Pathogenesis of Avian Lymphoid Leukosis. I. Histogenesis2

Inbred White Leghorn chickens, line 15l, were killed and examined monthly after infection at hatching with the avian tumor virus, RPL12. The earliest neoplastic changes of lymphoid leukosis were found in the bursa of Fabricius by age 8 weeks. After a latency period of approximately 14 weeks lymphomatous foci occurred in peripheral visceral organs. Removal of the bursa at 12 weeks prevented dissemination of lymphoid leukosis. Although histologic evidence of neoplastic transformation was found in the bursas of approximately 70% of males and females, only 40% of RPL12-inoculated birds developed disseminated lymphoid leukosis, and most of these were females. Physiologic regression of the bursa occurred serveral weeks earlier in males than in females. The observations indicate that avian lymphoid leukosis begins in the bursa of Fabricius and is featured by an alteration of differentiation of the bursal line of lymphoid cells. The results also suggest that, while the lymphoma is localized to the bursa, the physiologic influences which induce bursal regression may also bring about a regression of the tumor.