Effect of vaccination with modified vaccinia Ankara (ACAM3000) on subsequent challenge with Dryvax

Abstract

Background: Despite the success of smallpox vaccination, the immunological correlates of protection are not fully understood. To investigate this question, we examined the effect of immunization with modified vaccinia Ankara (MVA) on subsequent challenge with replication-competent vaccinia virus (Dryvax).

Methods: Dryvax challenge by scarification was conducted in 36 healthy subjects who had received MVA (n = 29) or placebo (n = 7) in a previous study of doses and routes of immunization. Subjects were followed up for clinical take, viral shedding, and immune responses.

Results: MVA administration attenuated clinical takes in 21 (72%) of 29 subjects, compared with 0 of 7 placebo recipients (P = .001). Attenuation was most significant in MVA groups that received 1 x 10(7) median tissue culture infective doses (TCID(50)) intradermally (P = .001) and 1 x 10(7) TCID(50) intramuscularly (P = .001). Both duration and peak titer of viral shedding were reduced in MVA recipients. Peak neutralizing antibody responses to vaccinia virus or MVA previously induced by MVA immunization were associated with attenuated takes (P = .02) and reduced duration (P = .001) and titer (P = .005) of viral shedding.

Conclusions: MVA immunization results in clinical and virologic protection against Dryvax challenge. Protection is associated with prior induction of neutralizing antibodies to MVA or vaccinia virus. MVA administered intradermally has protective and immunologic responses similar to those of a 10-fold-higher dose given subcutaneously.

Trial registration: ClinicalTrials.gov NCT00133575.

Figure 1

A, Magnitude and duration of viral shedding in relation to Dryvax take category* in subjects who had received MVA vaccination. Swabs of Dryvax-induced lesions were obtained at the indicated timepoints following challenge, and viral shedding was determined by tissue culture pfu assay. Data are presented as the median pfu/ml with interquartile ranges per group, and the dashed line represents the limit of detection (pfu =10/ml). Subjects who received placebo all had full takes* and are plotted separately. * Take categories defined in text [9, 10] B, Prior vaccination with MVA correlates with reduced titer and duration of viral shedding following Dryvax challenge. Data are presented as individual swab titers (pfu/ml) from individuals grouped as MVA or placebo recipients. A lowess fit line is plotted for each group. The assay limit of detection was 10 pfu/ml.

Figure 2

Neutralizing antibody responses following Dryvax challenge. Serum samples were obtained at days 0, 4, 7, 14, and 28 following Dryvax challenge. Serial dilutions were tested for neutralizing activity against VV:Luc. Data are presented as median ID₅₀ titers with interquartile ranges for each dose and route-of-immunization group, with the number of individuals in each group indicated. The dashed line represents the assay limit of detection (serum ID₅₀ titer =10).

Figure 3

Anti-EEV neutralizing antibody activity. Serum samples were obtained from vaccinated subjects two weeks following Dryvax challenge and tested in a comet reduction assay at a 1:50 dilution. Anti-vaccinia hyperimmune serum and matched pre-immune sera were used as positive and negative controls, respectively. Data are presented as the percent comet reduction observed from individual volunteers in each dose and route-of-immunization group, with bars indicating the median response. PL indicates the placebo group.

Figure 4

Antibody responses to vaccinia IMV- and EEV-associated antigens following Dryvax challenge. Serum samples were obtained on days 14 and 28 following Dryvax challenge. Serial dilutions were tested for antibody binding activity against two IMV (A27L and LIR) and two EEV (A33R and B5R) associated protein antigens by ELISA. Data are presented as individual endpoint titers with bars indicating median titer per group.

Figure 5

Cellular immune responses elicited by Dryvax challenge. PBMC were isolated at days 0, 4, 7, 14, and 28 following Dryvax scarification and tested in an IFN-γ ELISPOT assay against autologous VV:WR-infected target cells isolated at day 4 following the first MVA (or placebo) inoculation. Data are presented as median SFC per 10⁶ effector PBMC with interquartile ranges for each dose and route-of-immunization group following subtraction of responses to medium alone. SFC at baseline are subtracted from each data point.

Figure 6

Magnitude of vaccine-elicited anti-VV:WR neutralizing antibody responses prior to challenge correlate with reduced titer and duration of viral shedding following Dryvax challenge. Swabs of Dryvax lesions were obtained at the indicated timepoints following challenge, and viral shedding was determined by pfu assay. Data are presented as individual swab titers (pfu/ml) from individuals grouped according to peak anti-VV:WR neutralizing antibody titers measured following MVA or placebo vaccination as follows: ID₅₀ ≤ 20 (black circles), ID₅₀ 21–100 (red triangles), or ID₅₀ >100 (blue squares). A lowess fit line is plotted for each group. The assay limit of detection is 10 pfu/ml.