Plasmidic CpG sequences induce tumor microenvironment modifications in a rat liver metastasis model

Area of Science:

• Immunology
• Molecular Biology
• Oncology

Background:

• Bacterial DNA's unmethylated cytosine-phosphate-guanine (CpG) motifs trigger immune responses.
• CpG motifs are used in cancer treatment via CpG oligodeoxynucleotides (CpG-ODN).
• Naked DNA in gene therapy trials is increasing, yet CpG effects are mainly studied with CpG-ODN.

Purpose of the Study:

• To analyze early tumor microenvironment changes after intratumoral plasmid injection in a rat liver metastasis model.
• To investigate the role of plasmidic CpG motifs in immune cell recruitment and activation.

Main Methods:

• Intratumoral injection of native or SssI-treated plasmid (CpG motifs inactivated by methylation) in a rat liver metastasis model.
• Analysis of plasmidic CpG motif activity by measuring IFN-gamma secretion from rat splenocytes.
• Flow cytometry and mRNA expression analysis of tumor-infiltrating immune cells and cytokines/chemokines 24 hours post-injection.

Main Results:

• Plasmidic CpG motifs were confirmed to be immunostimulatory, inducing IFN-gamma secretion.
• Active plasmidic CpG sequences decreased conventional dendritic cell numbers but increased plasmacytoid dendritic cells and natural killer cells.
• Upregulation of CCR7 and increased expression of IL-1beta, IL-10, and IL-18 were observed in tumors with active CpG motifs.

Conclusions:

• Active plasmidic CpG motifs in tumors recruit and activate immune cells crucial for antitumor responses.
• These early immune events, driven by plasmidic CpG, can enhance immune responses against tumor antigens.
• While not sufficient for therapeutic effect alone, CpG-driven immune modulation shows potential in cancer gene therapy.