BABOON-TO HUMAN BONE MARROW TRANPLANTATION

BABOON-TO HUMAN BONE MARROW TRANPLANTATION

Egal Gudal, Les Pullen, Christa Skow

Background to Bone Marrow Tranplantation (1).

A number of factors has recently inspired the research to improve baboon to human bone marrow tranplantation(BMT). Among these is the dramatic shortage of human donor organs available for human to human transplantation and the apperent baboon resistance to certain viruses, such as hepatitis B and HIV-1. This project focuses on several key areas of research and devolopment including the differences in the major histocompatability complex (MHC) in baboons and humans. Such differences can effect the success of xenotransplantation since they can induce an auto immune response eventually leading to graft versus host disease (GVHD). In addition, BMT is explored as a cure for sickle-cell anemia, and its role in gene therapy in stem cells.

Making Baboon to Human Bone Marrow Transplantation a reality.

The hypothesis supporting baboon to human BMT in HIV-1 infected human is that the transplanted immunocompetent baboon lymphocytes will rid the host of HIV-1 infected cells. Two things are critical in the success of baboon to human BMT to cure an HIV-1 infected patient. First, the tranplanted baboon T cells need to recognize human MHC molecules in the thymus to educate the pluripotent xenogeneic baboon T cells and second, recognize the HIV-1 peptides presented by human MHC molecules. It has been found that baboon and human MHC molecules are close to 90% identical. The differences mostly lie at the amino terminal regions at positions in the antigen binding groove. The carboxy-terminal regions of the Class I heavy chain appears to be more conserved between the two species. Regardless, no data exists that indicate how T cell mediated responses to MHC Class I and Class II xenoantigens are quantitatively or qualitatively different from responses to self or allo-antigens (2).

Improvements in immunosuppressive drugs and the management of the immune response, as well as improvement in HLA typing have increased the success rate of human to human BMT. Future studies will make baboon to human BMT a reality.

Bone Marrow Tranplantation as a Cure for Sickle Cell Disease.

Sickle cell anemia is a disease stemming from the malformation of red blood cells, which leads to a series of physical ramification eventually ending in death. Although BMT has been proven to be an effective measure, it is a risky procedure and can cause severe damage by leading to such conditions as sterility, and increased chance of leading to chemotherapy related diseases and as previously mentioned, GVHD. Therefore, it is debatable whether bone marrow transplants are the best course of action for young patients.

It is thought that a valine susbstitution for glutamine in the Beta chain of hemaglobin is responsible for sickle-cell anemia. This mutation results in the formation of insoluble fibers within the structure of the red blood cell, forming a sickle shaped cell. These mutant red blood cells are more susceptible to breakage and have difficulty crossing capillaries which damage such organs as the spleen, brain, kidney, eye and heart.

In a study conducted by an extensive collaborative group, twenty-two patients below the age of sixteen, suffering from the extreme symptoms of sickle-cell anemia were given bone marrow tranplants. The first step in these procedures is finding donors with identical or near identical HLA antigens as the recipeints. The recipeints immune system is then suppressed so initially it will be unable to reject the donor marrow. Of the twenty-two patients, twenty survived the tranplantation procedure, two died of GVHD or central nervous system hemorrhage and three rejected the donor marrow and sickle cell disease reappeared (3).

The conclusion was drawn, therefore, that BMT can cure sickle cell anemia. It was found that repeated transfussions cause a progressively more sensitive response by the patients immune system. Further, studies have shown that the more symptomatic a paiteint is, the less likely, GVHD will occur. Therefore, those patients with no history of transfusions and with serious symptomatic complications are the best candidates for BMT.

Bone Marrow Tranplantation Research in patients with Lymphoma.

Bone Marrow Tranplantation is a promising treatment for patients with lymphoma. There is a great deal of research being done to further improve the results of BMT. In particular, the work performed by Atkinson and Fay is to accelerate bone marrow recovery after a BMT procedure.

The study used twelve patients with malignancies and administered to them recombinant human granulocyte stimulating factor (rHuG-CSF) at 5mu/g/kg/day for seven days prior to bone marrow harvest. The patients were also treated with lenograstim which is a therapy for bone marrow and cancer patients until their neutrophil count was 1.0 times 10-9/1. The patients were then split into two groups of six. Both recieved lenograstim stimulated marrow. Group one received stem cells that were encriched with CD34 stem/progenitor cells and T cells depleted on an absorption column. Group two recieved lenograstim treated marrow only.

The results showed that all twelve patients had an increase in circulating white blood cells one day after tranplantation. This is significant because with treatment before bone marrow harvest a patient would have an incresed white blood cell count. Therefore a stronger immune system is attained that can better cope with the post chemotherapy problems that may arise as a side effect of the treatment (4). This study was compared to control patients in previous experiments. The control group with rFuG-CSF has a higher rate of neutrophil engraftment in patients.

Bone Marrow Tranplantation in Gene Therapy.

High doses of chemotherapy can eradicate cancer, but in the process it can destroy the bone marrow and consequently damage blood cells and the immune system as a whole. Therefore, by harvesting a patients stem cells before chemotherapy and then tranplanting them back into the patient post chemotherapy, the damaged immune system is boosted and better prepared to deal with the damage inflicted by chemotherapy.

Scientists recently worked with totipotent hematopoietic stem cells (THSC). A gene that can confer proliferative advantage in the expansion of the hematopoitic stem cells was used in this experiment. Transgenic mice expressing a receptor for the growth factor erythropoietin was generated. The mice appeared phenotipically normal. But when they were treated with erythropoietin they showed a big increase in multipotent, clonogenic, hematopoietic cells. These cells also gave rise to granulocytes, erythroid cells, macrophages, and megakaryocytes within the same colony. The erythropoietin receptor greatly enhanced the differentiation capabilities of the stem cells. This erythropoietin receptor gene shows great promise in obtaining cytokine-inducible hematopoeitic stem cell proliferation (5).

An important use in gene-therapy transplant is the ability to monitor the progress of these stem cells. Scientists have used small viruses, as vectors that carried marker genes into the stem cells. The marker genes are then used as a diagnostic tool to help researchers follow the proliferation of these cells. Easy to detect genes are integrated into stem cells. Once these harmless viruses carrying the genes incorporate into the cell, the genes start dividing along with the cells and can be in any cells that descended form the original. This recent experimend used a marker gene which encodes resistance to the antibiotic neomycin. The marker genes are then used as a diagnostic tool to help reaserchers follow the proliferation of these stem cells. Since there are two types of stem cells derived, one from the blood, and the other from bone-marrow, it is important to differentiate them. Whether these stem cells have descended from marrow-derived stem cells, or blood derived stem cells, will help reserchers understand the process of blood-cell differentiation, a crucial step for cancer patients (6).