Cytomegalovirus Vaccine Development - Human Cytomegalovirus - page 364

plethora of additional, previously unrecognized CD4 + and CD8 + T lymphocyte targets
encoded by the HCMV genome. Other potential CD8 + T cell targets, in addition to
pp65 and IE1, are just beginning to be investigated. A recently described bioinformat-
ics and ex vivo functional T cell assay approach revealed that CD8 + T cell responses
to HCMV often contained multiple antigen-specific reactivities, which were not just
constrained to pp65 or IE1 antigens. These studies identified structural, early/late
antigens, and HCMV-encoded immunomodulators (pp28, pp50, gH, gB, US2, US3,
US6, and UL18) as potential targets for HCMV-specific CD8 + T cell immunity
(Elkington et al. 2003). An elegant and comprehensive analysis of T cell responses to
HCMV infection was recently conducted using cytokine flow cytometry in conjunc-
tion with overlapping peptides comprising 213 HCMV open reading frames: this
study demonstrated that 151 HCMV ORFs were immunogenic for CD4 + and/or CD8 +
T cells (Sylwester et al. 2005). Recently, an approach to vaccination has been exam-
ined in the MCMV model in which essential, nonstructural proteins that are highly
conserved among the CMVs were explored as a novel class of T cell targets. These
studies found that DNA immunization of mice with the murine CMV (MCMV)
homologs of HCMV DNA polymerase (M54) or helicase (M105) was protective
against virus replication following systemic challenge, and that gamma interferon
staining of CD8 + T cells from mice immunized with either the M54 or M105 DNAs
showed strong primary responses that recalled rapidly after viral challenge (Morello
et al. 2007). These conserved, essential proteins thus may represent a novel class of
CD8 + T cell targets that could contribute to successful HCMV vaccine design.
Dense Body Vaccines
A novel candidate for vaccination against HCMV currently in preclinical development
is the dense body vaccine. Dense bodies (DBs) are enveloped, replication-defective
particles formed during replication of CMVs in cell culture. These structures are a
potentially promising vaccine because they contain both envelope glycoproteins and
large quantities of pp65 protein, two key targets of the protective immune response to
infection. DBs are noninfectious and therefore, in principle, would be immunogenic,
but incapable of establishing latent HCMV infection in the vaccine recipient, provid-
ing a useful safety feature. DBs have been shown to be capable of inducing virus
neutralizing antibodies and T cell responses after immunization of mice, including
human HLA-A2.K(b) transgenic mice, in the absence of viral gene expression. Based
on these studies, the utilization of DBs may represent a promising, novel approach to
the development of a subunit vaccine against HCMV infection (Pepperl et al. 2000).
Peptide-Based Vaccines
Another potential approach to HCMV vaccination is the use of peptide vaccination
employing synthetic peptides comprising immunodominant cytotoxic T cell