Vanessa Kibel

Background: Malignant melanoma, treatable with resection if diagnosed early,¹ is the most dangerous form of skin cancer and results from malignant transformation of melanocytes. Melanoma accounts for 80% of skin cancer deaths even though it is only 2% of cancer diagnoses. Current melanoma staging looks at the thickness of the tumor, the expansion to a regional lymph node, and the expansion of metastasis. There are five stages of melanoma invasion ranging from stage 1 being <0.75 mm and stage 5 being >3.0 mm.² Although there are multiple mechanisms that can cause melanoma, we investigate the molecular mechanism using BRAF.³ BRAF protein is a part of the MAPK pathway, which regulates proliferation, growth, apoptosis, and differentiation. The most common mutant BRAF allele is V600E that results in a 480-fold increase in kinase activity. To diagnose the BRAF V600E mutation, monoclonal antibody VE1, sanger sequencing, and PCR are used. Current BRAF-targeted therapies are Vemurafenib and Dabrafenib.¹ Combined treatment shows a 76% positive response rate. Despite combined therapies, many patients relapse several years after finishing treatment. Reactivation is due to acquiring secondary resistance to BRAF and MEK inhibitors leading to reactivation of MAPK signaling.⁴

Objective: In this analysis, we investigate the molecular mechanism of immune evasion elucidated in BRAF V600E mutant melanomas.

Search Methods: A literature review was performed through the PubMed database from 2018-2023 using the following key words: “Malignant Melanoma,” “BRAF Mutant Melanoma,” “BRAF V600E Melanoma,” “Melanoma and the Immune System.”

Results: An intact immune system that includes T-cells is required for BRAF + MEK dual inhibitor therapies to have efficacy. Activation of T-cells and regression of tumors are dependent on pyroptosis of the tumor cells. BRAF + MEK inhibitors work by lowering the immune-suppressive cells in the BRAF V600E melanoma. Cells that are resistant to the BRAF + MEK inhibitor therapy are not inducing pyroptosis and are associated with anti-tumor immune responses.⁴ The immune system is doing constant surveillance. Regulatory T-cells (Treg) impede immune surveillance in the lymph nodes, suppressing the CD8 ⁺ T-cells. In BRAF V600E mutant melanoma, there is an increase in the ratio of Treg:CD8 ⁺ T-cells. Treg recruitment mediated by chemokine signaling restrains the CD8⁺ T-cells in immune surveillance in early tumorigenesis.⁵ Activin A, a member of the TGF-B family also promotes cancer progression by reducing CD8⁺ T-cells. Activation results in shift from CD8⁺ T-cells and NK cells to CD4⁺ T-cells, dendritic cells, and monocytes.⁶ Immune evasion of the tumor is followed by proinflammatory interferons. The increase in tumor growth depends partially on STING pathway activity. IFN signaling in tumor cells is upregulated but there is a diminished amount of CD8⁺ T-cells inside the tumor compared to the CD4⁺ T-cells.⁷ Interferon gamma and TNF alpha are antitumor cytokines that are inhibited by human melanoma-derived exosomes (HMEX). HMEX secrete IL-10 which inhibits the anti- tumor cytokines allowing the tumor to grow. Consequently, anti-IL-10 immunosuppressive effect on the HMEX exosomes could be used for a therapy.⁸ Vaccination containing lipid-coated nanoparticles (LCP) as delivery to tumors NP-delivery of aggressive BRAF mutant melanoma induced strong antigen-specific response which enhanced T-cell response into tumor microenvironment.⁹

Conclusion: BRAF mutant melanoma is one of the most common and aggressive forms of skin cancer. Its aggressiveness is due to its ability to evade the immune system through the decrease in CD8⁺ T-cells. Development of a drug that targets Activin A via LCP vaccination may be a novel treatment option against BRAF V600E mutant melanoma.

Works Cited:

1. Castellani G, Buccarelli M, Arasi MB, et al. BRAF Mutations in Melanoma: Biological Aspects, Therapeutic Implications, and Circulating Biomarkers. Cancers. 2023;15(16):4026.
2. Ottaviano M, Giunta EF, Tortora M, et al. BRAF Gene and Melanoma: Back to the Future. Int J Mol Sci. Mar 27 2021;22(7)doi:10.3390/ijms22073474
3. Davis LE, Shalin SC, Tackett AJ. Current state of melanoma diagnosis and treatment. Cancer Biol Ther. 2019;20(11):1366-1379. doi:10.1080/15384047.2019.1640032
4. Erkes DA, Cai W, Sanchez IM, et al. Mutant BRAF and MEK Inhibitors Regulate the Tumor Immune Microenvironment via Pyroptosis. Cancer Discov. Feb 2020;10(2):254-269. doi:10.1158/2159-8290.Cd-19-0672
5. Shabaneh TB, Molodtsov AK, Steinberg SM, et al. Oncogenic BRAF(V600E) Governs Regulatory T-cell Recruitment during Melanoma Tumorigenesis. Cancer Res. Sep 1 2018;78(17):5038-5049. doi:10.1158/0008-5472.Can-18-0365
6. Pinjusic K, Ambrosini G, Lourenco J, et al. Inhibition of anti-tumor immunity by melanoma cell-derived Activin-A depends on STING. Front Immunol. 2023;14:1335207. doi:10.3389/fimmu.2023.1335207
7. Pinjusic K, Dubey OA, Egorova O, Nassiri S, Meylan E, Faget J, Constam DB. Activin-A impairs CD8 T cell-mediated immunity and immune checkpoint therapy response in melanoma. J Immunother Cancer. May 2022;10(5)doi:10.1136/jitc-2022-004533
8. Shu S, Matsuzaki J, Want MY, et al. An Immunosuppressive Effect of Melanoma-derived Exosomes on NY-ESO-1 Antigen-specific Human CD8(+) T Cells is Dependent on IL-10 and Independent of BRAF(V600E) Mutation in Melanoma Cell Lines. Immunol Invest. Oct 2020;49(7):744-757. doi:10.1080/08820139.2020.1803353
9. Liu Q, Zhu H, Liu Y, Musetti S, Huang L. BRAF peptide vaccine facilitates therapy of murine BRAF-mutant melanoma. Cancer Immunol Immunother. Feb 2018;67(2):299-310. doi:10.1007/s00262-017-2079-7