Th17 Cells Play a Key Role in COPD Complicated by Invasive Fungal Infection

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Th17 Cells Play a Key Role in COPD Complicated by Invasive Fungal Infection: Mouse Study Offers New Clues

Over the past two decades, a dangerous trend has emerged: more people with chronic obstructive pulmonary disease (COPD)—a leading cause of death worldwide—are developing invasive pulmonary aspergillosis (IPA), a severe fungal lung infection caused by Aspergillus fumigatus. For COPD patients, IPA is especially deadly: mortality rates reach 70–100%. But why these patients are so vulnerable, and how their immune systems fail to fight the fungus, has remained unclear. A 2021 study from Chinese researchers now points to Th17 cells—an immune cell type critical for fungal defense—and their signal molecule IL-17 as key players in this life-threatening combination.

What Is COPD, and Why Does IPA Strike Hard?

COPD damages the lungs over time, usually from smoking or air pollution, making breathing difficult. IPA, meanwhile, targets people with weakened immune systems—though COPD patients were once thought “low-risk” for this infection. Today, COPD is recognized as a top risk factor for IPA, with up to 13% of critically ill COPD patients developing the fungus.

The immune system’s ability to fight Aspergillus depends heavily on Th17 cells, a type of white blood cell that produces IL-17. IL-17 acts like a “call to arms,” recruiting infection-fighting cells (like neutrophils) to attack fungi. Th17 cells also help regulate inflammation in COPD—but how they behave when COPD and IPA collide was unknown.

The Study: Testing Th17 Cells in COPD-IPA Mice

Researchers from Beijing Chao-Yang Hospital, Capital Medical University, and other institutions designed a mouse study to answer this question. They created four groups of mice:

  1. Healthy controls
  2. COPD-only (exposed to cigarette smoke for 16 weeks)
  3. IPA-only (given Aspergillus spores directly into the trachea)
  4. COPD+IPA (smoke-exposed mice with Aspergillus infection)

To isolate the role of IL-17, they also tested IL-17 knockout (KO) mice—animals missing the gene for IL-17—with both COPD and IPA.

The team measured:

  • Th17 cell levels in blood and lungs
  • RORγt (a protein that drives Th17 cell development)
  • IL-17 and IL-23 (signals that keep Th17 cells active)
  • Fungal load in lungs (how much Aspergillus was growing)

Key Results: Th17 Cells and IL-17 Protect Against IPA—Even in COPD

The study’s most striking findings:

  1. COPD+IPA mice had fewer Th17 cells than IPA-only mice: Compared to mice with just IPA, those with both diseases had 44% fewer Th17 cells in blood and 68% fewer in lungs. They also had lower levels of IL-17 and IL-23—signals that help Th17 cells fight infections.
  2. RORγt was higher in blood but not lungs: Even though the “driver” for Th17 cells (RORγt) was more abundant in the blood of COPD+IPA mice, it didn’t translate to more Th17 cells in the lungs—where the infection actually occurs.
  3. IL-17 is critical for fungal defense: IL-17 KO mice with COPD+IPA had twice as much Aspergillus in their lungs as normal mice with both diseases. This proves IL-17 plays a direct, protective role in fighting the fungus—even when Th17 cell numbers are low.

Why Does COPD Weakened the Th17 Response to IPA?

The study suggests two main reasons COPD patients struggle to fight IPA:

  1. Immune imbalance: Chronic inflammation from COPD may throw the immune system out of whack, making it harder for Th17 cells to respond to new infections like IPA.
  2. T cell exhaustion: Long-term exposure to smoke and inflammation can “wear out” T cells (including Th17 cells), so they can’t multiply or fight fungi effectively.

Even though Th17 cells tried to respond (higher RORγt in blood), the damage from COPD left too few cells to stop the infection. But IL-17 still made a difference—when it was missing, the fungus ran wild.

What This Means for COPD Patients

This research is a first step in understanding why COPD patients get severe IPA—and how to treat it. The findings suggest:

  • Boosting IL-17 could help COPD patients fight fungal infections.
  • Targeting “exhausted” T cells (e.g., with drugs that reactivate them) might improve immune responses.

But important caveats: The study was done in mice, so human trials are needed to confirm these effects. Still, it’s a promising lead for a condition with few treatments.

Conclusion

For COPD patients, IPA is a silent threat—but this study offers hope. By linking Th17 cells and IL-17 to fungal defense in COPD, researchers have uncovered a new pathway to explore for treatments. While more work is needed, the findings highlight the importance of the immune system’s “fungal fighters” in one of the world’s deadliest lung diseases.

Study Details

  • Original Publication: Chinese Medical Journal (2021)
  • Authors: Wan-Ru Geng, Hang-Yong He, Qing Zhang, Zhao-Hui Tong
  • Funding: National Natural Science Foundation of China (Grant No. 81400003)
  • DOI: doi.org/10.1097/CM9.0000000000001183

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