Thesis Explaining Fatigue and Post-Exertional Malaise in Long COVID and ME/CFS: From Skeletal Muscle To Exercise Immunology, 2026, Charlton

[TiredMathematician]

Explaining Fatigue and Post-Exertional Malaise in Long COVID and ME/CFS

Charlton, Braeden Trey

Abstract
The SARS-CoV-2 viral outbreak in 2020 resulted in a record number of global hospitalizations and deaths. Although most people recover from an initial COVID infection within 3 months, around 10% of people have symptoms that do not improve after 3 months, which is called long COVID. Other viruses and bacterial infections can also result in long-term symptoms, which is known as myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). One of the most common symptoms of long COVID or ME/CFS is post-exertional malaise, which is the worsening of symptoms or development of new symptoms after physical, mental or emotional stress. This can lead to patients leading less physically active lifestyles.

Physical activity is known to be beneficial for overall health and wellbeing, mostly by increasing exercise capacity, which is a significant predictor in long-term health. Physical inactivity. It is thought that many symptoms of long COVID and ME/CFS can be explained by lower physical activity of patients. However, people who are less physically active don’t experience post-exertional malaise, implying there are disease-specific changes.

Many of the benefits that come from exercise come from a persons ability to adapt to an exercise session. Usually, exercise will create a short energy deficit, which then may result in small amounts of muscle damage. This then triggers a short-lived immune response, led first by neutrophils and macrophages, which result in activation of fibroadipogenic progenitor cells (FAP cells) and satellite cells (muscle stem cells). FAP cells and satellite cells communicate to repair damage to the muscle itself, while immune cells clear debris surrounding the muscle. In healthy situations, this creates a stronger, more resilient muscle. It is unknown whether these responses are still seen in long COVID or ME/CFS.

The goal of this thesis then to 1) understand muscle changes in patients with long COVID and ME/CFS, 2) to better understand how these changes may relate to PEM, 3) to understand whether the muscle changes in patients are a result of physical inactivity.

Within the thesis, I find that patients with long COVID and ME/CFS have specific patterns of skeletal muscle that separate them from healthy individuals. This is highlighted by more fast-twitch fibre types, slow-twitch muscle atrophy, lower mitochondrial respiration that is separate from just mitochondrial loss, less metabolites related to oxidative capacity and antioxidation, less lipids that are required for cell membrane maintenance, and lower amounts of satellite cells and FAP cells. Beyond this, I find that ME/CFS can be distinguished from long COVID by less capillaries, less slow-twitch fibres, less local immune cells, and lower amounts of several essential amino acids.

As well, I find evidence that physical inactivity alone cannot explain long COVID and ME/CFS symptoms. Where all of long COVID, ME/CFS, and physical inactivity have lower whole-body aerobic capacity, physical inactivity causes a decrease in muscle fibre size, loss of mitochondria, increases in muscle lipotoxic lipids, and decreases in metabolites related to carnitine metabolism. These changes are not seen in either long COVID nor ME/CFS, and support the idea that long COVID and ME/CFS have disease specific muscle changes.

Overall, I find that the reduced whole-body capacity of long COVID and ME/CFS patients is not cause by physical inactivity, and is defined by reduced muscle oxidative ability. The lower amounts of local cells that would normally support exercise and damage responses are lower in both long COVID and ME/CFS, which partially explains the occurrence of PEM. This also suggests that patients experiencing PEM should not undertake normal rehabilitation, however it might be that more individually tailored, and severely monitored approaches may still be beneficial.

Supervisors: Richard Jaspers, Michele van Vugt, Rob Wüst.

Web | DOI | Vrije Universiteit Amsterdam

 

[SNT Gatchaman]

Chapter 10: General Discussion —

When assessing peripheral skeletal muscle alterations, it was apparent that long COVID and ME/CFS exhibited similar adaptations, namely increased proportions of fast-twitch, glycolytic fibres, reduced oxidative phosphorylation capacity (even when normalized to a proxy for mitochondrial content), increased fibrosis surrounding the myofibre, decreased satellite cell populations, reduced oxidative and antioxidant metabolites, and reductions to lipid species associated with cell membrane integrity.

The key differences that distinguished long COVID patients from ME/CFS was the lower capillary fibre ratios found in ME/CFS patients, decreased SDH activity (a proxy for mitochondrial content), and lower resident immune cell populations, possibly indicative of immune exhaustion. In many of these measures, it appeared to be near step-wise effects moving from healthy controls to long COVID patients to ME/CFS patients. The exception being collagen IV deposition around myofibres, in which long COVID patients had more compared to ME/CFS patients.

Phosphorylethanolamine on the other hand similarly increased following exercise in both healthy controls and long COVID patients, but failed to increase in ME/CFS patients. Phosphorylethanolamine is implicated in osmolarity regulation and the synthesis in phospholipids, and therefore may be central to the eventual disruption of phospholipids observed in patients. This appeared to be a distinguishing feature between ME/CFS and long COVID patients, where ME/CFS patient phospholipids decreased following exercise, where healthy controls and long COVID patients remained relatively unchanged.

In chapter 8, I found that ME/CFS patients exhibited reduced capillarization compared to both long COVID patients and healthy controls, with no difference in the capillary sizes. As capillarization is typically reflective of the area needing to be perfused (i.e. larger fibre cross sectional areas require higher capillarization), I initially expected FCSA to be subsequently reduced in ME/CFS patients. However, FCSA in both ME/CFS and long COVID patients were comparable to those of healthy controls.

No evidence exists to suggest that oxygen solubility differs between different disease states, and thus we are left with the oxygen gradient and oxygen diffusion distance as the primary determinants of oxygen diffusion in patients.

Recent evidence further suggests that not only is the number of capillaries affected, but capillary basement membrane are thickened in patients with long COVID and ME/CFS (Slaghekke et al. in prep). The thickening of capillary basement membrane in combination with the increased myofibre collagen IV deposition I reported in chapter 9, increases the diffusion distance of oxygen.

Further, chapter 9 also provides evidence for altered membrane structure, due to the lower amount of membrane-associated lipid species in patients. It is suggested that cholesterol is the main determinant for oxygen permeability across membranes, and while I did not find evidence of altered cholesterols in patients, there was evidence of reduced phosphatidylcholines and phosphatidylethanolamines in patients. Both phosphatidylcholines and phosphatidylethanolamines, but specifically phosphatidylcholines interact with cholesterol to surround neutral lipids in order to form lipid droplets, which are thought to enhance oxygen solubility.

 

[SNT Gatchaman]

Chapter 9 is unpublished, so acting as a preprint. (Definitely worth a read!)

An Inability to Recover: Unresolved Regeneration and Satellite Cell Depletion in Post-Acute Infectious Diseases Associated with Altered Metabolic Landscape
Braeden T. Charlton, Anouk Slaghekke, Jelle Y. Huijts, Brent Appelman, Brian Po Lin Chen, Frank Bloemers, Wendy Noort, Richie P. Goulding, Michèle van Vugt, Richard T. Jaspers, Rob CI Wüst

INTRODUCTION
Myalgic encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) and long COVID patients frequently report with myalgia and post-exertional malaise (PEM), which is the worsening of symptoms upon exertion. However, it is currently unknown whether PEM results in higher amounts of muscle damage, and whether any damage or regenerative features are present in patient skeletal muscle.

METHODS
Sections of vastus lateralis biopsies of 26 ME/CFS patients, 25 long COVID patients, and 30 healthy age- and sex-matched controls were stained for Pax7 to visualize satellite cells, PDGFRα to visualize fibroadipogenic progenitor (FAP) cells, and collagen IV and embryonic myosin heavy chain (eMHC) to assess damage and regeneration. Skeletal muscle metabolomics and lipidomics were performed by liquid chromatography mass spectrometry.

RESULTS
Both patient groups had less Pax7 + cells (P<0.001), while FAP cells were lower in ME/CFS patients (P=0.017). Both patient groups had more collagen IV surrounding myofibers (P<0.001). Expression of eMHC was not different between groups. Oxidative and antioxidant metabolites were lower in patients, while several essential amino acids were lower in patients with ME/CFS. Membrane-associated lipids were significantly lower in both patient groups (P<0.001), predominantly phospholipid species and cardiolipin. Pax7 + satellite cell density was positively associated with membrane lipids (r=0.39, P<0.001), cardiolipin (r=0.28, P=0.004), taurine (r=0.36, P=0.006), CoA-glutathione (r=0.36, P=0.010), creatine (r=0.34, P=0.013), creatinine (r=0.33, P=0.019), FAD (r=0.31, P=0.026), and mitochondrial respiration (r=0.21, P=0.034), and negatively associated with collagen IV (r=-0.35, P<0.001). Collagen IV was negatively associated to membrane-associated lipids (r=-0.31, P<0.001).

CONCLUSIONS
ME/CFS and long COVID patients have a reduced potential regenerative capacity, marked by signs of unresolved regeneration, collagen deposition, and altered membrane-associated lipids, possibly impacting organelle function.

 

[SNT Gatchaman]

Selected, abridged quotes from introduction —

Acute exercise will often result in the activation of neutrophils, T-cells, and M1 macrophages within the first few hours after exercise. As local tissue damage is resolved, neutrophil and T-cells are removed locally, and macrophages are polarized to M2 phenotypes, with macrophages being the dominant immune cell two to several days following muscle damage. The M2 subset of macrophages is necessary for the activation of fibroadipogenic progenitor (FAP) cells, and subsequently satellite cells.

FAP cells activated via the IL4R will further release myogenic factors to promote satellite cell activation and differentiation into myonuclei […] important regulators of ECM composition, wherein activation of FAP cells generally increases collagen composition in the extracellular matrix […] FAP cells if stimulated by TGF-β aids in transient ECM remodelling, however long-term exposure leads to the chronic deposition of ECM proteins, adipose, and hinders overall muscle regeneration.

A necessary component of muscle regeneration is the activation of satellite cells by FAP cells […] or mechanical stimuli. Once activated, satellite cells will begin to undergo proliferation and differentiation, in which a handful of the satellite cells will incorporate into the myofibre as new myonuclei, while the rest of the satellite cell pool will return to quiescence to maintain the overall satellite cell pool. Remodelling of the myofibre occurs throughout regeneration, often with transient expression of embryonic myosin heavy chain, increases in non-fibrillar collagens (IV, VI, etc.), and restructuring of the extracellular matrix.

We hypothesized that resident immune cells are elevated in both patients with long COVID and ME/CFS, while satellite cells and FAP cells are lower in patients compared to controls. Upon PEM induction, we expect to see a further increase in macrophages, and an increase in satellite cell content, due to their proliferation in response to exercise. As basal levels of satellite cells and FAP cells are expected to be lower in patients, we then expect to see increased signs of damage in patient skeletal muscle, notably increase in eMHC expression, collagen deposition surrounding myofibres, and increased lipid content, consistent with damaged and fibrotic tissue.

 

[SNT Gatchaman]

Selected, abridged quotes from results —

(FCSA is fibre cross-sectional area. FAP is fibroadipogenic progenitor.)

we found that the area of collagen IV present per area was higher in both patient groups compared to healthy controls, with no differences between groups. […] we corrected the collagen IV for myofibre FCSA, which yielded the same results.

As we previously reported that both patients with long COVID and ME/CFS have higher proportions of glycolytic fibres compared to healthy controls, which are more susceptible to both oxidative and mechanical damage, we hypothesized that both patient groups would exhibit more eMHC compared to controls. However, the amount of eMHC-positive fibers was not different between groups.

As incomplete regeneration and fibrosis often coincides with fatty infiltration, we assessed hydrophobic and neutral lipid formation […] Unexpectedly, lipids per myofibre did not differ between groups. However, due to tissue availability only a small subset of patients and controls were assessed. Additionally, the number of centralized myonuclei that could be seen was not different between groups.

We then aimed to determine the relative proportions of resident mononuclear cells, focusing on CD68+ macrophages, CD3+ T-cells, PAX7+ satellite cells, and PDGFRα+ FAP cells. The proportion of nuclei expressing CD68, was markedly lower in ME/CFS compared to both healthy controls and long COVID patients. The proportion of CD68+ nuclei increased one day after a maximal exercise test in all groups. Prior to exercise, there was no difference in CD3+ nuclei between groups. However, following exercise, only healthy controls and patients with long COVID had increases in CD3+ nuclei.

The proportion of PAX7+ nuclei was markedly lower in both patients with ME/CFS and long COVID, in which PEM induction did not change the relative proportions […]. FAP cell numbers (PDGFRα+) were lower in ME/CFS patients compared to healthy controls, while patients with long COVID had a tendency to have less FAP cells compared to controls. Neither PAX7+ nor FAP cell number was altered following PEM induction.

 

[SNT Gatchaman]

Selected, abridged quotes from results (cont'd) —

In both patient groups, most metabolites were reduced compared to those in healthy controls, with those related to the tricarboxylic acid (TCA) cycle, pyrimidine metabolism, and cell stress panel being the most commonly affected metabolite groups.

After post-hoc corrections, 14 metabolites were significantly altered in either patient group […]. Metabolites that were lower in both patient groups […] included citric acid, FAD2+, glutamate, creatine, which are implicated in the TCA cycle or ATP production, and hydroxyphenyllactic acid, taurine, and CoAGlutathione, which have been implicated in cell redox status. Whilst glutamate was the only common amino acid low in both patient groups, patients with ME/CFS also exhibited lower levels of the essential and large amino acids phenylalanine, tyrosine, methionine, isoleucine, and leucine, most prominently following exercise.

all lipid families were either not different or were lower in both long COVID and ME/CFS patient skeletal muscle. Lipid families that were significantly lower were cardiolipins, sphingomyelins, PE, and PC, all of which are typically implicated in lipid membrane integrity. No lipid species were significantly altered following 24 hours following exhaustive exercise.

Collagen IV was negatively associated with membrane associated lipids, whilst CD3+ T-cells, FAP cells and satellite cells were positively associated with membrane lipids. […] Collagen IV was negatively associated with satellite cell count. Conversely, satellite cell prevalence was positively associated with both CD3+ cell prevalence and oxidative phosphorylation capacity.

 

[SNT Gatchaman]

Selected, abridged quotes from discussion —

FAP cells are necessary for proper satellite cell activation, and typically reside in greater proportions to satellite cells in skeletal muscle. […] intact FAP cell signalling is necessary for non-differentiated satellite cells to replenish the satellite cell pool and return to quiescence. Thus, the combination of lower number of satellite cells and FAP cells suggests a depletion of the satellite pool either by recurring damage or by improper restoration of the satellite cell pool via FAP cell signalling.

we observed a clear distinction between both patient groups and healthy controls in the amount of collagen IV surrounding muscle fibres. While abnormal collagen accumulation is typically indicative of fibrosis, and thus unresolved damage repair, which may relate to chronic inflammation, we found no differences between groups with respect to eMHC expression or myofibre lipid accumulation. Fibrosis, the result of abnormal collagen accumulation typically consisting of collagen I and III, is mediated by TGF-β and TNF-α. However, how collagen IV is regulated in these scenarios is less completely understood

we found that collectively the amount of collagen IV surrounding the myofibre was inversely related to lipids that contribute predominantly to cell membrane structure. Specifically, phosphoethanolamines and phosphatidylcholines, which collectively account for ~80% of all cell membranes […] the data suggests a disruption of cellular membrane structure integrity and a potential compensation by increasing components of the extracellular matrix to maintain skeletal myofibre structure.

A striking pattern observed in both long COVID and ME/CFS was the reduction of metabolites implicated in oxidative phosphorylation.

Several amino acids were lower in skeletal muscle in both patient groups, but most specifically in patients with ME/CFS, and more prominently following exercise. Notably, glutamate was lower in both patient groups, while tyrosine tended to be lower in long COVID patients (P=0.053), but was significantly lower in ME/CFS patients compared to healthy controls. ME/CFS patients also exhibited lower phenylalanine, methionine, isoleucine and leucine, all of which are essential amino acids.

Specifically, leucine is known to be one of two amino acids distinctly implicated in mTOR activation, and thus the lower levels of leucine in ME/CFS patients may indicate anabolic resistance. Additionally, all amino acids that were altered in either patient group are large amino acids and typically hydrophobic, which all use the same transporter: LAT1. This then may indicate, particularly in ME/CFS patients, impairments in skeletal muscle uptake of large amino acids, impairing protein synthesis following damage or in response to cell stressors.

Multiple membrane lipid classes were reduced in patients compared to controls, similarly to other studies. Notably, three significantly reduced classes also account for ~90% of the inner mitochondrial membrane lipids: phosphatidylcholines, phosphatidylethanolamines, and cardiolipins. (Normalized) oxidative phosphorylation capacity both correlated with multiple membrane lipid species, particularly cardiolipins and phosphatidylethanolamines, suggesting that mitochondrial integrity in particular may be affected in patients.

 

[Dude]

It would have been interesting if they had also addressed the delayed onset of PEM. Why do symptoms often worsen hours after exertion, and what biological processes could explain this delay? How does this fit into the overall hypothesis proposed in the thesis?

 

[Trish]

Thank you @SNT Gatchaman for those quotes. It seems to be significant research findings.

I am interested in the timescale over which the observed changes take place. For example, does someone newly infected with EBV or Covid immediately experience this change in muscle biology that perhaps helps to explain sickness behaviour, and return to normal muscle biochemistry is quick in those who recover fully from the acute infection, slower = PVFS, becomes long term = ME/CFS.

And do similar metabolic changes occur in brain, gut, cardiovascular and other tissues?

 

[neophyte32]

Thank you so much for everything you've just done for us. I have to admit that I don't understand half of these mechanisms, but strangely enough, a lot of it resonates with me, as if some pieces of the puzzle were finally starting to come together at the cellular level.

This also seems to echo the results of the RNA sequencing I had done, which revealed significant abnormalities in several parameters, particularly in certain cell populations.

One question comes to mind, though: how do these findings fit with the DecodeME results, which seem to point more towards the involvement of the central nervous system? Could there be a connection between these two mechanisms, or are they completely separate lines of investigation?

Once again, thank you so much for this fascinating work and for sharing all this information!

 

[V.R.T.]

Chapter 9 is unpublished, so acting as a preprint. (Definitely worth a read!)

An Inability to Recover: Unresolved Regeneration and Satellite Cell Depletion in Post-Acute Infectious Diseases Associated with Altered Metabolic Landscape
Braeden T. Charlton, Anouk Slaghekke, Jelle Y. Huijts, Brent Appelman, Brian Po Lin Chen, Frank Bloemers, Wendy Noort, Richie P. Goulding, Michèle van Vugt, Richard T. Jaspers, Rob CI Wüst

INTRODUCTION
Myalgic encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) and long COVID patients frequently report with myalgia and post-exertional malaise (PEM), which is the worsening of symptoms upon exertion. However, it is currently unknown whether PEM results in higher amounts of muscle damage, and whether any damage or regenerative features are present in patient skeletal muscle.

METHODS
Sections of vastus lateralis biopsies of 26 ME/CFS patients, 25 long COVID patients, and 30 healthy age- and sex-matched controls were stained for Pax7 to visualize satellite cells, PDGFRα to visualize fibroadipogenic progenitor (FAP) cells, and collagen IV and embryonic myosin heavy chain (eMHC) to assess damage and regeneration. Skeletal muscle metabolomics and lipidomics were performed by liquid chromatography mass spectrometry.

RESULTS
Both patient groups had less Pax7 + cells (P<0.001), while FAP cells were lower in ME/CFS patients (P=0.017). Both patient groups had more collagen IV surrounding myofibers (P<0.001). Expression of eMHC was not different between groups. Oxidative and antioxidant metabolites were lower in patients, while several essential amino acids were lower in patients with ME/CFS. Membrane-associated lipids were significantly lower in both patient groups (P<0.001), predominantly phospholipid species and cardiolipin. Pax7 + satellite cell density was positively associated with membrane lipids (r=0.39, P<0.001), cardiolipin (r=0.28, P=0.004), taurine (r=0.36, P=0.006), CoA-glutathione (r=0.36, P=0.010), creatine (r=0.34, P=0.013), creatinine (r=0.33, P=0.019), FAD (r=0.31, P=0.026), and mitochondrial respiration (r=0.21, P=0.034), and negatively associated with collagen IV (r=-0.35, P<0.001). Collagen IV was negatively associated to membrane-associated lipids (r=-0.31, P<0.001).

CONCLUSIONS
ME/CFS and long COVID patients have a reduced potential regenerative capacity, marked by signs of unresolved regeneration, collagen deposition, and altered membrane-associated lipids, possibly impacting organelle function.

This is fascinating, thanks for pulling it out of the thesis!

If these are bona fide findings it could be huge! They seem to have found cellular mechanisms/abnormalities behind their biopsy findings. Which afaik makes them less likely to be lab artefacts. My brain fog is too heavy for me to figure the implications of this out right now but there is even a mention of TGF beta, which is frequently seen raised in ME/CFS.

Raised CD68+ expressing nuclei (is that meaning macrophages?) after excercise [edit: apparantly normal after excercise and happened in all groups including controls, my bad]. As are the lower FAP cells and the other one I can't remember now [edit: these two were general findings not post exercise].

Does anyone know if anybody is working on independently replicating Wust/Charton's data? I feel like it's beyond time someone tried, we need to know if this is real yesterday.

I wonder what @Jonathan Edwards and @jnmaciuch think of the findings presented in chapter 9? And all the other smart people on here I don't have capacity to tag right now!

 

[V.R.T.]

Some info on CD68:

https://www.sciencedirect.com/topics/neuroscience/cd68

 

[Jonathan Edwards]

I do not know enough about musclehistochemistry to know whether these findings are significant. I keepcoming back to the question of why, if muscle damage is occurring enough to contribute to symptoms, why are simple things not abnorkal like serum creatine kinase?

Raised CD68+ expressing nuclei (is that meaning macrophages?) after excercise is a really interesting finding.

I think that is a normal observation on post-exercise muscle.

 

[V.R.T.]

I think that is a normal observation on post-exercise muscle.

Oops, yes it says all groups including controls. Colour me embarrassed! I was thinking the timescale looked interesting for PEM and of course macrophages- innate immunity etc.

I really need to start those Khan Academy courses in earnest!

 

[neophyte32]

I do not know enough about musclehistochemistry to know whether these findings are significant. I keepcoming back to the question of why, if muscle damage is occurring enough to contribute to symptoms, why are simple things not abnorkal like serum creatine kinase?


I think that is a normal observation on post-exercise muscle.

I agree that the increase in CD68+ cells after exercise is probably a normal response. What I found more interesting was the difference between the groups. ME/CFS patients had significantly fewer CD68+ macrophages in their muscles than both healthy controls and Long Covid patients, while the Long Covid group was comparable to healthy controls.
CD68+ cells increased after exercise in all three groups, so it's really their lower abundance in ME/CFS that caught my attention.

There's also an interesting difference regarding CD3+ T cells. Their numbers increased after exercise in healthy controls and Long Covid patients, but not in ME/CFS patients.
Of course, this doesn't tell us whether these immune cells are functionally impaired, but I find these differences between ME/CFS and Long Covid quite intriguing, particularly regarding the immune response involved in muscle repair and recovery.

 

[V.R.T.]

I agree that the increase in CD68+ cells after exercise is probably a normal response. What I found more interesting was the difference between the groups. ME/CFS patients had significantly fewer CD68+ macrophages in their muscles than both healthy controls and Long Covid patients, while the Long Covid group was comparable to healthy controls.
CD68+ cells increased after exercise in all three groups, so it's really their lower abundance in ME/CFS that caught my attention.

There's also an interesting difference regarding CD3+ T cells. Their numbers increased after exercise in healthy controls and Long Covid patients, but not in ME/CFS patients.
Of course, this doesn't tell us whether these immune cells are functionally impaired, but I find these differences between ME/CFS and Long Covid quite intriguing, particularly regarding the immune response involved in muscle repair and recovery.

Shows how bad my brain fog is today that I got that mixed up!

What could  less CD68 macrophages mean? And no increase in CD3 T cells? This reminds me of the Hanson proteomics that showed that certain markers in ME/CFS patients were not changing as normal post exercise - that we DON'T see changes we expect to see in healthy people.

Are they studying LC with PEM or LC more broadly?

 

[Jonathan Edwards]

I would expect relatively unchallenged muscle tohave fewer macrophages at baseline. That could also explain lack of T cell recruitment since it is likely that macrophages call T cells in to the tissue.

 

[neophyte32]

I would expect relatively unchallenged muscle tohave fewer macrophages at baseline. That could also explain lack of T cell recruitment since it is likely that macrophages call T cells in to the tissue.

That's a plausible hypothesis, but the activity levels of the two patient groups were actually not that different in this study: a median of 4,718 steps per day for Long Covid patients versus 3,704 for ME/CFS patients. The difference wasn't reported as statistically significant either.

Yet ME/CFS patients had significantly fewer CD68+ macrophages than Long Covid patients, whose levels were comparable to healthy controls.

Physical inactivity could certainly play a role, but I find it difficult to attribute this difference to inactivity alone. Ideally, the results should be adjusted for physical activity levels, but also for disease duration, which was considerably longer in the ME/CFS group.

 

[Jonathan Edwards]

Physical inactivity could certainly play a role, but I find it difficult to attribute this difference to inactivity alone. Ideally,

I think there may be a difference between inactive and unchallenged muscle. People with ME/CFS have learned to do everything gently so that even the steps they take are likely a modest strain on the muscle. And cohorts in these studies often reflect artefacts of referral patterns in clinics and treatment units.

An absence of macrophages and T cells, after all, seems to be an indication of nothing much happening, rather than some pathology.

 

[V.R.T.]

Chapter 9 is unpublished, so acting as a preprint. (Definitely worth a read!)

An Inability to Recover: Unresolved Regeneration and Satellite Cell Depletion in Post-Acute Infectious Diseases Associated with Altered Metabolic Landscape
Braeden T. Charlton, Anouk Slaghekke, Jelle Y. Huijts, Brent Appelman, Brian Po Lin Chen, Frank Bloemers, Wendy Noort, Richie P. Goulding, Michèle van Vugt, Richard T. Jaspers, Rob CI Wüst

INTRODUCTION
Myalgic encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS) and long COVID patients frequently report with myalgia and post-exertional malaise (PEM), which is the worsening of symptoms upon exertion. However, it is currently unknown whether PEM results in higher amounts of muscle damage, and whether any damage or regenerative features are present in patient skeletal muscle.

METHODS
Sections of vastus lateralis biopsies of 26 ME/CFS patients, 25 long COVID patients, and 30 healthy age- and sex-matched controls were stained for Pax7 to visualize satellite cells, PDGFRα to visualize fibroadipogenic progenitor (FAP) cells, and collagen IV and embryonic myosin heavy chain (eMHC) to assess damage and regeneration. Skeletal muscle metabolomics and lipidomics were performed by liquid chromatography mass spectrometry.

RESULTS
Both patient groups had less Pax7 + cells (P<0.001), while FAP cells were lower in ME/CFS patients (P=0.017). Both patient groups had more collagen IV surrounding myofibers (P<0.001). Expression of eMHC was not different between groups. Oxidative and antioxidant metabolites were lower in patients, while several essential amino acids were lower in patients with ME/CFS. Membrane-associated lipids were significantly lower in both patient groups (P<0.001), predominantly phospholipid species and cardiolipin. Pax7 + satellite cell density was positively associated with membrane lipids (r=0.39, P<0.001), cardiolipin (r=0.28, P=0.004), taurine (r=0.36, P=0.006), CoA-glutathione (r=0.36, P=0.010), creatine (r=0.34, P=0.013), creatinine (r=0.33, P=0.019), FAD (r=0.31, P=0.026), and mitochondrial respiration (r=0.21, P=0.034), and negatively associated with collagen IV (r=-0.35, P<0.001). Collagen IV was negatively associated to membrane-associated lipids (r=-0.31, P<0.001).

CONCLUSIONS
ME/CFS and long COVID patients have a reduced potential regenerative capacity, marked by signs of unresolved regeneration, collagen deposition, and altered membrane-associated lipids, possibly impacting organelle function.

Noticed that @RichieG123 is listed as an author on this preprint. It looks really interesting and potentially quite significant for the field. I wondered if you had any comments to make on the paper, discussion here, or your team's future research directions based on these findings?