Genetically modified tumour vaccines: an obstacle race to break host tolerance to cancer

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Abstract
The development of genetically modified tumour vaccines (GMTV) has been prompted by a better understanding of antitumour immune responses and genetic engineering technologies, as well as the identification of numerous tumour antigens (TA) in several malignancies which occasionally induce spontaneous tumour regressions. Cellular vaccines are based on autologous or allogeneic tumour cells genetically engineered to secrete different cytokines, co-stimulatory molecules, or allogeneic HLA molecules in order to provide a strong stimulatory signal together with the presented TA. Another promising approach that is targeted towards breaking immune tolerance to TA, exploits dendritic cells (DC) loaded or genetically modified with TA (and sometimes cytokines). Effective nonviral and viral gene delivery systems have been constructed including a third generation of adenoviral, lentiviral and hybrid vectors. Studies in mice demonstrated that therapeutic, curative immune responses might be elicited by GMTV. Promising results from animal studies are rarely seen in human trials. Several reasons, such as numerous escape mechanisms of slowly evolving spontaneous tumours and immune incompetence of advanced patients, are major concerns. Improved monitoring of immune responses to GMTV is essential to distinguish between responders and non-responders in order to tailor immune therapy strategy to the individual patient.