Identification of a radio-resistant and cycling dermal dendritic cell population in mice and men

The IL-4Rα Q576R polymorphism is associated with increased severity of atopic dermatitis and exaggerates allergic skin inflammation in mice

BACKGROUND

Atopic dermatitis (AD) is characterized by TH2-dominated skin inflammation and systemic response to cutaneously encountered antigens. The TH2 cytokines IL-4 and IL-13 play a critical role in the pathogenesis of AD. The Q576->R576 polymorphism in the IL-4 receptor alpha (IL-4Rα) chain common to IL-4 and IL-13 receptors alters IL-4 signaling and is associated with asthma severity.

OBJECTIVE

We sought to investigate whether the IL-4Rα R576 polymorphism is associated with AD severity and exaggerates allergic skin inflammation in mice.

METHODS

Nighttime itching interfering with sleep, Rajka-Langeland, and Eczema Area and Severity Index scores were used to assess AD severity. Allergic skin inflammation following epicutaneous sensitization of mice 1 or 2 IL-4Rα R576 alleles (QR and RR) and IL-4Rα Q576 (QQ) controls was assessed by flow cytometric analysis of cells and quantitative RT-PCR analysis of cytokines in skin.

RESULTS

The frequency of nighttime itching in 190 asthmatic inner-city children with AD, as well as Rajka-Langeland and Eczema Area and Severity Index scores in 1116 White patients with AD enrolled in the Atopic Dermatitis Research Network, was higher in subjects with the IL-4Rα R576 polymorphism compared with those without, with statistical significance for the Rajka-Langeland score. Following epicutaneous sensitization of mice with ovalbumin or house dust mite, skin infiltration by CD4+ cells and eosinophils, cutaneous expression of Il4 and Il13, transepidermal water loss, antigen-specific IgE antibody levels, and IL-13 secretion by antigen-stimulated splenocytes were significantly higher in RR and QR mice compared with QQ controls. Bone marrow radiation chimeras demonstrated that both hematopoietic cells and stromal cells contribute to the mutants' exaggerated allergic skin inflammation.

CONCLUSIONS

The IL-4Rα R576 polymorphism predisposes to more severe AD and increases allergic skin inflammation in mice.

Allogeneic stem cell transplantation in the treatment of acute myeloid leukemia: An overview of obstacles and opportunities

As an important treatment for acute myeloid leukemia, allogeneic hematopoietic stem cell transplantation (allo-HSCT) plays an important role in reducing relapse and improving long-term survival. With rapid advancements in basic research in molecular biology and immunology and with deepening understanding of the biological characteristics of hematopoietic stem cells, allo-HSCT has been widely applied in clinical practice. During allo-HSCT, preconditioning, the donor, and the source of stem cells can be tailored to the patient’s conditions, greatly broadening the indications for HSCT, with clear survival benefits. However, the risks associated with allo-HSCT remain high, i.e. hematopoietic reconstitution failure, delayed immune reconstitution, graft-versus-host disease, and post-transplant relapse, which are bottlenecks for further improvements in allo-HSCT efficacy and have become hot topics in the field of HSCT. Other bottlenecks recognized in the current treatment of individuals diagnosed with acute myeloid leukemia and subjected to allo-HSCT include the selection of the most appropriate conditioning regimen and post-transplantation management. In this paper, we reviewed the progress of relevant research regarding these aspects.

Incomplete chimerism following myeloablative and anti-thymocyte globulin-conditioned hematopoietic cell transplantation is a risk factor for relapse and chronic graft-versus-host disease

BACKGROUND AIMS

The value of routine chimerism determination after myeloablative hematopoietic cell transplantation (HCT) is unclear, particularly in the setting of anti-thymocyte globulin (ATG)-based graft-versus-host disease (GVHD) prophylaxis.

METHODS

Blood samples were collected at 3 months post-HCT from 558 patients who received myeloablative conditioning and ATG-based GVHD prophylaxis. Chimerism was assessed using multiplex polymerase chain reaction of short tandem repeats in sorted T cells (CD3+) and leukemia lineage cells (CD13+CD33+ for myeloid malignancies and CD19+ for B-lymphoid malignancies). ATG exposure was determined using a flow cytometry-based assay. The primary outcomes of interest were relapse and chronic GVHD (cGVHD).

RESULTS

Incomplete (<95%) T-cell chimerism and leukemia lineage chimerism were present in 17% and 4% of patients, respectively. Patients with incomplete T-cell chimerism had a significantly greater incidence of relapse (36% versus 22%, subhazard ratio [SHR] = 2.03, P = 0.001) and lower incidence of cGVHD (8% versus 25%, SHR = 0.29, P < 0.001) compared with patients with complete chimerism. In multivariate modeling, patients with high post-transplant ATG area under the curve and any cytomegalovirus (CMV) serostatus other than donor/recipient seropositivity (non-D+R+) had an increased likelihood of incomplete T-cell chimerism. Patients with incomplete leukemia lineage chimerism had a significantly greater incidence of relapse (50% versus 23%, SHR = 2.70, P = 0.011) and, surprisingly, a greater incidence of cGVHD (45% versus 20%, SHR = 2.64, P = 0.003).

CONCLUSIONS

High post-transplant ATG exposure and non-D+R+ CMV serostatus predispose patients to incomplete T-cell chimerism, which is associated with an increased risk of relapse. The increased risk of cGVHD with incomplete B-cell/myeloid chimerism is a novel finding that suggests an important role for recipient antigen-presenting cells in cGVHD pathogenesis.

New Key Players in Irritant Contact Dermatitis: Residential Skin Cells and Neutrophils Drive Inflammation

The chemokine CCL2 is a potential biomarker for progression of inflammatory skin disease. In a new article of the Journal of Investigative Dermatology, Shibuya et al. (2021) use murine experimental models to show that CCL2‒CCR2‒dependent IL-1β secretion by local skin cells and skin-infiltrating neutrophils are key drivers of skin irritation.

Intercrypt sentinel macrophages tune antibacterial NF-κB responses in gut epithelial cells via TNF

Intestinal epithelial cell (IEC) NF-κB signaling regulates the balance between mucosal homeostasis and inflammation. It is not fully understood which signals tune this balance and how bacterial exposure elicits the process. Pure LPS induces epithelial NF-κB activation in vivo. However, we found that in mice, IECs do not respond directly to LPS. Instead, tissue-resident lamina propria intercrypt macrophages sense LPS via TLR4 and rapidly secrete TNF to elicit epithelial NF-κB signaling in their immediate neighborhood. This response pattern is relevant also during oral enteropathogen infection. The macrophage-TNF-IEC axis avoids responses to luminal microbiota LPS but enables crypt- or tissue-scale epithelial NF-κB responses in proportion to the microbial threat. Thereby, intercrypt macrophages fulfill important sentinel functions as first responders to Gram-negative microbes breaching the epithelial barrier. The tunability of this crypt response allows the induction of defense mechanisms at an appropriate scale according to the localization and intensity of microbial triggers.

A Human Dectin-2 Deficiency Associated With Invasive Aspergillosis

Immunocompromised patients are highly susceptible to invasive aspergillosis. Herein, we identified a homozygous deletion mutation (507 del C) resulting in a frameshift (N170I) and early stop codon in the fungal binding Dectin-2 receptor, in an immunocompromised patient. The mutated form of Dectin-2 was weakly expressed, did not form clusters at/near the cell surface and was functionally defective. Peripheral blood mononuclear cells from this patient were unable to mount a cytokine (tumor necrosis factor, interleukin 6) response to Aspergillus fumigatus, and this first identified Dectin-2–deficient patient died of complications of invasive aspergillosis.

Gold nanorods enhance different immune cells and allow for efficient targeting of CD4+ Foxp3+ Tregulatory cells

Gold nanoparticles (AuNPs) hold great promise in nanomedicine, yet their successful clinical translation has not been realized. Some challenges include effective AuNP targeting and delivery to improve modulation of immune cells of interest while limiting potential adverse effects. In order to overcome these challenges, we must fully understand how AuNPs impact different immune cell subsets, particularly within the dendritic cell and T cell compartments. Herein, we show that polyethylene glycol coated (PEG) gold nanorods (AuNRs) and PEG AuNRs covered with a thin layer of silver (AuNR/Ag) may enhance the immune response towards immune suppression or activation. We also studied the ability to enhance CD4+ Foxp3+ Tregs in vitro using AuNRs functionalized with interleukin 2 (IL2), a cytokine that is important in Treg development and homeostasis. Our results indicate that AuNRs enhance different immune cells and that NP composition matters in immune targeting. This knowledge will help us understand how to better design AuNRs to target and enhance the immune system.

Generation of Murine Bone Marrow and Fetal Liver Chimeras

The generation of radiation chimeras allows researchers to substitute the hematopoietic system of a mouse with that of one or more donors. A suspension of hematopoietic stem cells (HSCs) is prepared from the bone marrow (BM) or the fetal liver (FL) of a donor mouse and adoptively transferred into an irradiated recipient. Within days, the donor's HSCs will engraft, and their progeny will quickly replace the blood cells of the recipient. This simple tool, together with the large availability of genetically modified mouse lines, can be harnessed to manipulate and study various aspects of blood cell biology in vivo. We present here protocols to generate three types of radiation chimera: (1) BM chimeras, which can assist in determining whether the origin of a genetically based phenotype is the hematopoietic or radio-resistant compartment and which are also conducive for studying the ecology of blood cells and for manipulating the environment hematopoietic cells live; (2) FL chimeras, which allow the study of hematopoietic systems from animals that carry genetic modifications incompatible with postnatal life; and (3) mixed BM chimeras, in which the hematopoietic system comprises blood cells of two different genotypes. Mixed BM chimeras can be used to identify genes that affect hematopoietic cell fitness and to establish whether secreted factors mediate a phenotype of interest. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Generation of bone marrow chimera Basic Protocol 2: Generation of fetal liver chimera Basic Protocol 3: Generation of mixed bone marrow chimera Support Protocol 1: Isolation of bone marrow cells Support Protocol 2: Cell counting by flow cytometry Support Protocol 3: Assessment of chimerism.

Fate mapping analysis reveals a novel murine dermal migratory Langerhans-like cell population

Dendritic cells residing in the skin represent a large family of antigen-presenting cells, ranging from long-lived Langerhans cells (LC) in the epidermis to various distinct classical dendritic cell subsets in the dermis. Through genetic fate mapping analysis and single-cell RNA-sequencing, we have identified a novel separate population of LC-independent CD207⁺CD326⁺ LC^like cells in the dermis that homed at a slow rate to the lymph nodes (LNs). These LC^like cells are long-lived and radio-resistant but, unlike LCs, they are gradually replenished by bone marrow-derived precursors under steady state. LC^like cells together with cDC1s are the main migratory CD207⁺CD326⁺ cell fractions present in the LN and not, as currently assumed, LCs, which are barely detectable, if at all. Cutaneous tolerance to haptens depends on LC^like cells, whereas LCs suppress effector CD8⁺ T-cell functions and inflammation locally in the skin during contact hypersensitivity. These findings bring new insights into the dynamism of cutaneous dendritic cells and their function opening novel avenues in the development of treatments to cure inflammatory skin disorders.

Our immune cells are constantly on guard to defend and protect us against invading pathogens, such as bacteria and viruses. Specialized immune cells, known as antigen-presenting cells, or APCs, have a key role in this process. They engulf invaders, chew them up, and travel to the closest local lymph node to stimulate other immune cells with small fragments of these pathogens. This ramps up the immune response to control infection and disease.

APCs are a large and diverse family of immune cells, which includes dendritic cells and macrophages. Some APCs work as mobile surveillance units, travelling around the body to find new threats. Others embed themselves in particular organs and tissues, such as the skin, to provide local, on-the-spot surveillance. Langerhans cells are one of the main types of APC in the skin and are found in the thin outer layer of the epidermis. While it is commonly believed that Langerhans cells can move from the epidermis to the skin-draining lymph nodes, some seemingly contradictory evidence exists to suggest that this may not be the case.

Now, Sheng et al. have investigated this issue by tracking APCs, including Langerhans cells, in the skin of mice. A powerful genetic cell labelling technique allowed them to track the movement of immune cells inside a living mouse. Sheng et al. found that majority of 'real' Langerhans cells did not leave the skin. Yet, a second lookalike cell that shared many of the same features of a Langerhans cell was found in the dermal layer of skin, and this cell could travel to local lymph nodes. Both the original and lookalike cells had distinct and separate roles in the skin.

This research, which has uncovered a new type of Langerhans-like immune cell in the skin, may be extremely useful for developing new targeted therapies to boost immune responses during infection; or to suppress inappropriate immune activation that can lead to autoimmune diseases, such as psoriasis.

GILT in Thymic Epithelial Cells Facilitates Central CD4 T Cell Tolerance to a Tissue-Restricted, Melanoma-Associated Self-Antigen

Central tolerance prevents autoimmunity, but also limits T cell responses to potentially immunodominant tumor epitopes with limited expression in healthy tissues. In peripheral APCs, γ-IFN-inducible lysosomal thiol reductase (GILT) is critical for MHC class II-restricted presentation of disulfide bond-containing proteins, including the self-antigen and melanoma Ag tyrosinase-related protein 1 (TRP1). The role of GILT in thymic Ag processing and generation of central tolerance has not been investigated. We found that GILT enhanced the negative selection of TRP1-specific thymocytes in mice. GILT expression was enriched in thymic APCs capable of mediating deletion, namely medullary thymic epithelial cells (mTECs) and dendritic cells, whereas TRP1 expression was restricted solely to mTECs. GILT facilitated MHC class II-restricted presentation of endogenous TRP1 by pooled thymic APCs. Using bone marrow chimeras, GILT expression in thymic epithelial cells (TECs), but not hematopoietic cells, was sufficient for complete deletion of TRP1-specific thymocytes. An increased frequency of TRP1-specific regulatory T (Treg) cells was present in chimeras with increased deletion of TRP1-specific thymocytes. Only chimeras that lacked GILT in both TECs and hematopoietic cells had a high conventional T/Treg cell ratio and were protected from melanoma challenge. Thus, GILT expression in thymic APCs, and mTECs in particular, preferentially facilitates MHC class II-restricted presentation, negative selection, and increased Treg cells, resulting in a diminished antitumor response to a tissue-restricted, melanoma-associated self-antigen.

Fetal origin confers radioresistance on liver macrophages via p21cip1/WAF1

BACKGROUND & AIMS

Cells of hematopoietic origin, including macrophages, are generally radiation sensitive, but a subset of Kupffer cells (KCs) is relatively radioresistant. Here, we focused on the identity of the radioresistant KCs in unmanipulated mice and the mechanism of radioresistance.

METHODS

We employed Emr1- and inducible CX3Cr1-based fate-mapping strategies combined with the RiboTag reporter to identify the total KCs and the embryo-derived KCs, respectively. The KC compartment was reconstituted with adult bone-marrow-derived KCs (bm-KCs) using clodronate depletion. Mice were lethally irradiated and transplanted with donor bone marrow, and the radioresistance of bone-marrow- or embryo-derived KCs was studied. Gene expression was analyzed using in situ mRNA isolation via RiboTag reporter mice, and the translatomes were compared among subsets.

RESULTS

Here, we identified the radioresistant KCs as the long-lived subset that is derived from CX3CR1-expressing progenitor cells in fetal life, while adult bm-KCs do not resist irradiation. While both subsets upregulated the Cdkn1a gene, encoding p21-^cip1/WAF1 protein, radioresistant embryo-derived KCs showed a greater increase in response to irradiation. In the absence of this molecule, the radioresistance of KCs was compromised. Replacement KCs, derived from adult hematopoietic stem cells, differed from radioresistant KCs in their expression of genes related to immunity and phagocytosis.

CONCLUSIONS

Here, we show that, in the murine liver, a subset of KCs of embryonic origin resists lethal irradiation through Cdkn1a upregulation and is maintained for a long period, while bm-KCs do not survive lethal irradiation.

LAY SUMMARY

Kupffer cells (KCs) are the tissue-resident macrophages of the liver. KCs can be originated from fetal precursors and from monocytes during the fetal stage and post-birth, respectively. Most immune cells in mice are sensitive to lethal-irradiation-induced death, while a subset of KCs resists radiation-induced death. These radioresistant KCs continue to live in the irradiated mice. We discovered that this relatively radioresistant KC subset are the fetal-derived KCs, and they achieve this through cell-cycle arrest. Understanding the radiobiology of KCs will provide valuable insights into the mechanisms that elicit radiation-induced liver disease.

Immune Function and Diversity of Osteoclasts in Normal and Pathological Conditions

Osteoclasts (OCLs) are key players in controlling bone remodeling. Modifications in their differentiation or bone resorbing activity are associated with a number of pathologies ranging from osteopetrosis to osteoporosis, chronic inflammation and cancer, that are all characterized by immunological alterations. Therefore, the 2000s were marked by the emergence of osteoimmunology and by a growing number of studies focused on the control of OCL differentiation and function by the immune system. At the same time, it was discovered that OCLs are much more than bone resorbing cells. As monocytic lineage-derived cells, they belong to a family of cells that displays a wide heterogeneity and plasticity and that is involved in phagocytosis and innate immune responses. However, while OCLs have been extensively studied for their bone resorption capacity, their implication as immune cells was neglected for a long time. In recent years, new evidence pointed out that OCLs play important roles in the modulation of immune responses toward immune suppression or inflammation. They unlocked their capacity to modulate T cell activation, to efficiently process and present antigens as well as their ability to activate T cell responses in an antigen-dependent manner. Moreover, similar to other monocytic lineage cells such as macrophages, monocytes and dendritic cells, OCLs display a phenotypic and functional plasticity participating to their anti-inflammatory or pro-inflammatory effect depending on their cell origin and environment. This review will address this novel vision of the OCL, not only as a phagocyte specialized in bone resorption, but also as innate immune cell participating in the control of immune responses.

Macrophages Switch Their Phenotype by Regulating Maf Expression during Different Phases of Inflammation

Macrophages manifest distinct phenotype according to the organs in which they reside. In addition, they flexibly switch their character in adaptation to the changing environment. However, the molecular basis that explains the conversion of the macrophage phenotype has so far been unexplored. We find that CD169⁺ macrophages change their phenotype by regulating the level of a transcription factor Maf both in vitro and in vivo in C57BL/6J mice. When CD169⁺ macrophages were exposed to bacterial components, they expressed an array of acute inflammatory response genes in Maf-dependent manner and simultaneously start to downregulate Maf. This Maf suppression is dependent on accelerated degradation through proteasome pathway and microRNA-mediated silencing. The downregulation of Maf unlocks the NF-E2-related factor 2-dominant, cytoprotective/antioxidative program in the same macrophages. The present study provides new insights into the previously unanswered question of how macrophages initiate proinflammatory responses while retaining their capacity to repair injured tissues during inflammation.

Polyphenolic Compounds Alter Stress-Induced Patterns of Global DNA Methylation in Brain and Blood

SCOPE

Stress is a known contributor to various forms of disease in humans and animals, although mechanisms are still unknown. In animals, psychosocial stress-induced depression/anxiety phenotypes are coincidental with increased inflammation in both brain and blood. The authors recently showed that a novel treatment with a select bioactive polyphenol preparation promotes resilience to stress-mediated depression/anxiety phenotypes mice. Moreover, selective bioactive phenolic compounds within the polyphenol preparation are identified that are effective in mitigating the behavioral effects of bone marrow transplantation from stressed mice.

METHODS AND RESULTS

Here, an animal model of adult stress and bone marrow transplantation is used to identify an epigenetic signature of repeated social defeat stress (RSDS) that is passed through bone marrow hematopoietic progenitor cells to naïve mice, revealing the maintenance of epigenetic memory following stress both centrally and peripherally. Further, polyphenols are administered to naïve and stress-susceptible mice, demonstrating that polyphenol treatment in mice from both susceptible and naïve donors alters global DNA methylation in the central nervous system and periphery and likewise has an effect on human blood cells after immune challenge.

CONCLUSIONS

Findings highlight the enduring molecular memory of stress and the possible mechanism by which select bioactive polyphenols may promote resiliency to stress. Polyphenols may be an efficacious alternative to traditional pharmacological treatments in psychiatry.

Epigenetic modulation of inflammation and synaptic plasticity promotes resilience against stress in mice

Major depressive disorder is associated with abnormalities in the brain and the immune system. Chronic stress in animals showed that epigenetic and inflammatory mechanisms play important roles in mediating resilience and susceptibility to depression. Here, through a high-throughput screening, we identify two phytochemicals, dihydrocaffeic acid (DHCA) and malvidin-3′-O-glucoside (Mal-gluc) that are effective in promoting resilience against stress by modulating brain synaptic plasticity and peripheral inflammation. DHCA/Mal-gluc also significantly reduces depression-like phenotypes in a mouse model of increased systemic inflammation induced by transplantation of hematopoietic progenitor cells from stress-susceptible mice. DHCA reduces pro-inflammatory interleukin 6 (IL-6) generations by inhibiting DNA methylation at the CpG-rich IL-6 sequences introns 1 and 3, while Mal-gluc modulates synaptic plasticity by increasing histone acetylation of the regulatory sequences of the Rac1 gene. Peripheral inflammation and synaptic maladaptation are in line with newly hypothesized clinical intervention targets for depression that are not addressed by currently available antidepressants.

Polyphenols have partial antidepressant effect without known mechanism. Here, the authors identify two phytochemicals from bioactive dietary polyphenols, show their antidepressant effect in a rodent model of depression, and that this effect is mediated by epigenetic and anti-inflammatory mechanisms.

MicroRNA-146a reduces MHC-II expression via targeting JAK/STAT signaling in dendritic cells after stem cell transplantation

Acute Graft-versus-host disease (GVHD) is a major immunological complication after allogeneic hematopoietic cell transplantation and a better understanding of the molecular regulation of the disease could help to develop novel targeted therapies. Here we found that a G/C polymorphism within the human microRNA-146a (miR-146a) gene of transplant-recipients, which causes reduced miR-146a levels, was strongly associated with the risk of developing severe acute GVHD (n=289). In mice, deficiency of miR-146a in the hematopoietic system or transfer of recipient-type miR 146a^-/- dendritic cells (DCs) enhanced GVHD, while miR-146a mimic-transfected-DCs ameliorated disease. Mechanistically, lack of miR-146a enhanced JAK2 STAT1-pathway activity, which led to higher expression of class II-transactivator (CIITA) and consecutively increased MHCII-levels on DCs. Inhibition of JAK1/2 or CIITA knockdown in DCs prevented miR-146a^-/- DC-induced GVHD exacerbation. Consistent with our findings in mice, patients with the miR-146a polymorphism rs2910164 in hematopoietic cells displayed higher MHCII levels on monocytes, which could be targeted by JAK1/2-inhibition.

Our findings indicate that the miR-146a polymorphism rs2910164 identifies patients at high risk for GVHD before allo HCT. Functionally we show that miR-146a acts as a central regulator of recipient-type DC activation during GVHD by dampening the pro-inflammatory JAK-STAT/CIITA/MHCII axis, which provides a scientific rationale for early JAK1/2-inhibition in selected patients.

Expression profiling of constitutive mast cells reveals a unique identity within the immune system

Mast cells are evolutionarily ancient sentinel cells. Like basophils, mast cells express the high-affinity receptor for immunoglobulin E (IgE) and have been linked to host defense and diverse immune-system-mediated diseases. To better characterize the function of these cells, we assessed the transcriptional profiles of mast cells isolated from peripheral connective tissues and basophils isolated from spleen and blood. We found that mast cells were transcriptionally distinct, clustering independently from all other profiled cells, and that mast cells demonstrated considerably greater heterogeneity across tissues than previously appreciated. We observed minimal homology between mast cells and basophils, which shared more overlap with other circulating granulocytes than with mast cells. The derivation of mast-cell and basophil transcriptional signatures underscores their differential capacities to detect environmental signals and influence the inflammatory milieu.

Pivotal role for skin transendothelial radio-resistant anti-inflammatory macrophages in tissue repair

Heterogeneity and functional specialization among skin-resident macrophages are incompletely understood. In this study, we describe a novel subset of murine dermal perivascular macrophages that extend protrusions across the endothelial junctions in steady-state and capture blood-borne macromolecules. Unlike other skin-resident macrophages that are reconstituted by bone marrow-derived progenitors after a genotoxic insult, these cells are replenished by an extramedullary radio-resistant and UV-sensitive Bmi1⁺ progenitor. Furthermore, they possess a distinctive anti-inflammatory transcriptional profile, which cannot be polarized under inflammatory conditions, and are involved in repair and remodeling functions for which other skin-resident macrophages appear dispensable. Based on all their properties, we define these macrophages as Skin Transendothelial Radio-resistant Anti-inflammatory Macrophages (STREAM) and postulate that their preservation is important for skin homeostasis.

DOI:http://dx.doi.org/10.7554/eLife.15251.001

The skin forms an essential barrier that defends our body from external damage. For this reason, it is important to understand the complex mechanisms involved in healing wounds and maintaining healthy skin. This could allow us to find effective treatments for skin diseases such as atopic dermatitis and psoriasis.

Immune cells called macrophages can protect the body in different ways depending on the signals they receive. Their protective roles include killing microbes that may cause disease, and helping to repair damaged tissues. Barreiro et al. have now investigated the roles of the macrophages in the skin by means of a number of complementary techniques, including a method called intravital microscopy that can image cells in a living organism.

The experiments revealed that a division of labor exists among the macrophages that reside in the skin of mice. Some macrophages help to trigger inflammatory responses in the skin. These immune cells disappear after being exposed to ionizing radiation (such as that used to treat cancer) but can be replaced via a bone marrow transplant. Other macrophages that help to repair tissues can survive being exposed to ionizing radiation but cannot resist high levels of ultraviolet light. Both types of macrophages perform unique and essential roles, and both types are necessary for maintaining healthy skin.

Barreiro et al. also discovered that the skin macrophages that help to repair tissues have the ability to move into blood vessels and take up substances from the blood. A question for future investigation is whether the macrophages perform this scavenging process to trigger a protective immune response in the nearby skin.

DOI:http://dx.doi.org/10.7554/eLife.15251.002

CD11c.DTR mice develop a fatal fulminant myocarditis after local or systemic treatment with diphtheria toxin

To assess the role of alveolar macrophages (AMs) during a pulmonary Aspergillus fumigatus infection AMs were depleted by intratracheal application of diphtheria toxin (DTX) to transgenic CD11c.DTR mice prior to fungal infection. Unexpectedly, all CD11c.DTR mice treated with DTX died within 4-5 days, whether being infected with A. fumigatus or not. Despite measurable impact of DTX on lung functional parameters, these constrictions could not explain the high mortality rate. Instead, DTX-treated CD11c.DTR animals developed fulminant myocarditis (FM) characterized by massive leukocyte infiltration and myocardial cell destruction, including central parts of the heart's stimulus transmission system. In fact, standard limb lead ECG recordings of diseased but not healthy mice showed a "Brugada"-like pattern with an abnormally high ST segment pointing to enhanced susceptibility for potential lethal arrhythmias. While CD11c.DTR mice are extensively used for the characterization of CD11c(+) cells, including dendritic cells, several studies have already mentioned adverse side effects following DTX treatment. Our results demonstrate that this limitation is based on severe myocarditis but not on the expected lung constrictions, and has to be taken into consideration if this animal model is used. Based on these properties, however, the CD11c.DTR mouse might serve as useful animal model for FM.

Inflammasome-Dependent Induction of Adaptive NK Cell Memory

Monobenzone is a pro-hapten that is exclusively metabolized by melanocytes, thereby haptenizing melanocyte-specific antigens, which results in cytotoxic autoimmunity specifically against pigmented cells. Studying monobenzone in a setting of contact hypersensitivity (CHS), we observed that monobenzone induced a long-lasting, melanocyte-specific immune response that was dependent on NK cells, yet fully intact in the absence of T- and B cells. Consistent with the concept of "memory NK cells," monobenzone-induced NK cells resided in the liver and transfer of these cells conferred melanocyte-specific immunity to naive animals. Monobenzone-exposed skin displayed macrophage infiltration and cutaneous lymph nodes showed an inflammasome-dependent influx of macrophages with a tissue-resident phenotype, coinciding with local NK cell activation. Indeed, macrophage depletion or the absence of the NLRP3 inflammasome, the adaptor protein ASC or interleukin-18 (IL-18) abolished monobenzone CHS, thereby establishing a non-redundant role for the NLRP3 inflammasome as a critical proinflammatory checkpoint in the induction of hapten-dependent memory NK cells.

In vivo depletion and genetic targeting of mouse intestinal CX3CR1(+) mononuclear phagocytes

Mononuclear phagocytes (MPs) are an essential component of the intestinal immune system. They are comprised of a few dendritic cell and macrophage subsets, all with the common ability to sample extracellular milieu and to discriminate between dangerous and innocuous signals. Despite the commonality, each MP subset acquires distinct developmental pathways and unique functions, likely to fulfill needs of the tissue in which they reside. Some MP subsets develop from monocytes and are distinguished by their expression of CX3C-chemokine receptor 1 (CX3CR1). This manuscript summarizes our expertise in vivo targeting of intestinal CX3CR1(+) MP subsets. The described tools might be useful for studies of CX3CR1(+) MP function in various murine experimental models, particularly under non-inflammatory conditions.

Selective Treg reconstitution during lymphopenia normalizes DC costimulation and prevents graft-versus-host disease

Regulatory T cells (Tregs) have been shown to enhance immune reconstitution and prevent graft-versus-host disease (GVHD) after hematopoietic stem cell transplantation; however, it is unclear how Tregs mediate these effects. Here, we developed a model to examine the mechanism of Treg-dependent regulation of immune reconstitution. Lymphopenic mice were selectively reconstituted with Tregs prior to transfer of conventional CD4+ T cells. Full Treg reconstitution prevented the rapid oligoclonal proliferation that gives rise to pathogenic CD4 effector T cells, while preserving the slow homeostatic form of lymphopenia-induced peripheral expansion that repopulates a diverse peripheral T cell pool. Treg-mediated CTLA-4-dependent downregulation of CD80/CD86 on DCs was critical for inhibition of rapid proliferation and was a function of the Treg/DC ratio achieved by reconstitution. In an allogeneic BM transplant model, selective Treg reconstitution before T cell transfer also normalized DC costimulation and provided complete protection against GVHD. In contrast, cotransfer of Tregs was not protective. Our results indicate that achieving optimal recovery from lymphopenia should aim to improve early Treg reconstitution in order to increase the relative number of Tregs to DCs and thereby inhibit spontaneous oligoclonal T cell proliferation.

Dendritic cell chimerism in oral mucosa of transplanted patients affected by graft-versus-host disease

OBJECTIVE

Graft-versus-host disease (GVHD) is one of the main complications after haematopoietic stem cell transplantation. Clinical features of GVHD include either an acute (aGVHD) or a chronic (cGVHD) condition that affects locations such as the oral mucosa. While the involvement of the host's dendritic cells (DCs) has been demonstrated in aGVHD, the origin (donor/host) and mechanisms underlying oral cGVHD have not been completely elucidated. In this study, we intend to determine the origin of DCs present in mucosal tissue biopsies from the oral cavity of transplanted patients affected by cGVHD.

METHODS

We purified DCs, from oral biopsies of three patients with cGVHD, through immunobeads and subsequently performed DNA extraction. The origin of the obtained DCs was determined by PCR amplification of 13 informative short tandem repeat (STR) alleles. We also characterised the DCs phenotype and the inflammatory infiltrate from biopsies of two patients by immunohistochemistry.

RESULTS

Clinical and histological features of the biopsies were concordant with oral cGVHD. We identified CD11c-, CD207- and CD1a-positive cells in the epithelium and beneath the basal layer. Purification of DCs from the mucosa of patients affected by post-transplantation cGVHD was >95%. PCR-STR data analysis of DCs DNA showed that 100% of analysed cells were of donor origin in all of the evaluated patients.

CONCLUSION

Our results demonstrate that resident DCs isolated from the oral tissue of allotransplanted patients affected by cGVHD are originated from the donor. Further research will clarify the role of DCs in the development and/or severity of oral cGVHD.

Purification of dendritic cell and macrophage subsets from the normal mouse small intestine

Mononuclear phagocytes are essential for protecting against pathogens breaching the intestinal mucosa and maintaining the integrity of the gastrointestinal tract. The mononuclear phagocyte family of the healthy intestine is represented by a small population of hematopoietic cells including dendritic cells and macrophages. Distinct mononuclear phagocyte subsets strategically accumulate within and below the mucosal epithelium and are distributed in the submucosa and muscularis externa. Shaped by its unique microenvironment, each mononuclear phagocyte subset is developmentally and functionally unique and phenotypically distinct. Here we summarize our recent advances on identifying and purifying various intestinal mononuclear phagocyte subsets by flow cytometry in the context of their developmental properties and location within the intestinal tissue.

The NF-κB regulator Bcl-3 modulates inflammation during contact hypersensitivity reactions in radioresistant cells

Bcl-3 is an atypical member of the IκB family. Bcl-3 functions as a cofactor of p50/NF-κB1 or p52/NF-κB2 homodimers in nuclei, where it modulates NF-κB-regulated transcription in a context-dependent way. Bcl-3 has tumorigenic potential, is critical in host defense of pathogens, and has been reported to ameliorate or exacerbate inflammation, depending on disease model. However, cell-specific functions of Bcl-3 remain largely unknown. Here, we explored the role of Bcl-3 in a contact hypersensitivity (CHS) mouse model, which depends on the interplay between keratinocytes and immune cells. Bcl-3-deficient mice exhibited an exacerbated and prolonged CHS response to oxazolone. Increased inflammation correlated with higher production of chemokines CXCL2, CXCL9, and CXCL10, and consequently increased recruitment of neutrophils and CD8(+) T cells. BM chimera experiments indicated that the ability of Bcl-3 to reduce the CHS response depended on Bcl-3 activity in radioresistant cells. Specific ablation of Bcl-3 in keratinocytes resulted in increased production of CXCL9 and CXCL10 and sustained recruitment of specifically CD8(+) T cells. These findings identify Bcl-3 as a critical player during the later stage of the CHS reaction to limit inflammation via actions in radioresistant cells, including keratinocytes.

Disease severity and mortality can be independently regulated in a mouse model of experimental graft versus host disease

Graft versus host disease (GVHD) is the major limitation of allogeneic hematopoietic stem cell transplantation (HSCT) presenting high mortality and morbidity rates. However, the exact cause of death is not completely understood and does not correlate with specific clinical and histological parameters of disease. Here we show, by using a semi-allogeneic mouse model of GVHD, that mortality and morbidity can be experimentally separated. We injected bone marrow-derived dendritic cells (BMDC) from NOD2/CARD15-deficient donors into semi-allogeneic irradiated chimaeras and observed that recipients were protected from death. However, no protection was observed regarding clinical or pathological scores up to 20 days after transplantation. Protection from death was associated with decreased bacterial translocation, faster hematologic recovery and epithelial integrity maintenance despite mononuclear infiltration at day 20 post-GVHD induction with no skew towards different T helper phenotypes. The protected mice recovered from aGVHD and progressively reached scores compatible with healthy animals. Altogether, our data indicate that severity and mortality can be separate events providing a model to study transplant-related mortality.

The chemokine system in innate immunity

Chemokines are chemotactic cytokines that control the migration and positioning of immune cells in tissues and are critical for the function of the innate immune system. Chemokines control the release of innate immune cells from the bone marrow during homeostasis as well as in response to infection and inflammation. They also recruit innate immune effectors out of the circulation and into the tissue where, in collaboration with other chemoattractants, they guide these cells to the very sites of tissue injury. Chemokine function is also critical for the positioning of innate immune sentinels in peripheral tissue and then, following innate immune activation, guiding these activated cells to the draining lymph node to initiate and imprint an adaptive immune response. In this review, we will highlight recent advances in understanding how chemokine function regulates the movement and positioning of innate immune cells at homeostasis and in response to acute inflammation, and then we will review how chemokine-mediated innate immune cell trafficking plays an essential role in linking the innate and adaptive immune responses.

Langerhans cell homeostasis and turnover after nonmyeloablative and myeloablative allogeneic hematopoietic cell transplantation

BACKGROUND

Langerhans cells (LCs) are self-renewing epidermal myeloid cells that can migrate and mature into dendritic cells. Recipient LCs that survive cytotoxic therapy given in preparation for allogeneic hematopoietic cell transplantation may prime donor T cells to mediate cutaneous graft-versus-host disease (GVHD). This possible association, however, has not been investigated in the setting of nonmyeloablative allografting.

METHODS

We prospectively studied the kinetics of LC-chimerism after sex-mismatched allogeneic hematopoietic cell transplantation with nonmyeloablative (n=23) or myeloablative (n=25) conditioning. Combined XY-FISH and Langerin-staining was used to assess donor LC-chimerism in skin biopsies obtained on days 28, 56, and 84 after transplant. The degree of donor LC-chimerism was correlated with the development of skin GVHD.

RESULTS

We observed significantly delayed donor LC-engraftment after nonmyeloablative transplantation compared with other hematopoietic compartments and compared with LC-engraftment after myeloablative conditioning. In most recipients of nonmyeloablative transplants, recipient LCs proliferated in situ, recruitment of donor-LCs was delayed by two months, and full donor LC-chimerism was only reached by day 84 after transplant. Although persistence of host LCs on day-28 after transplant was not predictive for acute or chronic skin GVHD, the recruitment of donor-derived LCs was associated with nonspecific inflammatory infiltrates (P=0.009).

CONCLUSIONS

These results show that LCs can self-renew locally but are replaced by circulating precursors even after minimally toxic nonmyeloablative transplant conditioning. Cutaneous inflammation accompanies donor LC-engraftment, but differences in LC conversion-kinetics do not predict clinical or histopathological GVHD.

Monocyte recruitment to the dermis and differentiation to dendritic cells increases the targets for dengue virus replication

Dengue virus (DENV) causes the most prevalent arthropod-borne viral disease in humans. Although Aedes mosquitoes transmit DENV when probing for blood in the skin, no information exists on DENV infection and immune response in the dermis, where the blood vessels are found. DENV suppresses the interferon response, replicates, and causes disease in humans but not wild-type mice. Here, we used mice lacking the interferon-α/β receptor (Ifnar(-/-)), which had normal cell populations in the skin and were susceptible to intradermal DENV infection, to investigate the dynamics of early DENV infection of immune cells in the skin. CD103(+) classical dendritic cells (cDCs), Ly6C(-) CD11b(+) cDCs, and macrophages in the steady-state dermis were initial targets of DENV infection 12-24 hours post-inoculation but then decreased in frequency. We demonstrated recruitment of adoptively-transferred Ly6C(high) monocytes from wild-type and Ifnar(-/-) origin to the DENV-infected dermis and differentiation to Ly6C(+) CD11b(+) monocyte-derived DCs (moDCs), which became DENV-infected after 48 hours, and were then the major targets for virus replication. Ly6C(high) monocytes that entered the DENV-infected dermis expressed chemokine receptor CCR2, likely mediating recruitment. Further, we show that ∼ 100-fold more hematopoietic cells in the dermis were DENV-infected compared to Langerhans cells in the epidermis. Overall, these results identify the dermis as the main site of early DENV replication and show that DENV infection in the skin occurs in two waves: initial infection of resident cDCs and macrophages, followed by infection of monocytes and moDCs that are recruited to the dermis. Our study reveals a novel viral strategy of exploiting monocyte recruitment to increase the number of targets for infection at the site of invasion in the skin and highlights the skin as a potential site for therapeutic action or intradermal vaccination.

Individual differences in the peripheral immune system promote resilience versus susceptibility to social stress

Depression and anxiety disorders are associated with increased release of peripheral cytokines; however, their functional relevance remains unknown. Using a social stress model in mice, we find preexisting individual differences in the sensitivity of the peripheral immune system that predict and promote vulnerability to social stress. Cytokine profiles were obtained 20 min after the first social stress exposure. Of the cytokines regulated by stress, IL-6 was most highly up-regulated only in mice that ultimately developed a susceptible behavioral phenotype following a subsequent chronic stress, and levels remained elevated for at least 1 mo. We confirmed a similar elevation of serum IL-6 in two separate cohorts of patients with treatment-resistant major depressive disorder. Before any physical contact in mice, we observed individual differences in IL-6 levels from ex vivo stimulated leukocytes that predict susceptibility versus resilience to a subsequent stressor. To shift the sensitivity of the peripheral immune system to a pro- or antidepressant state, bone marrow (BM) chimeras were generated by transplanting hematopoietic progenitor cells from stress-susceptible mice releasing high IL-6 or from IL-6 knockout (IL-6(-/-)) mice. Stress-susceptible BM chimeras exhibited increased social avoidance behavior after exposure to either subthreshold repeated social defeat stress (RSDS) or a purely emotional stressor termed witness defeat. IL-6(-/-) BM chimeric and IL-6(-/-) mice, as well as those treated with a systemic IL-6 monoclonal antibody, were resilient to social stress. These data establish that preexisting differences in stress-responsive IL-6 release from BM-derived leukocytes functionally contribute to social stress-induced behavioral abnormalities.

Tailored immunity by skin antigen-presenting cells

Skin vaccination aims at targeting epidermal and dermal antigen-presenting cells (APCs), indeed many subsets of different origin endowed with various functions populate the skin. The idea that the skin could represent a particularly potent site to induce adaptive and protective immune response emerged after the success of vaccinia virus vaccination by skin scarification. Recent advances have shown that multiple subsets of APCs coexist in the skin and participate in immunity to infectious diseases. Induction of an adaptive immune response depends on the initial recognition and capture of antigens by skin APCs and their transport to lymphoid organs. Innovative strategies of vaccination have thus been developed to target skin APCs for tailored immunity, hence this review will discuss recent insights into skin APC subsets characterization and how they can shape adaptive immune responses.

Individual differences in the peripheral immune system promote resilience versus susceptibility to social stress

Depression and anxiety disorders are associated with increased release of peripheral cytokines; however, their functional relevance remains unknown. Using a social stress model in mice, we find preexisting individual differences in the sensitivity of the peripheral immune system that predict and promote vulnerability to social stress. Cytokine profiles were obtained 20 min after the first social stress exposure. Of the cytokines regulated by stress, IL-6 was most highly up-regulated only in mice that ultimately developed a susceptible behavioral phenotype following a subsequent chronic stress, and levels remained elevated for at least 1 mo. We confirmed a similar elevation of serum IL-6 in two separate cohorts of patients with treatment-resistant major depressive disorder. Before any physical contact in mice, we observed individual differences in IL-6 levels from ex vivo stimulated leukocytes that predict susceptibility versus resilience to a subsequent stressor. To shift the sensitivity of the peripheral immune system to a pro- or antidepressant state, bone marrow (BM) chimeras were generated by transplanting hematopoietic progenitor cells from stress-susceptible mice releasing high IL-6 or from IL-6 knockout (IL-6(-/-)) mice. Stress-susceptible BM chimeras exhibited increased social avoidance behavior after exposure to either subthreshold repeated social defeat stress (RSDS) or a purely emotional stressor termed witness defeat. IL-6(-/-) BM chimeric and IL-6(-/-) mice, as well as those treated with a systemic IL-6 monoclonal antibody, were resilient to social stress. These data establish that preexisting differences in stress-responsive IL-6 release from BM-derived leukocytes functionally contribute to social stress-induced behavioral abnormalities.

Origin of Langerhans cells in normal skin and chronic GVHD after hematopoietic stem-cell transplantation

Chronic graft-versus-host disease (cGVHD) is a common complication following allogeneic stem-cell transplantation (SCT). Past studies have implicated the persistence of host antigen-presenting cells (APCs) in GVHD. Our objective was to determine the frequency of host Langerhans cells (LCs) in normal skin post-SCT and ask if their persistence could predict cGVHD. Biopsies of normal skin from 124 sex-mismatched T-cell-replete allogenic SCT recipients were taken 100 days post-transplant. Patients with acute GVHD and those with <9 months of follow-up were excluded and prospective follow-up information was collected from remaining 22 patients. CD1a staining and X and Y chromosome in-situ hybridization were performed to label LCs and to identify their host or donor origin. At 3 months, 59 ± 5% of LCs were host derived. The density of LCs and the proportion of host-derived LCs were similar between patients that did or did not develop cGVHD. Most LCs in the skin remained of host origin 3 months after SCT regardless of cGVHD status. This finding is in line with the redundant role of LCs in acute GVHD initiation uncovered in recent experimental models.

Tracking and quantification of dendritic cell migration and antigen trafficking between the skin and lymph nodes

Skin-derived dendritic cells (DCs) play a crucial role in the maintenance of immune homeostasis due to their role in antigen trafficking from the skin to the draining lymph nodes (dLNs). To quantify the spatiotemporal regulation of skin-derived DCs in vivo, we generated knock-in mice expressing the photoconvertible fluorescent protein KikGR. By exposing the skin or dLN of these mice to violet light, we were able to label and track the migration and turnover of endogenous skin-derived DCs. Langerhans cells and CD103⁺DCs, including Langerin⁺CD103⁺dermal DCs (DDCs), remained in the dLN for 4–4.5 days after migration from the skin, while CD103⁻DDCs persisted for only two days. Application of a skin irritant (chemical stress) induced a transient >10-fold increase in CD103⁻DDC migration from the skin to the dLN. Tape stripping (mechanical injury) induced a long-lasting four-fold increase in CD103⁻DDC migration to the dLN and accelerated the trafficking of exogenous protein antigens by these cells. Both stresses increased the turnover of CD103⁻DDCs within the dLN, causing these cells to die within one day of arrival. Therefore, CD103⁻DDCs act as sentinels against skin invasion that respond with increased cellular migration and antigen trafficking from the skin to the dLNs.

Fcγ receptor III and Fcγ receptor IV on macrophages drive autoimmune valvular carditis in mice

OBJECTIVE

Arthritis and valvular carditis coexist in several human rheumatic diseases, including systemic lupus erythematosus, rheumatic fever, and rheumatoid arthritis. T cell receptor-transgenic K/BxN mice develop spontaneous autoantibody-associated arthritis and valvular carditis. The common Fc receptor γ (FcRγ) signaling chain is required for carditis to develop in K/BxN mice. FcRγ pairs with numerous receptors in a variety of cells. The aim of this study was to identify the FcRγ-associated receptors and Fcγ receptor (FcγR)-expressing cells that mediate valvular carditis in this model.

METHODS

We bred K/BxN mice lacking the genes that encode activating Fcγ receptors (FcγRI, FcγRIII, and FcγRIV), and we assessed these mice for valvular carditis. We similarly evaluated complement component C3-deficient K/BxN mice. Immunohistochemistry, bone marrow transplantation, and macrophage depletion were used to define the key FcRγ-expressing cell type.

RESULTS

Genetic deficiency of only one of the activating Fcγ receptors did not prevent carditis, whereas deficiency of all 3 activating Fcγ receptors did. Further analysis demonstrated that FcγRIII and FcγRIV were the key drivers of valve inflammation; FcγRI was dispensable. C3 was not required. FcRγ expression by radioresistant cells was critical for valvular carditis to develop, and further analysis indicated that macrophages were the key candidate FcγR-expressing effectors of carditis.

CONCLUSION

FcγRIII and FcγRIV act redundantly to promote valvular carditis in K/BxN mice with systemic autoantibody-associated arthritis. Macrophage depletion reduced the severity of valve inflammation. These findings suggest that pathogenic autoantibodies engage Fcγ receptors on macrophages to drive valvular carditis. Our study provides new insight into the pathogenesis of cardiovascular inflammation in the setting of autoantibody-associated chronic inflammatory diseases.

Second-generation Langerhans cells originating from epidermal precursors are essential for CD8+ T cell priming

In vivo studies questioned the ability of Langerhans cells (LCs) to mediate CD8(+) T cell priming. To address this issue, we used intradermal immunization with plasmid DNA, a system in which activation of CD8(+) T cells depends on delayed kinetics of Ag presentation. We found that dendritic cells (DCs) located in the skin at the time of immunization have limited ability to activate CD8(+) T cells. This activity was mediated by a second generation of DCs that differentiated in the skin several days after immunization, as well as by lymph node-resident DCs. Intriguingly, CD8(+) T cell responses were not affected following treatment with clodronate liposomes, immunization of CCR2(-/-) mice, or local neutralization of CCL20. This suggests that local, rather than blood-derived, DC precursors mediate CD8(+) T cell priming. Analysis of DC differentiation in the immunized skin revealed a gradual increase in the number of CD11c(+) cells, which reached their maximum 2 wk after immunization. A similar differentiation kinetics was observed for LCs, with the majority of differentiating LCs proliferating in situ from epidermal precursors. By using B6/Langerin-diphtheria toxin receptor chimeric mice and LC ablation, we demonstrated that epidermal LCs were crucial for the elicitation of CD8(+) T cell responses in vivo. Furthermore, LCs isolated from lymph nodes 2 wk after immunization contained the immunization plasmid and directly activated Ag-specific CD8(+) T cells ex vivo. Thus, these results indicate that second-generation Ag-expressing LCs differentiating from epidermal precursors directly prime CD8(+) T cells and are essential for optimal cellular immune responses following immunization with plasmid DNA.

Chemokines and chemokine receptors: positioning cells for host defense and immunity

Chemokines are chemotactic cytokines that control the migratory patterns and positioning of all immune cells. Although chemokines were initially appreciated as important mediators of acute inflammation, we now know that this complex system of approximately 50 endogenous chemokine ligands and 20 G protein-coupled seven-transmembrane signaling receptors is also critical for the generation of primary and secondary adaptive cellular and humoral immune responses. Recent studies demonstrate important roles for the chemokine system in the priming of naive T cells, in cell fate decisions such as effector and memory cell differentiation, and in regulatory T cell function. In this review, we focus on recent advances in understanding how the chemokine system orchestrates immune cell migration and positioning at the organismic level in homeostasis, in acute inflammation, and during the generation and regulation of adoptive primary and secondary immune responses in the lymphoid system and peripheral nonlymphoid tissue.

CD301b⁺ dermal dendritic cells drive T helper 2 cell-mediated immunity

Unlike other types of T helper (Th) responses, whether the development of Th2 cells requires instruction from particular subset of dendritic cells (DCs) remains unclear. By using an in vivo depletion approach, we have shown that DCs expressing CD301b were required for the generation of Th2 cells after subcutaneous immunization with ovalbumin (OVA) along with papain or alum. CD301b⁺ DCs are distinct from epidermal or CD207⁺ dermal DCs (DDCs) and were responsible for transporting antigen injected subcutaneously with Th2-type adjuvants. Transient depletion of CD301b⁺ DCs resulted in less effective accumulation and decreased expression of CD69 by polyclonal CD4⁺ T cells in the lymph node. Moreover, despite intact cell division and interferon-γ production, CD301b⁺ DC depletion led to blunted interleukin-4 production by OVA-specific OT-II transgenic CD4⁺ T cells and significantly impaired Th2 cell development upon infection with Nippostrongylus brasiliensis. These results reveal CD301b⁺ DDCs as the key mediators of Th2 immunity.

Subcutaneous cholera toxin exposure induces potent CD103⁺ dermal dendritic cell activation and migration

CD103⁺ dermal dendritic cells (dDCs) are a recently described DC subset of the skin shown to be the principal migratory DCs capable of efficiently cross-presenting antigens and activating CD8⁺ T cells. Harnessing their activity would promote vaccine efficacy, but it has been unclear how this can be achieved. We tested a panel of adjuvants for their ability to affect dDCs. In comparison to the other adjuvants tested, the capacity of cholera toxin (CT) to induce the migration of dDCs was unique. Within 24 h of CT injection, large numbers of highly activated dDCs (including CD103⁺ dDCs) migrated to the draining lymph nodes and cross-presented coinjected antigens, potently activating naïve CD8⁺ T cells. Peptide vaccines adjuvanted with CT induced T-cell responses uniquely characterized by dynamic cytokine responses including the production of IL-2, and such vaccines were protective in situations reliant on CD8⁺ T-cell responses, including liver-stage Plasmodium challenge, or tumor challenge. This study is the first to examine the effects of adjuvants on CD103⁺ dDCs and identifies CT as a prototypical adjuvant for the activation of CD103⁺ dDCs, opening the way to development of vaccines and adjuvants that specifically target dDCs and generate effective CD8⁺ T-cell responses.

Microglia during development and aging

Microglia are critical nervous system-specific cells influencing brain development, maintenance of the neural environment, response to injury, and repair. They contribute to neuronal proliferation and differentiation, pruning of dying neurons, synaptic remodeling and clearance of debris and aberrant proteins. Colonization of the brain occurs during gestation with an expansion following birth with localization stimulated by programmed neuronal death, synaptic pruning, and axonal degeneration. Changes in microglia phenotype relate to cellular processes including specific neurotransmitter, pattern recognition, or immune-related receptor activation. Upon activation, microglia cells have the capacity to release a number of substances, e.g., cytokines, chemokines, nitric oxide, and reactive oxygen species, which could be detrimental or beneficial to the surrounding cells. With aging, microglia shift their morphology and may display diminished capacity for normal functions related to migration, clearance, and the ability to shift from a pro-inflammatory to an anti-inflammatory state to regulate injury and repair. This shift in microglia potentially contributes to increased susceptibility and neurodegeneration as a function of age. In the current review, information is provided on the colonization of the brain by microglia, the expression of various pattern recognition receptors to regulate migration and phagocytosis, and the shift in related functions that occur in normal aging.

Immunogenicity of subcutaneously administered therapeutic proteins--a mechanistic perspective

The administration of therapeutic proteins via the subcutaneous route (sc) is desired for compliance and convenience, but could be challenging due to perceived immunogenic potential or unwanted immune responses. There are clinical and preclinical data supporting as well as refuting the generalized notion that sc is more immunogenic. We provide a mechanistic perspective of immunogenicity of therapeutic proteins administered via the sc route and discuss strategies and opportunities for novel therapeutic approaches to mitigate immunogenicity.

The dendritic cell lineage: ontogeny and function of dendritic cells and their subsets in the steady state and the inflamed setting

Dendritic cells (DCs) form a remarkable cellular network that shapes adaptive immune responses according to peripheral cues. After four decades of research, we now know that DCs arise from a hematopoietic lineage distinct from other leukocytes, establishing the DC system as a unique hematopoietic branch. Recent work has also established that tissue DCs consist of developmentally and functionally distinct subsets that differentially regulate T lymphocyte function. This review discusses major advances in our understanding of the regulation of DC lineage commitment, differentiation, diversification, and function in situ.

Immunobiology of dendritic cells and the influence of HIV infection

Recent progress in phenotyping of human dendritic cells (DCs) has allowed a closer alignment of the classification and functions of murine and human dendritic cell subsets. Marked differences in the functions of these human DC subsets and their response to HIV infection have become apparent, relevant to HIV pathogenesis and vaccine and microbicide development. Systems biology approaches to studying HIV uptake and infection of dendritic cells has revealed how markedly HIV subverts their functions, especially in relation to the trafficking pathways and viral transfer to T cells. Furthermore the interactions between DCs and other innate immune cells, NK cells, NKT cells and gamma delta T cells are now known to influence DC and T cell function and are also disturbed by HIV infection in vitro and in vivo. Such cellular interactions are potential targets for vaccine adjuvants and immunotherapy.

Advantages and limitations of mouse models to deplete dendritic cells

Dendritic cells (DCs) play a key role in regulating innate and adaptive immunity. Our understanding of DC biology has benefited from studies in CD11c.DTR and CD11c.DOG mouse models that use the CD11c promoter to express a diphtheria toxin (DT) receptor transgene to inducibly deplete CD11c(+) cells. Other models to inducibly deplete specific DC subsets upon administration of DT have also been generated. However, most models suffer from limitations such as depletion of additional cell types or the requirement to be used as radiation chimeras. Moreover, CD11c.DTR and CD11c.DOG mice have recently been reported to display neutrophilia and monocytosis upon DT injection. We discuss here some of the limitations that should be taken into consideration when interpreting results obtained with mouse models of DC ablation.

The dendritic cell lineage: ontogeny and function of dendritic cells and their subsets in the steady state and the inflamed setting

Dendritic cells (DCs) form a remarkable cellular network that shapes adaptive immune responses according to peripheral cues. After four decades of research, we now know that DCs arise from a hematopoietic lineage distinct from other leukocytes, establishing the DC system as a unique hematopoietic branch. Recent work has also established that tissue DCs consist of developmentally and functionally distinct subsets that differentially regulate T lymphocyte function. This review discusses major advances in our understanding of the regulation of DC lineage commitment, differentiation, diversification, and function in situ.

Aeroallergen challenge promotes dendritic cell proliferation in the airways

Aeroallergen provocation induces the rapid accumulation of CD11c(+)MHC class II (MHC II)(+) dendritic cells (DCs) in the lungs, which is driven by an increased recruitment of blood-derived DC precursors. Recent data show, however, that well-differentiated DCs proliferate in situ in various tissues. This may also contribute to their allergen-induced expansion; therefore, we studied DC proliferation in the airways of mice in the steady state and after local aeroallergen provocation. Confocal whole-mount microscopy was used to visualize proliferating DCs in different microanatomical compartments of the lung. We demonstrate that in the steady state, CD11c(+)MHC II(+) DCs proliferate in both the epithelial and subepithelial layers of the airway mucosa as well as in the lung parenchyma. A 1-h pulse of the nucleotide 5-ethynyl-2'-deoxyuridine was sufficient to label 5% of DCs in both layers of the airway mucosa. On the level of whole-lung tissue, 3-5% of both CD11b(+) and CD11b(-) DC populations and 0.3% of CD11c(+)MHC II(low) lung macrophages incorporated 5-ethynyl-2'-deoxyuridine. Aeroallergen provocation caused a 3-fold increase in the frequency of locally proliferating DCs in the airway mucosa. This increase in mucosal DC proliferation was later followed by an elevation in the number of DCs. The recruitment of monocyte-derived inflammatory DCs contributed to the increasing number of DCs in the lung parenchyma, but not in the airway mucosa. We conclude that local proliferation significantly contributes to airway DC homeostasis in the steady state and that it is the major mechanism underlying the expansion of the mucosal epithelial/subepithelial DC network in allergic inflammation.

Targeting cells causing split tolerance allows fully allogeneic islet survival with minimal conditioning in NOD mixed chimeras

Donor-specific tolerance induced by mixed chimerism is one approach that may eliminate the need for long-term immunosuppressive therapy, while preventing chronic rejection of an islet transplant. However, even in the presence of chimerism it is possible for certain donor tissues or cells to be rejected whereas others from the same donor are accepted (split tolerance). We previously developed a nonmyeloablative protocol that generated mixed chimerism across full major histocompatability complex plus minor mismatches in NOD (nonobese diabetic) mice, however, these chimeras demonstrated split tolerance. In this study, we used radiation chimeras and found that the radiosensitive component of NOD has a greater role in the split tolerance NOD mice develop. We then show that split tolerance is mediated primarily by preexisting NOD lymphocytes and have identified T cells, but not NK cells or B cells, as cells that both resist chimerism induction and mediate split tolerance. Finally, after recognizing the barrier that preexisting T cells impose on the generation of fully tolerant chimeras, the chimerism induction protocol was refined to include nonmyeloablative recipient NOD T cell depletion which generated long-term mixed chimerism across fully allogeneic barriers. Furthermore, these chimeric NOD mice are immunocompetent, diabetes free and accept donor islet allografts.

Identification of lineage relationships and novel markers of blood and skin human dendritic cells

The lineage relationships and fate of human dendritic cells (DCs) have significance for a number of diseases including HIV where both blood and tissue DCs may be infected. We used gene expression profiling of human monocyte and DC subpopulations sorted directly from blood and skin to define the lineage relationships. We also compared these with monocyte-derived DCs (MDDCs) and MUTZ3 Langerhans cells (LCs) to investigate their relevance as model skin DCs. Hierarchical clustering analysis showed that myeloid DCs clustered according to anatomical origin rather than putative lineage. Plasmacytoid DCs formed the most discrete cluster, but ex vivo myeloid cells formed separate clusters of cells both in blood and in skin. Separate and specific DC populations could be determined within skin, and the proportion of CD14(+) dermal DCs (DDCs) was reduced and CD1a(+) DDCs increased during culture, suggesting conversion to CD1a(+)-expressing cells in situ. This is consistent with origin of the CD1a(+) DDCs from a local precursor rather than directly from circulating blood DCs or monocyte precursors. Consistent with their use as model skin DCs, the in vitro-derived MDDC and MUTZ3 LC populations grouped within the skin DC cluster. MDDCs clustered most closely to CD14(+) DDCs; furthermore, common unique patterns of C-type lectin receptor expression were identified between these two cell types. MUTZ3 LCs, however, did not cluster closely with ex vivo-derived LCs. We identified differential expression of novel genes in monocyte and DC subsets including genes related to DC surface receptors (including C-type lectin receptors, TLRs, and galectins).

Reduced efficacy of mesenchymal stromal cells in preventing graft-versus-host disease in an in vivo model of haploidentical bone marrow transplant with leukemia

Mesenchymal stromal cell (MSC) immunosuppressive properties have been applied to treat graft-versus-host disease (GVHD) in allogeneic hematopoietic stem cell transplants (HSCTs). We have previously demonstrated that MSC infusions early after haplo-HSCT prevent GVHD in a haploidentical-HSCT mouse model. Now, we investigated the impact that MSCs' immunosuppressive properties have on the graft-versus-leukemia (GVL) effect. First, to mimic a chronic myeloid leukemia (CML) relapse after a haploidentical HSCT, lethally irradiated mice were coinfused with haploidentical donor bone marrow cells plus syngenic hematopoietic progenitors transduced with a retroviral vector encoding both the BCR/ABL oncogene and the ΔNGFR marker gene. As expected, a CML-like myeloproliferative syndrome developed in all the recipient animals. The addition of haploidentical splenocytes to the transplanted graft prevented CML development by a GVL effect, and all transplanted recipients died of GVHD. This GVL mouse model allowed us to investigate the impact of MSCs infused to prevent GVHD on days 0, 7, and 14 after HSCT, on the GVL effect, expecting an increase in leukemic relapse. Strikingly, a high mortality of the recipients was observed, caused by GVHD, and only few leukemic cells were detected in the recipient animals. In contrast, GVHD prevention by MSCs in the absence of BCR/ABL leukemic cells resulted in a significant survival of the recipients. In vitro data pointed to an inability of MSCs to control strong CTLs responses against BCR/ABL. Our results show that, although an evident increase in leukemic relapses induced by MSCs could not be detected, they showed a reduced efficacy in preventing GVHD that precluded us to draw clear conclusions on MSCs' impact over GVL effect.