The Spoils of War — How T Cells Refuel to Wage War on Pathogens

A killer T cell attacks an infected cell.

In humans, T cells fight viruses and other invaders. Scientists have long thought T cells simply killed an enemy and then moved on to fight others, but new research suggests that some T cells use the spoils of their battles to win the larger war.

White blood cells are part of the immune system, the human body’s main defense against viruses and other infectious agents, all of which are called pathogens. Pathogens take over the machinery of a cell and cause it to manufacture more pathogens, leading to infection. The body has six major classes of white blood cells, each offering specific weaponry and tactics for combating infection. One class of white blood cells is the lymphocytes, which include T cells and B cells.

Killer T Cells

One type of T cell, the CD8+ T cell, is known more commonly as a “killer” T cell. It attacks infected cells with cytoxins, which cause the plasma membrane of an infected cell to open, allowing water, ions, and toxins to rush in. The infected cell then swells and bursts, destroying the pathogens that have taken over the cell. Until recently, scientists thought that T cells left their victims after killing them and simply moved on to attack other infected cells. Results of recent research, however, show that the interaction between killer T cells and infected cells is more like that of predator and prey.

Caught “Green-Handed”

Mark Slifka and Carol Beadling, researchers at Oregon Health & Science University’s Vaccine and Gene Therapy Institute, happened upon this discovery while observing interactions between killer T cells and cells infected by a virus.

Slifka and Beadling dyed infected cells with a fluorescent green dye so that these cells could be more easily seen under a microscope. They then unleashed killer T cells that were specific to the virus of the infected cells, to observe their interactions. They watched the small T cells attack and kill the large green infected cells as expected. But Slifka and Beadling noticed something strange. The T cells, which had not been dyed, were themselves turning green as the infected cells broke apart. Slifka and Beadling realized that the T cells were actually eating portions of the infected cells’ membranes. Like a child whose tongue is stained purple from a popsicle, the fluorescent dye was a telltale sign that the killer T cells weren’t just killing the infected cells, they were feeding on them. They had been caught “green-handed.”

A Well-Fed Army

A fluorescent infected cell is killed and partially devoured by killer T cells. (Credit: David Parker and Scott Wetzel/OHSU)

“This is truly a case of microscopic cannibalism,” Slifka says. “And this is the first time we’ve seen virus-specific killer T-cells ingest parts of infected cells.” Slifka thinks that the benefit of this behavior is that T cells can refuel themselves before fighting other infected cells. He compares this to an army of warriors that invades a city, destroys it, but takes care to gather resources that could help it maintain strength in its ongoing war. “Not only do you have this warrior cell coming in and attacking these virus factories, but it’s able to take away nourishment from this in order to help it to continue the fight against the infection,” he says.

A similar response to virus-infected cells by CD4+ T cells, also known as “helper” T cells, was observed by researcher David Parker, also of the Oregon Health & Science University. The value, Slifka says, of these discoveries about T cells is that the same experimental techniques used to study interactions with pathogens could be applied to observe and measure the response of T cells to a vaccine.

Hope for Vaccines?

A vaccine is a substance that carries the identifying markers, or antigens, of a virus but does not have the destructive capabilities of the actual virus. Some vaccines are made from dead viruses. Others are weakened versions of a virus. A vaccine triggers an immune response. The body produces lymphocytes such as B and T cells that will be able to recognize the real virus should it ever appear in the body. In other words, a vaccine teaches the human body how to identify and defeat a virus, but without putting the body through the danger of the real viral infection.

Slifka’s findings suggest that if a vaccine were marked with a fluorescent green dye, and killer T cells were then unleashed to attack and consume the vaccine, scientists could accurately measure the interaction between the vaccine and the killer T cells. This could help determine a vaccine’s effectiveness, as well as the dosage needed to immunize a person.

More to Explore

Check out the following sites to hear from Dr. Slifka, read more about his discovery, and watch killer T cells as they kill and eat an infected cell.