Leukemia and lymphoma are generally treated by high-dose chemotherapy and/or radiotherapy. In certain patients, the treatment is consolidated with a transplantation of donor stem cells and immune cells (allogeneic stem cell transplantation). Treatment with chemotherapy and/or radiotherapy is limited by the damage that is afflicted to the healthy tissues of the patients. An improved therapy of hematological malignancies should therefore be specific for malignant cells and spare the healthy cells. Immune therapy and gene therapy are the two most innovative treatment strategies developed over the last years, aiming at a more specific treatment of hematological malignancies. Adoptive immune therapy is the application of donor immune cells (T cells) to a patient, as in allogeneic stem cell transplantation. The donor T cells can track down and eliminate remaining malignant cells in the host (graft-versus-leukemia/lymphoma). Unfortunately, also the healthy tissues of the host can be attacked. This serious and potentially life-threatening complication is called graft-versus-host disease. Through genetic manipulation of T cells in the laboratory before application to the patients, the T cells can be provided with a gene that makes it possible to selectively recognize and eliminate the T cells in case of graft-versus-host disease. This is called suicide gene therapy. Another specific therapy is antibody immune therapy. Antibodies can attach to a specific molecule on the malignant cell and directly kill the cell or activate other immune mechanisms of the body to eliminate the cells. The most famous and successful type is antibody therapy with rituximab directed against the CD20 molecule on the membrane of lymphoma cells. The goal of this thesis is to improve antibody-mediated immune therapy and gene therapy of hematological malignancies by investigation of the human CD20 molecule. The first part describes how retroviral vectors are able to provide donor T cells with the human CD20 gene and how CD20-positive T cells can be eliminated by rituximab. The second part of this thesis demonstrates the direct relationship between the number of CD20 molecules on the cell membrane and the complement-mediated activity of rituximab. In addition, it shows how improved human anti-CD20 antibodies can kill rituximab-resistant cells. An important point concerning the field of gene therapy in general, is safety. It has become clear in the past that the insertion of a vector into the genome of a cell can induce malignant transformation. It will be necessary in the future to protect against this insertional mutagenesis by for instance providing the retroviral vectors with an insulator element or a suicide gene. For anti-CD20 antibody therapy, it can be concluded that the maximum effect of treatment does not seem to be reached. The use of improved second generation antibodies or ways to increase the CD20 expression can lead to a more efficient therapy and may overcome disease resistance. In conclusion, this thesis shows the development of the CD20/anti-CD20 suicide system for adoptive T cell immunotherapy and the investigation of the mechanism of CD20/anti-CD20 antibody therapy. CD20 immuno-gene therapy may lead to a more effective treatment of hematological malignancies
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