These research were approved by the Emory University Institutional Animal Care and Use Committee. Platelet depletions, platelets counts, and thrombopoietin treatment Platelets were depleted by intraperitoneal injection into the lower-left abdominal quadrant of 0.2 mL of antibodies against GPIIb (MWReg30, BD Bioscience), GPIb (R300, Emfret), or Aspercetin (isolated as previously described)31 diluted to concentrations indicated in Results. Capillary blood was collected directly from the tip of the tails of the mice into a BD Unopette system test to count platelets in a hemocytometer according to the manufacturer’s instructions. only in those animals receiving the stronger depletion treatment. Furthermore, we showed that the nonhemorrhagic but Namitecan partially platelet-depleted mice were unable to control the viral replication because of generalized splenic necrosis, affecting innate and adaptive immune cells. These data suggest that, by their supportive roles in hemostasis, platelets may be preventing the severe pathology observed in human arenaviral infections. Introduction Viral hemorrhagic fevers (VHFs) are a group of distinct infectious diseases with similar clinical manifestations in humans. The acute phase of these infections is characterized by a flu-like syndrome accompanied by fever, headache, and general malaise. Severe or fulminant cases develop into hemorrhagic fevers (HFs) leading to mucocutaneous bleedings, thrombocytopenia, leukopenia, uncontrolled viral replication, internal-organ hemorrhages, immunosuppression, multiple organ dysfunction, shock, and death. Lipid-enveloped, single-stranded RNA viruses from the families Arenaviridae (Lassa [LASV], Junin, and lymphocytic choriomeningitis virus [LCMV]), Bunyaviridae (Hanta, Crimean-Congo, and Rift Valley), Filoviridae (Ebola and Marburg), and Flaviviridae (Yellow Fever and Dengue) are the best known etiologic agents of VHFs.1 Even though these viruses infect millions of individuals annually, our understanding of their pathophysiology is currently limited. Unfortunately, animal models do not fully recapitulate the clinical manifestations of infection with VHFs, and this together with Namitecan the fact that most of these viruses must be studied under high biosafety containment, represents a major roadblock to enhanced understanding.2 The 2 2 main clinical manifestations for all severe VHF cases in humans and nonhuman primates are defects in hemostasis that leads to a hemorrhagic/shock syndrome, high viral titers, and a suboptimal immune response. Thrombocytopenia is the most dramatic alteration in hemostasis. The mechanisms underlying its development are not fully understood, Rabbit Polyclonal to RNF144B but it seems to be the combined result of a maturation arrest Namitecan and/or apoptosis of megakaryocytes in the bone marrow in response to high levels of type I interferons (IFNs)C/,3,4 and Namitecan a platelet consumption process in the periphery.5 As an example of the latter, evidence of disseminated intravascular coagulation has been consistently reported in Ebola and Marburg infections.6,7 In addition, high serum viral titers are frequently associated with leukopenia and deficient immune responses. Lymphopenia in the arenavirus Lassa and Argentine HFs strongly correlates with disease severity and widespread necrosis in the splenic marginal zone and cortical and paracortical areas of the lymph nodes.8 Large numbers of lymphocytes undergoing apoptosis are seen in Ebola and Marburg infections.7,9 The sporadic severe human and nonhuman primate cases of LCMV infections resemble LASV infections, with thrombocytopenia, leukopenia, high viral titers, involvement of liver, lungs, and kidneys, and neurologic abnormalities that were overshadowed by the severity of the systemic illness.10C13 In mice, LCMV infection generates a completely different disease probably because of adaptations gained during the long virus/natural-host coevolution. LCMV is a noncytolytic virus, which indicates that any sign of acute pathology is exclusively mediated by the host response against the infection. 14 When inoculated intracranially into adult mice, a fulminant meningitis develops mediated by the migration of LCMV-specific cytotoxic T lymphocytes (CTLs) into the central nervous system (CNS). Arriving CTLs release cytokines and chemokines that attract a strong myelomonocytic infiltrate that disrupts the meningeal vasculature, causing vascular leakage, seizure, and death.15 On the other hand, when the virus is inoculated subcutaneously, intraperitoneally, or intravenously, innate immune mechanisms limit the infection until the development of a CTL response that purges the infection. Lymphoid isolates (eg, clone-13), in contrast to CNS isolates (eg, Armstrong 53b) of LCMV, replicate persistently at high viral titers in multiple organs, with a deficient CTL response in adult mice.16.