Human B and T lymphocytes have similar amounts of CD21 mRNA, but differ in surface expression of the CD21 glycoprotein

CD21, the complement receptor type 2 (CR2), binds the complement fragments iC3b, C3dg and C3d, interacts with CD23 (the low-affinity receptor for IgE), and binds IFN-alpha. This 145 kDa glycoprotein merits particular interest because it plays a pivotal role in the activation and proliferation of B cells by lowering the signal threshold. In human disease CD21 is important as a receptor for Epstein- Barr virus and HIV. CD21 is primarily expressed on B lymphocytes and follicular dendritic cells, but has also been reported on T cells. We established a semi-quantitative PCR and compared the CD21 mRNA levels of B and T lymphocytes with the expression of the CD21 glycoprotein on the surface of the respective cells by flow cytometry. The B cell lines Raji and Ramos and the T cell lines Jurkat and Molt4 expressed equal amounts of CD21 mRNA, but differed in surface staining. To address the question to which extent primary human B and T lymphocytes express CD21 mRNA and membrane-bound CD21 glycoprotein, we separated B cells, CD4+ and CD8+ T cells from peripheral blood mononuclear cells of healthy donors. B lymphocytes and CD4+ or CD8+ T cells expressed similar amounts of CD21 mRNA. Nevertheless only B cells, but not CD4+ or CD8+ T cells, expressed detectable amounts of CD21 on their cell surface. Expression of the CD21 exon 11 has been reported being restricted to follicular dendritic cells only. To the contrary, we found that both purified B and T cell subpopulations expressed CD21 mRNA with and without exon 11.