Abstract C102: Combination of immune checkpoints DNA vaccines and radiation enhances melanoma control
Authors: Lan Keng-Hsueh , Wei Ming-Feng , Lan Keng-Li , Cheng Ann-Lii , Kuo Sung-Hsin 
Published: May 2016

Background:

We intend to develop DNA-based vaccines targeting immune checkpoint proteins in order to enhance immune response to tumors. Ionizing radiation, similar to some conventional chemotherapy, possesses immunomodulatory properties. Therefore, we reason that in combination with radiation, which may expose tumor-related antigens to the T cells, these immune-modulating DNA vaccines may reverse tumor-induced immune evasion and create a sustainable anti-tumor immunity.

Materials and methods:

The DNA sequences encoding murine CTLA-4 or PD1 were PCR amplified using a cDNA library obtained respectively from leukocyte of mouse and human as template. The resulting PCR products were fused with a transmembrane domain sequence of placental alkaline phosphatase (PLAP) into a mammalian expression plasmid, pVAC-1, forming CTLA-4 and PD1 DNA vaccines. These c57BL/6 mice (5 mice/group) were injected with DNA vaccine or control plasmid intramuscularly with the aid of electroporation weekly for 3 times. One week after the last vaccination, sera from the immunized mice was subjected to ELISA assay for detection of antibody titers against the respective immune checkpoint protein. B16F10 murine melanoma tumors were established on the c57BL/6 mice. The radiation regimen used (when applicable) was 2 weekly fractions of 5 Gy after the tumors were established. The mice were divided into groups, each treated with either radiation, radiation plus CTLA-4 vaccine, radiation plus PD1 vaccine, radiation plus CTLA-4 and PD1 vaccines, or no treatment (control group).

Results:

The mice vaccinated with the CTLA-4 or PD1 DNA vaccines demonstrated increased antibody titers against respective proteins, compared with the control groups (p &lt; 0.005). Compared with radiation alone, the addition of the CTLA-4 vaccine led to enhanced tumor regression (a 53.2% reduction of tumor volume relative to that of radiation alone group) 1 month after radiation started. However, immunization with PD1 vaccine did not significantly enhance the tumor suppression of radiation (a 14.7% reduction of tumor volume; p = 0.55). Interestingly, near total regression of B16 tumor was observed when the mice received radiation plus immunization with both CTLA-4 and PD1 vaccines, a 92.4% decrease of tumor volume relative to that of radiation alone group (p = 0.037).

Conclusion:

We have developed potent DNA vaccines targeting CTLA-4 and PD1 immune checkpoints, enhancing the tumor control effects of radiation in the melanoma animal model. These DNA-based vaccines have the advantages of being less expensive and easier to manipulate as opposed to the antibody-based reagents. The blockade of PD1 checkpoint alone is futile in aiding tumor control of radiation in the c57BL/6 melanoma model. The interplay of various aspects of immunity seems crucial in the effort to recruit its anti-tumor ability.

Citation Format: Keng-Hsueh Lan, Ming-Feng Wei, Keng-Li Lan, Ann-Lii Cheng, Sung-Hsin Kuo. Combination of immune checkpoints DNA vaccines and radiation enhances melanoma control. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C102.