Differential susceptibility to Marek's disease is associated with differences in number, but not phenotype or location, of pp38+ lymphocytes.

Flow cytometric and immunocytochemical techniques were used to quantify, identify and locate Marek's disease herpesvirus (MDV)-infected lymphocytes in lymphoid organs of infected chickens, by expression of the virus antigen pp38. Two closely related lines of chicken, one susceptible to Marek's disease (line 7(2)) and another resistant (line 6(1)), were infected at 2 weeks of age and compared at 10 sampling times between 0 and 50 days post-infection. In both lines 6(1) and 7(2), pp38+ lymphocytes were detected at 4-6 days in the spleen, thymus and bursa. pp38+ cells could not be detected from day 8 onwards. In both lines, pp38+ lymphocytes were located in the peri-ellipsoidal area of the spleen, the medulla of the thymic lobes and the medulla of the bursal follicles. In both lines, pp38+ cells were predominantly B lymphocytes, but CD4+ and CD8+ TCR alphabeta+ T lymphocytes were also detected in the thymus and spleen. For each organ, the mean number of pp38+ lymphocytes was greater in line 7(2) than in line 6(1). pp38+ lymphocytes were not detected in the peripheral blood at any time. The data show that the differential susceptibility of lines 6(1) and 7(2) to the development of Marek's disease lymphoma is not attributable to differences in phenotype or location of pp38+ lymphocytes, or the time of expression of pp38. However, susceptibility is associated with greater numbers of pp38+ lymphocytes.