Hematopoietic stem-cell transplantation (HSCT) is an established therapeutic platform for selected malignant and nonmalignant hematologic diseases, but hematopoietic stem cells and mesenchymal stromal/stem cells (MSCs) are biologically distinct populations. MSCs have been investigated as supportive or immunomodulatory adjuncts around HSCT rather than as substitutes for hematopoietic grafts. Proposed roles include support of marrow stromal function, modulation of alloreactive immunity, and treatment of transplant complications. Clinical interpretation must separate established HSCT efficacy from investigational MSC or extracellular-vesicle interventions. Outcomes depend on donor type, conditioning, graft source, HLA matching, disease, infection risk, immunosuppression, and transplant-center practice.
Figure 1 placeholder — Hematopoietic transplantation
Marrow failure
Marrow failure includes heterogeneous disorders in which production of one or more blood-cell lineages is inadequate. Causes include acquired aplastic anemia, inherited marrow-failure syndromes, toxic or treatment-related injury, immune-mediated destruction, clonal disorders, and other conditions. MSCs are components of the marrow microenvironment and can influence hematopoietic-cell support experimentally, creating a biological rationale for investigation. That rationale does not establish that exogenous MSCs restore durable hematopoiesis. Standard management is disease-specific and may include supportive transfusion care, immunosuppressive therapy, hematopoietic transplantation, growth factors in selected settings, or targeted therapies. Investigational cell or EV approaches must be evaluated against those established options.
Figure 2 placeholder — Marrow failure
Hemoglobinopathies
Hemoglobinopathies such as sickle cell disease and beta-thalassemia are genetic disorders of hemoglobin production or structure. Curative-intent approaches target the hematopoietic system through allogeneic HSCT or, for selected patients and jurisdictions, gene-based autologous hematopoietic-cell therapies. MSCs do not correct the pathogenic globin gene in a patient's hematopoietic stem cells. MSC or EV research may explore inflammatory, vascular, marrow-niche, or tissue-injury pathways, but these supportive mechanisms are fundamentally different from correcting or replacing the disease-causing hematopoietic lineage. Claims should therefore distinguish disease modification, supportive tissue effects, and curative-intent hematopoietic or gene therapy.
Figure 3 placeholder — Hemoglobinopathies
Graft-versus-host disease
Graft-versus-host disease (GVHD) occurs when donor-derived immune cells recognize recipient tissues after allogeneic transplantation. Acute and chronic GVHD differ in timing, organs involved, immunobiology, and treatment. MSCs have been investigated extensively for steroid-refractory or steroid-resistant GVHD because of their immunomodulatory properties. Clinical results have varied across products, studies, age groups, endpoints, and jurisdictions, illustrating the importance of product identity and trial design. Some MSC products have received jurisdiction-specific regulatory authorizations for defined GVHD settings, but regulatory status must be reported separately from the strength and generalizability of clinical evidence. Standard immunosuppressive and targeted therapies remain central to care.
Figure 4 placeholder — Graft-versus-host disease
Immune reconstitution
Immune reconstitution is the recovery of innate and adaptive immune function after transplantation or intensive hematologic therapy. Neutrophils, monocytes, natural killer cells, B cells, and T-cell subsets recover on different timelines, and functional recovery can lag behind cell counts. Conditioning intensity, graft source, donor characteristics, GVHD, immunosuppression, infections, age, and thymic function all influence reconstitution. MSC-associated immunomodulation could theoretically alter this process, but beneficial control of alloreactivity must be balanced against antimicrobial and antitumor immunity. Measurements of lymphocyte subsets or cytokines are mechanistic biomarkers; clinically meaningful assessment also requires infection outcomes, vaccine responses, relapse, GVHD, survival, and functional immune competence.
Figure 5 placeholder — Immune reconstitution
Clinical takeaway
Hematology contains both established stem-cell medicine and investigational regenerative-cell approaches. Hematopoietic transplantation is established for defined indications; MSC and EV interventions must not be conflated with hematopoietic stem-cell replacement.
Biological rationale
The marrow niche contains stromal, endothelial, immune, neural, and extracellular-matrix components that regulate hematopoiesis. MSC biology is relevant to this environment, but ex-vivo MSC products are not equivalent to the native marrow niche.
Human clinical evidence
The strongest clinical MSC experience in hematology has centered on transplant complications, particularly GVHD. Evidence for marrow-failure restoration, hemoglobinopathy correction, or generalized immune reconstitution is substantially less established and is product- and indication-dependent.
Trial design and limitations
Hematology trials can be confounded by concurrent immunosuppression, conditioning regimens, donor selection, infection prophylaxis, transfusions, graft source, disease severity, and rapidly evolving standard care. Controlled designs and prespecified endpoints are essential.
Manufacturing and product identity
MSC source, donor, expansion, passage, culture conditions, cryopreservation, viability, potency assays, dose, and route can affect biological activity. Results from one product should not be generalized automatically to another.
Dose and administration
MSC studies in hematology have used heterogeneous dosing schedules and repeated administrations. No universal MSC or EV dose exists across hematologic indications, and dose should be interpreted within the exact product and trial.
Safety and contraindications
Potential concerns include infusion reactions, infection in immunocompromised patients, thrombo-inflammatory interactions, alloimmunization questions, and risks specific to the underlying hematologic disease and transplant setting. Safety evaluation must account for concomitant therapy.
Regulatory status
Regulatory status is jurisdiction- and product-specific. Authorization of a defined cell product for a defined indication does not establish class-wide approval for MSCs or EVs and does not extend to unrelated hematologic diseases.
Standard-care alternatives
Standard hematologic care may include HSCT, immunosuppression, targeted drugs, transfusion support, antimicrobial prophylaxis, gene therapy, or other disease-specific treatment. Investigational regenerative interventions should be compared with—not substituted rhetorically for—appropriate standard care.
Knowledge gaps
Key gaps include validated potency markers, optimal dose and timing, patient selection, comparative effectiveness, long-term safety, interactions with infection and relapse risk, and whether cell-free products can reproduce clinically meaningful effects.
Bottom-line evidence statement
Hematology demonstrates why precise terminology matters: hematopoietic stem-cell transplantation can be established therapy while MSC and EV applications remain investigational, product-specific, and indication-specific.
Additional controlled terminology
A hematopoietic stem cell is distinct from a mesenchymal stromal cell. Allogeneic transplantation uses donor cells, whereas autologous transplantation returns a patient's own collected cells. A conditioning regimen and HLA matching influence transplant outcomes. The marrow niche supports hematopoiesis. Aplastic anemia is one form of marrow failure. Sickle cell disease and beta-thalassemia are hemoglobinopathies for which selected curative-intent approaches include transplantation or gene therapy. Alloreactivity contributes to acute GVHD and chronic GVHD; steroid-refractory GVHD remains a major treatment challenge requiring immunosuppression and other therapies. Neutrophil engraftment, lymphocyte recovery, natural killer cell recovery, T-cell reconstitution, and B-cell reconstitution are different dimensions of immune recovery. Infection risk, relapse risk, transfusion support, and functional immune competence are clinically important outcomes.