Epithelial Heparan Sulfate Promotes Staphylococcus aureus Corneal Infection by Inhibiting Cathelicidins

Cathelicidins are short cationic peptides with potent microbicidal activities and comprise an important arm of host innate immunity. Many cell types can produce cathelicidins, but they are mainly expressed by recruited immune cells and are induced in epithelial cells during infection. Although the mechanisms of bacterial killing by cathelicidins have been largely elucidated in vitro, those that regulate their activities in vivo are less well understood. Bacterial pathogens often co-opt host extracellular matrix (ECM) components and their functions to escape host defense; however, it is unclear whether such mechanisms exist against cathelicidins. Several studies have demonstrated that host heparan sulfate (HS) inhibits LL-37, the human cathelicidin, suggesting that bacteria might exploit HS to evade killing by cathelicidins. However, precisely how HS inhibits LL-37 and possibly other cathelicidins remains unknown, and the role of the HS-cathelicidin interaction in infectious disease has not been rigorously studied. Here, we found that deleting CRAMP, the murine cathelicidin, significantly increases the susceptibility of mice to Staphylococcus aureus corneal infection. We also determined that heparan compounds bind to CRAMP with low nanomolar affinity, the secondary structure of CRAMP is required for HS binding, and HS binding to CRAMP inhibits CRAMP binding to target bacterial cells. Furthermore, we found that heparan compounds inhibit the killing of S. aureus by cathelicidins derived from several mammalian species in a 2-O-sulfate-dependent manner. Additionally, we demonstrate for the first time that conditional deletion of HS2ST, the enzyme responsible for 2-O-sulfation of HS, in corneal epithelial cells significantly reduces the susceptibility of mice to corneal infection. Altogether, these data uncover an endogenous inhibition mechanism of cathelicidins where 2-O-sulfated epithelial HS tightly binds and neutralizes the antibacterial activity of cathelicidins.