Long-lived poxvirus immunity, robust CD4 help, and better persistence of CD4 than CD8 T cells.

The currently used smallpox vaccine is associated with a high incidence of adverse events, and there is a serious need for a safe and effective alternative vaccine. Here, we carried out a longitudinal evaluation of vaccinia virus-specific CD4 and CD8 T cells in smallpox-vaccinated individuals by using a highly sensitive intracellular cytokine staining assay. Our results demonstrate that, in addition to the CD8 response, the smallpox vaccinations raised a robust CD4 response with a Th1-dominant cytokine profile. These CD4 T cells were stable and exhibited only a twofold contraction between peak effector and memory phases compared with an approximate sevenfold contraction for CD8 cells. A significant proportion of vaccinated individuals lost detectable CD8 memory while maintaining CD4 memory. After a booster immunization, these individuals generated a robust CD8 response, which some of them rapidly lost. Thus, the current smallpox vaccine provides long-lasting CD4 help that may be critical for long-lived B-cell memory. We suggest that the provision of adequate CD4 help for CD8 and humoral effector functions will be critical to the success of the next generation of smallpox vaccines.