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Analysis of transcriptional regulatory elements of the human CD23 gene

CD23 (FcgammaRII), the low affinity receptor for IgE, is a single chain 45kDa Type II membrane glycoprotein member of the C-type lectin family. The CD23 molecule is a multifunctional receptor/ligand and cytokine, playing a role in antigen presentation, macrophage activation and cell adhesion. There are two distinct isoforms of human CD23, termed CD23a and CD23b, with the only difference being noted in a 6 or 7 amino acid region in the N-terminal cytoplasmic domain of the protein. These isoforms are generated from distinct transcription start sites and differential RNA splicing of the single CD23 gene, located on chromosome 19. CD23a is constitutively and cell type-specifically expressed on B cells, whereas CD23b can be expressed on B cells, monocytes and other cells of haematopoietic lineage when activated by stimuli including IL-4 and IL-13. A large body of evidence suggests that CD23 plays a regulatory role in IgE production, and cross-linking of CD23 at the cell surface with IgE delivers a negative feed-back signal for IgE production and inhibits the release of soluble CD23 (sCD23). Cleaved sCD23 fragments larger than 25kDa are known to promote IgE production. Allergic disease is thought to be due to the dysregulation of CD23/IgE feed-back mechanisms, probably through increased cleavage of CD23 from the cell surface, leading to increased IgE production and release of inflammatory mediators. Soluble CD23 is considerably elevated in atopic and neoplastic individuals, and this sCD23 can originate from either the a or b isoform on B cells or just from the b isoform on other cells of haematopoietic origin. The patterns of CD23 gene expression and functional diversity has led to the assumption that CD23a and b are involved in B cell function and IgE-mediated I immunity, respectively, with divergence in the signalling pathways attributed to the N-terminal amino acid differences. It was of importance and interest, therefore, to study and elucidate those events which lead to the expression of each CD23 isoform and to pin-point these to differences in promoter activation and regulation. In this research, the activation of the promoter region of each CD23 isoform was studied in great detail, and reporter vector construct studies have shown that each isoform promoter is differentially activated by distinct stimuli. The CD23a promoter contains two IL-4 response elements (IL-4RE), an NFKB site, a glucocorticoid response element (GRE), a B-cell-specific activator protein (BSAP) or Pax-5 binding site, and is activated by IL-4 only. The CD23b promoter, on the other hand, contains two IL- 4RE's separated by an NFKB site, two AP-1 binding sites and is stimulated by IL-4, anti-CD40, anti-p. and PMA. Reporter vector constructs containing truncated promoters revealed that IL-4 stimulation of the CD23a promoter requires the presence of the first IL-4RE/STAT-6 site, while successful IL-4 stimulation of the CD23b promoter is dependent on the second IL-4RE/STAT-6 site. The discovery of these differentially regulated isoform promoter regions confirms the theory that CD23a and CD23b are involved in functionally different roles, and it is hypothesised that these include the prevention of plasma cell formation in B cells and IgE negative regulation, respectively.