Abstract

By definition, stem cells are capable of self-renewal, proliferation and differentiation. Stem cells can be categorized based on their tissue of origin and their differentiation potential (pluripotent or multipotent). Pluripotent stem cells are capable of differentiating into all cell types of the body whereas multipotent stem cells have a more limited differentiation. Pluripotent stem cells include embryonic stem cells and induced pluripotent stem cells. Multipotent stem cells include hematopoietic stem cells and mesenchymal stem cells (MSC). Most all tissues of the body also have a population of resident tissue stem cells ensuring tissue-specific cell replacement (i.e., gastrointestinal or hepatic). Current “stem cell products” include culture expanded, adult-derived MSCs, suspensions of embryonic-like cells as well as nucleated cells from tissues including fat (stromal vascular fraction [SVF]) or bone marrow mononuclear cells. In this talk, there will be a basic review of different stem cell types; however, the emphasis will be on MSCs as they are the most common stem cell being used in current medical clinical applications.

MSCs are defined by their ability to adhere to plastic and be induced to undergo tri-lineage differentiation in vitro, notably adipogenic, osteogenic and chondrogenic differentiation, and by a specific surface protein expression (Cluster of Differentiation [CD] CD90+, CD105+, CD29+, CD45-, CD34-). MSCs with these characteristics have been isolated from almost every tissue. Both in human and veterinary medicine, MSCs promote tissue regeneration and return to function by dampening the immune response, decreasing inflammation, increasing blood flow, secreting growth factors to support resident stem cell populations and decreasing local cell death. They promote normal healing rather than scarring mostly by secreting soluble mediators. In veterinary medicine, MSCs are harnessed primarily for their immunomodulatory functions (inflammatory and immune-mediated diseases) and their tissue reparative properties (orthopedic injuries, spinal cord repair).

MSCs have been shown to communicate with nearly all cells of the immune system. Once MSCs are activated by the inflammatory environment, they decrease proliferation and activation of pro-inflammatory immune cells and promote expansion of inhibitory and immunomodulatory cells. Depending on the animal species and tissue source of MSCs, MSCs secrete a variety of soluble mediators including prostaglandin E₂ (PGE₂), interleukin-6 (IL6), transforming growth factor beta (TGFβ), nitric oxide (NO), hepatocyte growth factor (HGF) and indoleamine 2,3-dioxygenase (IDO). Overall, the administration of MSCs inhibits T-cell proliferation, alters B-cell function, down-regulates MHC II and inhibits dendritic cell maturation and differentiation. These anti-inflammatory properties provide the rationale to use MSCs for clinical trials for diseases including osteoarthritis, and inflammatory bowel, liver and pulmonary diseases. Their immunomodulatory properties, notably their regulation of T cell activation and phenotype, drives the use of MSCs for clinical trials involving autoimmune diseases or diseases that result from chronic antigenic stimulation including feline chronic gingivostomatitis and immune-mediated dermatologic diseases. The stimulation of local progenitor cells and the secretion of potent growth and anti-apoptotic factors are likely partially responsible for their role in tendon, cartilage and ligament healing. MSCs can also be activated by microbes, including a wide range of bacteria and viruses. Engagement with bacteria may prime or enhance MSC functions including stimulating neutrophil or macrophage phagocytic activity and priming MSCs to be even more potent immunomodulatory agents. This interaction with microbes suggests that MSCs may be indicated for infected as well as inflamed lesions (they have been investigated for the treatment of sepsis in rodent models). MSCs are also being used in neural repair in veterinary medicine including spinal cord injury. They are thought to enhance neural repair by decreasing T cell activation, increasing neural blood flow, enhancing remyelination, reducing local cell death, secreting neurotrophic factors and increasing neuronal differentiation.

Stem cells have also attracted significant interest for tissue engineering purposes due to their ability to differentiate into tissues of mesodermal lineage (bone, tendon, cartilage, etc.). For these tissue engineering applications, stem cells are generally administered within scaffolds and with additional growth factors. Mechanical cues, including pre-conditioning with mechanical stimulation, are increasingly considered for the generation of orthopedic tissues. Various scaffold printing technologies, including three-dimensional (3D) printing permit complex scaffold manufacturing applications.

A deeper understanding of how MSCs function in vitro and in vivo helps us design rational, focused clinical trials to improve outcome for veterinary patients and inform human clinical trials. During the session, I will provide an overview of some current clinical trials using MSCs in veterinary medicine and how we harness how MSCs function in vitro to target our trials towards naturally occurring diseases in veterinary patients. Some of the targeted disease processes could be very relevant and translatable to captive wild animals. Working in collaborative teams of research scientists and clinicians, we can begin to answer questions on appropriate cell dose (including the number of doses), route of administration (intravenous, intralesional, regional), timing of cell administration (early, acute versus more delayed treatment), cell tissue source (product, autologous or allogenic), and cell distribution using in vivo cell labeling techniques. Ideally, cell products should meet minimal release criteria including cell phenotype, cell number, cell viability, product sterility and, ideally, some measure of cell function for the intended use (differentiation, suppression of lymphocyte proliferation, etc.).