Hypothesis: Increasing serum soluble CD40 ligand (sCD40L) may be a biomarker of ME/CFS and chronic Long COVID progression

by Vijay Iyer, Ph.D.[1,2]

1 Relevant affiliation: Independent Patient-Researcher
2 Other affiliation: Principal Neuroscience Specialist at MathWorks, Inc.

Cite as: Iyer, V. (2023). Hypothesis: Increasing serum soluble CD40 ligand (sCD40L) may be a biomarker of ME/CFS and chronic Long COVID progression. Patient-Generated Hypotheses Journal for Long COVID & Associated Conditions, Vol. 1, 44-48


To date, no single blood lab test exists to diagnose or track ME/CFS or chronic Long COVID. Based on existing literature, this article brings together evidence that a molecule secreted by the immune system called sCD40L tends to become increasingly elevated in ME/CFS, Long COVID, and Multiple Sclerosis. These studies, along with what’s known about the role of sCD40L in health and other diseases, suggest sCD40L may be useful to track over time in ME/CFS and Long COVID patients.


Many studies into myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and Long COVID (LC) have sought to identify serum or plasma biomarkers. Most have focused on a single point in time, with a growing number identifying multi-marker “signatures” which can accurately classify patients versus healthy controls1-3. Some studies have investigated biomarkers at distinct time points with respect to condition, such as before and after exercise which is known to provoke ME symptoms4,5. Few if any LC and/or ME/CFS biomarker studies have been longitudinal, i.e., tracking the evolution of candidate biomarkers in individual patients over time.

One study from Hornig et al. (2015) explored candidate biomarker time-evolution at the group level, comparing plasma cytokine levels for ME/CFS patient cohorts that were early (< 3 years) and later (> 3 years) in their disease course6. They remarked on one cytokine in particular: soluble CD40 ligand (sCD40L) between the early and late stage ME/CFS patient cohorts, which showed a highly significant progression (p<0.00001 via null hypothesis testing).

ME/CFS is often a post-viral illness7, and recently further evidence has emerged for the progression of sCD40L levels in other post-viral illnesses:

  • Wu et al. (2021) reported on sCD40L as a marker of progression in multiple sclerosis (MS), with a clear increase between cohorts in the earlier (relapsing-remitting) and later (secondary progressive) forms of the disease8. Recent evidence indicates MS also has a viral trigger, specifically Epstein-Barr virus9
  • Patterson et al. studied sCD40L levels in chronic Long COVID cohorts (with predominantly ME/CFS-like symptoms), reporting significant but modest sCD40L elevation in an early study10 and a stronger significant correspondence of sCD40L levels to LC severity (across 4 of 5 symptom questionnaires) in a later study11.

These studies each point to increasing sCD40L correspondence with progression (time and/or symptom), giving rise to the hypothesis that sCD40L levels progressively increase on average during the course of chronic Long COVID and ME/CFS. Whereas Hornig et al. (2015) and Patterson et al. (2021) found opposing effects at an early timepoint for ME/CFS and LC, respectively, both showed evidence for a progressive increase in sCD40L levels. Such progression on average may give indications about the disease mechanism(s), while such progression individually may be an indicator of disease presence and/or severity.

Soluble CD40 ligand (sCD40L), alternatively known as CD154, is a mediator of CD40 receptor immune and inflammatory responses ubiquitous across immune cell types. The ligand form was originally found on the surface of activated T-cells12. But more recently it is recognized platelets are likely the largest source of soluble (circulating) CD40L; and that sCD40L may in turn be the most ubiquitous signaling molecule in the platelet repertoire13,14

The platelet origin of sCD40L may comport with multiple findings of abnormal platelet activation for LC and/or ME/CFS using various experimental methods:

  • Microscopic investigation of platelet-poor plasma from a cohort (n=80) of LC patients identified platelet hyperactivation as a candidate sign15, with similar findings in a report of a smaller cohort (n=25) of ME/CFS patients16.
  • Flow cytometry study of LC patients (n=24) with confirmed cardiopulmonary exercise test (CPET) findings found two markers of platelet activation—P-selectin and platelet-leukocyte aggregates (PLA)—persistently elevated at 6 months post-infection17.
  • RNAseq study of ME/CFS patients (n=30) found abnormally enriched gene sets for platelets (but not other immune cells) post-exercise18.

Beyond platelet activity dysfunction, several suspected pathological pathways for LC and ME/CFS have been linked to sCD40L signaling, including endothelial cell activation19, metabolism-associated cell danger signaling via monocytes20, pathogen-associated molecular pattern (PAMP) activation of B-cell adaptive immunity21, and neurocognitive impairment22.

Taken all together, sCD40L levels in LC and ME/CFS appear to be a readily achievable (via blood sample) and potentially fruitful longitudinal measurement, which may serve as a progressive biomarker and/or an indicator of underlying pathological mechanisms.

Given its progression appears in MS, one caveat is in order: sCD40L is not apt to be a specific marker for LC and/or ME/CFS. It has also been implicated as a marker of cardiovascular disease including stroke23,24 and for several other neurological disorders including Alzheimer’s disease25,26. Any studies of this marker for LC and/or ME/CFS should be designed and interpreted accordingly.


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