Liesbet Martens

Bone marrow-derived monocytes give rise to self-renewing and fully differentiated Kupffer cells

Self-renewing tissue-resident macrophages are thought to be exclusively derived from embryonic progenitors. However, whether circulating monocytes can also give rise to such macrophages has not been formally investigated. Here we use a new model of diphtheria toxin-mediated depletion of liver-resident Kupffer cells to generate niche availability and show that circulating monocytes engraft in the liver, gradually adopt the transcriptional profile of their depleted counterparts and become long-lived self-renewing cells. Underlining the physiological relevance of our findings, circulating monocytes also contribute to the expanding pool of macrophages in the liver shortly after birth, when macrophage niches become available during normal organ growth. Thus, like embryonic precursors, monocytes can and do give rise to self-renewing tissue-resident macrophages if the niche is available to them.

Yolk Sac Macrophages, Fetal Liver, and Adult Monocytes Can Colonize an Empty Niche and Develop into Functional Tissue-Resident Macrophages

Tissue-resident macrophages can derive from yolk sac macrophages (YS-Macs), fetal liver monocytes (FL-MOs), or adult bone-marrow monocytes (BM-MOs). The relative capacity of these precursors to colonize a niche, self-maintain, and perform tissue-specific functions is unknown. We simultaneously transferred traceable YS-Macs, FL-MOs, and BM-MOs into the empty alveolar macrophage (AM) niche of neonatal Csf2rb(-/-) mice. All subsets produced AMs, but in competition preferential outgrowth of FL-MOs was observed, correlating with their superior granulocyte macrophage-colony stimulating factor (GM-CSF) reactivity and proliferation capacity. When transferred separately, however, all precursors efficiently colonized the alveolar niche and generated AMs that were transcriptionally almost identical, self-maintained, and durably prevented alveolar proteinosis. Mature liver, peritoneal, or colon macrophages could not efficiently colonize the empty AM niche, whereas mature AMs could. Thus, precursor origin does not affect the development of functional self-maintaining tissue-resident macrophages and the plasticity of the mononuclear phagocyte system is largest at the precursor stage.

The unfolded-protein-response sensor IRE-1α regulates the function of CD8α + dendritic cells

The role of the unfolded protein response (UPR) and endoplasmic reticulum (ER) stress in homeostasis of the immune system is incompletely understood. Here we found that dendritic cells (DCs) constitutively activated the UPR sensor IRE-1α and its target, the transcription factor XBP-1, in the absence of ER stress. Loss of XBP-1 in CD11c+ cells led to defects in phenotype, ER homeostasis and antigen presentation by CD8α+ conventional DCs, yet the closely related CD11b+ DCs were unaffected. Whereas the dysregulated ER in XBP-1-deficient DCs resulted from loss of XBP-1 transcriptional activity, the phenotypic and functional defects resulted from regulated IRE-1α-dependent degradation (RIDD) of mRNAs, including those encoding CD18 integrins and components of the major histocompatibility complex (MHC) class I machinery. Thus, a precisely regulated feedback circuit involving IRE-1α and XBP-1 controls the homeostasis of CD8α+ conventional DCs.

Passenger Mutations Confound Interpretation of All Genetically Modified Congenic Mice

Targeted mutagenesis in mice is a powerful tool for functional analysis of genes. However, genetic variation between embryonic stem cells (ESCs) used for targeting (previously almost exclusively 129-derived) and recipient strains (often C57BL/6J) typically results in congenic mice in which the targeted gene is flanked by ESC-derived passenger DNA potentially containing mutations. Comparative genomic analysis of 129 and C57BL/6J mouse strains revealed indels and single nucleotide polymorphisms resulting in alternative or aberrant amino acid sequences in 1,084 genes in the 129-strain genome. Annotating these passenger mutations to the reported genetically modified congenic mice that were generated using 129-strain ESCs revealed that nearly all these mice possess multiple passenger mutations potentially influencing the phenotypic outcome. We illustrated this phenotypic interference of 129-derived passenger mutations with several case studies and developed a Me-PaMuFind-It web tool to estimate the number and possible effect of passenger mutations in transgenic mice of interest.

Nlrp6- and ASC-Dependent Inflammasomes Do Not Shape the Commensal Gut Microbiota Composition

&NA; The gut microbiota regulate susceptibility to multiple human diseases. The Nlrp6‐ASC inflammasome is widely regarded as a hallmark host innate immune axis that shapes the gut microbiota composition. This notion stems from studies reporting dysbiosis in mice lacking these inflammasome components when compared with non‐littermate wild‐type animals. Here, we describe microbial analyses in inflammasome‐deficient mice while minimizing non‐genetic confounders using littermate‐controlled Nlrp6‐deficient mice and ex‐germ‐free littermate‐controlled ASC‐deficient mice that were all allowed to shape their gut microbiota naturally after birth. Careful microbial phylogenetic analyses of these cohorts failed to reveal regulation of the gut microbiota composition by the Nlrp6‐ and ASC‐dependent inflammasomes. Our results obtained in two geographically separated animal facilities dismiss a generalizable impact of Nlrp6‐ and ASC‐dependent inflammasomes on the composition of the commensal gut microbiota and highlight the necessity for littermate‐controlled experimental design in assessing the influence of host immunity on gut microbial ecology. HighlightsNon‐littermate controls fail to define host genetic impacts on the gut microbiotaLittermate separation does not reveal ASC‐Nlrp6 impacts on the gut microbiotaLifetime littermate separation does not reveal Nlrp6 impacts on DSS colitis &NA; Inflammasomes were proposed to shape gut ecology based on dysbiosis in mutant mice versus non‐littermate wild‐types. Mamantopoulos et al. show that inflammasomes do not affect gut microbiota composition when controlling for non‐genetic confounders. This finding dismisses the suggested role for inflammasomes in controlling host health through regulation of intestinal ecology.

The transcription factor Zeb2 regulates development of conventional and plasmacytoid DCs by repressing Id2

Lambrecht et al. show that the transcription factor Zeb2 regulates commitment toward both the pDC and cDC2 lineages by repressing Id2.

TGFβR signalling controls CD103+CD11b+ dendritic cell development in the intestine

CD103+CD11b+ dendritic cells (DCs) are unique to the intestine, but the factors governing their differentiation are unclear. Here we show that transforming growth factor receptor 1 (TGFβR1) has an indispensable, cell intrinsic role in the development of these cells. Deletion of Tgfbr1 results in markedly fewer intestinal CD103+CD11b+ DCs and a reciprocal increase in the CD103−CD11b+ dendritic cell subset. Transcriptional profiling identifies markers that define the CD103+CD11b+ DC lineage, including CD101, TREM1 and Siglec-F, and shows that the absence of CD103+CD11b+ DCs in CD11c-Cre.Tgfbr1fl/fl mice reflects defective differentiation from CD103−CD11b+ intermediaries, rather than an isolated loss of CD103 expression. The defect in CD103+CD11b+ DCs is accompanied by reduced generation of antigen-specific, inducible FoxP3+ regulatory T cells in vitro and in vivo, and by reduced numbers of endogenous Th17 cells in the intestinal mucosa. Thus, TGFβR1-mediated signalling may explain the tissue-specific development of these unique DCs.Developmental cues for the different dendritic cell (DC) subsets in the intestine are yet to be defined. Here the authors show that TGFβR1 signalling is needed for development of CD103+CD11b+ intestinal DCs from CD103−CD11b+ cells and that they contribute to the generation of Th17 and regulatory T cells

The Transcription Factor ZEB2 Is Required to Maintain the Tissue-Specific Identities of Macrophages

SUMMARY Heterogeneity between different macrophage populations has become a defining feature of this lineage. However, the conserved factors defining macrophages remain largely unknown. The transcription factor ZEB2 is best described for its role in epithelial to mesenchymal transition; however, its role within the immune system is only now being elucidated. We show here that Zeb2 expression is a conserved feature of macrophages. Using Clec4f‐cre, Itgax‐cre, and Fcgr1‐cre mice to target five different macrophage populations, we found that loss of ZEB2 resulted in macrophage disappearance from the tissues, coupled with their subsequent replenishment from bone‐marrow precursors in open niches. Mechanistically, we found that ZEB2 functioned to maintain the tissue‐specific identities of macrophages. In Kupffer cells, ZEB2 achieved this by regulating expression of the transcription factor LXR&agr;, removal of which recapitulated the loss of Kupffer cell identity and disappearance. Thus, ZEB2 expression is required in macrophages to preserve their tissue‐specific identities. Graphical Abstract Figure. No caption available. HighlightsZEB2 is highly expressed across the macrophage lineageZEB2 preserves the tissue‐specific identities of macrophages across tissuesZEB2 deficient macrophages are outcompeted by WT counterpartsLXR&agr; is crucial for Kupffer cell identity and is maintained by ZEB2 &NA; Scott et al. demonstrate that ZEB2 is critical for maintaining the tissue identities of macrophages. Loss of ZEB2 results in tissue‐specific changes in different macrophage populations and their subsequent disappearance. In Kupffer cells, ZEB2 maintains LXR&agr; expression, loss of which reproduces the change in Kupffer cell identity and their disappearance.