Regulation of Necroptosis and Autophagy in T cell Homeostasis and Function
Development of functional adaptive and innate immune responses requires strict regulation of programmed cell death signaling pathways. These signaling pathways are essential for shaping and maintaining the immune system, and ensuring functional immune responses. Programmed death is required for clonal deletion of lymphocytes during an infection, and dysregulation results in defective clearance of autoreactive T cells and autoimmune disease.
Death Receptor (DR) pathways regulate apoptotic signaling and are essential for immune homeostasis and tolerance. Ligation of DR, CD95/Fas/Apo-1, triggers formation of a cytosolic complex known as the "death-inducing signaling complex" (DISC), which includes adaptor molecule FADD (Fas associated with death domain), Receptor Interacting Protein (RIP) Kinase 1, and caspase-8. Caspase 8 is required for extrinsic apoptosis upon DR ligation, while promoting clonal expansion in T cells following T cell receptor stimulation. In the absence of caspase 8, cells succumb to a programmed necrosis-like death process facilitated by the generation of RIPK1-RIPK3 "necrosomes". The aim of this dissertation is to elucidate the crosstalk between necroptosis and apoptosis in T lymphocytes in the context of caspase 8. By generating RIPK3-/- x FADDdd double mutant mice, we show in Chapter 2 that FADD, caspase 8 and RIP kinases are all essential for clonal expansion, contraction, and antiviral responses.
FADDdd T cells also display hyper-autophagy upon activation; thus, another aim of this dissertation is to uncover the role of autophagic signaling in T lymphocytes. Using mice genetically deficient in autophagy protein, Atg5, we demonstrate in Chapter 3 the differential roles for autophagy in naïve and proliferating T cells. Autophagy is required for clearing excess mitochondria in naïve T cells, while activated cells are capable of diluting mitochondria through rapid division. In Chapter 4, we characterize the platform(s) that promote recruitment of RIPK1-RIPK3 complex in T cells by identifying the kinetics, constituency, and subcellular localization of the necrosome. Understanding the paradigms of necroptosis and autophagy and defining the switch between apoptosis and necrosis will allow us to manipulate death pathways to modulate desired immune responses to various diseases and infections.