Coligation of the B Cell Receptor with Complement Receptor Type 2 (CR2/CD21) Using Its Natural Ligand C3dg: Activation without Engagement of an Inhibitory Signaling Pathway

Abstract

C3dg is a cleavage product of the C3 component of complement that can facilitate the coligation of the complement receptor 2 (CR2/CD21) with the BCR via C3dg/Ag complexes. This interaction can greatly amplify BCR-mediated signaling events and acts to lower the threshold for B cell activation. Although previous studies have used anti-CR2 Abs or used chimeric Ags in the context of BCR transgenic mice as surrogate C3d-containing ligands, we have used a physiological form of C3d to study signaling in B cells from wild-type C57BL/6 mice. We find that while CR2-enhanced BCR signaling causes intracellular Ca2+ mobilization and total pTyr phosphorylation of an intensity comparable to optimal BCR ligation using anti-IgM Abs, it does so with limited activation of inhibitory effectors (such as CD22, Src homology region 2 domain containing phosphatase 1, and SHIP-1) and without substantial receptor cross-linking. In summary, we demonstrate that CR2-enhanced BCR signaling may proceed not only through the previously described amplification of positive signaling pathways, but is potentially augmented by a lack of normal inhibitory/feedback signaling.

Uncoupling CD21 and CD19 of the B-cell coreceptor

“…1B) (37). WT B cells proliferated in response to mixed tetramers and anti-CD40, as demonstrated previously (37), whereas Cr2 Ϫ/Ϫ and Cr2 ⌬/⌬gfp B cells were unresponsive. Therefore, CR binding of C3d tetramers and signaling via CD19 are functionally uncoupled in the Cr2 ⌬/⌬gfp B cells, leading to an impaired proliferation in vitro.…”

“…To test coreceptor signaling, mixed tetramers, composed of a suboptimal level of anti-IgM and C3d, and anti-CD40 were cultured with B cells for 3 days (Fig. 1B) (37). WT B cells proliferated in response to mixed tetramers and anti-CD40, as demonstrated previously (37), whereas Cr2 Ϫ/Ϫ and Cr2 ⌬/⌬gfp B cells were unresponsive.…”

Defective B cell ontogeny and immune response in human complement receptor 2 (CR2, CD21) transgenic mice is partially recovered in the absence of C3

“…The potential for direct signaling through CR2 has not been totally ruled out (Delcayre et al, 1987;Frade et al, 1992) and despite overwhelming data confirming the positive signaling potential of CR2 (Fearon and Carroll, 2000;Fearon and Carter, 1995), negative signaling in response to CR2 ligation has also been demonstrated (Chakravarty et al, 2002;Lee et al, 2005). CR2 dependent BCR signal amplification has been shown to depend on CD19 (Lyubchenko et al, 2005), but CD19 independent effects could result from CR2 being free to associate and/or interfere with other signaling proteins or processes that it would not normally have access to or depend on CD19 during this phase of B cell development. Furthermore, the role of the other two identified members of the CR2 signaling complex, namely CD81 and CD225, are obvious candidates for modulation of hCR2 signals.…”

Expression of complement receptors 1 (CR1/CD35) and 2 (CR2/CD21), and co-signaling molecule CD19 in cattle

“…In B lymphocytes, cross--linking CR2 with surface IgM leads to calcium Blux despite suboptimal B cell receptor complex (BCR; membrane bound immunoglobulin with Igα/Igβ) ligation (Lyubchenko et al, 2005) and stimulates increased antigen presentation (Cherukuri et al, 2001). CR2 has a short cytoplasmic domain and signaling is dependent upon the presence of the co--signaling molecule CD19.…”

“…The requirement for complement activation in addition to BCR ligation reduces the probability of aberrant B cell activation. BCR co--ligation with CD21 leads to reduced engagement of inhibitory molecules, such as CD22 and SHP--1 further enhancing activation (Lyubchenko et al, 2005). Antigens associated with complement are also subject to improved processing and presentation on MHC--II (Cherukuri et al 2001).…”