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SummaryMonocytes are heterogeneous innate effector leukocytes generated in the bone marrow and released into circulation in a CCR2-dependent manner. During infection or inflammation myelopoiesis is modulated to rapidly meet demand for more effector cells. Danger signals from peripheral tissues can influence this process. Herein we demonstrate that repetitive TLR7 stimulation via the epithelial barriers drove a potent emergency bone marrow monocyte response. This process was unique to TLR7 activation and occurred independently of the canonical CCR2 and CX3CR1 axes or prototypical cytokines. The monocytes egressing the bone marrow had an immature Ly6C-high profile and differentiated into vascular Ly6C-low monocytes and tissue macrophages in multiple organs. They displayed a blunted cytokine response to further TLR7 stimulation and reduced lung viral load after RSV and influenza virus infection. These data provide insights into the emergency myelopoiesis likely to occur in response to the encounter of single-stranded RNA viruses at barrier sites.

RUNX1 is a transcription factor that plays key roles in haematopoietic development and in adult haematopoiesis and lymphopoiesis. Here we report that RUNX1 is also involved in controlling the dynamics of cell cycle entry of naïve resting B cells in response to stimulation of the B cell receptor (BCR). Conditional knockout ofRunx1in mouse resting B cells resulted in accelerated entry of the cells into S-phase following BCR engagement. Our results indicate that Runx1 regulates the cyclin D2 (Ccnd2) gene, the immediate early genes,Fosl2,Atf3andEgr2, and the Notch effectorRbpj, in B cells, reducing the rate at which transcription of these genes increases following BCR stimulation. RUNX1 interacts with the chromatin remodeller SRCAP, recruiting it to promoter and enhancer regions of theCcnd2gene. BCR-mediated activation triggers switching between binding of RUNX1 and its paralog RUNX3 and between SRCAP and the SWI/SNF remodelling complex member BRG1. We also find that RUNX1 regulates expression of a number of immunomodulatory genes in resting B cells. These include the interferon receptor subunit geneIfnar1, which is upregulated in B cells from lupus patients, thePtpn22gene, which has been identified as a major lupus risk allele, and theLrrk2gene, which is mutated in familial Parkinson’s disease. The hyperresponsiveness of theRunx1knockout B cells to antigen stimulation and its role in regulating a suite of genes that are known to be associated with autoimmune disease suggest that RUNX1 is a major regulator of B cell tolerance and autoimmunity.

IgE is the least abundant circulating antibody class but is constitutively present in healthy tissues bound to resident cells via its high-affinity receptor, FcεRI. The physiological role of endogenous IgE is unclear but it is suggested to provide host protection against a variety of noxious environmental substances and parasitic infections at epithelial barrier surfaces. Here we show that skin inflammation enhances levels of IgE with natural specificities and with a similar repertoire, VDJ rearrangements and CDRH3 characteristics as in healthy tissue. IgE-bearing basophils are recruited to inflamed skin via CXCL12 and TSLP/IL-3-dependent upregulation of CXCR4. In the inflamed skin, IgE/FcεRI-signalling in basophils promotes epithelial cell growth and differentiation, partly through histamine engagement of H1R and H4R. Furthermore, this natural IgE response strongly drives tumour outgrowth of epithelial cells harbouring oncogenic mutation. These findings indicate that natural IgE support skin barrier defences however during chronic tissue inflammation this may be subverted to promote tumour growth.

Embryo-derived Langerhans cells (eLC) are maintained within the sealed epidermis without contribution from circulating cells. When the network is perturbed by transient exposure to ultra-violet light, short-term LC are temporarily reconstituted from an initial wave of monocytes, but thought to be superseded by more permanent repopulation with undefined LC precursors. However, the extent to which this mechanism is relevant to immune-pathological processes that damage LC population integrity is not known. Using a model of allogeneic hematopoietic stem cell transplantation, where allo-reactive T cells directly target eLC, we have asked if and how the original LC network is ultimately restored. We find that donor monocytes, but not dendritic cells, are the precursors of the long-term LC in this context. Destruction of eLC leads to recruitment of a single wave of monocytes which engraft in the epidermis and undergo a sequential pathway of differentiation via transcriptionally distinct EpCAM+precursors. Monocyte-derived LC acquire the capacity of self-renewal, and turn-over in the epidermis was remarkably similar to that of steady state eLC. However, we have identified a bottleneck in the differentiation and survival of epidermal monocytes, which together with the slow turn-over of mature LC limits repair of the network. Furthermore, replenishment of the LC network leads to constitutive entry of cells into the epidermal compartment. Thus, immune injury triggers functional adaptation of mechanisms used to maintain tissue-resident macrophages at other sites, but this process is highly inefficient in the skin.HighlightsImmune injury leads to recruitment of a single wave of monocytes to replace resident Langerhans cells (LC).DC lineage cells cannot become long-term replacement LC.The size of the re-emerging network is controlled by density-dependent division of mature LC.Immune injury and inefficient repopulation by monocyte-derived cells lead to a permanently altered LC niche.