Ronnie Youssef

Introduction. Melanoma is the subset of skin cancer that has the highest mortality rate. Distant-stage melanoma, in which tumors have metastasized, has a 5-year survival rate of 20%¹. Research has shown that the interaction between the immune checkpoint programmed death-1 (PD-1) and its ligand PD-L1 has hindered T-cell immunotherapy for melanoma. PD-1 is expressed on T cells and PD-L1 is highly expressed on tumor cells. Their binding initiates an inhibitory signal that downregulates the T cell effector response². Checkpoint inhibition immunotherapy studies using PD-1 monoclonal antibodies, such as Nivolumab, exhibited marked melanoma regression in vitro and in vivo^3-5. Despite its success, system checkpoint blockade is not tumor-specific and immune-related adverse effects have been reported. More recent research indicates that CRISPR/Cas9-engineered melanoma specific T-cells with a PD-1-knockout (KO) are safer and more specific⁶. Methods. Anti-PD-1 antibodies were used to determine its effects on T cells in vitro and on tumor volume in vivo using mice inoculated with B16 melanoma cells^3,4. In a human clinical trial, 107 patients with melanoma were treated with Nivolumab over the course of 8 weeks⁵. In another study, melanoma specific gp100 PD-1-deficient CD8+ T cells were generated using a Cas9/PD-1-specific guide RNA (gRNA) ribonucleoprotein (RNP) in order to knock-out the PD-1 gene. The engineered cells were analyzed during in vitro and in vivo killing assays of B16 melanoma cells⁶. Results. T-cells treated with Nivolumab showed a significant increase in IFN-l production and cell proliferation³. In immunocompetent, PD-1-KO, and immunodeficient mice, the anti-PD-1 antibody caused a decrease in tumor volume⁴. In the human clinical trial, 20-30% of patients receiving Nivolumab exhibited tumor regression and the tumors continued to regress after discontinuation of Nivolumab⁵. Lastly, in the CRISPR/Cas9 study mice with B16 melanoma that received PD-1 deficient T-cells showed a greater reduction in tumor volume as compared to those who received PD-1(+) T-cells. No adverse effects were noted in the mice, and there was no evidence of any off-target cleavage by the CRISPR-Cas9 gene-editing method⁶. Conclusion. The results of these experiments show that anti-PD-1 antibodies such as Nivolumab have proven to be effective in reducing tumor volume and restoring the T-cell effector response^3-5. However, systemic check-point inhibition may cause adverse immune effects. The development of the CRISPR/Cas9 method of genetically editing PD-1 out of melanoma specific T-cells may be a solution to that problem. The success of the pre-clinical model provides promise for its potential use in humans⁶.

1. American Cancer Society: Cancer Facts and Figures 2018. Atlanta, Ga: American Cancer Society, 2018.
2. John LB, Devaud C, Duong CPM, et al. Anti-PD-1 Antibody Therapy Potently Enhances the Eradication of Established Tumors By Gene-Modified T Cells. Clinical Cancer Research. 2013;19(20):5636-5646.
3. Wang C, Thudium KB, Han M, et al. In Vitro Characterization of the Anti-PD-1 Antibody Nivolumab, BMS-936558, and In Vivo Toxicology in Non-Human Primates. Cancer Immunology Research. 2014;2(9):846-856
4. Kleffel S, Posch C, Barthel SR, et al. Melanoma Cell-Intrinsic PD-1 Receptor Functions Promote Tumor Growth. Cell. 2015;162(6):1242-1256.
5. Topalian SL, Sznol M, McDermott DF, et al. Survival, Durable Tumor Remission, and Long-Term Safety in Patients With Advanced Melanoma Receiving Nivolumab. Journal of Clinical Oncology. 2014;32(10):1020-1030
6. Ouchi Y, Patil A, Tamura Y, et al. Generation of tumor antigen-specific murine CD8+ T cells with enhanced anti-tumor activity via highly efficient CRISPR/Cas9 genome editing. International Immunology. 2018;30(4):141-154.