The CD52 antigen was extracted from human spleens with organic solvents and purified by immunoaffinity and reverse-phase chromatography. The latter step resolved two CD52 species, called CD52-I and CD52-II. Both species were found to contain similar N-linked oligosaccharides and glycosylphosphatidylinositol (GPI) anchor glycans. The N-linked oligosaccharides were characterized by methylation linkage analysis and, following exhaustive neuraminidase and endo-beta-galactosidase digestion, by the reagent array analysis method. The results showed that the single CD52 N-glycosylation site is occupied by large sialylated, polylactosamine-containing, core-fucosylated tetraantennary oligosaccharides. The locations of the phosphoryl substituents on the GPI anchor glycan were determined using a new and sensitive method based upon partial acid hydrolysis of the GPI glycan. The difference between CD52-I and CD52-II was in the phosphatidylinositol (PI) moieties of the GPI anchors. The phosphatidylinositol-specific phospholipase C-sensitive CD52-I contained exclusively distearoyl-PI, while the PI-phospholipase C-resistant CD52-II contained predominantly a palmitoylated stearoyl-arachidonoyl-PI, as judged by electrospray ionization mass spectrometry. Tandem mass spectrometric studies indicated that the palmitoyl residue of the CD52-II anchor is attached to the 2-position of the myo-inositol ring. Both the CD52-I and CD52-II PI structures are unusual for GPI anchors and the possible significance of this is discussed. The alkali-lability of the CD52 epitope recognized by the Campath-1H monoclonal antibody was studied. The data suggest that the alkali-labile hydroxyester-linked fatty acids of the GPI anchor are necessary for antibody binding.

CAMPATH-1 antibodies recognize a unique molecule on human lymphocytes and are unusually efficient at causing cell lysis with homologous complement. They have been successfully used for lymphocyte depletion in vivo in a variety of diseases. We find that the antigen is a very small glycosylphosphatidylinositol (GPI)-anchored glycoprotein with a mature peptide comprising only 12 amino acids. It can be separated into two distinct antigenic fractions which differ in their susceptibility to phosphatidylinositol-specific phospholipase C. There is one N-linked glycosylation site, but no evidence for O-glycosylation despite the presence of several serine and threonine residues. The antibodies were found to bind, albeit with a generally reduced affinity, to a proteolytic fragment containing the C-terminal tripeptide and the GPI anchor. We postulate that one of the reasons why the CAMPATH-1 antibodies are so good for cell lysis is because they bind to an epitope which is likely to be very close to the lipid bilayer.

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