03/09 - Exhaustion Can be Fatal - MSMR: What A Year!

Think back to the last time you were really sick with an infection. What did it feel like? How long did it last? Most sicknesses make you feel pretty crummy and last for about a week. A pathogen, a foreign agent, has invaded your body. The sickness will last as long as it takes your body to fight off the infection, and then you’ll start feeling better. You may think that feeling sick is caused by the infection, but in fact much of that sick feeling is caused by your own body working to fight it. Let’s take a closer look at how your body responds to infection.

The system of the body responsible for keeping you healthy and preventing infection is known as the immune system. The immune system identifies and destroys foreign agents (such as viruses or bacteria) that could be harmful to the body. When your body identifies an infection, there are various mechanisms set into action to combat the invader. Special immune cells known as T-cells carry out many of these mechanisms.

There are several types of T-cells. Two important types of T-cells are known as CD4 and CD8 cells. CD4 cells can be thought of as the generals of the immune system. They coordinate the immune reaction and produce toxins and other soluble factors to combat infection. CD8 cells, on the other hand, are the foot soldiers of the immune system, attacking and destroying viruses or infected cells. Because of their ability to attack and destroy cells, CD8 cells are often called killer T-cells. But this impressive immune response is a two-edged sword, because sometimes T-cells can’t differentiate between healthy cells and infected cells. This means that the same immune responses that prevent and combat infections can also make you pretty sick when you do get an infection.

Biologically, you are a large multi-cellular organism. From an evolutionary perspective, the idea was that as a large multi-cellular organism, you can withstand the immune response better and longer than the bacteria or virus infecting you. You would feel sick as the infection is destroyed and then feel better again.

At some point, the immune system has to find a balance so that it doesn’t cause so much damage that it kills the host … you! In these cases it may be better to live with the infection than to die of the immune response. For this reason, CD8 cells contain many pathways that can shut down cells or induce cell death: these negative regulators weaken the immune response so it does not cause irreparable damage to the body.

You will hear scientists talk about “genetic pathways” and may get the impression that genes are passing along little sidewalks in your body. That is not the case. Here is the Wikipedia explanation: “A genetic pathway is the set of interactions occurring between a group of genes who depend on each other's individual functions in order to make the aggregate function of the network available to the cell.” So when you hear or read about genetic pathways, you will understand that it is really a collection of genetic interactions, not a stroll along a leafy walkway.

Scientists have observed that the T-cells of people infected for a long time become “tired” and do not respond as well to an infection. One reason for this exhaustion is that many well-known chronic diseases have a devious trick. They take advantage of the negative regulators and cell death mechanisms: they turn off or even induce cell death in the T-cells that are supposed to be fighting the infection. Thus they reduce T-cell effectiveness against the pathogen causing the disease. This is one reason why chronic infections can be so difficult to treat and are often deadly. Among the diseases that behave this way are Human Immunodeficiency Virus/Acquired Immunodeficiency Syndrome (HIV/AIDS), hepatitis C, and some types of cancer.

Dr. E. John Wherry of The Wistar Institute has been studying these immune system mechanisms of cell death for over a decade. “There are dozens of these pathways in the immune system,” said Dr. Wherry. “We’re trying to figure out how the immune system uses these pathways.” His initial experiments were focused on finding the main pathway responsible for turning off (=inactivating) T-cells. To do this he compared the CD8 T-cells of healthy mice to the CD8 T-cells of mice that had been fighting a chronic infection. Dr. Wherry used a mouse model of a chronic infection to study the immune system. He used a microarray to compare the genes activated by CD8 T-cells of the infected mice with the genes activated by CD8 T-cells of healthy mice. By comparing the 39,000 or so known genes of each group of mice, Dr. Wherry and his team of researchers identified the genes expressed by the CD8 T-cells in each group.

As a result of this initial survey of CD8 T-cell genes, Dr. Wherry and his colleagues were able to narrow the field of key pathways to several hundred genes. Over the course of many years, the Wherry team tested these genes to determine which ones caused immune dysfunction and cell death. In particular, they identified a role for a pathway known as the Programmed Death-1 pathway or PD-1. “There are about half a dozen pathways similar to PD-1,” Dr. Wherry said. He decided to use PD-1 as a starting point to understand how the PD-1 pathway negatively regulates the immune response.

The researchers continued experimenting with the PD-1 pathway in CD8 T-cells of mice. They treated infected mice with an antibody to block the PD-1 receptor so the PD-1 pathway could not be activated and weaken the immune response. The control mice received a simple solution that did not interfere with the immune system. Dr. Wherry then compared the CD8 T-cells of the treated mice to those of the control mice. Rather than comparing individual cells, he compared whole populations of CD8 T-cells. He found two clear sub-groups of CD8 T-cells among the population of treated T-cells. One group of CD8 T-cells responded well to the treatment and the other group did not. The group that responded to the treatment was able to fight the chronic infection. The CD8 T-cells that did not respond were the focus of the next set of experiments.

The CD8 T-cells that did not respond to the treatment were examined to figure out why they did not respond to blocking the PD-1 receptor. Dr. Wherry found that the cells that did not respond to treatment had many pathways, similar to PD-1, working to reduce the immune response. This meant that blocking only one inhibitory pathway was insufficient to prevent exhaustion. “This is an interesting and rather unexpected result with wide applications,” Dr. Wherry said. These experiments have shown that it may be possible to screen patients to determine the effectiveness of a given treatment based on the number of PD-1 receptors on their CD8 T-cells.

This is an extremely important discovery for many patients suffering from chronic infections. For example, hepatitis C is a chronic infection of the liver. There are drugs used as treatment for the disease, but they can cause bad side effects and are only effective about 50 percent of the time. During both hepatitis C and HIV infection, CD8 T-cells responding to the infections express PD-1.

Preliminary studies in the lab suggest that blocking the PD-1 pathway can also improve immunity to these two infections in people, which is consistent with the experiments in mice. However, there has been a wide range of response to this type of blockade in early studies and it is not yet clear if this approach will provide benefit to patients at all stages of hepatitis C and HIV disease progression. If there was a way to screen these patients to determine if the treatment would be effective, it could improve the quality of life of many people suffering from hepatitis C. Another possibility is that, if there was a way to renew exhausted CD8 T-cells fighting chronic infections, it could improve the effectiveness of the immune system and help restart the immune systems of patients fighting the disease. Dr. Wherry says the PD-1 pathway may also be important in autoimmune diseases and cancer. The PD-1 pathway has been shown to be active in malignant melanoma.

So far Dr. Wherry’s experiments have focused on CD8 T-cells, the foot soldiers of the immune system. He has begun experiments on CD4 T-cells, and these experiments are ongoing. “We are interested and excited to see the differences or similarities in the responses of CD4 and CD8 cells to blocking the PD-1 receptor,” he said. “These results could improve our understanding of the PD-1 receptor and enable us to treat patients with chronic illnesses better.”

He is also interested in the larger picture of the immune system. “We know that different pathways regulate different properties of T-cells.” It is possible that with a better understanding of these pathways it may be possible to block multiple pathways to improve T-cell response. For now, Dr. Wherry hopes to develop drugs to block the PD-1 receptor and help the immune system fight diseases that use the PD-1 pathway.

Dr. E. John Wherry is an Assistant Professor in the Wistar Institute’s Immunology Program where he studies T-cells and their role in the immune response. Dr. Wherry said that he has always had a scientific mind and knew he wanted to be a scientist. He had trouble deciding exactly which part of science he wanted to study, though. “I had intended to be a physics major in college,” he said, “until I saw how much math there is in physics. So I turned to immunology instead.” Dr. Wherry received his Ph.D. in immunology and “never looked back.” His advice to budding scientists is to find an area of interest, something that makes you excited to wake up and go to work each morning. When he’s not in the lab, Dr. Wherry enjoys running, being outdoors, and spending time with his family.

Blackburn, S., H. Shin, G. Freeman, and E. Wherry. “Selective expansion of a subset of exhausted CD8 T cells by αPD-L1 blockade.” Proceedings of the National Academy of Sciences, 2008(105): 15016-21.