Autologous Cell Therapy: Current Treatments and Future Prospects
Abstract: Autologous cell therapy (ACT) is a novel therapeutic intervention that uses an individual’s cells, which are cultured and expanded outside the body, and reintroduced into the donor. Advantages of such an approach include the minimization of risks from systemic immunological reactions, bio-incompatibility, and disease transmission associated with grafts or cells not cultivated from the individual. So far, this form of therapy has been used successfully to bioengineer skin substitutes, aid wound healing, counteract chronic inflammation, treat burns and pressure ulcers, and improve postoperative healing. The authors will review the promising outcomes of various therapeutic interventions using ACT, as well as the concerns raised with using explanted material, and any potential alteration through the cultivation process. This review will discuss its role in assisting the healing process of conditions such as a damaged myocardium, developing hyaline cartilage, and in the treatment of neurodegenerative diseases and other ailments that benefit from the immediate availability of a donor. The use of ACT for cosmetic enhancement or corrective surgery is also gaining recognition as a creditable form of treatment and has been shown to reduce the risk of rejection and to have longer lasting effects than conventional treatments. This form of treatment is under intense investigation with the hope that it will eventually be able to replace conventional forms of plastic surgery to improve the repair process of aging or damaged tissues.
Address correspondence to: Batool Kazmi, PhD Eastman Dental Institute University College of London 256 Gray’s Inn Road London WC1X 8LD Phone: 079 4046 0423 Email: firstname.lastname@example.org
Over the past few decades, since the bioengineering revolution, autologous cell therapy (ACT) has become a rapidly evolving field. From the discovery of plasmids as vectors for bulk protein cultivation in 1973, and the production of recombinant human insulin in 1978, to the construction of the world’s first artificial cornea in 1999, biosynthetic ailments to human conditions have been of great revelation and interest in the scientific industry. One of the greatest success stories of modern medicine has been the advent of organ transplantation. As life expectancy in the developed world increases, the expiration of body tissues and organs through attrition, disease, or even trauma, is inevitable. Despite its ability to either restore a normal standard of living or extend life, organ transplantation is plagued with its fair share of problems. Almost a victim of its own success, the body’s immune system can sometimes recognize the foreign entity and reject the new tissue or organ regardless of patient matching and immunosuppressive drugs. Such consequences and shortages in supply have increased interest in regenerative and autologous therapies. Although many cells respond to signaling and stimuli in the body, they can also be cultivated outside the human body in Petri dishes and culture flasks. Tissue engineering techniques allow cell types to be grown in isolation using defined media for optimal growth. Such a capacity allows for the culture and study of cells more closely and independently of the organ from which they are a part. More impressively, this enables a small number of cells taken from an individual to be expanded outside the body and reintroduced into the donor for therapeutic intervention. Autologous cell therapy is composed of various technologies and disciplines ranging from cellular and molecular biology to virology; however, this review will focus primarily upon cell and tissue based therapies. The use of ACT has been reported from as early as 1985 where autologous conjunctival transplants were used for the restoration of damaged ocular surfaces.1 Currently, this form of therapy has broad applications in modern medicine and plastic surgery ranging from the treatment of various ailments, improvement of wound healing, to life saving operations, and even cosmetic surgery.
Current Range of Application
The scope for ACT is vast and has immense potential for rejuvenation. Much of this form of therapy is currently under investigation or in clinical trials. These include the treatment of limb ischemia2; bone marrow mononuclear cell transplantation treatment for acute radiation victims3,4; ischemic stroke5,6 and ischemic heart disease7–9; cell-mediated immunotherapy after chemotherapy10; autologous hematopoietic cells as targets for gene transfer to treat various blood disorders11,12 and autoimmune diseases13,14; the use of autologous macrophages to treat acute complete spinal cord injury15–17; cell therapy with autologous lymphocytes to treat various cancers18,19; cell therapy for inner ear cell degeneration20; and autologous serum for treatment of ocular disorders.21 Advantages. Some of the challenges facing any cellular transplant include the cells ability to integrate and function alongside resident cells without interfering with other cells or cellular reactions. The major disadvantage of using allogeneic grafts or transplants from non-self is the possibility of systemic immunological reactions. At best, such a response could be uncomfortable and at worst life-threatening, either of which would require a lifetime of immunosuppressive drug therapy. Another concern is the risk of disease infection from animal transplants; for example, porcine endogenous retrovirus (PoERV) has caused concerns with xenotransplantation. Present in the genome of all pigs, this virus is genetically transmitted and is able to infect human cells.22 Concerns surround the release of the virus from the xenograft, subsequent infection of host cells, and eventual production of lymphomas. Disadvantages. Commercial demands for any biotechnological practice require that the process and products are cost-effective, fit for purpose, and either easily produced or accessible. Cell cultivation often employs the use of undefined components, such as animal serum, in culture media in order to accomplish desired cell growth while controlling costs. The potential risks are that animal-derived products will contaminate or alter cells, and the inability to categorically define the nature and expression of biomolecules and factors that are present. Other aspects of tissue culture being called in to question include the use of various growth factors, extrinsic factors, such as temperature, CO2, and humidity, which may influence the metabolic activity of the cells and therefore the function, specificity, and efficacy. One significant disadvantage of ACT is that it is not immediately available for treatment in many cases. A significant lag exists between the extraction and cultivation of the cells and their use, which is dependent on the type of treatment. Ultimately, this treatment is not rapid and relies heavily upon cell growth and cultivation; any problems with this process could further delay treatment by weeks. In some cases, a temporary measure would also be required to subdue symptomatic problems and pain in the interim. Treatment of heart disease. In addition to skin tissue or organs transplants, autologous blood vessels and artery transplants have been used for a considerable length of time. Heart bypass surgery has been the gold standard surgery for treating coronary artery disease. Relatively safe, a segment of healthy vessel is harvested from either the leg (saphenous vein), from the inside of the breast (internal mammary artery), or from the forearm (radial artery), which is used to connect the aorta and the coronary artery beyond the blockages. High success rates can mostly be attributed to the autologous nature of the transplant. Such results have inspired work for further autologous treatments. Due to a population of mainly terminally differentiated myocytes and an insufficient regenerative response, the adult heart is rendered inept to fully repopulate any reduced cardiomyocyte levels after, for example, a myocardial infarction.23,24 Autologous treatments available involve taking skeletal muscle from the patient’s back or abdomen to be enveloped over the weak heart, and subsequently using a device similar to a pacemaker, to electrically stimulate the transplant. Autologous cells can also be transplanted to aid an ailing heart and involve the use of bone marrow derived stem cells or skeletal myoblasts. Although such work is still undergoing clinical examination, the aim is to replace the damaged myocardium or to assist in the healing process.25–28 Many practical considerations regarding cell therapy remain, such as the timing of transplant into the post infarct heart, whether the patient is fit to undergo surgery, and the method of cell delivery (injection into the coronary sinus, artery or directly into the damaged myocardium).29 The advantage of using skeletal myoblasts over other cell types is that they can be obtained from muscle with relatively little stress put upon the patient after heart trauma. The harvest of hematopoietic stem cells, on the other hand, requires numerous painful bone marrow biopsies that may be questionable in the days or even weeks following a major heart attack. The long-term outcomes of these transplants have yet to be assessed, as this form of treatment is in its developmental phase. However, as with any heart surgery, there are associated risks of arrhythmia, cardiac arrest, pain, or death. Advances in the potential of stem cells to induce tissue regeneration and neovascularization has increased interest for use in treatment of cardiovascular diseases.30,31 The injection of autologous mononuclear bone marrow cells into areas of ischemic myocardium has been shown to improve the condition of a patient during phase I clinical trials.10,11,30 Moreover, tissue engineering uses a defined and specific cell population cultivated for a purpose, and therefore, unlike surgical reconstruction using other body tissues, these cultivated tissues can reinstate lost function to a better degree. Effectively, these cells can also be cultivated and combined with gene therapy, removing or rectifying deficient genes to correct the operative defect.
Commercially Available Autologous Treatments
Examples of commercially available autologous treatments are discussed throughout this review (Tables 1, 2). This section briefly discusses two such successful treatments. Commercially available since 1987, epidermal autografts consisting of autologous keratinocytes co-cultured with irradiated murine cells has been on the market (Epicel®, Genzyme Biosurgery, Cambridge, MA).32,33Successful treatment and coverage of large burn wounds has been reported (Table 1). In 1997, the US Food and Drug Administration (FDA) approved a product consisting of autologous cultured chondrocytes (Carticel®, Genzyme Biosurgery). Derived from the in-vitro culture of a patient’s normal femoral articular cartilage, this product has shown clinical benefits in developing hyaline cartilage that is lost in the course of acute or recurring trauma.34 More than 5000 cases have been reported since the clinical use of autologous chondrocyte transplantation in the treatment of articular defects, most outcomes after a 3-year follow up.35,36 However, adverse reactions, most commonly hypertrophic tissue development, have been reported both intraoperatively and postoperatively.
Cell Therapy and Neurogenerative Diseases
Alzheimer’s disease. The treatment of neurodegenerative diseases has also been an avenue of research for autologous therapies. Cholinergic neuron and synapse loss is a core feature associated with the neurodegenerative disease, Alzheimer’s. Great interest surrounds the use of gene transfer into autologous cells to treat Alzheimer’s disease, as well as spinal cord injury.37–40 In an uncontrolled Phase I clinical trial, primary autologous fibroblasts obtained from skin biopsies were genetically modified to secrete human nerve growth factor and transplanted into the brain of eight individuals.41 The authors reported significant clinical benefits such as improved memory, reduced cognitive deterioration and cholinergic degeneration, and amyloid over expression. No long-term adverse effects have been reported to date.41 The results of this clinical trial, despite not being able to reverse the condition, demonstrate a significant retardation in its progression. These results are promising if they can be recreated without a significant loss of life. Multiple sclerosis. Following intensive immunosuppression to destroy neurodegenerative immune cells, “antigen naïve” autologous hematopoietic cells (tolerant to the nervous system) are transplanted to repopulate the system. Despite being in the early stages of investigation, the use of such an immunosuppressive therapy has shown to halt progression and the results show longevity. These findings hold potential benefits for patients with active inflammatory disease before irremediable axonal damage occurs.42–45 Parkinson’s disease. Iris pigment epithelial cells have been investigated as a possible source for the replacement of degenerated dopaminergic neurons. Such cells have the capacity to secrete dopamine or dopamine precursors and can be obtained in ample amounts from the posterior layer of the retina.46
Autologous Cell Therapy and Wound Healing
Different types of autologous treatments exist that seek to improve wound healing, counteract chronic inflammation, and aid wound closure. Various medical conditions have benefited from such modes of therapy including the management of burns, pressure ulcers, postoperative healing, and various skin disorders. Bioengineered skin equivalents have been used to improve wound-healing outcomes and re-epithelialization in several clinical situations.47 The first living skin equivalent was developed in 1981 using sheets of cultured autologous keratinocytes to treat burn victims. Although not entirely successful due to problems with skin contracture, scarring, poor epithelialization, and lack of a dermal component, the development forefronted a new line of treatment by introducing the ability to produce large sheets of keratinocytes from a smaller section of skin.48 Since then, various non-autologous bioengineered substitutes have come onto the market such as Dermagraft® (Advanced Biohealing, Westport, CT), which consists of a dermal component only; and Apligraf® (Organogenesis, Canton, MA), which consists of both a dermal and epidermal layer derived from fibroblasts and keratinocytes originating from human neonatal foreskin (Table 1).47 Autologous material is utilized as both a stand-alone treatment or in composite with other materials. Biosynthetic composites using both autologous cells and synthetic or allogeneic materials (eg, Epicel) have been used successfully in the treatment of burns and in wound healing. Larger burns necessitate quick coverage to provide a barrier to infection and desiccation. The cultivation of autologous cells takes more time than is initially available, and therefore until final coverage can be achieved, such a composite mode of treatment holds significant benefit. Indeed, Integra® (Integra LifeSciences, Plainsboro, NJ) has been used as an immediate dressing to promote dermal regeneration while providing a receptive bed for autologous fibroblast addition, followed by epidermal application at a later date. The time for wound closure has reportedly correlated positively with the number of seeded fibroblasts. Sites that receive the greatest number of fibroblasts do not show scar lines, experience faster wound healing, and have faster restoration of functional movement.49 Indeed, it could be argued that it is the time lag between cultivation and the application itself that attributes to this treatment’s success. It could also be suggested that without a “cool-down” period, the transplanted cells could be destroyed in the hostile environment created during the inflammatory response. Composites that utilize synthetic or allogeneic components are also being employed for growth factor or autologous fibroblast delivery to promote dermal regeneration prior to epidermal seeding.50,51 Upon treatment comparison of full thickness skin defects with an allogeneic, acellular or autologous dermal substitute, the observed healing and scarring is reported to be markedly improved with autologous therapy and poorest with allogeneic dermal substitute therapy. Despite better results in terms of reduced scarring, less inflammation and fibroblast longevity being obtained with autologous seeded fibroblasts, allogeneic cell use is valuable for emergency or rapid treatment because the culture of autologous fibroblasts takes several weeks.50
Autologous Therapy and Cosmetic Surgery
Cosmetic enhancement or corrective surgeries are on the increase, and despite the availability of a multitude of injectable materials for soft tissue augmentation, the search for a more clinically efficacious and longer lasting dermal filler continues. Since as early as 1899, the use of materials for superficial soft tissue wrinkles and contour imperfections has been sought after. For many years, materials for the treatment of superficial facial tissue imperfections such as dermal depressions (pock marks, acne scars, etc.), and rhytids (contour defects) have involved the use of various autologous materials. Autologous fat transplants have been reportedly used for over a century, being first described by Neuber in 1893. However, due to a high reabsorption rate, a degree of necrosis at the donor site and necessary pre-treatment of the fat to remove any pro-inflammatory mediators have hindered the use of this procedure as a widely used corrective treatment.52 Currently, bovine collagen is the most common commercially available and administered material. This product, although not a stand alone, is used as the benchmark for all injectable materials that are already available on the market or currently in development. This soft implant receives excellent scores in terms of efficacy, longevity, cost effectiveness, and the magnitude and frequency of allergic response. The implant is not without its drawbacks—the effects do not last indefinitely (approximately 6–8 months due to reabsorption by the body) and require skin tests prior to injection in order to rule out allergic reaction. The advantage of autologous cellular fillers is that the potential for hypersensitivity reactions is eliminated, thus mandatory intra-dermal skin testing is not required. Previous studies have reported allergic reactions with collagen including local site abscess and granuloma formation in response to collagen implants.53–56 The pursuit for a new injectable material in the mid- 1990s spawned the research and development on Isolagen Therapy™ (Isolagen, Inc., Exton, PA), which consists of autologous dermal fibroblasts (Table 2).22 These cells, as major components of the dermis, are responsible for the synthesis and secretion of the extracellular environment (collagen, elastin, hyaluronic acid, and glycosaminoglycans), and when injected, can recognize and replenish self-diminished areas.52,56,57 Another cosmetic treatment currently in Phase II clinical trials is Trichocyte® (Intercytex Ltd, UK), which is composed of in-vitro cultivated dermal papilla cells to treat male androgenic alopecia and female diffuse alopecia (Table 2). These cells, found at the base of the hair shaft surrounding the follicle, have the potential to regenerate new hair follicles when injected intradermally into the scalp.58 This product is marketed primarily for administration in dedicated hair transplant centers by dermatologists or plastic surgeons. This product has the potential to take a large sector of the hair regeneration market if proven effective. Autologous therapy has also been used as a cosmetic treatment for vitiligo (Table 2). Occurring in approximately 1% of the population, this hypopigmentation disorder affects individuals indiscriminately. Various autologous transplantation techniques exist, including mini-grafting, epidermal blister grafting, split-thickness grafting, of which the latter has shown reduced scarring. Although a promising course of treatment, it still remains to be seen whether such treatment is a permanent solution to the underlying problem or just a temporary fix; further studies and investigations in to the plausibility of melanoma promotion are warranted.59 The application of ACT to cosmetic treatments is proving to be highly effective and increasing in demand. With reports appearing on television and in newspapers, public support and intrigue look to make this form of treatment a strong competitor for other corrective or cosmetic enhancement surgeries.
Much of the work and treatments discussed in this review are merely the tip of the iceberg. Autologous therapy is under vast exploration in pursuit of other avenues and ventures that may benefit from such an approach and immediate availability of a donor. As discussed earlier in this article, the future of autologous therapies lies in a multitude of tangents. One such avenue is that of “cell printing,” a novel endeavor uses a modified inkjet printer to create cellular libraries, as well as cellular constructs, and potentially whole organs.60–62 The use of such equipment could prove to be an asset for regenerative and reconstructive surgery and pardon the use of a lifetime of immunosuppressive drugs. Such an initiative could also eliminate the use of poor surgical substitutes, such as reconstruction of the oesophagus with the colon, which leads to poor post-operative quality of life,62,64 and where different cell types are unable to perform to maximum effect in a new environment. Regenerative medicine is now a global venture as opposed to being a predominantly US phenomenon. Although the interest in stem cell research and regenerative medicines is increasing, teething problems in this field combined with companies experiencing a boom and bust level of success, have made funding in this area largely private rather than public. The first products to make it on to the commercial market may not be revolutionary to medical practice, but are slowly beginning to make an impact. With millions of people every year undergoing cosmetic surgery, the market for treatments that have an increased longevity or improve healing are highly sought after. Success with the use of skin cultures from autologous cells replacing various forms of skin grafts is also promising. Fresh ideas, novel technology, and an increasing need continue to revitalize and strengthen interest in this field. Nevertheless, it may be that the use and availability of autologous therapies are governed more by factors such as labor intensity and cost rather than the success of the actual treatments.