T cell memory and protective immunity by vaccination: is more better?

Protection against intracellular pathogens or tumor antigens requires T-cell mediated responses. Recently, it has become apparent that protection against disease correlates with T cells of the central memory type in many instances. Here, we analyze current data to distill a set of rules for the induction and maintenance of central memory T-cell responses. Recent studies show that T-cell help and the lack of overt inflammation at the time of priming are prerequisite for the induction, maintenance and expansion of memory T cells. Central to our hypothesis is that, in addition to these factors, successful vaccination in the immunologically inexperienced individual should be based on low antigen dose, to decelerate replicative senescence in responding cells and favor lineage differentiation of central memory T cells. In the immunologically experienced individual, it will be necessary, in addition, to abate the antigen load in plasma before vaccination. These guiding principles might help to raise improved protective T-cell responses by vaccination in humans.