Roles of Group B Streptococcal membrane proteins in virulence and protection

Abstract

Group B Streptococcus (GBS) asymptomatically colonizes the rectovaginal tract in up to 20% of women worldwide. During pregnancy, GBS is a leading cause of invasive infections associated with adverse pregnancy outcomes including preterm births, neonatal infections, and stillbirths. In addition to its role as a pregnancy-associated pathogen, GBS infection rates in non-pregnant adults are also rising. While the field has made significant progress in defining GBS virulence factors and their roles in infection, many remain undefined. GBS membrane-associated and secreted proteins are more likely to interact with the host, making these proteins especially important during infection. Importantly, the localization of these proteins also makes them attractive targets for therapeutic design, which is relevant because current GBS treatment regimens are limited to antibiotics and no prophylactic vaccine is approved for use. Here, I describe the importance of two GBS membrane proteins: the serine protease HtrA and the alpha-like surface proteins. Many bacterial pathogens encode homologs of high temperature response A (HtrA) serine proteases, which regulate virulence and stress responses. While the function of GBS HtrA was previously unknown, here I show that HtrA-deficient GBS displayed attenuated virulence during systemic infection and resulted in fewer adverse pregnancy outcomes in mice. Proteomic analysis of isogenic HtrA-proficient and -deficient strains identified 110 proteins that may be HtrA regulated, including the novel Streptococcal surface immunogenic protein (Sip). Interestingly, HtrA‘s virulence effect is partially linked to Sip, since deletion of both proteins led to a similar rate of adverse pregnancy outcomes compared to either single deletion. Further, I observed that Sip can be directly cleaved by HtrA. These findings support a system wherein HtrA, partially through the effects of Sip, can regulate GBS disease pathogenesis. Alpha-like proteins (Alps) are a family of adhesins found exclusively on the surface of Streptococci. In GBS, there are six chimeric Alps variants that are allelically expressed., with alpha C (αC) and Rib being the most abundant. Here, I assessed the immunogenicity and efficacy of the clinical GBS-NN vaccine produced by fusion of the N-termini of surface exposed αC and Rib proteins. GBS-NN immunized mice exhibited heightened vaccine-specific antibody titers in serum compared to adjuvant controls. GBS-NN-specific antibodies also bound to native conformations of αC and Rib on GBS clinical isolates. Mice immunized with GBS-NN experienced reduced disease severity during both systemic and pregnancy-associated challenge. In addition to improving fetal survival in immunized dams, maternally-derived GBS-NN-specific antibodies were detected in full-term pups and contributed to improved pup survival following intranasal GBS challenge. Together, the insights gained from this thesis highlight the importance of membrane associated proteins in GBS pathogenesis and provide examples of the broad range of host-pathogen interactions they can confer. They may also inform the development of novel anti-GBS strategies such as a small molecule inhibitor targeting HtrA and lend support for further testing of maternal vaccination strategies utilizing GBS-NN, which is currently under human clinical trials. Together with antibiotics, these novel strategies could improve GBS disease outcomes worldwide.