Increased per cell IFN-γ productivity indicates recent in vivo activation of T cells

Immunization with vaccinia virus causes long-term immunity. Efforts have been made to characterize the T cells responsible for this protection. Recently, T cell subsets were described that not only co-express multiple cytokines, but also show increased per cell cytokine productivity. These highly productive cells are often considered to be the most protective. We used ELISPOT assays to measure per cell IFN-γ productivity of vaccinia-specific T cells in childhood immunized adults immediately before and at different time points after vaccinia re-vaccination. Apart from an increase in frequency, we found a marked increase of IFN-γ productivity following vaccinia re-vaccination. However, these changes were short-lived as both parameters quickly returned to baseline values within 22 days after re-vaccination. Therefore, increased per cell IFN-γ productivity seems to be a sign of recent in vivo T cell activation rather than a stable marker of a distinct T cell subset responsible for long-term immune protection.