Hepatitis E is an important public health disease with a high mortality rate of up to 28% in infected pregnant women. Recently, chronic HEV infection has become an emerging and significant clinical problem worldwide in immunocompromised individuals such as organ transplant recipients and patients with HIV, lymphoma and leukemia with considerable morbidity and mortality. Approximately 58-92% of the HEV-infected organ transplant recipients developed chronic HEV infection but the mechanism leading to the progression into chronicity remains unknown. The long-term goal of this project is to develop an animal model for chronic hepatitis E to study the mechanisms of HEV immunopathogenesis and intervention strategies. In aim 1, we hypothesize that HEV infection in pigs under immunosuppressive conditions will mimic the course of chronic HEV infection in immunocompromised individuals, which will enable us to delineate the predictive immunological factors leading to the progression into chronicity. Pigs will be immunosuppressed to mimic the immunosuppressive conditions in organ transplant recipients and then experimentally infected with HEV. The immunological, pathological, biochemical, virological and clinical parameters of HEV infection and hepatitis will also be analyzed and compared to identify the broad predictive factors for progression into chronicity. In aim 2, we hypothesize that impairment of T-cell responses, particularly CD4+ and CD8+ T-cell responses, is responsible for the progression into chronicity in immunocompromised individuals. We will determine if depletion of specific CD4+ or CD8+ T cell subsets in chickens will lead to chronic infection and severe hepatitis in chickens infected with avian HEV. Chickens depleted of either CD4 or CD8 T cells will be experimentally infected with avian HEV, and assessed for pathological, biochemical, virological, immunological and clinical parameters of hepatitis. Adoptive transfer CD4+ or CD8+ T cells into immune cell-depleted chickens will also be performed. In aim 3, we hypothesize that the humoral immune response is a double-edged sword in that it is important in control of HEV infection but it also involves in immune-mediated hepatic damages. B cell- depleted chickens as well as immunoglobulin (Ig) heavy chain homozygous knockout (-/-) piglets and wild-type littermate (+/+) will be experimentally infected with HEV, hepatic lesions, immunological, biochemical and virological parameters of hepatitis will be assessed and compared. The Ig heavy chain(-/-) knockout piglets will also be passively transferred with a HEV-specific antiserum followed by HEV challenge. By completing this project, we anticipate that a unique animal model for chronic hepatitis E will be established, and that the predictive immunological factors leading to chronic infection will be identified, the T cell epitopes responding to HEV infection mapped, and the mechanisms of HEV immunopathogenesis and chronic infection delineated. The results will be important for devising effective prevention and treatment strategies against HEV.