Elucidating the role of peptidoglycan from Borrelia burgdorferi in Lyme disease pathogenesis

As of 2024, more than 50,000 people suffer from Lyme arthritis — a debilitating late-stage symptom of Lyme disease. Symptoms remain even after the completion of antibiotic therapy and when there is no longer any indication of an active infection. Studies have found that a portion of the bacterial cell wall from the causative agent, Borrelia burgdorferi, is a persistent antigen in Lyme arthritis patients, lingering within the synovial fluid. This antigen, peptidoglycan, is recognized by the immune system in numerous ways. Multiple publications have shown that peptidoglycan is proinflammatory and can cause arthritis when injected in vivo. The same was found to be true for B. burgdorferi peptidoglycan. Studies focused on the structure of peptidoglycan from B. burgdorferi have shown atypical differences in both glycan and peptide chemistry that likely alter immune recognition. Due to a lack of necessary enzymes and transporters B. burgdorferi are unable to recycle their peptidoglycan as they elongate and produce daughter cells. This leads to a 45% reduction of their total cell wall that is released into the environment. The work detailed below focuses on this antigen to further our knowledge as to its in vivo biodistribution pattern, half-life, and ability to induce arthritis. For these experiments B. burgdorferi peptidoglycan (pBb PG) was purified, fluorescently labeled, and tracked in vivo to study its clearance pattern and rate. Three different mouse models for Lyme arthritis were utilized in these studies and all experienced persistence of B. burgdorferi peptidoglycan in their liver for upward of 20 days. There were differences in the rate of clearance between types of mice, suggesting the involvement of host genetics. Serum collected weekly throughout this experiment showed over a log fold change in the abundance of ALT and AST levels, which indicates liver dysfunction. Proteomic analysis of the livers of mice post pBb PG injection showed altered levels of proteins important for mitochondrial function and iron homeostasis. When human PBMCs were stimulated with PG from various bacteria it was found that at 12 h pBb PG differentially expressed genes involved in energy metabolic pathways, including oxidative phosphorylation and the citric acid cycle. A subset of Lyme disease patients continue to experience symptomology even after completion of multiple rounds of antibiotics. These patients are termed to have post treatment Lyme disease syndrome and typically experience fatigue as their most common symptom. This symptom in combination with the findings of this dissertation regarding the link between pBb PG and energy metabolism warrants further investigation. Especially since this biopolymer has been found to persist in the synovial fluid of Lyme arthritis patients. Better understanding how these processes are connected could allow for the eventual development of a way to target this material for clearance, or ways to inactivate it. Both options have the potential to help alleviate the devastating symptomology experienced by patients.