An effective cytomegalovirus (CMV) vaccine could prevent the majority of birth defects caused by congenital CMV infections. Candidate vaccines in clinical evaluation include live attenuated, protein subunit, DNA, and viral-vectored approaches. Subunit approaches have focused on the CMV proteins pp65 and IE1 as important inducers of cytotoxic T cells and glycoprotein B (gB) as an important inducer of neutralizing antibodies. A vaccine comprised of recombinant gB protein with MF59 adjuvant reduced the incidence of primary infection by 50%. Recent revelations regarding CMV entry pathways into different cell types suggest a possible course for improvement. A 5-subunit pentameric complex is uniquely required for endothelial and epithelial cell entry. Sera from naturally infected subjects contain high-potency neutralizing activities specific for this complex, whereas the gB/MF59 vaccine fails to induce comparable neutralizing activities. A vaccine's ability to induce salivary antibodies that neutralize epithelial cell entry may be especially important for preventing oral transmission as the first cells infected are presumably epithelial cells of the oral mucosa. In addition, recent evidence suggests that antibodies can inhibit postentry CMV spread between endothelial and epithelial cells. Such activities may serve to limit viral replication in tissues or impair dissemination to the placenta and fetus. Thus, inclusion of epitopes derived from the pentameric complex may provide enhanced efficacy by inducing potent neutralizing/spread-inhibiting antibodies that target virus replication in a broad spectrum of cell types. Next-generation vaccine candidates in preclinical development incorporate peptides, subunits, or multisubunit complexes representing parts or all of the pentameric complex. Approaches include peptides, recombinant proteins, DNA, replication-defective viral vectors, genetically disabled CMV, and inactivated CMV virions. The diversity of novel strategies under development engenders optimism that a successful candidate will emerge.
Keywords: congenital infection; cytomegalovirus; vaccine.