Platelets Mediate Clearance of Lymphocytic Choriomeningitis Virus Infection Preventing Lethal Hemorrhage.

Lymphocytic choriomeningitis virus (LCMV) is a noncytopathic mouse pathogen of the arenaviridae family. Acute LCMV infection in adult mice has been extensively studied and found to be systemic, essentially asymptomatic and associated with a bone marrow aplasia that produces a transient pancytopenic state. The initial lymphopenia is rapidly reversed, such that within one week of LCMV exposure the mice display lymphocytosis and clear the infection through a response mediated by virus-specific cytotoxic T cells (CTLs). In spite of the thrombocytopenia, the occurrence of hemorrhage in these animals has not been previously investigated, likely because of the lack of overt bleeding symptoms. Other arenaviruses (such as Lassa and Junin) produce systemic infections in humans and cause hemorrhagic diseases that are often lethal. Hemorrhage, mostly mucosal and cutaneous, occurs in the context of profound thrombocytopenia (characteristic of Junin infection) and/or platelet dysfunction (characteristic of Lassa infection), but without disseminated intravascular coagulation (DIC) or other coagulation defects. The pathogenesis of arenavirus infections in humans remains elusive, although disease severity has been associated with the extent of hemorrhage, impaired cellular immunity and lack of viral clearance. We recently showed that platelets may contribute to viral pathogenesis by facilitating the accumulation of virus-specific CTLs at sites of infection. Thus, we reasoned that the thrombocytopenia and/or platelet dysfunction that typify arenavirus infections in humans might not only predispose to the development of hemorrhage but also compromise CTL-mediated viral clearance. Here we report our studies based on the model of LCMV infection in mice. We found that normal inbred mice infected with different isolates of LCMV develop thrombocytopenia associated with decreased platelet function, but show only limited mucosal hemorrhage prior to CD8+ T cell-mediated viral clearance. In contrast, mice depleted of platelets, but not those given anticoagulant treatment, fail to produce a normal CD8+ T cell response or clear LCMV; instead, they develop an interferon (IFN)-α/β-dependent lethal hemorrhagic infection. Transfusion of normal but not activation-blocked platelets into these animals restored the CD8+ T cell responses and allowed clearance of the infection, preventing hemorrhage and death. These results indicate that activated platelets are required for CD8+ T cells to clear LCMV infection and for protecting the host against the induction of an IFN-α/β-dependent, lethal hemorrhagic diathesis.