Date of Award

Spring 2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Immunobiology

First Advisor

Medzhitov, Ruslan

Abstract

The extracellular matrix (ECM) is an organized assembly of proteins and polysaccharides that is produced by cells and forms the physical environment in which cells reside. Together, diverse cell types and their surrounding extracellular matrix form units of organization known as tissues, which make up organs. The ECM gives rise to the particular architecture and mechanical properties of each organ. During tissue repair, cells known as myofibroblasts deposit large quantities of ECM, in order to reconstruct the injured tissue. In normal tissue repair, that reparative phase is followed by a resolution phase, in which cells such as macrophages degrade and remodel excess extracellular matrix, returning the tissue to a homeostatic state. In fibrotic diseases, however, tissue repair persists, leading to the progressive accumulation of dense, stiff extracellular matrix that prevents normal organ function and leads to organ failure. Macrophages are immune cells that reside within all tissues and have important non-immunologic functions, including sensing and regulating features of the tissue environment. They often act as sensors within a homeostatic circuit, monitoring a variable of interest and communicating with other cells, known as effectors, that can correct the variable when it deviates from the desired range. During tissue repair, monocytes from the blood enter the tissue and differentiate into macrophages, where they play a critical role both in driving fibroblast ECM production and in resolving tissue repair through ECM degradation. We hypothesized that macrophages act as sensors of the extracellular matrix within tissues, both to maintain ECM homeostasis under normal conditions and to monitor the progression of tissue repair to ensure an appropriate transition to resolution and avoid fibrosis. In the studies presented in this dissertation, using in vitro hydrogel systems to mimic essential elements of tissue biology, we find that macrophages can sense changes in the extracellular matrix and that they respond by regulating a specific subset of their gene expression program involved in tissue repair. This program includes the protein FIZZ1, which drives fibroblast ECM deposition and, we find, is suppressed by increased ECM, suggesting that macrophages may be involved in a negative feedback loop to control tissue repair. We determine that macrophages sense, in particular, the mechanical properties of the extracellular matrix, and that they employ a novel, integrin-independent mechanosensing mechanism. Macrophage mechanosensing is mediated by intracellular changes in the dynamics of the actin cytoskeleton, which ultimately control chromatin availability and binding of the transcription factor C/EBP to specific genomic targets. Furthermore, we identify that the macrophage growth factor, macrophage colony stimulating factor (MCSF), converges on these cytoskeletal dynamics and downstream regulatory mechanisms to control the same gene expression program. Thus, we find that macrophages integrate mechanical and biochemical information about the tissue environment through changes in their actin cytoskeleton, in order to regulate their tissue repair program. In the final chapters, we present some of the implications of these findings, as well as broader perspectives on tissue biology, homeostasis, and inflammation.

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