Lineage Tracing Provides Novel Insights Into the Cellular Mechanisms of Atherosclerosis

Atherosclerosis has been the focus of biomedical researchers for nearly a century. Our understanding of the disease has followed a stepwise function – such that the development of a new technology results in a large improvement in our understanding. Inflection points along this timeline have included techniques for human histopathological analysis, cell culture, large epidemiologic datasets, hypercholestolemic animal models, genome-wide association studies, and many others. When I entered Gary Owens’ research lab, they had just published one of the first studies implementing cell-specific lineage tracing of smooth muscle cells in atherosclerosis. During the years of my PhD, cell-specific lineage tracing has been extended to multiple projects, the summation of which has contributed many novel insights into atherosclerosis that would have been impossible to observe without the use of this technique.
Chapter 1 will introduce the key foundational knowledge required to understand the studies conducted during my PhD including basic vascular biology, a classic description of atherosclerosis and where our studies have filled in gaps, expanded the current understanding, or resulted in unexpected new insights, and then finally a description of inflammation’s role in atherosclerosis. Chapters 2 and 3 are published works. I have included the main figures in line with the text and the supplementary material can be found online. Chapter 4 is a manuscript in preparation with the main figures included. Following each of these chapters, there is an additional discussion section where I describe ongoing experiments, future directions, and additional considerations and implications for the field. Finally, Chapter 5 attempts to unify the many lessons we have learned through lineage tracing to ask two fundamental questions about atherosclerosis. I will briefly describe each chapter below.
Chapter 2 describes a serendipitous result with critical ramifications for bone marrow transplantations, an essential technique in the biomedical sciences and clinical arsenal. Specifically, high dose gamma radiation completely abrogated the ability of SMC to accumulate in the neointima following vascular injury or atherosclerosis. Interestingly, this appeared to be limited to vascular beds where SMC were derived from the neural crest. This has important clinical and research implications but also fuels discussion of a fascinating biomedical science quagmire that I describe in detail in the additional discussion section.
The main component of Chapter 3 is our collaboration with Novartis to understand the effect of IL1β neutralization on late-stage atherosclerotic lesions. These results are critical to our understanding of the inflammatory hypothesis of atherosclerosis as well as the recently completed CANTOS trial. Our data suggest a counter-intuitive, protective role for IL1 signaling in late-stage disease. Specifically, it appears that SMC rely on IL1 to maintain their protective functions at the fibrous cap of advanced lesions. The additional discussion section includes unpublished experiments that attempt to reconcile our results in mouse models to the CANTOS trial patients.
Chapter 4 attempts to apply our experience with SMC lineage tracing to a completely different cell type – the vascular endothelial cell – to elucidate its role in late-stage in atherosclerosis. This study challenges the view that endothelial cells are passive, cellular gatekeepers of atherosclerosis but rather an active participant. In addition, our study is the first to provide evidence for a protective role for endothelial-to-mesenchymal transitions.
Finally, in Chapter 5, I have attempted to integrate the many lessons I have learned in the last four years of lineage tracing cells in atherosclerosis to answer two fundamental questions: 1) is there really such a thing as cellular identity in the context of chronic inflammation, and; 2) does cellular origin matter?