Hepatitis C is a viral infection of the liver. Jaundice, i.e. the yellowish pigmentation of the skin or the white of the eye, is one of several symptoms of the disease. With approximately 150,000 infected people in Germany, hepatitis C is a common disease for which therapies, but no vaccine, are available. Dr. Tobias Böttler from the Freiburg University Medical Center and his team are exploring the body’s immune response to the hepatitis C virus (HCV) and have found valuable information that could be used for vaccine development.
Haemoglobin, the molecule that gives red blood cells their characteristic red colour, is broken down in the liver, resulting in bilirubin that is excreted in the urine. If this process is defective, for example as a result of a liver inflammation, bilirubin accumulates in skin and mucous membranes and causes the typical yellow pigmentation of the skin. Liver infections can be caused by hepatitis A, B or C viruses. Prophylactic vaccines are available against hepatitis A and B viruses, but not against infection with the hepatitis C virus. Around 100 million people have the hepatitis C virus (HCV) worldwide, in developing countries significantly more than in industrialised countries. HCV is transmitted through infected blood; before 1990, transmission occurred mainly through blood transfusions, nowadays, the virus is mainly transmitted through drug injection needles or non-sterile tattoos or piercings.
Chronic HCV infection can lead to liver cancer
Hepatitis C infections are difficult to diagnose in the acute phase of the disease when patients have only mild or no symptoms at all. Fatigue, loss of appetite and joint pain are often perceived as flu-like infections. Many patients are completely unaware that they might be HCV infected. “In contrast to other viral infections, hepatitis C can be both acute and self-limiting as well as chronic,” says Tobias Böttler from the Freiburg University Medical Center. In 30 percent of cases, the disease resolves spontaneously, and the virus is eliminated by the immune system. In the remaining 70 percent of cases, liver disease develops slowly but gradually over several decades, and a doctor may only suspect the presence of a hepatitis C infection when the patient has abnormal liver values. If left untreated, one third of those infected will develop liver cirrhosis, which in turn represents an increased risk of hepatocellular carcinoma. However, the disease usually clears with treatment – at least when it is detected at an early stage.
Standard treatment consists of a combination of at least two to three drugs. So-called direct acting antivirals (DAA) have been available for hepatitis C infection treatment for around two years now. DAA are polymerase, protease and NS5a inhibitors that interfere with the replication of the viral genome by blocking the proteins the virus needs for replication. Treatment involving DAA works well, but is also very expensive. A twelve-week course of therapy costs around 60,000 euros, a price people in developing countries cannot afford to pay. “In Europe and North America we treat around one percent of all patients worldwide with these drugs,” says Böttler. A vaccine could help improve this situation considerably.
The body must recognise a virus before it can fight against it. “Two things may happen – the viruses are either eliminated by the immune system or they hide from the immune response,” says Böttler. A virus that is able to evade the immune system can cause chronic infections.
Body cells then die either because they are destroyed by the virus or damaged by the immune system in its effort to eliminate the virus. In the case of hepatitis C, the body itself is the culprit. When large numbers of viruses are present, the immune system must decide between killing all liver cells or retreating. In the latter case, the virus has won. “But if it kills all liver cells, there will eventually be no liver cells left, which is not a good solution either,” says Böttler. A compromise is usually best: the body accepts that the virus stays in the liver, and does not destroy it in order to protect the liver against damage. “The virus itself does not cause any damage,” says the researcher. Hepatitis C infections become chronic in 70 percent of cases because the immune system continues to fight the infection on a low flame. “As the body is constantly trying to fight the infection, the inflammation continues over time and will eventually destroy the liver tissue,” explains Böttler. The successive incorporation of more and more connective tissue over many years will eventually lead to liver cirrhosis, resulting in the disruption of tissue architecture. The scarred tissue alterations prevent blood from flowing through the organ. The greatest risk factor for liver cancer is therefore not the virus itself, but rather the liver cirrhosis it has caused.
The human body can fight an HCV attack in two ways. The virus proteins are recognised as foreign and trigger an immune response in the liver and lymph nodes. On the one hand, the virus is directly attacked by T cells, and on the other, T cells join forces with B cells and trigger an antibody reaction. Böttler and his team are studying the cell type that connects T and B cells. They have managed to detect specialised follicular T helper cells (Tfh), which were known to exist but whose role had not previously been studied in detail. Tfh are specialised T cells that migrate into the B-cell zones relatively early during infection and help B cells fight the pathogens. They provide maturation signals to B cells, which then produce pathogen-specific antibodies.
The researchers identified Tfh cells based on the presence of the surface proteins CXCR5, PD-1 and ICOS and discovered that patients with acute HCV infection had far more virus-specific Tfh cells. They also found an association between the large number of Tfh cells and the large number of antibodies. Tfh cells are a very exciting cell population that is, however, difficult to study,” says Böttler. These difficulties can be put down to the lack of methods to isolate these cells from the lymph nodes, their low frequency and the narrow time window in which the cells are present. These problems are pretty obvious considering that there must be a specific cell type for each virus. Tfh cells and the markers on their surface disappear from patients’ blood during the course of infection and in patients with chronic infection.
The three markers that characterise follicular T helper cells do not however provide information on the type of virus at which the cells are directed. Therefore, Böttler has also analysed the T cell receptor, a molecule found on the surface of T cells that recognises specific antigen fragments and thus also HCV-specific ones. This is a good starting point for further research as the knowledge of the exact course of the virus-specific Tfh response in acute and chronic infections provides valuable information that can be used for developing a prophylactic vaccine. The study suggests that Tfh cells activate B cells, which produce antibodies and so-called memory cells that protect the body against re-infection. In order to find out more, Böttler and his team will compare hepatitis B virus infections for which a vaccine exists with hepatitis C infections. “We want to find out what an effective Tfh response is like in people who are vaccinated and then find ways to modulate the T cell response to make it effective against HCV.”