T-CELL-MEDIATED, IFN-GAMMA-FACILITATED REJECTION OF MURINE B16 MELANOMAS

The murine melanoma cell line B16.F10 (H-2(b)) was used to study speci fic T cell responses that reject tumors. Stable B16 transfectants were established that express viral Ags, either the hepatitis B surface Ag (HBsAg) or the large tumor Ag (T-Ag) of SV40. B16 cells and their tra nsfected sublines were CD40(+)CD44(+) but expressed no (or low levels of the) costimulator molecules CD154 (CD40L), CD48, CD54, CD80, and CD 86. Surface expression of MHC class I (K-b, D-b) and class II (I-A(b)) molecules by B16 cells was low, but strikingly up-regulated by IFN-ga mma. CD95 (Fas) and CD95 ligand (CD95L (FasL)) were ''spontaneously'' expressed by B16 cells growing in vitro in serum-free medium; these ma rkers were strikingly up regulated by IFN-gamma. B16 cells coexpressin g CD95 and CD95L were irreversibly programed for apoptosis, In vitro, noninduced B16 transfectants stimulated a specific IFN-gamma release r esponse, but no cytolytic response (in a 4-h assay) in MHC class I-res tricted CTL; in contrast, IFN-gamma-induced B16 targets were efficient ly and specifically lysed by CTL, In vivo, B16 transfectants were spec ifically rejected by DNA-vaccinated syngeneic hosts through a T-depend ent immune effector mechanism. The tumors showed evidence of massive a poptosis in vivo during the rejection process. The data suggest that C TL-derived IFN-gamma enhances an intrinsic suicide mechanism of these tumor cells in addition to facilitating lytic interactions of effecter s with tumor targets.