Stimuli-Responsive Substrates to Control the Immunomodulatory Potential of Stromal Cells

Abstract

Mesenchymal stromal cells (MSCs) have broad immunomodulatory properties that range from regulation, proliferation, differentiation, and immune cell activation to secreting bioactive molecules that inhibit inflammation and regulate immune response. These properties provide MSCs with high therapeutic potency that has been shown to be relevant to tissue engineering and regenerative medicine. Hence, researchers have explored diverse strategies to control the immunomodulatory potential of stromal cells using polymeric substrates or scaffolds. These substrates alter the immunomodulatory response of MSCs, especially through biophysical cues such as matrix mechanical properties. To leverage these cell-matrix interactions as a strategy for priming MSCs, emerging studies have explored the use of stimuli-responsive substrates to enhance the therapeutic value of stromal cells. This review highlights how stimuli-responsive materials, including chemo-responsive, microenvironment-responsive, magneto-responsive, mechano-responsive, and photo-responsive substrates, have specifically been used to promote the immunomodulatory potential of stromal cells by controlling their secretory activity.

Figure 1.

Divalent cations can modulate the formation and stiffness properties of hydrogels that control the immunomodulatory potential of stromal cells. (A) Egg-box structure for alginate gelation after adding Ca²⁺ ions. Reproduced or adapted with permission under a Creative Commons CC BY 4.0 from ref . Copyright 2020 μ_BC Part of Springer Nature. (B) Controlling stiffening of an alginate hydrogel with encapsulated MSCs by adding Ca²⁺ ions. (C) Experimental design for examining the effect of stiffness and dynamic environment on MSCs secretory activity. Soft hydrogel condition was designated as Static-Soft, hydrogel stiffened on day 0 was established as Static-Stiff, and conditions at 3 and 5 days were recognized as dynamic. (D) MSCs’ paracrine secretion of VEGF, FGF, HGF, and SDF evaluated in each condition (n = 3; *p < 0.05, **p < 0.01, ***p < 0.001). All data are shown as mean ± SD. Reproduced or adapted with permission from ref . Copyright 2020 Royal Society of Chemistry Biomaterials Science.

Figure 2.

ROS-scavenging injectable hydrogel to raise the therapeutic potential of MSCs in skin flap regeneration. (A) (i) Schematic representation of gelatin and κ-Car hydrogels with encapsulated MSCs and MnO₂ nanoparticles (nGk@MSCs). (ii) nGk@MSCs accelerate mouse skip flap regeneration by promoting angiogenesis, reducing inflammation, and attenuating necrosis. (B) (i) Live/Dead study in hydrogels without nanoparticles (Gk@MSCs) and hydrogels with nanoparticles (nGk@MSCs). (ii) Relative protein expression of cells in nGk@MSCs in an oxidative stress microenvironment. (iii) Relative gene expression of MSCs in nGk@MSCs. Data were presented as mean ± SD. Unpaired t tests were used to compare statistically significant differences between two groups, while one-way analysis of variance was utilized to compare multiple groups. *p < 0.05, **p < 0.01, and ***p < 0.001. Reproduced or adapted with permission from ref . Copyright 2024 American Chemical Society.

Figure 3.

(A) Illustration of pH-driven continuous cell production, (B) SEM pictures of the surface topography and the cell attachment activity on each substrate, including (i) polyamide-66 (PA), (ii) chitosan (CS), (iii) 1:5 PA:CS (PA15CS), (iv) 1:6 PA:CS (PA16CS), and (v) 1:7 PA:CS (PA17CS), and (vi) size measurements of the surface microstructures shown in PA/CS blends. Different letters (a, b, and c) indicated statistically significant differences between any pairs of groups (p < 0.05 was considered significant). (C) Cell proliferation and viability: (i) Dependence of ASC cell number on cycle increment in different blends. Data were calculated from four independent experiments and expressed as mean standard deviation, (ii) Cell viability study: ASCs were plated onto different blends at pH 6.99 for the indicated time duration before the viability study. Data were measured in quadruplicate samples and normalized to the PA15CS groups, and expressed as mean standard deviation of four independent experiments. The asterisks shown in the plots represented significant differences between groups (*p < 0.05). (D) VEGF and HGF expression or secretion: (i) qRT-PCR results of the angiogenesis regulator gene expressions, and (ii) Measurement of the secreted angiogenesis-related growth factors in ASC conditioned medium using ELISA. Data were measured in quadruplicate samples pooled from four independent cultures of each group and expressed as mean standard deviation. The asterisks shown in the plots represented significant differences between groups (*p < 0.05). Reproduced or adapted with permission under a Creative Commons CC BY-NC-ND 4.0 from ref . Copyright 2023 Elsevier.

Figure 4.

Immune-suppressive agarose hydrogels for MSC and Fas ligand delivery into traumatic brain injury regulate expression of inflammatory/anti-inflammatory cytokines. (A) Schematic representation of composite hydrogel containing lipid microtubes embedded in agarose hydrogel to release Fas ligand (FasL) and encapsulated MSC. (B) Protein expression of IL-10, TNF-α, IL-12β, IL-1β, IL-1β RA, and TGF-β. Reproduced or adapted with permission from ref . Copyright 2021 Elsevier.

Figure 5.

Diagram of magnetic field initiating permanent stiffening of a soft hydrogel. Reproduced or adapted with permission from ref . Copyright 2022 Elsevier.

Figure 6.

Pulsed electromagnetic field (PEMF) decreases the pro-inflammatory cytokine secretion and increases anti-inflammatory cytokine secretion of hTDCs. (A) Gene expression of pro-inflammatory cytokines TNFα, IL-6, IL-8, and COX-2. (B) Gene expression of anti-inflammatory cytokines IL-10 and IL-4. (C) Cytokine release of IL-6 and TNFα in cultured medium. Symbols $, #, γ, α, &, and p denote statistical differences * for p < 0.05; ** for p < 0.01; *** for p < 0.001; and **** for p < 0.0001. Reproduced or adapted with permission from ref . Copyright 2022 Elsevier.

Figure 7.

MSC spheroids subjected to greater and longer cyclical compression yielded increased inflammatory cytokines/response and decreased anti-inflammatory cytokines. (A) Different load and load times used to compress alginate hydrogels with MSC spheroids encapsulated. (B) Luminex assay portraying the secretory profile of MSC spheroids with IL-6, IL-8, IL-10, and VEGF represented graphically. p-Values ≤0.05 were considered significant. Bars with different letters are considered significant in comparison to each other. Reproduced or adapted with permission under a Creative Commons CC BY 4.0. Copyright 2022 AIP Publishing APL Bioengineering.

Figure 8.

Photocontrol process of the hydrogels and repair of cartilage defects in an animal model. (A) Experimental setup for exploring the impact of matrix stiffness on MSCs under light irradiation. (B) Photographic representation of articular cartilage defect repairs at 12 weeks. Scale bar: 2.0 mm. (C) Evaluation of GAG contents in the repaired cartilage (n = 6 per group). A p-value of less than 0.05 was considered statistically significant (*p < 0.05, **p < 0.01, and ***p < 0.001). Reproduced or adapted with permission from ref . Copyright 2024 Elsevier.

Figure 9.

Formation of the methacrylated alginate, dopamine-grafted alginate, and polydopamine-functionalized Ti₃C₂ MXene nanosheets (AMAD/MP) hybrid hydrogels. Reproduced or adapted with permission from ref . Copyright 2023 Wiley Online.

Scheme 1.. Stimuli-Responsive Substrates That Have Been Used to Control the Immunomodulatory Potential of Stromal Cells