Response of embryonic chicken lymphocytes to in ovo exposure to lymphotropic viruses

Objective - To examine effects of virus exposure on embryonic lymphoid organ structure, apoptosis, and lymphoid cell subpopulations. Animals - Eggs of specific pathogen free (SPF) White Leghorn chickens at embryonation day (ED) 17. Procedures - Eggs were inoculated with 2,000 plaque-forming units (PFU) of serotype 1 herpesvirus (Marek's disease virus [MDV 1]), 2,000 PFU of herpesvirus of turkeys (MDV 3), or 1,000 embryo infectious doses (EID₅₀) of infectious bursal disease virus (IBDV). On post-inoculation days (PID) 3 and 5, lymphoid organ to body weight ratios were determined, and bursa of Fabricius, thymus, and spleen were evaluated for lesions and apoptosis. Proportions of lymphoid cell subpopulations of PID-3 chicken embryos and 7-to 10-day-old chicks were quantitated by flow cytometry. Results - Lymphoid organ weights were similar in virus-free, MDV 1, and IBDV groups. Embryos inoculated with 2,000 PFU MDV 3/egg had lower bursal weights than virus-free controls. In a repeated trial, MDV 3 (1,000 PFU to 4,000 PFU) did not reduce bursal weights among groups. Histologic changes were seen in bursae after MDV 1 and IBDV inoculation. Apoptosis was greater in bursae of MDV 1-infected embryos than controls. Lymphoid cell subpopulations were similar among all groups with the exception of CD8⁺ and IgM⁺ cells in spleens of IBDV-infected 10-day-old chicks. Conclusions and Clinical Relevance - Infection with pathogenic strains of MDV 1 and IBDV did not alter lymphocyte subpopulations in embryos or cause complete destruction of lymphoid organs. Changes in lymphoid cell subpopulations exposed as embryos to IBDV were seen only after hatching.