Next-generation smallpox vaccines like LC16m8, which have never been tested in the setting of natural variola, rely on surrogate markers for evaluation of effectiveness [14
]. The LC16m8 virus has a 1-base deletion within the B5R gene that results in a premature stop codon that truncates this protein and encodes for only 29% of the residue protein. B5R is a vaccinia gene product with a highly related homologue in variola virus. Deletion of the B5R results in decreased production of EEV [15
], a target for EEV neutralizing antibodies [16
]. Loss of B5R would predict attenuation of the virus. However, the success of vaccinia-based smallpox vaccines in protecting humans against variola major lies in the ability of the vaccinia to induce strong neutralizing antibody responses across multiple viral proteins with immunological homology to variola proteins, resulting in a “safety net” of highly redundant neutralizing antibody responses [18
This phase I/II trial assessed the safety and immunogenicity of LC16m8 compared with Dryvax in vaccinia-naive adult volunteers. There were no vaccine-related serious AEs or AEs graded greater than mild associated with either vaccine; other AEs (local or systemic) were generally equivalent in both groups. LC16m8 vaccine lesions were significantly smaller with less swelling or erythema than Dryvax lesions. In comparison to recently published data from a larger study in which 1529 nonnaive and 1692 vaccinia-naive volunteers received LC16m8, the extent of AEs, both systemic and local, were quite similar. Cardiac toxicity was not apparent in our study. US military and historical studies suggest a cardiac toxicity rate of between 0.1% and 0.3% of vaccine recipients [1
LC16m8 was immunogenic, although with lower humoral and cellular immunity kinetic responses than with Dryvax. The virus strain used in the PRNT assays had a significant impact on the results. In PRNT assays conducted using variola or monkeypox, Dryvax produced higher titers compared with LC16m8. Our data suggest that deletion of individual proteins, particularly B5R, results in different patterns of humoral immunity and that proteins targeted by the immune system may be different depending on the strain of vaccinia used. Our data suggest that vaccine prototypes that alter EEV production will have some attenuation of antivariola neutralization.
While the GMTs achieved by LC16m8 in the Dryvax-based PRNT assay were less than that of Dryvax at all measured time points, the kinetic profile was similar, suggesting that single-dose vaccination with LC16m8 will afford immunity in a manner paralleling that of the less attenuated vaccines. More important than absolute antibody titer is the specificity of antibody required for protection. Although the absolute immunological correlates of protection have yet to be defined, consensus has been that neutralizing antibody titers >1:40 against vaccinia are protective [22
]. In addition, the antivariola titers achieved with LC16m8, although semiquantitative due to variability in control samples, elicited neutralizing antibody titers that are associated with protection. However, duration of effective titer response after vaccination needs to be evaluated.
CMI was assessed in a subset of 47 participants. At day 13, 95% of LC16m8 and 67% of Dryvax vaccinees had lymphocyte proliferative responses, increasing to 97% and 89%, respectively, by day 180 (P
= .04 and .34, respectively). All vaccinees produced IFN-γ peak response by day 13, although 1 LC16m8-vaccinated individual lost detectable IFN-γ response at day 30. Memory responses following vaccination with LC16m8 appear to be durable, even at 180 days. It is interesting to note the lower IFN-γ ELISPOT responses given the B5R deletion in LC16m8. The IFN-γ response difference may reflect contribution of epitopes from B5R in the overall antivaccinial T-cell repertoire after vaccination. Previous publications have shown this protein to be a source of dominant T-cell epitopes although whether these are essential for protective immunity is unclear [24
]. In addition, robust T-cell responses could be contributors to AEs, particularly myocarditis as shown with other viruses but not to date with vaccinia [26
Our findings suggest that LC16m8 is well tolerated with similar reactogenicity compared with Dryvax. LC16m8, despite its deletion in B5R, elicited potent immune responses after single inoculation capable of neutralizing variola virus. Overall, the study demonstrated comparable safety between LC16m8 and Dryvax and immune responses that would support further development of LC16m8.