Siglec-G/10 in self-nonself discrimination of innate and adaptive immunity.

Siglec-G/10 is broadly expressed on B cells, dendritic cells and macrophage subsets. It binds strongly to CD24, a small glycosyl-phosphatidylinositol-anchored sialoprotein, in a sialylation-dependent manner. Targeted mutation of Siglecg dramatically elevates the level of natural IgM antibodies and its producer, B1 B cells. Incorporation of Siglec-G ligands to both T-dependent and T-independent immunogens reduces antibody production and induces B-cell tolerance to subsequent antigen challenges. By interacting with CD24, Siglec-G suppresses inflammatory responses to danger (damage)-associated molecular patterns, such as heat-shock proteins and high mobility group protein 1, but not to Toll-like receptor ligands. By a CD24-independent mechanism, Siglec-G has been shown to associate with Cbl to cause degradation of retinoic acid-inducible gene 1 and reduce production of type I interferon in response to RNA virus infection. The negative regulation by Siglec-G/10 may provide a mechanism for the host to discriminate between infectious nonself and noninfectious self, as envisioned by the late Dr. Charles A. Janeway.