DNA Ligase 4 Inhibition Sensitizes Prostate Cancer to Immune Checkpoint Blockade In Vivo.

BACKGROUND/AIM: Prostate cancer is a common malignant tumor in men. DNA ligase IV (LIG4) expression correlated with poor prognosis in prostate cancer patients. LIG4 joins DNA double strand breaks and is essential for repair of these genetic lesions. Prostate cancers have not demonstrated clinically significant responses to anti-PD-1 immunotherapy. Prostate cancers express low PD-L1 levels and exhibit limited cytotoxic T lymphocyte infiltrates. To determine the effects of LIG4 inhibition on prostate tumorigenesis, we created a new genetically engineered in vivo model.

MATERIALS AND

METHODS

Lig4+/+;TAg and Lig4+/-;TAg prostate glands and tumors were processed for histopathology. Separate groups of prostate tumor-bearing mice were treated with anti-PD1 antibody or preimmune IgG. LIG4 and PD-L1 expression was determined by quantitative reverse transcription polymerase chain reaction. Expression of DNA damage repair proteins, cell senescence, and cell death markers was determined by immunohistochemistry and immunofluorescence microscopy. The prostate cancer stem cell fraction was analyzed by Sca1/CD49f flow cytometry and tumorsphere culture. PD-L1 protein expression was determined by western blot.

RESULTS

LIG4 inhibition induced DNA double strand breaks and cellular senescence in prostate glands and cancers and significantly reduced prostate intraepithelial neoplasia and tumorigenesis. LIG4 inhibition reduced the prostate cancer stem cell fraction and proliferation in stem cell cultures. Prostate cancers resistant to LIG4 inhibition evaded anti-tumor immune response due to increased PD-L1 expression. PD-1 antibody treatment of these cancers induced CD8+ T lymphocyte infiltration and reduced tumor volume.

CONCLUSION

Inhibition of LIG4 sensitized prostate cancers to immune checkpoint inhibition.