The genomic organization of the CD28 gene. Implications for the regulation of CD28 mRNA expression and heterogeneity.

CD28 is a 90-kDa homodimeric glycoprotein present on the surface of a large subset of T cells that appears to play an important role in the modulation of T cell activation. Although a number of physiologic effects associated with CD28 stimulation have been defined, relatively less is known about the structure and expression of the CD28 gene itself. We now show that CD28 is expressed in both Th cells and plasma cells as a series of four distinct CD28 mRNA species: 1.3-, 1.5-, 3.5-, and 3.7-kb transcripts. The steady state expression of all four transcripts in CD28+ T cells was stimulated by PMA, suggesting that they might share a common phorbol-sensitive promoter. Consistent with this hypothesis, CD28 was found to be encoded by a single copy gene organized into four exons, each exon defining a functional domain of the predicted protein. All CD28 transcripts appear to initiate within a 61-bp palindrome. Generation of the four CD28 mRNA species from the CD28 gene involves two distinct posttranscriptional events. The longer pair of transcripts (3.5/3.7 kb) is generated by the use of an alternate nonconsensus polyadenylation signal. This results in the addition of 2167 bp beyond the first polyadenylation site utilized by the shorter (1.3/1.5 kb) pair of transcripts. The size difference between the 3.7- and 3.5-kb messages and between the 1.5- and 1.3-kb messages is generated by an internal splicing event that deletes 252 bp within exon 2, which encodes the extracellular domain. This deletion would result in the loss of 84 amino acids, including 4 of 5 extracellular cysteine residues. Although this deletion would result in significant disruption of CD28 secondary structure, it would not be expected to interfere with the ability of the resultant protein to be expressed on the cell surface. These findings suggest that variant isotypes of CD28 may be expressed on the cell surface with potentially different physiologic roles.