The immunosuppressive properties of MSCs were found not to be constitutively manifested, but to be stimulated by paracrine mediators typical of the inflammatory microenvironment, which became the basis for the development of in vitro preconditioning protocols to increase the MSC immunomodulatory potential by “priming” with inflammatory cytokines TNFa, IFNg, IL-1α, IL-1β, etc. (Gornostaeva et al., 2016; de Cássia Noronha et al., 2019; Micheli et al., 2021). Both in vitro and in vivo experimental models have demonstrated the enhancement of MSC immunosuppression after priming (Bartholomew et al., 2002; Crop et al., 2010; Wobma et al., 2018; Ragni et al., 2020).

In addition to immunosuppression, the ability of MSCs to stimulate the activity of endogenous cell sources in situ due to the secretion of growth factors is in high demand. Therefore, the increased levels of these mediators in inflammatory-activated MSCs would be a very significant bonus. However, mainly immunosuppression-related outcomes are usually evaluated after priming of MSCs with IFN-γ, TNF-α, and other inflammatory cytokines. Commonly, IDO, PGE2, TFG-β production and antiproliferative effects of above are measured (Noone et al., 2013; Guan et al., 2017; Redondo-Castro et al., 2017; Wobma et al., 2018).

We believe that such a narrowed view on the inflammatory priming results, in particular the underestimation of growth factor production, is not entirely justified. It is assumed that resident MSCs are both involved in reparation processes and inhibit tissue inflammatory responses (Nimiritsky et al., 2019; Girousse et al., 2021). From an applied point, the administration of MSCs in damage tissue should not only help suppress the excessive immune response, but also activate endogenous cell populations. It is reasonable that the inflammatory microenvironment should stimulate not only the immunomodulatory, but also the reparative MSC properties. Very few studies are available in this area. Screening the Pubmed database for the 2006-2021 period using the “priming”, “MSC”, and “growth factors” keywords, we found ten publications that had evaluated the production of growth factors after an inflammatory stimulation of MSCs (Table 1).

MSCs treated with IFN-γ, TNF-α, IL-1α, IL-1β, and IL-8 alone or in combination were found to increase the production of growth factors such as G-CSF, BMP-2, VEGF, FGF2, HGF, IGF-1, NGF, and GDNF (Wang et al., 2008; Hess et al., 2009; Liu et al., 2011; Lu et al., 2013; Hou et al., 2014; Li et al., 2016; Gray et al., 2016; Redondo-Castro et al., 2017). Also, the genes encoded growth factors were found to be upregulated when MSCs were primed with conditioned media from LPS- and IFN-γ-polarized macrophages (Maldonado-Lasunción et al., 2021). Meanwhile, Ragni et al. (2020) found a downregulation of ANG, HGF, TGFB1, and FGF7 after IFN-γ priming, and Redondo-Castro et al. (2017) did not detect any changes in the levels of VEGF, BDNF, and NGF in MSC stimulated with IL-1α, IL-1β, TNF-α, or IFN-γ). In above studies, the cytokines, concentrations and time intervals for MSC priming differed significantly, which could result in data heterogeneity. Despite the fewness and some inconsistency of data, it can be concluded that production of growth factors in MSCs may be stimulated with inflammatory priming, while the selection of optimal conditions is still to be done.

We have supposed that the stimulation of growth factor secretion would be more stable and pronounced in case of priming with inflammatory cocktail produced by activated immune cells directly interacted with MSCs.

Recently, we analyzed the growth factor levels in MSC monoculture and in coculture with PHA-stimulated allogeneic T cells (Fig. 1). A number of growth factors including epidermal growth factor (EGF), platelet-derived growth factor-AA and AB/BB (PDGF-AA, PDGF-AB/BB), fibroblast growth factor-2 (FGF-2), transforming growth factor-α (TGF-α), and vascular endothelial growth factor (VEGF) were identified in the conditioned medium from monocultured MSCs. These mediators play an important role in the implementation of MSC reparative potential. TGF-α treatment was shown to enhance MSC-mediated cardioprotection (Herrmann et al., 2010). MSC preconditioning with FGF-2 enhanced vascularization in vivo (Gorin et al., 2016). PDGF-AB/BB was found to be involved in resident cardiac MSC migration to the damage site (Windmolders et al., 2014). bEGF, VEGF and FGF accelerated wound closure, promoted angiogenesis, reepithelization and collagen deposition (Padeta et al., 2017; Tarcisia et al., 2018). TGFβ-1, VEGF, FGF, HGF, and PDGF reduced transepidermal water loss and accelerated healing (Zhou et al., 2013). These factors can also interact with one another, mutually enhancing their effects; e.g., FGF-2 increased HGF and VEGF secretion (Gorin et al., 2016) and TGF-α caused an increased production of VEGF in MSCs via MEK and PI3-K-dependent mechanisms (Wang et al., 2008).

Interaction with activated T cells, was followed by a significant increase in the levels of EGF, PDGF-AB/BB, FGF-2, and VEGF by MSCs (p < 0.05). TGF-a tended to increase and PDGF-AA production remained unchanged (Fig. 1). Therefore, we found that interaction with immune cells resulted in an increased production of a number of growth factors in MSCs, and hence contributing to the increase of reparative potential.

Thereby, at the inflammatory microenvironment created by activated immune cells, not only MSC immunosuppressive properties are enhanced, but their regenerative properties as well. The above phenomenon is in demand for the practical use of MSCs and requires a comprehensive study. The development of the MSC priming approach for the induction of both immunomodulatory and growth factors production will make it possible not only to optimally “tune” the MSC potential for therapeutic purposes, but also to obtain cell-free products for clinical use.