Lymphocyte and Antibody Responses Reduce Enterovirus 71 Lethality in Mice by Decreasing Tissue Viral Loads

Enterovirus 71 (EV71) infects the central nervous system and causes death and long-term neurological sequelae in hundreds of thousands of young children, but its pathogenesis remains elusive. Immunopathological mechanisms have been suspected to contribute to the pathogenesis of neurological symptoms, so anti-inflammatory agents have been used to treat patients with neurological symptoms. The present study was therefore designed to investigate the functions of lymphocyte and antibody responses in EV71 infection using a mouse model. Immunohistochemical staining analysis revealed virus and three types of lymphocytes, B cells, CD4 T cells, and CD8 T cells, in the spinal cord of an EV71-infected patient who died. A study of mice showed that the levels of virus and lymphocytes in brains and antibody titers in sera were elevated during the time when the mice succumbed to death in a phenomenon analogous to that observed in patients. Further studies demonstrated that after infection, the disease severity, mortality, and tissue viral loads of mice deficient in B, CD4 T, or CD8 T cells were significantly higher than those of wild-type mice. In addition, treatment with a virus-specific antibody, but not a control antibody, before or after infection significantly reduced the disease severity, mortality, and tissue viral loads of mice deficient in B cells. Our results show that both lymphocyte and antibody responses protect mice from EV71 infection. Our study suggests the use of vaccines and virus-specific antibodies to control fatal outbreaks and raises caution over the use of corticosteroids to treat EV71-infected patients with neurological symptoms.Enterovirus 71 (EV71), a member of the family Picornaviridae, infects humans by the fecal-oral route and induces mild symptoms, such as herpangina and hand, foot and mouth disease. It can also infect the central nervous system (CNS) and induce fatal neurological manifestations, such as aseptic meningitis, brain stem encephalitis, encephalomyelitis, and acute flaccid paralysis, with cardiopulmonary complications, especially in young children. Most fatalities occur in cases with brain stem encephalitis and fulminant pulmonary edema complications (6, 7, 9, 12, 15, 19). Survivors of severe cases are often left with long-term neurologic sequelae (6, 14, 15).EV71 outbreaks have been reported periodically throughout the world (7, 9, 16). In the past decade, the Asia-Pacific region has experienced more frequent and widespread fatal outbreaks (16). The largest and most severe outbreak occurred in Taiwan in 1998 when 129,106 cases of herpangina and hand, foot and mouth disease, 405 cases of neurological and cardiopulmonary complications, and 78 deaths were reported (7). Since then, EV71 infection has become endemic in Taiwan and caused >40, >40, and 14 deaths in 2000, 2001, and 2008, respectively (3, 11). In addition, 42 deaths have been reported in China by June in 2008 (16). Although it has been estimated that EV71 infects millions of children and causes thousands of cases of neurologic sequelae and >200 deaths in the past decade (3, 7, 11, 16), there are no effective vaccines and specific antiviral therapies available to control fatal outbreaks due in part to the lack of understanding of viral pathogenesis.Infants and young children are very susceptible to EV71 infection. Immature immunity is therefore suspected to associate with increased morbidity and mortality (6, 7, 9). This is supported by the findings of lymphopenia, depletion of CD4 and CD8 T lymphocytes, and decreased cellular immunity in the peripheral blood of patients with brain stem encephalitis and pulmonary edema (4, 17). However, some clinical studies showed that elevated cellular immunity was linked with unfavorable outcomes (5, 8). High levels of white blood cell counts in blood and cerebrospinal fluid with a predominance of lymphocytes were detected in patients with fatal or severe sequelae (5, 8, 19, 22). In addition, autopsy reports revealed not only virus but also severe mononuclear cell infiltrates in the CNSs of patients who died (12, 22). Moreover, a clinical study reported that a patient developed opsomyoclonus syndrome, which is an autoimmune disease resulting from lesions in the dentate nucleus of the cerebellum (14). In this patient, the high titer of virus-specific antibodies detected at the onset of neurological disease and the responsiveness of the condition to anti-inflammatory agents (corticosteroids) provide further evidence of an autoimmune etiology. Besides corticosteroids, intravenous immunoglobulin (IVIG), which has several anti-inflammatory properties and often contains neutralizing antibodies to enteroviruses, has been a mandatory treatment for patients with neurological symptoms in Taiwan, because it has been shown to improve the conditions of patients infected with other enteroviruses, coxackievirus B, and echovirus (1, 5, 13, 18).Although corticosteroids have been used to treat EV71-infected patients with neurological symptoms (14, 15), the significance of lymphocyte and antibody responses in the pathogenesis of EV71 remains to be determined. The present study was therefore designed to address this issue using a mouse model.