Novel costimulators in the immune gene therapy of cancer.

One of the major goals of cancer immunotherapy is the induction of tumour-specific T-lymphocyte responses that will be effective in the rejection of established tumours. The prospects for such therapy rely on the identification of tumour antigens, and although there is persuasive evidence for the presence of such antigens,1,2 the occurrence of the disease does illustrate that the immune system is at least, on some occasions, unable to recognise and destroy these targets. Tumour antigens may be novel proteins (from genetic lesions or viral infections), modified existing antigens (eg, abnormally glycosylated cell surface proteins), or inappropriately expressed normal gene products (eg, CA125, carcinoembryonic antigen, and alpha-fetoprotein).1 Involvement of the immune system in the normal surveillance and suppression of cancer is further suggested by the increased incidence of tumours in immunocompromised patients.3 However, recent evidence has shown that, at least in model systems, cancer cells can be modulated in such a way that they stimulate cells of the immune system to recognise and destroy these malignant cells. This review summarizes the costimulatory molecules involved in the activation of such cells, the principles and mechanisms underlying their activation, and how such knowledge can be used to persuade the immune system to challenge cancer.