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Author contributions: D.G. and A.U.: designed research; D.G., B.P., C.U., L.V., S.C., and S.B.: performed research; D.G., B.P., G.M., and A.U.: analyzed data; D.G. and A.U.: wrote the paper; A.U.: provided financial support to the study.
Disclosure of potential conflicts of interest is found at the end of this article.
First published online in STEM CELLSEXPRESS July 20, 2012.
Mesenchymal stem cells (MSC) display a remarkable ability to modulate the immune response and protect the central nervous system mainly through the release of soluble factors in a paracrine fashion, affecting the functional behavior of cells in the tissues. Here we investigated the effect of the interaction between MSC and microglia in vitro, and we dissected the molecular and cellular mechanisms of this crosstalk. We demonstrated that MSC impair microglia activation by inflammatory cues through the inhibition of the expression and release of inflammatory molecules and stress-associated proteins. We showed that MSC significantly increase microglial expression and release of molecules associated with a neuroprotective phenotype such as CX3CR1, nuclear receptor 4 family, CD200 receptor, and insulin growth factor 1. Interestingly, MSC can enhance functional changes on microglia as depicted by the increase of intracellular calcium concentration and phagocytic activity. This last event is associated with an increased expression of triggering receptor expressed on myeloid cells-2, an innate immune receptor involved in phagocytosis in the absence of inflammation. The observed effects on CX3CR1-expressing microglia are due to the release of CX3CL1 by MSC, driven by inflammatory signals, as demonstrated by the reversal of the observed results when CX3CL1 expression was silenced in MSC or its release was blocked. Finally, we showed that exogenous CX3CL1 induce phenotypic and functional changes of microglia similar to those induced by MSC. These findings demonstrate that MSC instruct, through the release of CX3CL1, microglia responsiveness to proinflammatory signals by modulating constitutive “calming” receptors, typically expressed by “steady-state microglia” thus switching microglia from a detrimental phenotype to a neuroprotective one. Stem Cells2012;30:2044–2053
Mesenchymal stem cells (MSC) are stromal progenitors of mesodermal cells also referred as multipotent mesenchymal stromal cells, since their true “stemness,” that is, on division one daughter cell remains stem and the other is able to replenish a whole tissue compartment, has yet to be demonstrated . In any case, MSC demonstrate classic adult stem cell multipotency in that they are capable of differentiating in vitro and in vivo to all mesenchymal lineages, including adipose, bone, cartilage, muscle, and myelosupportive stroma .
MSC initially attracted interest for their ability to differentiate into multiple cellular phenotypes both in culture and in vivo. However, recent observations indicate that, following i.v. administration, most cells are rapidly entrapped in the lungs and only a few engraft into injured tissues where they display negligible transdifferentiation capacity [3–5].
It has been demonstrated that MSC could provide an ideal cell source for repair of injured central nervous system (CNS) due to their striking therapeutic plasticity based on the capacity of modulating the immune response and on a wide range of bystander effects in the target tissues . Indeed MSC regulate effector functions of cells of the adaptive and innate immunity through the release of soluble factors such as prostaglandin E2 (PGE2), nitric oxide (NO), indoleamine 2,3-dioxygenase (IDO), and others  leading to the induction of peripheral tolerance . Moreover, MSC can protect axons and improve neuronal survival [4, 9, 10] releasing antiapoptotic  or antioxidant molecules . It has been also reported that MSC may foster endogenous neurogenesis  and oligodendrogenesis [14–16] not only by direct release of trophic factors but also by stimulating glial cells to secrete molecules leading to neuron survival and proliferation of the endogenous neural precursor cells .
These preclinical studies together indicate that MSC are bestowed with several features leading to amelioration of experimental autoimmune encephalomyelitis (EAE), a model of human multiple sclerosis, through mechanisms leading to inhibition of the pathogenic immune response and support of neural repair chiefly mediated by the release of soluble molecules in a “bystander” fashion .
Several studies showed that microglia play a critical role as resident immunocompetent cells within the CNS. They are highly active in their resting state, continually surveying the CNS microenvironment with extremely motile processes and protrusions. Furthermore, blood brain barrier disruption, brain injuries, systemic infections, and chronic diseases can induce the immediate and focal activation of microglia, switching its behavior from patrolling to shielding of the injured site [18, 19]. Upon activation, microglia produce a variety of effector molecules that have been closely associated with the pathogenesis of multiple sclerosis and neurodegenerative diseases [20–22]. Microglia reacts to infections and other insults activating Toll like receptors (TLRs), such as TLR4, by releasing proinflammatory molecules in the attempt to clear pathogens. Microglia can also be involved in the maintenance of CNS homeostasis phagocytizing apoptotic bodies and cellular debris and concur in the tissue repair through the release of neuroprotective molecules . An attractive approach to treat neurological diseases where inflammation associates with neurodegeneration lays in the possibility of modifying the behavior of microglia switching their functional phenotype from a detrimental to a protective one [22, 23]. Microglia with neuroprotective features have been associated with an increased surface expression of the fractalkine receptor CX3CR1  and may act through the nuclear receptor (NR) 4 family (NURR1) pathway to protect neurons by suppressing inflammatory gene expression .
In this study, we addressed the in vitro effect of MSC on microglia, and we dissected the molecular and cellular mechanisms of these interactions demonstrating that MSC can switch microglia from a detrimental behavior dominated by the release of proinflammatory molecules to a neuroprotective phenotype associated with the production of anti-inflammatory and trophic factors. Moreover, we showed that MSC induce functional changes on microglia as depicted by modifications in intracellular calcium concentration and phagocytic activity. Finally, we provided compelling evidence that CX3CL1 released by MSC plays a major role in inducing these beneficial effects on microglia.
MSC In Vitro Culture and Expansion
Bone marrow-derived MSC were isolated from 6-8-week-old C57BL/6J mice (Harlan Laboratories, San Pietro al Natisone (UD), Italy, http:///www.harlan.com) as described elsewhere . In brief, marrow cells, flushed out from tibias and femurs, were plated in 75 cm2 tissue culture flasks at the concentration of 0.3–0.4 × 106 cells per centimetre square using Murine Mesencult as medium (Stem Cell Technologies, Vancouver, BC, Canada, http://www.stemcell.com). Cells were cultured in plastic plates as adherent cells and kept in a humidified 5% CO2 incubator at 37°C, refreshing medium every 3 days for approximately 6 weeks when cells reached 80% confluence. Following treatment with 0.05% trypsin solution containing 0.02% EDTA (Euroclone, Milan, Italy, http://www.euroclone.it), marrow cells were plated in 75 cm2 flask at the density of 4 × 105 cells. Mature MSC, obtained after four to five passages in culture, were defined by the expression of CD9, Sca-1, CD73, and CD44 and the lack of the hematopoietic markers CD45, CD34, and CD11b on their surface. The phenotype was analyzed using the following monoclonal antibodies directed against mouse surface markers: phycoerythrin cyanine 5 (PE Cy5) conjugated rat anti-mouse CD45, PE-conjugated rat anti-mouse Sca-1, purified rat anti-mouse CD9, PE-conjugated rat anti-mouse CD73 (all purchased from BD Pharmingen, San Diego, CA, http://www.bdbiosciences.com/index). Fluoroscein isothiocyanate anti-rat IgG1/2a (BD PharMingen) was used as secondary reagent for an indirect staining of CD9 positive cells. Analysis was performed using a FACSCalibur flow cytometer, and data were analyzed with Cell Quest software (Becton Dickinson Immunocytometry Systems, Franklin Lakes, NJ, http://www.bd.com).
The murine microglial cell line N9, originally developed by Prof. P. Ricciardi-Castagnoli and kindly provided by Prof. E. Zocchi, Genoa, Italy, was obtained by immortalization of E13 mouse embryonic cultures with the 3RV retrovirus carrying an activated v-myc oncogene that is similar to primary microglia in producing substantial amounts of NO and various cytokines after stimulation . Cells were grown in Iscove's Modified Dulbecco's Medium (IMDM) containing 25 mM HEPES and L-glutamine, supplemented with 5% fetal calf serum (FCS), 100 IU/ml penicillin, 100 μg/ml streptomycin, and 50 nM β mercaptoethanol and were cultured in a humidified 5% CO2 atmosphere at 37°C. Primary microglial cells were prepared from cerebral cortex of newborn C57BL/6J mice. Following removal of meninges, brain tissue was trypsinized for 15 minutes and, after mechanical dissociation, cell suspension was washed and plated on poly-L-lysine coated (10 μg/ml, Sigma Aldrich, St. Louis, MO, http://www.sigmaaldrich.com) flasks (75 cm2). Mixed glial cells were cultured in Basal Medium Eagle (BME) (Invitrogen, Carlsbad, CA, http://www.invitrogen.com) supplemented with 10% FCS (Invitrogen), 2 mM glutamine (Sigma Aldrich), and 100 μM gentamicin sulfate (Sigma Aldrich). After 10–13 days, microglial cells were harvested from mixed primary glial cultures by mild shaking and collected by centrifugation. Purity of microglia was analyzed by flow cytometry using a CD11b antibody (> 98% surface expression) (R&D Systems, Minneapolis, MN, http://www.rndsystems.com). For microglia activation, 1 μg/ml lipopolysaccharide (LPS, Sigma Aldrich) or 10 ng/ml interferon γ (IFNγ) (R&D Systems) or 2.5 μg/ml non methylated CP oligonucleotides 2006 (CpG 2006) (5′-TCGTCGTTTTGTCGTTTTGTCGTT-3′), as described by  (Tib Molbiol, Genoa, Italy, http://www.tib-molbiol.com) was added to the culture medium for 24 hours. N9 and N9-activated cell lines were cocultured for 24 hours in the presence or absence of MSC at 1:1 ratio (8 × 105 cells) in a six-well plate in a transwell system (BD FALCON Cell Culture Inserts, BD Biosciences, San Diego, CA, http://www.bdbiosciences.com).
Cell proliferation was measured by the flow-cytometric detection of the intracellular incorporation of 5,6 carboxyfluorescein diacetate succinimidyl ester (CFSE, Molecular Probes, Invitrogen). 3 × 106 LPS-activated N9 cells were incubated for 4 days with 1 μM CFSE in the presence of absence of MSC at the same concentration. After this period, cells were harvested and cell proliferation was assessed by flow cytometry displaying CFSE dilution.
RNA Isolation and Real Time Quantification
Total RNA was isolated from N9 cells and primary microglia using TRIzol reagent (Invitrogen) according to the manufacturers' instructions. First strand cDNA was synthesized from 1 μg of total RNA using 60 μM random examer-primer, 20 U Transcriptor Reverse Transcriptase, 20 U Protector RNase Inhibitor, and 1 mM dNTPs in a final volume of 20 μl (Transcriptor First Strand cDNA Synthesis Kit, Roche, Basel, Switzerland, http://www.roche-applied-science.com). Real-time polymerase chain reaction (PCR) reactions were performed in LightCycler 480 (Roche) in duplicate in a final volume of 20 μl containing 50 ng cDNA, 1 μl of each primer pair 20 μM (TIB Mol Biol), 10 μl of LightCycler 480 SYBR Green I Master Mix (Roche). The hypoxanthine-guanine phosphoribosyltransferase (HPRT) was used as housekeeping gene to normalize the expression data. In RT-PCR experiments, we utilized primer pairs for the following genes: inducible Nitric Oxide Synthase (iNOS), heme oxygenase 1 (HOX1), metallothionein-1 and −2 (MT1 and MT2), poly(ADP-ribose) polymerase-1 (PARP-1), tumor necrosis factor (TNF), interleukin1β (IL1β), insulin growth factor 1 (IGF1), fractalkine receptor (CX3CR1), CD200 receptor (CD200R), nuclear receptor (NR) 4 family (NURR1), prostaglandin E2 receptor (EP2), prostaglandin E4 receptor (EP4), triggering receptor expressed on myeloid cells-2 (TREM2), and fractalkine (CX3CL1) (Supporting Information Table S1). The relative gene expression of target genes in comparison with the HPRT reference gene was conducted following the comparative CT threshold method. The normalized expression was then expressed as relative quantity of mRNA (fold induction) with respect to the control sample.
Cytokines, NO Metabolites, and CX3CL1 Quantification
Quantitative analysis of the levels of IGF1, IL1β, and TNF was performed by enzyme-linked immunosorbent assay (ELISA) using commercial available kits (R&D Systems) on supernatants derived from 24 hours transwell cultures. The concentration of nitrate and nitrite (NO metabolites) was determined by using a colorimetric reaction with the Griess reagent (total nitric oxide and nitrite/nitrate parameter assay kit, R&D Systems). Quantitative analysis of CX3CL1 was performed by ELISA utilizing the Quantikine Mouse CX3CL1/fractalkine Immunoassay (R&D Systems) on supernatants derived from MSC cultured for 24 hours in the presence of either 5 ng/ml IFNγ or 1 μg/ml LPS and in the presence of both stimuli according to the manufacturer's instructions. In parallel, the quantitative expression of CX3CL1, in MSC activated as previously described, was analyzed by real time PCR.
Determination of the Intracellular Cyclic ADP Ribose Levels
Adherent N9 cells (5 × 106) were incubated for 24 hours in IMDM with or without (control) LPS in the absence or presence of MSC. Cells were recovered by trypsin treatment, washed once with Krebs Ringers buffer, and cell pellets were lysed in 300 μl of deionized water and neutralized with perchloric acid (0.6 M final concentration). After centrifugation (1,500g for 10 minutes), the cyclic ADP Ribose (cADPR) content was measured on the neutralized cell extracts by means of a highly sensitive enzymatic cycling assay  and expressed as pmol/mg of cell protein.
Fluorimetric Determination of the Intracellular Calcium Concentration (Ca2+i)
Adherent N9 cells (8 × 105) were cultured on 20-mm diameter coverslips with or without LPS in the presence or absence of the same concentration of MSC for 24 hours. At the end of LPS treatment, cells were incubated with Fura-2-acetoxymethyl ester (Fura 2 AM) for 45 minutes at 37°C, and the Ca2+i was measured on an inverted microscope (Zeiss IM35, Zeiss S.p.A., Arese (MI), Italy, http://www.zeiss.com) as described elsewhere .
N9 cells, at a concentration of 8 × 105, were cultured on 35-mm glass bottom cell culture dishes with or without LPS in the presence or absence of the same concentration of MSC for 24 hours. At the end of LPS treatment, cells were incubated with 10 μl of red dyed fluoresbrite microspheres (Polysciences, Inc., Eppelheim, Germany, http://www.polysciences.com) for 30 minutes at 37°C. The phagocytic activity was then stopped by adding 2 ml of ice-cold phosphate buffered saline (PBS), cells were washed twice with ice-cold PBS, and 2 ml of medium was added. Microglia was analyzed by confocal microscopy (TCS SP5, LEICA Microsystems GmbH, Milan, Italy, http://www.leica-microsystems.com), and the area of the maximal confocal section of the cells was measured. Phagocytosis was quantified as percent of this area occupied by the microspheres as described elsewhere .
Blocking of CX3CL1 Produced by MSC
MSC were stably transfected with small interfering RNA (siRNA) for CX3CL1 using Lipofectamine 2000 transfection reagent (Invitrogen). The most efficient target sequence for RNA interference was selected among four sequences provided by Qiagen (Hilden, Germany, http://www1.qiagen.com; 5′-TTGAAGTTGTTGATCCTTTA-3′) (Supporting Information Fig. S1). For the Ca2+i and phagocytosis experiments requiring a longer blocking of fractalkine-produced by MSC, cells were treated with a CX3CL1 monoclonal neutralizing antibody for 24 hours (anti-mouse CX3CL1 antibody, R&D Systems).
Exogenous Administration of CX3CL1
LPS-activated N9 cells were cultured in the presence of 5 ng/ml recombinant mouse CX3CL1 (R&D Systems) for 24 hours, and the expression of TNF, IL1β, CX3CR1, NURR1, EP2, and TREM2 genes was measured by real time PCR. The Ca2+i and phagocytic activity of LPS-activated microglia in the presence of CX3CL1 was measured as described before.
Statistical analysis was performed on independent samples, namely N9 with or without LPS cultured in the presence of MSC versus cells cultured alone (controls), using unpaired two-tailed Student's t test, which takes into account mean values and standard deviation. Analysis for repeated measures was performed by ANOVA.
MSC Affect the Expression and Release of Effector Molecules by Microglia
We first investigated the effect of MSC on the expression and production of different inflammatory factors produced by microglia when activated by LPS. To this end, we utilized the immortalized murine microglial cell line N9, which has been widely utilized in the literature as cell line capable of producing large amount of cytokines following activation . We observed a significant decrease of iNOS and TNF mRNA expression when LPS-stimulated N9 cells were cultured in the presence of MSC (p < .05 and p < .01, respectively) (Fig. 1). Next, we investigated the influence of MSC on the production of NO metabolites (nitrites and nitrates) and the secretion of TNF by activated microglia. Resting microglia produced relatively small amount of nitrite and nitrate after 24 hours in culture (4 μM and 3 μM, respectively) compared to the amount produced following LPS stimulation (9–12 μM) (Supporting Information Fig. S2). When microglia was activated by LPS in the presence of MSC, production of NO metabolites significantly decreased, similar to the levels observed in resting microglia (5–6 μM, p < .05) (Supporting Information Fig. S2). Similarly, MSC significantly reduced TNF production by activated microglia after 24 hours of coculture (p < .01) (Supporting Information Fig. S2). Surprisingly, the expression and production of the proinflammatory cytokine IL1β by microglia activated in the presence of MSC was significantly upregulated compared to control cells (N9+LPS without MSC) (p < .001 and p < .05, respectively) (Fig. 1 for PCR experiments and Supporting Information Fig. S2 for ELISA).
Next, we asked whether MSC could regulate the expression of molecules involved in the cellular stress response such as MTs, HOX1, and PARP, which are upregulated following LPS activation. As shown in Figure 1, the expression of the two non-neuronal MT isoforms, MT1 and MT2, were strikingly downregulated by MSC compared to the levels observed in LPS-activated microglia (p < .001). Similarly, the expression of HOX1 and PARP, a typical marker of cellular damage and apoptosis was strikingly reduced compared to LPS-stimulated control cells (p < .001). In order to assess whether results obtained utilizing the cell line N9 are representative of what could be observed using microglia from primary cultures, we stimulated primary microglia with LPS as described above and measured gene expression and protein release of selected target molecules upon MSC coculture. Similarly to what observed using N9 cells, MSC significantly inhibited the upregulation and release of TNF and enhanced the expression and release of IL1β (Supporting Information Fig. S3).
MSC Increase Expression and Release of Neuroprotective Molecules by Microglia
We addressed the possible effects of MSC on three receptors associated with microglia neuroprotective phenotype such as CX3CR1, NURR1, and CD200R. As shown in Figure 2, the mRNA expression of CX3CR1, NURR1, and CD200R was significantly downregulated in LPS-activated microglia (p < .05), while the presence of MSC significantly upregulated their expression (p < .05). Similar trend was observed for the trophic factor IGF1 that was downregulated in LPS-activated cells and upregulated by coculture with MSC (p < .05). Moreover, this effect of MSC was confirmed by the increased release of IGF1 induced by the presence of MSC (p < .05) as shown in Supporting Information Figure S2.
Next, we analyzed the effect of MSC on activated N9 microglia expression of the PGE2 receptors EP2 and EP4, whose activation have been suggested to promote cell survival in several models of tissue damage [31, 32]. We observed that the expression of EP2 was significantly increased when LPS-activated N9 cells were cultured in the presence of MSC (p < .05, Fig. 2); MSC were also able to upregulate the expression of EP4 but only restoring its expression value to that observed in N9 in resting condition (p < .05, Fig. 2). Similarly to what observed when MSC were cocultured with LPS-activated N9 cells, MSC were also capable of inducing upregulation of the neuroprotective genes CX3CR1, NURR1, and IGF1 in LPS-activated primary microglia (Supporting Information Fig. S3). The similarity of results obtained with primary microglia and the N9 cell line suggests that the latter is a robust and reproducible tool to evaluate the effect of MSC on microglia. Therefore, from now on, experiments were carried out exclusively on N9 cells.
MSC Modulate the Profile of Genes Expression of Microglia when Activated by CpG and IFNγ
Next, we assessed whether MSC can have an impact also on gene expressed by microglia when activated by other proinflammatory stimuli such as IFNγ, a cytokine known to enhance microglial phagocytosis  and CpG, a TLR9 agonist, which regulates the microglia immune response . Overall, MSC displayed a similar ability to downregulate TNF, iNOS, and HOX1 expressed by microglia also when activated by IFNγ and CpG. Similarly to what observed when microglia was activated by LPS, MSC were able to significantly upregulate the expression of IGF1, CX3CR1, and NURR1 (Supporting Information Fig. S4). These results demonstrate that MSC can act on functions of microglia regardless the way it is activated.
MSC Induce Functional Changes on Microglia Without Affecting Its Proliferation Capacity
In order to assess whether MSC could affect the activation state of microglia, we investigated the proliferative activity, intracellular calcium concentration, and phagocytic capacity of N9 cells stimulated with LPS in the absence or in the presence of MSC. MSC were not able to interfere with proliferation of LPS-activated N9 cells as demonstrated by the fact that microglia proliferated at 96 hours at the same rate observed in the presence or absence of MSC (Supporting Information Fig. S5).
As indicated in Figure 3, MSC significantly increased the basal Ca2+ concentration of LPS-activated microglia and were able to enhance their phagocytic activity (p < .001). cADPR is a potent and universal Ca2+ mobilizer, which was demonstrated to mediate the LPS-induced Ca2+ increase in N9 cells .
In line with the finding that MSC increased the Ca2+i of LPS-activated microglia, we observed a significant increase of the intracellular concentration of cADPR when LPS-activated N9 were cultured in the presence of MSC compared to control (0.12 pmol/mg vs. 0.05 pmol/mg, p < .05). Then, we addressed whether MSC can affect the expression on activated microglia of TREM2, which was recently demonstrated to play a role in the maintenance of CNS homeostasis without inflammation . As shown in Figure 3, we detected a significant upregulation of TREM2 expression by activated microglia cultured in the presence of MSC (p < .05).
All together these findings suggest that MSC exposure impacts on microglia activation state affecting intracellular Ca2+ signaling and phagocytic capacity.
MSC Exert Beneficial Effect on Microglia Through the Release of CX3CL1
We next investigated the role of the chemokine fractalkine/CX3CL1 and its specific receptor CX3CR1 that are constitutively expressed in several region of the CNS and are reported to mediate neuron–microglial interaction, synaptic transmission, and neuronal protection from toxic insults . Recent data showed that augmenting CX3CR1 signaling protect against microglial neurotoxicity . As we observed that MSC can enhance CX3CR1 expression in LPS-stimulated microglia (Fig. 2), we hypothesized that MSC could exert their effect on microglia through the release of CX3CL1. We demonstrated that MSC express and release only minimal levels of CX3CL1 in the steady state condition but expression and release were significantly augmented following stimulation with two proinflammatory molecules such as IFNγ and LPS (Supporting Information Fig. S5). To address the possibility that MSC-produced CX3CL1 has a major impact on microglia functions, we examined the effect of CX3CL1 silencing in MSC using the siRNA technique. Indeed, when we utilized MSC transfected with siRNA for CX3CL1 in the coculture with activated microglia, we detected a partial restoration of TNF expression compared to LPS-stimulated N9 in the presence of MSC (p < .01) (Fig. 4). More importantly, silencing of CX3CL1 resulted in a striking inhibition of the upregulation observed for those molecules such as IL1β, CX3CR1, NURR1, and EP2 whose expression was increased in activated microglia following coculture with MSC (Fig. 4). In contrast, IGF1 was the only gene for which silencing of CX3CL1 in MSC did not vanished the effect of wild-type MSC on LPS-activated microglia (p < .01, Fig. 4), possibly suggesting that, in microglia, the IGF1 pathway is CX3CL1 independent.
To further validate these results, we assessed whether blocking of CX3CL1 could functionally affect microglia activation utilizing a monoclonal anti-mouse CX3CL1 antibody (R&D Systems), as described in Materials and Methods. We demonstrated that blockade of CX3CL1 on MSC treated with the anti-CX3CL1 antibody for 24 hours resulted in the downregulation of the basal Ca2+ concentration (p < .001), phagocytic activity (p < .001), and TREM2 RNA expression (p < .05) of stimulated microglia in the presence of treated MSC (Fig. 5).
Exogenous CX3CL1 Mimics the Effects on N9 Cells Observed Following Microglia Exposure to MSC
In order to confirm the key role of CX3CL1 in modulating function and activity of the LPS-activated microglia, we repeated some of the above-reported experiments by adding the recombinant CX3CL1 protein (R&D Systems) directly to microglia cultures during LPS treatment. The results reported in Figure 6 demonstrated that, in the presence of a concentration of CX3CL1 similar to that produced by IFNγ and LPS-activated MSC (5 ng/ml, Supporting Information Fig. S5), LPS-stimulated microglia expressed lower levels of TNF compared to control cells (N9+LPS) (p < .05). Conversely, when we measured the mRNA expression of both IL1β and of the neuroprotective genes under analysis such as CX3CR1, NURR1, and EP2, we demonstrated that CX3CL1 led to the upregulation of all genes as we observed when LPS-activated microglia was cultured in the presence of MSC (p < .05).
Finally, we showed that CX3CL1 can affect the functional state of microglia, as shown by the significant increase of the Ca2+i (p < .001), phagocytic activity (p < .001), and TREM2 RNA expression on LPS-activated microglia (p < .05) (Fig. 6). All together these findings demonstrate that exogenous CX3CL1 can reproduce most of the MSC-mediated effects observed on activated microglia.
In this article, we demonstrate that MSC, a subset of progenitor cells displaying a remarkable ability to modulate effector functions of cells of the innate and adaptive immunity , can affect several features of microglia following activation with proinflammatory stimuli. Microglia, the resident macrophages of the CNS, are exquisitely sensitive to brain injury and disease, altering their morphology and phenotype in response to brain injuries and contributing to the maintenance of brain homeostasis  . Upon activation, microglia have the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells . Various cytokines, inflammatory and also neuronal signaling molecules have been reported to actively control microglia function, thus instructing microglia toward a beneficial or detrimental phenotype. First, we observed that MSC reduce the increase of the expression of TNF, iNOS and oxidative stress-associated proteins, induced by LPS and other proinflammatory molecules, by microglia thus supporting their ability to inhibit the release of proinflammatory molecules typically occurring following classic activation as response to pathogens or during chronic brain inflammation. Second, we demonstrated that MSC can induce a significant upregulation of surface molecules playing a calming influence on microglia and associated with a neuroprotective phenotype. Among these, CD200R expressed on microglia negatively regulates effector functions on myeloid cells , while NURR1 has been reported to suppress inflammatory genes expression in microglia . Cardona et al.  reported that augmenting CX3CR1 signaling, through CX3CL1 binding, decreases microglia neurotoxicity. Recent data suggested also that activation of the PGE2 receptor EP2 promote cell survival in several models of tissue damage . Interestingly, we detected a significant upregulation of IL-1β, a cytokine not only often involved in the inflammatory response but also considered to play a key role in promoting repair of the CNS, possibly inducing the synthesis and release of IGF by glial cells [39, 40]. The reported upregulation of the above-mentioned molecules suggests that MSC can instruct microglia responsiveness to proinflammatory signals such as LPS, IFNγ, and CpG, by modulating constitutive “calming” receptors, typically expressed by “steady-state microglia” .
Of relevance, we demonstrated that MSC also induce functional modifications of microglia as depicted by the detection of enhanced intracellular calcium concentration and phagocytic activity without affecting the proliferation capacity of microglia. These results suggest that MSC appear to act on the ability of microglia to activate following LPS triggering and subsequently enter their “executive phase” by dissociating their capacity of releasing proinflammatory molecules from their phagocytic activity. This last event is associated with the acquisition of a neuroprotective phenotype by microglia as further confirmed by the upregulated expression of TREM2, an innate immune receptor expressed by microglia involved in phagocytosis in the absence of inflammation . More importantly, we demonstrated that, following stimulation with LPS and IFNγ, two molecules involved in CNS inflammatory events, MSC modulate microglia effector functions through the release of CX3CL1. Several soluble immunosuppressive factors have been reported to be involved in MSC-mediated immunoregulation, either produced constitutively by MSC or released following crosstalk with target cells. Examples of molecules belonging to the latter group are IDO and PGE2, which are released by MSC when licensed by IFNγ produced by target cells [41, 42].
The ability of CX3CL1 released by MSC to promote the switch of microglia from a detrimental, neurotoxic phenotype dominated by the release of proinflammatory molecules to a beneficial, neuroprotective phenotype associated with the production of anti-inflammatory and trophic factors, enhanced expression of calming receptors, increased intracellular calcium concentrations, and phagocytosis was confirmed by experiments where CX3CL1 was added exogenously to microglia activated with LPS in the absence of MSC. These results are in line with previous studies reporting on the ability of CX3CR1 expressing microglia to exert striking neuroprotective functions through the binding to CX3CL1 released by neurons , which leads to enhancing intracellular calcium concentration  and increased phagocytic activity .
These data demonstrate that MSC, through the release of CX3CL1, may enable microglia to dissociate phagocytic activity from the release of proinflammatory molecules  thus enhancing its homeostatic role in the maintenance of a healthy CNS, possibly through an efficient removal of apoptotic cells and phagocytosis of cellular debris and fostering a proregenerative environment within the CNS [30, 43].
These results are also in line with the current evidence that MSC act mainly through the release of soluble factors in a paracrine fashion, possibly affecting the functional behavior of tissue-resident cells. Interestingly, MSC can induce dendritic cells to acquire a tolerogenic phenotype in vitro and in vivo  and educate alternatively activated macrophages  which have been demonstrated to be essential players in CNS healing . Recently, Zhou et al.  demonstrated that MSC are able to inhibit LPS-stimulated microglia activation and the production of inflammatory factors through diffusible molecules and, within an inflammatory environment, can significantly increase the production of neurotrophic factors, possibly involved in tissue repair. In addition, MSC exert neuroprotective effects on dopaminergic neurons via an anti-inflammatory mechanism mediated by the modulation of microglia activation . More importantly, a recent study demonstrated that MSC alternatively activate microglia, thus enhancing its migratory features and ability of eliminating amyloid β.
These findings demonstrate that MSC are capable of functionally affecting myeloid cells behavior including microglia which is induced toward a protective phenotype, possibly accounting for some of the beneficial effects that have been associated with the therapeutic plasticity of MSC in EAE and other experimental models of neurological diseases . Overall these results further confirm the incumbent translation of MSC cell therapy from experimental models to human neurological diseases where microglia may play a major pathogenic role .
This study was supported by grants to A.U. from the Italian Foundation for Multiple Sclerosis (FISM), the Italian Ministry of Health (Ricerca Finalizzata), the Regione Liguria, the Italian Ministry of University and Scientific Research (MIUR), the Progetto Limonte, and the Fondazione CARIGE. We thank Dr. E. Polazzi for technical assistance on Primary microglial cells isolation.
Disclosure Of Potential Conflicts Of Interest
The authors indicate no potential conflict of interests.