ORCID
0000-0002-2220-3218
Date of Award
5-9-2024
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Despite their reputation for promoting disease, microorganisms have an immensely underappreciated capacity to predict, prevent, and ameliorate disease. For thousands of years, humankind and microbes have evolved alongside one another. Long before we had a mechanistic understanding of microbiology, humans across the world have benefitted from valuable functions of microbes. As the field of microbiology has evolved to elucidate these beneficial mechanisms, rational use of microbes in therapeutics has been pushed to the forefront of contemporary medicine. Over the past six years, I have focused my thesis work on the therapeutic potential of microbes to improve cancer treatments in a variety of contexts described herein. In Chapter 1, I provide the reader with contextual background underlying the motivation for my thesis work. In this chapter, I offer historical context illustrating the use of microbes throughout the ages to prevent and treat disease. I explain the importance of microbes in the world around us, including in the context of various microbiomes and their utility when directly manipulated for human benefit. Finally, I explore ways in which microbially-directed approaches could be applied to cancer therapy. Ultimately, Chapter 1 provides crucial context and background information supporting approaches used throughout my thesis work. Chapter 2 is a published review that I co-authored focusing on the complex dynamics between the gut microbiome, cancer, and cancer immunotherapy. This review includes a thorough description of techniques used to analyze the microbiome, including those used in subsequent chapters. It also summarizes the current body of literature related to how the microbiome impacts cancer development, modulates response to immune checkpoint blockade, and contributes to development of immunotherapy-associated toxicity. Lastly, it details approaches to modulating the microbiome, including antibiotics, prebiotics, probiotics, postbiotics, and fecal microbiota transplantation, to prevent cancer and improve cancer treatment outcomes. Chapter 2 builds on the concepts introduced in Chapter 1 with an emphasis on cancer, the disease of focus in this thesis. Chapter 3 is a published commentary that I wrote evaluating the results of a clinical trial that entailed the use of fecal microbiota transplantation to improve therapeutic response to immune checkpoint blockade. This commentary includes a summary of the study design and key results, as well as a discussion of the implications of the findings and limitations of the study. Chapter 3 illustrates a bench-to-bedside clinical translation of the principles described in Chapter 1 and Chapter 2. Chapter 4 is an article that I co-authored and am preparing for submission soon after completion of this dissertation, which characterizes the skin microbiome of patients undergoing mastectomy in conjunction with breast cancer treatment. These patients underwent mastectomy with immediate breast reconstruction procedures here at Washington University in St. Louis and provided skin and intraoperative samples at various timepoints before and after the procedure. Our goal was to determine whether there were genomic signatures of bacterial communities or commensal isolates that were associated with post-operative complications. We observed significant shifts in community composition immediately following the mastectomy procedure that varied across patient outcomes. Furthermore, we also identified genomic signatures of communities and individual isolates that were associated with specific patient outcomes, including some signatures that were present at pre-mastectomy timepoints and may have therapeutic potential as predictive biomarkers to inform patient care. In summary, Chapter 4 is a demonstration of the prognostic value of the skin microbiome in predicting and understanding the development of post-operative complications. Chapter 5 is an article that I co-authored and is currently undergoing peer review, which details the design, characterization, and testing of a synthetic yeast probiotic for the treatment of gastrointestinal cancers. Our goal was to bioengineer a probiotic yeast to constitutively secrete PD-L1 inhibitors to promote antitumor immunity. We extensively characterized the growth and secretion capacity of this construct in vitro. We also characterized the stability of the plasmid-based expression system, gut transit dynamics, and other key variables to demonstrate consistent and reliable performance of this synthetic probiotic platform. As a final proof of concept, we tested the tolerability and efficacy of this construct in a mouse model of small bowel and colorectal cancer that is recalcitrant to conventional PD-L1-directed therapy. We achieved a significant reduction in relative tumor count in the distal small intestine and observed accompanying shifts in immune cell populations and gut microbiota composition that were associated with our probiotic therapy. Altogether, Chapter 5 shows the rational design and testing of a synthetic probiotic as an alternative delivery mechanism to expand treatment options for gastrointestinal cancers that do not respond to conventional immune checkpoint blockade therapy. In Chapter 6, I summarize the contents of my thesis and place them in the broader context of the future outlook of microbially-based therapeutics for cancer. I also reflect on the key lessons that I learned during the course of my thesis work and how I plan to apply my skills and knowledge in a future career devoted to using microbes as tools to improve human health. Following the final chapter, I have also included a current version of my curriculum vitae summarizing my accomplishments thus far, including the successful completion of this doctoral degree.
Language
English (en)
Chair and Committee
Gautam Dantas
Recommended Citation
Gorushi, Olivia Nicole, "Leveraging Microbes to Improve Cancer Outcomes" (2024). Arts & Sciences Electronic Theses and Dissertations. 3025.
https://openscholarship.wustl.edu/art_sci_etds/3025