Local and Systemic Immunobiology of Muse-Cell-Derived Signaling in Ischemic Injury
I began researching Muse cells for a medical lecture I am giving in October, and I have to admit that one finding surprised me: the spleen appears to be intimately involved in how these stem cells function after ischemic injury.
Multilineage-differentiating stress-enduring, or Muse, cells are a rare, naturally occurring population of endogenous, non-tumorigenic, pluripotent-like reparative cells found in adult connective tissues and within mesenchymal stromal cell populations. In preclinical models of ischemic stroke and other tissue injuries, Muse cells have demonstrated selective migration to damaged tissue, prolonged survival in hostile microenvironments, spontaneous differentiation into tissue-compatible lineages, and a broad range of paracrine effects, including anti-apoptotic, trophic, anti-inflammatory, and anti-fibrotic activity.
What has become increasingly interesting is that their effects may extend well beyond the site of injury itself.
Recent studies suggest that Muse-cell-derived signaling can influence both the local tissue environment and distant immune organs. In an immunocompetent ischemic stroke model, intravenously administered Muse cells accumulated predominantly in the spleen rather than the brain, yet still improved motor recovery. When the spleen was removed, that therapeutic benefit disappeared. When splenocytes from Muse-treated animals were transferred into splenectomized recipients, recovery was restored.
That finding raises an important possibility: Muse cells may support repair through a coordinated local-systemic program that combines direct tissue regeneration with peripheral immune reprogramming.
What Makes Muse Cells Different?
Muse cells were originally identified as a rare, stress-tolerant, SSEA-3-positive subpopulation within mesenchymal stromal cells and fibroblast-like cell populations.
They express pluripotency-associated factors including OCT3/4, SOX2, and NANOG and can differentiate into derivatives of all three germ layers. Unlike embryonic stem cells and induced pluripotent stem cells, however, Muse cells have low telomerase activity, maintain a normal karyotype, and do not form teratomas in vivo. These characteristics, together with their ability to tolerate severe cellular stress, have made them particularly attractive candidates in regenerative medicine.
Early interest in Muse cells centered on their ability to home to areas of tissue damage and replace lost cells by spontaneously differentiating into tissue-compatible lineages. In stroke models, perilesional transplantation has been associated with long-term engraftment, neuronal and glial differentiation, neural circuit reconstruction, and functional recovery.
At the same time, direct cell replacement does not appear to explain the entire therapeutic effect.
Muse cells also demonstrate anti-inflammatory, anti-apoptotic, trophic, and anti-fibrotic effects across multiple organ systems. This suggests that secreted factors, cell-to-cell interactions, or both may contribute substantially to their reparative activity.
This is especially important in stroke because an injured brain does not exist in isolation. Cerebral ischemia triggers a systemic immune response involving the spleen, circulating leukocytes, cytokines, chemokines, and stress-response pathways. Evidence that intravenously administered Muse cells can improve outcomes through immunomodulation in the spleen broadens the concept of Muse-cell therapy from lesion-directed regeneration to a much larger local-systemic reparative biology.
The Biological Basis for Local and Systemic Effects
Several features of Muse cells make this dual-level activity biologically plausible.
First, Muse cells are highly stress-tolerant. This allows them to remain viable in ischemic, inflammatory, and oxidative environments in which many transplanted cells fail to persist.
Second, they demonstrate selective migration toward damaged tissue. In ischemic models, this homing behavior appears to be mediated in part by injury-related signals, including sphingosine-1-phosphate pathways.
Third, Muse cells maintain anti-inflammatory and trophic functions and have been described as having immune-privileged characteristics. These properties may facilitate allogeneic use without conventional HLA matching or immunosuppression.
Taken together, the current evidence supports at least three non-mutually exclusive mechanisms of action:
- Local structural repair: Muse cells reach damaged tissue, survive, and differentiate into tissue-compatible cells.
- Local microenvironmental modulation: Muse-cell-derived factors influence apoptosis, inflammation, angiogenesis, and glial or stromal responses within and around the lesion.
- Systemic immune modulation: Muse cells alter peripheral immune compartments, particularly the spleen, which can then influence the inflammatory response to injury from a distance.
All three mechanisms may occur, although their relative importance appears to vary according to the disease model, route of administration, treatment timing, and host immune context.
Local Repair Within the Ischemic Brain
The strongest evidence for direct local effects in the central nervous system comes from studies in which Muse cells were delivered into or near the lesion.
In a subacute lacunar stroke model, perilesional transplantation of human Muse cells resulted in prolonged survival within host brain tissue, differentiation into cells expressing neuronal and glial markers, reconstruction of neuronal circuitry, and functional improvement.
Importantly, mechanistic experiments using diphtheria toxin to selectively eliminate the transplanted human cells reversed those functional gains. This provides direct evidence that Muse cells can participate in structural repair rather than functioning solely through transient signaling.
Still, local benefit does not appear to depend entirely on replacing lost cells.
Muse-cell treatment has also been associated with reduced apoptosis, improved survival of host neural cells, and broader trophic support. Reviews of Muse therapy in stroke describe anti-apoptotic, anti-inflammatory, and angiogenic effects that may complement direct differentiation, although the specific mediators responsible for each effect remain incompletely defined.
A cautious interpretation is important here.
It is reasonable to say that Muse-cell-derived signaling may reduce inflammatory injury and support neuroprotection within the peri-infarct environment. However, the degree to which these effects can be experimentally separated from direct differentiation and the body’s own repair processes remains limited.
Most lesion-focused studies also use conditions that produce relatively high local cell concentrations. Those findings should not automatically be generalized to intravenous administration, where the majority of cells may remain outside the brain.
The Spleen and Systemic Immunobiology
Some of the most compelling recent evidence for systemic Muse-cell activity comes from the spleen.
In an immunocompetent stroke model, intravenously administered Muse cells accumulated predominantly in the spleen rather than the brain. Despite this distribution, treated animals demonstrated improved motor recovery.
The mechanistic experiments were particularly revealing. Splenectomy eliminated the therapeutic benefit. When splenocytes from Muse-treated animals were transferred into splenectomized recipients, the recovery benefit returned.
This indicates that direct engraftment within the brain is not required for meaningful benefit in every setting.
Instead, Muse cells may act through peripheral immune organs to reshape the systemic inflammatory response to cerebral ischemia. The spleen is already recognized as an important contributor to post-stroke inflammation because it serves as both a reservoir and regulator of immune cells capable of trafficking to the injured brain.
The preferential targeting of the spleen by Muse cells therefore fits within the broader framework of neuroimmunology while adding a distinctive stem-cell-based mechanism of repair.
Immune profiling in this model revealed changes in splenocyte populations, cytokine activity, and gene-expression patterns related to protein folding and cellular stress responses. These observations support the possibility that Muse cells alter immune-cell behavior in ways that become secondarily neuroprotective.
The precise molecular intermediates remain uncertain, however. It would be premature to describe Muse cells as producing a fixed or fully characterized immune phenotype within the spleen.
The most accurate conclusion at this point is that Muse cells can produce spleen-mediated immunomodulation after stroke and that this mechanism can be functionally important in at least one well-defined experimental paradigm.
Neutrophils, T Cells, Macrophages, and Microglia
Several immune-cell populations may be affected by Muse-cell-derived signaling, although the evidence continues to evolve.
Neutrophils
Neutrophils are among the earliest immune mediators involved in ischemic injury. They can worsen tissue damage through oxidative burst, protease release, and endothelial disruption.
In the spleen-focused Muse stroke model, researchers observed changes involving splenic neutrophils after treatment. This supports the possibility that Muse cells may dampen harmful neutrophil activation or trafficking.
Whether this effect consistently translates into reduced neutrophil infiltration within the brain across different models remains to be established.
T Cells
T-cell balance represents another plausible mechanism.
Muse cells have been discussed in the context of immune privilege and anti-inflammatory regulation, making it biologically plausible that treatment could shift immune activity toward more regulatory and less pro-inflammatory T-cell responses.
The stroke-spleen study reported immune changes consistent with altered cytokine activity and adaptive immune regulation. Additional research is still needed to determine whether Muse cells reliably produce greater regulatory dominance or suppress pro-inflammatory helper T-cell programs across different experimental settings.
Macrophages and Microglia
Macrophage and microglial polarization may also be targets of Muse-cell-derived signaling.
Across broader regenerative models, Muse-cell treatment has been associated with anti-inflammatory and pro-resolution effects. This raises the possibility that Muse cells may enhance M2-like macrophage polarization and improve the resolution of inflammation.
That interpretation is biologically reasonable, but it should remain provisional unless direct phenotyping confirms the effect within a specific disease model.
Local and Systemic Mechanisms Are Complementary
Local and systemic Muse-cell actions should not be viewed as competing explanations.
The current evidence instead supports an integrated model in which Muse cells may act at several levels simultaneously.
When delivered locally, or when sufficient numbers of cells reach the lesion, Muse cells can survive within damaged tissue, differentiate, and participate in structural repair.
When administered intravenously, they may still provide meaningful benefit through peripheral immune organs, particularly the spleen, even when direct lesion engraftment is limited.
These mechanisms may also reinforce one another.
Systemic immune reprogramming could reduce secondary inflammatory damage and create a more favorable environment for tissue repair. At the same time, local Muse-cell-derived signaling and direct differentiation may stabilize the lesion and support structural recovery.
This combined model helps explain an interesting feature of the existing data. Some Muse-cell benefits appear too early to be explained by large-scale cellular replacement alone, yet they may persist long enough to suggest that the effect involves more than a short-lived burst of cytokine activity.
Relatively few studies have directly compared local engraftment, local secretome effects, and systemic immune modulation within the same experimental conditions. Any attempt to rank these mechanisms is therefore still tentative.
For now, the literature supports complementarity rather than exclusivity.
Muse Cells Compared With Conventional Mesenchymal Stromal Cells
Many of the effects associated with Muse cells overlap with the established literature on conventional mesenchymal stromal cells, or MSCs.
Both cell types can exert anti-inflammatory, trophic, anti-apoptotic, and reparative effects. In both cases, the magnitude of therapeutic benefit can appear disproportionate to long-term engraftment, which points toward an important role for secretome-driven and immune-mediated mechanisms.
Conventional MSCs are already well recognized as potent immunomodulatory cells.
In ischemic stroke, MSC transplantation has been associated with reduced neuroinflammation, improved neurologic recovery, secretion of trophic factors, and modulation of peripheral immune responses. MSCs can influence T cells, B cells, dendritic cells, neutrophils, macrophages, and microglia through both soluble mediators and direct cell-to-cell interactions.
Much of the therapeutic effect of conventional MSCs is therefore thought to result from paracrine signaling rather than durable tissue integration.
In that sense, the concept of combining local tissue support with systemic immune modulation is not unique to Muse cells.
Muse cells, however, appear to have several properties that are not typical of unsorted MSC preparations.
They are a rare SSEA-3-positive subpopulation that can be isolated from mesenchymal populations and demonstrate pluripotent-like differentiation without teratoma formation, superior survival under severe cellular stress, selective injury-directed homing, and spontaneous differentiation into tissue-compatible cells after reaching sites of damage.
Reviews comparing Muse cells with conventional MSCs emphasize that MSCs frequently demonstrate limited homing, survival, and direct integration or differentiation within stroke models. Muse cells appear better able to survive hostile conditions and participate directly in structural repair.
This distinction may be particularly relevant in the brain, where perilesional Muse transplantation has been linked to long-term survival and neural circuit reconstruction.
How the Spleen Comparison With MSCs Fits In
The systemic immunobiology comparison is equally instructive.
Substantial evidence already indicates that peripheral immune organs, including the spleen, can contribute to the therapeutic effects of MSCs after neurologic injury.
In traumatic spinal cord injury and other experimental models, intravenously administered MSCs have been shown to home to and persist in the spleen, alter splenic cytokine expression, and lose some beneficial effects after splenectomy.
In stroke, MSCs have also been reported to modulate peripheral immune responses and reduce splenic injury or dysfunction.
The idea that a stem-cell therapy can work through a spleen-linked systemic immune mechanism is therefore not exclusive to Muse cells.
What appears more distinctive for Muse cells is the specificity and coherence of the mechanistic chain demonstrated in ischemic stroke.
In the Muse stroke study, intravenously administered cells preferentially accumulated in the spleen rather than the brain, improved motor recovery, lost their therapeutic effect after splenectomy, and regained that effect when splenocytes from Muse-treated animals were transferred into splenectomized recipients.
That sequence provides a particularly direct causal argument for spleen-mediated benefit.
Comparable spleen-centered mechanistic work exists for MSCs in related settings, but the stroke literature for MSCs is broader and more heterogeneous. It often emphasizes general peripheral immunomodulation rather than a single, experimentally unified, spleen-dependent pathway.
Another important distinction involves the relative weight given to local and systemic effects.
For conventional MSCs, the prevailing view is that benefit comes predominantly from paracrine and immunomodulatory mechanisms, with relatively limited durable engraftment or direct tissue replacement, particularly in the central nervous system.
For Muse cells, both direct differentiation and paracrine or systemic effects remain biologically credible and experimentally supported in different models.
This does not mean that Muse cells conceptually replace MSCs. Rather, Muse cells may represent a particularly reparative subpopulation within mesenchymal cell biology that combines core MSC-like immunomodulation with greater injury-directed integration and stress-enduring behavior.
Overall, the evidence supports a balanced interpretation.
Muse cells and MSCs both demonstrate local and systemic mechanisms of action and can influence recovery through immune regulation beyond the lesion itself. Muse cells, however, may provide a more integrated regenerative profile in ischemic injury by combining MSC-like paracrine immunobiology with stronger evidence for spontaneous tissue-compatible differentiation, prolonged survival in hostile microenvironments, and an explicitly demonstrated spleen-centered mechanism in stroke.
Direct head-to-head studies using harmonized models, dosing strategies, and immune profiling will be necessary to determine whether Muse cells are best understood as a truly distinct therapeutic class or as the most functionally potent reparative fraction within the broader MSC continuum.
Potential Relevance Beyond Stroke
Although the strongest evidence for spleen-mediated systemic Muse-cell activity currently comes from stroke, there are reasons to consider broader implications.
Muse cells have demonstrated beneficial effects in myocardial infarction and other ischemic conditions. In these settings, they can home to damaged tissue, reduce injury, improve function, and support angiogenesis and remodeling.
These findings support the broader idea that Muse cells participate in multi-level repair programs involving both cell replacement and paracrine support.
Still, extrapolation from stroke to other ischemic diseases should be approached carefully.
The spleen has a well-established role in post-stroke neuroinflammation, but the relative contribution of splenic versus local-organ effects in myocardial infarction has not been characterized to the same extent.
The most accurate statement at present is that systemic immunobiology may also be relevant in other ischemic diseases, but direct evidence for a spleen-centered mechanism remains strongest in stroke.
Clinical Translation and the Questions That Remain
Clinical translation of Muse-cell therapy for stroke has already begun.
A randomized, placebo-controlled trial evaluating an allogeneic Muse-cell product in subacute ischemic stroke reported an acceptable safety profile and encouraging signals of efficacy, although confirmatory studies are still needed.
These findings are consistent with the preclinical rationale that Muse cells may combine a favorable safety profile with meaningful reparative potential.
Important questions remain.
The molecular composition of the Muse-cell-derived secretome is still incompletely defined, and the relative contributions of soluble factors and extracellular vesicles have not been resolved.
Researchers also do not yet fully understand the circumstances that determine whether Muse cells preferentially localize to damaged tissue or to peripheral immune organs such as the spleen.
Other variables may also influence Muse-cell immunobiology, including donor variability, cell source, manufacturing conditions, and the immune status of the recipient.
Future studies should therefore focus on several areas:
- Comparing local and systemic Muse-cell effects within the same experimental model.
- Profiling splenic and lesion-level immune responses using harmonized methods.
- Identifying secretome components specifically linked to immune reprogramming.
- Determining whether cell-free products derived from Muse cells can reproduce both local and systemic benefits.
- Identifying biomarkers that can predict clinical response.
The Bottom Line
The current evidence supports a cautious but compelling view of Muse-cell biology in ischemic injury.
Muse cells appear capable of acting not only within damaged tissue but also through distant immune organs, particularly the spleen, to influence recovery.
In stroke, this local-systemic framework is supported by complementary lines of evidence. Lesion-focused studies demonstrate direct neural integration, neuronal and glial differentiation, circuit reconstruction, and local paracrine support. Intravenous administration studies demonstrate spleen-dependent immunomodulation even when brain engraftment is limited.
Compared with conventional MSCs, Muse cells share many core immunomodulatory and paracrine mechanisms but appear to combine them with greater stress endurance, more reliable injury-directed homing, spontaneous tissue-compatible differentiation, and stronger evidence for direct tissue integration.
At the same time, important uncertainties remain, particularly around the composition of the Muse-cell secretome and the biological conditions that determine whether cells localize predominantly to injured tissue or peripheral immune organs.
The most accurate conclusion today is that Muse-cell therapy likely works through a coordinated reparative program involving local tissue support, immune regulation, and, in some settings, direct differentiation.
Understanding how these processes interact will be critical to the rational development of Muse-cell-based therapies and to future efforts to translate Muse-derived secretome products into clinical practice.
The emerging picture is fascinating. What began as a story about stem cells finding damaged tissue may ultimately be a much broader story about how regenerative cells communicate with the immune system, the spleen, and the injured organ at the same time.
Dr. P
