Mechanisms of Group B Streptococcus Colonization and Ascending Infection

Thesis (Ph.D.)--University of Washington, 2017-06Group B Streptococcus (GBS), also known as Streptococcus agalactiae, are bacteria that commonly reside in the vagina of healthy women. GBS is not a major cause of infection in normal adults. However, newborns can acquire GBS from colonized mothers during birth leading to neonatal infections. Alternatively, GBS can migrate from the vagina to the uterus, during pregnancy leading to in utero infection and significant adverse pregnancy outcomes such as preterm birth and stillbirth. To successfully establish an in utero infection, GBS must bypass the host immune response to traffic from the vagina into the pregnant uterus in a process known as ascending infection. Despite the significant impact on perinatal and neonatal health, the mechanisms of GBS vaginal colonization and ascending infection are not completely understood. An increased understanding of these processes will enable the development of novel therapeutic strategies and interventions and are critical for the reduction of the burden of GBS disease. This dissertation summarizes our efforts to describe the interactions between GBS and the host during vaginal colonization, ascending in utero infection and preterm birth. First, we aimed to explore the host-pathogen interactions that mediate vaginal colonization. While significant work has been done to identify GBS virulence factors involved in vaginal colonization, little attention has been given to the host response during colonization. We demonstrate that the vaginal resident immune cells known as mast cells are critical for preventing colonization of hyper-virulent GBS, which are highly associated with invasive infections. GBS virulence is largely mediated by its unique hemolytic toxin which is a pigmented rhamnolipid (hereafter-called hemolytic pigment); yet hyper-pigmented GBS strains are rarely isolated from vagina. We found that the hemolytic pigment toxin activates a subset of host immune cells known as mast cells, leading to its clearance from the vagina. Interestingly, removal of the GBS hemolytic pigment from the bacteria, which renders it avirulent, also leads to inefficient vaginal colonization. Once GBS has ascended from the vagina to infect the placenta, the hemolytic pigment is required for resistance of the host immune response, and hyper-pigmented strains are more resistant to killing by host immune defenses. These data indicate that a delicate balance of hemolytic pigment expression, and therefore regulation of the host immune response, is necessary for successful colonization and ascending infection. Next, we sought to determine the mechanism by which GBS ascends from the vagina to the uterus. We show that GBS interactions with vaginal epithelial cells play an integral role in permitting ascending infection. GBS stimulate a process known as epithelial exfoliation that is critical for ascending infection. Epithelial exfoliation is a process wherein epithelial cells lose their junction properties and detach from the epithelial surface and basement membranes. To induce epithelial exfoliation, GBS actives a class of proteins called integrins, which leads to β-catenin signaling and epithelial-to-mesenchymal transition (EMT), resulting in a migratory cell phenotype. EMT drives loss of junctional and adherent properties, and permits bacterial dissemination into vaginal tissues for ascending infection. Reduction of integrin activation results in less epithelial exfoliation and a reduction in ascending infection and adverse pregnancy outcomes. This work is the first to outline a mechanism of GBS ascending infection. Finally, we endeavored to define the mechanism by which GBS establishes in utero infection. We observed that GBS isolated from cases of preterm birth had significantly higher levels of hyaluronidase activity than those isolated from commensal settings. The GBS hyaluronidase, HylB, is a secreted enzyme that cleaves the host extra-cellular matrix molecule hyaluronic acid into is disaccharide moiety. Hyaluronic acid disaccharides have the ability to block toll-like receptors, which are critical for the detection of pathogenic bacteria, and thus prevent immune recognition of GBS. Using a murine model of ascending infection, we show that GBS deficient for hylB have reduced ability to establish in utero infections due to increased immune recognition of the bacteria. The reduction in bacterial load in the uterus reduces bacterial invasion of placental and fetal tissues, leading to improved pregnancy outcomes. Together, the work in this dissertation describes how GBS successfully colonizes the vagina, ascends from the vagina into the pregnant uterus, and blunts the host immune response in the uterus, leading to placental and fetal infection and adverse pregnancy outcomes