Title

Multiscale Regulation of Lipid Metabolism by Cytokine Signaling in B Lymphocytes

Date of Award

Fall 10-1-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Immunobiology

First Advisor

Flavell, Richard

Abstract

At the onset of an immune response lymphocytes receive activating signals in the form of antigen receptor ligation, co-stimulation, and cytokines that drive cellular growth, proliferation, and differentiation. This activation involves a transition from quiescence and catabolic metabolism to a metabolic state with noted similarities to that of cancer cells such as heavy reliance on aerobic glycolysis for energy demands and increased nutrient requirements for biomass accumulation and division. Specific nutrient requirements may be met either by synthesis from metabolite precursors, or by uptake from the extracellular environment. However, much like the intratumoral environment, dense proliferation of cells in lymphatic tissues may lead to local nutrient depletion and a metabolically challenging microenvironment. The goal of this thesis is to define metabolic requirements for B cells during early activation and understand how these are modulated and met by different activating signals that the cell may receive. Herein, I present a broad introduction to B cell activation and metabolism in Chapter 1. In Chapter 2, I introduce trajectory analysis of single cell transcriptomic data from B cells undergoing early IL-4 dependent activation in response to influenza infection. We use this to model the transcriptional coordinates across which cells progress from naïve to activated states, and then to define transcriptional programs that coincide with IL-4 dependent activation within these coordinates in vivo. Through this, we identify the cholesterol biosynthetic pathway to be both transcriptionally upregulated and confirm that it is functionally important in early B cell expansion. The connection between IL-4 and cholesterol synthesis is expounded in Chapter 2, in which I describe our finding that cholesterol specifically is an essential lipid nutrient that enables activation and proliferation of naïve B cells, but that this requirement can be bypassed by IL-4R signaling that leads to a STAT6-dependent upregulation of the SREBP2 transcriptional program and enhanced cholesterol biosynthesis. In Chapter 3, I detail our investigation into the cell biological mechanism through which cholesterol enables B cell proliferation, and after ruling out a number of potential roles determine that cholesterol licenses expansion of the ER and progression from G0 to G1 phase of cell cycle. How IL-4 more broadly restructures global lipid metabolism in B cells is explored in Chapter 4, through which we found an unexpected role for IL-4 in driving an increase in peroxisome abundance. Finally, I report evidence that these peroxisomes function as a site of non-canonical cholesterol synthesis, on clinical observations that human patients with inherited peroxisome defects exhibit substantial defects in hepatic cholesterol synthesis, accordant with clinical observations that human patients with inherited peroxisome defects exhibit substantial defects in hepatic cholesterol synthesis. In sum, this work defines a novel role for IL-4 in transcriptionally upregulating SREBP2-target cholesterol synthesis genes and LXR-target efflux genes, where the activity of these biosynthetic enzymes is enhanced by an IL-4 dependent accumulation of peroxisomes where they are active. To address the questions that rise from this work I discuss implications and future directions in the final chapter.

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