Abnormal signal transduction by T cells of mice with parental tumors is not seen in mice bearing IL-2-secreting tumors

There is considerable evidence to demonstrate that immune function is abnormal in tumor-bearing mice, perhaps accounting, at least in part, for progressive tumor growth. In an attempt to generate an antitumor response, we used retroviral vectors to express IL-2 cDNA in CMS5, a murine fibrosarcoma. Mice inoculated with unmodified tumor cells suffered progressive tumor growth, whereas tumors secreting IL-2 were rejected or grew slowly. Animals bearing unmodified but not IL-2-secreting tumors also were immunosuppressed. On the basis of these observations, we were interested in how IL-2 secretion by the tumor cells prevented the onset of hyporesponsiveness. To identify biochemical differences between T cells of mice with parental vs slowly growing IL-2-secreting tumors, we examined signal transduction after activation through the CD3/TCR complex. Protein tyrosine phosphorylation was altered and calcium flux was reduced in cells of mice with parental tumors compared with animals with slowly growing IL-2-secreting tumors. In addition, levels of protein for the tyrosine kinases p56(lck) and p59(fyn), as well as the TCR-ζ-chain, were reduced. These differences in signal transduction were observed for T cells of mice with parental and IL-2-secreting tumors of the same size, demonstrating that differences in tumor size alone could not explain our findings. Thus, IL-2 secretion by tumors seems to be able to prevent immunosuppression by maintaining normal signal transduction in T cells, facilitating the generation of antitumor responses.