Monocyte-derived alveolar macrophages autonomously determine severe outcome of respiratory viral infection.

Various lung insults can result in replacement of resident alveolar macrophages (AM) by bone marrow monocyte-derived (BMo)-AM. However, the dynamics of this process and its long-term consequences for respiratory viral infections remain unclear. Using several mouse models and a marker to unambiguously track fetal monocyte-derived (FeMo)-AM and BMo-AM, we established the kinetics and extent of replenishment and their function to recurrent influenza A virus (IAV) infection. A massive loss of FeMo-AM resulted in rapid replenishment by self-renewal of survivors, followed by the generation of BMo-AM. BMo-AM progressively outcompeted FeMo-AM over several months, and this was due to their increased glycolytic and proliferative capacity. The presence of both naïve and experienced BMo-AM conferred severe pathology to IAV infection, which was associated with a proinflammatory phenotype. Furthermore, upon aging of naïve mice, FeMo-AM were gradually replaced by BMo-AM, which contributed to IAV disease severity in a cell-autonomous manner. Together, our results suggest that the origin rather than training of AM determines long-term function to respiratory viral infection and provide an explanation for the increased severity of infection seen in the elderly.

KappaBle fluorescent reporter mice enable low-background single-cell detection of NF-κB transcriptional activity in vivo.

Nuclear factor-κB (NF-κB) is a transcription factor with a key role in a great variety of cellular processes from embryonic development to immunity, the outcome of which depends on the fine-tuning of NF-κB activity. The development of sensitive and faithful reporter systems to accurately monitor the activation status of this transcription factor is therefore desirable. To address this need, over the years a number of different approaches have been used to generate NF-κB reporter mice, which can be broadly subdivided into bioluminescence- and fluorescence-based systems. While the former enables whole-body visualization of the activation status of NF-κB, the latter have the potential to allow the analysis of NF-κB activity at single-cell level. However, fluorescence-based reporters frequently show poor sensitivity and excessive background or are incompatible with high-throughput flow cytometric analysis. In this work we describe the generation and analysis of ROSA26 knock-in NF-κB reporter (KappaBle) mice containing a destabilized EGFP, which showed sensitive, dynamic, and faithful monitoring of NF-κB transcriptional activity at the single-cell level of various cell types during inflammatory and infectious diseases.

Gene therapy of Csf2ra deficiency in mouse fetal monocyte precursors restores alveolar macrophage development and function.

Tissue-resident macrophage-based immune therapies have been proposed for various diseases. However, generation of sufficient numbers that possess tissue-specific functions remains a major handicap. Here, we showed that fetal liver monocytes cultured with GM-CSF (CSF2-cFLiMo) rapidly differentiated into a long-lived, homogeneous alveolar macrophage-like population in vitro. CSF2-cFLiMo retained the capacity to develop into bona fide alveolar macrophages upon transfer into Csf2ra-/- neonates and prevented development of alveolar proteinosis and accumulation of apoptotic cells for at least 1 year in vivo. CSF2-cFLiMo more efficiently engrafted empty alveolar macrophage niches in the lung and protected mice from severe pathology induced by respiratory viral infection compared with transplantation of macrophages derived from BM cells cultured with M-CSF (CSF1-cBMM) in the presence or absence of GM-CSF. Harnessing the potential of this approach for gene therapy, we restored a disrupted Csf2ra gene in fetal liver monocytes and demonstrated their capacity to develop into alveolar macrophages in vivo. Altogether, we provide a platform for generation of immature alveolar macrophage-like precursors amenable for genetic manipulation, which will be useful to dissect alveolar macrophage development and function and for pulmonary transplantation therapy.

Differential sensitivity of inflammatory macrophages and alternatively activated macrophages to ferroptosis.

Acumulation of oxidized membrane lipids ultimately results in ferroptotic cell death, which can be prevented by the selenoenzyme glutathione peroxidase 4 (Gpx4). In vivo conditions promoting ferroptosis and susceptible cell types are still poorly defined. In this study, we analyzed the conditional deletion of Gpx4 in mice specifically in the myeloid cell lineages. Surprisingly, development and maintenance of LysM+ macrophages and neutrophils, as well as CD11c+ monocyte-derived macrophages and dendritic cells were unaffected in the absence of Gpx4. Gpx4-deficient macrophages mounted an unaltered proinflammatory cytokine response including IL-1ß production following stimulation with TLR ligands and activation of several inflammasomes. Accordingly, Gpx4fl/fl LysM-cre mice were protected from bacterial and protozoan infections. Despite having the capacity to differentiate to alternatively activated macrophages (AAM), these cells lacking Gpx4 triggered ferroptosis both in vitro and in vivo following IL-4 overexpression and nematode infection. Exposure to nitric oxide restored viability of Gpx4-deficient AAM, while inhibition of iNOS in proinflammatory macrophages had no effect. These data together suggest that activation cues of tissue macrophages determine sensitivity to lipid peroxidation and ferroptotic cell death.

A spontaneous leptin receptor point mutation causes obesity and differentially affects leptin signaling in hypothalamic nuclei resulting in metabolic dysfunctions distinct from db/db mice.

OBJECTIVE: Leptin (Lep) plays a crucial role in controlling food intake and energy expenditure. Defective Lep/LepRb-signaling leads to fat accumulation, massive obesity, and the development of diabetes. We serendipitously noticed spontaneous development of obesity similar to LepR-deficient (db/db) mice in offspring from a C57BL/6J breeding and transmittance of the phenotype in a Mendelian manner. Candidate gene sequencing revealed a spontaneous point mutation in the LepRb gene. We investigated leptin responsiveness, leptin receptor signaling and metabolic phenotype of this novel LepRb mutant mouse variant. METHODS: Overexpression and functional tests of the mutant LepRb in 3T3 cells. Measurement of leptin responsiveness in hypothalamic nuclei, glucose tolerance, food uptake and energy expenditure in the mutant mice. RESULTS: The mutation results in the exchange of a glycine for serine (G506S) and introduces an alternative splice acceptor which, when used, encodes for a protein with a 40aa deletion that is retained in the cytoplasm. LepRb signaling was abrogated in the hypothalamic ventromedial nucleus (VMN) and dorsomedial nucleus (DMN), but only partially reduced in the hypothalamic arcuate nucleus (ARC) of LepRbG506S/G506S mice, most likely due to differential splicing in neurons located in the respective regions of the hypothalamus. Extensive metabolic characterization of these mice revealed interesting differences in the control of food intake, glucose tolerance, energy expenditure, and fat accumulation in LepRbG506S/G506S compared with LepRb-deficient db/db mice. CONCLUSIONS: This study provides further insight into differences of the leptin responsiveness in VMN, DMN, and ARC and its metabolic consequences.