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• IV-delivered MSCs act fast and transiently, interacting with innate immune cells and triggering anti-inflammatory effects.
• Locally delivered MSCs behave differently, persisting longer in tissue as “living pharmacies” or that release therapeutic signals, and can be engineered for specific functions.
• With clearer rules for MSC fate established, the field is shifting from guesswork to engineering, using standardized cells, defined processes, and targeted modifications to design next-generation MSC therapies.

Today we have a clearer picture of what mesenchymal stem/stromal cells (MSCs) really do after administration. [1] First, IV-injected MSCs act through a fast, “hit-and-run” interaction with innate immune cells. Second, locally implanted MSCs act more like living pharmacies that stay longer in place and release therapeutically helpful “secretome” signals. This new clarity is finally turning MSC therapy into something reliable and engineerable, not guesswork.

“Rx-MSCs” can perform brilliantly in vivo, expressing fascinating properties that may provide clinical benefit, [2, 3] depending on how they are initially sourced and processed. [4] Yet it may be useful to nudge them a little towards a useful fate. How? The expert authors of a recent consensus review article in Cytotherapy, [5] “Fate and Function of Exogenously Administered MSCs: Current Insights and Future Directions,” Shokoohmand, et al. (2025) explain lessons learned. They then shine a light on a few remaining questions and the experimental approaches to address them. As regenerative medicine advances, technology to optimally control the fate of MSCs for novel cell therapies will benefit from these experimental initiatives.

The Great Escape of MSCs from IBMIR

Historically, systemically administered MSCs by IV were observed to face targeting by the innate immune system. Upon exposure to serum, a sizable proportion immediately lyse from complement activation, within hours, via the blood-mediated inflammatory reaction (IBMIR). Currently, IBMIR appears to offer no medical benefit to Rx-MSCs, but further research is ongoing. If more cells could remain IBMIR-resistant, the potency of an MSC dose could increase. [1] Genetic modification, priming, or adjuvants to counteract IBMIR are thus areas ripe for additional innovation. [6]

Following IBMIR, the MSC survivors are predominantly entrapped in the lung’s microvasculature. They show signs of apoptosis signals, with most cleared and/or losing viability within about a day. Tracking signals can sometimes be detected for 2-3 days, yet these may reflect debris or phagocytosed material. [7, 8] They are then “swallowed” by lung macrophages, which may polarize into an M2 phenotype that educates the immune system toward an anti-inflammatory stance. [9] The notable mild MSC pulmonary emboli formation seems due to MSCs’ larger size and less deformability than other therapeutic cell types, like CAR-Ts. Nevertheless, the article reports, “In human studies, there is little evidence of clinically significant emboli, supporting a favorable safety profile even in procoagulant conditions such as COVID-19–associated ARDS.” This is presumably from rapid clearance of MSCs. Further experimentation will soon illuminate exactly how MSC apoptotic bodies send a “cooling” signal to macrophages, and how it might be augmented with genetic programming. [10]

MSCs Wanted: For New Jobs in Localized, In Vivo Biofactories

Distinct from IV- and/or systemic injected MSCs is the category of non-systemic MSC therapies. These are where MSCs are placed directly into a tissue (-/+ scaffolding or matrix). They open a broad and less explored space for novel treatments. That is because they can work through more diverse mechanisms than the standard immune-calming effects seen with systemic delivery. Because locally injected Rx-MSCs don’t immediately touch blood, they can avoid the IBMIR reaction, and many survive longer. [11] With that extra time of viability, they don’t need to “die to heal” [7] or rely on a lung-based apoptotic signal. Instead, they can stay active and be engineered to perform with what have been called “MSC 2.0” functions inside the body. [12, 13, 14]

In this setting, MSCs can act like small, living bioreactors. Developers can prime and/or genetically modify an artificial implant to release helpful cytokines, enzymes, peptides, extracellular vesicles (EVs) or exosomes, decoy receptors, antibody fragments, or even whole organelles. Their homeostatic signals can extend to nearby cells or distant tissues. MSCs can also be placed into biomaterial scaffolds that support their survival, protect them from immune attack, and keep them situated. [15] In some cases, they can even be combined with other cell types to form small, functional tissue implants grown outside the body. [16, 17]

Because local MSC therapies enable so many diverse delivery routes and tissue targets, the science becomes more complex. More options mean more questions with new solutions to arrive at empirically.  Whether systemic- or local-injected, the authors of “Fate and Function…” (2025) outline the biggest questions this field still needs to answer.  We summarize some of our favorites in Table 1, below:

Key Question About IV MSCs	Suggested Approach to Answer
How many MSCs survive the immediate immune reaction after touching blood serum (IBMIR), and what features make those survivors different from the ones that die?	Expose a variety of MSCs from different donors & tissues to human blood in the lab; use single-cell tools to isolate living vs. dying cells. From this material, screen phenotypic markers like angiogenic and/or cytokine factor secretion profiles to identify sortable cells that light up with therapeutically modifiable survival traits. Next, identify via “omics” the pathways that can be augmented in accord with better viability.
Can MSCs be engineered or “primed” beforehand to survive IBMIR better or to produce a stronger therapeutic signal?	Pretreat MSCs with cytokines, hypoxia, transgenes, or gene edits (transfected with the aid of RoosterGEM). Then test whether these changes reduce IBMIR or improve potency study results.
Which natural molecules released by MSCs (like EVs, organelles & mitochondria, cytokines, or apoptotic bodies) meaningfully drive the therapeutic effect after systemic infusion?	With the help of upstream and cell downstream processing expertise, isolate the different products MSCs release—EVs, soluble secretome factors, apoptotic bodies—and test each one alone in potency assays to see which parts reproduce the benefits of MSC therapy.

Table 1. “Fate and Function of Exogenously Administered MSCs: Current Insights and Future Directions,” Shokoohmand, et al. (2025) list ~34 questions and suggest experimental approaches to address them. In our own words, we summarize a few key items here (above).

QwA: Questions with Answers

T cell and NK cell therapies became much more controllable once we learned how to upgrade them with CAR constructs. [14, 18, 19] Rx-MSCs will follow a similar path. “Reprogrammed” MSCs delivered by IV could act as short-lived messengers that send stronger signals to lung phagocytes. In turn, their command to the innate immune system would confer longer-lasting repression of autoimmune or age-related inflammation. On the other hand, when placed locally into tissue, engineered MSCs could work more like a small, controlled-release gland. [20] With transcriptionally targeted promoters acting as a rheostat, [21] the cells would release therapeutic factors only when disease signals appear. Local MSCs could also be designed to mature directly into bone, cartilage, fat, or other needed tissues in situ. [22]

“Use the right tools to do the right job”—say nearly everyone’s grandparents. Likewise, therapies from MSC toolkits administered systemically can do certain jobs quite well, like immunomodulation. MSCs injected into local depots are good for other distinct uses. Yet in lieu of niches (e.g., lung, or in situ scaffolds) that can trap whole MSCs, there exist even more unique options. Behold, EVs and/or exosomes. EVs, being ~100-fold smaller in diameter than whole cells, more circulate more broadly and penetrate deeply into target tissues. They are also known to embody a significant portion of MSCs’ therapeutic effect. Whether collected from bioproducer cells ex vivo or secreted continuously from MSCs in vivo, [23] EVs are now justifiably investigated as a drug delivery platform. This is because they can target via displayed ligands as well as encapsulate natural or artificial bioactive cargoes, and also tend to be inert to the immune system.

Regardless of their route of administration (ROA), MSC fate can be readily influenced from inception [24] with tools like RoosterGEM™. [25] Then, MSCs cultured using RoosterBio’s standardized cell banks and media can uniformly expand in flasks or bioreactors. [26, 27, 28] Because of adaptable, fit-for-purpose scale up schemes, novel therapeutic designs become practical for anyone who wishes to begin a cell therapy trial. Rx-MSCs are optimizable based on a set of reliable product (raw material) and process (unit operation) building blocks. With simplified access to clinically-relevant MSCs, key experiments outlined in Fate and Function (2025) will guide new developers on how to build the next generation of MSC therapies.

 

References

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