Identification of protective T-cell antigens for smallpox vaccines

Ando, Jun; Ngo, Minhtran C.; Ando, Miki; Leen, Ann; Rooney, Cliona M.

Abstract E3L is an immediate-early protein of vaccinia virus that is detected within 0.5 hours of infection, potentially before the many immune evasion genes of vaccinia can exert their protective effects. E3L is highly conserved among orthopoxviruses and hence could provide important protective T-cell epitopes that should be retained in any subunit or attenuated vaccine. We have therefore evaluated the immunogenicity of E3L in healthy VV vaccinated donors. Peripheral blood mononuclear cells from healthy volunteers (n=13) who had previously received a smallpox vaccine (Dryvax®) were activated and expanded using overlapping E3L peptides and their function, specificity and anti-viral activity was analyzed. E3L-specific T-cells were expanded from 7 of 12 (58.3%) vaccinated healthy donors. 25% of these produced CD8+ T-cell responses and 87.5% produced CD4+ T-cells. We identified epitopes restricted by HLA-B35 and HLA-DR15. E3L-specific T-cells killed peptide-loaded target cells as well as vaccinia-infected cells but only CD8+ T-cells could prevent the spread of infectious virus in virus inhibition assays. The epitopes recognized by E3L-specific T-cells were shared with monkeypox and although there was a single amino acid change in the variola epitope homolog, it was recognized by vaccinia-specific T-cells. Therefore it might be important to include E3L in any deletion mutant or subunit vaccine and E3L could provide a useful antigen to monitor protective immunity in humans.