In our lab (@LabRajagopal), we study Group B Streptococcus (GBS), a Gram-positive bacterium that typically colonizes the female lower genital tract as an asymptomatic member of the commensal flora but causes severe infections during pregnancy, in newborns, and increasingly in non-pregnant adults. Although GBS screening and antibiotic administration during birth have reduced the incidence of GBS disease in the first seven days of life, these protocols have little to no effect on preterm births, stillbirths, rates of late-onset disease in newborns (disease seven days after birth), let alone infection in non-pregnant adults. Notably, no FDA-approved vaccine exists to prevent GBS. A driving philosophy in our lab is that improved therapeutic strategies against GBS will arise from a greater understanding of its virulence factors.
The repertoire of virulence factors encoded by GBS allows this bug to deftly overcome host defenses and establish itself as an invasive pathogen. One such virulence factor is the hemolytic pigment, also known as granadaene, which kills several cells of the immune system and is critical for bacterial transmigration across host barriers such as the placenta and blood-brain barrier. Unlike many other bacterial hemolysins which are proteinaceous in nature, granadaene is comprised of a long, unsaturated lipid flanked on one side with a rhamnose sugar and on the other side an ornithine amino acid (Fig. 1).
Fig. 1. Structure of the GBS hemolytic pigment, granadaene
Due to granadaene’s instability and insolubility, structure-function studies have been limited, hindering our ability to completely understand the mechanistic underpinnings of toxic activity. For the work presented in our recent article in Nature Communications, we joined forces with an organic chemist, Dr. Juan Manuel Cuerva, and his group (@MOR_Fun_Group) at the University of Granada to resolve issues surrounding the insoluble granadaene. In collaboration with Dr. Cuerva’s group, we synthesized compounds similar to granadaene to understand which chemical components are important for hemolytic activity. We found that compounds that had shorter polyene chains were less able to lyse red blood cells, implicating the length of the polyene chain in toxin activity.
In addition to our structure-function studies, we were also curious if we could use the granadaene-inspired analogs to design a vaccine targeting granadaene. As granadaene is cytotoxic to cells of the adaptive immune system ex vivo, this indicated that incorporating granadaene itself as a vaccine antigen may be unsafe and/or ineffective.
So, we turned to one of our non-hemolytic synthetic compounds, R-P4 (Fig. 2), with the idea that we may be able to elicit an immune response that could cross-protect against granadaene during GBS infection. After confirming that R-P4 was well tolerated by T cells and B cells, we put our idea to the test. We saw that mice vaccinated with R-P4 produced granadaene-binding antibodies with neutralizing properties and were protected against systemic infection with a GBS strain that over-produces the toxin.
Fig. 2. Structure of the non-toxic synthetic analog, R-P4
Together, the work in this paper reveals the power of a collaborative, multidisciplinary approach in addressing key unanswered questions about GBS pathogenesis and anti-virulence factor vaccine strategies.
Blair Armistead is a graduate student in the laboratory of Dr. Lakshmi Rajagopal, Seattle Children’s Research Institute, Center for Global Infectious Disease Research.