CDB20:0002741 FCER2 — CR2

ABSTRACT: Human complement receptor type 2 (CR2/CD21) is a B lymphocyte membrane glycoprotein that plays a central role in the immune responses to foreign Ags as well as the development of autoimmunity to nuclear Ags in systemic lupus erythematosus. In addition to these three well-characterized ligands, C3d/iC3b, EBV-gp350, and CD23, a previous study has identified CR2 as a potential receptor for IFN-alpha. IFN-alpha, a multifunctional cytokine important in the innate immune system, has recently been proposed to play a major pathogenic role in the development of systemic lupus erythematosus in humans and mice. In this study, we have shown using surface plasmon resonance and ELISA approaches that CR2 will bind IFN-alpha in the same affinity range as the other three well-characterized ligands studied in parallel. In addition, we show that IFN-alpha interacts with short consensus repeat domains 1 and 2 in a region that serves as the ligand binding site for C3d/iC3b, EBV-gp350, and CD23. Finally, we show that treatment of purified human peripheral blood B cells with the inhibitory anti-CR2 mAb 171 diminishes the induction of IFN-alpha-responsive genes. Thus, IFN-alpha represents a fourth class of extracellular ligands for CR2 and interacts with the same domain as the other three ligands. Defining the role of CR2 as compared with the well-characterized type 1 IFN-alpha receptor 1 and 2 in mediating innate immune and autoimmune roles of this cytokine should provide additional insights into the biologic roles of this interaction.

ABSTRACT: Human CD21 has been described as a receptor for the C3d,g and iC3b proteins of complement, for the Epstein-Barr virus, and also for IFN-alpha. We reported recently that CD23, a low affinity receptor for IgE (Fc epsilon R2), is a new functional ligand for CD21. To determine the site of interaction of CD23 on CD21, we analyzed the ability of purified recombinant CD23 incorporated into fluorescent liposomes to bind CD21 mutants bearing various deletions of extracytoplasmic short consensus repeats (SCRs). We found that the site of interaction of CD23 on CD21 is on SCRs 5 to 8, with contribution of SCRs 1 and 2. Tunicamycin treatment of CD21-transfected K562 cells strongly inhibited the binding of CD23-liposomes, suggesting that an N-linked sugar, present on SCRs 5 to 8, is involved in the CD23/CD21 interaction. By mutating together or individually, the three asparagines present on SCRs 5 to 8, asparagines (Asn) 370 and 295, but not Asn 492, were shown to be involved critically in the binding of CD23. Furthermore, we mapped the binding sites of a panel of anti-CD21 mAbs and found that at least six epitopes can be detected on CD21. The mAbs that inhibit the most CD23 binding to CD21 map in SCRs 5 to 8. This study indicates that SCRs 5 to 8 represent a novel functional domain on the CD21 molecule, and is the first demonstration of an activity of an extracytoplasmic region of the CD21 outside of SCRs 1 to 4.