Modifications of antitumor defenses by tumor derived factors and by superantigens.

There is ample evidence that tumor development can be affected by the interactions between the growing neoplasms and the immune system. The balance of these interactions is tilted in favor of tumor growth in many cases due to the production of cytokines and other factors by the tumor cells. These factors can modulate the immune system either by direct interactions with immune cells or by indirect means, which include downregulation of the synthesis of other cytokines or products necessary for the activity of a given effector cell. In addition, endogenous retroviral superantigens, with their capacity of eliminating part of the T cell repertoire and possibly by other effects on cells of the T and B cell lineages, may provide the tumor cells an escape from the otherwise efficient antitumor host defenses.