Forms of innate immune memory: comparing immune priming and trained immunity

Immune memory in vertebrates, known as adaptive immunity, is a phenomenon that enhances the elimination of pathogens and parasites and thus increases survival upon subsequent exposures. Formerly regarded as an exclusive feature of vertebrates, the paradigm has shifted in the last two decades, with demonstrations of similar phenomena in invertebrates and plants. The terminology used to describe such phenomena has made use of terms like immune priming, innate immune memory, or trained immunity. However, these terms often lack clear definitions and are sometimes used interchangeably. After two decades of research in this field, the widespread occurrence and relevance of such phenomena are beyond doubt, but experts and students alike would benefit from a consensus on fundamental terminology. Such agreement would facilitate advancing our comprehension of the occurrence, mechanisms, and potential applications, including the development of "vaccines" for invertebrates and plants. In our workshop, we aim to critically consider whether the conflation of specific memory with non-specific responses and the inclusion of molecular mechanisms in its definitions hinders our understanding of the evolutionary processes that have led to immune responses becoming specific, non-specific, or semi-specific.