Smallpox vaccines: targets of protective immunity

Abstract

The eradication of smallpox, one of the great triumphs of medicine, was accomplished through the prophylactic administration of live vaccinia virus, a comparatively benign relative of variola virus, the causative agent of smallpox. Nevertheless, recent fears that variola virus may be used as a biological weapon together with the present susceptibility of unimmunized populations have spurred the development of new-generation vaccines that are safer than the original and can be produced by modern methods. Predicting the efficacy of such vaccines in the absence of human smallpox, however, depends on understanding the correlates of protection. This review outlines the biology of poxviruses with particular relevance to vaccine development, describes protein targets of humoral and cellular immunity, compares animal models of orthopoxvirus disease with human smallpox, and considers the status of second- and third-generation smallpox vaccines.

Fig. 1. Synergistic induction of neutralizing antibody following immunization with DNA encoding VACV A28 and H2 proteins

Plasmids expressing codon optimized A28 and H2 were adsorbed to gold beads and injected 4× at intervals into mice using a gene gun. Sera were tested for ability to neutralize VACV MVs and the half maximal inhibitory concentration (IC₅₀) values are indicated. EV, gold bead without empty vector plasmid; A28, gold bead with A28 plasmid; H2, gold bead with H2 plasmid; A28/H2, A28 and H2 plasmids on same gold bead; A28 + H2, mix of A28 and H2 beads injected into same site. Sera mix refers to a mixture of sera from mice immunized with A28-gold beads and mice immunized with H2-gold beads. Adapted from (118).

Fig. 2. Protection of mice, immunized with a single recombinant protein or combinations of recombinant proteins, to an intranasal challenge with 2 × 10⁷ PFU of VACV strain WR

Mice were immunized 4× with 10 µg of an individual recombinant protein or with mixtures containing 10 µg of each recombinant protein or with VACV Wyeth (derived from Dryvax). The percent of surviving mice (A) and percent of initial weight (B) are averages from two separate experiments with a total of 13 to 19 mice in each group. Abbreviations: Unimm, unimmunized; Unchall, unchallenged. From (223).

Fig. 3. Protection of mice, passively immunized with a single mAb or combinations of mAbs, to an intranasal challenge with 2 × 10⁷ PFU of VACV strain WR

Groups of four 14-week-old female BALB/c mice were immunized intraperitoneally with 100 µg of A33, B5, or L1 mAb or combinations as indicated. All immunized mice survived except for one mouse that received anti-B5 alone and all control mice that received mouse anti-K^b-ova mAb. Percent of initial mean weights +/− SEM are shown. Modified from (237).

Fig. 4. Comparisons of antibody responses and protection induced by prophylactic immunization with MVA and Dryvax

Monkeys were injected intramuscularly with 10⁸ PFU of MVA at 0 time and boosted at 8 weeks with MVA (M/M) or by skin scarification with Dryvax (M/D) or with Dryvax alone at 8 weeks (D) or were unimmunized (C). Antibodies that bound to immobilized purified VACV (Panel A), L1 protein (Panel B), A33 protein (Panel C), B5 protein (Panel D) or neutralized VACV infectivity (Panel E) are shown. At 16 weeks, the monkeys were challenged intravenously with a lethal dose of MPXV and the viral load was determined as the number of MPXV genomes (Panel F). Modified from (176).