GM-CSF AND B7-1 (CD80) COSTIMULATORY SIGNALS COOPERATE IN THE INDUCTION OF EFFECTIVE ANTITUMOR IMMUNITY IN SYNGENEIC MICE

B7-1 (CD80) co-stimulatory molecule gene-transduced Lewis lung carcino ma (LLC) cells (LLC/B7 cells) resulted in remarkable loss of tumorigen icity in syngeneic C57BL/6 mice (87.5% rejection) compared to B7-negat ive, wild-type LLC (LLC/wt) cells (0% rejection). However, mice that h ad rejected LLC/B7 cells developed almost no systemic immunity protect ive against challenge with wild-type tumor cells after 4 weeks (11.8% rejection). Enhancement of MHC class I (H-2K(b)) expression of LLC/B7 cells with in vitro interferon-gamma treatment did not result in enhan cement of protective immunity. In vivo depletion assay revealed that a brogation of tumorigenicity in LLC/B7 depended on CD8(+) T cells but n ot on CD4(+) T cells. However, vaccination of C57BL/6 mice with irradi ated LLC cells transduced with GM-CSF (LLC/GM) led to the induction of potent, specific immunity against challenge with the LLC/wt cells aft er 2 weeks (80.8% rejection). Next, we established a double transfecta nt of LLC cells expressing both B7-1 and GM-CSF (LLC/GM + B7). The tum origenicity of these clonal cells was also remarkably suppressed (90% rejection) to the same degree as LLC/B7, whereas that of LLC/GM was no t suppressed (0% rejection). Interestingly, mice that had rejected LLC /GM + B7 cells developed enhanced protective immunity against challeng e with LLC/wt cells after 4 weeks (55.6% rejection) compared to the re sults of LLC/B7 cells (11.8%). To evaluate whether coexpression of GM- CSF and B7-1 enabled the tumor cells to activate cytotoxic T cells mor e efficiently than B7-1 alone, we performed gn in vitro killing assay. We found that immunization with LLC/GM + B7 cells resulted in a 3-fol d stronger cytotoxic response than that with LLC/B7. Our data indicate that co-transfection of the B7-1 co-stimulatory molecule and GM-CSF g enes may be more effective for the induction of stronger protective im munity in this experimental system. (C) 1997 Wiley-Liss Inc.