Inhibitory effects of unmethylated CpG oligodeoxynucleotides on MHC class I‐deficient and ‐proficient HPV16‐associated tumours

Unmethylated oligodeoxynucleotides containing guanine–cytidine dimers (CpG ODN) have been described as potent inducers of selected antitumour immune responses and the immunotherapeutic efficacy of CpG ODN has been examined either alone or as a vaccine adjuvant. We hypothesized that CpG ODN therapy could be an effective tool for immunotherapy of not only conventional MHC class I + tumours but also of those tumours that have lost MHC class I expression during their progression. To address this hypothesis, we employed the animal model resembling MHC class I‐proficient and ‐deficient human papilloma virus (HPV) 16‐associated tumours. A cell line transformed with HPV16 E6 and E7 oncogenes, TC‐1, as a prototype of MHC class I‐positive line, and its MHC class I‐deficient sublines TC‐1/A9 and TC‐1/P3C10 were injected into syngeneic C57BL/6 mice and the growing tumours were subjected to immunotherapy with CpG ODN 1826. The therapy started either 1 day after the challenge with the tumour cells or later, when the tumours had reached a palpable size. In both settings, CpG ODN 1826 significantly reduced the growth of MHC class I‐proficient and ‐deficient tumours. Furthermore, we demonstrated that CpG ODN 1585, whose mechanism of action preferably involves indirect activation of the natural killer cells, induced regression of the MHC class I‐deficient tumours TC1/A9 but not of the MHC class I‐proficient tumours TC‐1. This study infers that synthetic CpG ODN have a potential for the therapy of both MHC class I‐proficient and ‐deficient tumours and thus could be also used against tumours that tend to down‐regulate their MHC class I expression. © 2005 Wiley‐Liss, Inc.