Autophagy is well equipped functionally to isolate microbial pathogens in autophagosomes and to carry out their clearance by dismemberment in the course of catabolic processes in the lysosome. Clearly, this is a non-metabolic function of autophagy that impacts strongly on the immune system. While in a preceding article on neutrophils, eosinophils, mast cells, and natural killer cells our focus was on the role of autophagy in regulating innate immune cell differentiation, degranulation, phagocytosis and extracellular trap formation, here we discuss monocytes/macrophages and dendritic cells, specifically, the influence of autophagy on functional cellular responses, such as phagocytosis, antigen presentation, cytokine production, control of inflammasome activation, tolerance and the consequences for overall host defense. Facts • The impact of autophagy on innate immunity extends beyond classical xenophagy, including extensive crosstalk with other host defense mechanisms. • Autophagy in macrophages has different functions, being responsible for xenophagy, LC3-associated phagocytosis (LAP), production and delivery of antimicrobial peptides, and control of inflammasome activation. • Many pathogens have developed strategies to escape xenophagy and LAP. • In dendritic cells (DCs), autophagy is involved in each step of functional maturation, such as antigen presentation, migration, and cytokine secretion.

Neutrophil extracellular trap (NET) formation is a cellular function of neutrophils thatfacilitates the immobilization and killing of invading microorganisms in the extracellular milieu. To form NETs, neutrophils release a DNA scaffold consisting of mitochondrial DNA binding granule proteins. This process does not depend on cell death, but requires glycolytic ATP production for rearrangements in the microtubule network and F-actin. Such cytoskeletal rearrangements are essential for both mitochondrial DNA release and degranulation. However, the formation of NETs has also been described as a distinct form of programed, necrotic cell death, a process designated "NETosis." Necrotic cell death of neutrophils is associated with the permeabilization of both plasma and nuclear membranes resulting in a kind of extracellular cloud of nuclear DNA. The molecular mechanisms eliciting necrotic neutrophil death have been investigated and appear to be different from those responsible for NET formation following mitochondrial DNA release. Here, we discriminate between the mechanisms responsible for the release of mitochondrial versus nuclear DNA and address their respective functions. Our aim is to clarify existing differences of opinion in the fields of NET formation and neutrophil death.

The importance of extracellular traps (ETs) in innate immunity is well established, but the molecular mechanisms responsible for their formation remain unclear and in scientific dispute. ETs have been defined as extracellular DNA scaffolds associated with the granule proteins of eosinophils or neutrophils. They are capable of killing bacteria extracellularly. Based mainly on results with phosphoinositide 3-kinase (PI3K) inhibitors such as 3-methyladenine (3-MA) and wortmannin, which are commonly used to inhibit autophagy, several groups have reported that autophagy is required for neutrophil extracellular trap (NET) formation. We decided to investigate this apparent dependence on autophagy for ET release and generated genetically modified mice that lack, specifically in eosinophils or neutrophils, autophagy-related 5 (Atg5), a gene encoding a protein essential for autophagosome formation. Interestingly, neither eosinophils nor neutrophils from Atg5-deficient mice exhibited abnormalities in ET formation upon physiological activation or exposure to low concentrations of PMA, although we could confirm that human and mouse eosinophils and neutrophils, after pre-treatment with inhibitors of class III PI3K, show a block both in reactive oxygen species (ROS) production and in ET formation. The so-called late autophagy inhibitors bafilomycin A1 and chloroquine, on the other hand, were without effect. These data indicate that ET formation occurs independently of autophagy and that the inhibition of ROS production and ET formation in the presence of 3-MA and wortmannin is probably owing to their additional ability to block the class I PI3Ks, which are involved in signalling cascades initiated by triggers of ET formation.

Eosinophils and their secretory mediators play an important role in the pathogenesis of infectious and inflammatory disorders. Although eosinophils are largely evolutionally conserved, their physiologic functions are not well understood. Given the availability of new eosinophil-targeted depletion therapies, there has been a renewed interest in understanding eosinophil biology as these strategies may result in secondary disorders when applied over long periods of time. Recent data suggest that eosinophils are not only involved in immunological effector functions but also carry out tissue protective and immunoregulatory functions that actively contribute to the maintenance of homeostasis. Prolonged eosinophil depletion may therefore result in the development of secondary disorders. Here, we review recent literature pointing to important roles for eosinophils in promoting immune defense, antibody production, activation of adipose tissue, and tissue remodeling and fibrosis. We also reflect on patient data from clinical trials that feature anti-eosinophil therapeutics.

Autophagy is an evolutionally conserved, highly regulated catabolic process that combines cellular functions required for the regulation of metabolic balance under conditions of stress with those needed for the degradation of damaged cell organelles via the lysosomal machinery. The importance of autophagy for cell homeostasis and survival has long been appreciated. Recent data suggest that autophagy is also involved in non-metabolic functions that impact the immune system. Here, we reflect in two review articles the recent literature pointing to an important role for autophagy in innate immune cells. In this article, we focus on neutrophils, eosinophils, mast cells, and natural killer cells. We mainly discuss the influence of autophagy on functional cellular responses and its importance for overall host defense. In the companion review, we present the role of autophagy in the functions performed by monocytes/macrophages and dendritic cells. Facts• What are the mechanisms by which autophagy increases NK cell infiltration into tumor tissues.• Do the autophagy pathways in innate immune cells offer novel therapeutic targets to control immune responses in inflammation and cancer?