Silencing of CD21 expression in synovial lymphocytes is independent of methylation of the CD21 promoter CpG island.

The complement receptor II (CD21) recognises the complement component C3d of immune complexes. Expression of the CD21 gene is tightly regulated during B lymphocyte differentiation. Only mature B lymphocytes express CD21 but not pro-, pre-, or plasma B lymphocytes. Previously we found that pro-, pre-, and intermediate B lymphocytes contain a methylated CpG island and do not express CD21. CD21-expressing mature B lymphocytes, plasma B lymphocytes, and nonlymphoid cells carried a demethylated CD21 CpG island. Furthermore, we found that synovial lymphocytes from patients with rheumatic disease show reduced expression of CD21. This observation tempted us to analyse the methylation status of the CD21 CpG island in peripheral blood mononuclear cells and synovial fluid mononuclear cells derived from these patients. While methylation is involved in silencing CD21 in early types of B lymphocytes, we found the CD21-CpG island to be demethylated in peripheral blood mononuclear cells and synovial fluid mononuclear cells of patient DNA.