CpG motifs in bacterial DNA and their immune effects

Design, synthesis, and immunostimulatory properties of CpG DNAs containing alkyl-linker substitutions: role of nucleosides in the flanking sequences

Bacterial and synthetic DNA containing unmethylated CpG dinucleotides activate the innate immune system and promote Th1-like immune responses. Recently, a receptor, TLR9, has been shown to recognize CpG DNA and activate immune cascade. But there have been no reports on the molecular mechanisms of recognition between CpG DNA and the receptor(s). Our earlier studies described a number of the chemical and structural characteristics of CpG dinucleotide and the sequences flanking the CpG dinucleotide that are critical for immunostimulatory activity. In the present study, we examined the effect of the presence and absence of a nucleoside in the flanking sequences by replacing one or two natural deoxyribonucleosides at various positions with one or more alkyl- (C2-C12), branched alkyl- (glyceryl or aminobutyryl-propanediol), or ethyleneglycol- (tri or hexa) linkers. The results suggest that a linker substitution at the first two nucleoside positions adjacent to the CpG dinucleotide on the 5'- or the 3'-side neutralizes the immunostimulatory activity, as determined by in vitro mouse spleen cell proliferation, cytokine secretion, and in vivo mouse spleen enlargement. The same substitutions placed about three to six nucleotides away from the CpG dinucleotide either did not affect or potentiated immunostimulatory activity compared with parent CpG-DNA without modifications. Substitution of deoxyribonucleosides with a C3 or C4 alkyl-linker was found to be optimal for potentiating immunostimulatory activity.

Potent CpG oligonucleotides containing phosphodiester linkages: in vitro and in vivo immunostimulatory properties

Bacterial and synthetic DNAs, containing CpG dinucleotides in specific sequence contexts, activate the vertebrate immune system. Unlike phosphorothioate (PS) CpG DNAs, phosphodiester (PO) CpG DNAs require either palindromic sequences and/or poly(dG) sequences at the 3(')-end for activity. Here, we report 'PO-immunomers' having two PO-CpG DNA molecules joined through their 3(')-ends. These PO-imunomers permitted us, for the first time, to assess immunostimulatory properties of PO-CpG DNAs in vitro and in vivo without the need for palindromic and/or poly(dG) sequences. In medium containing 10% fetal bovine serum, PO-immunomers were more resistant than PO-CpG DNAs to nucleases. Compared to PS-CpG DNA in BALB/c and C3H/HeJ mice spleen cell culture assays, PO-immunomers showed increased IL-12 secretion and minimal amounts of IL-6 secretion. PO-immunomers activated NF-kappa B and induced cytokine secretion in J774 cell cultures. In addition, PO-immunomers showed antitumor activity in nude mice bearing human breast (MCF-7) and prostate (DU145) cancer xenografts.

Conjugation of ligands at the 5'-end of CpG DNA affects immunostimulatory activity

Bacterial DNA and synthetic oligonucleotides containing unmethylated CpG dinucleotides (CpG DNA) activate the vertebrate immune system and promote Th1-like immune responses. Several CpG DNAs are currently being tested in clinical trials as either alone or in combination with vaccines, antibodies, and allergens separately or as conjugates for a number of disease indications including cancers, allergies, and asthma. In this paper, we show that conjugation of an oligonucleotide and a CpG DNA through their 5'-ends (5'-5'-linked DNA) significantly reduces the immunostimulatory activity of the CpG DNA. In addition, we found that the reduction in immunostimulatory activity of 5'-5'-linked CpG DNA depends on the size of the oligonucleotide conjugated to CpG DNA. Conjugation of a smaller group or molecule, such as a phosphorothioate group, at the 5'-end of CpG DNA has an insignificant effect on immunostimulatory activity. However, conjugation of a mononucleotide, tetra- or longer oligonucleotide or a fluorescein molecule to the 5'-end of a CpG DNA (5'-5'-linked DNA) significantly suppresses the immunostimulatory activity of CpG DNA. Surprisingly, conjugation of an oligonucleotide or a ligand through the 3'-end of CpG DNA (3'-3'-linked DNA) has an insignificant effect on immunostimulatory activity. Studies of cellular uptake and activation of transcription factor NF-kappaB in J774 cells using fluorescein-conjugated CpG DNAs suggest that the differences in the immune stimulation of 5'- and 3'-end-conjugated CpG DNAs is not as a result of differences in their cellular uptake properties. These results suggest that for optimal immunostimulatory activity, ligands should not be attached at the 5'-end of the CpG DNA.

Potential role of phosphatidylinositol 3 kinase, rather than DNA-dependent protein kinase, in CpG DNA-induced immune activation

Unmethylated CpG motifs present in bacterial DNA stimulate a strong innate immune response. There is evidence that DNA-dependent protein kinase (DNA-PK) mediates CpG signaling. Specifically, wortmannin (an inhibitor of phosphatidylinositol 3 kinase [PI3]-kinases including DNA-PK) interferes with CpG-dependent cell activation, and DNA-PK knockout (KO) mice fail to respond to CpG stimulation. Current studies establish that wortmannin actually inhibits the uptake and colocalization of CpG DNA with toll-like receptor (TLR)-9 in endocytic vesicles, thereby preventing CpG-induced activation of the NF-kappaB signaling cascade. We find that DNA-PK is not involved in this process, since three strains of DNA-PK KO mice responded normally to CpG DNA. These results support a model in which CpG signaling is mediated through TLR-9 but not DNA-PK, and suggest that wortmannin-sensitive member(s) of the PI3-kinase family play a critical role in shuttling CpG DNA to TLR-9.

Antitumor mechanisms of oligodeoxynucleotides with CpG and polyG motifs in murine prostate cancer cells: decrease of NF-kappaB and AP-1 binding activities and induction of apoptosis

Previous studies have shown that CpG oligodeoxynucleotides (ODNs) have substantial immunostimulatory effects with anticancer applications. The antitumor applications that have been described previously are mediated through the CpG-induced activation of the host immune system, not through direct antitumor effects. Using cytostasis and cell proliferation assays, we demonstrated that specific ODNs inhibit the proliferation of RM-1 cells, a murine prostate cancer cell line. Flow cytometry analysis using propidium iodide (PI) nuclear staining confirmed the direct proapoptotic effect of ODNs on prostate cancer cells. This effect was dose dependent. Further studies using Western blot analysis and electrophoresis mobility shift assay (EMSA) revealed that the treatment of prostate cancer cells with specific ODNs activated the caspase pathway(s) and decreased the binding activities of AP-1 and NF-kappaB in a time-dependent manner. Evaluation of a panel of ODNs containing different DNA motifs demonstrated that the optimal proapoptotic sequences required polyG sequences but that CpG motifs were not essential. Finally, in vivo antitumor studies showed that the proapoptotic polyG motifs significantly inhibited prostate tumor growth. PolyG motifs inhibited tumor growth, and the effects were enhanced by CpG immune activating sequences. ODN containing both polyG and CpG motifs may have enhanced efficacy in tumor therapy through multiple mechanisms of action, including direct antitumor activities and immune activation.

Highly immunostimulatory CpG-free oligodeoxynucleotides for activation of human leukocytes

Synthetic oligodeoxynucleotides (ODNs) bearing CpG dinucleotides can mimic the immunostimulatory effects of bacterial DNA in vertebrates. Besides the known CpG motifs, no other sequence motif has been shown to have independent immunostimulatory effects. Several past investigators have demonstrated that the nucleotide content or the phosphorothioate (PS) backbone may have effects independently of the sequence. However, the effect of both nucleotide content and PS backbone to stimulate human leukocytes is not well understood. We investigated the immunostimulatory activity of 34 PS-ODNs with different nucleotide contents, lengths, and methylation status on human leukocytes. The thymidine content showed strong CpG-independent contribution to immunostimulation. In contrast, ODNs rich in other nucleotides (guanosine, cytosine, or adenosine) induced no or much lower levels of immunostimulation. The observed effects were highly dependent on the PS backbone chemistry. In addition to the base content and the backbone chemistry, the length of the PS-ODN was directly related to the magnitude of its stimulatory effects, especially on B cells. In addition, methylation of CpG dinucleotides did not always cause an abrogation of the immunostimulation. Immunostimulatory effects could be observed with methylated CpG ODNs, specifically as the ODN length was increased from 18 to 24 or more nucleotides (nt). In contrast, PS-ODNs with inverted CpG dinucleotides showed some but only weak immunostimulation. Our results demonstrate that non-CpG ODNs rich in thymidine or ODNs with methylated CpG motifs have length-dependent immunostimulatory effects. Such ODNs can induce effects similar to those seen with CpG ODNs but are much less efficient in stimulating human immune cells.

Technological advances to increase immunogenicity of DNA vaccines

The latest clinical data obtained with DNA vaccines against HIV and malaria have shown promise, but it is clear that when DNA vaccines are compared with other vaccine vector delivery systems, there is still room for improvement. Further development is more than possible, based on the wealth of information accumulating on methods and approaches to increase immunogenicity of DNA vaccines. Thus, the goal of this review is to summarize some of the latest technological advances to increase immunogenicity of DNA vaccines administered by the im. and id. routes. By means of examples, the review will be intended to focus only on recent developments reported in the last 2 years and likely to go towards the improvement of mucosal, humoral and cellular immune responses mostly against cancer and infectious disease antigens.

Oblimersen Bcl-2 antisense: facilitating apoptosis in anticancer treatment

The components of the apoptotic program are targets for anticancer therapy. Bcl-2 protein inhibits apoptosis and confers resistance to treatment with traditional cytotoxic chemotherapy, radiotherapy, and monoclonal antibodies (mAb). Oblimersen sodium (G3139, Genasense, Genta Inc., Berkeley Heights, NJ) is an antisense oligonucleotide (AS-ON) compound designed to specifically bind to the first 6 codons of the human bcl-2 mRNA sequence, resulting in degradation of bcl-2 mRNA and subsequent decrease in Bcl-2 protein translation. Oblimersen is the first oligonucleotide to demonstrate proof of principle of an antisense effect in human tumors by the documented downregulation of the target Bcl-2 protein. A growing body of preclinical and clinical evidence suggests that oblimersen synergizes with many cytotoxic and biologic/immunotherapeutic agents against a variety of hematologic malignancies and solid tumors. Randomized clinical trials are currently underway to evaluate the efficacy and tolerability of oblimersen in combination with cytotoxic chemotherapy in chronic lymphocytic leukemia, multiple myeloma, malignant melanoma, and non-small cell lung cancer. In addition, nonrandomized trials are under way to evaluate oblimersen in non-Hodgkin's lymphoma, acute myeloid leukemia, and hormone-refractory prostate cancer. Preclinical data also support the clinical evaluation of oblimersen in additional tumor types, including chronic myelogenous leukemia and breast, small cell lung, gastric, colon, bladder, and Merkel cell cancers. Enhancement of the efficacy of anticancer treatments with oblimersen Bcl-2 antisense therapy represents a promising new apoptosis-modulating strategy, and ongoing clinical trials will test this therapeutic approach.