Eight months ago, the new coronavirus was unknown. But to some of our immune cells, the virus was already something of a familiar foe.
A flurry of recent studies has revealed that a large proportion of the population — 20 to 50 percent of people in some places — might harbor immunity assassins called T cells that recognize the new coronavirus despite having never encountered it before.
These T cells, which lurked in the bloodstreams of people long before the pandemic began, are most likely stragglers from past scuffles with other, related coronaviruses, including four that frequently cause common colds. It’s a case of family resemblance: In the eyes of the immune system, germs with common roots can look alike, such that when a cousin comes to call, the body may already have an inkling of its intentions.
The presence of these T cells has intrigued experts, who said it was too soon to tell whether the cells would play a helpful, harmful or entirely negligible role in the world’s fight against the current coronavirus. But should these so-called cross-reactive T cells exert even a modest influence on the body’s immune response to the new coronavirus, they might make the disease milder — and perhaps partly explain why some people who catch the germ become very sick, while others are dealt only a glancing blow.
“If you have a population of T cells that are armed and ready to protect you, you could control the infection better than someone who doesn’t have those cross-reactive cells,” said Marion Pepper, an immunologist at the University of Washington who is studying the immune responses of Covid-19 patients. “That’s what we’re all hoping for.” ...
In theory, cross-reactive T cells can “protect almost like a vaccine,” said Smita Iyer, an immunologist at the University of California, Davis, who is studying immune responses to the new coronavirus in primates. Previous studies have shown that cross-reactive T cells may guard people against different strains of the flu virus, and perhaps confer a trace of immunity against dengue and Zika viruses, which share a family tree.
The case for coronaviruses is less clear cut, said Alessandro Sette, an immunologist at the La Jolla Institute for Immunology who has led several studies examining cross-reactive T cells to the new coronavirus. Researchers have found people in the United States, Germany, the Netherlands, Singapore and the United Kingdom who have never been exposed to the new coronavirus but who carry T cells that react to it in the lab.
Researchers are eager to understand the history of these T cells, because that might help reveal who is more likely to have them. A growing body of evidence, including data published this week in Science by Dr. Sette and his colleagues, points to common-cold coronaviruses as a potential source. But even unrelated viruses can share similar features, and researchers may never know for sure what originally “drove their development,” said Avery August, an immunologist and T cell expert at Cornell University....
