Shattering barriers toward clinically meaningful MSC therapies

Abstract

More than 1050 clinical trials are registered at FDA.gov that explore multipotent mesenchymal stromal cells (MSCs) for nearly every clinical application imaginable, including neurodegenerative and cardiac disorders, perianal fistulas, graft-versus-host disease, COVID-19, and cancer. Several companies have or are in the process of commercializing MSC-based therapies. However, most of the clinical-stage MSC therapies have been unable to meet primary efficacy end points. The innate therapeutic functions of MSCs administered to humans are not as robust as demonstrated in preclinical studies, and in general, the translation of cell-based therapy is impaired by a myriad of steps that introduce heterogeneity. In this review, we discuss the major clinical challenges with MSC therapies, the details of these challenges, and the potential bioengineering approaches that leverage the unique biology of MSCs to overcome the challenges and achieve more potent and versatile therapies.

Fig. 1. Major factors affecting the heterogeneity and ultimately the clinical outcome of MSCs.

(A) Outlines the major variables associated with preparation of the MSC product. Donor variations such as the health status, genetics, gender, and age can affect the potency of MSCs (2). MSCs can be harvested from multiple different sources, such as bone marrow, AT, and UC, causing additional variations in potency (20). Furthermore, different methods of isolating cells (needle versus biopsy) from these tissues and obtaining cells (enzymatic dissociation versus mechanical dissociation) can affect the potency of MSCs (28). The culture conditions, including the medium composition, oxygen levels, confluence, culture surface, flasks/bioreactors, passage number, and cell surface modification, are also reported to affect potency/homing (26, 29). Last, cryopreservation and thaw/culture rescue protocols can affect the viability, function, and homing of MSCs (50, 52, 53). (B) Outlines the major variables associated with the administration of MSCs that can affect the therapeutic outcome. The administration route (local/systemic), injection site (dense/nondense tissue), injection device properties (needle size/geometry), injection/infusion buffer, and cell carrier materials can affect the residence time, viability, and homing of MSCs (81, 84). (C) Outlines the major factors associated with the MSC recipients that can affect the therapeutic outcome. Host cytotoxic responses against MSCs are shown to have strong correlations with the therapeutic outcome (58). The therapeutic outcome is also dependent on the host disease/severity, which can result in highly variable microenvironmental factors (inflammation status, hypoxia, and ECM) that shape the function of MSCs (151).

Fig. 2. Bioengineering solutions to boost the functions of MSCs.

(A) Priming MSCs with small molecules is a simple and promising approach to induce the secretion of immunomodulatory and regenerative molecules, but the effect of small molecules only lasts a few hours to a few days. (B) MSCs can also be engineered with drug-loaded particles. These particles are intracellularly loaded into MSCs to sustain their immunosuppressive profile for an extended period of time, regardless of the source of MSCs, but particle preparation can increase the cost and complexity when compared to the use of free small molecules. (C) MSCs can be genetically engineered to overexpress a variety of different therapeutic molecules, including anti-inflammatory cytokines and growth factors, either to boost their innate functions or to overexpress other therapeutics and broaden their application to other diseases such as cancer. Viral vector–based genetic engineering typically has more efficient and durable gene expression but has some safety concerns because genes are integrated into the target cell genome. Nonviral vectors are safer, but the transfection efficiency is typically lower and gene expression is less durable. (D) OVs have also been used to engineer MSCs. MSCs function by shielding viruses to avoid immunogenicity and by releasing the virus in tumor tissue to kill tumor cells. One limitation is that regular OVs have only moderate infectivity, although this can be overcome by using certain viral variants with higher infectious capacity.

Fig. 3. Bioengineering solutions for improving administration of MSCs.

(A) Priming MSCs with hypoxia, inflammatory cytokines, and small molecules have been shown to improve the survival of MSCs, but the effect of priming may not be preserved upon cryopreservation/thawing. (B) Hydrogel is one of the most common biomaterials used to encapsulate MSCs and enhance their survival to several weeks following local administration, but the bulk size of hydrogel is only suitable for local administration, not for systemic administration. (C) Microgels containing one or several MSCs is another bioengineering solution to enhance the residence time and survival of MSCs. Unlike bulk hydrogel, which is only suitable for local administration, microgel can be suitable for both local and systemic injections. One potential limitation is that the physical barrier of the microgel may mask the receptors on MSCs that are important for their homing to diseased sites, although this may be addressed by using additional homing ligands on the microgel. (D) To improve the homing of MSCs to the target sites, the surface of MSCs can be modified with different homing ligands. This can be achieved through genetic engineering, antibody conjugation, or polymer coating of MSCs, but more work is required to achieve a critical mass of MSCs at the target site that can predictably modulate the biological signaling pathways. (E) MSCs can be engineered with intracellular iron oxide to efficiently direct MSCs to reach the target sites under guidance by an external magnetic field. Iron oxide also makes it possible to monitor the biodistribution of MSCs using magnetic resonance imaging, but more work is needed to understand whether the properties of iron oxide–engineered MSCs can be maintained during cryopreservation/thawing, which can cause leakage of iron oxide from MSCs.

Fig. 4. Solutions relevant to host factors.

(A) Patient stratification based on the host cytotoxic responses against MSCs or the disease severity/stage can be used to recruit patients who can benefit from MSC therapies. (B) Priming the host with vitamin C can scavenge free radicals that compromise the potency of MSCs. Furthermore, priming the host with a vasodilator or irradiation can facilitate the homing of MSCs to the target sites, resulting in better therapeutic outcomes. (C) The identified host factors that affect the function of MSCs can guide the development of better MSCs, which can complement the host priming strategy and ultimately improve the therapeutic outcome. In particular, MSCs can be engineered to improve the homing to the target sites, engineered to maximum potency, or programmed to function regardless of the host environment. However, it is not clear yet if homing can be engineered to achieve a meaningful boost in efficacy in humans.