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While the roles of parenchymal microglia in brain homeostasis and disease are fairly clear, other brain-resident myeloid cells remain less well understood. By dissecting border regions and combining single-cell RNA-sequencing with high-dimensional cytometry, bulk RNA-sequencing, fate-mapping and microscopy, we reveal the diversity of non-parenchymal brain macrophages. Border-associated macrophages (BAMs) residing in the dura mater, subdural meninges and choroid plexus consisted of distinct subsets with tissue-specific transcriptional signatures, and their cellular composition changed during postnatal development. BAMs exhibited a mixed ontogeny, and subsets displayed distinct self-renewal capacity following depletion and repopulation. Single-cell and fate-mapping analysis both suggested that there is a unique microglial subset residing on the apical surface of the choroid plexus epithelium. Finally, gene network analysis and conditional deletion revealed IRF8 as a master regulator that drives the maturation and diversity of brain macrophages. Our results provide a framework for understanding host–macrophage interactions in both the healthy and diseased brain.Van Hove et al. reveal the diversity of macrophages at the brain’s border regions via single-cell analysis and fate-mapping. This also identified a microglial subset at the surface of the choroid plexus, in direct contact with cerebrospinal fluid.

While CNS microglia have been extensively studied, relatively little is known about macrophages populating the peripheral nervous system. Here we performed ontogenic, transcriptomic and spatial characterization of sciatic nerve macrophages (snMacs). Using multiple fate-mapping systems, we show that snMacs do not derive from the early embryonic precursors colonizing the CNS, but originate primarily from late embryonic precursors and become replaced by bone-marrow-derived macrophages over time. Using single-cell transcriptomics, we identified a tissue-specific core signature of snMacs and two spatially separated snMacs: Relmα+Mgl1+ snMacs in the epineurium and Relmα–Mgl1– snMacs in the endoneurium. Globally, snMacs lack most of the core signature genes of microglia, with only the endoneurial subset expressing a restricted number of these genes. In response to nerve injury, the two resident snMac populations respond differently. Moreover, and unlike in the CNS, monocyte-derived macrophages that develop during injury can engraft efficiently in the pool of resident peripheral nervous system macrophages.The authors identify two subsets of peripheral nerve macrophages residing in the endoneurium and the epineurium and displaying a distinct transcriptome and response to injury. These cells lack the main microglia identity and have a distinct origin.

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. Tissue-resident macrophages are mostly derived from embryonic progenitors. Scott et al . develop a mouse model to specifically deplete Kupffer cells (KC) in vivo and show that monocyte-derived cells can repopulate KC niche and behave similar to their embryonically-derived counterparts.

The Adjuvant System AS01 contains monophosphoryl lipid A (MPL) and the saponin QS-21 in a liposomal formulation. AS01 is included in recently developed vaccines against malaria and varicella zoster virus. Like for many other adjuvants, induction of adaptive immunity by AS01 is highly dependent on the ability to recruit and activate dendritic cells (DCs) that migrate to the draining lymph node for T and B cell stimulation. The objective of this study was to more precisely address the contribution of the different conventional (cDC) and monocyte-derived DC (MC) subsets in the orchestration of the adaptive immune response after immunization with AS01 adjuvanted vaccine. The combination of MPL and QS-21 in AS01 induced strong recruitment of CD26 + XCR1 + cDC1s, CD26 + CD172 + cDC2s and a recently defined CCR2-dependent CD64-expressing inflammatory cDC2 (inf-cDC2) subset to the draining lymph node compared to antigen alone, while CD26 - CD64 + CD88 + MCs were barely detectable. At 24 h post-vaccination, cDC2s and inf-cDC2s were superior amongst the different subsets in priming antigen-specific CD4 + T cells, while simultaneously presenting antigen to CD8 + T cells. Diphtheria toxin (DT) mediated depletion of all DCs prior to vaccination completely abolished adaptive immune responses, while depletion 24 h after vaccination mainly affected CD8 + T cell responses. Vaccinated mice lacking Flt3 or the chemokine receptor CCR2 showed a marked deficit in inf-cDC2 recruitment and failed to raise proper antibody and T cell responses. Thus, the adjuvant activity of AS01 is associated with the potent activation of subsets of cDC2s, including the newly described inf-cDC2s.

House dust mite (HDM)‐allergic asthma is driven by T helper 2 (Th2) lymphocytes, but also innate immune cells control key aspects of the disease. The precise function of innate natural killer (NK) cells during the initiation and propagation of asthma has been very confusing, in part because different, not entirely specific, strategies were used to target these cells. We show that HDM inhalation rapidly led to the accumulation of NK cells in the lung‐draining lymph nodes and of activated CD69 + NK cells in the bronchoalveolar lumen. However, genetically engineered Ncr1 ‐DTA or Ncr1 ‐DTR mice that constitutively or temporarily lack NK cells, still developed all key features of acute or chronic HDM‐driven asthma, such as bronchial hyperreactivity, Th2 cytokine production, eosinophilia, mucus overproduction, and Th2‐dependent immunoglobulin serum titers. The same results were obtained by administration of conventional NK1.1 or asialo‐GM1 NK cell‐depleting antibodies, antibody‐mediated blocking of the NKG2D receptor, or genetic NKG2D deficiency. Thus, although NK cells accumulate in allergen‐challenged lungs, our findings comprehensively demonstrate that these cells are not required for HDM‐driven asthma in the mouse. Synopsis The function of NK cells in allergic asthma development has been confusing, as targeting strategies with different specificities have generated conflicting results. Here, conventional NK cells are genetically and specifically depleted and shown to be dispensable for asthma development in mice. Allergic asthma to house dust mite or ovalbumin can be induced in genetically engineered Ncr1‐DTA mice that constitutively lack all NKp46 + cells. The asthma phenotype is neither affected by temporarily depleting NK cells in Ncr1‐DTR mice, nor by antibody‐mediated depletion of NK1.1 + or asialoGM1 + cells. Antibody‐mediated blocking or genetic depletion of the NKG2D receptor does not influence allergic asthma development. Graphical Abstract The function of NK cells in allergic asthma development has been confusing, as targeting strategies with different specificities have generated conflicting results. Here, conventional NK cells are genetically and specifically depleted and shown to be dispensable for asthma development in mice.