Characterizing immunological responses of vector and host to Borrelia burgdorferi
Date of Award
Spring 2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Microbiology
First Advisor
Jacobs-Wagner, Christine
Abstract
The overarching goal of the work presented herein is to deepen our understanding of the molecular interactions that permit Borrelia burgdorferi, the spirochetal bacterium responsible for Lyme disease, to colonize ixodid ticks and achieve transmission to vertebrate hosts upon subsequent tick feeding. In both cases, B. burgdorferi faces a formidable challenge: an innate immune system. During colonization of the vector, it is the tick’s innate immune mechanisms that must be avoided, subverted, and/or resisted. Then, during subsequent transmission, B. burgdorferi must confront the enormous arsenal of pattern recognition receptors that stand ready to alert the vertebrate host to the spirochete’s invasion. Understanding how B. burgdorferi achieves this feat to persist within nature necessitates better elucidation of the basic biology of not one, but two, non-model organisms: the spirochete itself and the ticks that harbor them. To that end, for the first chapter of this thesis, I collaborated with colleagues Joshua McCausland, Irnov Irnov, and Nicole Sonnert to better understand how B. burgdorferi may be able to remodel its cell wall during growth and release the resulting peptidoglycan (PG) fragments into the surrounding environment without triggering an innate immune response on the part of the human host. More specifically, we find that B. burgdorferi primarily sheds PG fragments whose MurNAc moiety is modified with a 1,6-anhydro bond and provide evidence that this modification is likely the result of lytic transglycosylase activity associated with the spirochete’s BB0259 gene product. Furthermore, and perhaps most importantly, we use different human cell lines to show that PG fragments modified in this way are incapable of stimulating a NOD2-dependent immune response. Given the wide distribution of genes encoding lytic transglycosylases across diverse bacterial phyla, it is intriguing to speculate that the 1,6-anhydro linkages these enzymes introduce into peptidoglycan turnover products may represent an evolutionarily ancient immune evasion strategy. In the second chapter, we turn our attention to B. burgdorferi’s interactions with its vector, the deer tick Ixodes scapularis. Using a hybrid selection RNA sequencing (hsRNA-seq) approach, we provide the first dual quantification of B. burgdorferi and tick gene expression in spirochete-colonized midgut tissue during both acquisition and transmission. Importantly, we show that hsRNA-seq not only provides minimally biased estimates of B. burgdorferi gene expression levels in tick tissue during spirochete transmission, but that it is also capable of capturing B. burgdorferi transcriptional signal even in the sparsely colonized midgut of spirochete-acquiring ticks. To our knowledge, this study also permits, for the first time, direct comparison of genome-wide tick transcriptional changes during B. burgdorferi acquisition versus transmission. Our differential expression analyses reveal that B. burgdorferi induces only subtle, and not obviously coordinated, changes in the expression of tick innate immune genes in the midgut during both acquisition and transmission. In addition to highlighting numerous tick genes that may be of interest for future study, our analyses also call attention to many plasmid-borne B. burgdorferi genes that encode genes of unknown function and that have not been previously implicated as essential for B. burgdorferi survival in the tick. We anticipate that hsRNA-seq may prove very fruitful, and be easily adapted, for the study of other tick-microbe associations.
Recommended Citation
Kloos, Zachary, "Characterizing immunological responses of vector and host to Borrelia burgdorferi" (2024). Yale Graduate School of Arts and Sciences Dissertations. 1259.
https://elischolar.library.yale.edu/gsas_dissertations/1259
