Development and composition of lymphoid lesions in the spleens of Marek's disease virus-infected chickens: association with virus spread and the pathogenesis of Marek's disease

Changes in lymphocyte distribution in spleens of Marek's disease virus (MDV ) infected White Leghorn chickens of line 7(2) (MD susceptible) and line 6( 1) (MD resistant) were studied by immunocytochemistry. Lymphocytes expressi ng the MDV antigen pp38 (predominantly B cells) were detected from 4 to 6 d ays post-inoculation (d.p.i,) but not at or after 8 d.p.i., and were more n umerous in line 7(2). In line 6(1), infection resulted in depletion of B ly mphocytes and an increase in T lymphocytes from 3 to 6 d.p.i., but no chang e in distribution of these cells. From 8 d.p.i., the B-dependent tissue beg an to recover and the T cells decreased in number, In line 7(2), infection caused a dramatic change in lymphocyte distribution, with formation of 'lym phoid lesions'. Diffuse, irregular patches of B lymphocytes, around the cap illaries, became surrounded by large aggregates of TCR alpha beta 1(+) CD8( +) and CD4(+) lymphocytes, bordered by a band of TCR gamma delta(+) lymphoc ytes, From 8 d.p.i., the B-dependent areas partially recovered, while TCR a lpha beta 1(+) CD4(+) and CD8(+) lymphocytes, potentially transformed, beca me extensively scattered throughout the spleen. We conclude that in line 7( 2), replication and spread of MDV is more efficient and T cell responses in early infection are greater, favouring the tumour stage of the disease.