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.