STAT5, STAT6 activate Th9 cells to drive allergic inflammation
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Key takeaways:
- The STAT5 and STAT6 transcription factors activate Th9 cells.
- Th9 cells produce the IL-9 cytokine, which causes allergic inflammation.
- Th9 levels may indicate which patients may respond to JAK inhibitors.
Th9 can drive allergic disease and may be targeted in precision medicine treating patients with high levels of this helper T cell, according to a study published in Nature Immunology.
When they encounter viruses, bacteria or other pathogens, T cells activate and ramp up production of cytokines, Daniella M. Schwartz, MD, an assistant professor of rheumatology at the University of Pittsburgh School of Medicine, and colleagues wrote.
As inflammatory proteins, the researchers continued, cytokines control a suite of immune responses via the JAK-STAT signaling pathway. Specifically, T cells activate when their receptor recognizes an antigen.
However, T cells also activate via bystander activation, which does not involve their receptors. Instead, bystander activation occurs when other types of dangerous signals indicate a threat, the researchers wrote.
Further, Th9 cells do not need these dangerous signals to be activated, and they do not need to see their antigen continue to function. But they do need a “perfect storm” of occurrences to appear, the researchers said, and they do not live very long.
Schwartz said that this “frustrating” instability is one of the reasons why the researchers chose to investigate Th9 cells.
“They are so short-lived that, in our mouse models of airway inflammation, we would have to come into the lab at 9 p.m. on the night before analyzing our mice in order to treat them with inflammatory stimuli,” Schwartz told Healio. “Any earlier than that, and the cells would start to lose their phenotype.”
Many researchers have noticed this very unusual behavior, but nobody knows how or why it happens, Schwartz said. She and her team kept thinking that there had to be some biological reason.
“While we were culturing Th9 cells in vitro, we noticed that our cultured cells were pumping out their inflammatory mediator, IL-9, like crazy, even when we were just letting them rest quietly,” she said. “So, we thought maybe this is a case of Occam’s razor: one related mechanism to explain two unique behaviors.”
Study design and results
Calling Th9 “the black sheep of helper T cells,” the researchers measured IL-9, which is a cytokine that Th9 cells produce, from healthy patients and from patients with atopic dermatitis. The Th9 cells from the patients with AD responded to bystander activation, but the cells from the healthy volunteers did not.
These results suggest that some kind of checkpoint prevents the non-specific activation of Th9 cells in healthy people, the researchers wrote. In patients with allergy, the researchers hypothesized, this checkpoint breaks down so the cytokine is produced without the cells getting restimulated with an antigen.
When an antigen binds to a T-cell receptor in most helper T cells, the researchers wrote, DNA in the T cell’s nucleus then unwinds and opens regions of DNA that encode the production of cytokines, which in turn unleash a host of immune responses.
After the threat is eliminated and there is no more antigen to stimulate the T cell receptors, the researchers said, the cells deactivate. The DNA structure remains open, however, so the cell can be ready for potential encounters in the future.
Regulation is different in Th9 cells, the researchers said. Transcription factors called STAT5 and STAT6, which bind to the open region of DNA around IL-9 to activate the gene, activate the cells. Unusually, the researchers said, the DNA closes over time, which shuts down IL-9 production.
This mechanism of opening and closing serves as a checkpoint to manage immune responses that are on all the time in healthy people. But in people with allergy, when this checkpoint breaks down, the DNA remains open and keeps the IL-9 gene activated, which drives allergic inflammation.
Disease symptoms improved when the researchers blocked JAK-STAT signaling with tofacitinib (Xeljanz, Pfizer), which treats rheumatoid arthritis, AD and other inflammatory disorders, in a mouse model of allergic asthma driven by Th9.
Analysis of the data from the patients with asthma revealed greater activation of STAT5- and STAT6-related genes in patients with higher levels of Th9 cells, indicating that Th9 may be a biomarker for predicting which patients may respond to JAK inhibitors and play a role in new approaches for precision medicine in allergy treatment.
Implications for treatment
Schwartz said that the STAT-dependent bystander activation was surprising since T cells usually are seen as responding to a specific antigen or pathogen or to danger signals, which includes Th2 cells, described by Schwartz as the classic T helper subset linked to allergic disease.
“But STAT5/6-activating cytokines aren’t really danger signals. You see them in the context of immune responses, but also during development, wound healing or even under homeostatic conditions,” Schwartz said. “So, you could imagine that if your cells are poised to respond to these cytokines, it could be quite undesirable.”
This helped the researchers understand why Th9 cells might not stick around for long during inflammatory responses.
“We were also surprised to see that while healthy volunteer T cells didn’t respond to these STAT-activating cytokines, allergic patient T cells did,” Schwartz said. “That’s what really made us ask whether this ‘negative checkpoint’ of lineage instability might break down in allergy and whether that might have implications for patient selection/therapy.”
Based on these findings, Schwartz said that Th9 cells could be both a target and a biomarker in allergy. If Th9 cells are poised to quickly respond to STAT-activating cytokines, then blocking that activation loop potentially could prevent flares of allergic disease, Schwartz said.
“We propose that, because of the importance of STAT-dependent signaling to Th9 acute activation, the presence of Th9 cells in the lung or circulation could be a biomarker to suggest that JAK inhibitors will be effective in an allergic patient,” she said.
JAK-STAT signaling is important for many different cytokines, Schwartz noted, adding that she did not want to suggest that JAK inhibitors do not block other pathways.
“But in our analysis, patients with high Th9 cells also had upregulated JAK-STAT signaling pathways in their blood and tissue and responded well to a JAK-SYK inhibitor in a previously published clinical trial,” she said.
Next, the researchers are interested in prospectively determining whether Th9 and IL-9 predict responses to JAK inhibition in allergic diseases.
“We are also really interested in following up on these mechanisms to understand how genetic variation of IL-9 contributes to inflammatory disease pathogenesis and how that interacts with this bystander activation pathway,” Schwartz said.
“Finally, we want to understand why the lineage instability of Th9 cells breaks down in allergy and whether there is any way to reduce the stability of allergic Th9 cells, getting them back to a healthy/repressed state,” she said.