Lung megakaryocytes are long-lived, arise from Flt3-negative bone marrow cells, and contribute to platelet recovery in thrombocytopenia

We previously characterized lung megakaryocytes (Mks) as largely extravascular cells with an immune modulatory phenotype (Pariser et al., 2021). Because bone marrow (BM) Mks are relatively short lived, it is assumed that extravascular lung Mks are constantly seeded from the BM, but there are no experimental data to validate this concept. To investigate lung Mk origin and how their origin may impact lung Mk functions, we developed novel models using CFSE dye delivered oropharyngeal (OP) and biotin labeling to specifically label lung Mks and identify lung Mk derived platelets. Labeled lung Mks were present for up to four months, while BM Mks had a less than 1 week lifespan. In a parabiosis model, lung Mks were only partially replaced by a circulating source over a 1-month time period. Unlike tissue resident lung macrophages, we determined using MDS1-Cre-ERT2 TdTomato mice that lung Mks arise from a hematopoietic stem cell (HSC) source. However, studies with FlkSwitch mTmG mice showed that lung Mks originate from a Flt3-negative cell lineage, that does not go through a multipotent progenitor stage. CFSE labeling of lung cells enabled us to track lung Mk-derived platelets and we found that about 10% of circulating platelets at steady-state are lung resident Mk-derived, but in the context of sterile thrombocytopenia there was a doubling of lung Mk derived platelets (about 20%). Lung-derived platelets were similarly increased in a murine malaria infection model (Plasmodium yoelii) typified by chronic thrombocytopenia. Taken together, our studies indicate that lung Mks arise from a Flt3-negative, HSC-dependent pathway and contribute relatively more platelets during thrombocytopenia.