Abstract
B cells express various inhibitory co-receptors including CD22, CD72, and Siglec-G. These receptors contain immunoreceptor tyrosine-based inhibition motifs (ITIMs) in the cytoplasmic region. Although many of the inhibitory co-receptors negatively regulate BCR signaling by activating SH2-containing protein tyrosine phosphatase 1 (SHP-1), different inhibitory co-receptors have distinct functional properties. CD22, Siglec-G, and CD72 preferentially regulate tonic signaling in conventional B cells, B-1 cell homeostasis, and development of lupus-like disease, respectively. CD72 recognizes RNA-related lupus self-antigen Sm/RNP as a ligand. This ligand recognition recruits CD72 to BCR in Sm/RNP-reactive B cells thereby suppressing production of anti-Sm/RNP autoantibody involved in the pathogenesis of lupus. In contrast, Siglec-G recognizes α2,3 as well as α2,6 sialic acids whereas CD22 recognizes α2,6 sialic acid alone. Because glycoproteins including BCR are dominantly glycosylated with α2,3 sialic acids in B-1 cells, Siglec-G but not CD22 recruits BCR as a ligand specifically in B-1 cells, and regulates B-1 cell homeostasis by suppressing BCR signaling in B-1 cells. Thus, recognition of distinct ligands determines functional properties of different inhibitory B cell co-receptors.
Highlights
Antigen-induced signaling through B cell receptor (BCR) plays a central role in B cell responses to antigens [1]
We demonstrated that the extracellular C-type lectin-like domain (CTLD) of CD72 recognizes the lupus self-antigen Sm/RNP as a ligand [32]
Recognition of α2,3 sialic acid by Sialic acid-binding Ig-like lectin (Siglec)-G induces association of Siglec-G and BCR in B-1 cells, which may induce phosphorylation of the Siglec-G inhibition motifs (ITIMs) by Lyn and activation of SH2-containing protein tyrosine phosphatase 1 (SHP-1) required for inhibition of BCR signaling (Figure 2C)
Summary
B cells express various inhibitory co-receptors including CD22, CD72, and Siglec-G These receptors contain immunoreceptor tyrosine-based inhibition motifs (ITIMs) in the cytoplasmic region. Many of the inhibitory co-receptors negatively regulate BCR signaling by activating SH2-containing protein tyrosine phosphatase 1 (SHP-1), different inhibitory co-receptors have distinct functional properties. CD72 recognizes RNA-related lupus self-antigen Sm/RNP as a ligand This ligand recognition recruits CD72 to BCR in Sm/RNP-reactive B cells thereby suppressing production of anti-Sm/RNP autoantibody involved in the pathogenesis of lupus. Because glycoproteins including BCR are dominantly glycosylated with α2,3 sialic acids in B-1 cells, Siglec-G but not CD22 recruits BCR as a ligand in B-1 cells, and regulates B-1 cell homeostasis by suppressing BCR signaling in B-1 cells. Recognition of distinct ligands determines functional properties of different inhibitory B cell co-receptors
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