【Introduction】
Regulatory T cells (Tregs) play essential roles in maintaining immunological self-tolerance, but suppress effective anti-tumor immune responses. Abundance of Tregs in the tumors of cancer patients closely associated with poor prognosis. Therefore, breaking this regulatory system is important for effective cancer immunotherapy. We previously reported that Nr4a family nuclear factors (Nr4a1, a2 and a3) play crucial roles in the development and function of Tregs. In this study, we investigated the role of Nr4a in Treg-mediated immune tolerance to cancer and possibility of pharmacological modulation of Nr4a activity for effective anti-tumor immunotherapy.
【Method】
Lewis lung cancer (3LL) or colon adeno carcinoma (MC38) cells were injected into syngenic Treg-specific Nr4a1/a2 conditional knockout (Nr4a1/2fl/fl Foxp3-cre ; DcKO) mice. Tumor tissues were harvested, and T cell subsets, population and cytokine production were examined by flow-cytometry. Nr4a inhibitors were screened by measuring transcriptional activity and expression levels of Nr4a. Administration of drugs was started after implanted tumors became palpable.
【Results】
In tumor transplantation model, in Nr4a-DcKO mice, tumor growth was significantly suppressed and CD8+T cell dependent tumor antigen-specific immune response was enhanced compared with WT mice. By a chemical library screening, we identified classical anti-tumor agent Camptothecin as Nr4a inhibitor. We also found that expression of Nr4a factors were up-regulated by the PGE2-PKA signaling and cox-2 inhibitor down-regulated Nr4a expression in Tregs in the tumor microenvironment.
Administration of Camptothecin derivative CPT-11 and cox-2 inhibitor SC-236 (a structural analogue of clinically-used celecoxib) synergistically induced potent anti-tumor immune responses in tumor bearing mice, which were dependent on CD8+ T cells. Adoptive transfer of Treg cells abrogated the effect of these Nr4a inhibitors.
【Conclusion】Our study indicates that Nr4a factors play important roles in Treg-mediated suppression of anti-tumor effects. Nr4a factors could be therapeutic targets to potentiate anti-tumor immunity.