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The complement receptor II (CD21) serves as a receptor for the complement component C3d of immune complexes on B lymphocytes. Expression of the CD21 gene is tightly regulated during B lymphocyte differentiation. Only mature B lymphocytes, but not pro-, pre- or plasma B lymphocytes, express CD21. There is evidence that cell type-specific expression is mediated by a silencer element located in the first intron. The CD21 promoter region contains a CpG island adjacent to the ATG start codon. We have analyzed the methylation status of this CpG island in B lymphoid cell lines representing the various differentiation stages of B lymphocyte development and primary lymphocytes. We found that the pro-, pre- and intermediate B lymphocytes contain a methylated CpG island and do not express CD21, whereas CD21-expressing mature B lymphocytes, plasma B lymphocytes and non-lymphoid cells carry a demethylated CD21 CpG island. To analyze whether the lack of CD21 expression in early B lymphocytes is due to inhibition by CpG methylation we have used 5-aza-2'-deoxycytidine to inhibit DNA methyltransferase activity. Treatment of pro-B lymphocytes with the drug resulted in expression of CD21. We have also applied Trichostatin A (TSA), an inhibitor of histone deacetylation, to determine whether the state of histone deacetylation affects the expression of CD21. We found that TSA induces expression of CD21 in early B lymphocytes. Thus CD21 expression is controlled by both methylation of the CD21 CpG island and chromatin modification through histone deacetylation in early B lymphocyte development.

Regulation of CD21 expression by DNA methylation and histone deacetylation