Neutrophils are critical for antifungal defense, but the mechanisms that clear hyphae and other pathogens that are too large to be phagocytosed remain unknown. We show that neutrophils sense microbial size and selectively release neutrophil extracellular traps (NETs) in response to large pathogens, such as Candida albicans hyphae and extracellular Mycobacterium bovis aggregates, but not small yeast and single bacteria. NETs are fundamental in countering large pathogens in vivo. Phagocytosis via dectin-1, acts as a sensor for microbial size preventing NETosis by downregulating neutrophil elastase (NE) translocation to the nucleus. Dectin-1 deficiency leads to aberrant NETosis and NET-mediated tissue damage during infection. Size-tailored neutrophil responses clear large microbes and minimize pathology when microbes are small enough to be phagocytosed.

Neutrophils are the foot soldiers of the immune system. They home in to the site of infection and kill pathogens by phagocytosis, degranulation, and the release of web-like structures called neutrophil extracellular traps (NETs) that trap and kill a variety of microbes. NETs have been shown to play a multitude of additional roles in immunity but have also been implicated in inflammatory and autoimmune disease. Here, we discuss the role of NETs in these various contexts with a particular emphasis on the molecular mechanisms that regulate NET release and clearance. We highlight the comprehensive concepts and explore the important open questions in the field.

TLRs play an essential role in the induction of immune responses by detecting conserved molecular products of microorganisms. However, the function of TLR8 is largely unknown. In the current study, we investigated the role of TLR8 signaling in immunity in mice. We found that Tlr8 -/-DCs overexpressed TLR7, were hyperresponsive to various TLR7 ligands, and showed stronger and faster NF-κB activation upon stimulation with the TLR7 ligand R848. Tlr8 -/-mice showed splenomegaly, defective development of marginal zone (MZ) and B1 B cells, and increased serum levels of IgM and IgG2a. Furthermore, Tlr8 -/-mice exhibited increased serum levels of autoantibodies against small nuclear ribonucleoproteins, ribonucleoprotein, and dsDNA and developed glomerulonephritis, whereas neither Tlr7 -/-nor Tlr8 -/-Tlr7 -/-mice showed any of the phenotypes observed in Tlr8 -/-mice. These data provide evidence for a pivotal role for mouse TLR8 in the regulation of mouse TLR7 expression and prevention of spontaneous autoimmunity. IntroductionTLRs, mammalian homologs of the Drosophila receptor Toll, detect evolutionarily conserved structures expressed by different groups of microbes and play a major role in the elimination of infections by coactivation of the immune system (1). TLRs are composed of an ectodomain of leucine-rich repeats, which are involved in ligand binding, a transmembrane domain, and a cytoplasmic Toll/IL-1 receptor (TIR) domain that interacts with TIR domain-containing adaptor molecules. The signaling pathways activated by different TLRs involve a family of 5 adaptor proteins, which couple to downstream protein kinases and ultimately lead to the activation of transcription factors such as NF-κB, MAPKs, and members of the IFN-regulatory factor family (2).The family of TLRs consists of 10 members in humans (TLR1-TLR10) and 12 members in mice (TLR1-TLR9 and TLR11-TLR13). The mammalian TLRs that are located in the plasma membrane recognize bacterial membrane components, whereas the TLRs that detect nucleic acid-based ligands are predominately located within endosomal compartments (3). The nucleic acid-sensing TLRs include TLR3, TLR7, TLR8, and TLR9. TLR3 senses viral and synthetic double-stranded RNA, TLR7 recognizes viral single-stranded RNA (ssRNA) and synthetic imidazoquinoline compounds, and TLR9 detects unmethylated CpG containing DNA motifs, found in bacterial and viral DNA (3). Although TLR7 and TLR8 are phylogenetically very close, their natural ligand viral ssRNA stimulates human TLR7 and TLR8 and mouse TLR7, but not mouse TLR8, leading to the belief that TLR8 is biologically inactive in mice (4). However, recent studies have shown that TLR8 is dynamically expressed during mouse brain development and functions as a negative regulator of neurite outgrowth and an inducer of neuronal apoptosis (5). Moreover, stimulation of murine TLR8-transfected HEK293 cells with a combination of imidazoquinoline immune response modifiers and polyT oligonucleotides leads to NF-κB activation, which suggests that mouse

Significance Macrophages—cells crucially involved in defense against infections—exhibit, depending on their anatomical location, distinct biological properties. Studies of the underlying mechanisms are of scientific and clinical interest, but are hampered by the difficulty of obtaining primary tissue macrophages in sufficient numbers and purity. Here, we report the generation of nontransformed murine macrophages, which are similar to alveolar macrophages and can be grown continuously without change of phenotype and in unlimited amounts. Such macrophages helped us to recognize several innate immune properties of alveolar macrophages that are involved in the pathogenesis of infectious lung inflammation.

SummaryHow the number of immune cells recruited to sites of infection is determined and adjusted to differences in the cellular stoichiometry between host and pathogen is unknown. Here, we have uncovered a role for reactive oxygen species (ROS) as sensors of microbe size. By sensing the differential localization of ROS generated in response to microbes of different size, neutrophils tuned their interleukin (IL)-1β expression via the selective oxidation of NF-κB, in order to implement distinct inflammatory programs. Small microbes triggered ROS intracellularly, suppressing IL-1β expression to limit neutrophil recruitment as each phagocyte eliminated numerous pathogens. In contrast, large microbes triggered ROS extracellularly, amplifying IL-1β expression to recruit numerous neutrophils forming cooperative clusters. Defects in ROS-mediated microbe size sensing resulted in large neutrophil infiltrates and clusters in response to small microbes that contribute to inflammatory disease. These findings highlight the impact of ROS localization on signal transduction.

Streptococcus pneumoniae coinfection is a major cause of influenza-associated mortality; however, the mechanisms underlying pathogenesis or protection remain unclear. Using a clinically relevant mouse model, we identify immune-mediated damage early during coinfection as a new mechanism causing susceptibility. Coinfected CCR2−/− mice lacking monocytes and monocyte-derived cells control bacterial invasion better, show reduced epithelial damage and are overall more resistant than wild-type controls. In influenza-infected wild-type lungs, monocytes and monocyte-derived cells are the major cell populations expressing the apoptosis-inducing ligand TRAIL. Accordingly, anti-TRAIL treatment reduces bacterial load and protects against coinfection if administered during viral infection, but not following bacterial exposure. Post-influenza bacterial outgrowth induces a strong proinflammatory cytokine response and massive inflammatory cell infiltrate. Depletion of neutrophils or blockade of TNF-α facilitate bacterial outgrowth, leading to increased mortality, demonstrating that these factors aid bacterial control. We conclude that inflammatory monocytes recruited early, during the viral phase of coinfection, induce TRAIL-mediated lung damage, which facilitates bacterial invasion, while TNF-α and neutrophil responses help control subsequent bacterial outgrowth. We thus identify novel determinants of protection versus pathology in influenza–Streptococcus pneumoniae coinfection.

Summary Innate lymphoid cells (ILC) are a recently identified group of tissue‐resident innate lymphocytes. Available data support the view that ILC or their progenitors are deposited and retained in tissues early during ontogeny. Thereby, ILC become an integral cellular component of tissues and organs. Here, we will review the intriguing relationships between ILC and basic developmental and homeostatic processes within tissues. Studying ILC has already led to the appreciation of the integral roles of immune cells in tissue homeostasis, morphogenesis, metabolism, regeneration, and growth. This area of immunology has not yet been studied in‐depth but is likely to reveal important networks contributing to disease tolerance and may be harnessed for future therapeutic approaches.

Innate and adaptive immunity are two complementary systems that work together to protect the host organism. A new study unravels how innate lymphoid cells and adaptive T lymphocytes act sequentially to establish microbial commensalism and ensure tissue and metabolic homeostasis.