With lisence to kill

The next movie about James Bond, the hero with lisence to kill, will be the 27th film in the series. Immunologists don’t need to wait several years between each time we get to see and hear new stories about secret agents with lisence to kill. We’ve got the killer T-cells.

One of the major challenges to the immune system is to detect whether one of the body’s cells are infected with a virus. Virus is a chemical information package that can penetrate cells and take over the cell’s machinery to replicate itself. These new virus particles will eventually slip out of the host cell and can then infect new cells. What initially was a small problem (namely a few viruses and a few virus infected cells) may quickly become an insurmountable problem because of the need to handle large quantities of new virus particles and new virus infected cells. This is where T cells with a lisence to kill enters the narrative.

T cell recognizes virus infected cell and kills it

Peptides from a virus bind to HLA molecules. The HLA-peptide complex is  brought to the cell surface, allowing T cells to recognize the virus infected cell and kill it.

A virus infected cell will always contain some virus proteins. Some of these viral proteins will be cut into shorter pieces or peptides. The peptides are then pumped into the cell compartment where HLA molecules are made. HLA molecules actually need peptides in order to be fully made. Without a peptide bound to the groove, an HLA molecule is unstable and may fall apart like a house of cards. When an HLA molecule has been produced and the groove contains a peptide, it is transported out to the cell surface. All cells in the body except for red blood cells have HLA molecules on their surface. This is the cell’s way of reporting on the latest hours events inside the cell.

Killer T-cells constantly patrol the body. They check all body cells they pass. Each of these secret agent cells are in particular looking for a specific virus. If a T cell finds a body cell that displays a peptide from this particular virus, it gets turned on. The killer T-cell establishes physical contact with the body cell. It then produces holes in the virus-infected cell so it dies. This method of killing is effective. When the job is done, the T cell go on searching for more virus-infected cells to kill.

The first time we become infected by a virus, there are not very many T cells with the lisence to kill cells that are infected with this particular virus. It will therefore take some time before all the virus infected cells are tracked down and killed. Meanwhile, the virus has had plenty of time to multiply and damage body cells.

Thus, the first time you are infected with a given virus, it is therefore likely that you will become ill. Next time, however, your killer T-cells are better prepared. They are more numerous and they attack faster. Most often you do not know that you have become infected by the same virus again. The immune system clears away the danger before you notice anything.

Blog written by Anne Spurkland, first published in Norwegian October 13 2012
English version publishsed November 19 2016