Myeloid cells, particularly macrophages, play a key role in combating respiratory infections: they help eliminate pathogens and limit inflammation. The lungs contain several specialized populations of tissue macrophages, each with distinct functions. Although a robust immune response is necessary to control viral infection, studies show that in severe COVID-19, adverse outcomes are often associated more with impaired immune regulation than with high viral load. Therefore, understanding the mechanisms by which different macrophage populations control inflammation is important for developing effective treatments.
Nerve- and airway-associated interstitial macrophages – NAMs – are of particular interest. They are known to limit inflammation during influenza virus infection, but their role during SARS-CoV-2 infection remained unknown until recently.
Scientists at New York University (USA) used a mouse model of SARS-CoV-2 infection. They found that selective depletion of NAMs led to only a slight increase in viral load but to a substantial increase in viral spread throughout the lungs. At the same time, inflammation increased sharply: levels of the proinflammatory cytokines IL-1α, IL-1β, and IL-6, as well as chemokines, increased. These changes were accompanied by 100% mortality among infected mice. By contrast, control mice with an intact NAM population effectively limited inflammation, controlled the virus, survived, and recovered from the infection.
NAMs require signaling through the type I interferon receptor IFNAR to perform their functions. In mice with IFNAR deficiency in CD169+ macrophages, which include NAMs, the characteristic increase in NAM numbers after SARS-CoV-2 infection was impaired, inflammation intensified, and mortality increased.
NAMs Are Necessary for Survival During Coronavirus Infection
The researchers conducted an experiment in which NAM-deficient mice and wild-type control mice were infected with SARS-CoV-2. Unlike the control mice, all NAM-deficient mice died 4–5 days after infection. Viral load increased only slightly, whereas lung damage increased dramatically.
NAM-deficient mice developed severe lung inflammation, vascular damage, and hemorrhaging. As early as the second day after infection, the alveolar structure was disrupted, followed by extensive inflammation and destruction of lung architecture.
These results show that NAMs play a key role in controlling the inflammatory response and protecting lung tissue, thereby determining the outcome of SARS-CoV-2 infection.
NAMs Limit Coronavirus Spread in the Lungs
On the second day after infection, the virus was found primarily in the epithelial cells of the large airways in wild-type mice. By contrast, in NAM-deficient mice, the virus spread from the airway epithelium into the lung tissue, even though the overall increase in viral load remained small. Viral antigen was not detected in the NAMs themselves, whereas alveolar macrophages showed signs of infection. These results indicate that NAMs are not infected by SARS-CoV-2 and thus limit the virus’s spread in the lungs.
NAMs Limit Inflammation During Coronavirus Infection
The absence of NAMs was accompanied by a sharp increase in the inflammatory response after infection. The lungs showed increased numbers of neutrophils, neutrophil extracellular traps, inflammatory monocytes, and mucus. At the same time, levels of proinflammatory cytokines and chemokines, including IL-1α, IL-1β, IL-6, TNF-α, MIP-1α, MIP-1β, and MCP-1, as well as the neutrophil growth factor G-CSF, increased. Production of type I and type II interferons was not reduced; on the contrary, it was elevated during the early stages of infection.
Following SARS-CoV-2 infection, the population of alveolar macrophages decreased, whereas the number of interstitial macrophages (IMs), including NAMs and CD169− IMs, increased. This indicates that NAMs do not die as a result of viral infection and increase in number either through proliferation or through the differentiation of bone marrow-derived monocytes in response to the infection.
These findings indicate that NAMs are key regulators of inflammation and prevent the development of an excessive immune response that causes lung damage and severe coronavirus disease.
Type I Interferons Are Necessary for NAM Expansion and Recovery from Coronavirus Infection
NAMs express the type I interferon receptor IFNAR at significantly higher levels than alveolar macrophages. To determine its role, the researchers selectively deleted IFNAR from CD169+ macrophages in mice and then infected the animals with SARS-CoV-2. Compared with wild-type mice, mice lacking IFNAR in CD169+ macrophages lost more weight, failed to recover from infection, and died 4–7 days after infection. Despite only a moderate increase in viral load, they developed severe lung damage, pneumonia, vascular abnormalities, and hemorrhaging.
Thus, the protective function of NAMs during SARS-CoV-2 infection depends on IFNAR signaling, which is necessary to limit inflammation and prevent severe lung damage.
IFNAR Signaling Is Necessary to Increase NAM Numbers and Control Coronavirus Spread
In mice with IFNAR deficiency in CD169+ macrophages, the virus spread from the epithelium of the large airways into the lung parenchyma as actively as it did in the absence of NAMs, with viral antigen detected throughout the lung tissue. In wild-type mice, the virus was detected only in the large airways and adjacent lung tissue.
The absence of IFNAR was also accompanied by increased neutrophil infiltration and the formation of neutrophil extracellular traps.
As before, NAMs were not infected with SARS-CoV-2. However, the characteristic increase in NAM numbers after infection did not occur, while the number of alveolar macrophages remained similar to that in wild-type mice. The failure of the NAM population to expand was associated with reduced proliferation rather than cell death.
Type I Interferon Signaling in NAMs Limits Inflammation During Coronavirus Infection
The absence of IFNAR signaling in CD169+ macrophages did not affect type I interferon production. Still, it led to a sharp increase in the levels of proinflammatory cytokines and chemokines, including IFN-γ, IL-1α, IL-1β, IL-6, MCP-1, and G-CSF. At the same time, production of the regulatory cytokine IL-10 was impaired. The expansion of the NAM population was also disrupted, which was accompanied by increased inflammation, lung damage, and more active viral spread.
Human lungs also contain a macrophage population similar to mouse NAMs. In patients who died from severe COVID-19, the expression of NAM-associated genes was reduced, whereas airway proinflammatory cytokine levels were elevated. These findings point to a similar immunoregulatory role for NAMs in mice and humans, underscoring their importance in protecting the lungs during coronavirus infection.
