Introduction

1.1  The Immune System

One of the major problems in organ or cell transplants is rejection of the transplanted material by the host organism. Transplants will be identified by the body as “non-self”. The immune system recognizes “self” from “non-self” in order to protect the body from disease, bacteria, etc. The system consists of a network of organs, several types of specialized cells, and the lymphatic circulatory system. These components work together to ensure that all foreign materials are recognized and eliminated from the body before they can cause us harm.

The lymphatic system is a critical part of immunity. It is similar to the circulatory system, being composed of lymph vessels, lymph nodes, and several organs. All organs which are part of the immune system are known as lymphoid organs. The most important of these are the bone marrow, spleen, thalamus, and lymph nodes. Lymph nodes are present all over our bodies and provide a meeting place for lymphocytes. There are several types of lymphocytes. The two major classes of lymphocytes are B cells and T cells.

The Activation of a B cell to Create an Ig

B cells become plasma cells which produce immunoglobulins (Ig), or antibodies when triggered by an antigen. These antibodies are released into the blood stream and the lymph stream. Each antibody is specific to one type of antigen, and will react only with this antigen. Antibodies attach themselves to antigens which they find in the body and mark them so that they can be recognized and destroyed by other lymphocytes. There are many different antibodies, each with a different purpose. Antibodies are the reason why our bodies build up immunity to illnesses. They are also the reason that vaccines work. When we inject a small amount of a disease into our bodies, our immune system is able to effectively combat the disease. In the process, our B cells produce antibodies specific to this ailment. The next time our body encounters this illness, it will be able to recognize it and will already have the right type of antibodies, so it will be more prepared to fight the infection.

T cells, the second major class of lymphocytes, are crucial to the immune response. In fact, they coordinate and regulate the overall immune response. When macrophages identify and devour antigens, they display a piece of the antigen on their surface as a marker. The first type of T cells, helper T cells, recognizes the antigen and binds to the macrophage. This bond stimulates helper T cells to produce interleukin-2 (IL-2) while the macrophage produces interleukin-1 (IL-1). IL-2 causes T cells to multiply which then signals B cells to multiply and to start producing antibodies. The second type, cytotoxic (killer) T cells, attack and kill infected cells thereby ridding the body of the harmful antigen.

1.2  Immunosuppressants 

As stated above, a key concern when transplanting cells or organs is that the transplanted material will be rejected by the immune system as “non-self”. In order to avoid this problem, immunosuppressant drugs are prescribed to prevent the immune system from recognizing the foreign material. Immunosuppressant drugs are used not only after a transplant, but also to treat certain other diseases such as rheumatoid arthritis and Crohn’s disease, which are named autoimmune disorders (meaning that the immune system is attacking the body itself).  The danger with these drugs is that they reduce the body’s ability to combat disease, so people taking these drugs are much more susceptible to infection.  

One example of an immunosuppressant agent is cyclosporin (CsA). Cyclosporin prevents the release of interleukin-2 by helper T cells so that they cannot signal the B cells to produce antibodies which would attack the transplanted material. CsA has been a widely used anti-rejection drug for many years. It is known to have several side effects including trembling hands and headaches. It also has long term effects, as it is frequently taken for several years. Despite these effects, it has been working well as an immunosuppressant drug for many years.

Another common immunosuppressant drug is tacrolimus (LTAC). Tacrolimus inhibits the activities of leukocytes. It thereby inhibits both B cells and T cells, as they are both types of leukocytes. It is stronger than cyclosporin, but reportedly has fewer side effects. LTAC must be capsulated in phospholipic liposomes, otherwise it will pass through the body too quickly. These liposomes regulate the release of the drug so that it remains in the system longer. LTAC is a newer drug, and less widely used than CsA.

Though CsA and LTAC have very similar immunosuppressive properties, they have other properties which differ, causing the differences between their potencies and side effects.

1.3  Stem Cells

In recent years, stem cells have come to the forefront of medical research as a potential source for cell restoration strategies aimed at repairing the brain. Stem cells are able to self renew through cell proliferation, the increase in the number of cells due to growth and division, and have not yet undergone differentiation; they do not have a specific function. Stem cells have the potential to become a certain type of cell, a skin cell, blood cell, brain cell, through differentiation.

Adult stem cells are found among already differentiated cells. They are most likely to differentiate into the type of cell that surrounds them, for example adult stem cells found in the brain may differentiate into neurons, astrocytes or oligodendrocytes. There are, however, cells known to transdifferentiate, or differentiate into cell types outside their area. For example,  hematopoetitic stem cells are able to differentiate into major brain cells. There are also pluripotent stem cells that are able to differentiate into multiple cells types.

Embryonic stem cells come from the embryo of an organism. They are totipotent; able to differentiate into any type of cell. Through division, embryonic stem cells are capable of reproducing a whole organism. There are many ethical issues surrounding the use of embryonic stem cells in research and patient treatment, therefore current research is gravitating towards the use of adult stem cells.

In many neurodegenerative diseases, physical disabilities occur because a certain cell necessary to carry out vital control functions is missing, for example, patients suffering from Parkinson’s disease (PD) lack the dopamine producing brain cells that regulate smooth motor control. The possibility of isolating, harvesting and manipulating adult stem cells to differentiate into a desired cell type in vitro and then transplanting the cells into a patient in order to repair the brain, has emerged as a possible therapy for diseases. Additionally, if the adult stem cells to be transplanted were taken from the patient, differentiated in vitro and transplanted back into the patient, rejection would not be an issue.

Much is still unknown about what causes stem cells to differentiate. In a functioning organism, an adult stem cell would be triggered to differentiate to repair degenerate cells or replace dead cells. Environmental factors may also have an effect. In the lab, growth factors are used in vitro to sway a stem cell down a certain pathway to differentiate into a specialized cell. Ongoing research is pointing to genes as critical to the differentiation process but the signals instructing the genes are still unclear. Currently, cell lines are used in laboratories, but these are very difficult to produce.

Once stem cells have differentiated, Ig staining techniques can be used to determine which type of cell they have become. Ig staining works by isolating a specific protein which exists only in a certain type of cell. This protein is treated as an “antigen” and antibodies are used to mark it. The three proteins marked in our experiment are Tyrosine Hydroxylase (TH) which exists in dopaminergic cells, Beta III Tubulin which marks immature neurons, and Glial Fibrillary Acidic Protein (GFAP) which is present in astrocytes. Several steps are required to ensure that the desired effect is produced. If, at the end of the procedure, a colour is visible in the viable cells, this is an indication that that the cell containing the specified protein is present. If not, the stem cells have clearly differentiated into a different type of cell.

As in the case of any other transplant, immunosuppressant drugs must be prescribed in order to ensure that the differentiated stem cells transplanted are not rejected by the body. Both genetic factors and environmental factors can affect the differentiation of stem cells. An intriguing question arises from all this knowledge. If the transplantation of stem cells is to become a routine treatment to heal the brain and spinal cord after injury or disease, what effects do immunosuppressant drugs have on stem cell survival and differentiation? Could these drugs be harming the transplanted cells rather than helping the body accept them?