During the PLAT-IL-1 project, we discovered that human and mouse platelets are likely not cellular sources of IL-1 cytokines. Using a series of complementary techniques and experiments in human cells and mouse models, we demonstrated that platelets do not express inflammasome proteins and are unable to produce IL-1 or many other pro-inflammatory cytokines that drive innate immune responses.
However, while studying the interactions of platelets with human innate immune cells, we found that, despite not expressing inflammasomes themselves, platelets are essential for the inflammasome function in other immune cells, such as macrophages, neutrophils, and monocytes. Co-culture with platelets boosted the inflammasome activation and production of IL-1α, IL-1β, and IL-18 from human macrophages and neutrophils and were crucial for the optimal production of IL-1 cytokines by human monocytes. Platelets influenced the inflammasome activation of these cells by enhancing the transcription of NLRP3 and pro-IL-1β. In line with their importance in vitro, platelet depletion in mice attenuated LPS-induced IL-1β responses in vivo, and blood platelet counts correlated positively with plasma levels of IL-1β in human cohorts. Moreover, we found an enriched platelet gene signature in the whole blood transcriptomics of pediatric patients with IL-1-driven auto-inflammatory diseases. Thus, in this first part of the project, we discovered a pivotal role for platelets in the development of IL-1-driven autoinflammatory diseases (Rolfes et al., 2020).
Another discovery during the PLAT-IL-1 action began after we noticed that primary monocytes, isolated from human blood using commercially available immuno-magnetic isolation kits, contain platelets as contaminants. We then noticed that the immune function of monocytes was impaired when contaminant platelets were removed. Removal of platelets caused monocyte immunoparalysis, characterized by a impaired in their ability to produce cytokines in response to inflammatory triggers. The anergic state of platelet-depleted monocytes was not permanent and was reversed by their supplementation with autologous platelets. Notably, cytokine functions in monocytes from patients with immune thrombocytopenia (ITP), an autoimmune condition characterized by low blood platelet counts, were naturally impaired. Strikingly, the supplementation with healthy platelets reverted immunoparalysis in ITP monocytes, revealing the possibility of using platelet therapy for these patients. Mechanistically, we discovered that platelets are cellular sources of transcription factors that they supply to monocytes, and that the platelet-derived transcription factor MAPK14, was necessary for the full cytokine function of human monocytes. These findings are of fundamental clinical relevance, as platelet transfusion, a worldwide therapy used to counteract thrombocytopenia, is highly inflammatory and known to cause numerous adverse and fatal effects. This study was published as a pre-print (Hawwari et al., 2022) and is currently undergoing peer review.
The experimental setup and animal models used in the PLAT-IL-1 action allowed us to investigate the inflammasomes' activation in vivo and raised the possibility of testing novel biologicals able to interfere with its activity. Together with Dr. Florian Schmidt, a cell biologist with expertise in innate immunity and virology, we developed camelid-derived nanobodies targeting ASC, a central component of the inflammasomes. ASC nanobodies were highly efficient in disaggregating pre-formed inflammasomes and ameliorating arthritis in mice (Bertheloot et al., 2022). This approach represents a novel biologic-based treatment for inflammasome-initiated inflammatory diseases. The study was published and highlighted in the June cover of EMBO Molecular Medicine in 2022.
Overall, the PLAT-IL-1 project was fundamental to establishing my group as a reference in platelet immunology in Bonn and the international community. It also increased the visibility of our research and consolidated me as an independent scientist in a prestigious German university.
REFERENCES:
Bertheloot, D., et al. 2022. EMBO Mol Med e15415.
Hawwari, I., et al. 2022. bioRxiv 2022.2008.2010.503291.
Rolfes, V., et al. 2020. Cell Rep 31:107615.
