Differential increase of an alternatively polyadenylated mRNA species of murine CD40 upon B lymphocyte activation.

CD40 is an integral membrane glycoprotein found on the surface of human B lymphocytes. Antibodies specific for CD40 have been shown to augment proliferation of activated B lymphocytes, prevent B lymphocyte apoptosis, and prolong the maintenance of normal B lymphocytes in culture. As a step toward developing an in vivo system to examine CD40 function, a molecular clone encoding the murine homologue of the human CD40 B lymphocyte surface Ag was isolated and characterized. Throughout their open reading frames, the murine and human proteins shared 62% predicted amino acid identity. Within the cytoplasmic domain, which includes a completely conserved region known to be important for signaling by human CD40, the CD40 homologues are 78% identical. The human and murine proteins are members of a new cytokine receptor family, which includes the receptors for nerve growth factor and TNF-alpha, that are homologous in their cysteine-rich extracellular domains. The murine CD40 gene is expressed in B lymphocytes as two mRNA species generated by alternative usage of polyadenylation signals in the 3' untranslated region. The activation of B lymphocytes differentially increases the relative levels of these two mRNA transcripts suggesting a posttranscriptional mechanism for the regulation of CD40 surface expression.