23.3 Gene Therapy principles:

A. Immunomodulation

Several studies have exhibited that tumors show a certain degree of immunogenicity. As a result the human immune system recognises specific tumor antigens and starts mounting humoral and cellular responses. During cancer development, these responses are of very less intensity and for limited duration. The cancer cells use various mechanisms to escape detection by the immune system. These mechanisms have been elucidated thus enabling development of more and more strategies to design an effective antitumor immune response. In the last decade tremendous increase in the knowledge of the immunobiology of cancer has made immunological approaches, like immunomodulation, a very important strategy in cancer gene therapy.

The immunomodulation studies can be categorized on the basis of

• •  The target cells involved(can be tumor cells, host cells, T-cells, or APC's such as dendritic cells or other cells)

•  The mode of gene delivery involved (type of vector used, type of delivery used which maybe in vivo or in vitro or ex vivo )

•  The transgenes involved (can be genes coding for cytokines, co-stimulatory molecules, and tumor-associated antigens).

Of the different target cells involved T lymphocyte is one of the most attractive target cell types for genetic modification. The earliest clinical protocols for gene therapy involved the application of cytokine-transduced, tumor-infiltrating T-lymphocytes (TILs). For ex vivo genetic modification T-lymphocytes have been targeted. The genetic modification in T-lymphocytes is being done by the transfer of cytokine gene whereas the redirection is done by the tumor antigen specific T-cell receptor genes also called chimeric receptor genes which encodes for the protein with extracellular domain involved in antigen binding and the intracellular domain involved in the cell signalling. They have also been isolated from genetically modified tumors or their draining lymph nodes. Other approaches enhance T-cell reactivity with antibodies that are targeted directly at the respective receptors on T-cells. These approaches are complemented by different methods to enhance antigen recognition on the surface of tumor cells.

One of the immunological methods to invoke a local inflammatory response is based on the short-range molecular interaction which enables communication between immune and non immune cells. As a result, direct actuation of specific as well as nonspecific immune cells by the cytokine transfected tumor cells or fibroblasts takes place. Therapeutic index increased in this case with the constant local release of cytokines. Similarly transfer of co-stimulatory molecules may also lead to increase in therapeutic index. Down-regulation of surface expression of MHC class I /MHC class II and co-stimulatory molecules on tumor cell surface could be up-regulated by transducing the tumor cells with a wild-type cDNA responsible for reactivating the antigen recognition on the transfected tumor cells. In the past few years antigens derived from tumor have become an attractive target for gene transfer methods. These tumor derived antigens are delivered as naked DNA or by the viral vectors directly to the tumors or via dendritic cells.

Tumor-derived antigens / peptides are being explored as an important target for gene transfer methods and have been extensively defined during the last decade. In order to elicit an immune response against the tumor cells these antigens are being delivered directly, to the tumor cells or through dendritic cells, by using viral vectors or in the form of naked DNA. To detect tumor antigens, other than those already being explored, efficient and promising methods like serological analysis of tumor antigens by recombinant expression cloning also called SEREX as well as microarray technique (which enables the detection of differential gene expression) are being used.

Another important approach worth mentioning in this context is the antibody based immunotherapy because profound benefits in clinical trials have been observed while using recombinant antibodies. The basic concept behind antibody based immunotherapy is to develop monoclonal antibodies which target the tumor antigens that are either soluble, expressed on the malignant cell surface or are present on the tumor stroma.