Loss of tissue function, whether by disease or accident, remains a major health problem. Heart and brain injuries, for example, are two of the leading causes of death and disability throughout the world. In the United States, cardiac disease accounts for 40% of all deaths and is the leading cause of congestive heart failure (American Heart Association. Heart and Stroke Update. Dallas, Tex.: American Heart Association; 2003). Cardiac disease that leads to acute myocardial infarction or chronic myocardial ischemia can also cause significant degradation in cardiac function. If the ischemic episode is limited in severity or duration, then cardiomyocytes survive and are protected from further ischemic insult through several preconditioning mechanisms. However, with acute and prolonged severe periods of ischemia, cardiomyocyte death occurs (Kloner R A, et al., Consequences of brief ischemia: stunning, preconditioning, and their clinical implications: part 1. Circ. 2001; 104:2981-2989). Under normal conditions, adult human cardiomyocytes lack the capability to regenerate, and over time, damaged myocardial cells are replaced by connective scar tissue along with a compensatory hypertrophy of the remaining viable cardiomyocytes (Gulbins H, et al., Cell transplantation—A potential therapy for cardiac repair in the future? Heart Surg Forum 2002; Vol 5(4):E28-34). This replacement of infarct by scar tissue leads to a loss of functional myocardium within the ischemic area, a progressive remodeling of the non-ischemic area, or border zone, and an overall reduction in cardiac performance.
Stroke is the third leading cause of death in the United States and the number one cause of adult disability. Ischemic stroke caused by blood flow interruption to the brain due to blockage of an artery by a blood clot accounts for about 70-80 percent of all strokes. A loss of blood flow to the brain deprives an area of brain cells of oxygen and nutrients which results in cell death. Body functions controlled by the area of the brain that has been damaged are lost. These functions include speech, movement and memory.
Repair of injured tissue is a complex process that begins at the moment of injury and can continue for months to years. This process can be broken down into three major phases; inflammatory, proliferative and remodeling. (Witte M B, et al. General principals of wound healing. Surg Clin North Am 1997, 77:509-528).
The inflammatory phase is immediate and can last for 5 to 7 days. During this phase, if there is tissue damage and/or cellular disruption as with trauma, vasoconstriction occurs and a clot forms which serves as a temporary protective shield for the exposed or damaged tissues. The clot provides cytokines and growth factors released by activated platelets that initiate the wound closure process and chemotactic signals to recruit circulating inflammatory cells to the wound site. Vasodilation follows and phagocytosis is initiated.
The proliferative phase is next and can last up to three weeks. During this phase granulation commences and involves the formation of a bed of collagen by fibroblasts which results in the filling of the defect. New capillaries are formed in a process called granulation tissue formation, which is followed by contraction in which the wound edges come together to reduce the lesion. The last stage of the proliferative phase is re-epithelialization. In skin wound healing, keratinocytes move in all directions from a point of origin across a provisional matrix to cover the wound.
The final phase of tissue repair is the remodeling phase. It can last up to two years and includes the production of new collagen which continues to increase the tensile strength of the wound.
The immune system has been recognized as an important regulator of tissue repair. It is composed of two parts, humoral and cellular defenses. The humoral arm includes antibodies and complement and little is known about the role that this arm plays in the process of tissue repair. On the other hand, much is known of the cellular arm which includes neutrophils, macrophages and T lymphocytes. These cell populations migrate into the wound in an ordered timeframe and contribute to the repair process through the secretion of signaling molecules in the forms of cytokines, lymphokines and growth factors. (Witte M B, et al., General principals of wound healing. Surg Clin North Am 1997, 77:509-528). Neutrophils are the first cells to appear at the wound site and are responsible for phagocytosis and debridement. Macrophages are the next cells to migrate into the wound. They complete the inflammatory and debridement processes and deliver critical tissue repair cytokines and growth factors. T lymphocytes are the last cells to migrate into the lesion and appear during the proliferative phase and their role includes the downregulation of the inflammatory response and growth state as this phase of the process concludes (Barbul A., Role of T-cell-dependent immune system in wound healing. Prog Clin Biol Res 1988, 266:161-175). Not all cells in the immune system are believed to participate in the tissue repair process. The role of B cells (B lymphocytes) in tissue repair, for example, is unclear and is presumed by those knowledgeable in the field to be inconsequential since helper T2 cell cytokines and B lymphocyte activating factors have not been detected at the wound site. What little amount of evidence exists on the role of B cells in tissue repair suggests B cells have a pathogenic role (Zhang M, et al., Identification of a specific self-reactive IgM antibody that initiates ischemia/reperfusion injury. Proc Natl Acad Sci USA 2004, 101:3886-3891).
Instead, B cells are best known for the role they play in the production of antibodies. They are generated from hematopoietic stem cells (HSCs) throughout life, first in the fetal liver and then in the adult bone marrow. Cytoplasmic cascades are initiated in response to tissue microenvironment signals that result in altered expression of proteins required for B cell maturation. The mature bone marrow B cell expresses IgD on its surface membrane which protects it from self antigen induced death. This mature cell moves into the periphery where it can be activated by antigen to become either an antibody-secreting plasma cell or a memory B cell.
While treatment options available to patients who have lost tissue function have increased recently, these options remain limited in their effectiveness. New therapies that can limit the amount of cell death and restore loss of body function are greatly needed. An appealing concept for the treatment of tissue injury is cell-based therapy. Evidence of cells engrafting into the damaged tissue coupled with an improvement of function supports this approach. However, the most appropriate cell type has yet to be defined. While many groups are eager to begin treating patients with various cells, researchers are just now beginning to understand some of the mechanisms of how these cells repair injured tissue. What is needed is an identification of which cell, or combination of cells, is most appropriate for the repair of damaged tissue.
While many different cell therapy methods are being tried, the common goal in cell therapy is the introduction into injured tissue of a cell that is either functionally related to the targeted tissue, such as with delivery of skeletal myoblasts into damaged myocardium, or primordial cells (stem or progenitor cells) that are hoped will regenerate new tissue and structures thereby returning function to the injured organ. The bone marrow is a well understood source of stem cells for a variety of tissue but primarily for the blood system. Early attention was given to the bone marrow as a source of potentially therapeutic cells after several studies demonstrated that animals with labeled bone marrow cells that were subjected to a tissue injury such as a myocardial infarction were found to have some of these labeled bone marrow cells integrated into the healing tissue. However, while integration of bone marrow derived cells into healed tissue was demonstrated, many questions remain unanswered including what cell type from the bone marrow integrated into the tissue and the extent to which these cells contributed to the functional recovery of the injured tissue. Nonetheless, there was sufficient potential of a therapeutic effect for research to proceed in this field including human experimentation. Experimentation into bone marrow derived cell therapy has utilized either the entire bone marrow, also known as unfractionated bone marrow, or the isolation of the endothelial progenitor, hematopoietic (CD34+, AC133+) and nonhematopoietic (CDstro1+) stem cells contained within it. While the experimental use of unfractionated bone marrow, bone marrow derived progenitor, and stem cells continues, early results from their use have been disappointing due to only modest improvement or negative outcomes, questioning the relevance of the earlier animal experimentation and the therapeutic value of bone marrow derived cells.