The immune system in action
A particularly “hungry” immune cell
A macrophage (green cell membrane, orange cell nucleus) engulfs dead neutrophils (pink). / Confocal fluorescence microscopy
A highly complex defence system involving various types of immune cells keeps our bodies healthy and balanced. We are especially interested in macrophages. A type of phagocyte (from the ancient Greek phagein, meaning to eat), macrophages are found in all organs and eliminate billions of cells that die every day as a result of natural cell turnover. They engulf the cell material with their cell membrane and “digest” it. The macrophage that can be seen here in a cell culture comes from the peritoneum of a mouse, in other words the tissue that lines the walls of the abdomen and pelvis and surrounds the organs within. By comparing different types of tissue, we have observed that these macrophages are particularly efficient at eliminating dead cell material. We have found out that this has to do with a receptor in their cell membrane. The cells produce particularly large quantities of this protein, allowing them to rapidly identify dead cells.
As well as studying processes in healthy organisms, we are investigating the role that macrophages play in inflammation. An inflammation occurs when a tissue is damaged in the body, for example as the result of infection, infarction or autoimmune diseases. Immune cells then migrate from the blood into the tissue. The tissue-resident macrophages produce signals that facilitate the recruitment of these immune cells and trigger further inflammatory reactions. Neutrophils arrive on the scene first, followed by monocytes. Like macrophages, both are types of phagocyte. Monocytes differentiate into macrophages in the tissue. We are interested in studying how macrophages adapt at the molecular level to different tissues – and how they contribute to resolving inflammation. After all, if this does not happen, inflammation can become life-threatening or chronic.
Irene Aranda Pardos and Noelia Alonso Gonzalez
Institute of Immunology, University of Münster
Waste disposal cells in the meninges
A view inside the head of a zebrafish. We have discovered that certain cells in the meningeal layer can absorb waste products. / Confocal fluorescence microscopy
The brain is enveloped by several membranes, the meninges, that form a protective shell. It was only discovered a few years ago that the outer layer of the meninges in fish, mice and humans has lymphatic vessels running through it. It is known that these vessels transport fluids and substances out of the tissue. This raised the fascinating question in research of how the brain is cleared of waste products and foreign substances. We have long known that the brain has specialised immune cells – microglia – that can actively absorb and neutralise such substances. We have found that a particular population of cells in the meninges of zebrafish performs a similar absorption function. The meninges presumably afford the brain additional protection with these special cells. We have demonstrated that the cells in the meninges are even able to eliminate small substances such as viruses better than microglia can, whereas microglia are more efficient when it comes to dealing with larger substances such as bacteria.
The cells we have been studying are similar to the cells that line the lymphatic vessels. In the zebrafish pictured here, we stained molecules (reddish and green) that are characteristic of such lymphatic vessel cells. It can be seen in the left and right halves of the image that cells have formed lymphatic vessels. However, the lymphatic cells that were the subject of our research remain as single cells (green, in the middle of the image) rather than forming vessels. While we were conducting our study, this cell population was also simultaneously discovered by two other research groups. Complementing our own findings, they were able to prove that the cells release signals that are important for the healthy growth of blood vessels. Mice and humans were also found to have cells that appear to be similar. Further research is being conducted to learn what other functions they may have.
Katharina Uphoff and Stefan Schulte-Merker
Institute for Cardiovascular Organogenesis and Regeneration, University of Münster
Immune cells activated following a stroke
Microglial cells (brown) in the brain of a mouse, 14 days after a stroke. The nuclei of all other cells are shown in purple. / Transmitted light microscopy
Microglia are a specific type of immune cells in the brain. As macrophages, they eliminate pathogens and dead cells and play a role in healthy neuronal function. We studied how microglia reacted after a stroke. During a stroke, part of the brain suddenly stops being properly supplied with blood – normally due to the blockage of a blood vessel. The resulting tissue damage activates immune cells, causing an inflammation. As “first responders”, microglia help neurons to survive and begin the process of removing dead cells. However, the tissue damage spreads very quickly during a severe stroke, with the result that the microglia are unable to limit the damage. They then produce more signals to guide additional immune cells from the blood to the site of the inflammation. If the inflammation becomes too widespread, further neurons will die.
We have shown that the shape of the microglia enables us to tell whether they are activated or not. It can be seen in the tissue cross section that some cells have long protrusions they use to scan their environment for changes. Other cells (top left) have significantly shorter protrusions – they have adapted their metabolism and now assume a different function. We can use a clinical imaging method known as positron emission tomography to make these activated microglia visible in stroke patients. By adapting their metabolism, these cells produce more of a particular protein, which allows us to identify them. However, our research has shown that this does not mean that these cells already have a negative impact on the disease process. We want to find out how we can make it possible to see whether an inflammation takes a positive or negative course and during which time window it would make sense to use drugs to modulate the course of the inflammation.
Cristina Barca Romero
European Institute for Molecular Imaging, University of Münster
Making inflammations in the body visible
Lab insights
Smaller versions of many of the imaging technologies used to examine patients in hospitals are used at the Multiscale Imaging Centre to examine mice. These include PET, SPECT, MRI, CT and ultrasound.© Uni Münster - Michael Kuhlmann 
A view of our small-animal PET-MRI. This hybrid device combines highly sensitive positron emission tomography (PET) with magnetic resonance imaging (MRI) in a very strong magnetic field. Unlike other hybrid devices, it does not conduct its scans one after another but simultaneously – and thus without any time lag.© Uni Münster - Michael Kuhlmann 
Preclinical imaging techniques make it possible to examine mice several times during a set period of time. This allows a large amount of coherent information about disease processes and the effectiveness of new therapies to be obtained with as few laboratory animals as possible.© Uni Münster - Michael Kuhlmann 
During the examination the mouse is anaesthetised and placed on a warming mat. Its heart rate, respiratory rate and body temperature are monitored constantly, as they are when human patients are anaesthetised. © Uni Münster - Michael Kuhlmann 
The mice are kept in an environment in which the temperature, humidity and light rhythm remain constant all the time. They live in permanent social groups and their wellbeing is monitored daily by specially trained staff.© Uni Münster - Michael Kuhlmann
A newly developed tracer (yellow-red signal) reveals an inflammation in the ear of a mouse. The tracer has spread all around the body in the blood and can also be seen in the liver, because that’s where it will be excreted from. / Rendered 3D image of PET-MRI data – positron emission tomography (PET, yellow-red signal) and magnetic resonance imaging (MRI, grey signal)
When an inflammation flares up in the body, different types of immune cells go into action. Neutrophils are the first cells to arrive on the scene en masse. They are always circulating in the blood, which allows them to reach any tissue very quickly. The activated cells secrete signal molecules, thus bringing in other immune cells, including monocytes. These migrate from the blood and from the bone marrow where they are produced, and from the spleen where they are stored, towards the site of the inflammation. We have developed a new tracer that shows when these cell types are active in the organism when an inflammation occurs. The tracer binds to a particular protein that is released by the immune cells – protein S100A9, which is found in a pair with protein S100A8. This protein complex is a typical marker of inflammation and can be measured in patients’ blood samples. We have been able to show a clear correlation in mice between the amount of this protein complex in tissue and the degree of inflammation.
The imaging technique we use to make the tracer visible – positron emission tomography (PET) – is clinically well established. It is possible therefore that the new tracer could one day be used in humans to locate inflammatory processes, assess their intensity and tailor medication to individual requirements. In our research we use the new imaging technique to analyse in disease models how well new therapies will work – for example to treat arthritis, an inflammatory joint disorder. We are also looking for novel ways to modulate the course of the inflammatory processes. We already know that other receptors of the immune cells are activated when the protein complex S100A8/A9 occurs not in a pack of two but in a pack of four. It is important to note that this changes the inflammatory activity of the immune cells. Our aim now is to change the way the protein complex S100A8/A9 interacts with those specific receptors of the immune cells so that we can adjust the inflammatory activity to the right level and exert a beneficial influence on the course of the disease.
Sven Hermann
European Institute for Molecular Imaging, University of Münster
Thomas Vogl
Institute of Immunology, University of Münster