Insights into the mechanism and activation of nonclassical monocyte patrolling in murine models of atherosclerosis and metastatic melanoma

Patrolling is a unique surveillance phenotype carried out by a subset of monocytes (nonclassical; Ly6C- mouse or CD16+ human) on the endothelium of blood vessels and interstitium. It is characterized by slow crawling for long periods of time under high shear stress or even against blood flow through the interactions of LFA-1 integrin and its ligands ICAM-1 and ICAM-2. Additionally, patrolling does not necessarily terminate with extravasation into local tissues as other immune cells do after short-term crawling in response to damage or infection. These patrolling monocytes have high levels of the Gαi chemokine receptor CX3CR1, which is necessary for their homeostasis in the periphery. The CX3CR1 ligand, fractalkine (CX3CL1), also helps to signal nonclassical patrolling monocytes to the endothelium and to tissues, but is not necessarily required to execute the patrolling behavior. This suggests that whatever extracellular signals are activating the patrolling mechanism in nonclassical monocytes, it is distinct from homing and homeostatic maintenance of these monocytes. When this monocyte subset was first defined, it was thought that their sole purpose was as an accessory cell to the endothelium, scavenging endothelial debris and resolving vascular inflammation in coordination with other innate immune cells. Subsequent reports have focused on recruitment of nonclassical monocytes to tissues, especially in the context of sterile wound healing and inflammatory diseases, but few studies have looked at the mechanism of patrolling, what induces that particular behavior in the context of a disease, and what the functional consequences are of the patrolling phenotype. In order to begin to address these questions, we employed intravital microscopy, fluorescence-activated cell sorting, RNA sequencing and dimensionality reduction algorithms. The aim of this thesis is to understand what activates patrolling in the setting of atherosclerosis and metastatic cancer and how patrolling contributes to the function of nonclassical monocytes in these disease states. The following studies elucidate the ability of scavenger receptors to induce patrolling by nonclassical monocytes and the intracellular proteins that contribute to mediating the patrolling behavior in two murine models of vascular inflammation: atherosclerosis and metastatic cancer