“This could well be contributing to severe disease,” Dr. Catherine Blish, a professor at Stanford University Medical Center and one of the report’s two senior authors, said. “We’re seeing the same inflammatory cytokines that I see in the blood of the really sick patients being produced in response to infection of those tissues.”
Body fat used to be thought of as inert, a form of storage. But scientists now know that the tissue is biologically active, producing hormones and immune-system proteins that act on other cells, promoting a state of nagging low-grade inflammation even when there is no infection.
Inflammation is the body’s response to an invader, and sometimes it can be so vigorous that it is more harmful than the infection that triggered it.
Fat tissue is composed mostly of fat cells, or adipocytes. It also contains pre-adipocytes, which mature into fat cells, and a variety of immune cells, including a type called adipose tissue macrophages.
Dr. Blish, with colleagues at Stanford and in Germany and Switzerland, carried out experiments to see if fat tissue obtained from bariatric surgery patients could become infected with the coronavirus, and tracked how various types of cells responded.
The fat cells themselves could become infected, the scientists found, yet did not become very inflamed. But certain immune cells called macrophages also could be infected, and they developed a robust inflammatory response.
Even stranger, the pre-adipocytes were not infected, but contributed to the inflammatory response. (The scientists did not examine whether particular variants were more destructive in this regard than others.)