K. Le Blanc

Minimal criteria for defining multipotent mesenchymal stromal cells. The International Society for Cellular Therapy position statement

The considerable therapeutic potential of human multipotent mesenchymal stromal cells (MSC) has generated markedly increasing interest in a wide variety of biomedical disciplines. However, investigators report studies of MSC using different methods of isolation and expansion, and different approaches to characterizing the cells. Thus it is increasingly difficult to compare and contrast study outcomes, which hinders progress in the field. To begin to address this issue, the Mesenchymal and Tissue Stem Cell Committee of the International Society for Cellular Therapy proposes minimal criteria to define human MSC. First, MSC must be plastic-adherent when maintained in standard culture conditions. Second, MSC must express CD105, CD73 and CD90, and lack expression of CD45, CD34, CD14 or CD11b, CD79alpha or CD19 and HLA-DR surface molecules. Third, MSC must differentiate to osteoblasts, adipocytes and chondroblasts in vitro. While these criteria will probably require modification as new knowledge unfolds, we believe this minimal set of standard criteria will foster a more uniform characterization of MSC and facilitate the exchange of data among investigators.

Clarification of the nomenclature for MSC: The International Society for Cellular Therapy position statement.

The plastic-adherent cells isolated from BM and other sources have come to be widely known as mesenchymal stem cells (MSC). However, the recognized biologic properties of the unfractionated population of cells do not seem to meet generally accepted criteria for stem cell activity, rendering the name scientifically inaccurate and potentially misleading to the lay public. Nonetheless, a bona fide MSC most certainly exists. To address this inconsistency between nomenclature and biologic properties, and to clarify the terminology, we suggest that the fibroblast-like plastic-adherent cells, regardless of the tissue from which they are isolated, be termed multipotent mesenchymal stromal cells, while the term mesenchymal stem cells is used only for cells that meet specified stem cell criteria. The widely recognized acronym, MSC, may be used for both cell populations, as is the current practice; thus, investigators must clearly define the more scientifically correct designation in their reports. The International Society for Cellular Therapy (ISCT) encourages the scientific community to adopt this uniform nomenclature in all written and oral communications.

Mesenchymal Stem Cells Inhibit and Stimulate Mixed Lymphocyte Cultures and Mitogenic Responses Independently of the Major Histocompatibility Complex

We aimed to study the effects of mesenchymal stem cells (MSCs) on alloreactivity and effects of T‐cell activation on human peripheral blood lymphocytes (PBLs) in vitro. MSCs were expanded from the bone marrow of healthy subjects. MSCs isolated from second to third passage were positive for CD166, CD105, CD44, CD29, SH‐3 and SH‐4, but negative for CD34 and CD45. MSCs cultured in osteogenic, adipogenic or chondrogenic media differentiated, respectively, into osteocytes, adipocytes or chondrocytes. MSC added to PBL cultures had various effects, ranging from slight inhibition to stimulation of DNA synthesis. The stimulation index (SIu2003=u2003(PBLu2003+u2003MSC)/PBL) varied between 0.2 and 7.3. The SI was not affected by the MSC dose or by the addition of allogeneic or autologous MSCs to the lymphocytes.

Immunomodulation by mesenchymal stem cells and clinical experience

Mesenchymal stem cells (MSCs) from adult marrow can differentiate in vitro and in vivo into various cell types, such as bone, fat and cartilage. MSCs preferentially home to damaged tissue and may have therapeutic potential. In vitro data suggest that MSCs have low inherent immunogenicity as they induce little, if any, proliferation of allogeneic lymphocytes. Instead, MSCs appear to be immunosuppressive in vitro. They inhibit T‐cell proliferation to alloantigens and mitogens and prevent the development of cytotoxic T‐cells. In vivo, MSCs prolong skin allograft survival and have several immunomodulatory effects, which are presented and discussed in the present study. Possible clinical applications include therapy‐resistant severe acute graft‐versus‐host disease, tissue repair, treatment of rejection of organ allografts and autoimmune disorders.

Immunomodulatory effects of fetal and adult mesenchymal stem cells

Mesenchymal stem cells (MSC) derived from adult BM or fetal liver form several mesenchymal tissues after appropriate stimulation. Reports indicate that MSC have unique immunologic properties, making them ideal for cellular therapy. MSC are not immunogenic, they do not stimulate alloreactivity, and they escape lysis by cytotoxic T-cells and natural killer (NK)-cells. Thus, MSC may be transplantable between HLA-mismatched individuals without the need for host immunosuppression. Furthermore, adult MSC appear to be immunosuppressive as they reduce alloreactivity and the formation of cytotoxic lymphocytes in vitro. In vivo, adult MSC prolong the time to rejection of mis-matched skin grafts in baboons. The immunosuppressive properties of first trimester fetal MSC are less pronounced, but inducible with IFNγ. These findings imply a potential role for MSC, not only in the repair of damaged tissues, but also in the manipulation of immune responses.

Mesenchymal stem cells: progress toward promise

Despite having access to embryonic stem cells, many laboratories choose to study adult stem cells, not because of philosophical reasons but because of the practical aspects and day-to-day progress necessary for developing cellular therapeutics. There is certainly the ethical desire and responsibility to provide patients with therapies where few options exist. Multipotential cells have been isolated from adult tissues in many laboratories, characterized and their multipotentiality examined. Mesenchymal stem cells (MSC) can be isolated from several tissues but easily accessible BM seems to be the most common source. These adult stem cells may not be as powerful or diverse as embryonic stem cells may one day become, but at present they offer many advantages for developing cellular therapeutics: ease of isolation, expansion potential, stable phenotype, shippability, and compatibility with different delivery methods and formulations. Their potential use as cellular therapeutics has prompted the investigation of interactions of allogeneic MSC with the immune response. The great importance of cardiovascular medicine has demanded that MSC also be tested in this discipline. We believe MSC continue to provide a substantial scientific and therapeutic opportunity, and have reviewed some of the recent developments in the field.

Mesenchymal Stem Cells Inhibit the Expression of CD25 (Interleukin-2 Receptor) and CD38 on Phytohaemagglutinin-Activated Lymphocytes

Mesenchymal stem cells (MSC) are immunomodulatory and inhibit lymphocyte proliferation. We studied surface expression of lymphocyte activation markers and secreted cytokines, when lymphocytes were activated in the presence of MSC. MSC suppressed the proliferation of phytohaemagglutinin (PHA)‐stimulated CD3+, CD4+ and CD8+ lymphocytes. MSC significantly reduced the expression of activation markers CD25, CD38 and CD69 on PHA‐stimulated lymphocytes. Mixed lymphocyte culture (MLC) supernatants containing MSC suppressed proliferation of MLC and PHA‐stimulated lymphocytes dose‐dependently. MSC secrete osteoprotegerin (OPG), but not hepatocyte growth factor (HGF) or transforming growth factor‐β (TGF‐β). Stromal‐cell‐derived factor‐1 (SDF‐1) is not expressed on the cell surface. A recent report suggested that T‐cell suppression by MSC is mediated by HGF and TGF‐β. MSC suppression was not restored by the addition of neutralizing antibodies against SDF‐1, OPG, HGF or TGF‐β, alone or in combination. Addition of guanosine to PHA‐stimulated lymphocyte cultures containing MSC did not affect lymphocyte proliferation. The immunosuppressive effects of cyclosporine and MSC did not interfere, when present in the cultures of PHA‐activated lymphocytes. In summary, human MSC suppress proliferation of both CD4+ and CD8+ lymphocyte and decrease the expression of activation markers.

Analysis of Tissues Following Mesenchymal Stromal Cell Therapy in Humans Indicates Limited Long-Term Engraftment and No Ectopic Tissue Formation†‡§

Mesenchymal stromal cells (MSCs) are explored as a novel treatment for a variety of medical conditions. Their fate after infusion is unclear, and long‐term safety regarding malignant transformation and ectopic tissue formation has not been addressed in patients. We examined autopsy material from 18 patients who had received human leukocyte antigen (HLA)‐mismatched MSCs, and 108 tissue samples from 15 patients were examined by PCR. No signs of ectopic tissue formation or malignant tumors of MSC‐donor origin were found on macroscopic or histological examination. MSC donor DNA was detected in one or several tissues including lungs, lymph nodes, and intestine in eight patients at levels from 1/100 to <1/1,000. Detection of MSC donor DNA was negatively correlated with time from infusion to sample collection, as DNA was detected from nine of 13 MSC infusions given within 50 days before sampling but from only two of eight infusions given earlier. There was no correlation between MSC engraftment and treatment response. We conclude that MSCs appear to mediate their function through a “hit and run” mechanism. The lack of sustained engraftment limits the long‐term risks of MSC therapy. STEM CELLS2012;30:1575–1578

Mesenchymal stem cells stimulate antibody secretion in human B cells.

Mesenchymal stem cells (MSC) have immunomodulatory effects and inhibit T‐cell responses to alloantigens and mitogens in vitro and in vivo. We wanted to examine the effect of MSC on human B cells. MSC stimulated IgG production, measured in an enzyme‐linked immunospot (ELIspot) assay in blood and spleen lymphocytes. MSC only induced a low proliferation. When a semipermeable membrane separated MSC and mononuclear cells, the IgG production was stimulated in unfractionated lymphocytes. In contrast, enriched B cells required cell contact with MSC to produce IgG. Co‐cultures of MSC and lymphocytes increased IFN‐γ production. MSC produce IL‐6, and addition of MSC to spleen cells dramatically increased IL‐6 levels. After lymphocyte stimulation with lipopolysaccharide (LPS), cytomegalovirus or varicella zoster virus, MSC either stimulated or inhibited IgG response, depending on the level of stimulation by LPS or the viral antigens. Similar results were obtained for enriched B cells. To conclude, MSC stimulate B‐cell antibody secretion. The IgG secretion by activated B cells may be stimulated or inhibited by the addition of MSC, depending on the level of stimulation.

Mesenchymal stromal cells: tissue repair and immune modulation

BM-derived mesenchymal stromal cells (MSC) differentiate along the mesenchymal lineage to bone, fat and cartilage. In vitro, MSC induce little, if any, proliferation of allogeneic lymphocytes. MSC inhibit the proliferation of activated T cells and the formation of cytotoxic T cells. In vivo, they appear to have anti-inflammatory effects. Preliminary studies suggest that MSC preferentially home to damaged tissue and therefore have therapeutic potential. Possible clinical indications include therapy-resistant severe acute GvHD, treatment of rejection of organ allografts and autoimmune disorders.