Inorganic phosphate exporter heterozygosity in mice leads to brain vascular calcification, microangiopathy, and microgliosis.

Calcification of the cerebral microvessels in the basal ganglia in the absence of systemic calcium and phosphate imbalance is a hallmark of primary familial brain calcification (PFBC), a rare neurodegenerative disorder. Mutation in genes encoding for sodium-dependent phosphate transporter 2 (SLC20A2), xenotropic and polytropic retrovirus receptor 1 (XPR1), platelet-derived growth factor B (PDGFB), platelet-derived growth factor receptor beta (PDGFRB), myogenesis regulating glycosidase (MYORG), and junctional adhesion molecule 2 (JAM2) are known to cause PFBC. Loss-of-function mutations in XPR1, the only known inorganic phosphate exporter in metazoans, causing dominantly inherited PFBC was first reported in 2015 but until now no studies in the brain have addressed whether loss of one functional allele leads to pathological alterations in mice, a commonly used organism to model human diseases. Here we show that mice heterozygous for Xpr1 (Xpr1WT/lacZ ) present with reduced inorganic phosphate levels in the cerebrospinal fluid and age- and sex-dependent growth of vascular calcifications in the thalamus. Vascular calcifications are surrounded by vascular basement membrane and are located at arterioles in the smooth muscle layer. Similar to previously characterized PFBC mouse models, vascular calcifications in Xpr1WT/lacZ mice contain bone matrix proteins and are surrounded by reactive astrocytes and microglia. However, microglial activation is not confined to calcified vessels but shows a widespread presence. In addition to vascular calcifications, we observed vessel tortuosity and transmission electron microscopy analysis revealed microangiopathy-endothelial swelling, phenotypic alterations in vascular smooth muscle cells, and thickening of the basement membrane.

Lactation-associated macrophages exist in murine mammary tissue and human milk.

Macrophages are involved in immune defense, organogenesis and tissue homeostasis. Macrophages contribute to the different phases of mammary gland remodeling during development, pregnancy and involution postlactation. Less is known about the dynamics of mammary gland macrophages in the lactation stage. Here, we describe a macrophage population present during lactation in mice. By multiparameter flow cytometry and single-cell RNA sequencing, we identified a lactation-induced CD11c+CX3CR1+Dectin-1+ macrophage population (liMac) that was distinct from the two resident F4/80hi and F4/80lo macrophage subsets present pregestationally. LiMacs were predominantly monocyte-derived and expanded by proliferation in situ concomitant with nursing. LiMacs developed independently of IL-34, but required CSF-1 signaling and were partly microbiota-dependent. Locally, they resided adjacent to the basal cells of the alveoli and extravasated into the milk. We found several macrophage subsets in human milk that resembled liMacs. Collectively, these findings reveal the emergence of unique macrophages in the mammary gland and milk during lactation.

BAM! Pathogen control at the brain border.

Border-associated macrophages (BAMs) reside at the interface between the brain and the periphery, including the meninges and choroid plexus. In this issue of Immunity, two studies report the dynamics, diversity, and fate of murine BAMs during infection, assigning these cells a neuroprotective role.

The quality of energy- and macronutrient-balanced diets regulates host susceptibility to influenza in mice.

Modulation of individual macronutrients or caloric density is known to regulate host resistance to infection in mice. However, the impact of diet composition, independent of macronutrient and energy content, on infection susceptibility is unclear. We show that two laboratory rodent diets, widely used as standard animal feeds and experimental controls, display distinct abilities in supporting mice during influenza infection. Mice placed on the highly processed AIN93G showed increased mortality to infection compared with those on a grain-based chow diet, suggesting a detrimental role for highly processed food in host defense. We further demonstrate that the heightened susceptibility of AIN93G-fed mice was associated with the failure in homeostasis restoration mediated by the cytokine interferon (IFN)-γ. Our findings show that diet composition calibrates host survival threshold by regulating adaptive homeostasis and highlights a pivotal role for extrinsic signals in host phenotype and outcome of host-pathogen interaction.

Diversity and function of brain-associated macrophages.

Different tissue-resident macrophages are present in and at the borders of the brain. Microglia reside in the central nervous system parenchyma, whereas non-parenchymal macrophages dwell at the interface between the brain and the periphery, including the perivascular spaces, the choroid plexus, and the meninges. Microglia display regional heterogeneity in phenotype and function, and fulfill critical roles in neuronal development and regulation of brain homeostasis in the healthy brain. In recent years, it became clear that the non-parenchymal macrophages also exhibit transcriptional diversity and different dynamics but less is known about their functional specification at their respective anatomical locations. This review discusses recent advances describing the heterogeneity of the brain macrophage compartment and potential physiological functions with a focus on non-parenchymal macrophages.

TCR Affinity Controls the Dynamics but Not the Functional Specification of the Antimycobacterial CD4+ T Cell Response.

The quality of T cell responses depends on the lymphocytes' ability to undergo clonal expansion, acquire effector functions, and traffic to the site of infection. Although TCR signal strength is thought to dominantly shape the T cell response, by using TCR transgenic CD4+ T cells with different peptide:MHC binding affinity, we reveal that TCR affinity does not control Th1 effector function acquisition or the functional output of individual effectors following mycobacterial infection in mice. Rather, TCR affinity calibrates the rate of cell division to synchronize the distinct processes of T cell proliferation, differentiation, and trafficking. By timing cell division-dependent IL-12R expression, TCR affinity controls when T cells become receptive to Th1-imprinting IL-12 signals, determining the emergence and magnitude of the Th1 effector pool. These findings reveal a distinct yet cooperative role for IL-12 and TCR binding affinity in Th1 differentiation and suggest that the temporal activation of clones with different TCR affinity is a major strategy to coordinate immune surveillance against persistent pathogens.

Pericytes regulate vascular immune homeostasis in the CNS.

Pericytes regulate the development of organ-specific characteristics of the brain vasculature such as the blood-brain barrier (BBB) and astrocytic end-feet. Whether pericytes are involved in the control of leukocyte trafficking in the adult central nervous system (CNS), a process tightly regulated by CNS vasculature, remains elusive. Using adult pericyte-deficient mice (Pdgfbret/ret ), we show that pericytes limit leukocyte infiltration into the CNS during homeostasis and autoimmune neuroinflammation. The permissiveness of the vasculature toward leukocyte trafficking in Pdgfbret/ret mice inversely correlates with vessel pericyte coverage. Upon induction of experimental autoimmune encephalomyelitis (EAE), pericyte-deficient mice die of severe atypical EAE, which can be reversed with fingolimod, indicating that the mortality is due to the massive influx of immune cells into the brain. Additionally, administration of anti-VCAM-1 and anti-ICAM-1 antibodies reduces leukocyte infiltration and diminishes the severity of atypical EAE symptoms of Pdgfbret/ret mice, indicating that the proinflammatory endothelium due to absence of pericytes facilitates exaggerated neuroinflammation. Furthermore, we show that the presence of myelin peptide-specific peripheral T cells in Pdgfbret/ret ;2D2tg mice leads to the development of spontaneous neurological symptoms paralleled by the massive influx of leukocytes into the brain. These findings indicate that intrinsic changes within brain vasculature can promote the development of a neuroinflammatory disorder.

Monocytes promote UV-induced epidermal carcinogenesis.

Mononuclear phagocytes consisting of monocytes, macrophages, and DCs play a complex role in tumor development by either promoting or restricting tumor growth. Cutaneous squamous cell carcinoma (cSCC) is the second most common nonmelanoma skin cancer arising from transformed epidermal keratinocytes. While present at high numbers, the role of tumor-infiltrating and resident myeloid cells in the formation of cSCC is largely unknown. Using transgenic mice and depleting antibodies to eliminate specific myeloid cell types in the skin, we investigated the involvement of mononuclear phagocytes in the development of UV-induced cSCC in K14-HPV8-E6 transgenic mice. Although resident Langerhans cells were enriched in the tumor, their contribution to tumor formation was negligible. Equally, dermal macrophages were dispensable for the development of cSCC. In contrast, mice lacking circulating monocytes were completely resistant to UV-induced cSCC, indicating that monocytes promote tumor development. Collectively, these results demonstrate a critical role for classical monocytes in the initiation of skin cancer.

STOP floxing around: Specificity and leakiness of inducible Cre/loxP systems.

Cre and CreER mouse strains are powerful tools that have proven invaluable for investigating the function of genes and for the fate-mapping of cell populations. The fidelity of these systems however are becoming more and more contested. In this issue of the European Journal of Immunology, Van Hove et al. and Chappell-Maor et al. carefully dissect the cellular specificities of two commonly used CreER mouse strains for the study of CNS macrophages; Cx3cr1CreER and Sall1CreER . Both studies elegantly highlight that CreER strains, as well as the "floxed" allele to be targeted, need to be carefully selected and properly characterized in order to ensure reproducible and robust data and interpretations. These studies are a cautionary tale for this technology, but also highlight that we must continuously question and improve our experimental approaches.

Emerging roles of IL-34 in health and disease.

Macrophages are part of the innate immune system and are present in every organ of the body. They fulfill critical roles in tissue homeostasis and development and are involved in various pathologies. An essential factor for the development, homeostasis, and function of mononuclear phagocytes is the colony stimulating factor-1 receptor (CSF-1R), which has two known ligands: CSF-1 and interleukin-34 (IL-34). While CSF-1 has been extensively studied, the biology and functions of IL-34 are only now beginning to be uncovered. In this review, we discuss recent advances of IL-34 biology in health and disease with a specific focus on mononuclear phagocytes.

Visualizing the Selectivity and Dynamics of Interferon Signaling In Vivo.

Interferons (IFN) are pleiotropic cytokines essential for defense against infection, but the identity and tissue distribution of IFN-responsive cells in vivo are poorly defined. In this study, we generate a mouse strain capable of reporting IFN-signaling activated by all three types of IFNs and investigate the spatio-temporal dynamics and identity of IFN-responding cells following IFN injection and influenza virus infection. Despite ubiquitous expression of IFN receptors, cellular responses to IFNs are highly heterogenous in vivo and are determined by anatomical site, cell type, cellular preference to individual IFNs, and activation status. Unexpectedly, type I and II pneumocytes, the primary target of influenza infection, exhibit striking differences in the strength and temporal dynamics of IFN signaling associated with differential susceptibility to the viral infection. Our findings suggest that time- and cell-type-dependent integration of distinct IFN signals govern the specificity and magnitude of IFN responses in vivo.

CXCR6-Deficiency Improves the Control of Pulmonary Mycobacterium tuberculosis and Influenza Infection Independent of T-Lymphocyte Recruitment to the Lungs.

T-lymphocytes are critical for protection against respiratory infections, such as Mycobacterium tuberculosis and influenza virus, with chemokine receptors playing an important role in directing these cells to the lungs. CXCR6 is expressed by activated T-lymphocytes and its ligand, CXCL16, is constitutively expressed by the bronchial epithelia, suggesting a role in T-lymphocyte recruitment and retention. However, it is unknown whether CXCR6 is required in responses to pulmonary infection, particularly on CD4+ T-lymphocytes. Analysis of CXCR6-reporter mice revealed that in naïve mice, lung leukocyte expression of CXCR6 was largely restricted to a small population of T-lymphocytes, but this population was highly upregulated after either infection. Nevertheless, pulmonary infection of CXCR6-deficient mice with M. tuberculosis or recombinant influenza A virus expressing P25 peptide (rIAV-P25), an I-Ab-restricted epitope from the immunodominant mycobacterial antigen, Ag85B, demonstrated that the receptor was redundant for recruitment of T-lymphocytes to the lungs. Interestingly, CXCR6-deficiency resulted in reduced bacterial burden in the lungs 6 weeks after M. tuberculosis infection, and reduced weight loss after rIAV-P25 infection compared to wild type controls. This was paradoxically associated with a decrease in Th1-cytokine responses in the lung parenchyma. Adoptive transfer of P25-specific CXCR6-deficient T-lymphocytes into WT mice revealed that this functional change in Th1-cytokine production was not due to a T-lymphocyte intrinsic mechanism. Moreover, there was no reduction in the number or function of CD4+ and CD8+ tissue resident memory cells in the lungs of CXCR6-deficient mice. Although CXCR6 was not required for T-lymphocyte recruitment or retention in the lungs, CXCR6 influenced the kinetics of the inflammatory response so that deficiency led to increased host control of M. tuberculosis and influenza virus.

A proline deletion in IFNAR1 impairs IFN-signaling and underlies increased resistance to tuberculosis in humans.

Type I interferons (IFN), best known for their anti-viral functions, have been shown to impair host resistance to intracellular bacteria in mice. However, the precise role of type I IFN signaling in bacterial infection in humans is unclear. Here, we show that genetic variation in the human IFNAR1 gene is associated with decreased susceptibility to tuberculosis and an increased risk of viral hepatitis in Chinese populations. Receptor mutagenesis and cell signaling studies establish that the IFNAR1 mutation corresponding to a proline deletion in the hinge region of the membrane-proximal domain of IFNAR1 decreases the binding affinity of IFNAR1 to IFN-ß, impeding type I IFN signaling. Our findings suggest that IFNAR1 signaling underlies an increased risk of tuberculosis in humans and reveals a function for the IFNAR1 inter-domain region in cytokine-cytokine receptor interaction and signal transduction.

Defining the distinct, intrinsic properties of the novel type I interferon, IFNϵ.

The type I interferons (IFNs) are a family of cytokines with diverse biological activities, including antiviral, antiproliferative, and immunoregulatory functions. The discovery of the hormonally regulated, constitutively expressed IFNϵ has suggested a function for IFNs in reproductive tract homeostasis and protection from infections, but its intrinsic activities are untested. We report here the expression, purification, and functional characterization of murine IFNϵ (mIFNϵ). Recombinant mIFNϵ (rmIFNϵ) exhibited an α-helical fold characteristic of type I IFNs and bound to IFNα/ß receptor 1 (IFNAR1) and IFNAR2, but, unusually, it had a preference for IFNAR1. Nevertheless, rmIFNϵ induced typical type I IFN signaling activity, including STAT1 phosphorylation and activation of canonical type I IFN signaling reporters, demonstrating that it uses the JAK-STAT signaling pathway. We also found that rmIFNϵ induces the activation of T, B, and NK cells and exhibits antiviral, antiproliferative, and antibacterial activities typical of type I IFNs, albeit with 100-1000-fold reduced potency compared with rmIFNα1 and rmIFNß. Surprisingly, although the type I IFNs generally do not display cross-species activities, rmIFNϵ exhibited high antiviral activity on human cells, suppressing HIV replication and inducing the expression of known HIV restriction factors in human lymphocytes. Our findings define the intrinsic properties of murine IFNϵ, indicating that it distinctly interacts with IFNAR and elicits pathogen-suppressing activity with a potency enabling host defense but with limited toxicity, appropriate for a protein expressed constitutively in a sensitive mucosal site, such as the reproductive tract.

Pho4 Is Essential for Dissemination of Cryptococcus neoformans to the Host Brain by Promoting Phosphate Uptake and Growth at Alkaline pH.

Phosphate acquisition by fungi is regulated by the phosphate-sensing and acquisition (PHO) signaling pathway. Cryptococcus neoformans disseminates from the lung to the brain and is the commonest cause of fungal meningitis worldwide. To investigate the contribution of PHO signaling to cryptococcal dissemination, we characterized a transcription factor knockout strain (hlh3Δ/pho4Δ) defective in phosphate acquisition. Despite little similarity with other fungal Pho4 proteins, Hlh3/Pho4 functioned like a typical phosphate-responsive transcription factor in phosphate-deprived cryptococci, accumulating in nuclei and triggering expression of genes involved in phosphate acquisition. The pho4Δ mutant strain was susceptible to a number of stresses, the effect of which, except for alkaline pH, was alleviated by phosphate supplementation. Even in the presence of phosphate, the PHO pathway was activated in wild-type cryptococci at or above physiological pH, and under these conditions, the pho4Δ mutant had a growth defect and compromised phosphate uptake. The pho4Δ mutant was hypovirulent in a mouse inhalation model, where dissemination to the brain was reduced dramatically, and markedly hypovirulent in an intravenous dissemination model. The pho4Δ mutant was not detected in blood, nor did it proliferate significantly when cultured with peripheral blood monocytes. In conclusion, dissemination of infection and the pathogenesis of meningitis are dependent on cryptococcal phosphate uptake and stress tolerance at alkaline pH, both of which are Pho4 dependent. IMPORTANCE Cryptococcal meningitis is fatal without treatment and responsible for more than 500,000 deaths annually. To be a successful pathogen, C. neoformans must obtain an adequate supply of essential nutrients, including phosphate, from various host niches. Phosphate acquisition in fungi is regulated by the PHO signaling cascade, which is activated when intracellular phosphate decreases below a critical level. Induction of phosphate acquisition genes leads to the uptake of free phosphate via transporters. By blocking the PHO pathway using a Pho4 transcription factor mutant (pho4Δ mutant), we demonstrate the importance of the pathway for cryptococcal dissemination and the establishment of brain infection in murine models. Specifically, we show that reduced dissemination of the pho4Δ mutant to the brain is due to an alkaline pH tolerance defect, as alkaline pH mimics the conditions of phosphate deprivation. The end result is inhibited proliferation in host tissues, particularly in blood.

Functional Interplay between Type I and II Interferons Is Essential to Limit Influenza A Virus-Induced Tissue Inflammation.

Host control of influenza A virus (IAV) is associated with exuberant pulmonary inflammation characterized by the influx of myeloid cells and production of proinflammatory cytokines including interferons (IFNs). It is unclear, however, how the immune system clears the virus without causing lethal immunopathology. Here, we demonstrate that in addition to its known anti-viral activity, STAT1 signaling coordinates host inflammation during IAV infection in mice. This regulatory mechanism is dependent on both type I IFN and IFN-γ receptor signaling and, importantly, requires the functional interplay between the two pathways. The protective function of type I IFNs is associated with not only the recruitment of classical inflammatory Ly6Chi monocytes into IAV-infected lungs, but also the prevention of excessive monocyte activation by IFN-γ. Unexpectedly, type I IFNs preferentially regulate IFN-γ signaling in Ly6Clo rather than inflammatory Ly6Chi mononuclear cell populations. In the absence of type I IFN signaling, Ly6Clo monocytes/macrophages, become phenotypically and functionally more proinflammatory than Ly6Chi cells, revealing an unanticipated function of the Ly6Clo mononuclear cell subset in tissue inflammation. In addition, we show that type I IFNs employ distinct mechanisms to regulate monocyte and neutrophil trafficking. Type I IFN signaling is necessary, but not sufficient, for preventing neutrophil recruitment into the lungs of IAV-infected mice. Instead, the cooperation of type I IFNs and lymphocyte-produced IFN-γ is required to regulate the tissue neutrophilic response to IAV. Our study demonstrates that IFN interplay links innate and adaptive anti-viral immunity to orchestrate tissue inflammation and reveals an additional level of complexity for IFN-dependent regulatory mechanisms that function to prevent excessive immunopathology while preserving anti-microbial functions.

Interfering with immunity: detrimental role of type I IFNs during infection.

Type I IFNs are known to inhibit viral replication and mediate protection against viral infection. However, recent studies revealed that these cytokines play a broader and more fundamental role in host responses to infections beyond their well-established antiviral function. Type I IFN induction, often associated with microbial evasion mechanisms unique to virulent microorganisms, is now shown to increase host susceptibility to a diverse range of pathogens, including some viruses. This article presents an overview of the role of type I IFNs in infections with bacterial, fungal, parasitic, and viral pathogens and discusses the key mechanisms mediating the regulatory function of type I IFNs in pathogen clearance and tissue inflammation.

Purification and biological characterization of soluble, recombinant mouse IFNß expressed in insect cells.

Interferon ß (IFNß) is a member of the type I interferon family of cytokines widely recognised for their anti-viral, anti-proliferative and immunomodulatory properties. Recombinant, biologically active forms of this cytokine are used clinically for the treatment of multiple sclerosis and in laboratories to study the role of this cytokine in health and disease. Established methods for expression of IFNß utilise either bacterial systems from which the insoluble recombinant proteins must be refolded, or mammalian expression systems in which large volumes of cell culture are required for recovery of acceptable yields. Utilising the baculovirus expression system and Trichoplusia ni (Cabbage Looper) BTI-TN-5B1-4 cell line, we report a reproducible method for production and purification of milligram/litre quantities of biologically active murine IFNß. Due to the design of our construct and the eukaryotic nature of insect cells, the resulting soluble protein is secreted allowing purification of the Histidine-tagged natively-folded protein from the culture supernatant. The IFNß purification method described is a two-step process employing immobilised metal-ion affinity chromatography (IMAC) and reverse-phase high performance liquid chromatography (RP-HPLC) that results in production of significantly more purified IFNß than any other reported eukaryotic-based expression system. Recombinant murine IFNß produced by this method was natively folded and demonstrated hallmark type I interferon biological effects including antiviral and anti-proliferative activities, and induced genes characteristic of IFNß activity in vivo. Recombinant IFNß also had specific activity levels exceeding that of the commercially available equivalent. Together, our findings provide a method for production of highly pure, biologically active murine IFNß.

Interferon-ε protects the female reproductive tract from viral and bacterial infection.

The innate immune system senses pathogens through pattern-recognition receptors (PRRs) that signal to induce effector cytokines, such as type I interferons (IFNs). We characterized IFN-ε as a type I IFN because it signaled via the Ifnar1 and Ifnar2 receptors to induce IFN-regulated genes. In contrast to other type I IFNs, IFN-ε was not induced by known PRR pathways; instead, IFN-ε was constitutively expressed by epithelial cells of the female reproductive tract (FRT) and was hormonally regulated. Ifn-ε-deficient mice had increased susceptibility to infection of the FRT by the common sexually transmitted infections (STIs) herpes simplex virus 2 and Chlamydia muridarum. Thus, IFN-ε is a potent antipathogen and immunoregulatory cytokine that may be important in combating STIs that represent a major global health and socioeconomic burden.

MyD88 drives the IFN-ß response to Lactobacillus acidophilus in dendritic cells through a mechanism involving IRF1, IRF3, and IRF7.

Type I IFNs are induced by pathogens to protect the host from infection and boost the immune response. We have recently demonstrated that this IFN response is not restricted to pathogens, as the Gram-positive bacterium Lactobacillus acidophilus, a natural inhabitant of the intestine, induces high levels of IFN-ß in dendritic cells. In the current study, we investigate the intracellular pathways involved in IFN-ß upon stimulation of dendritic cells with L. acidophilus and reveal that this IFN-ß induction requires phagosomal uptake and processing but bypasses the endosomal receptors TLR7 and TLR9. The IFN-ß production is fully dependent on the TIR adapter molecule MyD88, partly dependent on IFN regulatory factor (IRF)1, but independent of the TIR domain-containing adapter inducing IFN-ß MyD88 adapter-like, IRF and IRF7. However, our results suggest that IRF3 and IRF7 have complementary roles in IFN-ß signaling. The IFN-ß production is strongly impaired by inhibitors of spleen tyrosine kinase (Syk) and PI3K. Our results indicate that L. acidophilus induces IFN-ß independently of the receptors typically used by bacteria, as it requires MyD88, Syk, and PI3K signaling and phagosomal processing to activate IRF1 and IRF3/IRF7 and thereby the release of IFN-ß.