Immune memory is a defining feature of the acquired immune system, but activation of the innate immune system can also result in enhanced responsiveness to subsequent triggers. This process has been termed ‘trained immunity’, a de facto innate immune memory. Research in the past decade has pointed to the broad benefits of trained immunity for host defence but has also suggested potentially detrimental outcomes in immune-mediated and chronic inflammatory diseases. Here we define ‘trained immunity’ as a biological process and discuss the innate stimuli and the epigenetic and metabolic reprogramming events that shape the induction of trained immunity.

Alzheimer’s-prone mice deficient in NLRP3 or caspase-1 fail to develop learning deficits and show reduced neuropathology. Alzheimer's disease is associated with activation of the innate immune system. It is known that amyloid-β can activate the NLRP3 inflammasome in vitro in microglia, and here it is shown that the inflammasome has a critical role in Alzheimer's disease pathology in a mouse model in vivo. In the absence of NLRP3 or caspase-1, amyloidosis and neuropathology in mice is reduced, and cognition and associated electrophysiological parameters improved. Examination of post-mortem human Alzheimer's brains supports the link between NLRP3 and brain inflammation. Taken together, these results suggest that amyloid-β-induced activation of NLRP3 enhances the progression of Alzheimer's disease by mediating a harmful chronic inflammatory tissue post-mortem response, and that agents that block the activity of the NLRP3 inflammasome, or inflammasome-derived cytokines, might slow the progression of Alzheimer's disease. Alzheimer’s disease is the world’s most common dementing illness. Deposition of amyloid-β peptide drives cerebral neuroinflammation by activating microglia1,2. Indeed, amyloid-β activation of the NLRP3 inflammasome in microglia is fundamental for interleukin-1β maturation and subsequent inflammatory events3. However, it remains unknown whether NLRP3 activation contributes to Alzheimer’s disease in vivo. Here we demonstrate strongly enhanced active caspase-1 expression in human mild cognitive impairment and brains with Alzheimer’s disease, suggesting a role for the inflammasome in this neurodegenerative disease. Nlrp3−/− or Casp1−/− mice carrying mutations associated with familial Alzheimer’s disease were largely protected from loss of spatial memory and other sequelae associated with Alzheimer’s disease, and demonstrated reduced brain caspase-1 and interleukin-1β activation as well as enhanced amyloid-β clearance. Furthermore, NLRP3 inflammasome deficiency skewed microglial cells to an M2 phenotype and resulted in the decreased deposition of amyloid-β in the APP/PS1 model of Alzheimer’s disease. These results show an important role for the NLRP3/caspase-1 axis in the pathogenesis of Alzheimer’s disease, and suggest that NLRP3 inflammasome inhibition represents a new therapeutic intervention for the disease.

Cell death is a fundamental physiological process in all living organisms. Its roles extend from embryonic development, organ maintenance, and aging to the coordination of immune responses and autoimmunity. In recent years, our understanding of the mechanisms orchestrating cellular death and its consequences on immunity and homeostasis has increased substantially. Different modalities of what has become known as ‘programmed cell death’ have been described, and some key players in these processes have been identified. We have learned more about the intricacies that fine tune the activity of common players and ultimately shape the different types of cell death. These studies have highlighted the complex mechanisms tipping the balance between different cell fates. Here, we summarize the latest discoveries in the three most well understood modalities of cell death, namely, apoptosis, necroptosis, and pyroptosis, highlighting common and unique pathways and their effect on the surrounding cells and the organism as a whole.