Improving melanoma immunotherapies by regulating Treg proliferation

By Samantha Black, PhD, ScienceBoard editor in chief

January 7, 2020 -- A new pathway to regulating anti-tumor immunity and increasing the effectiveness of PD-1 therapies was identified by scientists at Sanford Burnham Prebys Medical Discovery Institute. The results were published in Nature Communications on January 7.

Fundamentals of immunotherapy

Cancer immunotherapy treatments utilize an individual's immune system to destroy tumors. In some cases, such as advanced melanoma, this treatment has extended survival by years instead of months. This treatment utilizes a blocking agent against programmed death ligand-1 (anti-PDL-1) to interrupt the shield that protects tumor cells from the immune system. However, the treatment only works in around 40% of individuals with advanced melanoma. The research presented in this study presents a way to improve the effectiveness of this treatment who do not respond to the anti-PD-1 therapy.

The ubiquitin proteasome system controls several key cellular functions in cell- and tissue-dependent manner. E3 ubiquitin-protein ligase (Siah2) encodes a protein of the seven in absentia homolog (SIAH) family that is involved in the ubiquitination and proteasome-mediated degradation of specific proteins. Understanding the underlying mechanisms ubiquitin ligases in control of tumor immunity could help researchers identify new targets for therapeutic modalities.

Another key to understand anti-tumor immunity is the balance between effector T cells and regulatory T cells (Tregs). Tregs are a subpopulation of T cells that are immunosuppressive and generally suppress or downregulate induction and proliferation of effector T cells. Previous research suggests that modulation of Tregs can treat cancer, as they are often recruited to tumor sites to suppress tumor immunity, and thus have the potential to be utilized in immunotherapies.

Improving immunotherapies

Therefore, the researchers wanted to determine the role of Siah2 in controls T cell proliferation and modulating anti-tumor immunity. They used genetically engineered mice, Siah2−/− mice, and introduced BRAF-mutant melanoma (a mutation that occurs in over half of human melanomas). Using a battery of cell-based analysis such as flow cytometry, and molecular techniques including RNAseq were used to study the role of Siah2 in tumor proliferation.

Ze'ev Ronai, PhD - Sanford Burnham Prebys

"In our study, mice lacking the Siah2 gene were able to mount an immune attack against melanoma," explains Ze'ev Ronai, PhD, professor in Sanford Burnham Prebys' Tumor Initiation and Maintenance Program and senior author of the study. "Moreover, the effectiveness of Siah2 in immunotherapy was demonstrated for 'cold' tumors--those that do not respond to immunotherapy--which were effectively eliminated by a PD-1 blockade in Siah2-mutant mice."

To understand the role that Siah2 has in the tumor microenvironment, the researchers used Siah2 negative mice and found that in the absence of the Siah2 gene, melanoma tumors receded, whereas when the Siah2 gene was present, the tumors grew. Moreover, they found that when Siah2 negative mice were given the anti-PD-1 treatment, melanomas were effectively eliminated.

"While Siah2 is involved in control of activities that govern cancer development, this study offers the first direct evidence for its role in the immune system, namely in anti-tumor immunity," says Ronai. "Our study shows that a PD-1 inhibitor can be used to treat tumors that currently do not respond to this therapy, when administered in mice lacking the Siah2 gene, thereby offering a means to expand the effectiveness of immunotherapy. The findings also provide further justification for our efforts to find a drug that blocks Siah2."

With a deeper dive into the mechanisms behind the effect of Siah2, the researchers discovered that Siah2 regulation of Treg recruitment and cell cycle progression effectively controls melanoma development. In Siah2 negative mice, they observed increased expression of cyclin-dependent kinase inhibitor p27, which controls the proliferation of Tregs but not effector T cells. These changes lead to increased anti-tumor immunity.

"Understanding the basic mechanisms of tumor immunity will ultimately help us improve the effectiveness of immunotherapy," says Michael Rape, PhD, Howard Hughes Investigator and professor of Cell and Developmental Biology at University of California, Berkeley. "This study uncovers an important layer in the regulation of key immune cell components that impact the effectiveness of cancer immunotherapy, highlighting the need to develop inhibitors for Tregs, in which a Siah2 inhibitor holds promise."

"Our discovery only fuels our sense of urgency to find a drug that inhibits Siah2," says Ronai. "Using an arsenal of novel approaches should enable us to advance the targeting of Siah2 in both the tumors and their microenvironment."

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