key: cord-0994387-kizlpcjw authors: Battina, Hanisha L.; Alentado, Vincent J.; Srour, Edward F.; Moliterno, Alison R.; Kacena, Melissa A. title: The Interaction of the Inflammatory Response and Megakaryocytes in COVID-19 Infection date: 2021-09-23 journal: Exp Hematol DOI: 10.1016/j.exphem.2021.09.005 sha: a97d31aa11bcea62a7d9e44773795ca5b4cba3fa doc_id: 994387 cord_uid: kizlpcjw The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has spread rapidly across the world and has resulted in over 4.2 million global deaths as of July 30, 2021. For reasons that remain unknown, the coronavirus disease 2019 (COVID-19) clinically manifests itself in various severities, with a majority of COVID-19 positive patients being asymptomatic or having only mild symptoms. However, clinical studies in severely ill patients have implicated that manifestations of this infection are due, in part, to abnormal megakaryocyte (MK) behavior. Additionally, COVID-19 associated cytokine storms have been identified to induce aberrant MK formation, primarily through IL-6 and JAK-STAT signaling. Autopsy reports have demonstrated significantly higher rates of MKs in the pulmonary and cardiac systems, which may be responsible for the high rate of thrombotic complications and abnormal coagulopathies in patients with severe forms of COVID-19. This review examines MKs and their potential function in the clinical manifestations of the SARS-CoV-2 infection. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which originated in the Hubei Province of Wuhan, China in December 2019, is posing a public health threat to millions of individuals around the world due to its rapid transmittance rate and potentially severe clinical manifestations. As of July 30, 2021, a total of 197.31 million coronavirus disease 2019 (COVID- 19) cases had been reported with 4.2 million global deaths 1 . COVID-19 is an airborne disease that is transmitted by the inhalation of virus-containing droplets. The disease manifests itself in the infected individual after an incubation period that ranges from 2 to 14 days 2 . Clinical manifestations of COVID-19 are variable, but include sore throat, fever, nonproductive cough, difficulty breathing, and fatigue 3 . The disease severity and complications vary significantly, with the majority of individuals being asymptomatic or experiencing mild symptoms 2,3 . However, some individuals, particularly those with pre-existing health conditions (diabetes, obesity and cancer to name a few) and the elderly, are experiencing more severe complications such as pneumonia, multiple organ failure and dysfunction, hemodynamic shock, and death 3, 4 . Clinical studies have recognized that amongst those patients with severe forms of COVID-19, the cytokine storm plays a significant role in symptomatology and outcomes. Cytokine storms are inflammatory responses that are characterized by the uncontrolled and excessive release of cytokines that lead to hyperinflammation 5, 6 . In addition to contributing to the final prognosis of COVID-19 patients, these exacerbated responses have been demonstrated to play a critical role in increasing the numbers and causing an aberrant function of megakaryocytes (MKs) in SARS-CoV-2 infected patients 7 . There is growing recognition of abnormal MK behavior in many patients with severe illness due to COVID-19 4 . Autopsies of COVID-19 patients have demonstrated increased MK and platelet numbers with subsequent increased rates of thrombosis 4 . MKs are hematopoietic cells that reside within the bone marrow and are responsible for the production of platelets -the main mediators of thrombosis. The complex process of megakaryocytopoiesis commits hematopoietic stem cells (HSCs) to undergo a series of lineage commitment steps to produce MKs. These MKs then undergo unique maturation and differentiation processes including polyploidization and endomitosis to form platelets 5, 8 . However, abnormalities in the production of platelets from the precursor MKs can have significant clinical manifestations in the form of thrombocytopenia or thrombocytosis. Thrombocytopenia, defined as platelet counts less than 150,000/l, is associated with inadequate primary hemostasis and heightened risk of bleeding 9 . Conversely, thrombocytosis, defined as platelet counts greater than 350,000/uL, contributes to pathologic thrombosis and abnormal coagulopathy, thereby inducing deep vein thrombosis (DVT) and pulmonary thromboembolism (PTE) 10 . This review summarizes insights into the potential role of immune responses and their impact on MK behavior in the SARS-CoV-2 infection. The SARS-CoV-2 infection has highly variable clinical manifestations in different individuals, with the progression of the infection being largely dependent upon the host immune system's ability to recognize and react effectively to the virus 11 . The SARS-CoV-2 virus enters the body by binding to the angiotensin-converting enzyme 2 (ACE2) and facilitating a cascade of immune responses 12 . Following entry, viral infection elicits both innate and adaptive immune responses. There are several cytokines that have been demonstrated to be upregulated in the SARS-CoV-2 infection that also influence megakaryocytopoiesis ( Table 1) . One of the most prominent cytokines that is upregulated is interleukin-6 (IL-6), which is uniformly elevated in patients with severe COVID-19 infections 13 . IL-6 has been shown to increase levels of thrombopoietin (TPO) -the main megakaryocyte growth factor responsible for the production of MKs and platelets. Upon binding to its receptor, c-Mpl, TPO stimulates the production of precursor MKs in addition to accelerating the proliferation of MK progenitor cells [14] [15] [16] . Inhibition of TPO has been associated with neutralizing thrombocytosis and preventing thromboembolism resulting from elevated levels of IL-6 17 . Similarly, IL-9 has been determined to be upregulated in COVID-19 positive patients 18 . Elevated levels of this cytokine also induce increased signaling of the JAK-STAT pathway, leading to higher concentrations of MKs 14, 16 . Additional cytokines upregulated in the SARS-CoV-2 infection that are known to increase MK levels include granulocyte-macrophage colony-stimulating factor (GM-CSF), Interferon , and IL-1 in addition to IL-6 and IL-9 13, [18] [19] [20] . Irrespective of the exact pathway, the SARS-CoV-2 infection has been demonstrated to elicit immune responses that are associated with a significant increase in MK numbers 4, 21, 22 . Impact on MKs Impact on TPO Impact on JAK-STAT Pathway G-CSF  Table 1 . The COVID-19 associated cytokine storm results in the upregulation of various proinflammatory cytokines and chemokines. These cytokines have also been noted to increase and decrease MKs in addition to regulating TPO levels and the JAK-STAT pathway. Cytokines utilize the Janus kinase-signal transducer and activator of transcription (JAK-STAT) signaling pathway which is mediated by Janus kinases (JAKs) 29 . The JAK-STAT pathway is upregulated by pro-inflammatory cytokines involved in the COVID-19 associated cytokine storm that transduce extracellular signals from cytokines, lymphokines, and various growth factors 30 . Additionally, the JAK-STAT signaling pathway is involved in megakaryopoiesis and triggered when TPO binds to its receptor, c-Mpl 31, 32 . inhibitors to decrease the prevalence of cytokine storms which may also lower MK levels. Tocilizumab is one such anti-IL-6 monoclonal antibody. This is a humanized antibody that is clinically utilized to inhibit IL-6 accumulation during cytokine storms and has therefore been utilized in treating highly severe COVID-19 cases 34, 36, 48 . However, it is important to note that Tocilizumab has been found to be ineffective in reducing mortality rates in critically-ill COVID-19 patients 49 . Thrombocytopenia is a common consequence of many infectious diseases, including the The thrombosis-associated clinical manifestations of SARS-CoV-2 infections are varied. Indeed, as discussed above, some patients exhibit thrombocytopenia while others exhibit significant increases in thrombosis (Figure 1) . Recent studies have shown that coagulopathy occurs in nearly 50% of patients suffering from severe cases COVID-19 61 . The abnormal coagulopathy in critically-ill patients triggers various thrombotic complications that have also been identified to instigate adverse effects on the final prognosis of COVID-19 positive patients. In autopsies of COVID-19 patients, both microvascular and macrovascular thrombosis was identified in addition to many patients having thrombi in the lung parenchyma 62 The pathophysiology of the SARS-CoV-2 infection has been shown to resemble that of complement-mediated thrombotic microangiopathy (TMA) 75 . TMA is a condition that manifests itself through thrombocytopenia, organ dysfunction, and microangiopathic hemolytic anemia 75 . Studies have demonstrated that cells expressing ACE2 at high levels have become target cells for the SARS-CoV-2 infection 75 . ACE2 is highly expressed in podocytes and epithelial cells of the kidneys with even higher levels in patients with pre-existing cardiac injuries 75 . This suggests that thrombocytopenia characteristic in TMA is similar to that of COVID-19 in the lungs with regards to the pathway by which organ damage is impelled 75 . Because the lung tissue, particularly the lung alveolar epithelial cells, express ACE2, injury induced by the SARS-CoV-2 infection is similar to that of ACE2 expressed in the kidneys resulting in TMA injury 75 . The ACE2 molecules in lung tissue act as a receptor to which the SARS-CoV-2 infection can bind and therefore enter the host's immune system 63, 75 . As While not yet tested, it is interesting to speculate that the COVID-19 associated cytokine storm increases IL-6 and JAK-STAT signaling which in turn increases MK levels 14, 16, 39 . Additionally, thrombosis and abnormal coagulopathy in the presence of increased levels of MKs in the bone marrow, pulmonary, and cardiac systems indicate the possible role of MKs in worsening the final prognosis for COVID-19 patients. Therefore, a potential adjuvant therapeutic intervention could include limiting MK production in COVID-19 patients 80, 82 . 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We would like to thank the Indiana Clinical and Translational Sciences Institute, funded in part by the NIH (RR025761, TR000006), for their support of this work.