Mesenchymal Stem Cell Immunomodulation Mechani.pdf

ReviewMesenchymal Stem Cell Immunomodulation:Mechanisms and Therapeutic PotentialNa Song,1,2,3Martijn Scholtemeijer,1,2and Khalid Shah1,2,4,*Mesenchymal stem/stromal cells(MSCs)are multipotent cells that are emergingas the most promising means of allogeneic cell therapy.MSCs have inherentimmunomodulatory characteristics,trophic activity,high in vitro self-renewalability,and can be readily engineered to enhance their immunomodulatory func-tions.MSCs affect the functions of most immune effector cells via direct contactwith immune cells and local microenvironmental factors.Previous studies haveconfirmed that the immunomodulatory effects of MSCs are mainly communi-cated via MSC-secreted cytokines;however,apoptotic and metabolicallyinactivated MSCs have more recently been shown to possess immunomodula-tory potential,in which regulatory T cells and monocytes play a key role.Wereview the immunomodulatory aspects of nave and engineered MSCs,anddiscuss strategies for increasing the potential of successfully using MSCs inclinical settings.Mesenchymal Stem CellsMSCs are pluripotent T cells that have self-renewing,differentiation,and immunomodulatoryproperties.Their two most attractive features are plasticity(see Glossary)and tropism.Theyare distinguished from other cell types by the expression of cell-surface markers includingCD73,CD90,and CD105,and by the lack of expression of CD45,CD34,CD14,CD19,CD11b,and human leukocyte antigen DR isotype(HLA-DR)1,and play a central role in tissuerepair in addition to their antitumorigenic,antifibrotic,antiapoptotic,anti-inflammatory,proangiogenic,neuroprotective,antibacterial,and chemoattractive effects 2,3.This unique setofcharacteristicsmakesMSCsattractivefortheirtherapeuticpotentialinthefieldsofregenerativemedicine 4,inflammatory disorders 2,and,increasingly,cancer therapy 5,6.Initially,MSCs were mainly used for tissue repair and regeneration 3.However,they have beenincreasingly used for diseases including graft-versus-host disease(GVHD)and autoimmunediseases such as lupus and Crohns disease 2.In addition,the clinical potential of MSCs hasbeen extended to the treatment of myocardial infarction,stroke,multiple sclerosis,liver cirrhosis,diabetes,lunginjuries,andcancer2,3.MSCshavebeenharvestedand expanded fromavarietyofadultandperinataltissuessuchasbonemarrow7,adiposetissue7,8,peripheralblood,fetaltissues 9,dental pulp 8,umbilical cord tissue 7,8,and placental tissues 7.Most preclinicalstudies have been performed with bone marrow-derived MSCs(BM-MSCs);however,adiposetissue-derived MSCs(A-MSCs)and umbilical cord blood-derived MSCs(UC-MSCs)have alsoreceived considerable attention in recent years 9.Immunomodulation by MSCsMSCs have recently been shown to possess immunomodulatory potential in which interactionswith regulatory T cells(Tregs)and monocytes play a key role 2,10.Several studies have sug-gested that A-MSCs exertmore potent immunomodulatory effects than BM-MSCs,implying thatA-MSCs are a better alternative for immunomodulatory therapy 11.UC-MSCs,on the otherHighlightsMSCs are multipotent cells that areemerging as the most promising meansof allogeneic cell therapy.MSCs participate in both innate andadaptive immunity,and their immuno-modulatory functions are exerted mainlyviainteractions with immune cellsthrough cell-to-cell contact and para-crine activity.Engineering MSCs to express specificimmunomodulatory agents contributesto MSC capacity and pluripotency,andalso enables them to deliver largedoses of cancer-targeting biologics witha single dose.MSC administration has shown potentialefficacy in the treatment of several dis-eases that resist standard treatment.However,there are some challenges inefficiently translating MSC-based thera-peutics into the clinic.Efficient homing and migration of MSCsto the target tissue will be essential forthe future development of MSC-basedtherapies.1Center for Stem Cell Therapeutics andImaging(CSTI),Harvard Medical School,Boston,MA 02115,USA2Department of Neurosurgery,Brighamand Womens Hospital,Harvard MedicalSchool,Boston,MA 02115,USA3Department of Medical Oncology,theFirst Hospital of China MedicalUniversity,Shenyang 110001,China4Harvard Stem Cell Institute,HarvardUniversity,Cambridge,MA 02138,USA*Correspondence:kshahbwh.harvard.edu(K.Shah).Trends in Pharmacological Sciences,September 2020,Vol.41,No.9https:/doi.org/10.1016/j.tips.2020.06.009653 2020 Elsevier Ltd.All rights reserved.Trends in Pharmacological Scienceshand,have been suggested to show minimal risk of initiating an allogeneic immune responsewhenadministeredinvivo.This,aswellastheireaseofcollection,makesUC-MSCssuitablether-apeutic candidates 8.In the following we focus on immunomodulatory aspects of nave andengineered MSCs,and synthesize our current understanding of the effects of MSC modulationon immune cells.Immunomodulatory Actions through Cell-to-Cell ContactsMSCs participate in both innate immunity and adaptive immunity,and their immunomodula-tory functions are exerted mainly via interactions with immune cells through cell-to-cell contactand paracrine activity involving T cells,B cells,natural killer(NK)cells,macrophages,monocytes,dendritic cells(DCs),and neutrophils 12(Figure 1).Cell-to-Cell Contact with Immune Cells of Adaptive ImmunityIn vitro,MSCs have been shown to inhibit nave T cell and memory T cell responses tocommunicate with antigen-presenting cells 13 by upregulating intercellular adhesion molecule 1GlossaryAdaptive immunity:a subset of theimmune system that is activated byexposure to pathogens and acts on thethreat using an immunological memoryto enhance its effect.Cells of theadaptive immune system include B andT cells.Allogeneic:tissues or cells fromindividuals of the same species,butwhich are genetically dissimilar andhence immunologically incompatible.Dendritic cells(DCs):antigen-presenting cells in the mammalianimmune system.DCs act asmessengers between the innate and theadaptive immune systems.Eczema area and severity index(EASI):a clinical scoring system that isused to measure the extent and severityof actopic eczema.Granule polarization:a prelude to therelease of cytotoxic contents inresponse to target-cell binding.Innate immunity:a subset of theimmune system that is activated by thepresence of antigens and their chemicalproperties,using nonspecific defensemechanisms,meaningthatanythingthatis identified as foreign or non-self is atarget.Cells of the innate immunesystem include natural killer(NK)cells,macrophages,mast cells,neutrophils,and DCs.Lymphocyte:a subtype of white bloodcells that include NK cells,T cells,andB cells.M1 macrophages:polarizedmacrophages that encourageinflammation.M2 macrophages:polarizedmacrophages that decreaseinflammation and encourage tissuerepair.Macrophage polarization:refers tohow macrophages have been activatedat a given point in space and time inresponse to signals from theirmicroenvironment.Membrane particles(MPs):particlesof the membrane of a cell ranging from63 to 700 nm in diameter.Memory T cells:a subset of infection-and cancer-fighting T cells.They canreproduce to mount a faster andstronger immune response,andrecognize foreign invaders,such asbacteria or viruses,as well as cancercells.Monocytes:the largest type of whiteblood cells that can differentiate intomacrophages and myeloid-lineage DCs.TrendsTrends inin PharmacologicalPharmacological SciencesSciencesFigure 1.Multifaceted Immunomodulatory Interactions between Mesenchymal Stem Cells(MSCs)andImmune Cells.MSCs exert immunomodulatory functions mainly via interactions with immune cells,such as T cells,B cells,natural killer(NK)cells,macrophages,monocytes,dendritic cells(DCs),and neutrophils,as well as through cell-to-cell contacts(blue arrows)and paracrine activity(shown by secretome).The MSC secretome includes several cytokines,growth factors,and chemokines,and their immunomodulatory functions vary depending on the source of the MSCs,thetarget cells,and the microenvironment.Abbreviations:ICAM-1,intercellular adhesion molecule-1;IDO,indoleamine-pyrrole2,3-dioxygenase;IFN,interferon;IL,interleukin;PD-L1,programmed death ligand 1;PD-L2,programmed death ligand 2;PGE2,prostaglandin E2;TGF-,transforming growth factor-;TNF-,tumor necrosis factor-;VEGF,vascularendothelial growth factor.Figure drawn with BioRender(https:/ in Pharmacological Sciences654Trends in Pharmacological Sciences,September 2020,Vol.41,No.9(ICAM-1)and vascular cell adhesion molecule 1(VCAM-1)that are crucial for T cell activation andleukocyte(white blood cell,WBC)recruitment to the site of inflammation 14.Furthermore,coculture of BM-MSCs with activated T cells has been shown to induce lymphocytes that ex-press interleukin-17A(IL-17A)15.MSCs cocultured with CD4+T cells activate the Notch1/Forkhead box P3(FOXP3)pathway and increase the percentage of CD4+CD25 FOXP3+cells16.Furthermore,knockdown of galectin 1,a protein that is abundantly expressed intracellularlyand on the cell surface of MSCs,and that has effects on T lymphocytes and cytokine secretion17,in MSCs results in loss of immunomodulatory properties and restores the proliferation ofCD4+and CD8+T cells 17.In addition,BM-MSCs express high levels of Toll-like receptors(TLRs)3 and 4,which are responsible for nuclear factor B(NF-B)activity and cytokine produc-tion.Expression ofTLR3and TLR4inMSCs hasbeenshown torestoreanefficient T cellresponseto infection 18.Human placenta MSCs(PMSCs)have been shown to express high levels of thecell adhesion molecules programmed death ligand 1(PD-L1)and PD-L2,which can inhibit T cellproliferation by arresting the cell cycle 19.In vivo mouse modelshavealso shown interactiveimmuneregulation between MSCs and T cells.In a syngeneic orthotopic mouse model of ovarian cancer,compact bone-derived MSCs(CB-MSCs)have antitumor effects in combination with a fusion protein consisting of an anti-mesothelin single-chain antibody variable region(scFv)genetically fused to Mycobacteriumtuberculosis heat-shock protein 70(Hsp70)(designated VIC-008),which is involved in activatingCD4+and CD8+T cells and inhibiting Tregs in the tumor microenvironment(TME)20.In fetalabortion models,MSCs have been shown to enhance the suppressive regulation of T cells andmacrophages 21.Conversely,MSCs primed by activated T cells derived from IFN-null miceexhibited a dramatically reduced ability to suppress T cell proliferation,and this supports the cell-to-cell contact mechanism for the immunosuppressive function of MSCs 22.In addition to T cells,MSCs also affect B cells through cell-to-cell contact in vitro.A-MSCs havebeen shown to increase the survival of quiescent B cells via contact-dependent mechanisms andto facilitate B cell differentiation independently of T cells 23.In addition,A-MSCs not only inhibitcaspase 3-mediated apoptosis of B cells by upregulating vascular endothelial growthfactor(VEGF)24 but also inhibit cell proliferation by blocking the cell cycle of B lymphocytes inG0/G1 phase by activating p38 mitogen-activated protein kinase(MAPK)pathways 25.Cell-to-Cell Contacts with Immune Cells of Innate ImmunityIn addition to the adaptive immune system,MSCs also affect the innate immune system throughcell-to-cell contacts.Tracking studies reveal that infused UC-MSCs briefly reside in the lungsand are rapidly phagocytosed by monocytes,which subsequently migrate to other body sites.UC-MSC phagocytosis induces phenotypic and functional changes in monocytes,which in turnmodulate cells of the adaptive immune system and thus play a crucial role in mediating,distributing,and transferring the immunomodulatory effects of MSCs 26.Coculture studies of MSCs withdifferent types of NK cell lines(KHYG-1 and NK-92)have shownthat granule polarization is eithersuppressed or induced,indicating differential crosstalk between MSCs and cytotoxic NK cells 27.In addition,A-MSCs are known to switch activated inflammatory M1 macrophages to an M2macrophage-like phenotype via prostaglandin E2(PGE2)28.MSCs can also prevent neutrophildeath via an ICAM-1-dependent mechanism to further exert tissue-protective effects 29.Immunomodulatory Actions through Paracrine ActivityImportantly,MSCs also exert their immunomodulatory properties by secreting multifunctionalmolecules via paracrine mechanisms 12(Figure 1).This secretome is a diverse repertoire ofmultifaceted cytokines,growth factors,and chemokines,which combine to modulate theNave T cells:primary cellular effectorsin the adaptive immune system that playcrucial roles in antigen specificity.Natural killer(NK)cells:cytotoxiclymphocytes that are crucial for theinnateimmunesystemandproviderapidresponses to virus-infected cells andtumor cells.Neutrophils:the most abundant typeofwhite blood cells and an essentialpartof the innate immune system.Plasticity:the capacity to differentiateinto other cell types in response todifferent stimuli.Regulatory T cells(Tregs):formerlyknown as suppressor T cells,Tregs area subpopulation of T cells that modulatethe immune system todownregulatetheinduction and proliferation of effectorT cells.Secretome:the set of proteins that areexpressed byan organism and secretedinto the extracellular space,includingcytokines,growth factors,extracellularmatrix proteins,regulators,and shedreceptors.T helper 17(Th17)cells:a subset ofproinflammatory T helper cells that aredefined by the production of interleukin17(IL-17).Toll-like receptors(TLRs):receptorsthat sense invading pathogens orendogenous damage signals,andinitiate the innate and adaptive immuneresponse.TLR3 and TLR4 are highlyexpressed in BM-MSCs,and theiractivation induces nuclear factor B(NF-B)activityandcytokineproduction.Tropism:the ability to migrate todamaged or diseased tissues or cells.Trends in Pharmacological SciencesTrends in Pharmacological Sciences,September 2020,Vol.41,No.9655function of immune and cancer cells.These include transforming growth factor-1(TGF-1),tumor necrosis factor-(TNF-),PGE2,IFN-,hepatocyte growth factor(HGF),fibroblastgrowth factor(FGF),indoleamine-pyrrole 2,3-dioxygenase(IDO),and nitric oxide,among manyothers 30,31.These paracrine factors are encapsulated in cell-secreted extracellular vesicles(EVs),whichin turn are usually divided into exosomes,microvesicles(MVs),and apoptotic bodiesaccording to their size and cell of origin 30,32.Although MSC-derived EVs(MSC-EVs)displayimmunoregulatory functions similar to the parent MSCs 33,their paracrine actions varyaccording to the source of the MSCs,the target cells,and the microenvironment surroundingthe cells 34.Paracrine Activity on the Adaptive Immune SystemMSCs have been shown to act on the adaptive immune system,particularly T cells,via paracrinesecretion.MSCs inhibit T helper 17(Th17)cell differentiation by inducing the production of IL-10and PGE2,and by inhibiting IL-17,IL-22,and IFN-35.Coculture studies of BM-MSCs withT cells have shown that both priming with cytokines and the cell ratio of BM-MSCs influence theircytokineprofiles,andsuggestthatBM-MSCsmodulatetheTh17lymphocytepathwayinac