Monocyte Oxidative Stress Underlies Persistent Immune Activation in Long-COVID Postural Orthostatic Tachycardia Syndrome

BackgroundLong COVID postural orthostatic tachycardia syndrome (LCPOTS) is characterized by persistent orthostatic tachycardia and multiple constitutional symptoms, many of which suggest persistent inflammation. We sought to define mechanisms responsible for ongoing immune activation in LCPOTs and to determine if this is related to autonomic dysregulation. MethodsWe performed a case-control study of 25 patients with LCPOTS and 15 controls who recovered from COVID-19 without persistent autonomic sequelae. Peripheral blood mononuclear cells were analyzed by flow cytometry to quantify circulating CD3CD14 T cell-monocyte doublets, cytokine production, memory phenotype, mitochondrial ROS, and isolevuglandin (IsoLG)-adduct formation. Forster resonance energy transfer was used to assess T-cell receptor-HLA interactions within doublets. Single-cell RNA sequencing (scRNA-seq) was performed on a subset of participants, and autonomic phenotyping included orthostatic heart rate responses, heart rate variability, baroreflex sensitivity, and blood volume measurements. ResultsLCPOTS was linked to impaired cardiovagal function and greater autonomic symptom burden. It was also associated with roughly a threefold rise in circulating CD3CD14 doublets and enhanced T cell-monocyte interactions. These complexes demonstrated signs of genuine immune synapse formation and were enriched with effector-memory and TEMRA T-cell types. T cells in doublets produced higher levels of IFN-{gamma} and IL-17A, and the proportion of cytokine-producing doublets correlated with the severity of orthostatic tachycardia and total COMPASS-31 score. Monocytes from LCPOTS showed increased mitochondrial content, superoxide generation, and IsoLG-adduct accumulation, along with decreased expression of antioxidant genes, including those related to NFE2L2. ConclusionsOur findings suggest that ongoing immune activation contributes to LCPOTS pathogenesis. We propose that impaired cardiovagal regulation stimulates monocyte ROS production, promotes neoantigen formation, and T cell activation. This persistent immune response, together with disrupted mitochondrial function, likely contributes to the diverse symptoms linked to LCPOTS. Novelty and SignificanceO_ST_ABSWhat Is Known?C_ST_ABSO_LILong COVID postural orthostatic tachycardia syndrome is associated with persistent orthostatic tachycardia and disabling orthostatic intolerance symptoms after SARS-CoV-2 infection. C_LIO_LIImmune dysregulation and oxidative stress have been implicated in long COVID, but the cellular mechanisms linking inflammation to autonomic dysfunction are not well defined. C_LIO_LICirculating T cell: monocyte doublets are a recently recognized marker of ongoing immune activation. C_LI What New Information Does This Article Contribute?O_LIPatients with LCPOTS exhibit a marked increase in circulating CD3CD14 T cell-monocyte doublets. C_LIO_LIDoublet-associated T cells are enriched for inflammatory effector-memory/TEMRA phenotypes and produce IFN-{gamma} and IL-17A in proportion to orthostatic tachycardia and autonomic symptoms severity. C_LIO_LIImpaired cardiovagal activity, monocyte mitochondrial ROS, IsoLG-adduct formation, and suppression of antioxidant pathways identify a mechanistic axis linking oxidative injury to persistent immune activation in LCPOTS. C_LI Summary of Novelty and SignificanceThis study identifies a mechanistic link between impaired cardiovagal function, mitochondrial oxidative stress, and persistent immune activation in LCPOTS. We show that circulating CD3CD14 T cell-monocyte doublets are expanded in LCPOTS and form true immune synapses, as demonstrated by T-cell receptor-HLA proximity. These are enriched in inflammatory effector-memory/TEMRA T cells and are associated with increased IFN-{gamma} and IL-17A production that correlate with orthostatic tachycardia severity and symptom burden. We further identified increased mitochondrial ROS, accumulation of IsoLG adducts, and reduced antioxidant gene expression in monocytes, suggesting that oxidation-induced neoantigen formation sustains pathogenic T-cell engagement. Together, these findings move LCPOTS beyond a descriptive post-viral syndrome and define a biologically plausible immune mechanism with diagnostic and therapeutic implications. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=109 SRC="FIGDIR/small/26352776v1_ufig1.gif" ALT="Figure 1"> View larger version (68K): org.highwire.dtl.DTLVardef@12f15b0org.highwire.dtl.DTLVardef@38e9e9org.highwire.dtl.DTLVardef@84c229org.highwire.dtl.DTLVardef@1e72cae_HPS_FORMAT_FIGEXP M_FIG C_FIG