This suggests that siglec-E up-regulation on macrophages represents a negative feedback pathway that

limits the inflammatory response to LPS signalling. A potential limitation of receptor over-expression and the use of antibodies to cross-link siglecs is that they may trigger non-physiological signalling pathways. Siglecs are normally masked on the cell surface via cis interactions with cell-expressed sialic acids, which limits the ability of exogenous trans ligands to induce clustering at 3MA the cell surface. Furthermore, the natural siglec–sialic acid interactions are much weaker than the siglec–antibody interactions and typically in the affinity range of 100–1000 μm. Alternative in vitro approaches include the use of synthetic sialylated carbohydrates to cross-link siglecs, which might better approximate the natural interactions between siglecs and their ligands on other cells in terms of both affinity and avidity. Siglec-deficient mice are proving useful in determining the precise regulatory role of siglecs as discussed further

below. Siglec-G is predominantly expressed on B cells, including the B1a RG7204 supplier cell population that is important for making rapid T-independent IgM responses to bacterial carbohydrate antigens as well as natural antibodies.41 Hoffmann et al.41 showed that siglec-G-deficient mice had a large expansion of the B1a population which began early in development and this was independently confirmed by Ding et al.42 The expansion was specific to B1a B cells and not follicular B2 B cells, which also express siglec-G.41,42 Mixed radiation chimeras prepared with 1 : 1 ratios of wild-type and siglec-G-deficient bone marrow cells, demonstrated that

the effect of siglec-G in controlling cellular expansion is B-cell intrinsic.41 The B1a-cell expansion in siglec-G-deficient mice was not the result of increased cell cycling but rather reduced turnover rate as shown by lower bromodeoxyuridine incorporation.41 These data are suggestive of increased survival Histone demethylase of B1a cells in siglec-G−/− mice, possibly through increased B-cell receptor signalling. Over-expression of siglec-G inhibited B-cell-receptor-mediated Ca2+ signalling and the siglec-G-deficient B1a cells exhibited exaggerated calcium signalling and increased IgM production.41 A similar phenotype has been observed in SHP-1-deficient mice, which exhibit expansion of the B1-cell population and higher B-cell receptor-induced calcium signalling in B cells. This suggests that SHP-1 plays a role downstream of siglec-G to give rise to its inhibitory function.43 This newly defined role of siglec-G may explain the naturally muted signalling response of B1a cells when compared with the B2 population in which siglec-G does not seem to play a functional role despite relatively high levels of expression.