Australian researchers have discovered a key differentiation process that provides an essential immune function in helping to control cancer and infectious diseases.
The researchers at the Walter and Eliza Hall Institute of Medical Research (WEHI) are the first to show that a new transcription factor – DC-SCRIPT – is required for the function of type 1 conventional dendritic cells (cDC1), which are essential in controlling the immune response to infection.
Published in the April 2, 2021, issue of Science Immunology, the research defines the role for the new regulatory protein (DC-SCRIPT) in producing dendritic cells. Lead study authors Stephen Nutt, Michael Chopin and Shengbo Zhang have uncovered a key step in the formation of cDC1 in controlling the immune response.
The research highlights the importance of DC-SCRIPT in the production of cDC1 cells, which activate 'killer' T cells that clear viral infections and trigger an immune response to cancer tumors.
"What we found, is that without this new factor, the cells develop poorly, and their capacity to fight infection and cancer, or to clear a parasite, is diminished," said Stephen Nut, who is the laboratory head at the WEHI immunology division.
Despite the importance of dendritic cell lineage diversity, "its genetic basis is not fully understood," study authors said. Yet the study extends "our understanding of DC development by characterizing the function of a previously poorly known transcription factor, DC-SCRIPT," which is expressed in cDCs and their committed bone marrow progenitors but not in plasmacytoid DCs (pDCs), the study authors noted.
"Dendritic cells are quite important, because these are the cells that patrol the human body and identify the killer T cells to engage a response," study author Michael Chopin told BioWorld Science.
"It's a critical link between what we call the innate immune system and the adaptive immune system," said Chopin, who is a senior post doc fellow in Stephen Nut's lab.
With respect to DC-SCRIPT, what was missing was the marker that looks at this particular subset of dendritic cells – cDC1 – that essentially prime the CD8 T cells that will kill virus-infected cells or cancer cells, he said.
"Without the cDC1, you won't mount a proper CD8 T cell response, and you won't clear a virus or mount a proper immune response against cancer," he said.
"What we define here is a novel transcription fighter, and we use it as a biomarker to follow the behavior of those cDC1s."
CRISPR-Cas 9 makes the cut
The aim of the study was to understand the importance of DC-SCRIPT in the development and function of mouse cDCs. To that end, the group generated a Chimeric protein that it could follow in vivo, and only the cell that expresses that biomarker was labeled, Chopin said.
The group used genomic editing technology to generate reporter mice that were tagged with DC-SCRIPT, and it used CRISPR Cas 9 to generate a mouse that lacks this protein so it could follow the functionality of these cells that lack the protein.
Mice lacking DC-SCRIPT displayed substantially impaired development of interferon regulatory factor 8 (IF8)-dependent cDC1.
Chopin explained that cDC1 patrols the body and looks for threats and produces cytokines, including IL-12 that is regulated by DC-SCRIPT, Chopin said.
"Without IL-12 you can't launch a proper CD8 T-cell response," he added. "DC-SCRIPT is essential for promoting a cytokine response that is actually priming the CD8 T cells to produce interferon."
The study showed that "residual DC-SCRIPT-deficient cDC1s had impaired capacity to capture and present cell-associated antigens and to secrete IL-12p40, two functional hallmarks of this population. Genome-wide mapping of DC-SCRIPT binding and gene expression analyses revealed a key role for DC-SCRIPT in maintaining cDC1 identity via the direct regulation of cDC1 signature genes, including IRF8," the study says.
In fact, IF8 is the "lineage-defining transcription factor for cDC1s because it is highly expressed in all cDC1s and absolutely required for their development."
Harnessing the body's natural response to infection
The study showed that cDC1-produced IL-12 played a critical role in responding to Toxoplasma gondii infection. Results showed that infected Zfp366 mice lost weight more rapidly and harbored a higher parasite burden than control chimeras, and analysis of the serum confirmed that DC-SCRIPT is a "major regulator of cDC1 functionality, including promoting the production of IL-12p40 that is an essential for an efficient response to the parasite infection," the study said.
By developing a mouse reporter strain, the researchers defined DC-SCRIPT expression at the single-cell level. In fact, DC-SCRIPT expression "could be detected in a fraction of clonogenic progenitors committed to the cDC1 lineage in the BM [bone marrow], whereas it was absent in cDC2 progenitors," it said.
The next stage of the research is to "try and work out how we can get the body to produce these particular dendritic cells, cDC1s, in large volumes in order to boost the body's natural tumor response," Nutt said.
Chopin said he was confident cDC1s held the clues to improving immunity to viruses and tumors.
"This paper clearly shows DC-SCRIPT is one of the regulators of dendritic cell production. As a result of this study, we're now focused on ways we could harness this to increase dendritic cell production," he said.
"We now have a biomarker to follow when we expand this elusive cell type, which we will continue to test in preclinical models."
He noted that cDC1s are quite rare and represent only 0.1% of the hematopoietic cells in tissue.
"Given the fact that this population is important for controlling tumor burden, we want to find innovative ways to expand this in vitro as much as possible."
This research lays the foundation for future studies into dendritic cell production and their clinical applications in response to tumors.
Chopin said there has been some controversy in scientific circles about the use of dendritic cells in the context of vaccines. Monocytes are often used as a source of monocyte dendritic cells, "but those cells are not really efficient to launch a proper CD8 T-cell response, so you need access to these cDC1 cells."
"We have generated new tools, allowing us to trace these cells within the tumor and observe how they behave in the tumor environment," Chopin said.
"We now define the biomarker to allow us to understand the differentiation process of this particular subset," he said, and the next step is to drive the expansion of this particular transcription factor and expand the number of cDC1 cells