Model as a basis for new therapeutic approaches
In COVID-19, the immune response is decisive for the severity of the disease. But what exactly happens in the lungs and blood during the initial phase of the disease was unclear until now. Researchers at Charité – Universitätsmedizin Berlin, the Max Delbrück Center for Molecular Medicine in the Helmholtz Association (MDC) and Freie Universität Berlin have now investigated the cellular mechanisms at the beginning of inflammatory lung damage triggered by SARS-CoV-2 infection. According to the study, the damage that makes ventilation necessary is not caused by the direct destruction of the lungs by the multiplication of the virus. Rather, inflammatory processes and the endothelium of the lung are significantly involved in severe courses, as the researchers now describe in the journal Nature Communications*.
For one and a half years, researchers all over the world have been trying to understand the COVID-19 disease caused by the SARS-CoV-2 coronavirus. To date, there are almost no drugs to treat the disease, which can lead to acute respiratory failure and damage other organs and organ systems besides the lungs. One of the hurdles: COVID-19 is caused, among other things, by a misguided, sometimes exaggerated reaction of the body’s own immune system. In order to find therapeutic targets, it is necessary to understand in detail how and where exactly which adverse processes are triggered in the body. Patient-centred research reaches its limits here – especially when it comes to investigating disease mechanisms in the early phase of infection. Biomaterial as a basis for studies is usually only accessible after admission to a hospital. In the case of mild or moderate courses associated with pneumonia, it is usually not possible to obtain any tissue from the lungs at all – the risk would be too high for acutely ill patients. What remains is the analysis of cell tissue after the death of COVID-19 patients.
Using available patient samples, the team led by Prof. Dr. Martin Witzenrath, Deputy Director of the Medical Clinic with a focus on Infectiology and Pneumology at Charité, was able to gain valuable information about the mechanisms and course of the disease. In addition, the research team was looking for a suitable model to examine lung areas that cannot be easily viewed in patients and to gain insights into the early phase of the disease. In international COVID-19 research, as well as in SARS-CoV-1 research, hamster models have proven informative. „We wanted to know whether the models could be used to develop new therapeutic approaches and tried to find findings from samples from patients in them. This worked well,“ says Prof. Witzenrath, co-lead author of the study. „We were particularly interested in the endothelial cells of the lungs, i.e. the barrier that lines the blood vessels. This loses function in severe COVID-19, eventually leading to lung failure.“
Together with researchers at the Berlin Institute for Medical Systems Biology (BIMSB) at the MDC, virologists and veterinarians at Freie Universität and data experts at the Berlin Institute of Health (BIH) at Charité, the scientists have carried out a precise characterisation of the SARS-CoV-2 infection in the animal model in their current work and backed this up with data sets from samples taken from patients. The analysis should make the currently most important non-transgenic model for COVID-19, the golden hamster, useful for developing therapies. Hamsters are infected with the same virus variants as humans. They also develop similar disease symptoms and their lungs are damaged in severe disease. However, COVID-19 progresses differently in individual species. While it usually only takes a moderate course in golden hamsters, Roborovski dwarf hamsters become severely ill.
Among other things, single-cell analyses at the BIMSB have shown why this is so and which processes actually take place in the cells of the lung. The researchers run the individual cells of a sample over a chip. There they are packed into small aqueous droplets together with a barcode. In this way, the RNA – the part of the genetic material that the cell had just read – can be sequenced and later reassigned to the cell. From the data obtained, the function of the cell can be inferred with high precision. „For example, we were able to observe how certain cells of the immune system in the lungs – the monocytes and macrophages that develop from them – take up the virus and react very violently. They send out messenger substances that cause a strong inflammatory reaction. In our model, this is recaptured quite quickly, because other immune cells – the T cells – swarm out for this purpose. This does not happen in severe COVID-19 courses,“ explains Dr Geraldine Nouailles, a scientist at the Charité Medical Clinic with a focus on infectiology and pneumology and co-first author of the study. „For a successful recovery from COVID-19, a fast and efficient T-cell response is central.“
While the immune system goes into turmoil in the course of a COVID-19 attack, SARS-CoV-2 initially multiplies only weakly in the lungs and in the cells of the airways. „The destruction of lung tissue in severe COVID-19 courses is not directly caused by the multiplication of the virus in the cells, but by the strong inflammatory response,“ explains MDC scientist Dr Emanuel Wyler, also co-first author. „This seems to be true for the cells of the blood vessels as well, especially the endothelial cells in the lungs. They react strongly to the virus but are not infected by it and do not perish.“ If the disease takes a severe course, blocked blood vessels and unstable vascular walls can lead to acute lung failure. In moderate COVID-19 disease, on the other hand, vascular damage is very unlikely to play a role. „That the endothelium, a kind of protective shield in the vessels – which regulates, among other things, various processes in the microvessels of the lung – is activated in COVID-19, we had expected. The fact that these cells are at the same time the active motor of inflammation rather surprised us,“ says Prof. Witzenrath. „One could therefore therapeutically attack these cells, which are central to the course of the disease, in two ways. On the one hand, with substances that seal the endothelial barrier. On the other hand, with substances that calm the endothelium. We are already researching one of these in the Collaborative Research Centre SFB-TR84 and have been able to show that it is successful in pneumonia and ventilation.“ Anti-inflammatory drugs against COVID-19 that are also currently being tested target the immune reaction as such – they also act on monocytes and macrophages and slow down their activity.
The now confirmed model should contribute to the development of effective and safe therapies for COVID-19. The goal is to reduce the number of patients with severe lung damage. Currently, the multidisciplinary research team is analysing the reactions of the different cell types in the Roborovski dwarf hamster. The researchers want to find out why the infection is more severe here and not self-limiting as in the case of the golden hamster. „We hope that this will also provide us with an explanation as to why some people contract COVID-19 severely and others do not,“ says Dr Nouailles. However, the genome of the dwarf hamster still has to be decoded for this. Since hamsters were previously considered rather exotic in laboratory animal science, there are still some gaps in our knowledge. „We have now been able to close some of these with information from the current study. This is a great step forward, also in terms of the conscious and targeted use of animals in medical research,“ explains co-lead author Dr Jakob Trimpert, virologist and veterinarian at the Free University of Berlin. With his team, he developed the COVID-19 hamster models and, together with the animal pathologists at the FU Berlin led by Prof. Dr. Achim Gruber, carried out the microscopic examinations of infected lung tissue. These histopathological analyses contributed significantly to the findings of the study.
*Nouailles G et al. Temporal omics analysis in Syrian hamsters unravel cellular effector responses to moderate COVID-19. Nat Commun 12, 4869 (2021). DOI: https://doi.org/10.1038/s41467-021-25030-7
About the study
The work was funded, among others, by the German Research Foundation (DFG) in the Collaborative Research Centre SFB-TR84, the Federal Ministry of Education and Research (BMBF) with the CAPSyS-COVID and PROVID projects, and the Berlin Institute of Health (BIH) at the Charité with CM-COVID. The study was also made possible by the BMBF-funded National Research Network of University Medicine on Covid-19 (NUM), in the sub-project NAPKON, a comprehensive pandemic cohort network.
Medizinische Klinik mit Schwerpunkt Infektiologie und Pneumologie der Charité
Berliner Institut für Medizinische Systembiologie (BIMSB) des MDC
Fachbereich Veterinärmedizin der Freien Universität Berlin