Syndecan-1 Promotes Staphylococcus aureus Corneal Infection by Counteracting Neutrophil-mediated Host Defense

Abstract

Many microbial pathogens subvert cell surface heparan sulfate proteoglycans (HSPGs) to infect host cells in vitro. The significance of HSPG-pathogen interactions in vivo, however, remains to be determined. In this study, we examined the role of syndecan-1, a major cell surface HSPG of epithelial cells, in Staphylococcus aureus corneal infection. We found that syndecan-1 null (Sdc1 ؊/؊ ) mice significantly resist S. aureus corneal infection compared with wild type (WT) mice that express abundant syndecan-1 in their corneal epithelium. However, syndecan-1 did not bind to S. aureus, and syndecan-1 was not required for the colonization of cultured corneal epithelial cells by S. aureus, suggesting that syndecan-1 does not mediate S. aureus attachment to corneal tissues in vivo. Instead, S. aureus induced the shedding of syndecan-1 ectodomains from the surface of corneal epithelial cells. Topical administration of purified syndecan-1 ectodomains or heparan sulfate (HS) significantly increased, whereas inhibition of syndecan-1 shedding significantly decreased the bacterial burden in corneal tissues. Furthermore, depletion of neutrophils in the resistant Sdc1 ؊/؊ mice increased the corneal bacterial burden to that of the susceptible WT mice, suggesting that syndecan-1 moderates neutrophils to promote infection. We found that syndecan-1 does not affect the infiltration of neutrophils into the infected cornea but that purified syndecan-1 ectodomain and HS significantly inhibit neutrophil-mediated killing of S. aureus. These data suggest a previously unknown bacterial subversion mechanism where S. aureus exploits the capacity of syndecan-1 ectodomains to inhibit neutrophil-mediated bacterial killing mechanisms in an HS-dependent manner to promote its pathogenesis in the cornea.Microbial pathogens express a multitude of factors that interact with host components. Pathogens use these hostpathogen interactions to their advantage to survive in the host environment. Studies during the last several decades have proposed that many viral, bacterial, and parasitic pathogens bind to cell surface HSPGs 2 to facilitate their initial attachment and subsequent invasion of host cells (1-3). Evidence that the HSPG interaction is biologically important is provided by the finding that HS-binding pathogens show markedly attenuated attachment or invasion of host cells whose HS expression has been reduced by enzymatic treatment or mutagenesis. Furthermore, exogenous HS or heparin (pharmaceutical functional mimic of HS) inhibits pathogen attachment and entry. Furthermore, where examined, mutant strains lacking the HSPG adhesin are viable and show normal growth rates, suggesting that the ability to bind to HSPGs is strictly a virulence activity. However, the significance of HSPG-pathogen interactions in infectious diseases has yet to be clearly established in vivo.Syndecans comprise a major family of cell surface HSPGs (1, 4). Syndecans are type I transmembrane HSPGs composed of four members in mammals. At the cell surface, syndecans function pr...

2-O-Sulfated Domains in Syndecan-1 Heparan Sulfate Inhibit Neutrophil Cathelicidin and Promote Staphylococcus aureus Corneal Infection

“…However, neutrophils do not express syndecan-1 and do not bind to syndecan-1. Syndecan-1 also does not regulate neutrophil influx into corneas infected with S. aureus, and isolated neutrophils of both Sdc1 Ϫ/Ϫ and Wt mice similarly kill S. aureus (21). Moreover, Wt and Sdc1 Ϫ/Ϫ neutrophils are similar in size, granularity, and pattern of GR1 staining, Sdc1 Ϫ/Ϫ neutrophils do not have an inherent defect in their ability to migrate, and Sdc1 Ϫ/Ϫ mice contain normal numbers of circulating neutrophils (25,37).…”

“…Moreover, in mice, syndecan-1 ablation is a gain of function of mutation where the syndecan-1 null (Sdc1 Ϫ/Ϫ ) mice are significantly protected from P. aeruginosa (19) and S. aureus (20) lung infection, and S. aureus corneal infection (21) compared with wild type (Wt) mice, suggesting that syndecan-1 shedding promotes bacterial pathogenesis. Indeed, inhibition of shedding reduces bacterial virulence, whereas administration of purified syndecan-1 ectodomains or HS, but not other glycosaminoglycans or syndecan-1 core protein devoid of HS, enhances bacterial virulence in mouse models of infection (19,21). These results indicate that syndecan-1 ectodomains promote bacterial pathogenesis in an HS-dependent manner, but precisely how this is accomplished is incompletely understood.…”

“…Moreover, Wt and Sdc1 Ϫ/Ϫ neutrophils are similar in size, granularity, and pattern of GR1 staining, Sdc1 Ϫ/Ϫ neutrophils do not have an inherent defect in their ability to migrate, and Sdc1 Ϫ/Ϫ mice contain normal numbers of circulating neutrophils (25,37). Instead, syndecan-1 ectodomains have been shown to promote S. aureus corneal infection by inhibiting neutrophil-mediated S. aureus killing in an HS-dependent manner (21).…”

“…S. aureus strains were grown to late log growth phase in tryptic soy broth (TSB), and the bacterial concentration was approximated by turbidity measurement at 600 nm. After washing, the concentration was adjusted to ϳ5 ϫ 10 8 cfu/5 l. The exact bacterial concentra- (21). Briefly, isolated femurs and tibias were cleaned and flushed with Hank's Balanced Salt Solution (HBSS) containing 10 mM HEPES, pH 7.4 (HBSS/HEPES).…”

“…Percoll was from GE Healthcare Life Sciences (Pittsburgh, PA) and protein A-agarose and protein G-agarose beads were from Pierce. Syndecan-1 ectodomains were purified from the conditioned medium of normal mammary gland epithelial cells as described previously (21), whereas syndecan-4 ectodomains were partially purified from the conditioned medium by DEAE chromatography and sequential absorption to anti-syndecan-2 and -3 and anti-syndecan-1 affinity resins to immunodeplete other syndecans. The partially purified syndecan-4 preparation was determined to contain no syndecan-1, -2, and -3 and glypican-1 and -3 by dot immunoblotting.…”