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

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Abstract
Changes in lymphocyte distribution in spleens of Marek's disease virus (MDV) infected White Leghorn chickens of line 72 (MD susceptible) and line 61 (MD resistant) were studied by immunocytochemistry. Lymphocytes expressing the MDV antigen pp38 (predominantly B cells) were detected from 4 to 6 days post-inoculation (d.p.i.) but not at or after 8 d.p.i., and were more numerous in line 72. In line 61, infection resulted in depletion of B lymphocytes and an increase in T lymphocytes from 3 to 6 d.p.i., but no change in distribution of these cells. From 8 d.p.i., the B-dependent tissue began to recover and the T cells decreased in number. In line 72, infection caused a dramatic change in lymphocyte distribution, with formation of 'lymphoid lesions'. Diffuse, irregular patches of B lymphocytes, around the capillaries, became surrounded by large aggregates of TCRαβ1+ CD8+ and CD4+ lymphocytes, bordered by a band of TCRγδ+ lymphocytes. From 8 d.p.i., the B-dependent areas partially recovered, while TCRαβ1+ CD4+ and CD8+ lymphocytes, potentially transformed, became extensively scattered throughout the spleen. We conclude that in line 72, replication and spread of MDV is more efficient and T cell responses in early infection are greater, favouring the tumour stage of the disease.