Innate Immune Memory: The Latest Frontier of Adjuvanticity

Table 2

Definitions.

Adaptive Memory; Adaptive memory is long-term, antigen-specific ability of T and B lymphocytes to respond more rapidly and more efficiently to a specific antigen upon second encounter.
Innate Memory; Innate memory is the ability of an organism to adapt its immune response depending on a previous infections or vaccination, mediated by NK cells and monocytes/macrophages. This immunological re-programming can result in non-specific suppression (tolerance) or increased innate immune response (training) against reinfection by the same or different pathogens.
Trained Immunity/Memory; Trained immunity/memory is the enhanced nonspecific protection against infections after previous exposure to certain microbial components (e.g., β-glucans), possibly involving epigenetic and metabolic re-programming in the cell.
Tolerance; Tolerance is the refractory state of monocytes/macrophages, involving epigenetic remodelling, induced by microbial components (e.g., LPS). Upon subsequent challenge, even with a high dose of LPS, a less robust induction of pro-inflammatory cytokines ensues.
Nonspecific Effects; “Nonspecific” immune effects are induced by a vaccination or infection, against unrelated and antigenically diverse infectious agents. Nonspecific effects are mediated by cross-reactive lymphocytes and innate memory cells, and might be either beneficial or detrimental, depending on the type of memory of the cells involved.