Abstract
The ability of developing immunological memory, a characteristic feature of adaptive immunity, is clearly present also in innate immune responses. In fact, it is well known that plants and invertebrate metazoans, which only have an innate immune system, can mount a faster and more effective response upon re-exposure to a stimulus. Evidence of immune memory in invertebrates comes from studies in infection immunity, natural transplantation immunity, individual, and transgenerational immune priming. These studies strongly suggest that environment and lifestyle take part in the development of immunological memory. However, in several instances the formal correlation between the phenomenon of immune memory and molecular and functional immune parameters is still missing. In this review, we have critically examined the cellular and humoral aspects of the invertebrate immune memory responses. In particular, we have focused our analysis on studies that have addressed immune memory in the most restrictive meaning of the term, i.e., the response to a challenge of a quiescent immune system that has been primed in the past. These studies highlight the central role of an increase in the number of immune cells and of their epigenetic re-programming in the establishment of sensu stricto immune memory in invertebrates.
Highlights
The phenomenon of innate memory in mammals is known since last century, based on observations that animals or cells in culture react differently to a stimulus, in terms of innate/inflammatory responses, if previously primed with the same or with a different agent [1,2,3]
Gene expression, or up/down-regulation of surface markers in monocytes or macrophages challenged in culture after a priming in vivo or in vitro [7, 51]
It is important to note that the phenomena of potentiation and tolerance are those that we can observe at the level of the entire organism, while at the level of single cells/cell populations we can observe a re-programming of their activation in directions that cannot be immediately ascribed to overall potentiation or overall tolerance [52]
Summary
The phenomenon of innate memory in mammals is known since last century, based on observations that animals or cells in culture react differently to a stimulus, in terms of innate/inflammatory responses, if previously primed with the same or with a different agent [1,2,3]. The transcriptional analysis of changes induced in immunocompetent tissues by priming/activation [63], and the new gene expression repertoire evoked by challenge in primed individuals [64] provide an excellent molecular basis to the phenomenon of immune memory, which will need functional validation of identified genes and pathways.
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