Crosstalk between Megakaryocyte Niches Unveils the Origin of Immunoreactive Platelets

Platelets play a prominent role in the pathophysiology of cardiovascular disease (CVD) however, the precise origin and function of pathological platelets remain elusive. While the bone marrow has been long established as a central site of platelet production, other important niches of platelet biogenesis have recently emerged. The role of platelets produced outside the bone marrow and the cross-communication between these niches remain incompletely understood. Here, we investigate the pathological role of lung-derived platelets in the development of immunothrombotic diseases. First, we show that blunting the Thrombopoietin receptor (TPOR) in the lungs of mice using CRISPR/Cas9 lentivirus modulates circulating platelet levels, adding support to existing literature that establishes the lungs as an important site of platelet biogenesis. Additionally, we designed a custom lead shield protecting the thoracic region in mice that conserved the lung hematopoietic niche following lethal irradiation. Combined lineage tracing and bone marrow transplantation from thrombocytopenic TPOR deficient donor mice into platelet/megakaryocyte reporter recipient mice (TdTomatoPF4^Cre) receiving lethal irradiation with shielding of the thorax, revealed that protection of hematopoietic niches in the lungs is sufficient to maintain serum platelet counts. Next, we explore the immunomodulatory role of lung-derived platelets by performing transplantation of TdTomatoPF4^Cre lung into wild-type donor mice. In the recipient mice, TdTomato platelets originating from the lungs were actively recruited to inflamed aortic sites, hijacked monocytes from the circulation, and regulated vascular remodeling. Additionally, targeted deletion of TPOR in the lungs by CRISPR/Cas9 lentivirus is protective against aortic inflammation and aneurysm development in mice. Secretome analysis identified a molecular signature of platelets that stems from the lungs that confer immunomodulatory properties to this subset. Our results reveal the critical role of the lungs as instrumental sites of molecularly distinct platelet subsets that could have far-reaching impacts on many immunothrombotic diseases.