Continuous and sustained in vivo production of monoclonal antibodies by engineered cells might render long-term antibody-based treatments cost-effective, avoid side effects associated with infusion of massive doses of antibody, and circumvent possible antiidiotypic responses against the therapeutic agent. The FrCasE retrovirus induces a lethal neurodegeneration on infection of newborn mice. We report here that implantation of cellulose sulfate capsules containing cells secreting an ectopic monoclonal antibody neutralizing FrCasE can prevent animals from developing the disease. All treated mice showed reduced or undetectable viremia in addition to a lack of the histopathological lesions characteristic of FrCasE infection. This work paves the way for a novel gene/cell antibody-based immunotherapy of a variety of severe viral and nonviral diseases.