The siglecs (sialic acid-binding Ig-like lectins) are a family of transmembrane receptors expressed in the haemopoietic, immune and nervous systems. The CD33-related siglecs are a distinct subset mostly expressed in the innate immune system where they can function as inhibitory receptors by suppressing the signalling mediated by receptors coupled with ITAMs (immunoreceptor tyrosine-based activation motifs). CD33-related siglecs contain ITIMs (immunoreceptor tyrosine-based inhibitory motifs) that recruit and activate SHP-1 [SH2 (Src homology 2) domain-containing phosphatase-1] and SHP-2. In addition, the ITIMs of CD33-related siglecs can suppress siglec-dependent adhesion of sialylated ligands and mediate endocytosis. Siglec-H is a recently characterized murine CD33-related endocytic receptor that lacks intrinsic tyrosine-based signalling motifs and is expressed selectively on PDCs (plasmacytoid dendritic cells). Siglec-H depends on DAP12 (DNAX-activating protein of 12 kDa) for surface expression and cross-linking with anti-siglec-H antibodies can selectively inhibit interferon-α production by PDCs following TLR9 (Toll-like receptor 9) ligation. Thus CD33-related siglecs are able to mediate diverse inhibitory functions of leucocytes in the innate immune system via both ITIM-dependent and -independent pathways.

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