To engineer
superantigens (SAg) to express
tumor reactivity, we genetically fused the Fab-part of the
tumor-reactive MAb C215 and the bacterial SAg
staphylococcal enterotoxin A (SEA). Treatment of mice carrying established lung
micrometastases of the C215-transfected syngeneic
B16 melanoma with 3-4 daily
injections of C215Fab-SEA resulted in strong antitumor effects, while only moderate effects were seen when treatment was given every 4th day (intermittent treatment). High serum levels of
IL-2,
TNF-alpha, IFN-gamma and strong induction of CTLs (cytotoxic T lymphocytes) were noted after priming with the fusion
protein. T cells responded well to 3 daily
injections of C215Fab-SEA and then gradually entered a hyporesponsive state, characterized by a reduced ability to produce
IL-2,
TNF-alpha and IFN-gamma and failure to mediate cytotoxicity in vitro. Intermittent treatment was characterized by increased levels of
IL-10, concomitant with accentuated loss of
IL-2,
TNF-alpha and IFN-gamma production. A 10-fold increase in SEA-reactive TCR V(beta)3+ CD4+ cells was observed in the spleen, while a loss of TCR V(beta)3+ CD8+ and V(beta)11+ CD8+ cells was noted. This is in striking contrast to
injections of native SEA which induced a marked deletion of TCR V(beta)3+ CD4+ T cells, but not of CD8+ cells. Recovery of the TH1
cytokine profile occurred within 1-2 weeks, while restoration of cytotoxicity required several months and correlated with recovery of TCR V(beta)3+ CD8+ and TCR V(beta)11+ CD8+ T cells. These results show that the temporal relationship of SAg stimulations dictates the
cytokine profile. Moreover, different mechanisms appear to regulate hyporesponsiveness in CD4+ and CD8+ T cells.