Neutrophils, the most abundant human immune cells, are rapidly recruited to sites of infection, where they fulfill their life-saving antimicrobial functions. While traditionally regarded as short-lived phagocytes, recent findings on long-term survival, neutrophil extracellular trap (NET) formation, heterogeneity and plasticity, suppressive functions, and tissue injury have expanded our understanding of their diverse role in infection and inflammation. This review summarises our current understanding of neutrophils in host-pathogen interactions and disease involvement, illustrating the versatility and plasticity of the neutrophil, moving between host defence, immune modulation, and tissue damage.

Cerebral ischemia induces a profound neuro-inflammatory response, but the underlying molecular mechanisms are poorly understood. Inflammasomes (NLRP1, NLRP3, NLRC4, AIM2) are intracellular multi-protein complexes which can induce sets of pro-inflammatory cyto- and chemokines, and thereby guide inflammation. We, here, assessed the functional role of NLRP3 in ischemia/reperfusion (I/R) injury in a mouse model of transient cerebral ischemia. Ischemic stroke was induced in C57Bl/6 mice by 60 minutes transient middle cerebral artery occlusion (tMCAO) and 3, 7 or 23 hours of reperfusion, a paradigm of I/R injury. The expression patterns of inflammasomes in the ischemic hemispheres were evaluated by semiquantitative real-time PCR and Western Blot analysis accompanied by protein localization using immunocytochemistry. Finally, animals were treated with the inflammasome inhibitors Sulforaphane, Genipin, MCC950 or vehicle, directly before or upon recanalization after tMCAO. Stroke outcome was assessed, including infarct size and functional deficits, local inflammatory response, neuronal survival as well as blood-brain barrier function on day 1 after tMCAO. After tMCAO the relative gene expression levels of NLRP3 increased 20-30x within 1 day in the ischemic hemisphere which translated into an increased expression of NLRP3 in neurons. Accordingly, the gene expression levels of the NLRP3-modulator, Bruton's Tyrosine Kinase (BTK), and the NLRP3-inducible cytokine, IL-1β significantly rose. Lesser or non-significant changes were seen for the other inflammasomes. Application of inflammasome inhibitors covering all inflammasomes or specifically NLRP3 significantly reduced infarct volumes when given before or after tMCAO and was accompanied by clear evidence for reduced activation of caspase 1. This stroke attenuating effect coincided with less immune cell infiltration in the ischemic hemisphere and preservation of the blood-brain barrier integrity. Our data show that induction of the NLRP3 inflammasome in neurons drives neuroinflammation in acute ischemic stroke. Early blockade of NLRP3 protects from I/R injury by mitigating inflammation and stabilizing the blood-brain barrier.

The inducible expression of antimicrobial peptide genes in Drosophila melanogaster is regulated by the conserved Toll and peptidoglycan recognition protein LC/immune deficiency (PGRP-LC/IMD) signaling pathways. It has been proposed that the two pathways have independent functions and mediate the specificity of innate immune responses towards different microorganisms. Scattered evidence also suggests that some antimicrobial target genes can be activated by both Toll and IMD, albeit to different extents. This dual activation can be mediated by independent stimulation or by cross-regulation of the two pathways. We show in this report that the Toll and IMD pathways can interact synergistically, demonstrating that cross-regulation occurs. The presence of Spätzle (the Toll ligand) and gram-negative peptidoglycan (the PGRP-LC ligand) together caused synergistic activation of representative target genes of the two pathways, including Drosomycin, Diptericin, and AttacinA. Constitutive activation of Toll and PGRP-LC/IMD could mimic the synergistic stimulation. RNA interference assays and promoter analyses demonstrate that cooperation of different NF-κB-related transcription factors mediates the synergy. These results illustrate how specific ligand binding by separate upstream pattern recognition receptors can be translated into a broad-spectrum host response, a hallmark of innate immunity.

Psoriasis is an inflammatory skin disease with strong neutrophil (PMN) infiltration and high levels of the antimicrobial peptide, LL37. LL37 in complex with DNA and RNA is thought to initiate disease exacerbation via plasmacytoid dendritic cells. However, the source of nucleic acids supposed to start this initial inflammatory event remains unknown. We show here that primary murine and human PMNs mount a fulminant and self-propagating neutrophil extracellular trap (NET) and cytokine response, but independently of the canonical NET component, DNA. Unexpectedly, RNA, which is abundant in NETs and psoriatic but not healthy skin, in complex with LL37 triggered TLR8/TLR13-mediated cytokine and NET release by PMNs in vitro and in vivo. Transfer of NETs to naive human PMNs prompts additional NET release, promoting further inflammation. Our study thus uncovers a self-propagating vicious cycle contributing to chronic inflammation in psoriasis, and NET-associated RNA (naRNA) as a physiologically relevant NET component.

Bruton’s Tyrosine kinase (BTK) was initially discovered as a critical mediator of B cell receptor signaling in the development and functioning of adaptive immunity. Growing evidence also suggests multiple roles for BTK in mononuclear cells of the innate immune system, especially in dendritic cells (DC) and macrophages. For example, BTK has been shown to function in Toll-like receptor-mediated recognition of infectious agents, cellular maturation and recruitment processes, and Fc receptor signaling. Most recently, BTK was additionally identified as a direct regulator of a key innate inflammatory machinery, the NLRP3 inflammasome. BTK has thus attracted interest not only for gaining a more thorough basic understanding of the human innate immune system but also as a target to therapeutically modulate innate immunity. We here review the latest developments on the role of BTK in mononuclear innate immune cells in mouse versus man, with specific emphasis on the sensing of infectious agents and the induction of inflammation. Therapeutic implications for modulating innate immunity and critical open questions are also discussed.