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Advisory Committee Chair

Casey T Weaver

Advisory Committee Members

Charles O Elson

Laurie E Harrington

Robin D Hatton

Michael J Gray

Robinna G Lorenz

Document Type

Dissertation

Date of Award

2018

Degree Name by School

Doctor of Philosophy (PhD) Heersink School of Medicine

Abstract

When mammals leave the womb, they encounter a world teeming with microbial life. Their intestinal tract, which will ultimately harbor an ecosystem of trillions of bacteria, must progress through a process called ecological succession. Late-onset sepsis (LOS) commonly occurs in preterm infants when pioneer members of the intestinal microbiome cause systemic infections. Both antibiotics and probiotics are used as prophylaxis for LOS, but mechanisms underlying susceptibility to the disease remain unclear. This is in large part due to a lack of animal models that mimic the relevant pathophysiology of LOS. This study begins with the development of a novel model of LOS in neonatal mice using K. pneumoniae (Kp) engineered to express either bioluminescent or fluorescent reporter proteins. Tracking these reporters revealed three stages to the pathogenesis of LOS: dysbiosis, translocation, and progression to sepsis if translocated Kp could not be cleared. Altering the commensal microbiome through antibiotic exposure, gnotobioticrearing, probiotic supplementation, or fecal transplantation proved the microbiome was both necessary and sufficient to prevent susceptibility to LOS. Interestingly, maternal gentamicin exposure reduced relative and absolute lactobacilli in the neonatal microbiome and increased susceptibility to LOS. Prophylactic L. murinus, but not commercially available L. rhamnosus GG (LGG), restored resistance to LOS in susceptible pups by reducing intestinal oxygen levels. Further, intestinal oxygen was determined to be a major driver of colonization dynamics in the neonatal intestine. In response to infection, no intestinal inflammation or epithelial damage was observed. Colonic T cells became activated and the proportion of Foxp3+ CD4+ T regulatory cell increased. However, RAG1-/- pups were not more susceptible than wildtype pups or heterozygote littermates, so neonatal adaptive immunity was not obligatory for protection. Inhibition of Rorgt + innate lymphoid cells (ILCs) increased mortality to LOS but not via production of IL-17A, IL-22 or lymphotoxin-beta. Finally, gentamicin exposure did not alter ILC3 cytokine expression but decreased mucus production. Recombinant IL- 33 treatment in gentamicin-reared pups further increased their susceptibility to LOS. Collectively, these studies not only establish a novel murine model to explore the mechanisms by which innate immunity and the microbiome impact the pathogenesis of LOS, but suggest that the neonatal microbiome controls disease susceptibility by preventing dysbiosis of facultative anaerobes in early life.

Supplementary Table 1.xlsx (111 kB)
Supplemental Table 1

Supplementary Table 2.xlsx (37 kB)
Supplemental Table 2

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