MSCs secretome contains a variety of biologically active molecules and its composition is influenced by growth conditions and environment that MSCs face (Ahangar et al., 2020). Proteomic analysis of MSCs secretome shows that it mainly contains: 1) growth factors, such as vascular endothelial growth factor (VEGF), fibroblast growth factor 2 (FGF-2), hepatocyte growth factor (HGF), and insulin-like growth factor 1 (IGF-1); 2) immune-modulatory factors, such as complement factor H, IL-10 and indolamine-2,3-dioxygenase (IDO); 3) anti-fibrotic factors, such as milk fat globule-EGF factor 8 (MFGE8), miR-150, miR-21 and miR-29, and 5) Extracellular Vesicles such as exosomes, proteasomes, and free nucleic acids (Kehl et al., 2019). The MSCs derived cell-free secretome appears to be able to recapitulate many of the therapeutic properties that have been described for the MSCs themselves (Ferreira et al., 2018). Recently, the Extracellular Vesicles including the microvesicles (MVs) and exosomes which are loaded with a bunch of therapeutically effective micro-RNAs have gained special attention and validated by many pre-clinical and phase I/II clinical studies (Klyachko et al., 2020) (Fig. 1).

Several studies showed that secretome can modulate and modify vital biological processes such as angiogenesis, neurogenesis, immune-modulation, wound healing, and anti-fibrotic and anti-tumour processes, which are of paramount importance for tissue repair and regeneration (Hassanzadeh et al., 2021). The regenerative and curative potential of MSCs derived secretome has been validated by many disease models (Hassanzadeh et al., 2021). Paik et al. (2020) reported that miR-150 secretome released by miR-150 transfected adipose tissues-derived MSCs (AD-MSCs) abrogated the increase in systemic inflammatory cytokines associated with liver fibrosis, such as IL-6 and TNF-α, and it also induced the upregulation of anti-fibrotic, proliferation, and antioxidant markers in the liver. Mitchell et al showed in their study that the extracellular vesicles and nucleic acids found in the secretome derived from AD-MSCs were able to promote the regeneration of skeletal muscles following acute injury (Mitchell et al., 2019). The immunosuppressive effects of extracellular vesicles and exosomes found in MSCs derived secretome were effective in treating a wide range of autoimmune diseases, including systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA) (Wang et al., 2020b). These microvesicles found in the secretome of MSCs contain different micro-RNAs, including miR-26a, miR-146 and exosomes carrying latency-associated peptide (LAP), TGFβ, and thrombospondin-1 (TSP1). These compounds can suppress inflammation and excessive immune reaction against self-antigens by downregulating the serum levels of TNF-α, IL-1β, IL-6 and other inflammatory cytokines. They also restrain the overstimulated activity of various immune cells including natural killer (NK) and T and B lymphocytes (Asgarpour et al., 2020).

The broad array of paracrine factors that compose the MSCs derived secretome has been corroborated to repair the damaged heart and restore normal heart function (Maghin et al., 2020) Many biological molecules in MSCs secretome are involved in the regeneration of damaged myocardium, including: 1) angiopoietin-1 (AGPT-1), cysteine rich angiogenic inducer (Cyr61), fibroblast growth factors (FGF) 1, 2, 4, 5 and angiogenin, which induce neovascularization and angiogenesis in the damaged myocardium; 2) neurotrophin 4 (NT 4), pentraxin 3 (PTX3 or TSG-14), bone morphogenic protein 7 (BMP 7) and stanniocalcin-1 (STC-1), which attenuate the apoptotic signaling pathway and enhance the activation of survival pathways; 3) secreted frizzled related protein (SFRP) 2 and 4, adrenomedullin (ADM) and hepatocyte growth factor (HGF), which reduce fibrosis and scar size, reducing remodeling; 5) Dickkopf-related proteins (Dkk) and fibroblast growth factors (FGF) 6, 7, which stimulate the JNK signaling resulting in cardiomyogenesis and myogenesis. MSCs derived secretome also contains anti-inflammatory cytokines and immune-modulators, such as IDO, IL-10, interleukin-1 receptor antagonist (IL-1 Ra) and prostaglandin E₂ (PGE₂), which reduce inflammation and immune cell infiltration to the site of myocardial injury (Bagno et al., 2018; Gnecchi et al., 2008; Mirotsou et al., 2011).

The role of MSCs secretome in treating neurodegenerative disorders has been also reported. Different studies described MSCs secretome positive effects in Parkinson’s disease (PD) by increasing viability of dopaminergic cells through the effect of PGE₂. MSCs secretome preserves TH+ neurons by increasing brain derived neurotrophic factor (BDNF) and neurotrophin-3 expression (d’Angelo et al., 2020). These neurotrophic factors are crucial for neuronal survival in the substantia nigra and promote the differentiation of dopaminergic neurons (Baquet et al., 2005). Furthermore, MSCs-derived secretome was effective in reducing α-synuclein, nestin, and glial fibrillary acidic protein in the substantia nigra and striatum, which attenuated neuro-inflammation and excessive damage to the dopaminergic cells to restore motor and cognitive performance in PD patients (d’Angelo et al., 2020). MSCs-derived secretome possesses the ability to stimulate neurotrophic substances such as BDNF, which is a biomarker for many neurodegenerative disorders, and promote neuronal survival pathways to counterpoise neuronal death. This makes MSCs-derived secretome a potential cure for other neurodegenerative conditions, including Alzheimer’s disease and stroke (Wang et al., 2020a). Many studies have emphasized on the therapeutic potential of MSCs-derived secretome in inducing massive recovery following cerebral ischemia (Cunningham et al., 2018). The cytokines and growth factors cocktail presented in the MSCs secretome, including TGF-β, VEGF, IL-10, PDGF-AA, CXCL16 and BDNF found to inhibit neuro-inflammation, promote angiogenesis, decrease cerebral infarction and edema and inhibit the recruitment of microglial cells which all are crucial to potentiate the functional recovery (Cunningham et al., 2018). Spinal cord injury is another excruciating disorder which can benefit from MSCS-derived secretome infusion. Vawda et al found that the administration of HUC-MSCs-derived secretome 48 h after SCI injury can reduce the lesion volume and improve the vascularity mediated by modulating the JAK/STAT and MAPK/ERK signaling pathways, and inhibit immune cell recruitment (Vawda et al., 2020). Fernandes-Cunha et al interestingly showed that delivering a lyophilized MSCs-secretome carried on viscoelastic gel composed of hyaluronic acid (HA) and chondroitin sulfate (CS) can restore the vision and promote epithelial cells proliferation which significantly reduced the scar size and hemorrhage after alkaline corneal burns (Fernandes-Cunha et al., 2019). The broad range of translational applications of MSCs-derived secretome needs further investigations to explore new therapeutic mechanisms and outcomes that can be applied to other disease models (Khatab et al., 2018; Ogata et al., 2018). A summary of the therapeutic applications of MSCs secretome is shown in Fig. 2.

There are some concerns related to the use of MSCs secretome in therapeutic settings. One of the major considerations is the instability and short half-life of secretome proteins and micro-RNAs (Ahangar et al., 2020). Moreover, the number of MSCs that are required to produce sufficient quantities of secretome to exert equivalent regenerative effect to MSCs transplantation is about 10–25 times higher than directly administered MSCs (Ahangar et al., 2020). The heterogeneity of MSCs secretome composition is another part of the challenges regarding the clinical use of secretome (Ahangar et al., 2020; Teixeira and Salgado, 2020). The composition profile of MSCs secretome is highly influenced by the donor characteristics, the source of MSCs being isolated, and the culture and environmental conditions presented during MSCs growth and expansion (Teixeira and Salgado, 2020). Based on that, it is mandatory to standardize and optimize the secretome isolation protocol and characterize its composition profile for specific clinical applications. Moreover, there are assumed risk factors that can be associated with the use of MSCs secretome, such as diminished immune system functions following secretome administration, which may increase the risk of infection, immunodeficiency and tumor growth in treated patients (Vizoso et al., 2017). Many studies still cannot provide an updated guideline for large-scale manufacturing and production of MSCs-derived secretome and its subcomponents which is needed for clinical applications (Maumus et al., 2020).

Despite these challenges, cell-free secretome offers considerable advantages over conventional cellular therapy in terms of safety, stability and ease of handling, manipulation and storage. Further studies are recommended to discover new therapeutic applications of MSCs secretome and to find feasible approaches to increase the half-life of its micro-RNAs, proteins and growth factors, which can sustain and control post-infusion therapeutic effects. A graphical summary of the challenges associated with MSCs secretome’ therapeutic applications can be found in Fig. 3.

Future studies should focus on using various strategies to modify the MSCs-derived secretome composition based on understanding the pathophysiological characteristics of the targeted disease which allows preparing more specific MSCs secretome that it is therapeutically effective for particular disorder. The possible approaches to modify MSCs-derived secretome have been highlighted in Fig. 4.