Quantitation of CD8⁺ T Cell Responses to Newly Identified HLA-A*0201–restricted T Cell Epitopes Conserved Among Vaccinia and Variola (Smallpox) Viruses

Immunization with vaccinia virus resulted in long-lasting protection against smallpox and was the approach used to eliminate natural smallpox infections worldwide. Due to the concern about the potential use of smallpox virus as a bioweapon, smallpox vaccination is currently being reintroduced. Severe complications from vaccination were associated with congenital or acquired T cell deficiencies, but not with congenital agammaglobulinemia, suggesting the importance of T cell immunity in recovery from infection. In this report, we identified two CD8⁺ T cell epitopes restricted by the most common human major histocompatibility complex (MHC) class I allele, HLA-A*0201. Both epitopes are highly conserved in vaccinia and variola viruses. The frequency of vaccinia-specific CD8⁺ T cell responses to these epitopes measured by interferon (IFN)-γ enzyme-linked immunospot (ELISPOT) assay and HLA/peptide tetramer staining peaked 2 wk after primary immunization and then declined, but were still detectable 1 to 3 yr after primary immunization. 2 wk after immunization, IFN-γ–producing cells specific to these two epitopes were 14% of total vaccinia virus-specific IFN-γ–producing cells in one donor, 35% in the second donor, and 6% in the third donor. This information will be useful for studies of human T cell memory and for the design and analyses of the immunogenicity of experimental vaccinia vaccines.