D-107. Effect of Serotype and O-Acetylation on Group B Streptococcus Sialic Acid-Dependent Phenotypes

S. Weiman, S. Uchiyama, A. F. Carlin, A. Varki, V. Nizet, A. L. Lewis;
Univ. of California, San Diego, CA.

Background: The capsular polysaccharide (CPS) of the pathogen group B Streptococcus (GBS) is a virulence factor that impairs host immune clearance. Nine GBS serotypes are classified by architecture of the CPS repeat unit; each possessing a terminal alpha-2-3 linked sialic acid (Sia) that mimics a common sugar motif on human cell surfaces. GBS CPS Sia is further modified by variable degrees of O-acetylation (O-Ac). Methods: GBS Sia O-Ac levels were determined by HPLC. Isogenic GBS strains with loss of CPS Sia or modification of CPS Sia O-Ac were generated by allelic replacement or site-directed mutagenesis of the Sia-O-Ac transferase neuD and dual function CMP-Sia synthetase/Sia O-acetylase (neuA) genes in the capsule biosynthetic operon. Serotype switch between Ia and III strains was generated by expression of the heterologous capsule polymerase (cpsK) gene. In vitro assays were used to measure GBS adherence and invasion of host epithelial and endothelial cells, interaction with host complement, and binding to Siglec family receptors expressed by host neutrophils and macrophages. Results: Evaluation of 100 clinical GBS isolates shows that serotype III strains almost invariably express high levels of O-Ac (20-45%), whereas serotype Ia strains almost always express low levels (0–5%) (P < 0.0001). O-Ac phenotype correlated with a polymorphism in neuD but was unaltered by isogenic serotype switching; indicating O-Ac occurs prior to capsule polymerization. We report that serotype and/or O-Ac levels may influence host innate immunity by altering sialic acid-dependent interactions with leukocyte surface receptors (Siglecs), and complement. Finally, reduction of O-Ac hastened GBS type III mortality in a murine infection model. Conclusions: Variations in Sia O-acetylation impact GBS host immune system interactions and may contribute to serotype based differences in disease pathogenesis.