For decades, pediatric acute graft-versus-host disease (aGvHD) that fails steroids has carried a dismal prognosis. Standard second-line therapy with ruxolitinib helps some children, but roughly 30-40% remain refractory, facing organ failure and death within months. The December 2024 FDA approval of remestemcel-L (Ryoncil) changed the therapeutic landscape for these patients — marking the first mesenchymal stromal cell (MSC) product ever approved in the United States. Now, 2025-2026 data from real-world registries, new mechanistic insights, and next-generation MSC platforms are reshaping how clinicians think about cell therapy in transplant-related complications.

Why Pediatric aGvHD Needs More Than Steroids and Ruxolitinib

Acute GvHD occurs in up to 70% of children after allogeneic hematopoietic stem cell transplantation (allo-HSCT). First-line treatment — high-dose corticosteroids — achieves complete or partial response in roughly 50-60% of patients. Those who fail enter a steep mortality curve. Ruxolitinib, a JAK1/2 inhibitor approved for steroid-refractory aGvHD, produces overall response rates around 30-65% depending on the grade and organ involvement. But children who fail both steroids and ruxolitinib have almost no approved options left. Their six-month survival historically sits below 20%.

This is where MSCs enter the picture. Unlike conventional immunosuppressants that block a single cytokine pathway, MSCs secrete a broad cocktail of anti-inflammatory mediators — prostaglandin E2, indoleamine 2,3-dioxygenase (IDO), TGF-beta, and hepatocyte growth factor — that dampen T-cell activation, expand regulatory T cells, and promote tissue repair simultaneously. The challenge has been converting that biological rationale into consistent clinical efficacy at scale.

Ryoncil (Remestemcel-L): The Landmark FDA Approval

After years of failed Phase III trials in adult GvHD and Crohn’s disease, the International Society for Cell and Gene Therapy (ISCT) MSC committee described Ryoncil’s approval as ending “a long-lasting drought on approved MSC clinical products in the United States” (Blanc et al., Cytotherapy, 2025; DOI: 10.1016/j.jcyt.2025.01.005). Ryoncil is an allogeneic bone marrow-derived MSC product manufactured by Mesoblast, administered intravenously at 2 million cells/kg twice weekly for four weeks.

The pivotal Phase III trial enrolled 54 children with steroid-refractory aGvHD grades B-D. Day-28 overall response rate was 70%, with 29% achieving complete response. Importantly, the response was durable — at day 100, survival was 74% in responders versus 16% in non-responders. The product’s safety profile was clean, with no treatment-related serious adverse events or dose-limiting toxicities reported.

What made this approval possible, where previous MSC products had failed, was the iterative engagement between Mesoblast and the FDA on potency assays. The agency required demonstration that each batch consistently suppressed T-cell proliferation and secreted key anti-inflammatory cytokines. This rigor set a new standard for MSC manufacturing.

Tomostrocel: Real-World Data From 139 Heavily Pretreated Children

While Ryoncil was gaining FDA attention, a European MSC product called tomostrocel was accumulating real-world evidence in some of the hardest-to-treat pediatric GvHD cases. Vogelsang et al. published a multi-center retrospective analysis of 140 treatment episodes in 139 pediatric patients with steroid- or treatment-refractory aGvHD across 10 transplant centers in Germany, France, Hungary, and the Netherlands (Haematologica, 2026; DOI: 10.3324/haematol.2026.300837).

These were not treatment-naive patients. The median number of prior therapies was 3.5, and 76 of the 139 patients had already failed ruxolitinib. Over two-thirds presented with grade III/IV aGvHD, most commonly involving the lower gastrointestinal tract. The results:

• Day-28 overall response: 62.9% across all grades (74.1% for grade II, 75.0% for grade III, 53.2% for grade IV)
• Organ-specific improvement: skin 69.4%, liver 57.5%, lower GI 58.6%, upper GI 39.5%
• Ruxolitinib pre-treatment did not blunt the response — OR rates were similar with and without prior ruxolitinib
• 6-month overall survival: 64.1%, with younger age, lower grade, and day-28 response as favorable prognostic factors
• Safety profile consistent with prior MSC reports — no new signals in this heavily pretreated cohort

That 6-month survival of 64% in children who had failed a median of 3.5 prior lines is remarkable. Historical controls in ruxolitinib-refractory aGvHD report survival rates below 20% at the same timepoint.

Case Evidence: Grade IV GI GvHD Responding to MSC-FFM

Ernst et al. reported the case of a 12-year-old boy with high-risk acute myeloid leukemia who developed grade IV intestinal aGvHD after allo-HSCT (Annals of Hematology, 2025; DOI: 10.1007/s00277-025-06491-y). His condition failed triple immunosuppression with corticosteroids, cyclosporine A, and ruxolitinib. Four weekly infusions of allogeneic MSCs from the Frankfurt blood service (MSC-FFM) finally brought the intestinal inflammation under control. Serial ultrasound showed normalized bowel wall thickness, fecal calprotectin levels dropped, and the child tolerated modular enteral nutrition.

This case illustrates two practical points: (1) MSCs can work even after exhausting conventional immunosuppression chains, and (2) non-invasive biomarkers like intestinal wall thickness on ultrasound and fecal calprotectin can track treatment response without repeated endoscopies.

Understanding the Immunomodulatory Mechanism in GvHD

MSCs suppress GvHD through four overlapping mechanisms: (1) T-cell suppression via IDO-mediated tryptophan catabolism and PGE2-driven inhibition of Th1/Th17 polarization; (2) Treg expansion through TGF-beta and Notch signaling; (3) macrophage reprogramming from pro-inflammatory M1 to tissue-repairing M2 phenotype; and (4) paracrine tissue repair via VEGF, HGF, and KGF secretion that regenerates the GI mucosal barrier. This polypharmacology explains why MSCs can work even in patients who have failed JAK inhibitors targeting only one pathway.

One of the most important insights from the Shafqat et al. review in Cytotherapy (2026; DOI: 10.1016/j.jcyt.2026.102886) is the role of patient heterogeneity in MSC trial failures. Not all aGvHD patients have the same biology. Those with predominantly inflammatory/immune-activation profiles (high TNF-alpha, IL-6, ST2, REG3alpha) appear to respond better to MSCs than those with fibrotic or apoptotic-dominant disease. Shafqat et al. argue that future MSC trials must stratify patients using biomarker panels at baseline, rather than treating all steroid-refractory aGvHD as a single entity. The Mount Sinai Acute GvHD International Consortium (MAGIC) algorithm, which uses day-28 ST2 and REG3alpha serum levels to predict transplant outcomes, could serve as a stratification tool.

Understanding the Immunomodulatory Mechanism in GvHD

MSCs suppress GvHD through four overlapping mechanisms: (1) T-cell suppression via IDO-mediated tryptophan catabolism and PGE2-driven inhibition of Th1/Th17 polarization; (2) Treg expansion through TGF-beta and Notch signaling; (3) macrophage reprogramming from pro-inflammatory M1 to tissue-repairing M2 phenotype; and (4) paracrine tissue repair via VEGF, HGF, and KGF secretion that regenerates the GI mucosal barrier. This polypharmacology explains why MSCs can work even in patients who have failed JAK inhibitors targeting only one pathway.

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MSCs suppress GvHD through four overlapping mechanisms: (1) T-cell suppression via IDO-mediated tryptophan catabolism and PGE2-driven inhibition of Th1/Th17 polarization; (2) Treg expansion through TGF-beta and Notch signaling; (3) macrophage reprogramming from pro-inflammatory M1 to tissue-repairing M2 phenotype; and (4) paracrine tissue repair via VEGF, HGF, and KGF secretion that regenerates the GI mucosal barrier. This polypharmacology explains why MSCs can work even in patients who have failed JAK inhibitors targeting only one pathway.

The therapeutic algorithm for pediatric steroid-refractory aGvHD is shifting. After the December 2024 Ryoncil approval, clinicians in the US now have an FDA-approved MSC option before exhausting all conventional therapies. European centers with access to tomostrocel have been using MSCs as a bridge for ruxolitinib-refractory patients, with real-world data showing meaningful response rates even in the most heavily pretreated children.

The key clinical takeaways from the 2025-2026 literature are straightforward:

• MSCs produce 60-63% overall response rates in pediatric aGvHD patients who failed a median of 3.5 prior therapies, including ruxolitinib
• Skin and lower GI organs respond better than the liver or upper GI tract
• Prior ruxolitinib does not preclude MSC response — these therapies appear to work through complementary mechanisms
• Biomarker-guided patient stratification (ST2, REG3alpha, TNF-alpha) will likely improve response rates in future trials by selecting patients with inflammatory-dominant disease
• Next-generation MSC platforms — iPSC-derived MSCs, CAR-MSCs, EVs — are in early trials and may address batch variability and short engraftment persistence

FAQ

Is Ryoncil (remestemcel-L) approved for adults with GvHD?

Currently, Ryoncil is approved only for pediatric (age 2 months and older) steroid-refractory acute GvHD. A Phase III trial in adults (NCT07568535) is planned but not yet recruiting. Off-label use in adults is at the discretion of the treating physician but is not FDA-cleared.

How is Ryoncil different from tomostrocel?

Both are allogeneic MSC products, but they differ in donor source and manufacturing. Ryoncil uses bone marrow-derived MSCs from a single young donor expanded through proprietary processes. Tomostrocel uses random-donor bone marrow-derived MSCs pooled from multiple donors. Ryoncil is FDA-approved; tomostrocel is available in several European countries under hospital exemption or compassionate use pathways.

Can MSCs be used after ruxolitinib failure?

Yes. The Vogelsang et al. real-world study showed that 76 of 139 patients had failed ruxolitinib before receiving tomostrocel, and response rates were similar regardless of prior ruxolitinib use. The ISCT and European Group for Blood and Marrow Transplantation (EBMT) both recognize MSCs as a therapeutic option after ruxolitinib failure.

What are the most common side effects of MSC therapy in GvHD?

Across clinical trials and real-world registries, MSC infusions in pediatric GvHD have shown a favorable safety profile. Reported adverse events include transient fever during infusion, mild hypotension, and occasional headache. No treatment-related serious adverse events, dose-limiting toxicities, or signals of ectopic tissue formation have been observed in the GvHD setting.

Will MSC-derived extracellular vesicles replace live cell therapy for GvHD?

MSC-EVs are a promising cell-free alternative that carries the immunomodulatory cargo without the risks of live cell administration. However, they remain in early clinical development. The Ryoncil approval provides a regulatory and validation pathway for EV developers, but no EV product for GvHD has reached Phase III yet. Standardization of EV isolation, dosing, and potency assays is still an active area of work.

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References

• Blanc KL, Dazzi F, English K, et al. ISCT MSC committee statement on the US FDA approval of allogenic bone-marrow mesenchymal stromal cells. Cytotherapy. 2025;27(4):413-416. DOI: 10.1016/j.jcyt.2025.01.005
• Vogelsang K, Zirngibl F, Mezger M, et al. Real-world outcomes of the mesenchymal stromal cell product tomostrocel in heavily pretreated pediatric patients with refractory acute graft-versus-host disease. Haematologica. 2026. DOI: 10.3324/haematol.2026.300837
• Shafqat A, Kadri N, Zubair A, Hashmi SK. The future of clinical trials of mesenchymal stromal cells in acute graft-versus-host-disease. Cytotherapy. 2026;28(10):102886. DOI: 10.1016/j.jcyt.2026.102886
• Ernst J, Reinsch S, Mentzel HJ, Milde T, Gruhn B. Successful treatment with mesenchymal stromal cells-Frankfurt am Main in a pediatric patient with steroid-refractory and ruxolitinib-refractory acute gastrointestinal graft-versus-host disease. Ann Hematol. 2025;104(8):4237-4243. DOI: 10.1007/s00277-025-06491-y
• Giebel B. A milestone for the therapeutic EV field: FDA approves Ryoncil, an allogeneic bone marrow-derived mesenchymal stromal cell therapy. Extracell Vesicles Circ Nucl Acids. 2025;6(1):183-190. DOI: 10.20517/evcna.2025.02