Stanford Medicine investigators have discovered that a previously disregarded type of immune cell called an interstitial macrophage is actually the most susceptible to SARS-CoV-2 infection, the virus responsible for causing COVID-19. This unexpected finding has significant implications for the prevention and treatment of severe COVID-19. The virus enters this vulnerable cell through a surprising route, leading to potentially important medical consequences.The researchers have found that interstitial macrophages, which are deep in the lungs and are responsible for protecting the organs from viruses, bacteria, fungi, and dust particles, are the most vulnerable to SARS-CoV-2 infection. This study, to be published in the Journal of Experimental Medicine on April 10, shows that these cells can transition from being protective to potentially deadly when infected with SARS-CoV-2. The infected macrophages can also transform into virus producers.d release inflammatory and scar-tissue-inducing chemical signals, which could lead to pneumonia and harm the lungs to the point where the virus and these potent substances can escape the lungs and cause damage throughout the body.
The unexpected findings suggest new strategies for preventing a SARS-CoV-2 infection from becoming a life-threatening illness. They may also help explain why monoclonal antibodies designed to treat severe COVID were not very effective, and only worked when given early in the infection when the virus was still infecting cells.
It was found in the upper airways on its way to the lungs but had not yet settled into the lung tissue.
New findings about the virus
“We have challenged several incorrect assumptions about how the virus replicates in the human lung,” stated Catherine Blish, MD, PhD, who is a professor of infectious diseases and microbiology and immunology, as well as the George E. and Lucy Becker Professor in Medicine and associate dean for basic and translational research.
Blish co-authored the study with Mark Krasnow, MD, PhD, the Paul and Mildred Berg Professor of Biochemistry and the Executive Director of the Vera Moulton Wa.The Center for pulmonary vascular disease states that the virus infects interstitial macrophages, causing a massive inflammatory reaction that can flood the lungs and spread infection and inflammation to other organs. According to Krasnow, blocking this step could be a significant therapeutic advancement. However, the virus has an unusual method of getting inside these cells, which drug developers have not yet learned to block effectively. This alternative mechanism requires a new focus for drug development. This information was published in Nature.In early 2020, Krasnow and his colleagues, including then-graduate student Kyle Travaglini, PhD, who is also one of the new study’s co-lead authors along with MD-PhD student Timothy Wu, described a method they had developed for isolating fresh human lungs . They were able to dissociate the cells from one another and characterize them individually based on their active genes and levels of activity. Using this method, the Krasnow lab and collaborators identified over 50 distinct cell types, creating an atlas of healthy lung cells.
“We had just completed this atlas when the COVID-19 pandemic struck,” Krasnow said. Shortly after, he found out that Blish and Arjun Rustagi, MD, PhD, who is an instructor of infectious diseases and another lead co-author of the study, were constructing an ultra-safe facility for growing SARS-CoV-2 and infecting cells with it.
This led to a collaboration. Krasnow and Blish and their team acquired fresh healthy lung tissue removed from seven surgical patients and five deceased lung donors whose lungs were free of the virus but not suitable for transplants for various reasons. After infecting the lung tissue with SARS-CoV-2 and waiting one to three days for the infection to spread, they separated and identified the cells.In order to create a map of infected lung cells, similar to the one Krasnow’s team had made for healthy lung cells, the scientists observed most of the same cell types in the infected lung tissue. This allowed them to compare healthy and SARS-CoV-2-infected lung cells of the same type and analyze the differences. Their goal was to identify which cells the virus targeted, how easily it replicated within the infected cells, and which genes were activated or deactivated in the infected cells compared to the healthy ones. They were able to accomplish this for each of the numerous cell types they had identified.
The experiment was simple, and the researchers thought they knew what to expect,” Krasnow explained. “But the results were surprising.”
It has been believed that the cells most susceptible to SARS-CoV-2 infection in the lungs are the alveolar type 2 cells. This is because these cells, along with many other cell types in various organs including the heart and gut, have a high concentration of a molecule called ACE2 on their surfaces. SARS-CoV-2 has the ability to bind to ACE2 and manipulate it, allowing the virus to enter the cells.
The scientists discovered that alveolar type 2 cells are somewhat susceptible to SARS-CoV-2, but the most frequently infected cell types were two varieties of macrophages. These macrophages, whose name comes from the Greek terms meaning “big eater,” play a crucial role in the body by ingesting foreign particles such as dust, spores, bacteria, and viruses present in the air we breathe.
Virus factories
The airway
The airways leading to our lungs end in many tiny alveoli, which are thin-walled air sacs bordered by numerous capillaries. This area, known as the interstitium, is where oxygen from the air we breathe enters the blood and is then carried throughout the body by the circulatory system.
There are two types of lung-related macrophages that are susceptible to SARS-CoV-2, and they are located in different areas. Alveolar macrophages are found in the air spaces within the alveoli. When infected, these cells produce and release viral offspring at a slow pace, but they continue to function and maintain their resilience.The normal function of alveolar macrophages may actually help SARS-CoV-2 to progress by incubating and producing new viral particles that can escape and penetrate the layer of cells around the alveoli. Interstitial macrophages, another type of cell, are also easily and deeply infected by SARS-CoV-2 and patrol the far side of the alveoli where oxygen and red blood cells meet. If a viral particle manages to evade alveolar macrophages and penetrate the cells enclosing the alveoli, it puts the lungs at risk.
But when it comes to interstitial macrophages, they are usually prepared to protect the body. However, when they encounter SARS-CoV-2, the virus infects them instead of being eliminated by them.
Once infected, interstitial macrophages become overwhelmed as the virus takes control and hijacks the cell’s protein and nucleic acid production. This leads to the destruction of the cell’s boundaries and the production of large numbers of virus copies.The cell bursts open like a spatula breaking and spreading the yolk of a raw egg. The new virus leaves the old macrophage and go on to infect other cells.
But that’s not all. Unlike alveolar macrophages, infected interstitial macrophages release substances that signal other immune cells in the body to come to the lungs. According to Krasnow, this would cause an inflammatory influx of these cells in a patient. As the lungs fill with cells and fluid, oxygen exchange becomes impossible. The barrier that keeps the alveoli intact becomes increasingly damaged. Infected fluids leak from the damage.The movement of viral progeny from alveoli into the bloodstream helps to spread the infection and inflammation to other parts of the body. Additionally, substances released by SARS-CoV-2-infected interstitial macrophages can lead to the production of fibrous material in connective tissue, causing scarring of the lungs. This scarring can impair the ability of the lungs to exchange oxygen. Dr. Blish noted that while it is not certain that a lung cell in a laboratory dish will become infected with COVID-19, it is believed that this may be the stage at which the infection becomes unmanageable in an actual patient.
Another interesting finding is that SARS-CoV-2 uses a different method to infect interstitial macrophages compared to other types of cells. While the virus attaches to ACE2 on the surfaces of alveolar type 2 cells and alveolar macrophages, it enters interstitial macrophages using a different receptor that these cells have. The study found that blocking SARS-CoV-2’s binding to ACE2 protected the former cells but did not affect the latter cells’ susceptibility to SARS-CoV-2 infection.
“SARS-CoV2 was able to enter interstitial macrophages through a different entryway, making them susceptible to infection,” the researchers said. “This unexpected discovery complicates our understanding of the virus’s behavior in the body, and suggests a need for further research to fully understand how it functions.”“It’s not using ACE2 to get into interstitial macrophages,” Krasnow said. “It enters via another receptor called CD209.”
That would appear to clarify why monoclonal antibodies created specifically to block SARS-CoV-2/ACE2 interaction were unsuccessful in reducing or preventing severe COVID-19 cases.
It’s time to discover a completely new set of medications that can hinder SARS-CoV-2/CD209 binding. Now, Krasnow said.
The research was supported by the National Institutes of Health (grants K08AI163369, T32AI007502 and T32DK007217), the Bill & Melinda Gates Foundation, Chan Zuckerberg Biohub, the Burroughs Wellcome Fund, Stanford Chem-H, the Stanford Innovative MeResearchers from Stanford University, the Chan Zuckerberg Biohub, and the Howard Hughes Medical Institute conducted a study on the role of interstitial macrophages in viral takeover and inflammation in CO.
