Evidence for Fc receptors on rabbit yolk sac endoderm

APPLICATION of the erythrocyte-antibody rosette technique (EA rosette) to cell cultures of mouse yolk sac membranes has provided evidence of Fc receptors on yolk sac endoderm of this species 1 . Binding of immunogloibulin (Ig) to Fc receptors forms an integral part of a hypothesis proposed by Brambell 2 to account for selective materno–foetal transmission of antibodies. In this hypothesis the receptor protects attached Ig when a transporting endocytotic vesicle (be it in syncytiotrophoblast, yolk sac endoderm, or gut enterocytes, according to the species) contacts proteolytic enzymes after fusion with lysosomes. Little is known about selective transport of antibodies across mouse yolk sac endoderm and it is not clear 1 to what extent these results show binding specific for transport of Ig across the endodermal cells, or merely reflect some special mechanism for endocytosing this particular class of proteins. Studies of rabbit yolk sac endoderm have not shown whether specific binding of Ig takes place; thus bovine IgG, which is poorly transported across rabbit yolk sac endoderm 3,4 , although readily endocytosed 5 , binds to the same extent as rabbit IgG to membrane components isolated from the cells 6 . On the other hand, compared with rabbit IgG, relatively little specific binding of bovine IgG was found when these proteins were incubated in vitro with formalin-fixed rabbit yolk sac 7 . We have now applied the EA rosette technique to rabbit yolk sac endoderm and find Fc receptors which bind both rabbit and human IgG, but not bovine IgG, pointing to their association with selective Ig transport.