Summary
Streptococcus pneumoniae serotype 3 strains are highly resistant to opsonophagocytosis due to recruitment
of the complement inhibitor Factor H via Hic, a member of the pneumococcal surface
protein C (PspC) family. In this study, we demonstrated that Hic also interacts with
vitronectin, a fluid-phase regulator involved in haemostasis, angiogenesis, and the
terminal complement cascade as well as a component of the extracellular matrix. Blocking
of Hic by specific antiserum or genetic deletion significantly reduced pneumococcal
binding to soluble and immobilised vitronectin and to Factor H, respectively. In parallel,
ectopic expression of Hic on the surface of Lactococcus lactis conferred binding to soluble and immobilised vitronectin as well as Factor H. Molecular
analyses with truncated Hic fragments narrowed down the vitronectin-binding site to
the central core of Hic (aa 151–201). This vitronectin-binding region is separate
from that of Factor H, which binds to the N-terminus of Hic (aa 38–92). Binding of
pneumococcal Hic was localised to the C-terminal heparin-binding domain (HBD3) of
vitronectin. However, an N-terminal region to HBD3 was further involved in Hic-binding
to immobilised vitronectin. Finally, vitronectin bound to Hic was functionally active
and inhibited formation of the terminal complement complex. In conclusion, Hic interacts
with vitronectin and simultaneously with Factor H, and both human proteins may contribute
to colonisation and invasive disease caused by serotype 3 pneumococci.
Keywords
Complement - Hic - pathogenesis - PspC11 - serotype 3 -
Streptococcus pneumoniae
- vitronectin