A comparison of neurovirulence of vaccinia virus by intrathalamic and/or intracisternal inoculations into cynomolgus monkeys

Vaccinia Virus LC16m8∆ as a Vaccine Vector for Clinical Applications

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.

IMVAMUNE: modified vaccinia Ankara strain as an attenuated smallpox vaccine

Smallpox vaccines based on replicating vaccinia virus are known to elicit rare yet serious adverse events, particularly in human populations with immune deficiency, atopic dermatitis and at the extremes of age. A vaccine that induces protective immune responses equivalent to first-generation smallpox vaccines while reducing the risk for severe adverse events is critical for a national stockpile of smallpox vaccines. Modified vaccinia Ankara (MVA) has been proposed as an immediate solution for vaccination of high-risk individuals. Bavarian Nordic's vaccine MVA-BN (IMVAMUNE) is a MVA strain that is replication incompetent in mammalian cell lines. IMVAMUNE has been administered to more than 1900 human subjects to date, including high-risk populations (e.g., people diagnosed with atopic dermatitis or infected with HIV) in which standard replicating vaccines are contraindicated. We review the Phase I clinical trial safety profile and immune responses and compare them with other smallpox vaccines, including ACAM2000 and Dryvax.

Genetically stable and fully effective smallpox vaccine strain constructed from highly attenuated vaccinia LC16m8

A highly attenuated LC16m8 (m8) smallpox vaccine has been licensed in Japan because of its extremely low neurovirulence profile, which is comparable to that of replication incompetent strains of vaccinia virus. From 1973 to 1975, m8 was administrated to >100,000 infants where it induced levels of immunity similar to that of the originating Lister strain, without any serious side effects. Recently, we observed that m8 reverts spontaneously to large plaque forming clones that possess virulence equivalent to that of LC16mO, a parental virus strain of m8. Here, we report that the B5R gene is responsible for the reversion, and that we could construct a more genetically stable virus by deleting B5R from m8. The protective immunogenicity of the vaccine candidate proved to be equivalent to that of the U.S.-licensed product Dryvax, and much superior to modified vaccinia Ankara in a mouse model. Furthermore, the vaccine strain never elicited any symptoms in severe combined immunodeficiency disease mice, even at a dose 1,000-fold greater than that used in the immune protection experiments, which is in contrast to the lethal pathogenicity induced by Dryvax inoculation of severe combined immunodeficiency disease mice. Our results suggest that this vaccine strain is a good candidate as a suitable smallpox vaccine and a vector virus, and that B5R is not essential for protective immunity against smallpox.

ACAM2000 clonal Vero cell culture vaccinia virus (New York City Board of Health strain)--a second-generation smallpox vaccine for biological defense

The threat of smallpox as a biological weapon has spurred efforts to create stockpiles of vaccine for emergency preparedness. In lieu of preparing vaccine in animal skin (the original method), we cloned vaccinia virus (New York City Board of Health strain, Dryvax by plaque purification and amplified the clone in cell culture. The overarching goal was to produce a modern vaccine that was equivalent to the currently licensed Dryvax in its preclinical and clinical properties, and could thus reliably protect humans against smallpox. A variety of clones were evaluated, and many were unacceptably virulent in animal models. One clonal virus (ACAM1000) was selected and produced at clinical grade in MRC-5 human diploid cells. ACAM1000 was comparable to Dryvax in immunogenicity and protective activity but was less neurovirulent for mice and nonhuman primates. To meet requirements for large quantities of vaccine after the events of September 11th 2001, the ACAM1000 master virus seed was used to prepare vaccine (designated ACAM2000) at large scale in Vero cells under serum-free conditions. The genomes of ACAM1000 and ACAM2000 had identical nucleotide sequences, and the vaccines had comparable biological phenotypes. ACAM1000 and ACAM2000 were evaluated in three Phase 1 clinical trials. The vaccines produced major cutaneous reactions and evoked neutralizing antibody and cell-mediated immune responses in the vast majority of subjects and had a reactogenicity profile similar to that of Dryvax.

Mouse neurotoxicity test for vaccinia-based smallpox vaccines

The only US FDA licensed smallpox vaccine, Dryvax, was associated with rare but serious neurological adverse events. After smallpox was eradicated in the United States, mass vaccination ceased in 1971. As counter-bioterrorism/biowarfare measures, new smallpox vaccines are now being investigated. However, there are no established pre-clinical neurotoxicity assays with which to evaluate these new vaccines prior to licensure. Here we report the development and initial characterization of a small animal neurotoxicity assay for vaccinia-based smallpox vaccines using Dryvax virus as a reference vaccine strain and the neuroadapted Western Reserve (WR) strain as a neurotoxic positive control. In neonatally inoculated mice, the WR strain produced significantly greater and more rapid onset of mortality than the Dryvax vaccine reference. Expression of virus antigen in neural cells and infectious virus replication in the brain was also significantly different between the two strains. In addition, the appearance of high titer virus antibody correlated with the clearance of virus from brain. With further validation, this assay incorporating a licensed vaccine reference standard and positive control strain may provide important pre-clinical neurotoxicity data on new vaccinia-based smallpox vaccine strains.

Intravenous inoculation of replication-deficient recombinant vaccinia virus DIs expressing simian immunodeficiency virus gag controls highly pathogenic simian-human immunodeficiency virus in monkeys

To be effective, a vaccine against human immunodeficiency virus type 1 (HIV-1) must induce virus-specific T-cell responses and it must be safe for use in humans. To address these issues, we developed a recombinant vaccinia virus DIs vaccine (rDIsSIVGag), which is nonreplicative in mammalian cells and expresses the full-length gag gene of simian immunodeficiency virus (SIV). Intravenous inoculation of 10(6) PFU of rDIsSIVGag in cynomologus macaques induced significant levels of gamma interferon (IFN-gamma) spot-forming cells (SFC) specific for SIV Gag. Antigen-specific lymphocyte proliferative responses were also induced and were temporally associated with the peak of IFN-gamma SFC activity in each macaque. In contrast, macaques immunized with a vector control (rDIsLacZ) showed no significant induction of antigen-specific immune responses. After challenge with a highly pathogenic simian-human immunodeficiency virus (SHIV), CD4(+) T lymphocytes were maintained in the peripheral blood and lymphoid tissues of the immunized macaques. The viral set point in plasma was also reduced in these animals, which may be related to the enhancement of virus-specific intracellular IFN-gamma(+) CD8(+) cell numbers and increased antibody titers after SHIV challenge. These results demonstrate that recombinant DIs has potential for use as an HIV/AIDS vaccine.

Clonal vaccinia virus grown in cell culture as a new smallpox vaccine

Although the smallpox virus was eradicated over 20 years ago, its potential release through bioterrorism has generated renewed interest in vaccination. To develop a modern smallpox vaccine, we have adapted vaccinia virus that was derived from the existing Dryvax vaccine for growth in a human diploid cell line. We characterized six cloned and one uncloned vaccine candidates. One clone, designated ACAM1000, was chosen for development based on its comparability to Dryvax when tested in mice, rabbits and monkeys for virulence and immunogenicity. By most measures, ACAM1000 was less virulent than Dryvax. We compared ACAM1000 and Dryvax in a randomized, double-blind human clinical study. The vaccines were equivalent in their ability to produce major cutaneous reactions ('takes') and to induce neutralizing antibody and cell-mediated immunity against vaccinia virus.

Recombinant vaccinia virus LC16m0 or LC16m8 that expresses hepatitis B surface antigen while preserving the attenuation of the parental virus strain

Thymidine kinase-negative recombinant vaccinia virus LC16m0 or LC16m8 expressing hepatitis B surface antigen preserved almost the same pathogenicity as their parental thymidine kinase-positive attenuated virus strains. The results demonstrate the potential for using the LC16m0 or LC16m8 virus strain as a vector for recombinant vaccine.