Preprint Monocyte Oxidative Stress Underlies Persistent Immune Activation in Long-COVID Postural Orthostatic Tachycardia Syndrome, 2026, Abd-Eldayem et al.

[SNT Gatchaman]

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

Spoiler: Authors

Marwa A Abd-Eldayem; Meenakshi Vinayagam; Yuliya A Vance; Sachin Y Paranjape; Celestine N Wanjalla; Kuniko C Hunter; Sergey Dikalov; André Diedrich; Surat Kulapatana; Pouya E Mehr; Tatiana X Solis-Montenegro; Joshua D Simmons; David G Harrison; Cyndya A Shibao

BACKGROUND
Long 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.

METHODS
We 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 CD3⁺CD14⁺ 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.

RESULTS
LCPOTS was linked to impaired cardiovagal function and greater autonomic symptom burden. It was also associated with roughly a threefold rise in circulating CD3⁺CD14⁺ 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-γ 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.

CONCLUSIONS
Our 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.

Web | DOI | PDF | Preprint: MedRxiv | Open Access

 

[SNT Gatchaman]

NOVELTY AND SIGNIFICANCE

What Is Known?
Long COVID postural orthostatic tachycardia syndrome is associated with persistent orthostatic tachycardia and disabling orthostatic intolerance symptoms after SARS-CoV-2 infection.

Immune dysregulation and oxidative stress have been implicated in long COVID, but the cellular mechanisms linking inflammation to autonomic dysfunction are not well defined.

Circulating T cell: monocyte doublets are a recently recognized marker of ongoing immune activation.

What New Information Does This Article Contribute?
Patients with LCPOTS exhibit a marked increase in circulating CD3⁺CD14⁺ T cell–monocyte doublets.

Doublet-associated T cells are enriched for inflammatory effector-memory/TEMRA phenotypes and produce IFN-γ and IL-17A in proportion to orthostatic tachycardia and autonomic symptoms severity.

Impaired 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.

Summary of Novelty and Significance
This study identifies a mechanistic link between impaired cardiovagal function, mitochondrial oxidative stress, and persistent immune activation in LCPOTS. We show that circulating CD3⁺CD14⁺ 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-γ 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.

 

[wigglethemouse]

TEMRA T-cells have come up multiple times in Long Covid and in Dr. Cliffs 2019 and 2025 papers on ME/CFS.

 

[Murph]

here's the stand-out result. big difference, although not full separation.

A recently identified marker of ongoing T cell activation is the presence of circulating T cell:monocyte doublets. These are increased in humans with active TB, in diabetic subjects with HIV, following vaccination, and in humans with Dengue fever. Evidence suggests that these represent cells that have formed an immunological synapse, representing ongoing immune activation in these conditions.

Circulating T cell-monocyte doublets were identified as the CD3⁺CD14⁺ double-positive population using the gating strategy

I dug in a tiny bit on this idea of cd3+cd14+ doublets and it is not super well established, there seems to be a bit of debate about some aspects of it (https://pubmed.ncbi.nlm.nih.gov/35092640/). Doublets (t-cell stuck physically to a monocyte) used to just be seen as an artefact and some stick-in-the-muds continue to insist that's all they can possibly be. Perhaps true. Or perhaps there's something there.

Then I checked to learn about our lead author here, Shibao. She's a POTS person using t-cell techniques rather than a t-cell person checking on POTS. What was apparently found could be true and important. But I'd like to see what other currently working scientists think.

 

[SNT Gatchaman]

Doublets (t-cell stuck physically to a monocyte) used to just be seen as an artefact and some stick-in-the-muds continue to insist that's all they can possibly be.

Maybe, but reading through the lens of our genetic studies pinpointing synapse-related genes…

These conjugates display characteristics of true immunological synapses and are more prevalent during immune activation, such as in active tuberculosis, dengue infection, and post-vaccination. […] In the present study, we demonstrated increased TCR–HLA interactions within CD3⁺CD14⁺ doublets using fluorescence resonance energy transfer (FRET), supporting the presence of authentic immune synapses rather than random cellular aggregates.

 

[Murph]

I have learned today that immune synapse is just the connection between two immune cells. i.e. a functional connection, not just an adhesion. What I have not yet learned is how similar that is to a neuronal synapse? Shared moelcular structure? idk

Immunological synapse - Wikipedia

en.wikipedia.org

 

[SNT Gatchaman]

See thread/paper Immunological synapse: structures, molecular mechanisms and therapeutic implications in disease (2025)

 

[V.R.T.]

Immunological and Clinical Markers of Post-acute Sequelae of COVID-19: Insights from Mild and Severe Cases 6 Months Post-infection
William Mouton; Sophia Djebali; Marine Villard; Omran Allatif; Cécile Chauvel; Sarah Benezech; Philippe Vanhems; Jacqueline Marvel; Thierry Walzer; Sophie Trouillet-Assant

Post-acute sequelae of COVID-19 (PASC) are a complex clinical condition that requires a better understanding of its underlying biological mechanisms.

In this study, we assessed hundreds of virological, serological, immunological, and tissue damage biomarkers in two cohorts of patients who had experienced either mild (n = 270) or severe (n = 188) COVID-19, 6 to 9 months post-initial infection, and in which 40% and 57.4% of patients, respectively, developed PASC.

Blood analysis showed that the main differences observed in humoral, viral, and biological biomarkers were associated with the initial COVID-19 severity, rather than being specifically linked to PASC. However, patients with PASC displayed altered CD4+ and CD8+ memory T cell subsets, with higher cytokine-secreting cells and increased terminally differentiated CD45RA+ effector memory T cells (TEMRA). Elevated SARS-CoV-2-specific T cells responsive to nucleocapsid/membrane proteins with a TEMRA phenotype were also observed. A random forest model identified these features and initial symptom duration as top variables discriminating PASC, achieving over 80% classification accuracy.

Link | PDF | European Journal of Immunology [Open Access]

This paper from last year also involved TEMRA T cells.

 

[V.R.T.]

@Jonathan Edwards, @jnmaciuch tagging you in case you missed this one.

 

[SNT Gatchaman]

here's the stand-out result. big difference, although not full separation.

Probably also worth highlighting the correlations, which seem relatively tight for IFN-g against both objective and subjective measures. Admittedly the range on the latter is not large.

Showing T-cell/monocyte doublet IL-17A and IFN-g expression against HR rise on tilt (A&B) and COMPASS-31 (C&D).

 

[V.R.T.]

It seems that we've got several papers inc Jackie Cliffs suggesting links to TEMRA T Cells, in two of them secreting IFNy (in the Cliff paper IFNy secretion - and CD38 incidentally - was only seen in severe patients irrc correctly - here it is correlated with severity) . Obviously this LC paper is about POTS but could there be a shared pathology or could they have enrolled patients with PEM and accidentally be measuring LCMECFS?

We also have the LC paper from 2024 suggesting a different type of T cell secretes IFNy in long covid.

Random thought- if there is a T cell pathology, could it possibly be shared amongst different T cell types - like there is gamma delta t cell involvement, but due to some signalling quirk it's other T cells that secrete ifny (both TEMRA and the ones implicated in the 2024 paper from the Cambridge group) causing sickness behaviour - perhaps involving eccentric medium spiny neurons?

Completely throwing mud at the wall with my very limited knowledge there, but my question is really - is it possible that we are seeing pieces of one cohesive T cell story, or do we have fragments of a bunch of different T cell stories?

 

[Jonathan Edwards]

What I have not yet learned is how similar that is to a neuronal synapse? Shared moelcular structure? idk

Completely different in structure and function. The only real similarity is a local functional adhesion between cells. In neural synapses soluble transmitter small molecules are relaesed in vesicles and diffuse across to receptors attached to ion gates. In immune synapses surface macromolecules on both sides engage through an antigen-specific steric interaction and cells are activated through kinases linked to intracellular domains.

I would forget the fact that both are called synapses. There seems to be cross talk between neural and immune signalling systems but not because they are both called synapses.

 

[Jonathan Edwards]

@Jonathan Edwards, @jnmaciuch tagging you in case you missed this one.

I saw it. I cannot get terribly excited at the moment. I am not sure what T cells stuck to monocytes in circulation would mean. They are likely to be bust apart at the next passage through alveolar capillaries (or other tissues). They very likely form in tubes. They may be indicators of T cell activation but phenomena that depend on things happening in tubes are likely to be subject to systematic biasing effects from sample handling.

I don't know enough about the technique to give a useful answer.