Brachytherapy Combined with CpG ODN Enhances Development of a Tumor Antigen-Specific CD8 Response.

Background: CpG Oligodeoxynucleotides (CpG ODN) are TLR9 agonists that have potent immunostimulatory effects and function well as immune adjuvants. Preliminary data suggests that combining CpG ODN with apoptosis-inducing agents can enhance the development of a tumor specific immune response. We hypothesize that administration, in a temporally and spatially coordinated fashion, of a therapy that induces apoptosis of lymphoma cells with an immunostimulatory agent such as CpG ODN will enhance the development of an anti-lymphoma T-cell response.

Methods: C57Bl/6 mice were inoculated subcutaneously with 2x10^6 EG7 lymphoma cells (EL4 murine T cell lymphoma line with an Ova transgene). On day 11 mice were treated with brachytherapy (radiation therapy administered by temporary placement of an 125-I pellet within the tumor), intra-tumor CpG-ODN or both CpG ODN and brachytherapy. A standard radiation dose equivalent to exposure to a 5 mCi pellet for 10 minutes was used as brachytherapy for each mouse. Sham radiation (placement of a non-radioactive seed in the tumor) and control (non-CpG) ODN were used as controls. Seven days after treatment, spleens were harvested and stained with CD8a-FITC and Ova-tetramer-PE to identify Ova-specific CD8 cells. Non-specific tetramer binding was detected using LCMV-tetramer-PE as a control.

Results: The combination of brachytherapy and CpG ODN led to the development of an enhanced Ova-specific CD8 response as measured by the percent of CD8(+) cells that were positive for Ova-tetramer minus the percent that were positive for the control tetramer.

Percent of CD8 Cells that Stain with Ova Tetramer

Similar results were found in repeated experiments where brachytherapy and CpG ODN were administered daily times three.

Conclusions: Brachytherapy administered via temporary placement of a radioactive seed in a lymphoma, followed immediately by injection of immunostimulatory CpG ODN into the same tumor bed, results in enhanced development of a tumor antigen specific T-cell response. Ongoing studies are exploring the anti-tumor activity of this response, how it compares to the combination of external beam radiation and CpG ODN, whether such responses can be induced against native antigens, and how CpG ODN compare to other immunostimulatory agents when used in this capacity.