Covid-19: Wie der Körper selbst die Lunge zerstört und den Menschen tötet

Covid-19: Wie der Körper selbst die Lunge zerstört und den Menschen tötet
© Bereitgestellt von Berliner Kurier

Bei der Mehrheit der Patienten mit schwerem Covid-19-Verlauf vernarbt die Lunge außergewöhnlich stark. Das kam bei Untersuchungen  unter anderem der Charité heraus. Dabei spielen Fresszellen des Immunsystems eine zentrale Rolle. Wenn die Lunge wegen Corona versagt, ähneln die Vorgänge einer bislang unheilbaren Lungenvernarbung bei der sogenannten idiopathischen Lungenfibrose. Das dürfte erklären, warum die Lunge lange funktionsunfähig bleibt und häufig eine teilweise Wochen und Monate andauernde ECMO-Therapie erfordert. Dabei muss das Blut außerhalb des Körpers maschinell mit Sauerstoff angereichert werden, damit der Patient eine Überlebenschance hat. Allerdings nur eine von 50 Prozent: rund jeder zweite ECMO-Patient stirbt. In Berlin wurden zuletzt 29 Menschen so beatmet, statistisch werden davon nur etwa 15 weiterleben.

Als Grund für die lange Dauer des Lungenversagens hatte das Forschungsteam unter anderem eine spezielle Krankheits-Form im Verdacht, bei der das Lungengewebe vernarbt, dadurch verdickt und unelastisch wird. Schon relativ früh in der Pandemie war bei einzelnen Kranken ein solcher als Fibrose bezeichneter Umbau des Gewebes aufgefallen. Die aktuelle Studie bestätigt nun, dass das schwere Covid-19-Lungenversagen tatsächlich sehr häufig von einer ausgeprägten Vernarbung des Lungengewebes begleitet wird.

„Eine fehlgeleitete Reaktion sogenannter Makrophagen, die auch als Fresszellen des Immunsystems bekannt sind, könnte dafür mitverantwortlich sein“, konstatiert Dr. Antoine-Emmanuel Saliba, Arbeitsgruppenleiter am Helmholtz-Instituts für RNA-basierte Infektionsforschung (HIRI) in Würzburg und Co-Leiter der Studie neben Erik Sander von der Infektiologie und Pneumologie der Charité.

Für die Studie untersuchte das Team die Lungen verstorbener Patienten anhand verschiedener mikroskopischer Aufnahmen. „Bei fast allen Betroffenen haben wir enorme Schäden entdeckt: Die Lungenbläschen waren weitgehend zerstört, die Wände deutlich verdickt. Außerdem fanden wir ausgeprägte Ablagerungen von Kollagen, welches ein Hauptbestandteil von Narbengewebe ist“ beschreibt Peter Boor die Befunde. Er hat die Studie am Institut für Pathologie der Uniklinik RWTH Aachen geleitet. „Das könnte erklären, warum wir die Betroffenen so lange beatmen müssen.“

Der Grund für dieses Phänomen war zunächst unklar, unter anderem, weil das Lungenversagen in der Regel erst zwei bis drei Wochen nach den ersten Corona-Symptomen auftritt, wenn die Viruslast eigentlich schon wieder sinkt, erklärte Professor Sander. „Das weist darauf hin, dass nicht die unkontrollierte Virusvermehrung zum Versagen der Lunge führt, sondern nachgeschaltete Reaktionen, beispielsweise des Immunsystems, eine Rolle spielen.“ 

Das Team ermittelte, dass sich vor allem Fresszellen in großen Mengen ansammelten. Diese Zellen beseitigen zum Beispiel eingedrungene Erreger oder Zellabfall, sind aber auch an der Wundheilung und Reparatur von Gewebe beteiligt. Bei Covid-19 verbinden sich die  Makrophagen aus unbekannter Ursache mit bestimmten Zellen des Bindegewebes, die für die Narbenbildung verantwortlich sind. Wie bei der chronischen Form der Lungenvernarbung, der idiopathischen Fibrose, vernarbt die Lunge unaufhaltsam bis zum Verlust der Organfunktion, es bilden sich viele spezielle Eiweiße, Kollagene.  

Matthias Selbach vom  Berliner Max-Delbrück-Centrum für Molekulare Medizin: „Das Corona-Virus vermehrt sich dabei anscheinend nicht in den Immunzellen, sondern programmiert sie um.“

Die Parallelen zwischen Covid-19 und der chronischen Lungenfibrose auf könnte laut der Studie erklären, warum sich Risikofaktoren für bei Erkrankungen gleich sind: Die Patienten sind meistens Männer, über 60 Jahre alt, haben Grunderkrankungen und rauchten.

Es gebe allerdings eine kleine Hoffnung für die Covid-19-Patienten: Hier sei die Vernarbung zumindest potenziell reparabel, zeigten Computer-Tomografien der Lungen. Bei Covid-19-Erkrankten, die mit der ECMO behandelt wurden, zeigten die Aufnahmen zunächst typische milchglasartige Trübungen, die sich im Verlauf der Erkrankung verdichteten und vernarbten. Bei Betroffenen, die von der ECMO-Behandlung entwöhnt werden konnten und genasen, gelang es dem Körper, die Verdichtungen allmählich aufzulösen, auch wenn in manchen Fällen deutliche Vernarbungsreste zurückblieben.

Die Wissenschaftlerinnen und Wissenschaftler wollen nun genauer untersuchen, welche zellulären Prozesse dazu führen, dass sich eine Fibrose zurückbildet. „Wenn wir die Auflösung von vernarbtem Gewebe besser verstehen, können wir in Zukunft hoffentlich nicht nur Covid-19-Betroffenen, sondern auch Patienten mit bisher unheilbarer Lungenfibrose helfen“, sagt Prof. Sander.  

An der Studie beteiligt waren neben der Charité das Helmholtz-Instituts für RNA-basierte Infektionsforschung (HIRI), des Max-Delbrück-Centrums für Molekulare Medizin in der Helmholtz-Gemeinschaft (MDC), der Uniklinik RWTH Aachen und des Robert Koch-Instituts in einer aktuellen Studie.

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