Cellular immunity of “comb-structured mRNA” as a novel cancer mRNA vaccine

The Kawasaki Innovation Center for NanoMedicine (iCONM; Director: Kazunori Kataoka; Location: Kawasaki, Japan) have announced with Tokyo Medical and Dental University, and Kyorin University that a group led by Prof. Satoshi Uchida, Principal Research Scientist of iCONM (Professor, Department of Advanced Nanomedical Engineering, Medical Research Institute, Tokyo Medical and Dental University), has developed a novel mRNA derivative and demonstrated its high cellular immunity inducing anti-cancer activities in mice model. This novel mRNA, with comb-like structure, has double-stranded RNA in the shape of comb teeth, activates immune cells, and demonstrates high anti-tumor effects in experiments using melanoma and lymphoma model mice. The paper, titled “Comb-structured mRNA vaccine tethered with short double-stranded RNA adjuvants maximizes cellular immunity for cancer treatment,” was published online in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) dated on July 10, 2023 (Note 1).

The efficacy and safety of mRNA vaccines have been demonstrated against novel coronaviruses, and research is currently underway worldwide to target cancer cells as the next target. This cancer mRNA vaccine provides cellular immunity (Note 2) to attack cancer cells by administering mRNA that produces a protein specifically expressed in cancer cells (cancer antigen). However, cancer cells are difficult to distinguish from normal cells and have immunosuppressive effects (Note 3), making the development of cancer vaccines more challenging than vaccines against infectious diseases. Therefore, strategies to enhance the efficacy of cancer mRNA vaccines are necessary, and in this study, we focused on adjuvants (Note 4) for immune activation. If the adjuvant is too strong, it causes adverse reactions, while if it is too weak, it does not provide sufficient vaccine effect. The adjuvant function has been incorporated into previous mRNA vaccines empirically, lacking rational and practical methods to obtain controlled adjuvant activity so far.

In this study, we developed a method of incorporating adjuvant directly into the mRNA chain encoding the antigen without interfering with the ability of the antigen protein production, using our original RNA engineering technique. Short double-stranded RNA (dsRNA) targeting the innate immune receptor retinoic acid-inducible gene I (RIG-I) was designed and loaded into the mRNA strand by hybridization. We obtained optimal comb-structured RNA effectively activating RIG-I by changing the length and sequence of dsRNA. The resulting comb mRNA effectively activated dendritic cells, which play an important role in vaccine efficacy. Furthermore, by changing the number of dsRNAs bound to the mRNA strand, the immunostimulation intensity could be controlled. This is important to prevent excessive immune activation and ensure safety while achieving sufficient vaccine effect.

Next, we evaluated the efficacy of comb mRNA as a cancer vaccine using mice. When comb mRNA was loaded onto lipid nanoparticles, which are used in clinical trials for cancer vaccines, the activity of cellular immunity, which is necessary to attack cancer, was dramatically enhanced. As a result, tumor size was reduced in skin cancer and lymphoma models, and mice’s lives were prolonged. Another important practical aspect of this method is that comb mRNA can be loaded into various mRNA vaccine delivery systems to enhance their efficacy. In fact, we have succeeded in improving vaccine efficacy by loading the comb mRNA into lipid nanoparticles used in a commercial novel coronavirus vaccine and our original polymeric nano-micelles. Thus, the system we have developed is a simple and practical platform that can safely improve the efficacy of mRNA cancer vaccines in various formulations by freely controlling the adjuvant function of mRNA vaccines.

The novelty of this study

Until now, there has been no method to precisely control and rationally incorporate the adjuvant function, which is important for cancer mRNA vaccines, into vaccine design. As a result, we had to rely on an empirical method in which a vast number of candidate compounds are tested in animal experiments to find the optimal one. This approach complicates the development process. To solve this issue, this research succeeded for the first time in the world in incorporating reasonably necessary amounts of adjuvant function into an mRNA vaccine by using a unique method called mRNA engineering. Using this method, the adjuvant function can be easily incorporated into various mRNA vaccine delivery systems, resulting in enhanced functionality of cancer vaccines.

The future of this study

　　In this study, the utility of comb mRNA was demonstrated in various mRNA vaccine delivery systems, including lipid nanoparticles, such as those used in cancer vaccine trials and in the novel coronavirus vaccine that is now in practical use. In other words, comb mRNA is a versatile system that can enhance the efficacy of any mRNA vaccine already under development, and is expected to be put to practical use in the future by loading it with existing mRNA vaccine carriers. In addition, its effectiveness can also be enhanced by integrating it with vaccine technologies that are being developed independently. Cancer mRNA vaccines are being developed at an accelerated pace around the world as the next generation of cancer immunotherapy, and comb-shaped mRNA is expected to become a core fundamental technology for enhancing their efficacy.

Note 1: Proceedings of the National Academy of Science of the USA (PNAS): The official journal of the U.S. National Academy of Science, one of the most prestigious in the world. It publishes more than 3,000 papers a year on all fields of science (physical science, social science, and biological science). Along with Science and Nature, it is known as one of the most highly cited journals, but all articles must be written in a way that is “easy to understand for a wide range of scientific audiences. The Academy is run on a volunteer basis (pro bono) by experts in their fields who contribute their professional knowledge and skills to society. The latest impact factor of this journal is 12.779 (2022-2023). For scientists around the world, having a paper published in PNAS is a great status.

https://www.pnas.org/

The paper describing this presentation is as follows:

Theofilus A. Tockary, Saed Abbasi, Miki Matsui-Masai, Akimasa Hayashi, Naoto Yoshinaga, Eger Boonstra, Zheng Wang, Shigeto Fukushima, Kazunori Kataoka^*, Satoshi Uchida^*, “Comb-structured mRNA vaccine tethered with short double-stranded RNA adjuvants maximizes cellular immunity Proc. Natl. Acad. Sci. 2023 in press.

DOI: https://doi.org/10.1073/pnas.2214320120

Note 2 Cellular Immunity: Immunity obtained through vaccination is classified into humoral immunity and cellular immunity. The former protects the host from infection mainly through the production of neutralizing antibodies that suppress the action of viruses, but is ineffective against large foreign enemies such as infected cells and cancer cells. In contrast, immunity that repels large foreign substances by activating cytotoxic immune cells such as killer T cells or by stimulating the secretion of inflammatory cytokines is called cellular immunity.

Note 3: Immunosuppressive effects of cancer: The more aggressive the cancer is, the more it evades the immune system through various mechanisms such as abnormal proliferation of fibrous tissue, transformation of macrophages into immunosuppressive type (M2 macrophages), convening of bone marrow-derived immunosuppressive cells (MDSCs), and secretion of anti-inflammatory cytokines such as IL-10, and establishes its own habitat (cancer microenvironment), and create an environment (cancer microenvironment) in which they can live comfortably.

Note 4 Adjuvant: A supplemental substance that enhances the effectiveness of a vaccine. It enhances the immunogenicity of the antigen and facilitates more active operation of the immune response system.