's disease virus‐infected lymphocytes. II. In vitro infection

Lymphocyte cultures from chicken spleens had been shown to be susceptible to in vitro infection by Marek's disease virus (MDV) 4 in an earlier report from this laboratory. In that study, virus infection was evidenced by virus isolation and detection of viral internal antigen (VIA) 2 days post inoculation (DPI), and serial passage was accomplished by adding fresh spleen cells at 2‐day intervals. The susceptible cells were identified as bursa‐derived lymphocytes (B cells). Using a dual fluorescence technique to identify surface markers for B cells, thymus‐derived lymphocytes (T cells) or la antigen on VIA‐positive cells, we have now shown that a small proportion (generally less than 10%) of VIA‐positive cells observed 2 DPI are T cells, and that a low level of infection can be maintained by serial passage of MDV in cultures totally free of B cells. Most infected T cells in this study had Ia antigen. As the incubation period for infected cultures was extended from 2 to 4 or 5 days, the average number of viable cells decreased but the percentage of viable cells infected with MDV (VIA‐positive) increased. Also, both the proportion and the actual number of infected T cells increased, significantly more so in cultures from genetically susceptible P‐2 donors than from resistant N‐2 donors. Spleen‐cell cultures from resistant Line 6 chickens were markedly less susceptible than those from susceptible Line 7 chickens to in vitro MDV infection, as assessed by numbers of VIA‐positive cells at 5 DPI.