Apolipoprotein E governs lymph node stromal cell function and lipid metabolism during autoimmunity - Autoimmune disease, which affects over 20 million people in the US, is caused by the disruption of the intricate balance between protective and pathogenic immune activation. Patients with chronic autoimmunity also have major disruptions in plasma lipids, which lead to increased and earlier cardiovascular disease. The options for treatment of autoimmune disease and prevention of cardiovascular disease are limited, highlighting our incomplete understanding of the mechanistic links between lipid metabolism and immune regulation. Lymph nodes (LN) play an important role in immune homeostasis, surveilling for pathogens while maintaining tolerance to innocuous antigens and self. LN stromal cells, such as T cell zone fibroblastic reticular cells (TRCs), are critical for LN structural organization, and recent data also imply immunoregulatory functions. During infection, TRCs facilitate T cell activation and in response, activated T cells promote suppressive functions by TRCs, balancing the initiation and termination of the immune response. The ability of LN TRCs to both promote and restrain inflammation is critical for maintaining the balance between a functional immune response and autoimmunity, but what drives these opposing TRC functions remains unclear. We show that TRCs unexpectedly upregulate lipid metabolism transcriptional programs after activation, and uniquely express apolipoprotein E (apoE), a fat-binding protein that regulates plasma lipid and cholesterol metabolism. Loss of apoE in TRCs leads to significantly worse colitis and impaired T cell activation after gastrointestinal infection. Our data suggest that apoE is a novel regulator of canonical TRC functions and restricts the TRC-suppressive phenotype to promote T cell activation through the control of lipid metabolism. In Aim 1, we will determine how apoE inhibits TRC suppressive capacity by assessing its effects on stimulatory and inhibitory ligand expression, secretion of immunologically active molecules, and changes to the LN extracellular membranes and reticular networks. We will further establish whether apoE signals through extracellular or intracellular mechanisms, and whether this signaling is lipid dependent. In Aim 2, we will determine how apoE effects lipid metabolism in TRCs and T cells by assessing lipoprotein uptake, cholesterol efflux, lipid storage capacity, and bioenergetics. The outcomes of these aims will define the role of apoE in TRCs, reveal mechanisms driving TRC regulation of T cell activation; and define lipid metabolism in TRCs, potentially establishing a mechanistic link between lipid metabolism and immune regulation. Through this Career Development Award, Dr. Poholek, an MD/PhD trained pediatric rheumatologist and basic immunologist, will obtain enhanced training and scientific development under the guidance of a multidisciplinary mentorship and advisory team with expertise in lipid and lipoprotein metabolism, microscopy, immunology, and autoimmunity. By establishing expertise in lipid metabolism and advanced microscopy with the help of this award, Dr. Poholek will be poised to establish an independent research program focused on lipid metabolism and autoimmunity.
