Recent advances in immune modulation.

Successful gene therapy protocols rely on the hypo-responsiveness of the immune system to transgene products generated from gene transfer vectors. In order to prevent cytotoxic lymphocyte or antibody formation induced by transgene expression, various strategies derived from recent advances in immune tolerance induction protocols have been tested in gene therapy model systems. Current immunosuppressive drugs were used to nonspecifically target T-cell activation, clonal expansion, and differentiation into effector cells. Central tolerance can be induced from intrathymic deletion of T cells with thymically expressed antigens or generation of hematopoietic mixed chimerism. Peripheral tolerance to transgenes may be achieved by several different pathways including deletion of activated/effector T cells by depleting antibodies, generation of T cell apoptosis or anergy by costimulation blockade, and active suppression by T regulatory cells. This review outlines the development of these strategies using various immune modulation regimens and protocols to induce long-term immune tolerance specific to the transgene product.