Mesenchymal Cell Information Summary

Mesenchymal Cells: A Basic Review Mesenchymal Cells: A Basic Review

Mesenchymal Cells Information Summary* Edwin M. Horwitz, MD, PhD, Chairman ISCT Nonhematopoietic and Mesenchymal Stem Cell Committee

Mesenchymal cell biology and therapy are quickly expanding areas of investigation that has drawn intense scrutiny from the biomedical research community. The basic issue is that the biology is not well understood and therefore the literature is quite confusing. As such, a “Mesenchymal Cell Fast Facts” cannot be readily assembled. The following “Mesenchymal Cell Information Synopsis” is an effort to provide an up-to-date distillation of the field. I hope it is helpful.

*prepared in May 2002 Mesenchymal Cells: A Basic Review Mesenchymal Cells: A Basic Review

Introduction The last word…

Mesenchymal cell biology is a complex, rapidly evolving field. Obviously, there is much confusion regarding mesenchymal cells: Much of the confusion in this field results from the lack of stem cells, progenitor cells, or otherwise. In fact, these multipotent universally accepted defining characteristics of the mesenchymal mesenchymal cells may not be mesenchymal but simply stem cell. See the proceedings for the ISCT2000 workshop, multipotent cells. Also, do not forget that several groups report “Nonhematopoietic mesenchymal stem cells: what are they?” that nonadherent cells may differentiate to mesenchymal and published in Cytotherapy 2:387-388, 2000. other nonhematopoietic tissues. Finally, multipotent mesenchymal cells have been isolated from tissues other than marrow, e.g. Definition of Mesenchymal Stem Cells (or lack thereof…) adipose. The world of stem cell biology is rapidly evolving and spreading to virtually every tissue under study. Follow the As stated above, the definition of a mesenchymal stem cell is progress in Cytotherapy and other activities of our Society. unclear. A logical proposal for the definition of a marrow mesenchymal stem cell would be, “ the putative marrow cell that can self renew and give rise to a one or more mesenchymal tissues.” However, marrow stromal cells, the population of cells within which the mesenchymal stem cell is thought to exist, can also seemingly differentiate to tissues other than those that originate in the embryonic mesoderm, raising questions about the appropriateness of the term mesenchymal stem cell.

The current message is that there is no accepted definition of a mesenchymal stem cell, not even an operational one. In much of the literature, the adherent fibroblastic cells from bone marrow are termed mesenchymal stem cells or mesenchymal progenitor cells; however, this heterogeneous population is recognized, among leading investigators, to be too “crude” to be considered all mesenchymal stem cells and the thinking about these cells will likely be refined over the next few years

Nonhematopoietic Stem Cells

Some investigators use this term. Blood is also derived from the mesoderm, so that hematopoietic stem cells are technically, mesenchymal stem cells; however, the term for mesenchymal stem cells is usually reserved for nonhematopoietic stem cells of marrow. Therefore, the term nonhematopoietic stem cells has come into use. Mesenchymal Cells: A Basic Review Mesenchymal Cells: A Basic Review

The extraordinary importance such cellular therapy is likely to Adult Stem Cells have in the future of medicine underlies the importance of our Society to foster the development of cell therapy through The term adult stem cell is used for any stem cell derived from the laboratory research and clinical application. postnatal animal (especially the human) to differentiate it from embryonic stem cells. The adult stem cell is generally thought to Clinical trials to date be a multipotent stem cell, but is often used for any postnatally derived stem cell. Again, the main use of this term is to distinguish Four clinical trials have been reported, and many more are in the cells from embryonic stem cells. Mesenchymal stem cells are progress throughout the world. First, unmanipulated bone marrow adult stem cells. has been infused into children with a genetic disorder of bone, osteogenesis imperfecta (OI) in a study of BMT for children with History this disorder. This was the first study to demonstrate marrow mesenchymal cells could engraft outside of the bone marrow The concept of the mesenchymal stem cell was proposed by (Edwin Horwitz was the principal investigator). Second, Maureen Owen and colleagues based, in part, on the work of autologous mesenchymal cells have been infused in patients Alexander Friendenstein. The notion was popularized by Arnold undergoing PBSC autografting for breast cancer in an effort to Caplan in the late 1980’s and throughout the 1990’s. His enhance hematopoietic recovery. These investigators found that proposed mesenchymal stem cell was an adherent fibroblastic cell neutrophil engraftment was about somewhat faster than historical isolated by Percoll density centrifugation that expressed antigens controls (Omar Koc was the principle investigator). Third, reactive with monoclonal antibodies SH2 and SH3 (see below). autologous gene marked mesenchymal cells were infused into Over the past decade, the field has exploded with an extraordinary adults undergoing autografting for cancer and gene marked cells level of work focused on mesenchymal stem cells. The Caplan cell were found in the marrow 6-8 months after infusion, but not at 12 is not universally accepted as a mesenchymal stem cell. months. Although a small number of patients, this study is Moreover, many other cell preparations have been shown to have extremely important in that it is the first to document a time course mesenchymal progenitor activity; however no single cell type has for the duration of infused mesenchymal cells (Armand Keating emerged as the stem cell. was the principal investigator). Finally, isolated allogeneic gene marked mesenchymal cells were infused into the children with OI Controversies after their BMT (which was the first study described above). This study showed that the isolated cells also engraft outside the Since there is no universal antigenic definition of mesenchymal marrow space (Edwin Horwitz was the principal investigator). stem cells, analogous to CD34+ cells for hematopoietic stem cells, and there is no universal assay, analogous to hematopoietic repopulation assays for hematopoietic stem cells, several labs have developed novel methods of isolation of a mesenchymal cell with both self-renewal and multipotent differentiation capacity. Since each cell is unique, and no one lab is isolating cells by multiple methods and comparing them, it is difficult to state how these cells relate to one another. Additionally, several labs have reported that nonadherent cells can give rise to mesenchymal tissue and ISCT member Catherine Verfaille reported, at ASH Mesenchymal Cells: A Basic Review Mesenchymal Cells: A Basic Review

2001, that the (adherent) MAPC cells she has isolated seem to above. Thus, the method or preparation is often being used to give rise to blood. Finally, candidate mesenchymal stem cells define the cell and then the characteristic of that cell, such as self- have been reported to differentiate to tissues derived from renewal and multipotent differentiation are described. Of ectoderm and endoderm during embryonic development. Thus, importance, magnetic bead separation of mesenchymal cells from the whole concept of what is the true stem cell, especially the marrow is now under ongoing development. This technology mesenchymal stem cell is very unclear. depends on a know antigen defining the cells. The currently available system use an anti-CD105 antibody linked to the beads. As other antigens identifying mesenchymal stem/progenitor cells What we know and what we don’t know… are described, this technology will likely be more widely applied.

Morphology Transplantation

Mesenchymal cells are typically adherent marrow cells, generally Mesenchymal cells (stem or progenitor) may be infused considered to be spindle shaped, but some candidate cell intravenously and will engraft in the recipient. Whether the cells populations are more spherical and less spindle shaped. truly home to target tissues has not been completely determined. Furthermore, the level of engraftment within the marrow space Antigenic determinants and other tissues and the duration of biologic activity after engraftment is not yet fully understood. These issues are There are no universally accepted antigenic determinants. The analogous to hematopoietic stem cells homing to the marrow SH2 and SH3 antibodies originally developed by Caplan and space, giving rise to enough terminally differentiated tissue (i.e. colleagues recognize CD105 and CD73, respectively. Many blood) to maintain normal function, and persisting (as far as we investigators have explored CD105 as an important antigenic know) for the life of the recipient. determinant in the identification of mesenchymal stem cells. Pittenger has reported that CD29, CD44, and CD90 are important Differentiation determinants, while Prockop and colleagues have reported that their RS cells cannot be clearly distinguished from other adherent Mesenchymal progenitor cells can clearly give rise to mesenchymal cells solely by antigen expression. Simmons hematopoietic supportive stroma, bone, cartilage, fat, and skeletal described STRO-1 antibody that also seems to identity an muscle in experimental animal systems. There have also been immature population of mesenchymal cells. Other determinants reports of these cells giving rise to cardiac myoctyes and neurons, have been studied and interested persons should see Cytotherapy but this has yet to be widely accepted. 2:387-388, 2000. Applications Isolation The goal of mesenchymal cell therapy is to treat diseases of The first step in isolating mesenchymal cells is usually density nonhematopoietic tissues in an analogous fashion to treating centrifugation (e.g. Ficoll) to obtain the mononuclear fraction of leukemia or aplastic anemia with hematopoietic stem cell marrow cells. After that adherence to plastic is typically involved transplantation. Virtually any tissue may be amenable to cellular as this is one of the more general characteristics. After these therapy, and only preclinical models and clinical trials will identify steps, the approach to isolation varies widely, as suggested the susceptibility of a given tissue and disease to such therapy.