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.

The CAMPATH-1 (CDw52) antigen has been purified from human spleen. The antigenic epitope is heat stable but sensitive to mild alkali treatment. Experiments with phosphatidylinositol-specific phospholipase C indicate that it is anchored by a glycosylphosphatidylinositol (GPI) anchor. An N-terminal sequence of 11 amino acids was determined, followed by an abrupt stop. Using short overlapping mixed oligonucleotide primers, cDNA synthesized from the mRNA of a human B cell line was amplified by the polymerase chain reaction. The product was used to isolate cDNA clones and the full amino acid sequence of the CAMPATH-1 antigen was deduced. It consists of 37 amino acid residues plus a 24-residue signal peptide. It has all the features expected for a GPI-anchored membrane protein except that the predicted mature protein is remarkably short, comprising no more than 18 residues and possibly as few as 12 (depending on the GPI linkage site). Potential attachment sites for carbohydrate are present and it is shown that the antigen contains N-linked oligosaccharide(s). This structure accounts for the known properties of the antigen, though the exact reasons why it is such a good target for cell lysis in vitro and in vivo are not yet clear.

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