Selective integration of avian leukosis virus in different hematopoietic tissues

Hematopoietic tissues obtained from avian leukosis virus (ALV)-infected Hyline SC chickens were analyzed for the presence of integrated viral DNA sequences. Cells were prepared from bone marrow, bursa, spleen, thymus, and peripheral blood. Following the removal of erythrocytes, cellular DNAs from each of these tissues were examined by Southern analysis. During the first few weeks of infection, DNA from the bone marrow contained as many as 0.5 copies of viral DNA per haploid genome. Cells from the bursa and peripheral blood contained between 0.05 and 0.15 copies per haploid genome. In contrast, neither splenic nor thymic DNA contained significant levels of viral DNA sequences even though infected birds developed titers of circulating virus between 10(5) and 10(6) IU/ml of plasma. DNA prepared from erythrocytes isolated from the peripheral blood of these birds contained approximately 0.4 copies of integrated viral sequences per haploid genome at 2 weeks after infection. Despite greater levels of integrated sequences in other tissues, by 9 weeks after infection viral sequences were detected only in DNA from bursal lymphocytes. Cells prepared from the spleen and thymus of infected birds were also examined for their size distribution, their internal complexity and their surface expression of immunoglobulin. None of the populations examined differed from normal, uninfected control preparations. These results suggest that ALV infection occurs primarily in the bone marrow and that the different tissues of the hematopoietic system are selectively infected. Further, these results indicate that ALV infection persists longer in bursal lymphocytes than in other hematopoietic tissues. Previous studies have demonstrated that the lymphoid tumors that develop in white leghorn chickens following ALV infection are bursal-dependent B-cell lymphomas that express immunoglobulin M. The observations presented in this communication offer, in part, an explanation for the restricted phenotype of the lymphoid tumor that develops in the SC chicken. Further, the data suggest an explanation for the bursal-dependent nature of the ALV-induced lymphoma.