Gut fungus has long-term impact on the immunological response to severe COVID-19: Study

According to a new study led by Weill Cornell Medicine and New York-Presbyterian, specific gut-dwelling fungi thrive in severe cases of COVID-19, exacerbating the disease’s extreme inflammation while also causing long-term immune system changes.

This research indicates a subset of individuals who could benefit from specialised, yet-to-be-determined treatments.

Using patient samples and preclinical models, the researchers discovered that the proliferation of fungus in the intestine, particularly Candida albicans yeast strains, causes an increase in immune cells whose actions can exacerbate lung injury.Their findings, published in Nature Immunology on Oct 23, also elucidate that patients retain a heightened immune response and immune memory against these fungi for up to a year after the resolution of SARS-CoV-2 infection.

The research reveals a new dimension of the complex pathology unleashed by severe COVID-19, according to senior author Dr Iliyan Iliev, an associate professor of immunology in medicine in the Department of Medicine, co-director of the Microbiome Core Lab and a member of the Jill Roberts Institute for Research in Inflammatory Bowel Disease at Weill Cornell Medicine.

“Severe and long COVID-19 were not thought to involve fungal blooms in the intestines that, in addition to the virus, can impact patient’s immunity,” he said.

Dr Iliev, an immunologist who studies the microbiome and the chronic inflammatory conditions targeting the gastrointestinal tract, pivoted to COVID-19 during the pandemic. As researchers gained a better handle on the new viral infections, it became clear that in COVID-19 as in inflammatory bowel disease, the body’s own inflammatory immune response causes harm.

To investigate this errant immune response, Dr Iliev and Dr Takato Kusakabe, a postdoctoral fellow and the first author in the study, worked with numerous colleagues to acquire three large clinical cohorts of COVID-19 patients and develop a mouse model to study the disease.They collaborated with members of the Weill Department of Medicine and the Department of Pathology and Laboratory Medicine at Weill Cornell Medicine, including Dr Stephen Josefowitz, Dr Mirella Salvatore, Dr Melissa Cushing, Dr Lars Westblade, and Dr Adolfo Garcia-Sastre, a professor of microbiology and director of the Global Health and Emerging Pathogens Institute of the Icahn School of Medicine at Mount Sinai.

The team first made the connection when analysis of blood samples from patients at New York-Presbyterian/Weill Cornell Medical Center diagnosed with severe COVID-19 unveiled the presence of antibodies tuned to attack fungi common to the gut.The researchers then found that populations of yeast, and one species in particular, Candida albicans, increased in the intestines of the patients during the course of severe COVID-19.

When they looked at the patient’s immune systems, the researchers found a parallel increase in immune cells called neutrophils. In severe COVID-19, excessive numbers of neutrophils appear in the lungs, where their activity worsens the inflammatory response already damaging these organs.

Turning to preclinical models, the investigators found that mice bearing fungi from patients with severe COVID-19 produced more neutrophils in their blood and lungs, and had signs of heightened inflammation when infected with SARS-CoV-2. However, giving them an antifungal drug reduced these effects.

From within patients’ blood samples, researchers also uncovered evidence of persistent changes to the immune system they believe are related to a condition known as long COVID-19, in which symptoms linger, or new ones develop after an infection has cleared.When the team examined patients’ blood up to a year afterwards, they found it still contained anti-fungal antibodies. In addition, when they looked at the stem cells that give rise to neutrophils, the researchers found that these progenitors are primed to respond to fungi.They found that an immune protein called IL-6 that these fungi induce, appears to bolster both the neutrophils and the antibodies.