Multipotent Adult Progenitor Cells Prevent Rejection of Transplanted Tissue


Solid organ transplantation is a procedure that has saved untold millions of lives. Unfortunately, the tendency for an organ to be rejected by the immune system of the organ recipient is a formidable problem that is addressed in two ways. One of these is through tissue matching of the organ to the recipient. The other is through the use of immunosuppressive drugs that suppress the immune system. Neither one of these strategies is without caveats.

Tissue typing begins with a blood test to determine the organ recipient’s blood type. If the organ contains a blood type that is incompatible with the immune system of the organ recipient, the result will be catastrophic. Hyperacute rejection is the name given to organ rejection that occurs minutes to hours after the organ was transplanted. Hyperacute rejection occurs because the recipient has pre-existing antibodies in their body that recognizes and begins to destroy the graft. These antibodies can result from prior blood transfusions, multiple pregnancies, prior transplantation, or xenografts against which humans already have antibodies. Massive blood clotting within the capillaries of the organ clog the blood vessels and prevent perfusion of the graft with blood. The organ must come out or the patient will die.

Human cells have on their surfaces particular proteins that are encoded by genes located on the short arm of chromosome 6 called the major histocompatibility complex or MHC. the MHC genes encode human leukocyte antigens or HLAs. HLA proteins are extremely variable from person to person, and this seems to be the case because the more variation we have in our HLA proteins, the better job the immune system does recognizing foreign invaders.

Each individual expresses MHC genes from each chromosome. Therefore, your cells contain a mosaic of surface proteins, some of which are encoded by the HLAs encoded by the chromosome you inherited from your father and others of which are encoded by the chromosome your inherited from your mother.

The MHC molecules are divided into 2 classes. Class I molecules are normally expressed on all nucleated cells, but class II molecules are expressed only on the so-called “professional antigen-presenting cells” or APCs. APCs include cells that have names like dendritic cells, activated macrophages, and B cells. T lymphocytes only recognize foreign substances when they are bound to an MHC protein. Class I molecules present antigens from within the cell, which includes bits from viruses, tumors and things like that. Class II molecules present extracellular antigens such as extracellular bacteria and so on to a subclass of T cells called T helper cells, which express a molecule called “CD4” on their cell surface.

MHC-Class-I-Topology_3mhc_class2

All this might seem very confusing, but it is vital to ensuring that the organ is properly received by the organ recipient. Some types of MHC are very different and will elicit robust immune responses against them, but others are not as different and can be rather well tolerated. How does the doctor which are which? Through three tests: 1) Blood type is the first one. If this does not match, you are out of luck; 2) lymphocytotoxicity assay in which blood from a patient is tested to determine if it reacts with lymphocytes from the blood of the donor. A positive crossmatch is a contraindication to transplantation because of the risk of hyperacute rejection. This is used mainly in kidney transplantation; 3) Panel-reactive antibody (PRA) screens in which the the serum of a patient is screened for antibodies against the lymphocytes from the donor. The presence of such antibodies is contraindicated for transplantation. Finally, there is a test that is not used a great called the mixed lymphocyte reaction test that uses lymphocytes from the blood of the organ donor and the organ recipient to see if they activate one another. This test takes a long time and can be difficult to interpret.

Once the patient receives the transplant, they are usually put on immunosuppressive drugs. These drugs include cyclosporine, tacrolimus, sirolimus, mycophenolate, and azathioprine. Each of these drugs has a boatload of side effects that range from hair loss, diabetes mellitus, nerve problems, increased risk of illness and tumors, and so on. None of these side effects are desirable, especially since the drug must be taken for the rest of your life after you receive the transplant.

Enter a new paper from University Hospital in Regensburg, Germany from the laboratory of Marc Dahkle that used particular stem cells from bone marrow to induce toleration of grafted heart tissue in laboratory animals without any drugs. This paper was published in Stem Cells Translational Medicine and is potentially landmark in what it shows.

In this paper, Dahkle and his colleagues used stem cells from the bone marrow known as multipotential adult progenitor cells or MAPCs. MAPCs have been thought to be a subtype of mesenchymal stem cell in the bone marrow because they have several cell surface markers in common. However, there are some subtle differences between these two types of cells. First of all, the MAPCs are larger than their mesenchymal stem cell counterparts. Secondly, MAPCs can be cultured more long-term, which increases the attractiveness of these cells for therapeutic purposes.

In this paper, the Dahkle group transplanted heart tissue from two unrelated strains of rats. Four days before the transplantation, the donor rats received an infusion of MAPCs into their tail veins. There were a whole slew of control rats that were used as well, but the upshot of all this is that the rats that received the MAPCs before the transplantation plus a very low dose of the immunosuppressive drug mycophenolate did not show any signs of rejection of the transplanted heart tissue. If that wasn’t enough, when the transplanted heart tissue was then extirpated and re-transplanted into another rat, those grafts that came from MAPC-treated rats survived without any drugs, but those that came from non-MAPC-treated rats did not.

Because control experiments showed that the rats treated with cyclosporine did not reject their grafts, Dahkle and others used this system to determine the mechanism by which MAPCs prevent immune rejection of the grafted tissue. They discovered that the MAPCs seem to work though a white blood cell called a macrophage. Somehow, the MAPCs signal to the macrophages to suppress rejection of the graft. If a drug (clodronate) that obliterates the macrophages was given to the rats with the MAPCs, the stem cells were unable to suppress the immunological rejection of the graft.

In this paper, the authors conclude that “When these data are taken together, our current approach advances the concept of cell-based immunomodulation in solid organ transplantation by demonstrating that third-party, adherent, adult stem cells from the bone marrow are capable of acting as a universal cell product that mediates long-term survival of fully allogeneic organ grafts.” Revolutionary is a good word for this findings of this paper.  Hopefully, further pre-clinical trials will eventually give way to clinical trials in human patients that will allow human patients to have their lives saved by an organ transplant without the curse of taking immunosuppressive drugs for the rest of their lives.

Co-culturing Immune Cells with Umbilical Cord Stem Cells Reverses Type 1 Diabetes in a Small Study


Type 1 diabetes results from an inability to make sufficient quantities of insulin. Insulin is made by specific cells in the pancreatic islets (also known as the islet of Langerhans). Most type 1 diabetics have suffered destruction of their pancreatic beta cells. Beta cell destruction can result from physical trauma to the pancreas, which causes the digestive enzymes of the pancreas to destroy the beta cells. For example, pancreatitis, pancreatic surgery, or certain industrial chemicals can cause diabetes. Also, particular drugs can also cause temporary diabetes, such as corticosteroids, beta blockers, and phenytoin. Rare genetic disorders (Klinefelter syndrome, Huntington’s chorea, Wolfram syndrome, leprechaunism, Rabson-Mendenhall syndrome, lipoatrophic diabetes, and others) and hormonal disorders (acromegaly, Cushing syndrome, pheochromocytoma, hyperthyroidism, somatostatinoma, aldosteronoma) also increase the risk for diabetes.

Additionally, viral infections of pancreas can cause the immune response to destroy pancreatic cells, and this wipes out enough beta cells to cause the onset of type 1 diabetes. The coxsackievirus family of viruses is a family of enteric viruses that are cause infections that are sometimes associated with the onset of type 1 diabetes, as are mumps and congenital rubella. In most cases, genetic factors cause the immune system to view the pancreatic beta cells as foreign invaders, and the beta cells are attacked and destroyed. Researchers have found at least 18 genetic loci that are designated IDDM1 – IDDM18 that are related to type 1 diabetes. The IDDM1 region contains the “HLA genes” that encode proteins called “major histocompatibility complex”. HLA genes encode cell-surface proteins that act as “bar codes” for the immune system. When cells have the proper bar codes on their cell surfaces, the immune system recognizes those cells as being a part of the body in which they reside, and the immune system leaves them alone. Any cells that do not have the right bar codes are attacked and destroyed, which is known as the “graft versus host response.” Therefore, it is fair to say that HLA genes affect the immune response. New advances in genetic research are identifying other genetic components of type 1 diabetes. Other chromosomes and genes continue to be identified.

A recent paper attempts to cure type 1 diabetes by using umbilical cord stem cells. Umbilical cord stem cells (UCSCs) have the ability to greatly calm down the immune system. UCSCs secrete a wide variety of molecules that prevent immune cells from reacting to and destroying other cells, and also have many cell surface proteins that bind to the surfaces of immune cells and put them to sleep (see Abdi et al., Diabetes 2008;57:1759-67 & Aguayo-Mazzucato C. and Bonner-Weir S, Nature Reviews Endocrinology 2010;6:726-36).

Animal experiments have shown that co-culturing UCSCs with circulating immune cells alters the immune response against pancreatic beta cells and greatly increases the ability of the animal to regulate blood glucose levels (Zhao et al., PLoS ONE 2009;4:e4226). The UCSCs seem to “re-educate” the immune cells so that they do not recognize the pancreatic islets are foreign anymore. Therefore, Yong Zhao and his colleagues in Theodore Mazzone’s laboratory at the University of Chicago, IL, and collaborators at the General Hospital of Jinan Military Command, Shandong, China, used human UCSCs to re-educate immune cells in human type 1 diabetic patients.  See here for this paper.

To do this, they circulated the blood of each patient through a close-loop system that separated the immune cells (lymphocytes) from whole blood. Thee lymphocytes were then co-cultured with UCSCs for 2-3 hours and then returned to the patients.

The results were remarkable. Six patients in group A, who all had some residual beta cell function showed successively improved insulin production 12-24 weeks after treatment. They also showed a reduced need for insulin shots, and overall improvement of their fasting blood glucose levels. Six patients in group B, who had no residual beta cell function, showed increased production of insulin production 12 week after treatment. This is an incredible finding because those without beta cells essentially grew new ones that were not attacked by the immune system. The group B group also saw successively reduced requirements for injected insulin. The patients in the control, whose immune cells did not undergo re-education by UCSCs showed no improvement.

Furthermore, the patients whose immune cells were re-educated by the UCSCs, did not experience any adverse effects. This procedure seems to be quite safe and feasible.

There is a word of caution here. These patients must be followed over several years to establish that the re-education of the lymphocytes is maintained over time. Also, this study is quite small and despite the amazing results, a larger study is needed. All the same, this is an incredible result that reverses type 1 diabetes, and even though caution is needed, embryonic stem cells were not required to do this.