Periodic Reporting for period 2 - IMMUNOTHROMBOSIS (Cross-talk between platelets and immunity - implications for host homeostasis and defense)
Reporting period: 2021-03-01 to 2022-08-31
Clot formation – thrombosis – in vessels of the heart and brain are leading causes of death and disability globally. Despite advances in treatment and prevention, it remains a challenge to efficiently prevent vessel occlusions. Moreover, available inhibitors of coagulation or platelet function that directly target thrombosis carry a high risk of bleeding. My group and others were able to show synergy of platelets, the coagulation cascade and immune function in thrombosis, a novel concept named immunothrombosis. Specifically targeting axes of immune cell – coagulation cascade - platelet interplay might prove efficacious in the prevention of thrombosis without elevated bleeding risks.
Therefore, my project IMMUNOTHROMBOSIS seeks to investigate interplay between thrombus formation and immune function. Specifically, this project investigates the underlying molecular mechanisms, as well as its role in models of thrombotic as well as inflammatory diseases. I have previously identified a novel effector function of immune responsive platelets – the ability to migrate autonomously. Moreover, I identified an immune cell-mediated feedback loop modulating platelet production. We seek to deepen our understanding of these two key processes to elucidate crucial signaling axes of immunothrombosis ultimately highlight novel treatment options.
Therefore, my project IMMUNOTHROMBOSIS seeks to investigate interplay between thrombus formation and immune function. Specifically, this project investigates the underlying molecular mechanisms, as well as its role in models of thrombotic as well as inflammatory diseases. I have previously identified a novel effector function of immune responsive platelets – the ability to migrate autonomously. Moreover, I identified an immune cell-mediated feedback loop modulating platelet production. We seek to deepen our understanding of these two key processes to elucidate crucial signaling axes of immunothrombosis ultimately highlight novel treatment options.
In my Advanced grant “IMMUNOTHROMBOSIS” I was able to identify a novel function of platelets that are recruited to sites of tissue inflammation: They actively move – migrate – on the vascular surface. This movement follows substrate densities, helping platelets to identify sites of vascular injury. This re-positioning is important for platelets to find sites of vessel injury and prevent bleeding in the inflamed microvasculature. Moreover, platelets migrate to find and collect invading bacteria, and therefore prevent dissemination of bacteria in the blood stream. I also revealed that this process depends on the cytoskeletal regulator actin related protein 2/3 (ARP2/3). Platelets sense and react to the local micro-environment and form sheet-like lamellipodia in an ARP2/3 dependent process; lamellipodia formation is dispensable for classical thrombosis and haemostasis. To better visualize individual platelets in vivo, I validated a multi-colour reporter mouse strain enabling tracking of individual platelets in complex and dynamic environments like thrombi.
In addition, I discovered the interactions between megakaryocytes and immune cells as a previously unknown regulatory mechanism of platelet production. First, I was able to demonstrate that physical interactions of neutrophils with proplatelet protrusions are key to efficient platelet release (thrombopoiesis). Second, I identified plasmacytoid dendritic cells (pDCs) as crucial bone marrow niche cells that regulate the proliferation of megakaryocyte progenitors. When pDCs encounter mature megakaryocytes that undergo thrombopoiesis they release INF-alpha which in turn drives the proliferation of megakaryocyte progenitors (megakaryopoiesis). This fine-tuned coordination between thrombopoiesis and megakaryopoiesis is crucial for megakaryocyte and platelet homeostasis.
I was also able to show an important contribution of IMMUNOTHROMBOSIS to the pathology of severe COVID-19. In cooperation with multiple partners at the hospital we collected and analysed blood and tissue samples of patients with COVID-19 pneumonia. We were able to show that platelets become activated in severe disease and interact with neutrophils. This interaction leads to neutrophil activation which release so called neutrophil extracellular traps (NETs). NETs then form clots in the smaller blood vessels of patients. Interestingly, this is not limited to the lung, but also occurs in the liver, kidney and heart of individuals with severe disease.
In addition, I discovered the interactions between megakaryocytes and immune cells as a previously unknown regulatory mechanism of platelet production. First, I was able to demonstrate that physical interactions of neutrophils with proplatelet protrusions are key to efficient platelet release (thrombopoiesis). Second, I identified plasmacytoid dendritic cells (pDCs) as crucial bone marrow niche cells that regulate the proliferation of megakaryocyte progenitors. When pDCs encounter mature megakaryocytes that undergo thrombopoiesis they release INF-alpha which in turn drives the proliferation of megakaryocyte progenitors (megakaryopoiesis). This fine-tuned coordination between thrombopoiesis and megakaryopoiesis is crucial for megakaryocyte and platelet homeostasis.
I was also able to show an important contribution of IMMUNOTHROMBOSIS to the pathology of severe COVID-19. In cooperation with multiple partners at the hospital we collected and analysed blood and tissue samples of patients with COVID-19 pneumonia. We were able to show that platelets become activated in severe disease and interact with neutrophils. This interaction leads to neutrophil activation which release so called neutrophil extracellular traps (NETs). NETs then form clots in the smaller blood vessels of patients. Interestingly, this is not limited to the lung, but also occurs in the liver, kidney and heart of individuals with severe disease.
Despite the partly debilitating effects of COVID-19 on research activity, with limitations in animal breeding capacities as well as personnel working in the laboratory, we were able to achieve major scientific success, reaching our set objectives. We were able to define and specifically interfere with a novel platelet function – directional migration along substrate gradients. We expect our findings to impact the field profoundly, as it highlights a novel, specific function of platelets recruited to sites of inflammation. This might prove an important target in chronic inflammatory diseases, a fact that we are right now investigating in murine disease models. I consider our findings of the crucial relevance of IMMUNOTHROMBOSIS in COVID-19 pneumonia an important and unexpected scientific leap: We and others were able to confirm mechanisms of IMMUNOTHROMBOSIS discovered in animal studies in human disease and highlight novel, potentially targetable pathways as key drivers of respiratory failure. This greatly advances the relevance of IMMUNOTHROMBOSIS in human disease and places the concept at the nexus of host defense as well as immune system dysfunction. I want to utilize the second reporting period to further understand the underlying molecular mechanisms with a focus on chronic diseases.