The role of siglec-H as an endocytic receptor has been characterized by Zhang et al.,31 who targeted pDCs using anti-siglec-H IgG coupled to ovalbumin. Siglec-H-dependent uptake led to cross-presentation of ovalbumin antigens to CD8+ T cells via MHC class I molecules on pDCs, resulting in antigen-specific CD8+ T-cell expansion.31 Mouse CD33 differs from PS 341 human CD33 because it also encodes a charged transmembrane containing a lysine residue. To date, it has not been shown whether this feature enables murine CD33 to associate with adaptor molecules such

as DAP12. However, a preliminary analysis of CD33-deficient mice revealed no clear-cut differences in regulation of inflammatory responses.34 Negative regulatory functions of different CD33rSiglecs have been observed in studies of cell expansion, cytokine production, Crizotinib order cellular activation and induction of apoptosis

(reviewed in ref. 1). It is likely, although not directly demonstrated in most cases, that the cytoplasmic ITIM and ITIM-like motif are important in these functions via recruitment of downstream targets such as SHP-1 and SHP-2 tyrosine phosphatases as well as other SH2-domain-containing effector molecules.1,35 Below we summarize recent data supporting a role of CD33rSiglecs in the regulation of inflammatory and immune responses. Using over-expression in mouse RAW and human THP-1 macrophage-like cell lines, siglec-9 expression was shown to suppress the Toll-like receptor (TLR) -dependent production of pro-inflammatory cytokines, tumour necrosis factor-α (TNF-α)

and IL-6, in macrophages following lipopolysaccharide (LPS) or peptidoglycan stimulation.35 In contrast, production of the anti-inflammatory cytokine IL-10 Adenosine triphosphate was enhanced. These effects were abolished when the critical tyrosine residues in ITIM and ITIM-like motifs of siglec-9 were mutated.35 These observations are consistent with earlier studies of human monocytes in which siRNA-mediated knockdown of CD33 led to spontaneous secretion of pro-inflammatory cytokines36 and collectively they indicate that ITIM-bearing CD33rSiglecs may restrain the pro-inflammatory functions of macrophages. Cross-talk between CD33rSiglecs and TLR signalling pathways was also demonstrated for siglec-H.32,33 Following cross-linking of siglec-H expressed in pDCs with antibodies, type-I interferon production in response to TLR-9 ligation with CpG was strongly inhibited. This paradoxical inhibition of cytokine production via DAP12-coupled ‘activating’ receptors has been observed with several pDC-expressed receptors and may be the result of a signalling pathway in pDCs shared with B cells that suppresses type 1 interferon production.37 Siglec-E is a typical inhibitory murine siglec expressed on myeloid cells.38,39 Boyd et al.40 have recently demonstrated a TLR- and MyD88-dependent up-regulation of siglec-E on murine bone-marrow-derived macrophages.