Mesenchymal Stem Cells

Adult stem cell is the term used to describe postnatal stem cells that persist throughout life and function to repair or replace cells within certain tissues in response to traumatic events or natural cell turnover. Adult stem cells have been identified in many organs and tissues, including brain, bone marrow, peripheral blood, blood vessels, skeletal muscle, skin, teeth, heart, gut, liver, and the reproductive organs of males and females. They are thought to reside in a specific area of each tissue (called a “stem cell niche”). Among tissues identified to harbor stem cells throughout postnatal life, one of the most studied is bone marrow, which serves as a source of both hematopoietic stem cells (HSCs) and MSCs.

Stem cells may remain quiescent (nondividing) for long periods of time until they are activated by disease or tissue injury or by a normal need for additional cells to maintain tissues. When needed, these stem cells may differentiate into (1) HSCs, which give rise to all the types of blood cells (e.g., red blood cells [RBCs], white blood cells [WBCs]) including cells that regulate immune function; (2) MSCs, which give rise to a variety of cell types including osteocytes, chondrocytes, adipocytes, and other kinds of connective tissue cells (e.g., tendon) (discussed later); (3) neural stem cells, which give rise to its three major cell types: neurons, astrocytes, and oligodendrocytes; (4) epithelial stem cells in the lining of the digestive tract, which occur in deep crypts and give rise to absorptive cells, goblet cells, Paneth cells, and enteroendocrine cells; (5) skin stem cells, which occur in the basal layer of the epidermis and at the base of hair follicles; (6) epidermal stem cells, which give rise to keratinocytes; and (7) follicular stem cells, which give rise to both the hair follicle and epidermis.

Strategies that utilize these cells for therapy are known as cell-based therapies. The best-known clinical application of cell-based therapies is bone marrow transplantation to repopulate the marrow and restore blood cells after high-dose chemotherapy or radiotherapy. However, because the need for donated organs and tissues often used to replace ailing or destroyed tissue far outweighs the available supply, the use of stem cells directed to differentiate into specific cell types is becoming increasingly important. Stem cells offer the possibility of a renewable source of replacement cells and tissues to treat many diseases including spinal cord injury, burns, heart disease, diabetes, osteoarthritis, and rheumatoid arthritis. Blood transfusions, fresh frozen plasma, platelet-rich plasma, cryoprecipitates, and blood substitutes are discussed on page 36 and Table 4-5.

There are three basic methods for obtaining and preparing stem cells for administration. (1) The simplest technique is to collect fat or bone marrow and centrifuge it to concentrate the cells. This produces a mixture of both stem and nonstem cells and generally results in a relatively low yield of stem cells. Techniques that rely on centrifugation generally produce a dilute MSC population (1 in 10,000 to 1,000,000 of the nucleated cells are stem cells). This is the method currently being used for production of stem cells for veterinary patients. In humans, flow cytometry, based on MSC reactivity to a number of antibodies, is increasingly being used to select for expressed antigens to remove nonmesenchymal cells from the sample. For instance, antibodies against CD34, a surface marker found on hematopoietic cells, are frequently used to identify and remove nonmesenchymal cells from a marrow culture. (2) A second technique is to isolate the cells and treat them with growth factors/cytokines to stimulate their differentiation into the cell type of interest (e.g., chondrocyte, cardiac myocyte) (Fig. 14-2). Osteogenic differentiation of MSCs can be induced in vitro by treating a monolayer culture with a pro-osteogenic cocktail such as dexamethasone, ascorbic acid-2-phosphate, and beta-glycerophosphate. For chondrogenic differentiation, bovine insulin, transferrin, selenious acid, linoleic acid, bovine serum albumin, sodium pyruvate, proline, L-glutamine, or TGF-beta1 may be added. Recently, MSCs have been shown to have the ability to form muscle cells and cardiomyocytes when treated with the demethylating agent 5-azacytidine. This technique may still result in a relatively low yield of stem cells (albeit predifferentiated cells). (3) The most complex technique (which is considered to be the gold standard) is to obtain the cells, expand the cell population in culture, and then treat them with growth factors. This latter technique, in which an expanded cell population is administered, results in significantly greater cells being given. Billions of MSCs can be produced from a small bone marrow aspirate.

Numerous factors may affect the number and function of stem cells, including the age and physical status of the donor, tissue source (i.e., fat, bone marrow), method of harvest, and manipulation, handling, and storage of the sample. It has been shown that the proliferation and chondrogenic differentiation capabilities of a 4-month-old dog are higher than those in a 12-month-old dog (Li et al, 2007).

Autologous versus Allogeneic Stem Cells

Mesenchymal stem cells are multipotent adult stem cells that can differentiate into different tissues originating from mesoderm, ranging from bone and cartilage to cardiac muscle. MSCs are an excellent candidate for cell therapy because they are easily accessible (e.g., fat, bone marrow, skin), their isolation is straightforward, they can be greatly expanded (thus, many cells can be derived from a single donor), they can be biopreserved with minimal loss of potency, and there appears to be no significant adverse reactions to allogeneic versus autologous MSCs transplants. Therefore, MSCs are being explored to regenerate damaged tissue and treat inflammation, resulting from cardiovascular disease (e.g., myocardial infarction), brain and spinal cord injury, diabetes, cartilage defects, bone injury, and immune-mediated diseases (e.g., Crohn’s disease, graft-versus-host disease). Stem cells appear to have potent immunosuppressive properties, allowing them to modulate the function of all major immune cell populations, thus impeding immune responses; however, the underlying mechanisms of their differentiation and function are not thoroughly understood.

MSCs may be autologous (from self), allogeneic (from a donor of the same species), or xenogeneic (from another species). Although human MSCs have been shown to maintain their multipotential capacity after transplantation into sheep, the immune response to xenografts (adult) is much more vigorous than to allografts (Yang, 2004) because of the greater antigenic differences that exist among different species than within a species. Thus, most current research is being done on allogeneic and autologous MSCs. MSCs represent an advantageous cell type for allogeneic transplantation because evidence suggests that MSCs are immune-privileged with low MHC I and no MHC II expression (Uccelli et al, 2006), therefore reducing risks of rejection and complications for transplantation. Although MSCs may be protected from the immune system, many authors believe that autologous MSCs are preferred over allogeneic stem cells in this respect. There are advantages and disadvantages of each approach. The Food and Drug Administration (FDA) does not currently regulate autologous stem cells in veterinary medicine because they are minimally manipulated (i.e., not expanded in cell culture) and are given back to the same animal from whom the tissue (e.g., bone marrow, adipose) came from. Lack of federal regulation makes commercial production of allogeneic stem products substantially cheaper and easier than allogeneic products, which are regulated. However, controversy exists as to the number of stem cells that are being returned to the patient and their activity when autologous processes are used. Several studies have provided evidence that the ability of stem cells to respond to environmental demands may be diminished during aging (Rao and Mattson, 2001); thus, patients that are old or sick and perhaps most in need of stem cells may have the fewest or least functional stem cells with which to repair their own tissues (Chambers et al, 2007). Several companies are currently marketing autologous stem cells in veterinary medicine for treatment of joint, bone, and ligament injuries of horses and dogs.

Much of the recent stem cell research (including clinical trials) in humans has investigated the use of allogeneic or autologous MSCs from bone marrow or adipose tissue or autologous (or sibling) use of umbilical cord blood (UCB). Umbilical cord blood is blood that remains in the placenta and in the attached umbilical cord after childbirth. Although UCB does contain stem cells, there are generally not enough stem cells in a single unit of cord blood to treat an adult patient; placenta has up to 10 times more stem cells than cord blood. In the United States, the Food and Drug Administration regulates UCB under the category of “Human Cells, Tissues, and Cellular and Tissue-Based-Products.” Umbilical cord blood is primarily being used clinically to treat blood disorders (blood cancers and genetic disorders of the blood), but safety and efficacy studies of UCB stem cells for certain therapeutic uses beyond these applications is currently under way (e.g., human clinical trials in brain injury and type 1 diabetes, preclinical studies for stroke, and hearing loss).

Use of allogeneic MSCs is currently being studied by numerous groups and has been widely tested in clinical trials of cardiovascular, neurologic, and immunologic disease in humans, with encouraging results (Malgieri et al, 2010). There are examples of MSC utilization in the repair of ischemic heart disease, chronic kidney disease, and inflammatory lung disease (e.g., acute lung injury, chronic obstructive pulmonary disease, asthma, and pulmonary hypertension) and in the treatment of chronic wounds; however, much research is still needed before using these cells is considered a routine part of clinical practice. Of concern is whether differentiation of MSCs will lead to induced immunogenicity and thus limit their long-term benefit in diseases such as myocardial infarction (Huang et al, 2010). MSCs may be administered with or without culture expansion. Because only a small number of the nucleated cells in bone marrow are capable of differentiating into bone, cartilage, or muscle, administration of expanded cells may be most beneficial.