Abstract
Mesenchymal stem cells (MSCs) have emerged as a promising therapeutic tool in stem cell-based therapy due to their immunomodulatory or regenerative characteristics. Nowadays, controlled application of nonpathogenic bacterial cells and their derivatives has shown promise in preconditioning and manipulating MSC behavior. This approach is being explored in various fields, including immunotherapy, tissue engineering, and cell therapy. However, recent discoveries have elucidated the complex interactions between MSCs and microorganisms, especially bacteria and viruses, raising concerns regarding the utility of MSCs in clinical applications. In this review, we discussed the interactions between MSCs and microorganisms and highlighted both positive and negative aspects. We also examined the use of bacterial-derived compounds in MSCs-mediated interventions. The balanced colonization of the microbiome in organs, such as the oral cavity, not only does not hinder therapeutic interventions but also could be crucial for achieving desirable outcomes. On the contrary, disturbances in the microbiome have been found to disturb the biological potential of MSCs, such as migration, osteogenic differentiation, and cell proliferation. Evidence also suggests that commensal bacteria, following certain interventions, can transition to a pathogenic state when interacting with MSCs, leading to acute inflammation. Indeed, the maintenance of homeostasis through various approaches, such as probiotic application, results in an optimal equilibrium during MSCs-based therapies. However, further investigation into this matter is imperative to identify efficacious interventions.
Impact Statement
This article will allow researchers to understand the approaches for manipulating the potential of bacteria and bacterial derivatives to help medical fields studying mesenchymal stem cells (MSCs), as well as handling the undesirable mutual effects of dysregulated microbiome and MSCs on their respective biological potentials.
Background
There is currently a growing trend toward manipulating nonpathogenic bacterial cells to produce biomaterials for bioengineering purposes.1–4 Studies have established biocompatibility between mesenchymal stem cells (MSCs) and biopolymers, which can be used to develop in vitro tissues capable of renovating injuries.5–8 Moreover, the microbiome which is defined as microorganisms that colonize mucosal environments in the human body, has recently been investigated regarding their interaction with MSCs in various fields, from colon diseases to lung discomforts. 9 Some studies have focused on MSCs interaction with gut microbiome, due to the fact that the gut microbiome plays a pivotal role in host metabolism, systemic immune function, and also in maintaining homeostasis in distant target organs such as the brain, lung, liver, and cardiovascular system. 10
MSCs have a highly heterogeneous phenotype, but certain characteristics such as being fibroblast-like, plastic adherent, having colonogenic proliferation potential, and expressing surface markers like cluster of differentiation 105 (CD105), CD73, CD90, STRO-1 (the name of a cell surface marker expressed by stromal cells), CD106, and CD146 are particularly important in their identification.11–14 MSCs are also promising in the field of cell therapy and bioengineering due to their immune privilege, multipotent properties, reaction to danger signals, and low risk of tumorigenesis. They closely interact with both adaptive and innate immune system cells through cell–cell contact and the secretion of various soluble factors such as indoleamine-pyrrole 2,3-dioxygenase (IDO), prostaglandin E (PGE)-2, and fibroblast growth factor. 15
Tissue-resident MSCs detect pathogens even before conventional immune system cells are recruited. On the contrary, they have a particularly close relationship with the microbiome in renewing epithelial tissue, such as the oral cavity and gastrointestinal tract.16,17 Based on evidence, all these activities could potentially be initiated through the interaction between pattern recognition receptors (PRRs) on MSCs and their corresponding ligands.18,19 Studies show that MSCs are armed with a wide range of PRRs, especially toll-like receptors (TLRs). These PRRs can be stimulated by a wide set of structural motifs, pathogen-associated molecular patterns (PAMPs), naturally expressed by either prokaryotic or eukaryotic microorganisms. 20 The stimulation of TLRs, mediated by the ligation of PAMPs, regulates various functions of MSCs, including migration, differentiation, proliferation, cytokine secretion, and immune suppressive properties.21,22 The pathways from which the MSCs biological properties could be affected by microbiome need to be more studied. In this regard, it has been indicated that Lactobacillus reuteri extract could activate the process of cell migration, proliferation, and osteogenic differentiation of gingiva-MSCs mediated by activating the PI3K/AKT/β-catenin/TGFβ1 pathway. 23 Generally, it is becoming clear that microbiota influences MSCs in terms of proliferation, types of differentiation, and immunomodulation. For instance, in bone marrow (BM)-MSCs of germ-free mice, the proliferation rate, cell renewal capability, and transition to the osteogenic lineage were significantly more active than those in mice with microbiomes. While BM-MSCs from conventional microbiome niches had differentiated into the adipogenic linage and revealed the capability to reduce colitis. However, no reduction in intensity of colitis was observed in MSCs from germ-free niches. 24 Moreover, the type of microbiome plays an inevitable role in directing MSCs toward immunomodulatory or stimulatory actions, which leads to either the alleviation or exacerbation of inflammatory conditions. In this review, we elucidate the beneficial or detrimental effects of microorganisms and their components in MSCs-based therapies.
Engineering Microorganisms and Their Derivatives for Optimizing MSCs-Based Therapy
Several drug discoveries, as well as the development of their applications, have resulted from using microorganisms, especially bacteria. 25 Recent studies have investigated the application of bacteria in order to tackle some of the challenges associated with MSCs-based regenerative medicine. In the following sections, we describe some of the fields that have benefited from the controlled application of microbes and their components (Table 1 and Fig. 1).
Examples of bacterial cell exploitation in MSC-based therapy. Several studies (mentioned by references
Application of Microorganisms, from Gene Fragments to the Whole Cell, in Mesenchymal Stem Cells-Based Therapy
3D-BC/G, three-dimensional bacterial cellulose-graphene foam; BC, bacterial cellulose; BM, bone marrow; BMP-2, bone morphogenetic protein 2; BNC, bacterial nanocellulose; FN, fibronectin; GMSCs: gingival-derived mesenchymal stem cells, GV, gas vesicles; hMSCs, human mesenchymal stem cells; HUMSCs, human umbilical mesenchymal stem cells; hUSCs, human urine-derived stem cells; MAI, magnetosome gene island; MNP, magnetic nanoparticles; MPLA, monophosphoryl lipid A; MR, magnetic resonance; MSCs; Mesenchymal stem cells; MTX, methotrexate; MV130, heat-inactivated whole-cell bacterial species; RA, rheumatoid arthritis; SLA, supernatant of Lactobacillus acidophilus; UC, umbilical cord.
Bacterial Products for MSCs Applications
Tissue engineering applies materials and cells, to help either substitute tissues or promote endogenous regeneration. 37 Several biomaterials with varying physical and chemical properties have the potential to serve in vitro and in vivo tissue engineering. 38 The surface properties of scaffolds, which serve as supporting structures for cells in tissue engineering, can elicit cellular responses that significantly influence vital cellular functions, including proliferation and migration. These factors play a crucial role in defining the degree of rejection of medical implants. Moreover, there currently is a trend for engineering materials to demonstrate specific biological characteristics to control stem cell growth and differentiation.39,40 Current materials, however, are still limited in stem cell engineering due to complex biological milieu needed for the field that requires spatiotemporal control.41,42 Bacterial nanocellulose (BNC), a new emerging biomimetic material, has been extensively studied for its potential in various contexts including wound dressing material and drug delivery. It has been shown that biotechnologically produced BNC has the potential to be organized into semitransparent hydropolymer materials, which also benefit mechanical stability. Its nano-fibrillar and microporous structure make it attractive for application in tissue engineering contexts. 35 In vitro cytotoxicity and biocompatibility of these biomaterials have been investigated and the results suggest that bacterial cellulose is nontoxic. However, the biotoxicity of other nanoparticles of cellulose, such as cellulose nanocrystals and cellulose nanofibers is less resolved.43,44 Furthermore, there is evidence of the stimulatory effect of BNC on MSC expansion and triggering the cells toward osteogenic differentiation in BNC based scaffolds.35,45
Another example of this concept is utilizing the potentials of lactic acid bacteria, such as their ability to produce biofilm along with engineering the bacteria to express the proteins involved in MSC self-renewal and differentiation. This process creates a dynamic quality on materials to support stem cell technologies. 36
MSCs have also demonstrated influential roles in immune disorder treatments and tissue regeneration.46–50 However, their underlying mechanisms remain elusive, partly due to a shortage of tools used for real-time in vivo tracking of therapeutically used MSCs.51–53 To address this issue, MSCs impregnation with gas vesicles (GVs) and magnetization of MSCs have been explored. GVs are biosynthetic nanobubbles found in specific aquatic microbes with outstanding ultrasound imaging capabilities. Notably, a study has shown that MSCs impregnation with methotrexate-loaded GVs has improved their therapeutic effect in rheumatoid arthritis (RA) rats.26,54,55 Moreover, commonly used methods involving iron oxide nanoparticles (superparamagnetic iron oxide nanoparticles) were found to have negative effects on MSCs function. An alternative approach to magnetization was also developed, including genetic modification via genes derived from Magnetospirillum magneticum AMB-1. This method showed no adverse effects on MSCs’ biological potential and the magnetism did not decrease with cell proliferation. 27 Furthermore, permeability of MSCs with viruses has been used for gene expression in MSCs via viral vectors either in gene therapy or in bioengineering. 56
Wound-Healing Process
According to evidence, MSC treatment influences all phases of wound repair, including inflammation, epithelialization, and tissue remodeling. In addition, they have remarkable antimicrobial effects through either indirect or direct mechanisms, partly intervened by the synthesis and secretion of antimicrobial proteins and peptides (AMPs).57,58 These cells can regulate inflammation through suppressing tumor necrosis factor (TNF), blocking T cell proliferation, and triggering interleukin (IL)-10 and IL-4 production. MSCs have been demonstrated to act in the proliferation phase through the recruitment of keratinocytes, fibroblasts, and indigenous stem cells, also production of hepatocyte growth factor (HGF), vascular endothelial growth factor (VEGF), and platelet-derived growth factor. MSCs also perform in the remodeling phase by modulation of tissue inhibitor of metalloproteinase and matrix metalloproteinase, production of TGF-b3, keratinocyte growth factor, and regulation of collagen deposition, in addition, MSCs transdifferentiate into the epithelium, as well. 59
Probiotics, a type of commensal bacteria, could also accelerate the process of wound healing by contributing to all phases. These cells and their derivatives act as immunomodulatory agents that attract inflammatory cells such as macrophages and polymorphonuclear leukocytes toward the wound site.30,60 A study showed that L. reuteri extracts increase cell migration via PI3K/AKT/β-catenin/TGFβ1 pathway. 23
Probiotics also exert their antimicrobial activity through regulating the production of AMPs by the skin cells. On the contrary, some commensal bacterial strains can also produce AMPs themselves, thereby acting synergistically in wound healing. 61 Probiotics can stimulate the pro-inflammatory M1 macrophage and then switch it to M2, the anti-inflammatory response, promoting angiogenesis, and epithelization.30,60 Furthermore, it has been shown that probiotics lead to re-epithelialization of gingival epithelial cells through upregulating CXCL8-CXCR1/CXCR2 axis. 62 Studies also demonstrated that stimulation of angiogenesis by lactobacilli and their metabolites including Nisin, clausin, and amyloliquecidin is mediated by upregulating epidermal growth factor and VEGF expression. In addition, lactobacilli were proved to accelerate keratinocyte migration and proliferation, plus to initiate collagen III synthesis and deposition through TGF-β.30,60
Additionally, some studies proved that synergistic effect of MSCs and probiotics together could be more than each of them alone. 63 Mechanistically, the viability and antioxidant genes expression of MSCs improve through preconditioning by probiotic metabolites. 63 Besides, it has been suggested that a balance between pathogenic bacteria and parabiotics enhances the biological potential of MSCs, resulting in an intensified wound-healing process30,64,65 (Fig. 2).
General therapeutic connections between probiotics and MSCs therapy in wound healing involve three main desired functional effects. These include immunoregulation to reduce inflammation, tissue repair through proliferation and remodeling, and microbiome restoration, which includes eliminating pathogenic bacteria.
In the following, we illustrated the potential of probiotics to enhance the functionality of MSCs to improve the wound-healing process. High levels of lipopolysaccharide (LPS), a component of the gram-negative bacteria with endotoxicity capacity, under the influence of several factors such as overuse of antibiotics, can act as a stimulating factor for inducing inflammatory pathways. Consequently, this inhibits the function of MSCs and delays the wound-healing process. 65 It has been revealed that LPS in Porphyromonas gingivalis, as a pathobiont which is a part of the oral microbiome, has the capacity to active the NLRP3 inflammasome, which consequently inhibits the function of MSCs and delays the wound-healing process. The balance of the oral microbiome can be partly sustained by probiotics such as L. reuteri. Reuterin, an effective ingredient of the bacteria, neutralizes LPS in P. gingivalis and ameliorates the biological capacity of MSCs. Similarly, inoculation with L. reuteri extract leads to enhanced gingival-MSCs (G-MSCs) proliferation and improved oral wound healing in mice.30,65
Regarding wound dressing, it has been reported that foreskin treated with both monophosphoryl lipid A—a bacterial derivative and supernatant of Lactobacillus acidophilus attracted human umbilical cord MSCs.
Microbiome, Immune System, and MSCs Homeostasis
Several studies have demonstrated that MSCs can collaborate with immune system, acting as either pro-inflammatory or anti-inflammatory mediators. These properties of MSCs-controlled by factors such as dosage, duration of exposure, and the diversity of TLR stimuli, as well as the expression levels of various paracrine and autocrine chemokines and cytokines-are destined to modulate the immune system and maintain homeostasis.24,59 Moreover, there is a triangular relationship among MSCs, the microbiome, and components of immune system (Fig. 3). Due to the rising demand for therapeutic applications of MSCs, which often necessitate a significant quantity of cells, it is crucial to explore new methods for enhancing MSCs yields and their modulatory properties to facilitate clinical therapy.66–68 In this regard, the interaction between microbiome with the immune system has opened new avenues. As observed that heat-inactivated Propionibacterium acnes and its soluble polysaccharide have been found to enhance MSCs subpopulations and improve their immunomodulatory effects by activating the TLR2 pathway. 66
Microbiome, immune system, and MSCs; homeostasis 1; under typical healthy circumstances, the host microbiota has advantageous impacts on MSCs, while the MSCs regulate the sustainability of the host microbiome through their immunomodulatory and antipathogenic properties 2; MSCs can collaborate with immune system as either pro-inflammatory or anti-inflammatory mediators and the resulting effect depends on the presence or absence of inflammatory cytokines 3; The microbiome has crucial functions in training and developing key elements of the host’s innate and adaptive immune system. In return, the immune system maintains important aspects of the host–microbe symbiosis.
In the field of research on designing vaccines against bacterial infections, MSCs have also been utilized due to their immunemodulatory effects. For example, an experimental study aimed at identifying the best formulation of a vaccine against Vibrio cholerae indicated that mice injected with LPS and conditioned medium of preconditioned MSCs with LPS showed the highest IgG and IgA compared with mice injected with LPS alone. Moreover, this combination approach significantly modulated inflammatory responses by reducing TNF-α levels and elevating IL-10 and TGF-β levels. 69 Another study revealed that administration of MV130, a polyvalent bacterial sublingual preparation, enhances leukocyte recruitment, and T cell expansion. However, upon T cell activation, MV130 stimulation leads to the upregulation of immunosuppressive factors in MSCs. As a result, the immune-modulatory effects of MSCs reduced T lymphocyte proliferation, promoted the differentiation of dendritic cells with immunosuppressive properties, and triggered M2-like macrophage polarization, thereby mitigating the immune response. Furthermore, MSCs exposed to MV130 undergo functional alterations that augment their immunomodulatory capacity in response to subsequent stimuli. 21
This approach has also been validated to relieve and control destructive inflammatory conditions, such as in inflammatory bowel disease (IBD), that the MSCs, and fecal microbiota transplantation (FMT) as distinct interventions have been applied and compared. The results demonstrate that the two therapies share several therapeutic effects and exhibit enhanced functionalities through their mutual interaction. It is implied that an ameliorated clinical remission level using both FMT and MSC transplantation approaches could be achieved in IBD compared with the single FMT or MSC therapy. 19 The outcomes of the interaction between MSCs, microbiome, and immune system in various inflammatory conditions are indicated in Table 2.
Interaction of Mesenchymal Stem Cells with Immune System and Microbiome in Experimental Models of Inflammatory Conditions
ADMSCs, adipose-derived mesenchymal stem cells; AHR, aryl hydrocarbon receptor; ALI, acute lung injury; GALTs, gut-associated lymph node tissues; Hfp-Exos, exosome from placenta MSCs; hUC-Exos, exosome from umbilical cord MSCs; IFN-α, interferon-α; IL, interleukin; PLD, popliteal lymph node; TNF-α, tumor necrosis factor-α; Tregs, T regulatory cells.
Additionally, the microbiota plays a pivotal role in metabolic pathways such as lipid metabolism, which can be compromised in different situations such as nonalcoholic steatohepatitis (NASH).76,77 The therapeutic use of human MSCs could ameliorate NASH-related lesions, as well as recover gut microbiome and metabolic disorder in NASH. 78 Moreover, aryl hydrocarbon receptor (AHR), which is a ligand-activated transcription factor that coordinates signals from the environment, including diet and microbes to internal factors such as metabolic cues to regulate complex transcriptional processes. It has been suggested that AHR serves as an agent through which therapeutically applied MSCs facilitate the connection between the gut microbiota and host immune system. 79
Challenges Related to MSCs Interactions with Bacteria and Viruses
Despite significant advancements in regenerative medicine, particularly in manipulating interactions between bacterial component and MSCs,23,80 it remains essential to consider potential negative interactions between bacteria and MSCs to enhance the clinical applications of MSCs. Researchers have been exploring these important issues in both fields of bone regeneration and stomatology.
Regarding MSCs ability to act as safe harbor for pathobionts, studies have documented that certain intracellular pathogens can survive within MSCs for extended periods of time. In some cases, viruses that were internalized could be released from their host cells.81–86 However, the effects of bacteria and bacterial components on the biological characteristics of MSCs, as well as the impact of MSCs on bacterial pathogenicity are dependent on various factors such as specific bacterial strains and origin of the MSCs and have not been clearly determined. Here, we present the advancements in this regard. The immunoregulatory effects of MSCs can provide an optimal niche for opportunistic bacteria. Cutibacterium acnes is typically found as a commensal bacterium, implicated in infections related to the implantation of foreign medical devices. BM-MSCs could promote the pathogenic potential of C. acnes through the secretion of immunosuppressive agents such as IDO-1, PGE2, IL-6, and transforming growth factor-β (TGF-β) that may increase the risk of osteomyelitis infections.80,81 It has been indicated that MSCs of distinct origins exhibit varied behaviors in contact with microbiome. For example, DP-MSCs appeared to present a consistent ability to internalize all investigated strains of Staphylococcus aureus and C. acnes, in comparison to BM-MSCs and Wharton’s jelly (WJ)-MSCs. 83 DPSCs have also provided a safe niche for Streptococcus mutans (S. mutans), which aids in its survival and facilitates infection in dental pulps. Moreover, the infected DPSCs through the secretion of inflammatory cytokines could establish dental pulp damage. 87 In another view, bacteria are known to exert a broad range of biological changes on MSCs function, which could enhance their immunomodulatory or inflammatory features. In the study, Josse et al. indicated that S. aureus could be internalized into WJ-MSCs, leading to a reduced viability rate of WJ-MSCs. Additionally, S. aureus provoked MSCs to secrete IL-6 and TNF-α inflammatory cytokines, which play significant roles in bone-related inflammatory conditions such as osteoarthritis. 88 The interaction between S. aureus and BM-MSCs was potentially observed to depend on a virulence factor, fibronectin-binding protein (FnBP), as S. aureus with a deficient FnBP mutant failed to adhere effectively to MSCs. 89 Fusobacterium nucleatum, an opportunistic bacterium, exerted stimulatory roles on gingival-derived MSCs (GMSCs) including migration, chemokine release, upregulation of bone cancers-related genes, and an inhibitory effect on the osteogenic differentiation of GMSCs. 90 In addition, a microarray analysis on GMSCs following an extended period of exposure to F. nucleatum revealed an increased expression of cancer-related genes. 91
Mycobacterium tuberculosis (Mtb), an intracellular pathogenic bacterium, has demonstrated the ability to persist within BM-MSCs, primary through the inhibition of antimicrobial peptide cathelicidin. Meanwhile, nonpathogenic strains of mycobacterium via inducing TLR 2, 4 pathway lead to the activation of cathelicidin gene expression. 92 PG-E2, a known immunomodulatory lipid mediator, was also found as an essential immune-privilege characteristic of MSCs, which results in the survival of MSCs-resident Mtb through the inhibition of the antimicrobial effects of phagocytic cells and inflammatory cytokines. 93 Viruses are also potent agents in altering the functional nature of MSCs. BM-MSCs infected with Cytomegalovirus indicated deficiencies in both antimicrobial and immunomodulatory responses through the inhibition of the interferon (IFN)-γ-mediated IDO-1 expression pathway. 94
Avian influenza A H5N1 has the potential to change the immunomodulative activities of BM-MSCs by inducing the secretion of inflammatory mediators including IL-6, CCL-2, and CCL-4. 95 BM-MSCs extracted from a human immunodeficiency virus (HIV) transgenic mouse showed a reduced capability in proliferation, osteogenic differentiation, along with less efficacy in mitigating cisplatin-induced toxicity and renal injury. 86 In another study, MSCs infected with a high load HIV indicated a significant tendency toward adipogenic differentiation, but with a slight reduction in osteogenesis. The adipogenesis process notably attenuated after the antiviral treatment of the HIV-infected MSCs. 96 Some viral infections can induce immune-regulatory features in MSCs. As observed in respiratory syncytial virus-treated umbilical cord blood MSCs, the expression level of IDO-1 and IFN-β was dramatically increased compared with untreated cells. 97
MSCs in Bone Regeneration and Concerns with the Osteomyelitis
MSCs contribute to bone fracture healing through osteogenic differentiation. However, in bone infection, also known as osteomyelitis, the osteogenic ability of MSCs and osteoblasts is affected under a chronic inflammation. Therefore, the difficulty in controlling inflammation leads to progressive bone damage and loss and eventually bone deformity. 98 Several studies have shown that the commensal C. acne and S. aureus are the most prevalent bacteria found in cases of osteomyelitis. The mechanism by which the commensal bacteria become pathogenic still needs to be studied more. However, according to recent studies, these bacteria have the ability to interact with and internalize into bone cells and MSCs, with BM-SCs being well-documented. This might result in evolution of pathogenic C. acne, which has been found to acquire virulence factors through increased biofilm formation, influence on macrophage phagocytosis, and osteoblasts invasion. Infected MSCs may also exhibit an increase in the secretion of IL-6, IL-8, PGE-2, VEGF, TGF-β, IDO-1, and HGF immunomodulatory mediators, which altogether can contribute to the chronic implant-associated osteomyelitis.80–82 With a similar mechanism, S. aureus has also been identified as a contributor to an inflammatory environment in BM-MSCs, which impedes osteogenic differentiation and thus exacerbates osteomyelitis. 99
MSCs in Stomatology and the Related Concerns
Gingivitis and periodontitis are of the most prevalent diseases affecting humans. 100 As it has been well reviewed, the inflammation-related signaling pathways are involved in regulating the differentiation potential of MSCs. LPS, as a PAMP, at certain doses has been shown to induce inflammation, intervertebral disc degeneration, osteogenesis, and adipogenesis imbalance in MSCs which could in turn cause alveolar bone absorption and implant failure.101–105 For example, a study showed that under inflammatory conditions caused by LPS, the resulting upregulation of the TLR4 signaling pathway inhibited osteogenic differentiation and induced adipogenesis of the human PDLSCs (periodontal ligament stem cells). 105 Indeed, LPS-mediated inflammatory environment inhibits BMP (bone morphogenetic protein 2)-induced osteogenic differentiation through crosstalk between TLR4/MyD88 (myeloid differentiation primary response 88)/NF-κB (nuclear factor kappa B), and BMP/Smad signaling pathways. 102 Accordingly, it has been shown that interaction between NF-κB, MAPK (mitogen-activated protein kinase), and BMP/Smad signaling interposes dual effect of IL-1β on the osteogenesis of PDLSCs. Mechanistically, low amounts of IL-1β trigger the BMP/Smad signaling pathway, promoting the osteogenesis of PDLSCs. Conversely, higher amounts of IL-1β prevent BMP/Smad signaling via the activation of NF-κB and MAPK signaling, thus impeding osteogenesis. PDLSCs with defected osteogenesis, secreted inflammatory more inflammatory cytokines and chemokines, which induces macrophages recruitment, implicating the contribution of PDLSCs to the pathogenesis of periodontitis. 101 It has been observed that preconditioned MSCs with LPS plus TNF-α exerted an anti-inflammatory condition and showed an increased osteogenic differentiation. Another research also indicated that pretreatment with an appropriate dose of LPS could protect hUCMSCs from high-dose LPS-induced apoptosis. It upregulates the expression of cellular FADD-like IL-1β-converting enzyme-inhibitory protein (c-FLIP).106,107
By designing a long-time in vitro bacteria-cell coculture model, the relationship between F. nucleatum, the most frequent pathogenic bacteria causing periodontitis, and GMSCs was explored. The results indicated that persistent exposure of GMSCs to F. nucleatum promoted cell migration and chemokine/cytokine secretion such as CCL2, CXCL1, and IL-6 while inhibiting the proliferation and osteogenic differentiation of GMSCs in a dose-dependent manner. 90
Using the microarray significant profiles analysis method, it was shown that long-term exposure to F. nucleatum may activate the expression of cancer-related genes in normal GMSCs. In this regard, genes for cancer-related pathways were found in infected GMSCs. 91
Interventions in the Microbiome Dysregulation
Several research studies are underway to uncover the mechanisms through which imbalanced microorganisms interfere with normal metabolic events, aiming to identify strategies for targeted interventions in stem cell therapy. For instance, it has been revealed that S. aureus directly impacts WJ-MSCs to produce inflammatory agents such as IL-6 and TNF-α. 88 Based on the evidence, targeting inflammation-related pathways could be a potential therapeutic approach in cases of bacterial dysregulation, resulting in improved biological potential of GMSCs, including cell proliferation and osteogenic differentiation.
Currently, probiotics are recognized for their potential to beneficially modulate deregulated microbiomes and subsequent inflammation by producing bactericidal peptides, stimulating immune system, and reducing inflammatory responses. 60 As reported by Han et al., the inoculation of the extract from the probiotic L. reuteri results in a balance of oral pathogenic bacteria and enhances the biological potential of GMSCs through inhibiting the NLRP3 inflammasome. 30 To prevent the risk of sepsis following exposure to live probiotic cells, utilizing bacterial derivatives presents a safer choice in regenerative interventions, such as those for IBD or wound healing. 108
Furthermore, the immunoregulatory and antimicrobial potential of MSCs itself could exert a degree of anti-inflammatory effects. A substantial number of studies have addressed this issue. As an in vivo experiment showed that the therapeutic use of adipose-derived stem cells inhibited P. acnes-induced skin inflammation by blocking the NLRP3 inflammasome through reducing the secretion of IL-1β. 44 Moreover, the potential of MSCs-derived exosomes as a promising strategy for managing tissue damage and inflammation via the modulation of microbiome has been considered. For instance, exosomes-derived from fetal placenta and umbilical MSCs have been shown to ameliorate colitis symptoms by reducing the prevalence of intestinal bacteria and regulating the balance between Th17 and T regulatory cells. 75 Thus, targeting the inflammatory mediators may lead to a better management of bacterial inflammation, resulting in much more successful achievements in regenerative medicine.
Conclusion
In recent years, numerous experiments have been conducted to investigate the mutual effects of microbiota and MSCs on their respective biological potentials. The objective of these experiments is to ascertain whether microbiota can be beneficial or detrimental to MSCs-based treatments and if MSCs can transform commensal bacteria into pathogenic ones. The results of these studies have been intriguing, as they have highlighted the potential applications of bacteria in bioengineering and immunotherapy fields. When a controlled strategy is employed using nonpathogenic bacteria, such as Lactococcus lactis, thriving outputs can be achieved. This has been demonstrated in genetically engineered lactic bacterial cells that express extracellular matrix proteins required to induce MSCs into a specific lineage of interest. On the contrary, there is evidence to suggest that MSCs can act as a safe harbor for certain microorganisms, such as C. acne and S. aureus, thereby leading to chronic implant-associated osteomyelitis. On that basis, to avoid the potential harmful effects resulting from the application of bacterial cells, applying the probiotics extracts has been suggested. 23
There is a growing trend in utilizing biomaterial for MSCs-mediated therapy, including those derived from bacteria. LPS, which is the bacterial components, has been shown to enhance the immunomodulatory abilities of MSCs at a certain concentration. The fields of bioengineering and cell therapy have also incorporated various forms of bacterial cellulose, nano cellulose, and cell wall components of lactic acid bacteria with MSCs and their exosome. These materials have been demonstrated to be more compatible with MSCs than their synthetic counterparts.
Regarding the therapeutic use of MSCs in organs that are naturally colonized by microbiome, such as oral cavity, designing novel test methods, such as a long-time bacteria-cell coculture in vitro model that mimics real organ circumstances, seems to be more practical. Promising results have been achieved thus far, indicating that MSCs could still be functional in such environments and would not undergo apoptosis.109–111 However, given the warning reports related to chronic bacterial inflammation resulting from the interplay between MSCs and commensal bacteria, further consideration is required to discover the relationship between MSCs and the bacteria in more detail. Studies are currently being conducted to discover the molecular mechanisms contributing to the crosstalk between MSCs and bacteria. MSCs utilize TLRs located on their surface to detect the presence of microbes. Some theories suggest that TLR activation may promote MSC migratory abilities, shift lineage commitment toward proliferation and differentiation, and influence the pathways that govern MSC responses to injury, microbial products, and inflammation.
The therapeutic use of MSCs and their exosome promotes the proliferation of beneficial gut bacteria through providing a balanced immune response, which helps to ameliorate inflammatory conditions. When faced with a microbial challenge, MSCs as versatile immune privileged cells can act as a safeguard providing an atmosphere favorable to infection clearance by resident phagocytes through inducing various antibactericidal pathways such as reactive oxygen specious. There is also evidence that the interaction between MSCs and pathogens can result in MSC’s physiological implications, such as influencing key modulators of trilineage differentiation, increasing the likelihood of senescence, and reducing the ability to repair tissues, particularly under the influence of factors such as uncontrolled oxidative stress. 112
From the aspect of relation between inflammatory and differentiation pathways, research has demonstrated that incubation of MSCs with pathogens significantly increases the expression of various small inflammatory molecules, such as IL-8, IL-6, and IL-1β. These molecules are believed to have a dual effect on the differentiation of MSCs. For example, low doses of IL-1β can activate related pathways to promote the osteogenesis of PDLSCs, but higher doses of IL-1β can inhibit osteogenesis. 101
Therefore, providing a supporting environment for MSCs when using them in therapeutic approaches seems to be crucial. In this regard, probiotics, or even safer, their derivatives or extracts that have antimicrobial and antioxidant effects, could be suggested for use in conjunction with the application of MSCs in such situations.
To sum up, available evidence suggests that a well-balanced microbiota has advantageous impacts on MSCs in typical healthy circumstances. Additionally, there are promising outcomes of modifying bacterial cells and their byproducts to utilize them in MSCs-based therapeutic strategies. Nonetheless, it appears imperative to obtain a comprehensive understanding of the interactions between microbiome, particularly the impaired microbiome, and MSCs in order to address numerous challenges related to inflammation and degeneration.
Authors’ Contributions
Conception and design of study: E.F.S., N.H., and E.K-.P. Acquisition of data: E.K-.P. Analysis and/or interpretation of data: E.F.S., N.H., and E.K.P. Drafting the article: E.K.-P. Revising the article critically for important intellectual content: E.F.-S. and N.H. Approving the version of the article to be published: E.F.-S. and N.H.
Footnotes
Author Disclosure Statement
The authors declare that they have no competing interests.
Funding Information
No funding was received.
Author Confirmation Statement
All authors are from Babol University of Medical Sciences (Babol, Iran), where education and research are the primary functions.
Availability of Data and Materials
The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.