Biology Reference
In-Depth Information
protective efficacy of this vaccine in young women. A recombinant gB study is
also currently in progress in renal transplant patients. In this study, subjects await-
ing transplantation receive gB vaccine, to test whether the antibody responses
engendered will contribute to reduction of HCMV viral load following transplan-
tation (P.D. Griffiths, personal communication).
Another formulation of recombinant gB has also been evaluated in clinical tri-
als using a vectored vaccine expression system based on a canarypox vector,
ALVAC, an attenuated poxvirus that replicates abortively in mammalian cells.
Clinical trials have focused on using ALVAC-gB in a prime-boost approach, in
which ALVAC vaccine is administered to prime immune responses for subse-
quent boost with live, attenuated vaccine, or recombinant protein. In the first such
prime-boost study, ALVAC-gB was evaluated alone or in combination with live,
attenuated Towne vaccine. ALVAC-gB vaccine induced low neutralizing and
ELISA antibodies in seronegative adults, but subjects primed with ALVAC-gB
and then boosted with a single dose of Towne developed binding and neutralizing
antibody titers comparable to naturally seropositive individuals (Adler et al.
1999). A subsequent study compared three immunization regimens: subunit gB
vaccine, ALVAC-gB followed by gB/MF59, or both vaccines administered con-
comitantly (Bernstein et al. 2002). All three vaccine approaches induced high-
titer antibody and lymphoproliferative responses, but no benefit for priming was
detected. Thus, ALVAC-gB priming appears to result in augmented gB-specific
responses following a boost with Towne vaccine, but not subunit gB/MF59.
Another approach used to express gB as a vaccine is the use of an alphavirus
replicon system. This approach results in generation of virus-like replicon particles
(VRPs) based on an attenuated Venezuelan equine encephalitis (VEE) expression
system. Advantages of the VRP approach include the expression of high levels of
heterologous proteins, the targeting of expression to dendritic cells, and the induc-
tion of both humoral and cellular immune responses to the vectored gene products
of interest. HCMV gB has been expressed in the VRP system, and these VRPs have
undergone protein expression analyses in cell culture as well as immunogenicity
studies in mice. These studies demonstrated that protein expression levels are high-
est in VRPs expressing the extracellular domain of gB. BALB/c mice immunized
with VRP expressing gB developed high titers of neutralizing antibody to HCMV
(Reap et al. 2007). Based on these encouraging results, a phase I study of a trivalent
vaccine including gB has recently been commenced in humans.
A final expression approach that has been applied to HCMV gB is DNA
vaccination. In preclinical studies of HCMV gB DNA vaccines in mice, both the
full-length gB, as well as a truncated, secreted form expressing amino acids 1-680
(of a total of 906 gB residues), were evaluated. Immunization with both
constructs induced neutralizing antibodies, but titers were higher in mice immu-
nized with the DNA encoding the truncated form of gB, which predominately
elicited IgG1 antibody. In contrast, the full-length gB construct primarily elic-
ited IgG2a antibodies (Endresz et al. 1999). The gB plasmid vaccine that has
moved forward in human clinical trials is accordingly based on a construct
encoding a truncated, secreted form of the protein. For clinical trials, HCMV
