CD22 and Siglec-G: B-cell inhibitory receptors with distinct functions

Siglecs (sialic acid-binding immunoglobulin-like lectins) are sialic acid-binding proteins, which are expressed on many cell types of the immune system. B cells express two members of the Siglec family, CD22 (Siglec-2) and Siglec-G, both of which have been shown to inhibit B-cell signaling. CD22 recruits the tyrosine phosphatase Src homology 2 domain-containing phosphatase 1 (SHP-1) to immunoreceptor tyrosine-based inhibitory motifs (ITIMs) and inhibits B-cell receptor (BCR)-induced Ca2+ signaling on normal B cells. CD22 interacts specifically with ligands carrying alpha2-6-linked sialic acids. Interaction with these ligands in cis regulates the association of CD22 with the BCR and thereby modulates the inhibitory function of CD22. Interaction of CD22 to ligands in trans can regulate both B-cell migration as well as the BCR signaling threshold. Siglec-G is a recently identified protein with an inhibitory function restricted to a B-cell subset, the B1 cells. Siglec-G inhibits Ca2+ signaling specifically in these cells. In addition, it controls the cellular expansion and antibody secretion of B1 cells. Thus, both Siglecs modulate BCR signaling on different B-cell populations in a mutually exclusive fashion.