Vaccinia Virus LC16m8∆ as a Vaccine Vector for Clinical Applications

Abstract

The LC16m8 strain of vaccinia virus, the active ingredient in the Japanese smallpox vaccine, was derived from the Lister/Elstree strain. LC16m8 is replication-competent and has been administered to over 100,000 infants and 3,000 adults with no serious adverse reactions. Despite this outstanding safety profile, the occurrence of spontaneously-generated large plaque-forming virulent LC16m8 revertants following passage in cell culture is a major drawback. We identified the gene responsible for the reversion and deleted the gene (B5R) from LC16m8 to derive LC16m8Δ. LC16m8∆ is non-pathogenic in immunodeficient severe combined immunodeficiency (SCID) mice, genetically-stable and does not reverse to a large-plaque phenotype upon passage in cell culture, even under conditions in which most LC16m8 populations are replaced by revertants. Moreover, LC16m8∆ is >500-fold more effective than the non-replicating vaccinia virus (VV), Modified Vaccinia Ankara (MVA), at inducing murine immune responses against pathogenic VV. LC16m8∆, which expresses the SIV gag gene, also induced anti-Gag CD8⁺ T-cells more efficiently than MVA and another non-replicating VV, Dairen I minute-pock variants (DIs). Moreover, LC16m8∆ expressing HIV-1 Env in combination with a Sendai virus vector induced the production of anti-Env antibodies and CD8⁺ T-cells. Thus, the safety and efficacy of LC16m8∆ mean that it represents an outstanding platform for the development of human vaccine vectors.

Keywords: B5R; DIs; HIV; LC16m8; LC16m8∆; MVA; SIV; reversion; vaccinia virus.

Figure 1

Pathogenicity of B5R-defective viruses in severe combined immunodeficiency (SCID) mice. Figure modified from Kidokoro et al. [34].

Figure 2

Genetic stability of LC16m8∆ and LC16m8 upon serial passage in primary rabbit kidney cells at different temperatures (30 °C or 34 °C). Figure modified from Kidokoro et al. [34]. LPC, large-plaque-forming clone.

Figure 3

(A) Protective immune responses induced by m8∆ and derivative viruses in mice. (A–F) Average body weight of mice immunized (intramuscularly) with (10⁴–10⁶ PFU) vaccinia viruses (VVs) and then challenged intranasally with the Western Reserve (WR) strain. Crosses denote mice that either died or were sacrificed because they lost >30% of their body weight. Figure modified from Kidokoro et al. [34].