Fetal and neonatal murine skin harbors Langerhans cell precursors

Sexual dimorphism in skin immunity is mediated by an androgen-ILC2-dendritic cell axis

Males and females exhibit profound differences in immune responses and disease susceptibility. However, the factors responsible for sex differences in tissue immunity remain poorly understood. Here, we uncovered a dominant role for type 2 innate lymphoid cells (ILC2s) in shaping sexual immune dimorphism within the skin. Mechanistically, negative regulation of ILC2s by androgens leads to a reduction in dendritic cell accumulation and activation in males, along with reduced tissue immunity. Collectively, our results reveal a role for the androgen-ILC2-dendritic cell axis in controlling sexual immune dimorphism. Moreover, this work proposes that tissue immune set points are defined by the dual action of sex hormones and the microbiota, with sex hormones controlling the strength of local immunity and microbiota calibrating its tone.

Lrig1- and Wnt-dependent niches dictate segregation of resident immune cells and melanocytes in murine tail epidermis

The barrier-forming, self-renewing mammalian epidermis comprises keratinocytes, pigment-producing melanocytes and resident immune cells as first-line host defense. In murine tail skin, interfollicular epidermis patterns into pigmented ‘scale’ and hypopigmented ‘interscale’ epidermis. Why and how mature melanocytes accumulate in scale epidermis is unresolved. Here, we delineate a cellular hierarchy among epidermal cell types that determines skin patterning. Already during postnatal development, melanocytes co-segregate with newly forming scale compartments. Intriguingly, this process coincides with partitioning of both Langerhans cells and dendritic epidermal T cells to interscale epidermis, suggesting functional segregation of pigmentation and immune surveillance. Analysis of non-pigmented mice and of mice lacking melanocytes or resident immune cells revealed that immunocyte patterning is melanocyte and melanin independent and, vice versa, immune cells do not control melanocyte localization. Instead, genetically enforced progressive scale fusion upon Lrig1 deletion showed that melanocytes and immune cells dynamically follow epithelial scale:interscale patterns. Importantly, disrupting Wnt-Lef1 function in keratinocytes caused melanocyte mislocalization to interscale epidermis, implicating canonical Wnt signaling in organizing the pigmentation pattern. Together, this work uncovers cellular and molecular principles underlying the compartmentalization of tissue functions in skin.

Summary: Pigmentation and immune surveillance functions in murine tail skin are spatially segregated by Lrig1- and Wnt-Lef1-dependent keratinocyte lineages that control the partitioning of melanocytes and tissue-resident immune cells into distinct epidermal niches.

Langerhans Cells From Mice at Birth Express Endocytic- and Pattern Recognition-Receptors, Migrate to Draining Lymph Nodes Ferrying Antigen and Activate Neonatal T Cells in vivo

Antigen capturing at the periphery is one of the earliest, crucial functions of antigen-presenting cells (APCs) to initiate immune responses. Langerhans cells (LCs), the epidermal APCs migrate to draining lymph nodes (DLNs) upon acquiring antigens. An arsenal of endocytic molecules is available to this end, including lectins and pathogen recognition receptors (PRRs). However, cutaneous LCs are poorly defined in the early neonatal period. We assessed endocytic molecules expression in situ: Mannose (CD206)-, Scavenger (SRA/CD204)-, Complement (CD2l, CDllb)-, and Fc-Receptors (CD16/32, CD23) as well as CD1d, CD14, CD205, Langerin (CD207), MHCII, and TLR4 in unperturbed epidermal LCs from both adult and early neonatal mice. As most of these markers were negative at birth (day 0), LC presence was revealed with the conspicuous, epidermal LC-restricted ADPase (and confirmed with CD45) staining detecting that they were as numerous as adult ones. Unexpectedly, most LCs at day 0 expressed CD14 and CD204 while very few were MHCII+ and TLR4+. In contrast, adult LCs lacked all these markers except Langerin, CD205, CD11b, MHCII and TLR4. Intriguingly, the CD204+ and CD14+ LCs predominant at day 0, apparently disappeared by day 4. Upon cutaneous FITC application, LCs were reduced in the skin and a CD204+MHCII+FITC+ population with high levels of CD86 subsequently appeared in DLNs, with a concomitant increased percentage of CD3+CD69+ T cells, strongly suggesting that neonatal LCs were able both to ferry the cutaneous antigen into DLNs and to activate neonatal T cells in vivo. Cell cycle analysis indicated that neonatal T cells in DLNs responded with proliferation. Our study reveals that epidermal LCs are present at birth, but their repertoire of endocytic molecules and PRRs differs to that of adult ones. We believe this to be the first description of CDl4, CD204 and TLR4 in neonatal epidermal LCs in situ. Newborns' LCs express molecules to detect antigens during early postnatal periods, are able to take up local antigens and to ferry them into DLNs conveying the information to responsive neonatal T cells.

The late endosomal adaptor molecule p14 (LAMTOR2) regulates TGFβ1-mediated homeostasis of Langerhans cells

Langerhans cells (LCs), a sub-population of dendritic cells (DCs) in the skin, participate in the regulation of immunity and peripheral tolerance. The adaptor molecule p14 is part of the late endosomal/lysosomal adaptor and mitogen-activated protein kinase and mammalian target of rapamycin (mTOR) activator/regulator (LAMTOR) complex, which mediates the activation of lysosome-associated extracellular signaling-regulated kinase (ERK) and the mTOR cascade. In previous work, we demonstrated that CD11c-specific deficiency of p14 disrupts LC homeostasis by affecting the LAMTOR-mediated ERK and mTOR signaling. In this study, we extended our analysis on p14 deficiency specifically in LCs. Langerin-specific ablation of p14 caused a complete loss of LCs, accompanied by an increased maturational phenotype of LCs. The absence of LCs in p14-deficient mice reduced contact hypersensitivity (CHS) responses to the contact sensitizer trinitrochlorobenzene. Analysis using bone marrow-derived DCs (BMDCs) revealed that p14 deficiency in DCs/LCs interfered with the LC-relevant transforming growth factor β1 (TGFβ1) pathway, by lowering TGFβ receptor II expression on BMDCs and LCs, as well as surface binding of TGFβ1 on BMDCs. We conclude that p14 deficiency affects TGFβ1 sensitivity of LCs, which is mandatory for their homeostasis and subsequently for their immunological function during CHS.

Human embryonic epidermis contains a diverse Langerhans cell precursor pool

Despite intense efforts, the exact phenotype of the epidermal Langerhans cell (LC) precursors during human ontogeny has not been determined yet. These elusive precursors are believed to migrate into the embryonic skin and to express primitive surface markers, including CD36, but not typical LC markers such as CD1a, CD1c and CD207. The aim of this study was to further characterize the phenotype of LC precursors in human embryonic epidermis and to compare it with that of LCs in healthy adult skin. We found that epidermal leukocytes in first trimester human skin are negative for CD34 and heterogeneous with regard to the expression of CD1c, CD14 and CD36, thus contrasting the phenotypic uniformity of epidermal LCs in adult skin. These data indicate that LC precursors colonize the developing epidermis in an undifferentiated state, where they acquire the definitive LC marker profile with time. Using a human three-dimensional full-thickness skin model to mimic in vivo LC development, we found that FACS-sorted, CD207(-) cord blood-derived haematopoietic precursor cells resembling foetal LC precursors but not CD14(+)CD16(-) blood monocytes integrate into skin equivalents, and without additional exogenous cytokines give rise to cells that morphologically and phenotypically resemble LCs. Overall, it appears that CD14(-) haematopoietic precursors possess a much higher differentiation potential than CD14(+) precursor cells.

The clash of Langerhans cell homeostasis in skin: Should I stay or should I go?

Langerhans cells (LC), the skin epidermal contingent of dendritic cells (DC), possess an exceptional life cycle and developmental origin. LC, like all mature blood cells, develop from haematopoietic stem cells (HSC) through successive steps of lineage commitment and differentiation. However, LC development is different to that of other DC subsets and not yet fully understood. Haematopoietic cell fate decisions are instructed by specific growth factors and cytokines produced in specialized microenvironments or niches. Upon ligand binding the cognate surface receptors on HSC and further restricted progenitor cells regulate the signalling pathways that eventually leads to the execution of lineage-determining genetic programs. In this review we focus on a specific set of surface receptor kinases that have been identified as critical regulators of LC development using genetically modified mice. Recent studies suggest for some of these kinases to impact on LC/LC progenitor interaction with the local niche by regulating adhesion and/or migration. During embryonic development, in wound healing and aberrantly in tumour invasion the same kinase receptors control a genetic program known as epithelial-to-mesenchymal-transition (EMT). We will discuss how EMT and its reverse program of mesenchymal-to-epithelial-transition (MET) can serve as universal concepts operating also in LC development.

Identification of bone morphogenetic protein 7 (BMP7) as an instructive factor for human epidermal Langerhans cell differentiation

Bone morphogenetic protein 7 (BMP7) promotes the differentiation of Langerhans cells in the epidermis during prenatal development.

Human Langerhans cell (LC) precursors populate the epidermis early during prenatal development and thereafter undergo massive proliferation. The prototypic antiproliferative cytokine TGF-β1 is required for LC differentiation from human CD34⁺ hematopoietic progenitor cells and blood monocytes in vitro. Similarly, TGF-β1 deficiency results in LC loss in vivo. However, immunohistology studies revealed that human LC niches in early prenatal epidermis and adult basal (germinal) keratinocyte layers lack detectable TGF-β1. Here we demonstrated that these LC niches express high levels of bone morphogenetic protein 7 (BMP7) and that Bmp7-deficient mice exhibit substantially diminished LC numbers, with the remaining cells appearing less dendritic. BMP7 induces LC differentiation and proliferation by activating the BMP type-I receptor ALK3 in the absence of canonical TGF-β1–ALK5 signaling. Conversely, TGF-β1–induced in vitro LC differentiation is mediated via ALK3; however, co-induction of ALK5 diminished TGF-β1–driven LC generation. Therefore, selective ALK3 signaling by BMP7 promotes high LC yields. Within epidermis, BMP7 shows an inverse expression pattern relative to TGF-β1, the latter induced in suprabasal layers and up-regulated in outer layers. We observed that TGF-β1 inhibits microbial activation of BMP7-generated LCs. Therefore, TGF-β1 in suprabasal/outer epidermal layers might inhibit LC activation, resulting in LC network maintenance.

The late endosomal adaptor molecule p14 (LAMTOR2) represents a novel regulator of Langerhans cell homeostasis

Langerhans cells (LCs) are dendritic cells (DCs) residing in epithelia, where they critically regulate immunity and tolerance. The p14 adaptor molecule is part of the late endosomal/LAMTOR (lysosomal adaptor and mitogen-activated protein kinase and mammalian target of rapamycin [mTOR] activator/regulator) complex, thereby contributing to the signal transduction of the extracellular signaling-regulated kinase (ERK) and the mTOR cascade. Furthermore, p14 represents an important regulator for endosomal sorting processes within the cell. Mutated, dysfunctional p14 leads to a human immunodeficiency disorder with endosomal/lysosomal defects in immune cells. Because p14 participates in the regulation of endosomal trafficking, growth factor signaling, and cell proliferation, we investigated the role of p14 in mouse DCs/LCs using a conditional knockout mouse model. p14-deficient animals displayed a virtually complete loss of LCs in the epidermis early after birth due to impaired proliferation and increased apoptosis of LCs. Repopulation analysis after application of contact sensitizer leads to the recruitment of a transient LC population, predominantly consisting of short-term LCs. The underlying molecular mechanism involves the p14-mediated disruption of the LAMTOR complex which results in the malfunction of both ERK and mTOR signal pathways. Hence, we conclude that p14 acts as a novel and essential regulator of LC homeostasis in vivo.

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.

Two distinct types of Langerhans cells populate the skin during steady state and inflammation

Langerhans cells (LCs), the dendritic cells (DCs) in skin epidermis, possess an exceptional life cycle and developmental origin. Here we identified two types of LCs, short-term and long-term LCs, which transiently or stably reconstitute the LC compartment, respectively. Short-term LCs developed from Gr-1(hi) monocytes under inflammatory conditions and occurred independently of the transcription factor Id2. Long-term LCs arose from bone marrow in steady state and were critically dependent on Id2. Surface marker and gene expression analysis positioned short-term LCs close to Gr-1(hi) monocytes, which is indicative of their monocytic origin. We also show that LC reconstitution after UV light exposure occurs in two waves: an initial fast and transient wave of Gr-1(hi) monocyte-derived short-term LCs is followed by a second wave of steady-state precursor-derived long-term LCs. Our data demonstrate the presence of two types of LCs that develop through different pathways in inflammation and steady state.

Adult Langerhans cells derive predominantly from embryonic fetal liver monocytes with a minor contribution of yolk sac-derived macrophages

Langerhans cells (LCs) are the dendritic cells (DCs) of the epidermis, forming one of the first hematopoietic lines of defense against skin pathogens. In contrast to other DCs, LCs arise from hematopoietic precursors that seed the skin before birth. However, the origin of these embryonic precursors remains unclear. Using in vivo lineage tracing, we identify a first wave of yolk sac (YS)-derived primitive myeloid progenitors that seed the skin before the onset of fetal liver hematopoiesis. YS progenitors migrate to the embryo proper, including the prospective skin, where they give rise to LC precursors, and the brain rudiment, where they give rise to microglial cells. However, in contrast to microglia, which remain of YS origin throughout life, YS-derived LC precursors are largely replaced by fetal liver monocytes during late embryogenesis. Consequently, adult LCs derive predominantly from fetal liver monocyte-derived cells with a minor contribution of YS-derived cells. Altogether, we establish that adult LCs have a dual origin, bridging early embryonic and late fetal myeloid development.

Abnormalities of dendritic cell precursors in the pancreas of the NOD mouse model of diabetes

The non-obese diabetic (NOD) mouse is a widely used animal model for the study of human diabetes. Before the start of lymphocytic insulitis, DC accumulation around islets of Langerhans is a hallmark for autoimmune diabetes development in this model. Previous experiments indicated that an inflammatory influx of these DCs in the pancreas is less plausible. Here, we investigated whether the pancreas contains DC precursors and whether these precursors contribute to DC accumulation in the NOD pancreas. Fetal pancreases of NOD and control mice were isolated followed by FACS using ER-MP58, Ly6G, CD11b and Ly6C. Sorted fetal pancreatic ER-MP58(+) cells were cultured with GM-CSF and tested for DC markers and antigen processing. CFSE labeling and Ki-67 staining were used to determine cell proliferation in cultures and tissues. Ly6C(hi) and Ly6C(low) precursors were present in fetal pancreases of NOD and control mice. These precursors developed into CD11c(+) MHCII(+) CD86(+) DCs capable of processing DQ-OVA. ER-MP58(+) cells in the embryonic and pre-diabetic NOD pancreas had a higher proliferation capacity. Our observations support a novel concept that pre-diabetic DC accumulation in the NOD pancreas is due to aberrant enhanced proliferation of local precursors, rather than to aberrant "inflammatory infiltration" from the circulation.

Dendritic cell and macrophage heterogeneity in vivo

Macrophage and dendritic cell (DC) are hematopoietic cells found in all tissues in the steady state that share the ability to sample the environment but have distinct function in tissue immunity. Controversies remain on the best way to distinguish macrophages from DCs in vivo. In this Perspective, we discuss how recent discoveries in the origin of the DC and macrophage lineage help establish key functional differences between tissue DC and macrophage subsets. We also emphasize the need to further understand the functional heterogeneity of the tissue DC and macrophage lineages to better comprehend the complex role of these cells in tissue homeostasis and immunity.

Wnt signaling influences the development of murine epidermal Langerhans cells

Langerhans cells (LC) are distinct dendritic cells (DC) that populate stratified squamous epithelia. Despite extensive studies, our understanding of LC development is incomplete. TGFβ1 is required for LC development, but other epidermis-derived influences may also be important. Recently, EpCAM (CD326) has been identified as cell surface protein discriminating LC from Langerin⁺ dermal and other DC in skin. EpCAM is a known transcriptional target of the Wnt signaling pathway. We hypothesized that intraepidermal Wnt signaling might influence LC development. Addition of Wnt3A into cultures of bone marrow-derived cells in combination with TGFβ1, GM-CSF, M-CSF resulted in increased (33%; p<0.05) accumulation of EpCAM⁺ DC. In contrast, addition of the Wnt antagonist Dkk1 decreased numbers of EpCAM⁺ DC (21%; p<0.05). We used K14-KRM1; K5-rtTA; tetO-Dkk1 triple transgenic and K5-rtTA; tetO-Dkk1 double transgenic mice to test the in vivo relevance of our in vitro findings. Feeding doxycycline to nursing mothers induced expression of Dkk1 in skin of transgenic pups causing an obvious hair phenotype. Expression of Dkk1 reduced LC proliferation (40%; p<0.01) on P7, decreased LC densities (26%; p<0.05) on P14, and decreased EpCAM expression intensities on LC as well (33%). In aggregate, these data suggest that Wnt signaling in skin influences LC development.

Development and homeostasis of 'resident' myeloid cells: the case of the Langerhans cell

Langerhans cells (LCs) are myeloid cells of the epidermis, featured in immunology textbooks as bone marrow-derived antigen-presenting dendritic cells (DCs). A new picture of LC origin, homeostasis and function has emerged, however, after genetic labelling and conditional cell ablation models in mice. LC precursors are recruited into the fetal epidermis, where they differentiate and proliferate in situ. In adults, LCs proliferate at steady state, and during inflammation, in response to signals from neighbouring cells. Here we review the experimental evidence that support either extra-embryonic yolk sac (YS) macrophages or hematopoietic stem cells (HSCs) as the origin of LCs. Beyond LC biology, we propose that YS and HSCs can contribute to the development of distinct subsets of macrophages and DCs.

Langerhans cells and more: langerin-expressing dendritic cell subsets in the skin

Langerhans cells (LCs) are antigen-presenting dendritic cells (DCs) that reside in epithelia. The best studied example is the LC of the epidermis. By electron microscopy, their identifying feature is the unique rod- or tennis racket-shaped Birbeck granule. The phenotypic hallmark is their expression of the C-type lectin receptor langerin/CD207. Langerin, however, is also expressed on a recently discovered population of DC in the dermis and other tissues of the body. These 'dermal langerin(+) dendritic cells' are unrelated to LCs. The complex field of langerin-negative dermal DCs is not dealt with here. In this article, we briefly review the history, ontogeny, and homeostasis of LCs. More emphasis is laid on the discussion of functional properties in vivo. Novel models using genetically engineered mice are contributing tremendously to our understanding of the role of LCs in eliciting adaptive immune responses against pathogens or tumors and in inducing and maintaining tolerance against self antigens and innocuous substances in vivo. Also, innate effector functions are increasingly being recognized. Current activities in this area are reviewed, and possibilities for future exploitation of LC in medicine, e.g. for the improvement of vaccines, are contemplated.

Development of the prenatal cutaneous antigen-presenting cell network

The skin, and in particular the epidermis, is a physical barrier that protects the body from external threats and is critically involved in immune reactivity. Professional antigen-presenting cells, such as epidermal Langerhans cells and dermal dendritic cells, are gaining prominence as principal players orchestrating the decision between immunity and tolerance. A focus of research interest in recent years has been the investigation of these cells in mammalian prenatal skin. In this review, we will compare the recent progress in dissecting the phenotype and functional role of antigen-presenting cells in the developing human and mouse skin before birth and perinatally, and will discuss how this knowledge improves our understanding of the level of immunocompetence of the skin in utero.

Isolation of skin dendritic cells from mouse and man

Dendritic cells (DC) are crucial for the induction of immune responses and populate various tissues to fulfil their special role. The skin harbours different DC subsets, the Langerhans cells (LC) in the epidermis and the dermal DC in the dermis. The investigation of skin DC is cumbersome since these cells are rare in the skin. As a consequence, it is laborious to receive enough cells from the tissue for experiments. Several approaches have been developed to isolate skin DC based on either enzymatic digestion of the tissue or skin explant culture. Immature LC can be obtained by trypsinization of epidermis, cultured in vitro and be highly enriched with gradient centrifugation and magnetic bead sorting. Mature skin DC can be easily received from skin explant culture. For this purpose skin pieces are cultured for several days and migratory DC emigrate from epidermis and dermis. Both techniques are described for human and mouse skin in the following chapter of the book.

Langerhans cell (LC) proliferation mediates neonatal development, homeostasis, and inflammation-associated expansion of the epidermal LC network

Most tissues develop from stem cells and precursors that undergo differentiation as their proliferative potential decreases. Mature differentiated cells rarely proliferate and are replaced at the end of their life by new cells derived from precursors. Langerhans cells (LCs) of the epidermis, although of myeloid origin, were shown to renew in tissues independently from the bone marrow, suggesting the existence of a dermal or epidermal progenitor. We investigated the mechanisms involved in LC development and homeostasis. We observed that a single wave of LC precursors was recruited in the epidermis of mice around embryonic day 18 and acquired a dendritic morphology, major histocompatibility complex II, CD11c, and langerin expression immediately after birth. Langerin⁺ cells then undergo a massive burst of proliferation between postnatal day 2 (P2) and P7, expanding their numbers by 10–20-fold. After the first week of life, we observed low-level proliferation of langerin⁺ cells within the epidermis. However, in a mouse model of atopic dermatitis (AD), a keratinocyte signal triggered increased epidermal LC proliferation. Similar findings were observed in epidermis from human patients with AD. Therefore, proliferation of differentiated resident cells represents an alternative pathway for development in the newborn, homeostasis, and expansion in adults of selected myeloid cell populations such as LCs. This mechanism may be relevant in locations where leukocyte trafficking is limited.

Differential effects of corticosteroids and pimecrolimus on the developing skin immune system in humans and mice

Atopic dermatitis arises primarily in early infancy. In these patients, corticosteroids are used especially with great caution because of their side effects. Calcineurin inhibitors such as pimecrolimus (PIM) could be useful, but safety concerns have been raised in particular because of the lack of knowledge about their effects on the developing skin immune system. This study was designed to investigate the impact of PIM and corticosteroids on epidermal cells (EC) in infants and newborn mice. We found that the percentage of unfractionated viable infant ECs was significantly decreased in the presence of beta-methasone-17-valerate (BMV) but not PIM. Exposure of unfractionated infant ECs to BMV but not to PIM and vehicle control caused a significant inhibition of the upregulation of CD86 molecules on Langerhans cells (LC). The release of cytokines by LCs and ECs, cultured in the presence of BMV and PIM, was not significantly reduced compared with controls. Topical corticosteroid but not PIM application onto newborn mice induced apoptosis in some LC precursors. Our data suggest that similar to the situation in adult skin, corticosteroids may impair LC maturation as well as viability of ECs in infants, effects not seen with PIM.

Biology of Langerhans cells and Langerhans cell histiocytosis

Langerhans cells (LC) are epidermal dendritic cells (DC). They play an important role in the initiation of immune responses through antigen uptake, processing, and presentation to T cells. Langerhans cell histiocytosis (LCH) is a rare disease in which accumulation of cells with LC characteristics (LCH cells) occur. LCH lesions are further characterized by the presence of other cell types, such as T cells, multinucleated giant cells (MGC), macrophages (MPhi), eosinophils, stromal cells, and natural killer cells (NK cells). Much has been learned about the pathophysiology of LCH by studying properties of these different cells and their interaction with each other through cytokines/chemokines. In this review we discuss the properties and interactions of the different cells involved in LCH pathophysiology with the hope of better understanding this enigmatic disorder.

Targeted ablation of plectin isoform 1 uncovers role of cytolinker proteins in leukocyte recruitment

Plectin, a typical cytolinker protein, is essential for skin and skeletal muscle integrity. It stabilizes cells mechanically, regulates cytoskeleton dynamics, and serves as a scaffolding platform for signaling molecules. A variety of isoforms expressed in different tissues and cell types account for this versatility. To uncover the role of plectin 1, the major isoform expressed in tissues of mesenchymal origin, against the background of all other variants, we raised plectin isoform 1-specific antibodies and generated isoform-deficient mice. In contrast to plectin-null mice (lacking all plectin isoforms), which die shortly after birth because of severe skin blistering, plectin isoform 1-deficient mice were viable at birth, had a normal lifespan, and did not display the skin blistering phenotype. However, dermal fibroblasts isolated from plectin 1-deficient mice exhibited abnormalities in their actin cytoskeleton and impaired migration potential. Similarly, plectin 1-deficient T cells isolated from nymph nodes showed diminished chemotactic migration in vitro. Most strikingly, in vivo we found that leukocyte infiltration during wound healing was reduced in the mutant mice. These data show a specific role of a cytolinker protein in immune cell motility. Single isoform-deficient mice thus represent a powerful tool to unravel highly specific functions of plectin variants.

Flk2+ myeloid progenitors are the main source of Langerhans cells

Langerhans cells (LCs) are antigen-presenting cells (APCs) residing in the epidermis that play a major role in skin immunity. Our earlier studies showed that when skin is inflamed LCs are replaced by bone marrow-derived progenitor cells, while during steady-state conditions LCs are able to self-renew in the skin. Identification of the LC progenitors in bone marrow would represent a critical step toward identifying the factors that regulate LC generation as well as their trafficking to the skin. To determine LC lineage origin, we reconstituted lethally irradiated CD45.2 mice with rigorously purified lymphoid and myeloid progenitors from CD45.1 congenic mice. Twenty-four hours later, we exposed the mice to UV light to deplete resident LCs and induce their replacement by progenitors. Reconstitution with common myeloid progenitors (CMPs), common lymphoid progenitors (CLPs), granulocyte-macrophage progenitors (GMPs), or early thymic progenitors led to LC generation within 2 to 3 weeks. CMPs were at least 20 times more efficient at generating LCs than CLPs. LCs from both lineages were derived almost entirely from fetal liver kinase-2+ (Flk-2+) progenitors, displayed typical dendritic-cell (DC) morphology, and showed long-term persistence in the skin. These results indicate that LCs are derived mainly from myeloid progenitors and are dependent on Flt3-ligand for their development.

Differential involvement of PU.1 and Id2 downstream of TGF-beta1 during Langerhans-cell commitment

Langerhans cells (LCs) are highly abundant dendritic cells (DCs) in epidermal and mucosal tissues. The transcription factors PU.1 and Id2 have been implicated as positive regulators of LC development from hematopoietic progenitor cells. LC differentiation from progenitors is absolutely dependent on transforming growth factor beta 1 (TGF-beta1) in vitro as well as in vivo; however, downstream mechanisms are poorly defined. We found that both PU.1 and Id2 are induced by TGF-beta1 in human CD34+ monocyte/LC (M/LC) progenitor cells, and that neither ectopic PU.1 or Id2 alone, nor both together, could replace TGF-beta1 in its instructive function on LC commitment. However, both factors critically contributed to LC differentiation by acting at 2 distinct intersection points. Ectopic PU.1 strongly enhanced TGF-beta1-dependent LC development. Additionally, Notch-induced generation of interstitial-type DCs was associated with PU.1 up-regulation. Thus, PU.1 is generally increased during myeloid DC development. Ectopic Id2 inhibits the acquisition of early monocytic characteristics by cells generated in the absence of TGF-beta1 and also inhibits monocyte induction by alternative stimuli. Since TGF-beta1 represses a default monocyte pathway of common progenitor cells, PU.1 and Id2 seem to modulate lineage options of M/LC precursors, downstream of TGF-beta1.

Langerhans cells are strongly reduced in the skin of transgenic mice overexpressing follistatin in the epidermis

Activins are members of the transforming growth factor-beta (TGF-beta) family and are important for skin morphogenesis and wound healing. TGF-beta1 is necessary for the population of the epidermis with Langerhans cells (LC). However, a role for activin in LC biology is not known. To address this question, we analyzed skin from transgenic mice overexpressing the activin antagonist follistatin in the epidermis. Using immunofluorescence, we observed a striking decrease in the number of LC in the epidermis of transgenic mice in comparison to wild-type mice. Nevertheless, these LC expressed normal levels of major histocompatibility complex (MHC)-class II and Langerin/ CD207 in situ. In explant cultures of whole ear skin the number of dendritic cells (DC), which migrated into the culture medium, was reduced. This reduction was even more pronounced in cultures of epidermal sheets. Virtually all emigrated cutaneous DC displayed typical morphology with cytoplasmic "veils", showed translocation of MHC-class II to the surface membrane, and expressed the maturation marker 2A1. Thus, cutaneous DC from transgenic mice seemed to mature normally. These results demonstrate that overexpression of follistatin in the epidermis affects LC trafficking but not maturation and suggest a novel role of the follistatin-binding partner activin in LC biology.