Genomic analysis of chimeric human cytomegalovirus vaccine candidates derived from strains Towne and Toledo

Abstract

Human cytomegalovirus (HCMV) is an important opportunistic pathogen in immunocompromised patients and a major cause of congenital birth defects when acquired in utero. In the 1990s, four chimeric viruses were constructed by replacing genome segments of the high passage Towne strain with segments of the low passage Toledo strain, with the goal of obtaining live attenuated vaccine candidates that remained safe but were more immunogenic than the overly attenuated Towne vaccine. The chimeras were found to be safe when administered to HCMV-seronegative human volunteers, but to differ significantly in their ability to induce seroconversion. This suggests that chimera-specific genetic differences impacted the ability to replicate or persist in vivo and the consequent ability to induce an antibody response. To identify specific genomic breakpoints between Towne and Toledo sequences and establish whether spontaneous mutations or rearrangements had occurred during construction of the chimeras, complete genome sequences were determined. No major deletions or rearrangements were observed, although a number of unanticipated mutations were identified. However, no clear association emerged between the genetic content of the chimeras and the reported levels of vaccine-induced HCMV-specific humoral or cellular immune responses, suggesting that multiple genetic determinants are likely to impact immunogenicity. In addition to revealing the genome organization of the four vaccine candidates, this study provided an opportunity to probe the genetics of HCMV attenuation in humans. The results may be valuable in the future design of safe live or replication-defective vaccines that optimize immunogenicity and efficacy.

A fifty-year odyssey: prospects for a cytomegalovirus vaccine in transplant and congenital infection

“…As an attempt to improve the immunogenicity provided by Towne, four Towne/Toledo genome chimeras have been generated by substituting portions of the Toledo strain UL/b’ region with those of the Towne strain [97]. When tested in phase I clinical trials, the Towne/Toledo chimeras were attenuated relative to Toledo [98], showed acceptable safety profiles, and stimulated immune responses similar to those conferred by Towne [99].…”

Progress toward Development of a Vaccine against Congenital Cytomegalovirus Infection

“…What impact this would have on the potential efficacy of a congenital CMV vaccine is unclear. The recently reported DNA sequence analysis of the Towne-Toledo chimeras should aid in elucidating the molecular basis for the observed differences in immunogenicity seen in this clinical trial, as well as in identifying potential genetic markers conferring attenuation (101).…”

Advances in the treatment of cytomegalovirus

“…These chimeras showed strong safety profiles and induced conventional CD8 + T cell responses, but showed differences in immunogenicity between chimeras 74 . Unfortunately, DNA sequencing analysis showed that all four chimeras had disrupted copies of the UL128 gene and so could not express WT PC 77 . However, this sequencing data may provide useful information for understanding how genetic differences between attenuated viruses correlate with differences in immunogenicity between chimeras, which could inform future vaccine design.…”