An antigen in normal avian cells which confuses virological and serological tests.

A cytoplasmic antigen was detected by fluorescent antibody tests in normal lymphoid and fibroblastic cells from chickens and other avian species when explanted and grown in culture. Cells of the bursa of Fabricius were consistently positive but cells of a proportion of thymus, spleen, bone marrow, buffy coat and embryo fibroblast cultures also were positive. Eight of 19 fluorescein-conjugated chicken sera prepared for other studies on a variety of avian pathogens had antibody against the antigen. Uncultured tissues or cultured but unfixed tissues were always negative, and selective blocking and absorption tests confirmed that the fluorescence was due to an antigen-antibody reaction. The antigen was associated with cells transformed by any of 8 strains of Rous sarcoma virus (RSV) but not with cells infected by other avian pathogens, including avian leukosis virus. Tests showed that the antigen was not coded for by RSV or by endogenous (subgroup E) avian leukosis virus genome. It may be coded for by normally non-functional genes which are derepressed in some cells by in vitro cultivation or RSV infection. In vivo derepression in some birds may account for the presence of antibody in sera.