Historically, blood transfusions were the first type of cell therapy. Bone marrow transplantation has also become a well-established protocol. Bone marrow transplantation is the treatment of choice for many kinds of blood disorders, including anemias, leukemias, lymphomas, and rare immunodeficiency diseases. The key to successful bone marrow transplantation is the identification of a good "immunologically matched" donor, who is usually a close relative, such as a sibling. After finding a good match between the donor’s and recipient’s cells, the bone marrow cells of the patient (recipient) are destroyed by chemotherapy or radiation to provide room in the bone marrow for the new cells to reside. After the bone marrow cells from the matched donor are infused, the self-renewing stem cells find their way to the bone marrow and begin to replicate. They also begin to produce cells that mature into the various types of blood cells. Normal numbers of donor-derived blood cells usually appear in the circulation of the patient within a few weeks. Unfortunately, not all patients have a good immunological matched donor. Furthermore, bone marrow grafts may fail to fully repopulate the bone marrow in as many as one third of patients, and the destruction of the host bone marrow can be lethal, particularly in very ill patients. These requirements and risks restrict the utility of bone marrow transplantation to some patients. Cell therapy is expanding its repertoire of cell types for administration.
Cell therapy treatment strategies include isolation and transfer of specific stem cell populations, administration of effector cells, induction of mature cells to become pluripotent cells, and reprogramming of mature cells. Administration of large numbers of effector cells has benefited cancer patients, transplant patients with unresolved infections, and patients with chemically destroyed stem cells in the eye. For example, a few transplant patients can’t resolve adenovirus and cytomegalovirus infections. A recent phase I trial administered a large number of T cells that could kill virally-infected cells to these patients. Many of these patients resolved their infections and retained immunity against these viruses. As a second example, chemical exposure can damage or cause atrophy of the limbal epithelial stem cells of the eye. Their death causes pain, light sensitivity, and cloudy vision. Transplantation of limbal epithelial stem cells for treatment of this deficiency is the first cell therapy for ocular diseases in clinical practice.
Several diseases benefit most from treatments that combine the technologies of gene and cell therapy. For example, some patients have a severe combined immunodeficiency disease (SCID) but unfortunately, do not have a suitable donor of bone marrow. Scientists have identified that patients with SCID are deficient in adenosine deaminase gene (ADA-SCID), or the common gamma chain located on the X chromosome (X-linked SCID). Several dozen patients have been treated with a combined gene and cell therapy approach. Each individual’s hematopoietic stem cells were treated with a viral vector that expressed a copy of the relevant normal gene. After selection and expansion, these corrected stem cells were returned to the patients. Many patients improved and required less exogenous enzymes. However, some serious adverse events did occur and their incidence is prompting development of theoretically safer vectors and protocols. The combined approach also is pursued in several cancer therapies.