There is a dearth of understanding as to why some patients experience long-term symptoms after contracting COVID-19.

Unexpectedly, a new study shows that months after mild COVID-19, macrophages, a kind of immune cell, altered inflammatory and metabolic expression.

“We can show that the macrophages from people with mild COVID-19 exhibit an altered inflammatory and metabolic expression for three to five months post-infection,” says one of the study authors. “Even though the majority of these people did not have any persistent symptoms, their immune system was more sensitive than that of their healthy counterparts.”

Some people with mild COVID-19 infection may develop long-term symptoms. More research is needed to fully comprehend the long-term immunological abnormalities in patients who have recovered from the acute phase of illness.

To examine this aspect, the researchers in the current study analysed blood samples from 68 people with previous mild COVID-19 infection and a control group of 36 people who had not had COVID-19.

The researchers isolated the macrophages in the laboratory and stimulated them with spike protein, steroids and lipopolysaccharides (LPS), a molecule that triggers the immune system. The cells were then RNA sequenced to measure active genes. The researchers also measured the presence of eicosanoid signaling molecules, which are a fundamental feature of inflammation. 

“It is not surprising to find a large number of eicosanoid molecules in people with COVID-19 as the disease causes inflammation, but it was surprising that they were still being produced in high quantities several months after the infection,” add the authors.

The study also showed a higher concentration of leukotrienes, which are a type of pro-inflammatory molecules known for causing asthma. 

“It’s very striking that the concentration of leukotrienes remains elevated in macrophages in people who have had mild COVID-19,” says the study’s corresponding author Julia Esser-von Bieren. “Leukotrienes are key mediators of asthma, but they’re also involved in the antiviral host defence against influenza. A sustained increase after SARS-CoV-2 infection could cause a greater sensitivity to respiratory inflammation, but could also improve antiviral immunity to SARS-CoV-2 or other viruses.”

The blood samples were collected on two occasions, at three to five months after SARS-CoV-2 infection and after 12 months. At three to five months, around 16 percent reported persistent mild symptoms while the rest were symptom-free. At 12 months, none reported persistent symptoms and there was no longer any difference in inflammatory markers between those with previous COVID-19 infection and the healthy control group.

The researchers note that the post-COVID diagnosis was not specifically examined in the study and as such more research is needed to determine if these results can be directly linked to what is also known as long COVID.

Source: 10.1038/s41385-021-00482-8

Image Credit: Getty

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