In the course of exploring the antibody response in the unsensitized WF (u) female pregnant by a DA (a) male, we prepared a hybridoma that secreted an antibody (mAb 213) that was specific to the a haplotype but identified an antigen different from Pa. This antigen was designated RT11. It is present from the twelfth day of gestation on the collagen fibers of the placenta and of all organs in fetal and adult rats. It is particularly prominent on red blood cells; in the yolk sac epithelium; in the walls of the endodermal sinus, blood vessels and bronchioles; and in capsules and trabeculae. A very small amount is present on DA lymphocytes, since 17— 20% of them react with mAb 213 by cytofluorimetery. The RT11 antigen is absent from the basal and labyrinthine trophoblast cells, from the parenchymal cells of all organs, and from T and B cells. This distribution patternis completely different from that of the Aa and Pa antigens. Inhibition and absorption studies showed that RT11 is not an integral part of the collagen molecule. The SDS-PAGE analysis of the immunoprecipitates of RT11 from radioiodinated whole-membrane extracts of red blood cells and from the glycoprotein fraction thereof showed that it is an unglycosylated protein of molecular weight 29,000. The evidence to date suggests that RT11 is a blood group antigen. Studies on the genetic control of the expression of RT11 were undertaken to determine whether a gene linked to the MHC was involved and whether the control mechanism was unigenic or polygenic. Backcrosses generated using inbred strains—(DA×BN)F1×DA—and using complementary congenic strains—(DA.1N×BN.1A) F1×BN.1A—showed that the expression of RT11 was under polygenic control, and that both an MHC-linked gene (1.2 cM from RT1.Aa) and genes not linked to the MHC are involved. By contrast, the expression of the Pa antigen is under the control of an MHC gene only.
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