Chronic Activation of pDCs in Autoimmunity is linked to Dysregulated ER Stress and Metabolic Responses

Plasmacytoid dendritic cells (pDCs) chronically produce type I interferon (IFN-I) in multiple autoimmune diseases, such as systemic sclerosis (SSc) and systemic lupus erythematosus (SLE). Here, we report that the IRE1α-XBP1 branch of the unfolded protein response (UPR) inhibits the production of IFN-α by TLR7- or TLR9-activated pDCs. In SSc patients, pDCs demonstrated reduced expression of UPR marker genes, which inversely correlated with the levels of IFN-I-stimulated genes. CXCL4, a chemokine that is elevated in SSc patients, downregulated IRE1α-XBP1-controlled genes and promoted IFN-α production by pDCs. Mechanistically, IRE1α-XBP1 activation rewired glycolysis to serine biosynthesis by inducing phosphoglycerate dehydrogenase (PHGDH) expression. This process reduced pyruvate access to the tricarboxylic acid (TCA) cycle and blunted mitochondrial ATP generation and cAMP signaling, which are essential for pDC IFN-I responses. Notably, PHGDH expression was reduced in pDCs from patients with SSc and SLE, and pharmacological blockade of TCA cycle reactions inhibited IFN-I responses in pDCs from these patients. Hence, modulating the IRE1α-XBP1-PHGDH axis may represent a hitherto unexplored strategy for alleviating chronic pDC activation in autoimmune disorders. Gene expression analysis of RNA-seq data from healthy human pDCs comparing response of pDCs to ER-stress inducing agents (Tunicamycin and Thapsigargin) and TLR9 activating ligands.