The murine homologue of the T lymphocyte antigen CD28: Molecular cloning and cell surface expression

The human T lymphocyte Ag CD28 (Tp44) is a homodimeric glycoprotein expressed on the surface of a majority of human peripheral T cells and thymocytes. Although exposure of T cells to anti-CD28 mAb does not activate T cells, stimulation of CD28 can synergize with signals transmitted through the TCR or other stimuli to augment proliferation and lymphokine production. We have used a portion of the human CD28 cDNA to isolate a homologous murine cDNA from an EL4 T lymphoma library. The murine clone has 61% nucleotide identity with the human cDNA. Both human and murine sequences exhibit homology with members of the Ig supergene family and CTLA-4, a T cell specific murine gene. Many characteristics of the human CD28 molecule are conserved within the putative murine CD28 polypeptide. The murine cDNA sequence encodes a polypeptide of 218 amino acids that has 68% identity with the human sequence. Both the murine and human molecules are integral membrane glycoproteins with hydrophobic signal peptide sequences and transmembrane region. All five potential N-linked glycosylation sites are conserved and six of the seven cysteine residues of the mouse protein are found in the human CD28 polypeptide. The murine cDNA is encoded by a single copy nonrearranging gene whose expression at the mRNA level is restricted to T cells. A rabbit antiserum was raised against a synthetic peptide corresponding to a hydrophilic portion of the translated murine cDNA sequence. This antiserum identifies an 80-kDa homodimer consisting of disulfide-bonded subunits of 40 kDa that is expressed on splenic T cells, thymocytes, and several T cell tumors, but not on B cells. Deglycosylation studies indicate that four of the five N-linked glycosylation sites are used and that the mature core protein has a molecular mass of 25 kDa, close to that predicted by the cDNA sequence. Transfection of the murine cDNA into Chinese hamster ovary cells resulted in the expression of an 80-kDa dimeric molecule that was immunoprecipitated by the antipeptide antiserum. Taken together, these data provide strong support that we have identified the murine homologue of CD28.