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Abstract
1. The course of infection by exogenous avian leukosis virus was followed in a commercial strain of White Leghorn domestic fowls by measuring viral antigen in feather pulp and egg albumin. Ten days after hatching, 2 out of 360 birds tested positive and at 286 days of age about 60% of the birds had been antigen positive at least once.
2. Among the antigen positive birds, two groups could be distinguished: those which permanently and those which transiently expressed viral antigen. Permanent antigen expression was associated with low antibody titres, while transient antigen expression was associated with high antibody titres.
3. The strain segregated for the two endogenous viral genes ev6 and ev9, both of which express endogenous viral envelope protein, and have been implicated in affecting immune‐responsiveness. The antibody titre in individuals positive for both ev6 and ev9, was significantly lower than in those which had none or only one of the two ev‐genes. In addition, individuals positive for both ev‐genes occurred more frequently in the group permanently positive for viral antigen than in the group transiently antigen positive.
4. The results indicate that there was a strong synergism between ev6 and ev9 in reducing the antibody response to exogenous avian leukosis virus infection, perhaps by inducing immune tolerance or interfering with antibody formation.