Jennifer's research aims to elucidate the role of autophagy and necroptosis in T lymphocytes in the context of caspase 8. Caspase 8 is required for extrinsic apoptosis upon death receptor (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 RIP kinases. Cleavage of RIP kinases by caspase 8 prevents formation of the FADD/caspase 8/RIPK death-inducing complex (necrosome), thereby preventing necrotic death. by generating RIPK3-/- x FADDdd double mutant mice, we have recently shown that FADD, caspase 8 and RIP kinases are all essential for clonal expansion, contraction and antiviral responses.
RIPK1-RIPk3-containing necrosomes assemble not only in response to DR ligation, but also in dividing T cells. Although the necroptotic death pathway can be activated through various stimuli and different cell surface receptors (TNF, TCR, TLR3-4, genotoxic stress, RIG-1, DAI), these signaling pathways converge at the formation of the necrosome. These findings unveil additional situations where necroptosis is induced, thereby broadening the scope of intra- and extracellular events that are under regulation by the necrosome. Thus, the goal of her research is to 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. Our data suggests the necrosome does not assemble until 24h post-activation, and the next step is identifying the cellular events in which these complexes develop physiologically. Understanding the exact paradigms of necroptosis 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.
T cell undergoing RIPK-mediated necroptosis also display hyper-autophagy, thus, we seek to study the role of autophagic signaling in T lymphocytes. It has been suggested that autophagy is required in activated T cells for elmination of damaged mitochondria or for T cell metabolism. However, our data call into question these proposed requirements for autophagy in activated T cells, as deficiency of an essential autophagy gene, Atg5, via CD4-Cre mediation deletion instead promotes early defects in single positive thymocytes. Our results demonstrate that naïve T cells from Atg5fl/fl x CD4-Cre mice harbor a "pre-existing condition" where they posess a survival defect prior to TCR activation. We have established that autophagy is required for homeostatic T cell proliferation, while deletion of Atg5 following TCR stimulation resulted in no discernable death effects. Therefore, the second aim of her research focuses on delineating the role of autophagy in naïve vs. activated T cells. We choose to continue our studies using an inducible-deletion mouse model, by breeding Atg5fl/fl mice withRosa26-Cre-ERT2 mice.
This mouse model allows us to pinpoint the direct effect of the Atg5 deletion during various stages of T cell differentiation; in particular, we will utilize this system to study the memory cell population following a viral infection. Should we observe effective viral clearance but a reduced memory pool, we would then assess the role of autophagy in promoting memory cell differentiation and in supplying the bioenergetics of proliferating effector T cells. Characterization of the differential requirements for autophagy will provide insight into the basis of memory cell formation and metabolic requirements of effector cells, and will be highly applicable in designing effective vaccines, especially against chronic infections.
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Lu JV, Weist BM, van Raam B, Marro B, Bell BD, Luhrs, KA, Lane TE, Salvesen GS, Walsh CM 2011. Complementary roles of FADD and RIPK3 in T cell development and homeostasis. Proceedings of the National Academy of Sciences, USA, 108(37):15312-7.
Lu JV, Walsh CM. 2012. Programmed necrosis and autophagy in immune function. Immunological Reviews 249(1)205-17