Mesenchymal stromal cells (MSCs) are defined by the International Society for Stem Cell Research (ISSCR) as fibroblast-like non-hematopoietic cells (Ullah et al., 2015). They are multipotent but are CD40, CD80, and CD86 negative and have low major histocompatibility complex (MHC) I/II values; thus, they are less immunogenic than other stem cells. MSCs are therefore increasingly being utilized for tissue engineering and immunotherapy applications. In particular, MSCs have the potential to modulate inflammatory or immune-activated circumstances by secreting immune modulation factors, nucleic acids or proteins (Li et al., 2019). These immune-modulating factors include indoleamine 2,3-dioxygenase (IDO), heme oxygenase-1 (HO-1), transforming growth factor-β (TGF-β), TNF-α stimulated gene/protein 6 (TSG-6), cyclooxygenease 2 (COX2), prostaglandin E2 (PGE2), hepatocyte growth factor (HepGF), galectins-1 (Gal-1), iNOS, interleukin-6 (IL-6), interleukin-1 receptor antagonist (IL-1Rag), interleukin-10 (IL-10) and human leukocyte antigen-G (HLA-G) (Pittenger et al., 2019). Additionally, secretome from MSCs can regulate the proliferation, differentiation, and activity of most immune cells, including T cells, B cells, natural killer (NK) cells, dendritic cells (DCs), and macrophages (Lee and Song, 2018). MSCs can therefore alleviate inflammatory or immune-related diseases. Emphasis regarding regenerative medicine using MSCs has been shifting toward either producing cytokines or other factors (termed the paracrine effect), rather than the differentiation and rebuilding of damaged tissues using MSCs themselves. The potentiation of the immunomodulatory function of MSCs means that they constitute an emerging and potentially important tool for cell therapy.

To enhance the capacity of the immunomodulatory function of MSCs, multiple bioengineering techniques can be employed during their cultivation. Scaffolds can be utilized during cell culturing to preserve the tissue architecture and provide a three-dimensional (3D) biomimetic milieu to MSCs, similar to a stem cell niche. It has been known that the dimensionality, physical characteristics, topographical cues, surface chemistry of biomaterials, and micro-structure of scaffolds can regulate the immunomodulatory function of MSCs. Here, we review the link between the immunomodulatory function of MSCs and the properties of scaffolds, particularly topographical cues or substrate elasticities of biomaterials.

The ECM consists of networks of fibrous proteins, collagens, and elastic fibers, which are immersed in a viscoelastic gel that is rich in proteoglycans (Mecham, 2011). Thus, diverse synthetic polymers and natural polymers have been used to generate scaffolds with fibrous structures, so as to mimic the natural ECM. Synthetic polymers such as polycaprolactone (PCL), poly(lactic-co-glycolic acid) (PLGA), and polylactic acid (PLA) can be utilized to generate fine-tuned fibrous structures via electrospinning. Su et al. (2017) reported that electrospun PCL scaffolds promoted pro-angiogenic and anti-inflammatory paracrine factors in adipose-derived stem cells [MSCs(A)]. They also further analyzed the effects of aligned, randomized, or meshed patterns fabricated with PCL on MSCs(A), revealing that meshed scaffolds best enhanced their paracrine effects. This finding indicates that the orientations of the fibers and the substrate rigidity might also be important factors for controlling the functions of MSCs. biocompatible material that can be used to fabricate several forms, including fibers, gels, and films (Qi et al., 2017). A recent study demonstrated that the meshed scaffolds comprising silk fibroin nanofibers strongly elevated the levels of immunomodulatory factors such as IDO-1, COX2, and PGE2 in bone marrow-derived stem cells [MSCs(M)] (Kim et al., 2019a). Furthermore, MSCs(M) on mesh form of silk fibroin nanofibers reduced mouse mortality from polymicrobial sepsis through their improved immunomodulatory functions than MSCs(M) only (Kim et al., 2021). ECM like structured silk fibroin seems to provide a favorable environment for MSC cultivation through its structural and material properties. Vallés et al. also demonstrated the effect of topographical cue on immunomodulatory function of MSCs (Vallés et al., 2015). MSCs(M) on 3D polystyrene scaffolds showed smaller cell bodies, but secreted a higher level of soluble factors than MSCs(M) on two-dimensional (2D) polystyrene scaffolds, indicating that topographical cue affects cell function differently. The pore size in hydrogel-based scaffolds also can make a critical difference even in the same hydrogel. For example, sponge or foam porous scaffolds contain connected pore structures that favorably transport gas and nutrients into cells. Hydrogels made of natural materials such as fibrin, alginate, or silk fibroin have been used as scaffolds for cells to promote cell growth and function (He et al., 2020). However, if the pores are too small, the cellular penetration, ECM deposition, or neovascularization can be inhibited (Shruti et al., 2013). Meanwhile, pores that are excessively large can decrease the cell-to-cell contact ratio, as the cells exhibit 2D growth patterns on the substrate rather than adopting a 3D organization (Marrella et al., 2018). The effective pore sizes for cellular function in tissue engineering applications are listed in Table 1. The micro-architecture supporting MSCs seems to be an important cue to manage the paracrine effect of MSCs.

Since the effect of matrix elasticity on the MSC function and fate was demonstrated by Engler et al. (2016) for the first time, many studies have been performed to find the optimal substrate rigidity for MSC cultivation. Hydrogels are favorable for MSC cultivation as they can be tuned regarding to their substrate rigidity. Drzeniek et al. (2021) reported that a microporous 3D hydrogel composed of collagen significantly improved the paracrine effects of MSCs, compared with a 2D collagen-coated polystyrene. They used a customizable collagen I-hyaluronic acid (COL-HA)-based hydrogel to encapsulate MSCs for mimicking stem cell niches. Compared with a 2D surface coated with COL-HA, the 3D hydrogel comprising COL-HA elevated the secretion of angiogenesis-, immunomodulation-, hemostasis-, and ECM remodeling-related cytokines from MSCs. Similarly, Wong et al. (2020) also demonstrated that soft extracellular matrix (~2 kPa) mimicking bone marrow maximized the ability of MSCs(M) to produce paracrine factors and induce chemotaxis upon inflammatory stimulation. In addition to matrix elasticity, composite scaffolds that mimic natural conditions may greatly enhance the paracrine effect of MSCs. In order to maximize the similarity to the natural ECM, decellularized ECM (dECM) from various tissues and organs have been investigated and applied via a number of techniques that utilize chemical, enzymatic, or mechanical disruption. As the ECM is an essential non-cellular component of the tissue microenvironment, decellularized scaffolds composed entirely of ECM can help to reconstruct stem cell niches in vitro. Hydrogel forms of dECM can both affect stem cell differentiation and exert a functional effect, according to the origin of the organ or tissue in question. One disadvantage of ECM-derived hydrogels, however, is their poor self-supporting ability, which arises from their low viscosities and mechanical properties as scaffolds. Due to this characteristic, which hinders the possibility of making large and complex 3D structures with hydrogels (Yi et al., 2019), the combination with other ECM components, such as collagen, can be applied for enhancing the stiffness of hydrogels. There are, however, limitations to the use of dECM in standard clinical treatments, since there is no standard process for the dECM (Kim et al., 2019b) and the current process is unable to completely remove all cell materials in a practical setting (Gilbert et al., 2009). Thus, the general standard guideline for processing dECM will accelerate the clinical trials of MSCs and dECM-derived scaffolds.

Apart from the above mentioned factors, some reports have indicated that the cell source might affect the immunomodulatory function of MSCs, as they can be harvested from various adult tissues as well as neonatal tissues. Although MSCs(M) seem to be designated as the gold standard of MSCs (Hall et al., 2013), MSCs(A) and cord blood-derived MSCs [MSCs(CB)] have also been widely used in clinical studies due to their easy accessibility. Although it is hard to conclude which MSCs are more appropriate to use, it has been reported that human MSCs(A) are more genetically and morphologically stable in long-term culture, display a lower senescence ratio, show a higher proliferative capacity, and retain differentiation potential for a longer period in culture compared with human MSCs(M) (Elman et al., 2014). Further, the yield of MSCs(A) is approximately 500-fold greater than that of MSCs(M) when isolated from an equivalent amount of adipose tissue and bone marrow stroma, respectively (Strioga et al., 2012). Nonetheless, more intensive studies about the cell source are needed for arriving at a conclusion. The functions of the MSCs from different sources are listed in Table 2.

In conclusion, many methods for preparing scaffolds with various materials have been developed. In particular, scaffolds with favorable properties for improving MSC functions are actively being fabricated. Furthermore, enhancing the similarity of these substrates (in terms of their structures and properties) for MSC cultivation can improve both the paracrine effect and the differentiation of stem cells, thus enhancing the feasibility and efficacy of MSC therapies for clinical applications (Fig. 1). Now, it is important to consider how the improved function of MSCs can be maintained sustainably within scaffolds or how long the injected MSCs can survive after in vivo transplantation. Thus, thermo-responsive or injectable hydrogels and adhesion-related ligand-containing hydrogels are being actively developed (Hong et al., 2019). In this regard, more biocompatible scaffolds need to be further developed to take advantage of the paracrine products of MSCs. Such studies could help to promote cell therapy and achieve efficient translational research.