The major goal of this R21 proposal is to understand how Listeria monocytogenes (Lm) subverts syndecan-1 (Sdc1) functions to promote its pathogenesis. Host-pathogen interactions largely dictate the onset, progression, and outcome of infectious diseases. Many pathogens, including viruses, bacteria and parasites, bind to the heparan sulfate (HS) moiety of HS proteoglycans (HSPGs) in vitro, and this activity is thought to be critical in the attachment and invasion of host cells. However, there are surprisingly very few examples supporting the importance of HSPG-pathogen interactions in microbial pathogenesis in vivo. Furthermore HSPGs are expressed in different cells and tissues at different times and levels, and they are thought to function specifically, but the identity of HSPGs that facilitate attachment and invasion is unknown for most heparin-binding pathogens. In addition, although HSPGs are multi-functional molecules, most studies have so far focused on their role as attachment/internalization receptors, and have largely overlooked the possibility that HSPGs may have other functions in microbial pathogenesis. Our proposal is designed to address these gaps in our understanding of HSPG biology in bacterial infections, using Lm as a model heparin-binding bacterial pathogen. Lm is a food-borne Gram-positive bacterial pathogen that causes listeriosis, a rare but significant infectious disease associated with a high rate of hospitalization and mortality. Preliminary studies showed that Sdc1 null (Sdc1-/-) mice are significantly less susceptible to both intragastric and intravenous Lm infection compared to wild type (Wt) mice. This phenotype is not seen in Sdc3-/- or Sdc4-/- mice, indicating that ablation of Sdc1 causes a specific gain of function that enables mice to resist listeriosis. However, Sdc1 does not support Lm attachment or invasion of host cells, indicating that Sdc1 does not promote pathogenesis as a cell surface Lm receptor. Instead, Sdc1 inhibits the clearance of Lm before the bacterium gains access to its intracellular niche. Large intravascular aggregates of neutrophils and neutrophil extracellular traps (NETs) embedded with antimicrobial compounds are formed in Sdc1-/- livers, which trap and kill Lm. Lm infection induces Sdc1 shedding in hepatic tissues of Wt mice, which is directly associated with the decrease in size of intravascular aggregated NETs. Furthermore, administration of purified Sdc1 ectodomains or DNase inhibits the formation of intravascular aggregated neutrophils and NETs and significantly increases the liver bacterial burden in Sdc1-/- mice. Based on these preliminary data, this proposal will explore key pathways of how Lm induces Sdc1 shedding (Aim 1) and how Sdc1 ectodomains inhibit NETs in listeriosis (Aim 2).
Listeriosis is a rare but deadly infectious disease caused by Listeria monocytogenes (Lm), a facultative intracellular Gram-positive bacterium. Immunocompromised individuals, pregnant women, very young and elderly, and patients using biologic drugs are particularly at risk for serious Lm infections. The intracellular mechanisms of Lm have been extensively studied, but how the bacterium overcomes eradication prior to its intracellular life cycle remains largely unknown. This project will investigate how Lm uses a molecule in our body called syndecan-1 to counteract immune mechanisms that trap and kill bacteria. The proposed studies are expected to uncover previously unknown mechanisms of innate immune suppression during infection and suggest new prophylactic and therapeutic targets and approaches to combat listeriosis.