Travis Block
Dominic Marino
BME 262 – Cell and Tissue Engineering
Professor Hani Awad

Cellular Therapy for the Treatment of Moderate to
Severe Traumatic Brain Injury

Introduction:

Traumatic brain injuries occur when external forces cause traumatic injury to the brain. They directly affect 1.5 million Americans annually, and annually result in 100,000 cases of life-long impairment with substantial loss of function. Today there are 3.2 million Americans living with long-term disability as a result of brain injury (Congressional Brain Injury Task Force, 2012). It is estimated that 2.5 to 6.5 million individuals live with consequences of TBI (National Institue of Health). In addition to the devastating effects on the families and friends of those injured due to cognitive, social, emotional, and behavioral effects; TBIs are also responsible for significant economic cost to society. Annual societal economic impact of TBIs has been estimated at $60 billion. This cost accounts for 29% of the total cost of all injuries in the United States (Max, MacKenzie, & Rice, 1991). Even though the NIH considers the burden of disease when allocating funding, acute treatments for TBI are still limited to attempting to control intracranial pressure and optimizing cerebral perfusion to prevent further edema, inflammation and cell death, and chronic treatment is limited to motor, cognitive, and behavioral rehabilitation (Varmus, 1999). While these treatments are limited, there have been significant advancements made to recognition, acute care, and rehabilitative strategies. The improvements have lead to increased survival rates. The increased rate of survival, however, has resulted in more survivors with significant life-long motor, cognitive, and social impairments. Currently, no therapies aim to salvage, support, repair or replace the damaged tissue (Harting, et al., 2008). Congress has formed a brain injury task force, and there are currently many opportunities for federal funding relating to the treatment of TBI. Because there are no FDA approved therapies, the field is wide open for innovation. It is estimated that TBI results in $60 billion annually in societal impact. In addition to the significant economic costs, TBI has recently received significant media attention due to large numbers of war heros and high profile athletes dealing with the long-term effects of TBI. Because of these factors, there is significant motivation to seek better solutions for treating TBI, but there are still no definitive answers. In this paper we discuss in detail the traditional clinical approaches, their shortcomings, the current state of the art, and where the science of treating TBI may be headed in the years to come.

Traditional Clinical Approaches:

Individuals with moderate to severe traumatic brain injury typically present with any combination of symptoms associated with head injuries including headache, vomiting, dilation of pupils, slurred speech, weakness, confusion, or complete loss of consciousness (National Institute of Neurological Disorders and Stroke, 2012). Because nothing can be done to reverse damage caused by the trauma, emergency treatment aims at restoring haemostasis to prevent any further damage. This includes supplying oxygen, managing blood pressure, and relieving intracranial pressure. If an individual survive the TBI, it is likely that they will require rehabilitation for an extended period of time. It is widely known that a ‘general’ treatment for TBI is unrealistic as each person presents varying level of injury as well as side effects (TBI.com). Programs of acute rehabilitation as well as long-term acute care aim to integrate the patient back into life without nursing care. The treatment aims to increase the person’s capacity to function in all aspects of family and community life. This includes the ability to interact with friends and family, as well as work in some capacity, and function normally in society with little or no supervision. The treatments are classified as either restorative or compensatory. Restorative training focuses on improving a specific cognitive function, and compensatory training focuses on adapting to the presence of a cognitive deficit (National Institute of Health). While these treatments are widely used, there is limited data supporting the efficacy of these approaches due to the heterogeneity of the disease. While data does exist concerning cellular plasticity of neurons and the limited ability of the brain to recover, the brain cannot fully recover from TBI.

Anatomy and Physiology of the Brain:

The Brain is one of the largest and arguably the most complex organs in the body. It contains billions of nerves communicating with one another via trillions of synapses. Together, these nerves are responsible for fulfilling the many duties of the brain including, thinking and voluntary movements, basic functions like breathing, coordination and balance, and integrating signals from different parts of the brain. The neurons, or nerve cells, are the cells responsible for processing and transmitting electrical and chemical signals (WebMD- Brain). When a Traumatic Brain Injury occurs many neurons can experience necrosis due to trauma, many more may experience necrosis due to the stress of the simultaneous firing of large numbers of signals in the brain, and a third volume can experience necrosis due to ischemia, when bleeding and swelling in the brain prevents some parts of the brain from receiving adequate oxygen and glucose for cellular metabolism. The necrosis of the neurons can disrupt function of the brain and lead to serious side effects ranging from death to personality changes and including every possibility in between. This is especially troublesome because of the unique nature of neurons. Neurons, unlike other cells in the body, stop dividing shortly after birth. Only in the subventricular and subgranular zones has a meaningful amount of neurogenesis been shown to occur. In many parts of the brain there are actually more cells at birth than there are in adulthood, because cells die during life due to various causes, but are not replaced by cell division. Because of this, damage done to the brain by TBI is permanent. If TBI is minor enough, effects of the damage may be so minor that they are undetectable (we kill brain cells every time we have a beer), but in moderate to severe TBI, there is little to no hope of restoring completely normal function without some therapy aimed at addressing TBI at the cellular or subcellular level. In order to address these issues we must consider ways to revitalize damaged tissue by restoring cell activity in the region, and creating an appropriate extracellular matrix so that new connections can be made between neurons. Adult neuronal cells are fully differentiated and permanently post-mitotic. Some neuronal progenitors are generated in the subventricular zone, but cell division is largely arrested by birth. In order to change this, factors must be introduced to the extracellular matrix to facilitate angiogenesis and exchange genes and proteins with existing neuronal cells. The extracellular matrix of a healthy adult brain is unique to the rest of the body. Many proteins present in large quantity in the rest of the body are essentially absent in the matrix in the brain. Because so much of the brain is extracellular matrix, it is important to consider the makeup of the native matrix when attempting to revitalize damaged tissue. The extracellular matrix should promote differentiation and division of new neurons, promote angiogenesis so that these new cells may be nourished, and promote the establishment of new connections between these cells and the existing neural cells (Erkki 1996).

Current State of the Art:

The current theory states that stem cell therapy induces angiogenesis. Angiogenisis is the proliferation of a network of blood vessels from pre-existing vessels. (www.cancer.gov) Initial tests to examine this theory have taken place in rats. The rats were treated with an intravenous injection of mesenchymal stem cells after induced ischemic stroke. Caplan saw elevated levels of VEGF and VEGF receptor two (VEGFR2) as well as increased angiogenisis. The VEGFR2 serves as a major receptor for the VEGF signaling pathway. Upon autophosphorylation, VEGFR2 becomes activated and recruits adaptor proteins. Signaling of VEGFR2 is necessary for VEGF proliferation and angiogenisis. (Doughner, 1994) The increased network of blood vessels was also seen to proliferate in the transition zone. Caplan concluded that a combination therapy of MSC with nitric oxide, significantly increased blood vessel diameter and endothelial cell proliferation. (Caplan et al., 2006) The success of the preclinical data has led to several clinical trials focused on cell therapy in neurological diseases. (Chen et al., 2004) No current treatments repair or replace the damaged tissue, but the evidence from phase 1 trial, suggest that cellular therapies have some potential for efficacy in the treatment of moderate to severe traumatic brain injury. The trials have been started using bone marrow derived mononuclear cells. The initial trials were completed for children between the ages of five and fourteen years of age. The study is testing the safety in using stored autologous human umbilical cord blood (HUCB) to treat pediatric patients that sustain a severe TBI and have not fully recovered. This trial is ongoing and is currently the only clinical trial examining cellular therapeutics for traumatic brain injury. Recent phase 1 trials are determining if autologous HUCB transplantation is safe and free of infusion related toxicity. These trials have started at the beginning of 2012. Phase two is set to follow phase one and determine if autologous HUCB transplantation improves post-TBI neuropsychological outcomes.

Routes of Administration and biodistribution of stem cells

One of the most common methods for administration for infusion of liquid into the body is intravenous therapy. IV therapy utilizes the subject’s vein network for ease of access to the blood circulation for therapy. Currently this method is the fastest and most thorough method to distribute fluids throughout multiple tissues. Although this method is quite effective, IV therapy does have its complications. The major drawback is first-pass pulmonary sequestration. Pulmonary sequestration is described as a portion of lung that has no recognized communication with the normal bronchial tree and receives its blood supply from one or more irrelevant arteries. First pass pulmonary sequestration prevents the majority of stem cells from accumulating in the tissue cavity of significance (Intrathecal Implantation, 2001). Mesenchymal stem cells are relatively large, activated, and express adhesion molecules, which lead to sequestration in the lungs. Recent advances have aimed to manipulate IV therapy to produce more desirable delivery by decreasing the effect due to pulmonary sequestration. Studies have isolated and purified mesenchymal stem cells and then infused with sodium nitroprusside. Sodium nitroprusside served as a vasodilator to the subject expanding the diameter of blood vessels. Nitroprusside pretreatment decreased the first-pass pulmonary effect and allowed an increase of delivered mesenchymal stem cells (Orlic et al, 2001). Direct implantation of mesenchymal stem cells provides a direct transfer of fluid to the site of desire. This offers the greatest load at the site of disease or injury. For traumatic brain injury, direct intracerebral placement of stem cells has had some success with the development of engraftment with stem cell migration to area of injury. Direct implantation does show some benefits over IV therapy in the amount of cell proliferation in the boundary and subventricular zone of the lateral ventricle. The localization of progenitor cells is evident of the success of direct implantation therapy. Direct implantation also has some drawbacks. Most notably is the method of delivery. This therapy requires very invasive techniques that could result in surrounding tissue damage as well as further tissue damage to the site of desire (Walker et al, 2009). Intraatrerial administration (IAA) is another chosen method for artery infusion of stem cells. IAA allows the administrator to localize the placement of stem cells. This method is more effective than intravenous therapy in its ability to bypass the high pulmonary first pass effect. IAA treatment of mesenchymal stem cells has shown neurological improvements. IAA’s major limitation is its potential to form a cerebral artery emboli. When MSC’s have been administered via IAA, Walczak noted a rapid decrease in blood flow associated with rapid death of stem cells. With the decreased blood flow, the amount of cells delivered decrease as well. Although this method does have some promising future for the localization of stem cells, the side effects limit the role of intraatreial MSC and patient safety.

Stem cells and Inflammation:

Stem cells are widely known for their ability to migrate toward sites of inflammation. This characteristic of stem cells is sought after as the migration of stem cells mediate inflammatory markers and control the overall inflammation of an injury (Lichtman, 2004). When MSC are combined with purified immune cells, Dr. Aggarwal observed an increase in population of the anti-inflammatory interulukin IL-4,10. While IL-4,10 increased, the amount of tumor necrosis factor-alpha(TNF-a) and interferon-gamma (IFN-g) were simultaneously decreased. IFN-g inhibits cellular proliferation while inhibiting leukocyte trafficking. IL-4,10 are cytokines that induce differentiation of effector T-cells. The effector T-cells stimulate cell immunity as they migrate to infection site and eradicate the source. As immunogenic factors are increasing, TNF-a and IFN-g are decreasing. As IL-4,10 increase, a shift in helper T cells from Th1 to Th2, increased IL-4,5,6,10,13. An increase in interleukins lessens the maturation of dendritic cells while simultaneously promoting its own profile as a Th2 T-cell. (Aggarwal and Pittenger, 2005) Although this finding does control the inflammation, suppressing the proliferation of T cells under the same conditions as decreased IFN-g, can result in a decrease in overall inflammation and immune response. This property would allow the maximum life of stem cells by reducing the cell damage.

Conclusion:

Recent efforts have shown the therapeutic benefits of stem cell therapy in the treatment of traumatic brain injury through preclinical data in rats. Using multiple types of stem cells derived from different sources have provided great advances in treatment, but their value in the human body is largely unknown. IV therapy is arguably the most attractive method of treatment, but the first pass pulmonary pass effect must be overcome to allow full circulation of stem cells. With the side effects presented with the various intervention methods and cell types, much work is needed to translate preclinical information into clinical to ensure the safety and efficacy of therapy.

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Figure 1 Image of the stem cells sequestered in the lungs by the pulmonary first pass effect. This is an obstacle to the intravenous delivery of stem cells that must be further studied.

Figure 2 Image showing effect of cell therapy on inflammatory immune response. There is an increase in anti-inflammatory cells and a decrease in in cytotoxic T cell production. (Walker et al., 2009)