Defective Platelet Thromboxane A2 Signaling and Serotonin Release in the Pathogenesis of Bleeding during Viral Infection

Viral hemorrhagic fever (VHF) refers to a group of diverse acute human diseases caused by single stranded RNA viruses typically associated with fever, vascular damage and abnormal vascular barrier function. These infections are a continuing threat to public health due to the high case-fatality rate and potential bioterrorism threats. Critically infected VHF patients with unfavorable prognosis often present bleeding symptoms and shock. Thrombocytopenia associated with endothelial, coagulation and platelet dysfunction has been reported during these infections, but their relationship to bleeding is still poorly defined. We have previously established that infection by lymphocytic choriomeningitis virus (LCMV) in extremely thrombocytopenic mice causes severe mucocutaneous bleeding and death mediated by interferon α/β receptor (IFNAR) signaling. We now show in vivo that IFNAR is expressed on megakaryocytes but not on circulating platelets, contrary to previously reported evidence. To explain how increased IFN-I in LCMV-infected mice alters the function of platelets lacking IFNAR, we cross-transplanted WT or IFNAR^-/- bone marrow (BM) cells into IFNAR^-/- and WT mice. Upon platelet depletion and LCMV infection, only chimeras with WT bone marrow died, suggest that IFN-I-sensitive bone marrow-derived cells are responsible for the mucocutaneous bleeding observed in conditions of extreme thrombocytopenia. Moreover, the results of BM transplantation studies prove that endothelial cell alterations and infection-related inflammatory conditions are not alone sufficient to cause hemorrhage. Circulating platelets during LCMV infection show a generalized aggregation disfunction that persists longer for the arachidonic acid/thromboxane A2 (AA/TBXA2) activation pathway. These aggregation defects are associated, and possibly linked to an impaired secretory ability of both α- and δ-granule. Since circulating platelets are unresponsive to IFN-I, these findings are consistent with the possibility that bone marrow-resident megakaryocytes are targets of IFN-I stimulation during LCMV infection and are the origin of structural and functional platelet abnormalities observed in the circulation. Indeed, megakaryocytes from LCMV-infected mice exhibited fewer granules limited to the perinuclear zone rather than normally diffused throughout the demarcation membrane system; the latter was also underdeveloped. Moreover, platelet transcriptome evaluation of genes involved in platelet production and function showed that IFN-I downregulates bone marrow expression of NF-E2 - a key transcription factor for thrombocytopoiesis - as well as of cyclooxygenase-1 and thromboxane-A synthase. The latter enzymes are required for TBXA2 synthesis from AA amplifying platelet activation. Thus, in addition to being decreased in number, circulating platelets of LCMV-infected animals have impaired activation-induced cargo release from storage granules supporting hemostasis. Experimentally, we observed that bleeding during LCMV infection is increased following aspirin, but not clopidogrel, administration. This suggested that serotonin secretion may be the final effector required to control erythrocyte extravasation. Accordingly, bleeding associated with LCMV infection was markedly enhanced in mice lacking platelet serotonin (TPH1^-/-mice), and this was ameliorated by transfusion of normal platelets. In conclusion, we have identified a IFN-I targeting of megakaryocytes leading to reduced production of dysfunctional platelets with abnormal serotonin release as the cause of potentially lethal bleeding in a mouse model of arenavirus infection. Our demonstration that infusion of viable platelets can prevent lethal anemia in LCMV-infected susceptible mouse strains indicates that the same therapeutic intervention monitored by platelet response to AA stimulation may aid in treating severe VHF cases in human patients.