Cytotoxic T lymphocyte (CTL) responses to vaccinia virus (VV) were studied in human subjects receiving smallpox vaccine by dermal scarification. After in vitro restimulation with VV, peripheral blood mononuclear cells (PBMCs) from three of six vaccinated subjects killed virus-infected target cells. W-specific, HLArestricted CTL activity was mediated primarily by CD8 + cells, although low levels of lytic activity by CD4 + cells were observed in some experiments. Two of the three responders had no history of exposure to VV prior to vaccination, whereas all three non-responders were vaccinated against smallpox during childhood. PBMCs from an additional three subjects who had received smallpox vaccine at least 20 years previously were negative for CTL activity. These data suggest that the duration of memory CTL populations against W in the peripheral blood is limited, and that pre-existing immunity to VV may interfere with boosting of the CTL response upon revaccination.The design of vaccines that will prime virus-specific class I major histocompatibility complex (MHC)-restricted T lymphocytes is complicated by the requirement for processing of antigen within the cytoplasm or endoplasmic reticulum (Morrison et al., 1986;Moore et al., 1988;Germain, 1986). Recombinant viruses deliver antigens efficiently into the class I antigen presentation pathway (Bennink & Yewdell, 1990;Moss, 1991), and thus have been proposed as vectors for vaccine delivery in humans. Infection of experimental animals with recombinant vaccinia virus (VV) vectors elicits cytotoxic T lymphocyte (CTL) populations that recognize VV antigens as well as heterologous protein(s) encoded by the virus (Bennink & Yewdell, 1990). However, attempts to detect class I MHC-restricted, VV-specific CTL in humans immunized against smallpox have been unsuccessful, even after revaccination with the virus (Perrin et aI., 1977;Graham et al., 1991). These data suggested that recombinant VV vectors may have a limited ability to prime CTL responses in humans (Graham et al., 1991).To gain an understanding of factors that influence priming and detection of anti-VV CTLs in humans, we undertook a study of virus-specific CTL activity in 10 laboratory staff volunteers. Smallpox vaccination is currently indicated for laboratory workers directly involved with orthopoxviruses such as VV (Anonymous, 1980). Accordingly, subjects 1 to 6 who work with recombinant VV vectors were immunized with 0.2 ml of dried smallpox vaccine (Dryvax, Wyeth) by dermal scarification (Table 1). Vaccine takes were confirmed by visual examination of the inoculation site. Four control subjects (7 to 10) who do not use recombinant VV vectors were not vaccinated.To generate VV-specific CTLs, peripheral blood mononuclear cells (PBMCs) from study subjects were enriched on Ficoll-Hypaque gradients and washed three times in PBS. PBMCs were then infected with VV (strain WR) at a multiplicity of 1 for 60 min. After one wash, 2 × 10 ~ cells/ml were cultured in 1.5 ml of RPMI 1640 medium supplemented with 10 % hea...