Lectin-dependent localization of cell surface sialic acid-binding lectin Siglec-9.

Siglecs are immunoglobulin lectin group proteins that recognize the sialic acid moiety. We previously reported that the expression of Siglec-9 on the macrophage cell line RAW264 markedly enhanced Toll-like receptor (TLR)-induced interleukin (IL)-10 production and inhibited the production of proinflammatory cytokines. In this study, we examined the lectin-dependent anti-inflammatory activities of Siglec-9. IL-10 production was modestly reduced by a mutation that disrupted the lectin activity of Siglec-9, while the reduction in tumor necrosis factor-α was not affected. Membrane fractionation experiments revealed that a part of Siglec-9 resided in the detergent-insoluble microdomain, the so-called lipid raft fraction. The amount of Siglec-9 in the lipid raft fraction rapidly increased following TLR2 stimulation by peptidoglycan and peaked after 3-10 min. This time course was similar to that of TLR2. The double tyrosine mutant in immunoreceptor tyrosine-based inhibitory motifs moved to lipid rafts in a similar manner, while lectin-defective Siglec-9 was not detected in the lipid raft fraction. The production of IL-10 was partially reduced by cholesterol oxidase that disturbed lipid raft organization. Taken together, these results suggest that Siglecs exhibit lectin-dependent changes in cellular localization, which may be partly linked to its control mechanism that increases the production of IL-10.

Siglec-9 enhances IL-10 production in macrophages via tyrosine-based motifs.

We examined whether Siglec-9 modulates cytokine production in the macrophage cell line RAW264. Cells expressing Siglec-9 produced low levels of tumor necrosis factor (TNF)-alpha upon stimulation with lipopolysaccharide, peptidoglycan, unmethylated CpG DNA, and double-stranded RNA. On the other hand, interleukin (IL)-10 production was strongly enhanced in Siglec-9-expressing cells. Similar activities were also exhibited by Siglec-5. However, the up-regulation of IL-10 as well as the down-regulation of TNF-alpha was abrogated when two tyrosine residues in the cytoplasmic tail of Siglec-9 were mutated to phenylalanine. A membrane proximal ITIM mutant of Siglec-9 did not enhance IL-10 production but partly inhibited TNF-alpha production, indicating diverse regulation mechanisms of TNF-alpha and IL-10. Siglec-9 also enhanced the production of IL-10 in the human macrophage cell line THP-1. These results demonstrate that Siglec-9 enhances the production of the anti-inflammatory cytokine IL-10 in macrophages.