Trained immunity (innate immune memory)
In 2011, Netea et al. 3 first proposed the concept of “trained immunity” to describe the ability of mammalian innate immunity to exhibit immune memory, monocytes/macrophages, NK cells, and granulocytes that show altered immune responses to secondary stimulation originating from exogenous or endogenous insults. In terms of cell populations and molecular mechanisms, there are important differences between trained immunity and classic adaptive immune memory. While adaptive immune memory generally implies high specificity and cascade amplification, trained immunity is a non-specific or semi-specific phenomenon that is mainly characterized by alterations in the number and/or function of innate immune cells, and subsequently involves the initiation of a pro-inflammatory or anti-inflammatory secondary immune response 3, 9, 10. Depending on the nature and the dose of the primary stimulus, trained immunity responds to the secondary challenge via one of two contrasting functional programs, with enhanced immunological imprinting being manifested as training, and the suppressed program contributing to tolerance 10, 11.