The theory of immune surveillance is that a prime function of the immune system is to detect and eliminate neoplastic cells before a tumor forms. A basic principle of this theory is that cancer cells are antigenically different from normal cells and thus elicit immune reactions that are similar to those that cause rejection of immunologically incompatible allografts. Studies have confirmed that tumor cells differ, either qualitatively or quantitatively, in their expression of antigens. For example, "tumor-specific antigens" are antigens that are specifically associated with tumor cells but not normal cells. Examples of tumor specific antigens are viral antigens in tumors induced by DNA or RNA viruses. "Tumor-associated" antigens are present in both tumor cells and normal cells but are present in a different quantity or a different form in tumor cells. Examples of such antigens are oncofetal antigens (e.g., caricnoembryonic antigen), differentiation antigens (e.g., T and Tn antigens), and oncogene products (e.g., HER/neu).
Different types of cells that can kill tumor targets in vitro and in vivo have been identified: natural killer cells (NK cells), cytolytic T lymphocytes (CTLs), lymphokine-activated killer cells (LAKs), and activated macrophages. NK cells can kill tumor cells without having been previously sensitized to specific antigens, and the activity does not require the presence of class I antigens encoded by the major histocompatibility complex (MHC) on target cells. NK cells are thought to participate in the control of nascent tumors and in the control of metastatic growth. In contrast to NK cells, CTLs can kill tumor cells only after they have been sensitized to tumor antigens and when the target antigen is expressed on the tumor cells that also express MHC class I. CTLs are thought to be effector cells in the rejection of transplanted tumors and of tumors caused by DNA viruses. LAK cells are a subset of null lymphocytes distinct from the NK and CTL populations. Activated macrophages can kill tumor cells in a manner that is not antigen dependent nor MHC restricted once activated. Activated macrophages are through to decrease the growth rate of the tumors they infiltrate. In vitro assays have identified other immune mechanisms such as antibody-dependent, cell-mediated cytotoxic reactions and lysis by antibody plus complement. However, these immune effector mechanisms are thought to be less important in vivo than the function of NK, CTLs, LAK, and macrophages in vivo (for review see Piessens, W. F., and David, J., "Tumor Immunology", In: Scientific American Medicine, Vol. 2, Scientific American Books, N.Y., pp. 1-13, 1996.
One of the most complex phenomenon in cancer immunology relates to the failure of the immune system to eliminate tumors. In the 1970's, Hewitt articulated the notion that most tumors did not express any tumor-specific or neoantigens and, thus, could not be recognized as "foreign" by the immune system. Indeed, virtually no tumor cell surface antigens recognized by antibodies were found to be tumor specific, and furthermore, most spontaneous murine tumors were considered "poorly immunogenic" as defined by their failure to be eliminated when transferred into syngeneic hosts (Hewitt, et al., Br. J. Cancer, 33:241-259, 1976). However, these same tumors could be rendered "immunogenic" by mutagenesis (Van Pel and Boon, Proc. Natl. Acad. Sci. USA, 79:4718-4722, 1982) when new antigens were expressed on the tumor cells surface. It is possible that the immune system fails to eliminate tumors not because neoantigens are absent, but rather because in vivo the response to antigens is inadequate. Therefore, a method for enhancing immunogenicity of the tumor cells by potentiating the host's immune response to the tumor cells would provide a key advance in immunotherapy.
The goal of immunotherapy is to augment a patient's immune response to an established tumor. One method of immunotherapy includes the use of adjuvants. Adjuvant substances derived from microorganisms, such as bacillus Calmette-Guerin, heighten the immune response and enhance resistance to tumors in animals. Although bacillus Calmette-Guerin has been tested in many clinical trials, the results have been inconclusive, and the value of this type of bacterial adjuvant therapy remains uncertain (Piessens and David, 1996, supra).
A number of bacterial products, such as lipopolysaccharide, are known to stimulate mammalian immune responses. Recently, bacterial DNA itself has been reported to be one such molecule (e.g., Krieg, A. M., et al., 1995, Nature 374:546-9). One of the major differences between bacterial DNA, which has potent immunostimulator effects, and vertebrate DNA, which does not, is that bacterial DNA contains a higher frequency of unmethylated CpG dinucleotides than does vertebrate DNA. Select synthetic oligodeoxynucleotides (ODN) containing unmethylated CpG motifs (CpG ODN) have been shown to have an immunologic effects and can induce activation of B cells, NK cells and antigen-presenting cells (APCs) such as monocytes and macrophages (Krieg, A. M., et al., supra). It can also enhance production of cytokines known to participate in the development of an active immune response, including tumor necrosis factor-.alpha., IL-12 and IL-6 (e.g., Klinman D. M., et al., Proc. Natl. Acad. Sci. USA, 93:2879-83, 1996).
The binding of DNA to cells has been shown to be similar to a ligand receptor interaction: binding is saturable, competitive, and leads to DNA endocytosis and degradation into oligonucleotides (Benne, R. M., et al., J. Clin. Invest. 76:2182, 1995). Like DNA, oligodeoxyribonucleotides are able to enter cells in a process which is sequence, temperature, and energy independent (Jaroszewski and Cohen, Ad. Drug. Del. Rev. 6:235, 1991). Lymphocyte oligodeoxyribonucleotide uptake has been shown to be regulated by cell activation (Krieg, A. M., et al., Antisense Research and Development 1:161, 1991).
GM-CSF is known to regulate cell proliferation under basal and stress conditions, and is known to activate the tumoricidal activity of macrophages. Some studies indicate that simultaneous treatment with GM-CSF and standard induction chemotherapy may improve the efficacy of chemotherapy (Estey, E. H., Blood 83:2015, 1994). The major benefit of colony stimulating factors, such as GM-CSF, has been postulated to be their use in the treatment pancytopenia, one of the complications of chemotherapy (Piessens and David, 1996, supra).