Chicken strain G-B1 exhibits a relative resistance to avian osteopetrosis.

The disease induced by the avian myeloblastosis associated virus MAV-2-O in the susceptible chicken strains Brown Leghorn (BLH) and Prague CB (CB) was compared with that induced in the resistant G-B1 strain. Osteopetrosis, stunting and lymphoid organ atrophy were more severe in BLH than in CB chickens. G-B1 animals remained superficially normal until the end of the experiment. In contrast to the other two strains, the histopathological changes were very mild and there was no sign of immunosuppression. After 4 months, however, nephroblastomas could be detected in more than 50 per cent of the infected G-B1 chickens. Similar tumors were also found in CB birds kept for up to 5 months. Antibodies against MAV-2-O specific viral proteins were detected in plasma from infected G-B1 chickens but the titers were less than in plasma of convalescent birds. Virus could be demonstrated in peripheral blood until the end of the experiment (at 8 weeks). Therefore the resistance of the G-B1 strain is due neither to a restriction at the receptor level nor the result of a humoral immune reaction, but represents a new type of relative resistance at the cellular level. From (CC X G-B1)F1 and (CC X G-B1)F2 crosses the resistant phenotype is determined by a single genetic factor. This gene is not linked to the major histocompatibility complex. There is also a sex-dependent factor, possibly hormonal, involved in the resistant phenotype.