CTLA-4 and CD28: similar proteins, neighbouring genes

Subtractive cloning and screening yielded a cDNA clone corresponding to a molecule expressed in activated T cells, called CTLA-4. At the protein level, CTLA-4, a single-V-domain member of the immunoglobulin superfamily, was found very homologous to the lymphocyte activation molecule CD28. In particular, the hinge region included the hexamer MYPPPY, completely conserved for both molecules and in mice and humans. By immunizing mice with a human CTLA-4 peptide, an anti-CTLA-4 monoclonal antibody (MAb) was obtained, which enabled to establish the MW of the protein (26 and 40 kDa under reduced and non-reduced conditions respectively) and its preliminary tissue distribution. Also, CTLA-4 and CD28 were very similar at the message and at the gene structure level. The corresponding genes had previously been found to co-map on mouse chromosome IC and on human chromosome 2q33. We show that they can be found on the same yeast artificial chromosomes bearing human genomic DNA, and that they are 25 to 150 kb apart. These marked homologies and gene proximity strongly suggest that CTLA-4 and CD28 are the direct products of a duplication event, and raise the question of the function of CTLA-4.