Disappearing Act: Novel Population of Transient Macrophages Repair Lungs After Illness

At the height of the SARS-CoV-2 pandemic, researchers in Thomas Marichal’s immunophysiology laboratory at the University of Liège were studying samples collected from the lungs of patients who had recovered from severe COVID-19. They made a striking observation.

“We were expecting to see maybe neutrophils or T cells, but the amount of macrophages we saw in the samples was huge,” Marichal said. “They are not normally present in a healthy lung [in large numbers], so it was really interesting and intriguing.”

Monocyte-derived macrophages recruit to the lungs en masse during the short window immediately following viral infection, so Marichal and his colleagues were at a loss as to why they appeared in such large numbers in the lungs of people who had already recovered from the virus.¹

Around the same time, Marichal’s graduate student Cecilia Ruscitti was working with a mouse model of influenza A virus (IAV) and had become fascinated by a highly transient population of cells. The team initially assumed that these cells were neutrophils based on their expression of Ly6G, a surface marker used to sort neutrophils from other cells.² Similar to the macrophages that Marichal observed in the COVID-19 samples, these neutrophil-like cells appeared during the recovery phase after severe viral illness. Then, they vanished.

“The more we looked at these cells, the more I was convinced these actually were not neutrophils, but macrophages,” Marichal remarked.

Now, in a recent Science Immunology study, Marichal and his colleagues confirmed that these unusual cells that emerged following an IAV infection are macrophages, and that they facilitate the repair of alveoli in the lungs following severe viral infections in mice.³ The findings highlight a novel population of macrophages that could be targeted therapeutically to treat lung damage.

“The findings are very robust—they covered all their bases,” commented Kirsten Spann, a virologist from Queensland University of Technology who was not involved in the study. “The authors really investigated the mechanism of how this particular subtype of macrophages is induced in the flu infection model in mice, so the data and the methods they used are very solid.”

When the researchers ran flow cytometry on the atypical Ly6G⁺ cells collected from the mouse IAV lung samples, additional surface markers cropped up that signaled to the researchers that they were looking at something different from neutrophils.

“The main reason why I think these cells have never been seen before is that people in the macrophage field are not interested in neutrophils, so they remove them based on the Ly6G marker, and they also removed these macrophages from their analysis,” speculated Marichal.

Subsequent single-cell RNA sequencing and transmission electron microscopy experiments further confirmed that the Ly6G⁺ macrophages were distinct from neutrophils. During severe respiratory illness, such as COVID-19 or IAV, Marichal said, lungs sustain patches of damage to the alveoli, impairing gas exchange. Once the infection has been cleared, the lesions are repaired by progenitor cells, which differentiate into epithelial cells that form new alveoli. Using confocal microscopy to peer into the lungs of mice that had cleared IAV infections, the team found Ly6G⁺ macrophages located near the damaged tissue in the space between the alveoli, where they clustered with progenitor cells. The team hypothesized that the Ly6G⁺ macrophages were orchestrating damage repair by giving instructions to the progenitor cells.

Further experiments confirmed that Ly6G⁺ macrophages release soluble factors that instruct the progenitor cells to proliferate and differentiate. They found that this function was partly dependent on signaling via the interleukin-4 (IL-4) receptor, which is known to induce a repair phenotype in other macrophages.⁴

To explore the full capabilities of these interesting cells, the team turned their attention to other types of injury and different organs. They found that Ly6G⁺ macrophages repair the lungs and liver of mice after drug-induced injuries as well. The team also showed that monocyte-derived macrophages present in samples of lung fluid from humans with suspected pneumonia are transcriptionally similar to Ly6G⁺ macrophages in mice.

Marichal and his colleagues said that their results could pave the way for new therapies that promote lung regeneration after injury and illness. For example, IL-4-based treatments could encourage differentiation of Ly6G⁺ macrophages and subsequently boost progenitor cell proliferation and repair. “You have less control over that scenario, [because] monocytes are quite plastic,” Spann cautioned. “So, you wouldn’t always be assured that if you manipulated the monocytes, they would actually differentiate into helpful macrophages and not harmful macrophages.”