Cell-specific regulation of the CD21 gene.

The Complement Receptor Type 2 (Cr2-145,CR2, CD21) is an important receptor in the innate and acquired immune response. CD21 is produced by B cells and follicular dendritic cells, where it binds cleavage products of the C3 complement protein. CD21 facilitates internalization of immune complexes by B cells to enhance antigen presentation. CD21, in association with CD19/CD81, also serves as a coaccessory activation complex with the B-cell antigen receptor, permitting a lower antigen concentration to achieve maximal B-cell activation. CD21 traps immune complexes on the surface of follicular dendritic cells and displays them to activated B cells in germinal centers. Much work has been conducted to determine the transcriptional control mechanisms dictating CD21 expression. Appropriate transcriptional control of the CD21 gene evidently requires the CD21 promoter, as well as intronic sequences with enhancer and suppressor functions. Chromatin structure has been implicated in regulating the coordination of CD21 promoter and intronic control sequences by regulating access to them by putative transcription factors. This review assesses the past and current research into CD21 transcriptional regulation and offers insight into future experimental directions.