Wednesday, December 14, 2011

Overcoming chronic infection with immune revival

Infections with pathogens can become chronic after a while. The main reason is that our immune system has no unlimited supply of 'cellular soldiers' that can be send to battle viruses or bacteria. Eventually, our bodily defences have to give in, and pathogens find a permanent home somewhere in our organs, causing permanent dismay to the patient. Scientists from the Emory Vaccine Center have found a way to revitalize the immune system after it is depleted due to chronic infection. This could be beneficial for treatment of viral infections of HIV or hepatitis.

Virus infection
In the lab, the researchers studied infection of a virus called LCMV in mice. A few weeks after exposure, the immune system gives in, and a chronic infection is developed. This is a useful model for the human form of the disease, also because the mouse immune system uses the same kind of cells to fight off the infection: to clear away infected cells, a class of killers called lymphocytes are used. Specifically, it is the CD8 positive T lymphocyte that does the work, as this is the one meant for killing cells that are no longer functioning properly.

Rekindling inflammation
To boost the immune system in chronically infected mice, a different subset of T lymphocytes was used, known as CD4 positive. They are from the same family as the CD8 version, but instead of killing, they function as mere helpers. They help to elicit a widespread immune response. When injecting mice with a dose of CD4 T lymphocytes from a healthy mouse, the CD8 T lymphocytes were reactivated and started killing the virus-infected cells again. According to the scientists, they have never seen such a sharp reduction in viral levels before, when it comes to chronic infection.

There are some virus infections that people can carry their whole life. Some are relatively harmless, and find a home somewhere in our body without being noticeable, but some of them cause a lot of harm. Examples include hepatitis, where a chronic infection can lead to big problems in the liver. Additionally, a HIV infection exhausts the immune system because the CD4 positive helper cells are being infected. If a similar therapy can be deployed as was shown in mice, then we might be able to treat these infections more effectively.

Vaccine development
One of the main properties of vaccines is that they activate our immune system to fight off a certain pathogen. Usually, we vaccinate to prevent disease, but in this case, the immune system needs to be activated after it has already launched a response on its own. Vaccines do not only protect against viral or bacterial infections. Recently, scientists have found promising new ways to treat or prevent cancer by making use of vaccines.

The immune system
The CD8 and CD4 cells are part of a large network of cells that make up our immune system. Other important cells include the B lymphocytes, that predominantly function as little factories that churn out antibodies. These small molecules circulate in the bloodstream, to bind and neutralize pathogens they encounter.

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