The test detects the response of T cells to the virus — an arm of the immune system that may be just as important as antibodies to preventing reinfection.

A new type of test can detect a person’s immune response to the coronavirus better than a widely used antibody test, according to research released on Tuesday.

The test, if authorized by the Food and Drug Administration, would be the first commercial product to detect the response of a T cells — a type of immune cell — to the virus. Antibodies have dominated the conversation on immunity since the start of the pandemic, but scientists believe that T cells may be just as important in preventing reinfection.

The test was developed by Adaptive Biotechnologies, a company based in Seattle. The company used small blood samples from 1,000 people across 25 metropolitan areas in the United States as well as another 3,500 participants from Europe 

Several studies have suggested that T cells that remember the virus persist for at least six months. “There’s a growing realization that T cells are important and may even be a better indicator of clinical outcome” than antibodies, said Alessandro Settee, an immunologist at the La Jolla Institute for Immunology in California.
But isolating T cells is an elaborate and onerous process, severely limiting the information about their role.

Each of the trillions of T cells present at birth carries a unique receptor on its surface that can spot a different molecule, or antigen, from potential invaders. This enormous diversity among T-cell receptors enables the human body to recognize virtually any new pathogen that it may encounter (although a vast majority may never meet their match.) But it also makes it painfully onerous for scientists to identify the 20 or 30 T cells among the trillions that can recognize fragments of a specific pathogen like the coronavirus.

“One of the brutal things about immunology is that all white blood cells look the same,” said Shane Crotty, a virologist at the La Jolla Institute for Immunology. “You can’t even tell B cells from T cells, let alone T cells specific to a virus.” 

To find the T cells specific to a virus, researchers must first separate the bulk of immune cells from a few millilitres of blood, which takes time. Then they wash and count the cells, and stimulate them with pieces of the virus to identify the tiny pool of T cells that respond. The cells that are activated release a molecule called gamma interferon that serves as a signal of their activity.

“The better the assay, the more complicated it is,” Dr. Crotty said. “You get more and more information, but it becomes more and more complicated.”

A single researcher can process no more than five blood samples over the course of a day and a half.

But with the urgent realization of T cells’ importance in Covid-19, several research teams are trying to streamline the steps involved in a test. Dr. Settee and Dr. Crotty are identifying the ideal set of antigens with which to stimulate the T cells.

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Microbiology: Current Research