This document provides a linear mechanistic specification of the SMPDL3B-shedding phenotype in ME/CFS and related post-infectious conditions.
Its purpose is to define what biological processes are involved, how they interact, and where instability emerges, rather than to explain why the disease persists. The latter is addressed separately in the companion document, A Feedback-Loop Architecture Explaining Persistence, Oscillation, and Relapse in ME/CFS.
Relationship to companion documents
This framework is intended to be read alongside two related works:
GLA v2.1 (Gut–Liver–Autonomic Axis):
Provides the systems-level context in which these mechanisms operate (vascular, autonomic, hepatic, metabolic).
Feedback-Loop Architecture (Shedding Phenotype):
Explains how the mechanisms specified here interact in closed loops to produce delayed PEM, relapse, and threshold erosion.
How to interpret “loops” in this document
The numbered “loops” in this framework do not represent a strict temporal sequence.
Instead, they define vulnerability nodes—mechanistic processes that can be engaged, amplified, or partially resolved depending on physiological state, stress load, and recovery quality.
Key interpretive principles:
Loops are state-dependent, not continuously active
Partial recovery between episodes is expected
Not all loops are dominant in all patients or at all times
The framework is phenotype-specific (shedding ≠ deficient)
This document should therefore be read as a mechanistic scaffold, not a diagnostic algorithm or treatment protocol.
Figure 2. SMPDL3B shedding versus deficient phenotypes.
Figure 2. SMPDL3B shedding versus deficient phenotypes.
The shedding phenotype is characterized by episodic, stress-activated cleavage of SMPDL3B with partial recovery between events, producing oscillatory membrane and immune instability. In contrast, the deficient phenotype reflects sustained suppression of SMPDL3B expression with chronically reduced membrane stability and limited recovery capacity. Distinguishing these phenotypes is essential, as they produce overlapping symptoms via different mechanistic architectures and recovery dynamics.
Canonical artifact (v2.4).
Figure 2 and its caption are designated as canonical components of the SMPDL3B phenotype framework.
The contrast presented here is locked for citation and submission purposes and defines phenotype-specific usage throughout v2.4.
Biological mechanisms remain hypothesis-level and state-dependent; future revisions will be explicitly versioned.
2. Architectural Overview of the Shedding Phenotype
The SMPDL3B-shedding phenotype is characterized by episodic membrane instability driven by stress-activated cleavage of SMPDL3B rather than by sustained suppression of its expression.
Across sixteen defined loops, four broad mechanistic domains recur:
Membrane & Innate Priming
ER–Golgi stress and altered extracellular vesicle (EV) glycosylation bias the system toward immune alertness and proteolytic permissiveness without chronic inflammation.
Shedding Execution & Ion Amplification
Episodic SMPDL3B loss destabilizes lipid rafts, disrupts ion handling, sensitizes innate receptors, and engages PI-PLC–mediated amplification pathways.
Vascular & Autonomic Expression
Membrane and calcium instability propagate into brittle endothelial nitric oxide signaling, perfusion distribution failure, and maladaptive autonomic compensation.
Figure 1. Linear Mechanistic Spine of the SMPDL3B-Shedding Phenotype
A left-to-right schematic from membrane/innate priming → episodic SMPDL3B cleavage → lipid raft destabilization → TLR4/PI-PLC amplification → Ca²⁺/mitochondrial stress → endothelial NO brittleness → perfusion mismatch → delayed recovery-phase collapse (PEM).
Mechanisms shown are state-dependent and episodic rather than constitutively active, distinguishing the shedding phenotype from sustained SMPDL3B deficiency. The figure intentionally omits feedback topology; closed-loop interactions responsible for persistence, oscillation, and relapse are addressed separately in the companion Feedback-Loop Architecture.
Canonical artifact (v2.4).
Figure 1 and its accompanying caption are designated as a canonical component of the SMPDL3B-Shedding Systems Framework v2.4.
The structure, ordering, terminology, and scope of this figure are locked for citation and submission purposes.
Mechanistic relationships depicted remain hypothesis-level and state-dependent, consistent with the framework’s provisional biological claims.
Future revisions, if any, will be versioned explicitly and will not retroactively modify this canonical artifact.
Loop 1 — EV High-Mannose Glycome Signaling & Innate Priming
Loop summary.
Altered glycosylation of extracellular vesicles maintains innate immune alertness without sustained inflammation, biasing the system toward episodic SMPDL3B shedding rather than the persistent enzymatic suppression seen in the deficient phenotype.
Primary trigger.
Post-infectious or inflammatory stress induces altered glycosylation of secreted extracellular vesicles (EVs), resulting in a relative enrichment of high-mannose and immature N-glycan structures.
Mechanistic chain.
Cellular stress within immune and endothelial compartments disrupts glycan maturation within the ER and Golgi, leading to the release of EVs displaying high-mannose glycans. These EVs act as circulating stress-context signals, engaging mannose-recognition pathways (including lectin receptors and innate immune sensors). In the shedding phenotype, this signaling does not chronically suppress membrane enzymes; instead, it recurrently activates stress-responsive pathways, maintaining immune alertness while preserving the capacity for episodic enzymatic cleavage.
Amplification dynamics.
High-mannose EV signaling primes downstream innate immune pathways (JAK–STAT, NF-κB contextually), lowering activation thresholds for subsequent TLR engagement. Rather than producing stable suppression of SMPDL3B expression, repeated exposure biases cells toward episodic SMPDL3B cleavage and surface loss, generating dynamic instability rather than fixed deficiency.
Key biomarkers / signals
EV N-glycan profiling: high-mannose enrichment
Subtle interferon-stimulated gene activation (often low-grade or fluctuating)
Lectin pathway activation markers
Context-dependent elevation of inflammatory cytokine signaling without sustained suppression markers
Clinical expression
Patients often exhibit immune reactivity without classic autoimmune signatures: fluctuating inflammatory symptoms, sensitivity to immune triggers, and variability rather than monotonic decline. Symptoms may worsen after immune challenges (infection, exertion, inflammatory foods) but partially normalize between flares.
Intervention levers (context-sensitive)
Reducing upstream cellular stress and glycan processing load may dampen this loop. Over-aggressive immune suppression can destabilize compensation and provoke rebound activation. Timing relative to downstream membrane and calcium stability is critical.
Loop summary
ER–Golgi stress increases reliance on stress-activated proteolysis, biasing SMPDL3B toward episodic cleavage rather than sustained down-regulation.
Primary trigger
Sustained secretory demand combined with inflammatory signaling overwhelms ER folding capacity and Golgi glycan maturation pathways.
Mechanistic chain
ER stress impairs proper protein folding and post-translational modification, while Golgi stress limits complex N-glycan processing. In shedding-dominant systems, this does not lock cells into low SMPDL3B expression; instead, it shifts trafficking and surface residence time, increasing susceptibility of SMPDL3B to proteolytic cleavage by stress-activated sheddases and PI-PLC–linked mechanisms.
Amplification dynamics
ER–Golgi stress increases reliance on rapid membrane turnover and stress-responsive proteolysis. This favors episodic SMPDL3B shedding during inflammatory or metabolic stressors, temporarily destabilizing lipid rafts and innate signaling control, followed by partial recovery once stress abates.
Key biomarkers / signals
UPR activation markers (e.g., BiP, CHOP trends rather than sustained elevation)
Variability in membrane-anchored enzyme levels over time
Stress-linked protease activity signatures
Clinical expression
Patients experience state-dependent vulnerability: periods of relative stability interrupted by abrupt crashes triggered by metabolic, inflammatory, or autonomic stress. These crashes often feel disproportionate to the trigger but are not permanent.
Intervention levers (context-sensitive)
Supporting ER–Golgi throughput and reducing secretory overload can reduce shedding frequency. Interventions that increase trafficking demand or protease activation may worsen instability if introduced prematurely.
Loop summary
Episodic membrane instability disrupts ion handling, activating stress-responsive microRNA programs that reinforce reversible inflammatory and metabolic stress signaling.
Primary trigger
Membrane instability and stress-activated signaling disrupt ion channel regulation and intracellular electrolyte homeostasis.
Mechanistic chain
Transient loss of SMPDL3B from the membrane destabilizes lipid rafts, impairing tight regulation of ion channels and receptor signaling. This promotes intracellular sodium accumulation and secondary calcium dysregulation, which in turn activates stress-responsive microRNA programs. These miRNAs reinforce inflammatory and metabolic stress responses while remaining reversible between episodes.
Amplification dynamics
Calcium instability sensitizes downstream innate receptors and mitochondrial metabolism, feeding forward into ER stress and immune priming without producing permanent suppression. The system oscillates between compensation and decompensation rather than collapsing into a fixed low-function state.
Key biomarkers / signals
Functional evidence of ion handling instability (exercise intolerance, autonomic swings)
Stress-responsive microRNA signatures (variable rather than persistently elevated)
Secondary markers of calcium-dependent signaling activation
Episodic metabolic stress markers
Clinical expression
Patients report sudden symptom amplification with exertion or stress (palpitations, weakness, cognitive disruption), followed by partial recovery. This contributes to the characteristic unpredictability of the shedding phenotype.
Intervention levers (context-sensitive)
Stabilizing membrane signaling and ion handling reduces crash frequency. Interventions that increase calcium flux or autonomic load may precipitate shedding-linked decompensation if not sequenced carefully.
Loop 4 — TLR4 Sensitization via Lipid Raft Destabilization
Loop summary
Episodic SMPDL3B loss transiently destabilizes lipid rafts, lowering TLR4 activation thresholds without constitutive immune activation.
Primary trigger
Episodic loss of SMPDL3B from the membrane alters lipid raft composition and receptor compartmentalization.
Mechanistic chain
SMPDL3B normally stabilizes membrane microdomains that constrain innate receptor signaling. In the shedding phenotype, stress-induced cleavage of SMPDL3B transiently destabilizes lipid rafts, increasing lateral mobility and clustering propensity of TLR4. This does not constitutively activate TLR4 but lowers the activation threshold, making the receptor hyper-responsive to otherwise tolerable ligands (LPS fragments, endogenous danger signals, glycan-patterned EVs).
Amplification dynamics
Because SMPDL3B expression is not chronically suppressed, raft instability fluctuates. Each shedding episode briefly sensitizes TLR4, amplifying inflammatory signaling, which then feeds back to promote further protease activation and repeat shedding during stress windows.
Key biomarkers / signals
Heightened TLR4 responsiveness without sustained cytokine elevation
Increased sensitivity to endotoxin-like triggers
Variability in inflammatory markers rather than fixed elevation
Functional hypersensitivity disproportionate to ligand burden
Clinical expression
Patients often describe exaggerated inflammatory or “flu-like” responses to minor triggers (dietary changes, exertion, environmental exposures), followed by partial normalization. This creates a pattern of relapsing immune sensitivity rather than persistent inflammation.
Intervention levers (context-sensitive)
Reducing membrane instability and limiting unnecessary innate stimulation may blunt this loop. Broad TLR4 blockade without addressing membrane dynamics can destabilize compensation and worsen downstream signaling variability.
Loop 5 — PI-PLC Activation and GPI-Anchored Protein Shedding
Loop summary
Stress-activated PI-PLC intermittently cleaves GPI-anchored proteins, including SMPDL3B, executing episodic membrane destabilization without sustained transcriptional loss.
Primary trigger
TLR4 sensitization and calcium-dependent stress signaling activate phosphoinositide-specific phospholipase C (PI-PLC).
Mechanistic chain
PI-PLC cleaves GPI anchors from the cell surface, including SMPDL3B and other regulatory proteins. In the shedding phenotype, PI-PLC activity is intermittently upregulated, producing episodic loss of membrane-bound SMPDL3B rather than sustained depletion. This cleavage further destabilizes lipid rafts and removes local inhibitory control over innate signaling, without underlying transcriptional loss.
Amplification dynamics
Each PI-PLC activation event reinforces conditions that favor its own reactivation—calcium flux, inflammatory signaling, and membrane disorder. Because transcriptional suppression is absent, membrane proteins can be re-expressed between episodes, maintaining oscillatory instability rather than collapse.
Key biomarkers / signals
Evidence of increased GPI-anchor cleavage products (where measurable)
Calcium-dependent signaling activation
Fluctuating membrane enzyme levels
Stress-linked phospholipid turnover markers
Clinical expression
Symptoms often escalate rapidly during stress exposure, with sudden drops in tolerance and abrupt PEM onset. Recovery is possible but incomplete, leading to cumulative fragility over time.
Intervention levers (context-sensitive)
Limiting calcium-driven PI-PLC activation and stabilizing membrane phospholipids may reduce shedding frequency. Interventions that increase intracellular calcium or phospholipid turnover may precipitate acute worsening.
Loop 6 — Calcium-Dependent Amplification and Mitochondrial Stress Signaling
Loop summary
Episodic calcium overload links membrane instability to mitochondrial stress, ROS generation, and delayed metabolic recovery without persistent baseline dysfunction.
Primary trigger
Membrane instability and PI-PLC activation drive intracellular calcium influx and dysregulation.
Mechanistic chain
Transient SMPDL3B loss and PI-PLC signaling disrupt calcium channel regulation, leading to episodic intracellular calcium overload. Elevated calcium activates mitochondrial stress pathways, increases reactive oxygen species (ROS), and impairs ATP generation efficiency. Unlike the deficient phenotype, these effects are episodic and tied to shedding events rather than persistent baseline dysfunction.
Amplification dynamics
Calcium-driven mitochondrial stress feeds back into ER stress, innate signaling, and PI-PLC activation, reinforcing the shedding loop. Partial recovery occurs between episodes, but repeated stress progressively lowers the threshold for future decompensation.
Key biomarkers / signals
Exercise- or stress-induced lactate elevation
Transient increases in oxidative stress markers
Variable mitochondrial performance metrics
Delayed metabolic recovery following exertion
Clinical expression
Patients experience delayed PEM with sudden energy collapse, cognitive dysfunction, and autonomic instability following exertion or stress, often with partial recovery days later.
Intervention levers (context-sensitive)
Reducing calcium volatility and mitochondrial load can stabilize this loop. Early introduction of agents that increase metabolic throughput or calcium flux may worsen crash severity if upstream membrane instability is unaddressed.
Loop 7 — Endothelial NO Signaling Brittleness Under Shedding Stress
Loop summary
Repeated shedding-linked stress produces brittle endothelial nitric oxide signaling that is preserved at rest but unreliable under demand.
Mechanistic chain
In shedding-dominant systems, episodic SMPDL3B loss and TLR4/PI-PLC activation propagate into endothelial cells, disrupting membrane signaling platforms required for stable eNOS regulation. Calcium overload and oxidative stress reduce NO bioavailability through uncoupling and scavenging rather than transcriptional suppression. The result is brittle NO signaling: preserved at rest, unreliable under demand.
Amplification dynamics
Each stress-induced shedding episode transiently worsens endothelial responsiveness, impairing vasodilatory reserve. Partial recovery occurs between episodes, but cumulative stress progressively narrows the margin of perfusion control.
Key biomarkers / signals
Normal or near-normal resting vascular markers
Stress-induced endothelial dysfunction
Reduced flow-mediated dilation under challenge
Oxidative stress markers linked to exertion
Clinical expression
Patients often appear vascularly “normal” at baseline but develop pronounced symptoms during orthostatic stress or exertion, including lightheadedness, cognitive fog, and exertional intolerance.
Intervention levers (context-sensitive)
Supporting endothelial resilience and reducing oxidative load may stabilize this loop. Premature use of strong vasodilators can worsen perfusion mismatch if upstream instability persists.
Loop 8 — Perfusion Distribution Failure and Microvascular Mismatch
Loop summary
Brittle endothelial control produces uneven microvascular perfusion, generating regional ischemia despite normal global hemodynamics.
Primary trigger
Brittle NO signaling impairs adaptive redistribution of blood flow during demand.
Mechanistic chain
Inadequate endothelial responsiveness leads to uneven microvascular perfusion, with some beds over-perfused and others under-supplied during stress. This creates regional ischemia despite normal global hemodynamic measures. Ischemic tissues generate metabolic stress signals that feed back into inflammatory and autonomic pathways.
Amplification dynamics
Each episode of perfusion mismatch reinforces mitochondrial stress, ROS production, and calcium dysregulation, increasing the likelihood of subsequent shedding events. Because global measures often appear normal, this loop remains clinically under-recognized.
Key biomarkers / signals
Normal resting blood pressure and cardiac output
Reduced cerebral or muscular perfusion during stress
Exercise-induced metabolic derangements
Evidence of regional hypoxia without systemic hypotension
Clinical expression
Patients describe sudden weakness, cognitive shutdown, or limb heaviness during activity, often disproportionate to objective exertion levels. Recovery is delayed but incomplete.
Intervention levers (context-sensitive)
Improving microvascular stability and reducing ischemic triggers may help. Interventions that increase demand without improving distribution can precipitate rapid decompensation.
Loop 9 — Autonomic–Vascular Mismatch and Sympathetic Lock-In
Loop summary
Compensatory sympathetic activation stabilizes perfusion short-term but increases long-term instability through calcium and metabolic load.
Mechanistic chain
The autonomic nervous system attempts to correct perfusion mismatch via sympathetic activation. In shedding-dominant patients, this compensation becomes maladaptive, with persistent sympathetic tone required to maintain baseline perfusion. This raises heart rate, increases vascular tone heterogeneity, and further increases microvascular flow brittleness.
Amplification dynamics
Sympathetic activation increases calcium flux and metabolic demand, feeding back into membrane instability, PI-PLC activation, and subsequent SMPDL3B shedding. The system oscillates between compensation and exhaustion rather than stabilizing.
Key biomarkers / signals
Elevated resting or stress-induced sympathetic markers
Heart rate variability suppression during flares
Orthostatic intolerance without fixed hypotension
Autonomic testing showing exaggerated responses
Clinical expression
Patients report palpitations, anxiety-like sensations, temperature dysregulation, and worsening fatigue with standing or stress. Symptoms fluctuate but trend toward increased fragility.
Intervention levers (context-sensitive)
Careful modulation of autonomic tone and volume status may improve tolerance. Over-suppressing sympathetic activity without restoring perfusion stability can worsen symptoms.
Loop 10 — Renal Perfusion Variability and Volume Dysregulation
Loop summary
State-dependent renal hypoperfusion destabilizes sodium and water handling, amplifying orthostatic stress and autonomic drive.
Primary trigger
Autonomic–vascular mismatch and microvascular distribution failure reduce stable renal perfusion under stress.
Mechanistic chain
When perfusion distribution becomes unreliable, the kidney experiences intermittent under-perfusion—often without persistent systemic hypotension. This disrupts sodium and water handling, blunts consistent volume regulation, and increases sensitivity to posture, heat, and exertion. In shedding-dominant systems, renal handling may appear near-normal at rest but becomes unstable during flares.
Amplification dynamics
Volume instability worsens orthostatic stress and increases sympathetic drive, reinforcing autonomic–vascular mismatch. Reduced renal perfusion also increases metabolic stress signaling, raising systemic vulnerability to exertion and inflammatory triggers.
Key biomarkers / signals
Evidence consistent with functional renal hypoperfusion during symptomatic windows
Fluctuating electrolytes (often subtle)
Orthostatic intolerance worsened by dehydration or heat
Creatinine and renal indices trending high-normal during low-flow states
Clinical expression
Patients often report intolerance to standing, dehydration sensitivity, morning instability, and disproportionate symptom worsening with minor volume losses.
Intervention levers (context-sensitive)
Volume support and electrolyte stabilization can reduce crash frequency. Over-diuresis, aggressive vasodilation, or interventions that increase heat loss can worsen instability.
Mechanistic chain
The liver functions as a central metabolic buffer for bile-acid signaling, lipid handling, and inflammatory clearance. In the shedding phenotype, repeated stress episodes—immune flares, perfusion mismatch, and sympathetic lock-in—impose hepatic load in bursts rather than continuously. This shifts endocrine and metabolic signaling and reduces metabolic flexibility under load, without requiring intrinsic liver pathology.
Amplification dynamics
When hepatic buffering capacity is exceeded, downstream systems experience increased oxidative and metabolic stress. This raises susceptibility to PI-PLC activation, calcium instability, and renewed SMPDL3B shedding, closing a loop in which systemic instability and reduced recovery capacity reinforce one another.
Normal liver imaging and enzymes despite functional constraint
Episodic bile-acid signaling irregularities
Clinical expression
Patients describe “metabolic fragility,” with reduced tolerance to dietary variation, poor recovery after exertion, and symptom worsening after inflammatory meals or prolonged stress.
Intervention levers (context-sensitive)
Reducing systemic stress load and supporting metabolic buffering may improve recovery depth. Aggressive metabolic stimulation or rapid dietary shifts may worsen symptoms if perfusion and autonomic stability remain brittle.
Loop 12 — ATP Buffer Failure and Delayed Post-Exertional Collapse
Loop summary
Delayed ATP buffering failure during recovery—rather than exertion itself—drives classic delayed PEM.
Primary trigger
Perfusion mismatch and mitochondrial stress during exertion overwhelm cellular ATP buffering systems.
Mechanistic chain
During stress, regional ischemia and calcium overload drive inefficient ATP production and increased ROS. In shedding-dominant systems, compensation during exertion is often maintained, but failure emerges during recovery, when oxidative signaling, ion rebalancing, and mitochondrial repair demand peak. This delayed metabolic cost produces PEM as a downstream consequence rather than an immediate exertional crash.
Amplification dynamics
ATP buffer failure increases intracellular stress signaling and calcium dysregulation, feeding back into PI-PLC activation and membrane instability. Each delayed crash narrows tolerance thresholds for future stressors.
Key biomarkers / signals
Delayed lactate and acid–base disturbances
Prolonged post-exertional recovery kinetics
Oxidative stress markers during the recovery window
Autonomic suppression and HRV deterioration post-stressor
Clinical expression
Patients experience delayed PEM 12–48 hours after exertion or stress, characterized by cognitive dysfunction, flu-like malaise, pain amplification, and prolonged reduction in capacity.
Intervention levers (context-sensitive)
Strict pacing and protection of recovery windows are foundational. Interventions that increase exertional output or mitochondrial demand without stabilizing perfusion and ion handling can worsen PEM severity and duration.
Figure 3. Delayed emergence of post-exertional malaise during recovery.
Figure 3. Delayed emergence of post-exertional malaise during recovery.
In the SMPDL3B-shedding phenotype, compensation can remain adequate during exertion while latent stress accumulates (ischemic debt, Ca²⁺ loading, ROS signaling).
During recovery, compensatory capacity (ATP repletion, autonomic/perfusion control, ion rebalancing) drops as demand peaks, allowing latent stress to exceed capacity.
PEM emerges when latent stress load crosses above compensation capacity during the recovery window, distinguishing delayed PEM from immediate exertional intolerance.
Canonical artifact (v2.4).
Figure 3 and its caption are designated as canonical components of the SMPDL3B-Shedding Systems Framework v2.4.
The temporal structure and recovery-phase emphasis are locked for citation and submission purposes.
Physiological relationships depicted are hypothesis-level and state-dependent; no fixed thresholds or diagnostic criteria are implied.
Loop 13 — Persistent Innate Reactivity and Incomplete Immune Resolution
Loop summary
Repeated re-triggering of innate pathways prevents full immune resolution, maintaining a primed state without continuous inflammation.
Primary trigger
Cycles of TLR4 sensitization, calcium instability, and metabolic stress recur before immune resolution is complete.
Mechanistic chain
In the shedding phenotype, innate immune activation is episodic rather than sustained. SMPDL3B loss during stress windows removes local inhibitory control over innate receptors, leading to renewed cytokine and danger-signal release. Between episodes, inflammatory signaling partially resolves but does not fully reset.
Amplification dynamics
Incomplete resolution lowers activation thresholds for subsequent immune responses, increasing flare frequency even when baseline inflammatory markers remain modest.
Key biomarkers / signals
Fluctuating inflammatory cytokines without sustained elevation
Low-grade innate immune activation signatures
Stress-sensitive immune markers
Absence of classic autoimmune patterns
Clinical expression
Patients experience waxing and waning flu-like symptoms, pain, and malaise triggered by exertion, infection, or stress, with cumulative symptom burden over time.
Intervention levers (context-sensitive)
Supporting immune resolution and reducing repeated triggering may help. Chronic immune suppression risks destabilizing compensation and worsening relapse severity.
Loop summary
Recurrent ischemic and inflammatory stress sensitizes neuroimmune pathways, amplifying pain and sensory load without fixed tissue injury.
Primary trigger
Repeated ischemia, oxidative stress, and immune activation sensitizes peripheral tissues and neural pathways.
Mechanistic chain
Metabolic and inflammatory stress increase nociceptor sensitivity and alter neuroimmune communication. In shedding-dominant systems, this produces state-dependent pain and sensory amplification rather than structural pathology. Central and peripheral sensitization reinforce fatigue, cognitive dysfunction, and pain perception.
Amplification dynamics
Pain and sensory stress increase sympathetic tone and autonomic load, feeding back into perfusion instability and calcium dysregulation, indirectly promoting further SMPDL3B shedding.
Key biomarkers / signals
Functional pain sensitization markers
Neuroimmune activation signatures
Stress-related neurotransmitter imbalance
Lack of overt structural pathology
Clinical expression
Patients report diffuse pain, headaches, sensory hypersensitivity, and cognitive overload that worsen during flares and partially improve between episodes.
Intervention levers (context-sensitive)
Reducing tissue stress and neuroimmune amplification may improve quality of life. Overstimulating the nervous system or aggressive activity can exacerbate symptoms.
Loop 15 — Relapse Architecture and Progressive Threshold Lowering
Mechanistic chain
Each shedding-driven decompensation leaves residual buffering loss: incomplete membrane repair, lingering metabolic stress, and autonomic sensitization. Over time, this lowers the threshold for future shedding events, biasing the system toward relapse even in the absence of major new insults.
Amplification dynamics
Relapse probability increases with the frequency and severity of prior crashes. The system becomes easier to destabilize but remains theoretically reversible if upstream stressors are adequately reduced.
Key biomarkers / signals
Progressive reduction in exertional tolerance
Increasing flare frequency
Slower recovery kinetics
Accumulating autonomic and metabolic fragility
Clinical expression
Patients describe a “shrinking envelope” of activity tolerance, with crashes triggered by increasingly minor stressors.
Intervention levers (context-sensitive)
Relapse prevention, strict pacing, and upstream stabilization are critical. Attempts to “push through” accelerate threshold erosion.
Loop 16 — Final Common Pathway: Delayed PEM Maintenance via Shedding-Driven Instability
Loop summary
Delayed PEM is maintained by recovery-phase re-engagement of SMPDL3B shedding across interacting systems.
Primary trigger
Integration of membrane instability, perfusion mismatch, metabolic stress, and immune priming.
Mechanistic chain
The final common pathway in the shedding phenotype is delayed PEM maintenance driven by load-dependent SMPDL3B shedding. Each shedding episode destabilizes membrane signaling at critical stress points, disrupting immune control, endothelial function, ion handling, and metabolic buffering simultaneously. Recovery occurs, but not to pre-illness resilience.
Amplification dynamics
Because SMPDL3B expression is not permanently lost, the system remains oscillatory. Without intervention, repeated cycles progressively reduce recovery depth and stability.
Key biomarkers / signals
Delayed PEM signatures
Multi-system stress markers post-exertion
Autonomic suppression during recovery
Absence of a single dominant structural abnormality
Clinical expression
Patients experience delayed, prolonged PEM with multi-day symptom exacerbation, partial recovery, and high relapse risk.
Intervention levers (context-sensitive)
Breaking the cycle requires upstream stabilization across membrane, perfusion, metabolic, and autonomic domains. Single-axis interventions are often insufficient.
Figure 4 State-dependent loop engagement matrix.
Heuristic depiction of which mechanistic loops (1–16) tend to dominate across common physiological states (baseline, exertion, early recovery, delayed recovery window, relapse) in the SMPDL3B-shedding phenotype. This matrix is intended to orient readers to state-dependence and does not imply deterministic sequencing, fixed thresholds, or diagnostic classification.
This document specifies the linear mechanistic components of the SMPDL3B-shedding phenotype. It does not, on its own, explain disease persistence, oscillation, or relapse timing; those dynamics emerge from closed feedback interactions described in the companion Feedback-Loop Architecture paper.
Key boundary conditions:
The framework is phenotype-specific and does not apply uniformly to all ME/CFS presentations
Mechanisms described are state-dependent and reversible, not fixed defects
Loops represent vulnerability nodes, not deterministic progression
This framework is intended for mechanistic reasoning and hypothesis generation, not diagnosis
When integrated with the GLA v2.1 systems context and the feedback-loop architecture, this framework provides a coherent explanation for delayed PEM, relapse susceptibility, and heterogeneous clinical expression without invoking permanent damage or continuous immune activation.