Graduation Date

Spring 5-6-2023

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Programs

Immunology, Pathology & Infectious Disease

First Advisor

Tammy Kielian

Abstract

Staphylococcus aureus (S. aureus) is a leading cause of prosthetic joint infection (PJI). These infections are often intractable due to biofilm formation, which are complex bacterial communities that adhere to biotic and abiotic surfaces. Within a biofilm, there is considerable heterogeneity in bacterial metabolism and gene expression that contributes to the establishment an anti-inflammatory milieu at the site of infection, which favors bacterial persistence even in the context of an immunocompetent host. This is due, in part, to the preferential recruitment of granulocytic myeloid-derived suppressor cells (G-MDSCs), which are a population of pathologically activated immature leukocytes that develop and exert immunosuppressive features to inhibit the healthy function of other host cells. Early studies from our laboratory identified IL-10 production and secretion as a major mechanism by which G-MDSC-mediated suppression occurs. This finding was later expanded on by later studies that demonstrated that S. aureus biofilms were active participants in the events that lead up to IL-10 production. Biofilm-derived lactate was shown to be capable of influencing the chromatin landscape around the Il10 promoter in host cells. Despite these advances, it is still not entirely understood how G-MDSCs come to acquire their pathological function. Additionally, while IL-10 was found to be a major factor in G-MDSC-mediated suppression, there are likely a number of other ways that contribute to their inhibitory effects. Recent work from our laboratory began exploring the role of metabolism and its relation to the immunological function of leukocytes. This early venture into the field of immunometabolism was met with great success in finding that the effectiveness of monocytic cells in combating a biofilm-associated infection could be significantly enhanced by modulating metabolism. This prompted us to begin thinking about how G-MDSC metabolism might be behaving and if we could also selectively modulate it to achieve improved outcomes. To this end, we leveraged the power of single-cell sequencing platforms and bioinformatics algorithms to identify candidate pathways that might be implicated in the suppressive function of G-MDSCs. Transcriptomic analyses and metabolic modeling predicted that glycolytic metabolism as well as the action of the HIF-1⍺ transcription factor pathway were significantly associated with the expression of G-MDSC signature genes. Indeed, follow-up studies using either pharmacological or genetic means of inhibiting the function of either system reduced bacterial burdens and alleviated G-MDSC-mediated immunosuppression in a mouse model of infection. Furthermore, these findings were bolstered with the analysis of human patients with prosthetic joint infection (PJI), which indicated an enrichment in both glycolysis and HIF-1⍺-dependent transcription. Collectively, these findings demonstrate the importance of a glycolysis/HIF-1⍺ axis in the promotion of G-MDSC suppressive activity and biofilm persistence in the context of PJI.

Comments

2023 Copyright, the authors

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Available for download on Friday, April 25, 2025

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