Tissue-resident macrophages actively suppress IL-1beta release via a reactive prostanoid/IL-10 pathway.

The alarm cytokine interleukin-1ß (IL-1ß) is a potent activator of the inflammatory cascade following pathogen recognition. IL-1ß production typically requires two signals: first, priming by recognition of pathogen-associated molecular patterns leads to the production of immature pro-IL-1ß; subsequently, inflammasome activation by a secondary signal allows cleavage and maturation of IL-1ß from its pro-form. However, despite the important role of IL-1ß in controlling local and systemic inflammation, its overall regulation is still not fully understood. Here we demonstrate that peritoneal tissue-resident macrophages use an active inhibitory pathway, to suppress IL-1ß processing, which can otherwise occur in the absence of a second signal. Programming by the transcription factor Gata6 controls the expression of prostacyclin synthase, which is required for prostacyclin production after lipopolysaccharide stimulation and optimal induction of IL-10. In the absence of secondary signal, IL-10 potently inhibits IL-1ß processing, providing a previously unrecognized control of IL-1ß in tissue-resident macrophages.

Salmonella Flagellin Activates NAIP/NLRC4 and Canonical NLRP3 Inflammasomes in Human Macrophages.

Infection of human macrophages with Salmonella enterica serovar Typhimurium (S. Typhimurium) leads to inflammasome activation. Inflammasomes are multiprotein complexes facilitating caspase-1 activation and subsequent gasdermin D-mediated cell death and IL-1ß and IL-18 cytokine release. The NAIP/NLRC4 inflammasome is activated by multiple bacterial protein ligands, including flagellin from the flagellum and the needle protein PrgI from the S. Typhimurium type III secretion system. In this study, we show that transfected ultrapure flagellin from S Typhimurium induced cell death and cytokine secretion in THP-1 cells and primary human monocyte-derived macrophages. In THP-1 cells, NAIP/NLRC4 and NLRP3 played redundant roles in inflammasome activation during infection with S. Typhimurium. Knockout of NAIP or NLRC4 in THP-1 cells revealed that flagellin, but not PrgI, now activated the NLRP3 inflammasome through a reactive oxygen species- and/or cathepsin-dependent mechanism that was independent of caspase-4/5 activity. In conclusion, our data suggest that NLRP3 can be activated by flagellin to act as a "safety net" to maintain inflammasome activation under conditions of suboptimal NAIP/NLRC4 activation, as observed in THP-1 cells, possibly explaining the redundant role of NLRP3 and NAIP/NLRC4 during S. Typhimurium infection.

NLR in eXile: Emerging roles of NLRX1 in immunity and human disease.

NLRX1 is a member of the NOD-like receptor family, a set of pattern recognition receptors associated with innate immunity. Interestingly, NLRX1 exists in somewhat of an exile from its NLR counterparts with unique features that mediate atypical functions compared with traditional NOD-like receptors (NLRs). Aside from a mitochondrial targeting sequence, the N-terminal region is yet to be characterized. Mitochondrially located, NLRX1 sits within a subgroup of regulatory NLRs responsible for negatively regulating cellular inflammatory signalling. As well as modulating pathogen response, emerging evidence is implicating NLRX1 as a central homeostatic gatekeeper between mitochondrial biology and immunological response. More recently, NLRX1 has been implicated in a wide range of disease, both pathogen-driven and otherwise. Emerging links of NLRX1 in cancer biology, autoimmunity and other inflammatory conditions are raising the potential of targeting NLRX1 therapeutically, with recent studies in inflammatory bowel disease showing great promise. Within this review, we address the unique features of NLRX1, its roles in innate immune signalling and its involvement in a range of inflammatory, metabolic and oncology disease indications with a focus on areas that could benefit from therapeutic targeting of NLRX1.

Macrophage reprogramming for therapy.

Dysfunction of the immune system underlies a plethora of human diseases, requiring the development of immunomodulatory therapeutic intervention. To date, most strategies employed have been focusing on the modification of T lymphocytes, and although remarkable improvement has been obtained, results often fall short of the intended outcome. Recent cutting-edge technologies have highlighted macrophages as potential targets for disease control. Macrophages play central roles in development, homeostasis and host defence, and their dysfunction and dysregulation have been implicated in the onset and pathogenesis of multiple disorders including cancer, neurodegeneration, autoimmunity and metabolic diseases. Recent advancements have led to a greater understanding of macrophage origin, diversity and function, in both health and disease. Over the last few years, a variety of strategies targeting macrophages have been developed and these open new therapeutic opportunities. Here, we review the progress in macrophage reprogramming in various disorders and discuss the potential implications and challenges for macrophage-targeted approaches in human disease.

A Novel Strategy to Identify Haematology Patients at High Risk of Developing Aspergillosis.

Invasive Aspergillosis (IA), typically caused by the fungus Aspergillus fumigatus, is a leading cause of morbidity and mortality in immunocompromised patients. IA remains a significant burden in haematology patients, despite improvements in the diagnosis and treatment of Aspergillus infection. Diagnosing IA is challenging, requiring multiple factors to classify patients into possible, probable and proven IA cohorts. Given the low incidence of IA, using negative results as exclusion criteria is optimal. However, frequent false positives and severe IA mortality rates in haematology patients have led to the empirical use of toxic, drug-interactive and often ineffective anti-fungal therapeutics. Improvements in IA diagnosis are needed to reduce unnecessary anti-fungal therapy. Early IA diagnosis is vital for positive patient outcomes; therefore, a pre-emptive approach is required. In this study, we examined the sequence and expression of four C-type Lectin-like receptors (Dectin-1, Dectin-2, Mincle, Mcl) from 42 haematology patients and investigated each patient's anti-Aspergillus immune response (IL-6, TNF). Correlation analysis revealed novel IA disease risk factors which we used to develop a pre-emptive patient stratification protocol to identify haematopoietic stem cell transplant patients at high and low risk of developing IA. This stratification protocol has the potential to enhance the identification of high-risk patients whilst reducing unnecessary treatment, minimizing the development of anti-fungal resistance, and prioritising primary disease treatment for low-risk patients.

Evolutionary loss of inflammasomes in the Carnivora and implications for the carriage of zoonotic infections.

Zoonotic pathogens, such as COVID-19, reside in animal hosts before jumping species to infect humans. The Carnivora, like mink, carry many zoonoses, yet how diversity in host immune genes across species affect pathogen carriage is poorly understood. Here, we describe a progressive evolutionary downregulation of pathogen-sensing inflammasome pathways in Carnivora. This includes the loss of nucleotide-oligomerization domain leucine-rich repeat receptors (NLRs), acquisition of a unique caspase-1/-4 effector fusion protein that processes gasdermin D pore formation without inducing rapid lytic cell death, and the formation of a caspase-8 containing inflammasome that inefficiently processes interleukin-1ß. Inflammasomes regulate gut immunity, but the carnivorous diet has antimicrobial properties that could compensate for the loss of these immune pathways. We speculate that the consequences of systemic inflammasome downregulation, however, can impair host sensing of specific pathogens such that they can reside undetected in the Carnivora.