Members of the California serogroup of bunyaviruses are a cause of arthropod borne encephalitis in the United States and Europe. Infection of mice and rats provides an excellent experimental model to study the pathogenesis of this infection. The bunyavirus genome is divided into 3 segments and genetic reassortants can be made between certain members of the group, for instance some of the California serogroup viruses. This system has already been used to show that the middle genome segment, coding for the glycoprotein, is an important determinant of virulence (neuroinvasiveness). We plan to exploit this system to extend the genetic approach to viral pathogenesis. Proposed work includes: (1) Production of monoclonal antibodies of the LaCrosse and Tahyna viruses of the California serogroup. (2) The use of monoclonal antibodies to make subgenomic variants to further analyze and map the molecular sites on the viral glycoprotein which determine virulence. (3) The description or development of bunyavirus strains which differ in intracerebral virulence (rats), or in intraperitoneal virulence (mice), to study neurovirulence as well as neuroinvasiveness. (4) The possible use of monoclonal antibodies for rapid phenotyping of viral reassortants. (5) Comparison of the molecular differences between a few selected variant viruses, using RNA fingerprinting and tryptic digestion. (6) Descriptive and analytic studies of pathogenesis using selected virus-host combinations to identify the specific steps in pathogenesiswhich determine neurovirulence or neuroinvasiveness. (7) The identification of inbred strains of rats and mice which differ in susceptibility to bunyaviruses encephalitis, to study host genetic determinants of infection. (8) Use of monoclonal antibodies for taxonomic classification of bunyaviruses (collaborative study). (9) Establishment of an assay for cytolytic T cells (CTL) and the use of reassortants and subgenomic variant viruses to study the antigenic determinants of CTL specificity (collaborative study).