Characterizing host immune responses to Group B Streptococcus hemolysin during invasive infection

Abstract

Group B Streptococcus (GBS) is a gram-positive, ꞵ-hemolytic bacteria that asymptomatically colonizes the vaginal tract. However, during pregnancy maternal GBS colonization greatly predisposes the neonate to a wide range of adverse outcomes, including preterm birth (PTB), stillbirth, and fetal injury. Perinatal transmission results in life-threatening GBS disease including pneumonia, meningitis, and sepsis. Additionally, rates of invasive GBS disease in nonpregnant adults have been on the rise, especially those with advanced age and underlying medical conditions. Reports of fatal GBS infection in healthy adults and the steadily increasing rates of antibiotic resistance in GBS clinical isolates are concerning. Given that there are no FDA-approved GBS vaccines, the development of a safe and effective therapeutic is an important global health objective and requires a better understanding of virulence factors implicated in multiple facets of GBS disease. GBS encodes an arsenal of virulence factors that facilitates invasive disease in vulnerable hosts niches. However, one of the most important virulence factors that GBS deploys is hemolysin, also known as the hemolytic pigment or granadaene. GBS hemolysin is a surface-associated toxin that is responsible for the trademark hemolytic activity of GBS. Notably, hyperhemolytic GBS strains have been isolated from various disease manifestations of invasive GBS infection in neonates, pregnant women, and nonpregnant adults. However, unlike many other bacterial hemolytic or cytolytic toxins which are proteinaceous in nature, the GBS hemolysin is an ornithine rhamnopolyene. Here, we characterize interactions between hemolysin and host immune responses during invasive GBS infection in adults and neonates. In previous work, we synthesized a non-toxic analog of hemolysin and showed that immunization provided protection against systemic GBS infection in nonpregnant adults. In this thesis, we have further characterized the adaptive immune responses to the analog by demonstrating the importance of T cell and antibody responses to generating protection against GBS infection. Additionally, given that the lung serves as a portal of entry for GBS, we optimized an intranasal neonatal murine model to show that GBS utilizes hemolysin to disseminate from the lung and causes increased neonatal morbidity. Furthermore, we show that hemolytic membrane vesicles (MVs) released from GBS exacerbate invasive lung disease and sepsis in neonates. Collectively, these studies presented in this dissertation provide new insights critical for our understanding of GBS hemolysin and for the development of therapeutic strategies that target hemolysin.