Dense bodies of human cytomegalovirus induce both humoral and cellular immune responses in the absence of viral gene expression

Infection of fibroblast cell cultures with human cytomegalovirus (HCMV) lea ds to the production of significant amounts of defective enveloped particle s, termed dense bodies (DB), These noninfectious structures contain major a ntigenic determinants which are responsible for induction of both the humor al and the cellular immune response against HCMV. We tested the hypothesis that, by virtue of their unique antigenic and structural properties, DB cou ld induce a significant immune response in the absence of infectious virus. Mice were immunized with gradient-purified DB, which were either left untr eated or subjected to sequential rounds of sonication and freeze-thawing to prevent cellular entry. Titers of neutralizing antibodies induced by DB we re in a range comparable to levels present in convalescent human sera. The virus-neutralizing antibody response was surprisingly durable, with neutral izing antibodies detected 12 months following primary immunization. The HCM V-specific major histocompatibility complex class I-restricted cytolytic T- cell (CTL) response was assayed using mice transgenic for the human HLA-A2 molecule. Immunization with DB led to high levels of HCMV-specific CTL in t he absence of de novo viral protein synthesis. Maximal total cytolytic acti vity in mice immunized with DB was nearly as efficient as the cytolytic act ivity induced by a standard immunization with murine cytomegalovirus. Furth ermore, DB induced a typical T-helper 1 (Th1)-dominated immune response in mice, as determined by cytokine and immunoglobulin G isotype analysis. Indu ction of humoral and cellular immune responses was achieved without the con comitant use of adjuvant. We thus propose that DB can serve as a basis for the future development of a recombinant nonreplicating vaccine against HCMV . Finally, such particles could be engineered for efficient delivery of ant igens from other pathogens to the immune system.