mRNA Uses Unconventional Pathways in CD8+T Cell Priming to Help Vaccines Work

mRNA vaccines scored a stunning win against SARS-CoV-2 in 2020, and now the Nobel-prize-winning technology is out to conquer some cancers. Several mRNA vaccines are already in clinical trials for melanoma, small cell lung cancer, and bladder cancer, among others. Recently, a pancreatic cancer vaccine grabbed headlines after researchers shared that most Phase I trial participants were still alive after several years -- unprecedented in a disease that is considered incurable, and usually kills patients quickly.

But how exactly does mRNA work? A new study suggests a broader role for how T cells become activated after an mRNA vaccine. It's a process that engages both cDC1 and cDC2 cells redundantly. The study was led by researchers at Washington University School of Medicine in St. Louis (WashU) and could lead to improvements in mRNA vaccine design. The findings were published in Nature. The work was powered by a novel mouse model developed by the WashU team.

"My lab made them in 2019 and 2022. We put all of them in Jackson labs [database] so anyone can get them, no strings, and study them," senior author Kenneth M. Murphy, MD, PhD, told Inside Precision Medicine. "Thanks to them, we saw the question and were able to address it most quickly."

Until now, scientists assumed that cDC1, which is a classical type 1 dendritic cell, was required for mRNA vaccination to activate the immune system. But, in a lab study, these researchers found that even without cDC1 cells, the mRNA vaccine still triggers strong cancer‑killing responses. That's because they determined that cDC2, a cousin to cDC1, can also stimulate anti-tumor immune activity -- an unexpected finding given that this related subtype is not involved in responses to other vaccines.

"There is a lot of interest in applying the mRNA vaccine approaches used during the COVID-19 pandemic to the problem of inducing anti-tumor immunity," said Murphy, the Eugene Opie Centennial Professor in the department of pathology & immunology at WashU Medicine. "By dissecting which immune cells are involved and how they coordinate the response, we're offering vaccine developers some additional mechanistic insights to consider in their goal of optimizing these vaccines against tumor proteins."

Murphy is also a research member at Siteman Cancer Center, based at Barnes-Jewish Hospital and WashU Medicine.

mRNA vaccines work by delivering instructions, in the form of messenger RNA, for immune cells to produce bits of protein that trigger the immune system to destroy cells bearing these proteins. Dendritic cells produce the protein bits from the mRNA instructions, and T cells then find and destroy the invading proteins. To treat cancer, mRNA vaccines can be designed to generate protein bits unique to a tumor.

The work was done in collaboration with the study's co-corresponding author, William E. Gillanders, MD, the Mary Culver Professor of Surgery at WashU Medicine. Gillanders, a physician-scientist and surgical oncologist who has also developed an investigational vaccine against triple-negative breast cancer, treats patients at Siteman Cancer Center.

Murphy and members of his lab used their mouse models, which lacked cDC1 or cDC2, to tease out the role that different groups of dendritic cells play in priming T cells after mRNA cancer vaccination.

One of their findings was that mice immunized with an mRNA vaccine generated strong T-cell responses even in the absence of cDC1s. In addition, they found that immunized mice without cDC1s were able to clear sarcoma tumors -- cancers that develop in connective tissues such as fat, muscle, nerves, blood vessels, bone, and cartilage. This indicated that some other cell type must be stimulating the T-cell response.

Indeed, their study found that cDC2s also participate in generating an immune response from T cells and preventing tumor growth. Further, the study found that T cells turned on by cDC1s and cDC2s each showed slightly different molecular "fingerprints." These differences could help scientists design better versions of vaccines in the future.

Similarly, immunized mice lacking cDC2s and mice that had both cell subtypes produced an immune response and rejected tumor growth, demonstrating that mRNA vaccination uses both dendritic cell subtypes to stop cancer.

"This work uncovers a new way mRNA vaccines engage the immune system -- through both cDC1 and cDC2 -- which helps explain their power and gives researchers concrete targets for making future mRNA cancer vaccines more effective," said Gillanders. "It could improve vaccine formulation and dosing, potentially explain why some patients respond better to vaccines than others, and guide strategies for making vaccines more effective."