Occurrence and role of lung megakaryocytes in infection and inflammation

Megakaryocytes (MKs) are large cells giving rise to platelets. It is well established that in adults, MKs develop from hematopoietic stem cells and reside in the bone marrow. MKs are also rare but normal constituents of the venous blood returning to the lungs, and MKs are found in the lung vasculature (MK_circ), suggesting that these cells are migrants from the bone marrow and get trapped in lung capillaries where the final steps of platelet production can occur. An unprecedented increase in the number of lung and circulating MKs was described in coronavirus disease 2019 (COVID-19) patients, suggesting that lung thrombopoiesis may be increased during lung infection and/or thromboinflammation. In addition to the population of platelet-producing intravascular MKs in the lung, a population of lung-resident megakaryocytes (MK_L) has been identified and presents a specific immune signature compared to its bone marrow counterparts. Recent single-cell analysis and intravital imaging have helped us gain a better understanding of these populations in mouse and human. This review aims at summarizing the recent data on increased occurrence of lung MKs and discusses their origin, specificities, and potential role in homeostasis and inflammatory and infectious lung diseases. Here, we address remaining questions, controversies, and methodologic challenges for further studies of both MK_circ and MK_L.