Stimulation of human lymphocytes by cytomegalovirions and dense bodies

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) leads to the production of significant amounts of defective enveloped particles, termed dense bodies (DB). These noninfectious structures contain major antigenic determinants which are responsible for induction of both the humoral and the cellular immune response against HCMV. We tested the hypothesis that, by virtue of their unique antigenic and structural properties, DB could induce a significant immune response in the absence of infectious virus. Mice were immunized with gradient-purified DB, which were either left untreated or subjected to sequential rounds of sonication and freeze-thawing to prevent cellular entry. Titers of neutralizing antibodies induced by DB were in a range comparable to levels present in convalescent human sera. The virus-neutralizing antibody response was surprisingly durable, with neutralizing antibodies detected 12 months following primary immunization. The HCMV-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 the absence of de novo viral protein synthesis. Maximal total cytolytic activity in mice immunized with DB was nearly as efficient as the cytolytic activity induced by a standard immunization with murine cytomegalovirus. Furthermore, DB induced a typical T-helper 1 (Th1)-dominated immune response in mice, as determined by cytokine and immunoglobulin G isotype analysis. Induction of humoral and cellular immune responses was achieved without the concomitant 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 antigens from other pathogens to the immune system.

In vitro stimulation of lymphocytes by different strains of cytomegalovirus

The in vitro stimulation of lymphocytes from 15 healty adult donors by 5 different stains of CMV resulted in a positive stimulation ratio of lymphocytes from CMV seropositive donors with each of the 5 different strains. Differences in the absolute strength of the response was observed among the 5 strains for each individual, but we concluded that the use of antigen prepared with 1 strain of CMV is sufficient to obtain a positive or negative result. A maximum value for the in vitro lymphocyte stimulation test, however, will only be obtained using different antigen concentrations from several different CMV strains simultaneously.

Conditions for cytomegalovirus stimulation of lymphocytes

Lymphocytes from healthy donors or from patients with chronic lymphocytic leukaemia were subjected to live or inactivated cytomegalovirus (CMV) or the mitogen phytohaemagglutinin. No early or late CMV antigens could be demonstrated in the lymphocytes, indicating that neither abortive nor replicative CMV infection takes place. Only cells from CMV antibody-positive leukaemic and non-leukaemic donors were stimulated by CMV to DNA synthesis, with a maximum on day 5. Cells from all individuals responded to phytohaemagglutinin stimulation, the peak of activity occurring on day 3. The stimulation with CMV occurred in T cells and was independent of early CMV antigen production, viral DNA synthesis, or viral replication. CMV is thus not an in vitro lymphocyte mitogen like Epstein-Barr virus but is a very potent antigen for memory T cells.

Comparison of in vitro lymphocyte proliferations induced by cytomegalovirus-infected human fibroblasts and cell-free cytomegalovirus

In vitro lymphocyte reactivity (LR) to cytomegalovirus (CMV)-infected human fetal fibroblasts (CMVFF) and cell-free CMV were measured by using lymphocytes from healthy donors. Lymphocytes from all seropositive donors were stimulated by CMVFF, whereas lymphocytes from negative donors were not. The optimal stimulator cell-to-lymphocyte ratio was in the range of 1:5 to 1:50, dependent on the virus dose used. LR to cell-free CMV was positive for 15 out of 18 seropositive donors and negative for 14 out of 16 seronegative donors. In most cases LR to CMVFF was considerably higher than LR to cell-free CMV. Within the CMV seropositive group there was no significant correlation between the LR to either CMVFF or cell-free CMV and the levels of antibodies to CMV early antigens or CMV late antigens. There was no strict correlation between LR to CMVFF and to cell-free CMV, especially not in tests with lymphocytes from two patients with CMV mononucleosis. Our data suggest that CMVFF and cell-free CMV are recognized (partly) by different subpopulations of CMV-specific memory lymphocytes. We conclude that the use of CMV-infected cells, in addition to cell-free CMV, in LR tests gives more reproducible and possibly also additional information about CMV-specific cellular immunity.

Humoral immune response to virions and dense bodies of human cytomegalovirus determined by enzyme immunofluorescence assay

An enzyme immunoassay utilizing a fluorogenic substrate (EIFA) was employed to compare humoral antibody responses to purified virion and dense body antigens in human cytomegalovirus (CMV) infections. Results of these antibody assays were also compared to those obtained by plaque reduction neutralization and complement fixation (CF). In adults with CMV infections diagnosed by a significant rise in CF antibody titer, there was also a vigorous antibody response to the virion and dense body antigens. There was better correlation between antibody levels to the virion and dense body antigens than between antibody levels detected by other combinations of tests. Antibody responses to the virion and dense body antigens were similar, but not identical, suggesting that although the two structures share major antigens, they may also possess unique antigens to which the host can mount independent antibody responses. All individuals with CMV neutralizing antibody also had virion and dense body antibodies, but there was poor correlation between neutralization titers and those detected by EIFA, perhaps reflecting greater strain specificity of the neutralization test. Some individuals with serological diagnoses of Mycoplasma pneumoniae or influenza A infection showed fourfold or greater antibody titer rises to CMV in two or more assays, suggesting reactivation of latent CMV in these infections.