I- Introduction to Mesenchymal Stem Cells (MSCs)
I.I- Background Information about MSCs
I.II- Characterization of MSCs
I.III-Differentiation of MSCs in vitro
II-The Role of Mesenchymal Stem Cells in Immunology
II.I- The Relationship between Chemokines and MSCs
II.II- The Relationship between Adhesion Molecules and MSCs
III- Clinical Strategies By Utilizing Mesenchymal Stem Cells
III.I- Allogenic MSC Therapy,
II-Clinical Trials due Immunomodulatory Potential of MSCs
III.III- Limitations and Strategies to MSC Therapy IV- Concluding Remarks and New Perspectives
I- Introduction to Mesenchymal Stem Cells
1.1- Background Information about Mesenchymal Stem Cells (MSCs)
Mesenchymal stem cells (MSCs) were originally found by Friedenstein and colleagues where the cells within bone marrow have potential to differentiate into other mesenchymal cells.  They have showed that adherent cells within plastic culture dishes have an appearance like fibroblasts after several passaging. After these work was extended, these isolated cells were differentiated into osteoblasts, chondrocytes, adiposites and myoblasts where these transformed cells were in mesenchymal type.
After isolation of human MSCs (hMSCs) from the mononuclear layer of the bone marrow, adherence of MSCs to tissue culture can clearly be observable. Washing cells over time will remove non-adherent cells. Remained adherent cells were fibroblasts and currently MSCs have been isolated not only from bone marrow but also adipose tissue, amniotic fluid and fetal tissues by having heterogeniccharacteristics
1.2- Characterization of MSCs
Despite none of the markers that MSCs express are specific to MSC, adult human MSCs do not express CD45, CD34, CD14 or CD11 which are hematopoietic markers. Additionally, no expression of costimulatory molecules such as CD80, CD86, or CD40 or the adhesion molecules (PECAM-1), CD18 or CD56 has been shown in MSCs. However, VCAM-1, ALCAM, ICAM-1 adhesion molecules expressed by MSCs was shown in several reports. [3-8] Blood Derived Mesenchymal Precursor Cells (BMPCs) many of markers expressed by BMPCs are the same as bone morrow MSCs.
1.3- Differentiation of MSCs in vitro
MSCs can differentiate to for chondrocytes, tenocytes, skeletal myocytes and endothelial cells under appropriate in vitro conditions. (9)
II-The Role of Mesenchymal Stem Cells in Immunology
MSCs show MHCI + , MHCII -, CD40,CD80-, CD86- immune phenotype. Therefore immunosuppression does not seem when alllogenic host was transplanted. T cells become anergic in the absence of costimulatory molecules when T cells are activated by MHCI.
MHCs can modulate (a) T-cell function and cell activation and (b) inhibition of B cell proliferation, differentiation and chemotaxis. (10)
The immunosuppressive effects of MSCs for clinical use mentioned in the part III of this review in detail.
Consequently, in vitro differentiation of MSCs into several different cell types and their immunological feature make them potential source of stem cells in terms of usage in tissue repair and gene therapy.
2.1- The Relationship between Chemokines and MSCs
Chemokines are chemotactic cytokines that belongs to a large superfamily of small glycoproteins play role in (a) leukocyte trafickking, (b) hematopoiesis, (c) angionesis and (d) organogenesis.
The role of chemokines in leukocyte traficcking initiate contacting among endothelial cells and blood leukocytes via interacting adhesion molecules such as integrin, selectins and their lignads that lead to rolling motion of leukocytes.(10)
Recent paper showed that the transendothelial migration occurred is mediated by PECAM-1, CD99 and ICAM-1. (11) MSCs can migrate into tissues from the circulation according to signal response. Cell migration is controlled by chemokines and their receptors, and there has been reported that their role in migration of progenitor cells such as CXCL12 (stromal cell derived factor-1) and its receptor CXCR4 is important for bone marrow related processes such as retention, mobilization and are involved recruitment of endothelial cell progenitor cells to sites of ischemic issue. (11)These situations can be connected to expression of chemokine receptor on hMSCs.
However, despite CXCR4 intracellular expression were demonstrated, Von Lüttichau et all showed that CCR1,CCR4 ,CCR7 and CXCR5 but not CXCR4 receptors functionally driving MSC migration. (12)There is limitation on this paper where not the whole repertoire of functional chemokine receptors at protein level have not studied. Hence, the variety of chemokines are expressed by MSCs except CXCR4 that varies from different reports.
As a result, different chemokines expressed by MSCs indicates that MSCs are potential to home to different type of tissues and can be used to prevent inflammation and increase tissue repair.
2.2- The Relationship between Adhesion Molecules and MSCs
In mice having genetic P-selectin deficiency, it was observed that decreasing in intravenously administered rolling ability of hMSCs upon HUVECs and none of the p-selectin glyocoprotein ligand 1 or the alternative ligand CD24 were present on hMSCs. As we mentioned in earlier paragraph, MSCs also requires rolling upon endothelial cells like leukocytes. (13)
Most of integrins expressed on MSCs which are VCAM-1, ICAM-1, ALCAM and alpha1-5, beta1-3 , beta5. (14-15)
Adhesion of rat MSCs to TNFalpha or interleukin-1beta stimulates cardiac microvascular endothelial cells (CMVE) under in vivo conditions and VCAM-1 levels of CMVE demonstrated that when cytokine stimulation happened, inhibiton of adhered MSCs to the endothelial cells with a blockinmg antibody of highly inhibited agains VCAM-1. Therefore p- selectin, VCAM-1 and their counterligands are important for adhesion of MSCs to the endothelium. (16)
Consequently, MSCs play vital role in immunology and do this processes by means of integrins, selectins and chemokine receptor. The functional importance of P-selectin and VCAM-1 during leukocyte adhesion has been connected to adhesion to MSCs to endothelium. However, role of PECAM-1/CD31 in leukocyte transmigration across the endothelium can not be relate to MSCs due to MSCs do not express these molecules. Therefore, there may exist specific distinctions in the use of adhesion molecules according to cell types.
III- Clinical Strategies By Utilizing Mesenchymal Stem Cells
MSCs has been used to treat diseases such as myocardial infarction, stroke, chronic obstructive pulmonary disease (COPD), multiple sclerosis (MS), amytrophic lateral sclerosis (ALS), graft versus host disease (GvHD) and diabetes (17) using allogenic MSCs.
3.1 – Allogenic MSC Therapy
Bartholomew et al. demonstrated when the baboon MSCs to cultures of stimulated allogenic peripheral blood leukocytes, leukocyte proliferation in an MSC was suppressed in dose dependent manner. (18) This suppression occurred no matter what the donor origin was. The T-cell suppression was transient, even T-cells respond to allo-antigen upon removal from MSC coculture. (19) Later on, addition of interleukin-2 partially reversed the suppression that can be suggested that T-cell anergy were not induced by MSCs.
In other reports, donor derived MSCs increases the tolerance for pancreatic islets and heart allografts. (20-21) These results clearly show that the MSCs are potentially MHC- unrestricted.
As said in earlier paragraphs, hMSCs express MHCI but no MHCII or costimulatory molecules. Thus, the cell to cell contact mechanism occur in MSCs maybe not necessarily for immune suppression. However, it has been reported that close proximity between cells increase suppression while cell to cell contact is not necessary issue. (22)
3.2- Clinical Trials due Immunomodulatory Potential of MSCs
In 2004, a 9 year old boy that suffers from grade IV, treatment-resistant GvHD receieved haplo-identical MSCs from her mother. And at the end survival rate was 1 year after bone-marrow transplantation when compared the other 24 patients received bone marrow transplantation but no MSCs therapy. Then, beginning of usage of allo-MSCs for therapy has initiated for the treatment of acute GvHD and acute myocardial infarction (MI). (23)
The results from recent studies are encouraging and currently allo-MHC therapy has been approved in South Korea, Canada and New Zelland. For example, children treated with prophylactic treatment with MSCs at the time of bone marrow transplantation at grade III shows no incidence during transplantation.This situation seems more significant in phase II in patients having MSC therapy.
III.III- Limitations and Strategies to MSC Therapy
One limitation during MSC therapy is MSCs do not maintain for following infusion which means the major cell population dies within 2 days after infusion. These also have been proved from autopsy of patients death within 1 year.(24) Additionally, other factors such as growth factor deprivation, shear stress and more during transplantation clarifies more immunological processes are involved in limited persistence of allo-MSCs.
One strategy must be taken into account during usage of allo-MSCs are despite their nonimmunogenic characteristics, they cause humoral and cellular cause of immune response under in vivo conditions. Since infusion of allo-MSCs induce memory thus rejection may happen. Additionally, Boosting of MHC persistence is difficult since overcoming MHC- barriers is very difficult.
In order to prolong MHC persistence, the host or MSC modification is necessary. Due to, unknown immunosuppressive mechanism of immunosuppressive drugs, it may hard to correlate usage of them that interfere with the immunosuppressive feature of MSCs. More study is need when effects of immunosuppressant drugs and MSCs connected to MSC persistence. (25)
IV- Concluding Remarks and New Perspectives
The question remained behind allo-rejection of MSCs for clinical therapies. However, immunogenicity of MSC onto therapy needs to be taken into account.
First critical issue which is infused MSCs do not express MHCII can be considered during allo- MSC therapies. The other type of auto-MSCs can also be considered according to conditions in diseases. Additionally, in spite of not having major adverse affects on allo-MSCs , an observed anti -donor response may create problem (26).
MSCs still need to be clarified. Regulatory T Cells, formerly known as suppressor T-cells suppress induction or proliferation of effector T-cells. For me, researchers need to focus on interaction between MSCs and TREGs in which MSCs seem to have similar features like TREGS. For example Foxp3+ TREGs are found within the MHCII restricted CD4+ population and expresses high level of IL-2. In earlier paragraph of this paper, it has been told that T-cell suppression was transient, even T-cells respond to allo-antigen upon removal from MSC coculture. (19) Accordingly, addition of interleukin-2 partially reversed the suppression that can be suggested that T-cell anergy were not induced by MSCs. Maybe here, the reason for T-cell energy due to expressed TREGs in microenvironment they present. Still we need to investigate more and more about MSCs.
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