The Group B Streptococcus–Secreted Protein CIP Interacts with C4, Preventing C3b Deposition via the Lectin and Classical Complement Pathways

Abstract

The group B Streptococcus (GBS) is a leading cause of neonatal invasive disease. GBS bacteria are surrounded by a thick capsular polysaccharide that is a potent inhibitor of complement deposition via the alternative pathway. Several of its surface molecules can however activate the classical and lectin complement pathways, rendering this species still vulnerable to phagocytic killing. In this study we have identified a novel secreted protein named complement interfering protein (CIP) that downregulates complement activation via the classical and lectin pathways, but not the alternative pathway. The CIP protein showed high affinity toward C4b and inhibited its interaction with C2, presumably preventing the formation of the C4bC2a convertase. Addition of recombinant CIP to GBS cip-negative bacteria resulted in decreased deposition of C3b on their surface and in diminished phagocytic killing in a whole-blood assay. Our data reveal a novel strategy exploited by GBS to counteract innate immunity and could be valuable for the development of anti-infective agents against this important pathogen.

Evasion Mechanisms Used by Pathogens to Escape the Lectin Complement Pathway

“…The amino acid sequence of CIP is 15% identical to Eap and the function of CIP is also to bind C4b and obstruct C3 convertase formation (Pietrocola et al, 2016). …”

Novel Evasion Mechanisms of the Classical Complement Pathway

“…However, a secreted protein from Group B Streptococcus was recently discovered which shares a remarkable level of functional and mechanistic similarity to Eap. Pietrocola et al identified the gene COH1_1804 in a library of putative surface-retained antigens from S. agalactiae strain COH1 which lacked any further cell-surface retention motifs (93). A recombinant form of this ~15 kDa protein blocked C3b deposition by both the CP and LP in a dose-dependent manner, leading the authors to rename it CIP for C omplement I nterfering P rotein.…”

“…Remarkably, while CIP and Eap both bind C4b and interfere with formation of the C4b2 proconvertase, it does not appear that these proteins represent true homologs of one another. Not only does CIP share very limited amino acid identity with Eap (15%) (93), structure prediction suggests that CIP adopts a thioredoxin-class fold that is significantly different from the tandemly repeating structural domains characteristic of Eap (91, 94). Thus, it seems more likely that CIP and Eap are a product of distinct evolutionary lineages that have selected for potent inhibitors of the CP and LP.…”

The Streptococcus agalactiae complement interfering protein combines multiple complement‐inhibitory mechanisms by interacting with both C4 and C3 ligands

“…In a recent study focusing on GBS-secreted virulence factors that could potentially interfere with human complement, we identified a 153-residues polypeptide that was named complement interfering protein (CIP) (15). The protein showed partial homology to the well-studied Staphylococcus aureus complement modulator extracellular adherence protein (Eap) (16, 17).…”

“…CIP also shows partial homology to the Staphylococcus aureus –secreted proteins extracellular fibrinogen-binding protein Efb (18), extracellular fibrinogen-binding homologous protein (Ehp) (19), and staphylococcal-binding IgG (Sbi) (20, 21) (10–13% identity and 15–20% similarity) (15). These 3 proteins are incapable of binding C4 ligands, but can interact with the C3 central component of complement and its fragments C3b, iC3b, C3dg, and C3d (18–23).…”