Construction, expression and functional analysis of a glycolipid-linked form of CR1

By genetic engineering of human CR1 cDNA and its stable transfection into cells we have produced a cell line which expresses CR1 anchored to the cell surface by a glycolipid anchor. The glycosyl-phosphatidylinositol (GPI)-CR1 protects cells intrinsically from damage mediated by complement activated through the classical pathway. Cell surface GPI-CR1 is more efficient on a molar basis than soluble CR1 in the assay, but extrinsic protection of other cells was not obtained. Soluble CR1-protected cells extrinsically in the assay but was required at nearly ten fold higher amounts than the intrinsic protection conferred by GPI-anchored CR1. Additionally, GPI-CR1 was shown to act as a co-factor to Factor I in the generation of C3c from iC3b. Since GPI-anchored proteins can incorporate spontaneously into the membranes of living cells by virtue of their lipid tails, the isolated GPI-CR1 will be used to introduce CR1 on to the surfaces of many different types of cell so that its role in immunity can be further investigated.