Ablation of
syndecan-1 in mice is a gain of function mutation that enables mice to significantly resist
infection by several bacterial pathogens.
Syndecan-1 shedding is induced by bacterial
virulence factors, and inhibition of shedding attenuates bacterial virulence, whereas administration of purified
syndecan-1 ectodomain enhances virulence, suggesting that bacteria subvert
syndecan-1 ectodomains released by shedding for their pathogenesis. However, the pro-pathogenic functions of
syndecan-1 ectodomain have yet to be clearly defined. Here, we examined how
syndecan-1 ectodomain enhances Staphylococcus aureus virulence in injured mouse corneas. We found that
syndecan-1 ectodomain promotes S. aureus corneal
infection in an HS-dependent manner. Surprisingly, we found that this pro-pathogenic activity is dependent on 2-O-sulfated domains in HS, indicating that the effects of
syndecan-1 ectodomain are structure-based. Our results also showed that purified
syndecan-1 ectodomain and heparan compounds containing 2-O-sulfate motifs inhibit S. aureus killing by antimicrobial factors secreted by degranulated neutrophils, but does not affect intracellular phagocytic killing by neutrophils. Immunodepletion of antimicrobial factors with staphylocidal activities demonstrated that
CRAMP, a
cationic antimicrobial peptide, is primarily responsible for S. aureus killing among other factors secreted by degranulated neutrophils. Furthermore, we found that purified
syndecan-1 ectodomain and heparan compounds containing 2-O-sulfate units potently and specifically inhibit S. aureus killing by synthetic
CRAMP. These results provide compelling evidence that a specific subclass of
sulfate groups, and not the overall charge of HS, permits
syndecan-1 ectodomains to promote S. aureus corneal
infection by inhibiting a key arm of neutrophil host defense.