Hidden memories: frontline memory T cells and early pathogen interception.

Immunologic memory reflects the ability of a host to more effectively respond to a re-encounter with a particular pathogen than the first encounter, and when a vaccine mimics the first encounter, comprises the basis of vaccine efficacy. For T cells, memory is often equated with the anamnestic response, the ability of secondary lymphoid tissue-based (central) memory T cells to respond to pathogen exposure with a more rapid and higher magnitude production and infection-site delivery of pathogen-specific effector cells than observed in naive hosts. However, increasing evidence supports a fundamentally different kind of T cell memory in which differentiated, long-lived effector memory T cells, prepositioned in sites of potential pathogen invasion or rapidly mobilized to such sites from blood and marginated pools, intercept and potentially control/eliminate pathogen within hours of infection. In this article, we review the evidence for this "hidden" T cell memory and its implication for vaccine development.