Siglec interactions with a sialylated bacterial pathogen

Group B Streptococcus (Streptococcus agalactiae or GBS) is the leading cause of bacterial sepsis and meningitis in human newborns. The GBS capsular polysaccharide (CPS) is a major virulence factor that contains terminally linked sialic acids (Sias) that are important for escaping detection by the innate immune system. The structure, position, and linkages of the Sia-capped oligosaccharide subunits that make up the CPS of GBS are identical to some of the most common Sia-capped glycans that coat the cell surface of all human cells. The Sia recognizing immunoglobulin (Ig) superfamily lectins (Siglecs) are a family of type I transmembrane surface proteins many of which are expressed on the surfaces of human leukocytes. The human CD33/Siglec-3 Related Siglecs (hCD33rSiglecs) are a rapidly evolving subgroup of the Siglecs that are composed of (a) an extracellular V-set domain that engages Sia-capped glycans, and (b) an intracellular domain that contains inhibitory motifs that are classically involved in reducing leukocyte activation. The CD33rSiglecs are postulated, but not proven, to interact with "self" Sias present on the surface of human cells, thereby reducing leukocyte-mediated autoimmune responses. We hypothesized that sialylated microorganisms producing mimics of host cell glycans would engage CD33rSiglecs, thus causing inhibition of leukocyte activation. Here we demonstrate that the sialylated pathogen, GBS, functionally engages human CD33rSiglecs by two distinct mechanisms. The first mechanism by which GBS binds CD33rSiglecs is dependent on CPS sialylation as we had hypothesized. The second mechanism involves a novel interaction between a GBS surface protein and human Siglec-5 that is Sia- independent. This latter finding opens up the possibility that the capacity of microorganisms to target CD33rSiglecs may not be restricted to sialylated pathogens, but instead may be a more universal mechanism of immune evasion. Using a novel experimental system incorporating immobilized, synthetic sialylated glycans, we confirm that CD33rSiglec engagement of Sias in trans can cause inhibition of neutrophil activation. These findings further support the hypothesis that the natural function of CD33rSiglecs is to recognize endogenous glycans. but that certain microorganisms have evolved to target these cell surface receptors in order to subvert the immune syste