The presence of immune infiltration of tumour deposits and the existence of effective in vitro anti-tumour immune responses would suggest the possibility of therapeutic manipulation against tumour cells. However, clinical immunotherapy has shown little promise as a cancer treatment. Numerous explanations for this inefficacy have been proposed, one of which involves the elaboration of immunosuppressive moieties from tumour cells. The results of studies presented below show that serum from patients with gastrointestinal and other tumours have immunosuppressive influences on normal lymphocytes. The degree of this in vitro inhibition is related to tumour 'bulk' and may reflect a systemic immunosuppressive influence of the tumour. Isolation and culture of lymphocytes from gastrointestinal tumour deposits demonstrated that these immune cells are functionally inert, suggesting the existence of an immunosuppressive tumour microenvironment. The isolation and partial purification of an immunosuppressive moiety from conditioned culture medium of a variety of human tumour cell lines further supports the hypothesis of tumour-mediated immunosuppression. A number of protein tumour cell products have been described with potent immunosuppressive properties. These include transforming growth factor-beta, interleukin-10, and the retroviral envelope protein p15E. The surgical implications of the proposed tumour-host immune relationship includes the hypothesis that clinically apparent disease may not be amenable to immune attack owing to tumour-mediated immune suppression. The use of immunostimulatory strategies as adjuvant perioperative therapy would seem a more effective environment for the activation of antitumour immune responses in the surgical patient
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