Oral viridans streptococci are mostly harmless while in the human oral cavity, but can act as systemic pathogens if they enter the bloodstream. Serine rich repeat (SRR) adhesins are used by streptococci to bind to glycosylated proteins found in saliva and on the surface of blood cells. Our lab has isolated several strains of oral streptococci from humans that preferentially bind to glycan structures containing the sialic acid N-glycolylneuraminic acid (Neu5Gc). This is surprising because humans are unable to synthesize Neu5Gc, and only produce the sialic acid N-acetylneuramininc acid (Neu5Ac). Preliminary data from our lab show that a complete lack of sialic acid binding significantly reduces the phagocytosis of streptococci by neutrophils, the most common phagocytes in oral tissue. A reduction in phagocytosis is observed when sialic acid is removed from the neutrophils, or when the gene encoding the SRR adhesin is deleted from the streptococci. Our hypothesis is that streptococci carrying Neu5Gc-specific SRRs will be less susceptible to phagocytosis by human neutrophils, which have no Neu5Gc. We will test this hypothesis by first confirming that the sialic acid binding phenotypes of streptococcal cells are due to SRR adhesin binding preferences.
Aim 1 will use recombinant SRR adhesin binding regions (SRR-BRs) to show the extent to which SRR binding regions mediate the sialic acid binding phenotypes observed with whole streptococcal cells.
In Aim 2, we will expose our clinical isolates, SRR deletion mutants, and SRR-BR-coated beads to an assortment of phagocytes carrying specific sialic acids on their surfaces, including human neutrophils that carry only Neu5Ac, mouse neutrophils that naturally carry Neu5Ac and Neu5Gc, and neutrophils from mutant mice that can only synthesize Neu5Ac. If specific sialic acid binding does significantly increase phagocytosis, it could represent a unique glycan-mediated mechanism for the detection of streptococci by neutrophils that has not been widely studied. If the Neu5Gc-binding streptococci are less likely to be detected by neutrophils that do not carry Neu5Gc, such as human neutrophils, it could represent a unique mechanism for avoiding phagocytosis in a specific host. The current project builds upon the skills Dr. Cross acquired working with oral streptococci during his PhD dissertation research. The project will also require him to develop new skills and areas of knowledge, notably in glycobiology, mammalian cell biology, and immunology. During this project, Dr. Cross will develop professional and academic skills with the help of mentors, and through courses offered by the University at Buffalo. He will present the data from his research project at local and international meetings, and broaden his network of potential collaborators. The proposed project, with support from his advisory committee, will expand Dr. Cross's capabilities to a point where he will be comfortable starting an independent laboratory. The results from this study will serve as preliminary data for his future grant applications.
Streptococci, normally harmless bacteria found in the human mouth, can enter the blood stream through microlesions in the oral tissues, becoming pathogens and causing local and systemic disease. This project is investigating a unique mechanism, based on glycan binding of oral streptococci, which leads to phagocytosis and may assist in clearance of the bacteria. Understanding the molecular basis of this mechanism may eventually pave the way for the prevention or treatment of streptococcus-induced inflammation and systemic disease.
