[Commemorative lecture of receiving Imamura Memorial Prize. II. Mode of action of oligonucleotide fraction extracted from Mycobacterium bovis BCG].

A fraction extracted from Mycobacterium bovis BCG was found to exhibit strong antitumor activity. This fraction, which was designated MY-1, caused some animal tumors to regress and/or prevent metastasis very effectively. MY-1 after digestion with DNase had almost completely reduced activity, while MY-1 digested with RNase did not. MY-1 also augmented natural killer (NK) cell activity of mouse spleen cells in vitro, and produced factors which showed anti-viral activity and rendered macrophages cytotoxic towards tumor cells. The function of the factor to activate macrophages was destroyed by treatment with anti-interferon (IFN)-gamma antibody, while the anti-viral activity was destroyed by treatment with anti-INF alpha/beta antibody. The oligonucleotides contained in MY-1 distributed in a broad range of molecular size, and peaked at 45 nucleotides. We synthesized 13 kinds of 45-mer nucleotides with sequence present in the known cDNA encoding various BCG proteins. Six out of these oligonucleotides, which contained one or more hexameric palindromic structures, showed strong antitumor activity, while the other without palindrome did not. These active oligonucleotides possessed the capability to induce IFN and to augment NK cell activity of mouse spleen cells by coincubation in vitro. When a portion of the sequence of the inactive oligonucleotides was substituted with either palindromic sequence of GACGTC, AGCGCT or AACGTT, the oligonucleotide acquired the ability to augment NK activity. In contrast, the oligonucleotides substituted with another palindromic sequence such as ACCGGT was without effect. Furthermore, exchange of two neighboring mononucleotides within, but not outside, the active palindromic sequence destroyed the ability of the oligonucleotide to augment NK activity.(ABSTRACT TRUNCATED AT 250 WORDS)