Transformation of affected cells of a tissue by the gene of interest is the first step in classical gene therapy. In some cases, however, the gene is targeted deliberately to the cells of an unaffected tissue, specifically to a lymphoid tissue.
This has several advantages. Firstly, by targeting genes to the cells of the immune system, the immune response to some types of cancer cells and infectious organisms is enhanced. Secondly, the gene product (polypeptide) is delivered to the remotest corner of the body. For example, transformed myoblasts (progenitors of muscle cells, serving as non-target cells) have an advantage of being longer lived cells in comparison to other cells and secreting the product into the blood.
Prior to transformation or transfection, the remedial gene is cloned in a host cell to form multiple copies. Two strategies have been adopted: ex vivo and in vivo. Ex vivo gene therapy: In ex vivo method of delivery, the affected cells are removed from the body and transformed by the remedial gene, which has been cloned.
The transformed cells are grown in culture media to sufficient number and then returned to the body by transfusion or by transplantation. When the patient’s own cells (autologous cells) or closely matched donor cells are used, there is no immune reaction and hence, no rejection.
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If the transformed cells are heterologous, the immune system of the body will reject the infused or transplanted cells. This has been the major hurdle in the ex vivo gene therapy. Currently, research is underway to engineer universal donor cells, which will contain all possible cell surface antigens, such that these cells can be used as donor cells with a wide range of patients. This method is currently practiced with animal systems and small number of human subjects.
This approach is applicable to tissues that can be removed from the body, genetically engineered and returned to the body, where it will survive for a longer period of time (e.g. hematopoietic cells and skin cells). Hence, the method has a limited application.
In vivo gene therapy: This practice is still in an experimental stage. Here, the cloned genes are directly introduced into the affected cells of the patient.
This is an option for the tissues, where individual cells cannot be cultured in vitro to a sufficient number and re-implanted into the patient’s body. This method encounters several problems. Firstly, the remedial genes directly transferred to the target cells are sometimes unstable and the product is expressed transiently. Secondly, there is no effective method of selecting the transformed cells in vivo.