Convergent evolution of monocyte differentiation in adult skin instructs Langerhans cell identity.

Langerhans cells (LCs) are distinct among phagocytes, functioning both as embryo-derived, tissue-resident macrophages in skin innervation and repair and as migrating professional antigen-presenting cells, a function classically assigned to dendritic cells (DCs). Here, we demonstrate that both intrinsic and extrinsic factors imprint this dual identity. Using ablation of embryo-derived LCs in the murine adult skin and tracking differentiation of incoming monocyte-derived replacements, we found intrinsic intraepidermal heterogeneity. We observed that ontogenically distinct monocytes give rise to LCs. Within the epidermis, Jagged-dependent activation of Notch signaling, likely within the hair follicle niche, provided an initial site of LC commitment before metabolic adaptation and survival of monocyte-derived LCs. In the human skin, embryo-derived LCs in newborns retained transcriptional evidence of their macrophage origin, but this was superseded by DC-like immune modules after postnatal expansion. Thus, adaptation to adult skin niches replicates conditioning of LC at birth, permitting repair of the embryo-derived LC network.

Primitive haematopoiesis in the human placenta gives rise to macrophages with epigenetically silenced HLA-DR.

The earliest macrophages are generated during embryonic development from erythro-myeloid progenitors (EMPs) via primitive haematopoiesis. Although this process is thought to be spatially restricted to the yolk sac in the mouse, in humans, it remains poorly understood. Human foetal placental macrophages, or Hofbauer cells (HBC), arise during the primitive haematopoietic wave ~18 days post conception and lack expression of human leukocyte antigen (HLA) class II. Here, we identify a population of placental erythro-myeloid progenitors (PEMPs) in the early human placenta that have conserved features of primitive yolk sac EMPs, including the lack of HLF expression. Using in vitro culture experiments we demonstrate that PEMP generate HBC-like cells lacking HLA-DR expression. We find the absence of HLA-DR in primitive macrophages is mediated via epigenetic silencing of class II transactivator, CIITA, the master regulator of HLA class II gene expression. These findings establish the human placenta as an additional site of primitive haematopoiesis.

Localized immune surveillance of primary melanoma in the skin deciphered through executable modeling.

While skin is a site of active immune surveillance, primary melanomas often escape detection. Here, we have developed an in silico model to determine the local cross-talk between melanomas and Langerhans cells (LCs), the primary antigen-presenting cells at the site of melanoma development. The model predicts that melanomas fail to activate LC migration to lymph nodes until tumors reach a critical size, which is determined by a positive TNF-α feedback loop within melanomas, in line with our observations of murine tumors. In silico drug screening, supported by subsequent experimental testing, shows that treatment of primary tumors with MAPK pathway inhibitors may further prevent LC migration. In addition, our in silico model predicts treatment combinations that bypass LC dysfunction. In conclusion, our combined approach of in silico and in vivo studies suggests a molecular mechanism that explains how early melanomas develop under the radar of immune surveillance by LC.

Isolation of First-Trimester and Full-term Human Placental Hofbauer Cells.

The placenta is the crucial organ that regulates the health of both mother and fetus during pregnancy. The human placenta is composed of villous tree-like structures that embed into the maternal decidua. Within the stroma of the villi resides a population of fetally-derived macrophages, the Hofbauer cells (HBC). HBC are the only fetal immune cells found within the placenta in the steady-state and are thought to play a crucial role in placental function. From the 10th week of gestation, maternal blood flow into the intervillous space begins, resulting in the placental villi becoming bathed in maternal blood. To study HBC it is necessary to develop techniques that allow for their specific isolation and distinction from maternal blood monocytes and decidual macrophages. Here, we describe a protocol that explains step-by-step the strategy we have developed that allows the specific isolation of HBC.

The Ontogeny and Function of Placental Macrophages.

The placenta is a fetal-derived organ whose function is crucial for both maternal and fetal health. The human placenta contains a population of fetal macrophages termed Hofbauer cells. These macrophages play diverse roles, aiding in placental development, function and defence. The outer layer of the human placenta is formed by syncytiotrophoblast cells, that fuse to form the syncytium. Adhered to the syncytium at sites of damage, on the maternal side of the placenta, is a population of macrophages termed placenta associated maternal macrophages (PAMM1a). Here we discuss recent developments that have led to renewed insight into our understanding of the ontogeny, phenotype and function of placental macrophages. Finally, we discuss how the application of new technologies within placental research are helping us to further understand these cells.

Phenotypic and functional characterization of first-trimester human placental macrophages, Hofbauer cells.

Hofbauer cells (HBCs) are a population of macrophages found in high abundance within the stroma of the first-trimester human placenta. HBCs are the only fetal immune cell population within the stroma of healthy placenta. However, the functional properties of these cells are poorly described. Aligning with their predicted origin via primitive hematopoiesis, we find that HBCs are transcriptionally similar to yolk sac macrophages. Phenotypically, HBCs can be identified as HLA-DR-FOLR2+ macrophages. We identify a number of factors that HBCs secrete (including OPN and MMP-9) that could affect placental angiogenesis and remodeling. We determine that HBCs have the capacity to play a defensive role, where they are responsive to Toll-like receptor stimulation and are microbicidal. Finally, we also identify a population of placenta-associated maternal macrophages (PAMM1a) that adhere to the placental surface and express factors, such as fibronectin, that may aid in repair.

A Crohn's Disease-associated IL2RA Enhancer Variant Determines the Balance of T Cell Immunity by Regulating Responsiveness to IL-2 Signalling.

BACKGROUND AND AIMS: Differential responsiveness to interleukin [IL]-2 between effector CD4+ T cells [Teff] and regulatory T cells [Treg] is a fundamental mechanism of immunoregulation. The single nucleotide polymorphism [SNP] rs61839660, located within IL2RA [CD25], has been associated with the development of Crohn's disease [CD]. We sought to identify the T cell immune phenotype of IBD patients who carry this SNP. METHODS: Teff and Treg were isolated from individuals homozygous [TT], heterozygous [CT], or wild-type [CC] for the minor allele at rs61839660, and used for phenotyping [flow cytometry, Cytometry Time Of Flight] functional assays or T cell receptor [TCR] sequencing. Phosphorylation of signal transducer and activator of transcription 5 [STAT5] was assessed in response to IL-2, IL-7, and in the presence of basiliximab, a monoclonal antibody directed against CD25. Teff pro-inflammatory cytokine expression levels were assessed by reverse transcription quantitative polymerase chain reaction after IL-2 and/or TCR stimulation. RESULTS: Presence of the minor T allele enhances CD25 expression, leading to increased STAT5 phosphorylation and pro-inflammatory cytokine transcript expression by Teff in response to IL-2 stimulation in vitro. Teff from TT individuals demonstrate a more activated gut homing phenotype. TCR sequencing analysis suggests that TT patients may have a reduced clonal capacity to mount an optimal regulatory T cell response. CONCLUSIONS: rs61839660 regulates the responsiveness of T cells to IL-2 signalling by modulating CD25 expression. As low-dose IL-2 is being trialled as a selective Treg modulator in CD, these findings highlight the potential for adverse effects in patients with this genotype.