Langerhans Cells – The Macrophage in Dendritic Cell Clothing

ACKR3 in Skin Homeostasis, an Overlooked Player in the CXCR4/CXCL12 Axis

CXCL12 and its receptor CXCR4 emerge as critical regulators within the intricate network of processes ensuring skin homeostasis. In this review, we discuss their spatial distribution and function in steady-state skin; delve into their role in acute wound healing, with emphasis on fibrotic and regenerative responses; and explore their relevance in skin responses to commensals and pathogens. Given the lack of knowledge surrounding ACKR3, the atypical receptor of CXCL12, we speculate whether and how it might be involved in the processes mentioned earlier. Is ACKR3 the (a)typical friend who enjoys missing the party, or do we need to take a closer look?

Tolerogenic Monocyte-Derived Langerhans Cells Promote Melanoma Progression and Immunotherapy Resistance through Aryl Hydrocarbon Receptor-Cyclooxygenase-2 Activation

Langerhans cells (LCs), as epidermal-resident antigen-presenting cells, are among the first to encounter early carcinogenic changes in the skin, such as those seen in melanoma. During inflammation or cancer, circulating monocytes are recruited into the epidermis, replacing resident LCs that have migrated to draining lymph nodes. However, the specific roles of LC subsets in the progression of melanoma remain unclear. In this study, we observed a decrease in resident LCs and an increase in monocyte-derived LCs (moLCs) within melanoma tumor tissues as the disease progressed. Notably, the frequency of moLCs was higher in patients with anti-PD-1 therapy-resistant melanoma than in those responsive to the therapy. Using muLangerin-DTR transgenic and moLC-specific knockout mouse models, we demonstrated that the depletion of resident LCs accelerated melanoma growth, whereas the depletion of moLCs suppressed tumor progression. Mechanistically, moLCs exhibited elevated expression of cyclooxygenase-2-related genes compared with resident LCs. TGF-β was found to activate aryl hydrocarbon receptor-cyclooxygenase-2 signaling in moLCs, leading to increased production of prostaglandin E2 and expression of PD-L1, which collectively contributed to the immunosuppressive effects of moLCs on CD8+ cytotoxic T lymphocytes. In summary, our findings highlight the functional heterogeneity of LC subsets in melanoma progression, offering deeper insights into LC biology and potential therapeutic strategies.

Isolicoflavonol alleviates UVB-induced photodamage via protecting mitochondria and blocking the activation of NLRP3 inflammasome

Photodamage, a type of skin inflammation caused by excessive exposure to solar radiation, leads to skin redness, inflammation, and even the development of skin cancer, posing a severe threat to individuals living at high altitudes. UVB radiation is considered the primary factor contributing to photodamage. It stimulates macrophages within the epidermis, triggers inflammasome activation, and increases the inflammatory cytokine interleukin-1β (IL-1β) production. This study examined the protective effects of the compound isolicoflavonol (ILF) and its mechanism against UVB-induced photodamage. We irradiated UVB to create a photodamage model in mice and macrophages. Next, we assessed ILF's ability to protect the skin and cells from UVB photodamage and its inhibitory effects on UVB-mediated NLRP3 inflammasome. Our findings indicated that ILF reduced UVB-induced skin injury and inflammation in mouse skin, decreased cell death, NLRP3 inflammasome activation, ROS production, and mitochondrial dysfunction. These results suggest that ILF may be a potent agent for protecting the skin against UVB-induced photodamage.

An mRNA Vaccine for Herpes Zoster and Its Efficacy Evaluation in Naïve/Primed Murine Models

Background/Objectives: An overwhelming burden to clinics, herpes zoster (HZ), or shingles, is a painful disease that occurs frequently among aged individuals with a varicella-zoster virus (VZV) infection history. The cause of shingles is the reactivation of dormant VZV in the dorsal root ganglia/cranial nerves of the human body. Patients with HZ experience sharp, intense, electric shock-like pain, which makes their health-related quality of life (HRQoL) extremely low. Methods: Various mRNA constructs were designed based on intracellular organelle-targeting strategies and AI algorithm-guided high-throughput automation platform screening and were then synthesized by in vitro transcription and encapsulated with four-component lipid nanoparticles (LNPs). Immunogenicity was evaluated on a naïve mouse model, long-term mouse model, and VZV-primed mouse model. Safety was evaluated by a modified "nestlet shredding" method for potential adverse effects induced by vaccines. Comparison between muscular and intradermal administrations was conducted using different inoculated approaches as well. Results: The best vaccine candidate, CVG206, showed robust humoral and cellular immune responses, durable immune protection, and the fewest adverse effects. The CVG206 administered intradermally revealed at least threefold higher humoral and cellular immune responses compared to intramuscular vaccination. The manufactured and lyophilized patch of CVG206 demonstrated good thermal stability at 2-8 °C during 9 months of storage. Conclusions: The lyophilized mRNA vaccine CVG206 possesses remarkable immunogenicity, long-term protection, safety, and thermal stability, and its effectiveness could even be further improved by intradermal administration, revealing that CVG206 is a promising vaccine candidate for HZ in future clinical studies.

Neurogenic inflammation and itch in barrier tissues

Once regarded as distinct systems, the nervous system and the immune system are now recognized for their complex interactions within the barrier tissues. The neuroimmune circuitry comprises a dual-network system that detects external and internal disturbances, providing critical information to tailor a context-specific response to various threats to tissue integrity, such as wounding or exposure to noxious and harmful stimuli like pathogens, toxins, or allergens. Using the skin as an example of a barrier tissue with the polarized sensory neuronal responses of itch and pain, we explore the molecular pathways driving neuronal activation and the effects of this activation on the immune response. We then apply these findings to other barrier tissues, to find common pathways controlling neuroimmune responses in the barriers.

Langerhans Cells Drive Tfh and B Cell Responses Independent of Canonical Cytokine Signals

Dendritic cells (DCs) are key regulators of adaptive immunity, guiding T helper (Th) cell differentiation through antigen presentation, co-stimulation, and cytokine production. However, in steady-state conditions, certain DC subsets, such as Langerhans cells (LCs), induce T follicular helper (Tfh) cells and B cell responses without inflammatory stimuli. Using multiple mouse models and in vitro systems, we investigated the mechanisms underlying steady-state LC-induced adaptive immune responses. We found that LCs drive germinal center Tfh and B cell differentiation and antibody production independently of interleukin-6 (IL-6), type-I interferons, and ICOS ligand (ICOS-L) signaling, which are critical in inflammatory settings. Instead, these responses relied on CD80/CD86-mediated co-stimulation. Our findings challenge the conventional three-signal paradigm by demonstrating that cytokine signaling is dispensable for LC-mediated Tfh and B cell responses in steady-state. These insights provide a framework for understanding homeostatic immunity and the immune system's role in maintaining tolerance or developing autoimmunity under non-inflammatory conditions.

Targeted delivery of cytotoxic proteins via lipid-based nanoparticles to primary Langerhans cells

Targeted delivery has emerged as a critical strategy in the development of novel therapeutics. The advancement of nanomedicine hinges on the safe and precise cell-specific delivery of protein-based therapeutics to the immune system. However, major challenges remain, such as developing an efficient delivery system, ensuring specificity, minimizing off-target effects, and attaining effective intracellular localization. Our strategy utilizes lipid-based nanoparticles conjugated with a glycomimetic ligand. These nanoparticles selectively bind to langerin, a C-type lectin receptor expressed on Langerhans cells in the skin. We opted for cytotoxic proteins, namely cytochrome c and saporin, as model proteins to showcase the potential of delivering intact proteins to Langerhans cells. These proteins are recognized for their ability to induce apoptosis upon entry into the cytosol. We observed specific killing of cells expressing langerin in vitro, and in primary Langerhans cells isolated from mouse and human skin ex vivo with minimal off target effects. By delivering functional proteins within lipid nanoparticles to Langerhans cells, our approach offers new potential to deliver effective therapeutics with minimal side effects.

Epidermal maintenance of Langerhans cells relies on autophagy-regulated lipid metabolism

Macroautophagy (often-named autophagy), a catabolic process involving autophagy-related (Atg) genes, prevents the accumulation of harmful cytoplasmic components and mobilizes energy reserves in long-lived and self-renewing cells. Autophagy deficiency affects antigen presentation in conventional dendritic cells (DCs) without impacting their survival. However, previous studies did not address epidermal Langerhans cells (LCs). Here, we demonstrate that deletion of either Atg5 or Atg7 in LCs leads to their gradual depletion. ATG5-deficient LCs showed metabolic dysregulation and accumulated neutral lipids. Despite increased mitochondrial respiratory capacity, they were unable to process lipids, eventually leading them to ferroptosis. Finally, metabolically impaired LCs upregulated proinflammatory transcripts and showed decreased expression of neuronal interaction receptors. Altogether, autophagy represents a critical regulator of lipid storage and metabolism in LCs, allowing their maintenance in the epidermis.

Calcium dynamics of skin-resident macrophages during homeostasis and tissue injury

Immune cells depend on rapid changes in intracellular calcium activity to modulate cell function. Skin contains diverse immune cell types and is critically dependent on calcium signaling for homeostasis and repair, yet the dynamics and functions of calcium in skin immune cells remain poorly understood. Here, we characterize calcium activity in Langerhans cells, skin-resident macrophages responsible for surveillance and clearance of cellular debris after tissue damage. Langerhans cells reside in the epidermis and extend dynamic dendrites in close proximity to adjacent keratinocytes and somatosensory peripheral axons. We find that homeostatic Langerhans cells exhibit spontaneous and transient changes in calcium activity, with calcium flux occurring primarily in the cell body and rarely in the dendrites. Triggering somatosensory axon degeneration increases the frequency of calcium activity in Langerhans cell dendrites. By contrast, we show that Langerhans cells exhibit a sustained increase in intracellular calcium following engulfment of damaged keratinocytes. Altering intracellular calcium activity leads to a decrease in engulfment efficiency of keratinocyte debris. Our findings demonstrate that Langerhans cells exhibit context-specific changes in calcium activity and highlight the utility of skin as an accessible model for imaging calcium dynamics in tissue-resident macrophages.

Immunomodulatory potential of primary cilia in the skin

Primary cilia (PC) are essential signaling hubs for proper epithelial formation and the maintenance of skin homeostasis. Found on most cells in the human body, including skin cells, PC facilitate signal transduction that allows ciliated cells to interact with the immune system via multiple pathways, helping to maintain immune system homeostasis. PC can be altered by various microenvironmental stimuli to develop corresponding regulatory functions. Both PC and ciliary signaling pathways have been shown to be involved in the immune processes of various skin lesions. However, the mechanisms by which PC regulate cellular functions and maintain immune homeostasis in tissues are highly complex, and our understanding of them in the skin remains limited. In this paper, we discuss key ciliary signaling pathways and ciliated cells in the skin, with a focus on their immunomodulatory functions. We have compiled evidence from various cells, tissues and disease models to help explore the potential immunomodulatory effects of PC in the skin and their molecular mechanisms.

Effective production of snake antivenom by targeting epidermal dendritic cells via the 'low dose, low volume, multi-site' immunization

Snakebite envenomation (SBE) is a serious neglected tropical disease that affects about 3 million people every year and causes over 100,000 deaths annually, mostly in developing countries. WHO has pledged to cut in half the morbidity and mortality due to SBE by 2030. Animal plasma-derived antivenoms, produced mostly in horses and sheep, are the main treatment modality. However, for over a century, equine plasma antivenom production has faced many problems. These include: low neutralizing potency, failure of horses to develop a satisfactory immune response, and a long immunization period. These problems have led to antivenom shortages and higher costs resulting in otherwise avoidable morbidity and mortality in snake bite victims. Attempts have been made to improve the antivenom production process. For example, a number of adjuvants designed to improve the immune response have been tested. In 1997, an immunization protocol involving the use of multi-site, low-volume and venom doses was developed and is currently used in antivenom production. This protocol constituted a significant innovation that has resulted in highly potent antivenoms within much shortened immunization periods, with all the immunized horses responding and with much less venom immunogen used. It has resulted in an ample antivenom supply for use in Thailand and neighboring countries and has led to no reported deaths from snakebite in Thailand in the past few years. The effectiveness of this immunization protocol was the result of a strategy based on targeting dendritic cells which play a pivotal role in the immune response process. This communication summarizes the basis and results of this immunization strategy.

Indeterminate DC histiocytosis is distinct from LCH and often associated with other hematopoietic neoplasms

ABSTRACT

Indeterminate dendritic cell histiocytosis (IDCH) is a rare and poorly understood entity characterized by accumulation of CD1a+/S100+ histiocytes (as Langerhans cell histiocytosis [LCH]) but with reduced-absent expression of Langerin/CD207. We assembled 43 cases of IDCH (defined by CD1a+/CD207<20% immunophenotypic profile) examining the clinical, pathologic, and molecular landscape. Median age at presentation was 70 years (interquartile range, 44-80) with cutaneous (31/43; 72%) and nodal (11/43; 26%) involvement predominating. Eighteen (42%) individuals had an associated nonhistiocytic hematopoietic neoplasm ("secondary" IDCH) whereas 7 of 43 (16%) had a concurrent non-IDCH histiocytosis ("mixed" histiocytosis). Most cases exhibited morphology indistinguishable from LCH but with a CD1c+/CSF1R(CD115)- phenotype, mirroring the signature of normal indeterminate cells and conventional DC type 2. Mutational analysis revealed frequent KRAS (13/32; 41%) and BRAF p.V600E (11/36, 31%) mutations that were nearly mutually exclusive. RNA-sequencing analysis uncovered ETV3::NCOA2 fusion in 6 other patients presenting as a sole genetic alteration without any other concurrent histiocytic or hematopoietic neoplasm. BRAF and MAP2K1 alterations were significantly associated with partial/retained (1%-20%) Langerin expression (P = .005) and mixed histiocytosis (P = .002). Remarkably, myeloid alterations (DNMT3A, TET2, and SRSF2) co-occurred in IDCH tissues of several individuals. Paired sequencing of IDCH and concurrent non-IDCH hematopoietic neoplasm in 4 individuals revealed shared mutations. Age at diagnosis and any nodal involvement at diagnosis predicted inferior overall survival, but BRAF/RAS pathway alterations did not affect outcome. These data have implications for the diagnostic evaluation, classification, and therapeutic management of IDCH.

Monocytes give rise to Langerhans cells that preferentially migrate to lymph nodes at steady state

Current evidence suggests that ontogeny may account for the functional heterogeneity of some tissue macrophages, but not others. Here, we asked whether developmental origin drives different functions of skin Langerhans cells (LCs), an embryo-derived mononuclear phagocyte with features of both tissue macrophages and dendritic cells. Using time-course analyses, bone marrow chimeras, and fate tracing models, we found that the complete elimination of embryo-derived LCs at steady state results in their repopulation from circulating monocytes. However, monocyte-derived LCs inefficiently replenished the epidermal niche. Instead, these cells preferentially migrated to skin-draining lymph nodes. Mechanistically, we show that the enhanced migratory capability of monocyte-derived LCs is associated with higher expression of CD207/Langerin, a C-type lectin involved in the capture of skin microbes. Our data demonstrate that ontogeny plays a role in the migratory behavior of epidermal LCs.

Unique insights into morphological characterization and functional adaptation of the scaly shank skin in aquatic and terrestrial birds

The avian skin is a vital barrier against external effects and undergoes modification to adapt to the different ecosystems. The current study focused on the comprehensive study of the scaly shank skin of aquatic birds, Egyptian Balady Duck (EBD) as well as terrestrial birds, Broad Breasted White Turkey (BBWT) via gross anatomy, histology, and scanning electron microscopy with ED-XRF analysis. The shank skin color was yellow to black in EBD and creamy-white in BBWT. Gross anatomy exhibited two types of scales around the shank: scute and scutella scales in EBD and four types: scute, scutella, reticula, and cancella (interstitial) scales in BBWT. Most scales were non-overlapped and separated by sulci in both birds except those on the dorsum of the shank of BBWT. SEM of the descaled skin revealed an irregular surface due to keratinocytes defining a polygonal texture in EBD and Langerhans cells (a large oval-shaped cell body with abundant long projections) attached to neighboring keratinocytes in BBWT. Histologically, the epidermal and dermal layers varied among the different skin aspects of the shanks of both birds. Langerhans cells were seen within the basal cell layer of the stratum germinativum and collagen fibers of the dermal stratum superficiale. Melanocytes were observed in the stratum basale in EBD. In both birds, abundant telocytes and fibroblasts were distributed within the dermal layers, with excessive adipose tissue in the dermis of the shank skin of EBD. According to the results of the ED-XRF analysis of the scaly shank skin, elements and oxides were present in both species. In conclusion, the findings of the present study reflect the correlations between the functional morphology of shank skin and the bird habitats.

The role of macrophage migratory behavior in development, homeostasis and tumor invasion

Tumor-associated macrophages (TAMs) recapitulate the developmental and homeostatic behaviors of tissue resident macrophages (TRMs) to promote tumor growth, invasion and metastasis. TRMs arise in the embryo and colonize developing tissues, initially to guide tissue morphogenesis and then to form complex networks in adult tissues to constantly search for threats to homeostasis. The macrophage growth factor, colony-stimulating factor-1 (CSF-1), which is essential for TRM survival and differentiation, is also responsible for the development of the unique motility machinery of mature macrophages that underpins their ramified morphologies, migratory capacity and ability to degrade matrix. Two CSF-1-activated kinases, hematopoietic cell kinase and the p110δ catalytic isoform of phosphatidylinositol 3-kinase, regulate this machinery and selective inhibitors of these proteins completely block macrophage invasion. Considering tumors co-opt the invasive capacity of TAMs to promote their own invasion, these proteins are attractive targets for drug development to inhibit tumor progression to invasion and metastasis.

Molecular mechanisms of neuropathic pain

Peripheral neuropathic pain, which occurs after a lesion or disease affecting the peripheral somatosensory nervous system, is a complex and challenging condition to treat. This chapter will cover molecular mechanisms underlying the pathophysiology of peripheral neuropathic pain, focusing on (1) sensitization of nociceptors, (2) neuro-immune crosstalk, and (3) axonal degeneration and regeneration. The chapter will also emphasize the importance of identifying novel therapeutic targets in non-neuronal cells. A comprehensive understanding of how changes at both neuronal and non-neuronal levels contribute to peripheral neuropathic pain may significantly improve pain management and treatment options, expanding to topical application that bypass the side effects associated with systemic administration.

Preliminary Characterisation of Immune Cell Populations in the Oral Mucosa of a Small Cohort of Healthy Dogs (Canis lupus familiaris)

Pre-determined anatomical locations in the oral cavity were biopsied, and their histomorphology was characterised using haematoxylin and eosin staining (H&E). The most abundant cell type was of dendritic morphology. Lymphocyte foci were not evident in the palatoglossal folds or the gingiva. Immunohistochemical staining (IHC) for validated leukocyte markers followed, including CD3, CD20, CD79α, CD204, and Iba1. Consistent with H&E findings, CD204 immunoreactivity predominated amongst all niches. With the exception of the alveolar mucosa and palatoglossal folds, we also demonstrate a significant difference in the population of macrophages by region for only the Iba1 antigen (p < 0.0001). B lymphocytes were found, and a significant difference was noted in the sub-epithelium where CD20-positive cells outnumbered those labelled as CD79a positive (p = 0.001), suggesting the possibility that these cells are in an active state in health. A similar significant difference was found in the subepithelial tissue for myeloid cells, as there were more cells labelled as CD204 positive over Iba1, which, along with their distribution pattern, indicates a possible functional and morphological overlap between these cells. No significant difference was found in epithelial tissues for cells of either myeloid or lymphoid origins. The results from this study suggest different regions of the oral cavity exhibit variations in the distribution of immune cells, particularly macrophages and B lymphocytes. Though more studies would be needed to confirm these findings, these differences may have implications for the immune response and overall health of the oral mucosa.

The impact of irritant challenge on the skin barrier and myeloid-resident immune cells in women who are postmenopausal is modulated by hormone replacement therapy

BACKGROUND

Sex hormone changes during menopausal transition contribute to declining skin health. However, how menopause and its treatment by hormone replacement therapy (HRT) impact the skin barrier and immune system is unclear.

OBJECTIVES

To examine how menopause and HRT affect the skin barrier and immune cell composition in postmenopausal women following irritant challenge.

METHODS

Two cohorts of postmenopausal women were recruited to the study. The first cohort consisted of 10 untreated women [HRT-; mean (SEM) age 56.5 (1.6) years (range 48-63)] and the second was composed of 8 women receiving HRT [HRT+; mean (SEM) age 54.0 (2.1) years (range 48-63)]. Skin irritation was induced by applying topical sodium lauryl sulfate (SLS) 1.25% to occluded buttock skin for 48 h. Clinical assessment was conducted after 24 h, followed by biopsy of both SLS-challenged and unchallenged skin for analysis of skin barrier proteins and immune cell distribution using immunofluorescence.

RESULTS

Clinically, there were no significant differences in skin irritant responses between those taking or not taking HRT (including increased skin redness and blood flow). In response to SLS challenge a significant increase in transepidermal water loss (P < 0.05), filaggrin deposition and cytokeratin 10 (K10)+ cell layers (P < 0.01) was observed in individuals receiving HRT compared with the HRT- group. Following SLS challenge in individuals taking HRT, a significant (P < 0.01) reduction in CD207+ cells in the epidermis was observed, accompanied by an increase of CD207+ cells in the dermis, indicative of migrating Langerhans cells (LCs). Significantly fewer migrating LCs were found in those who were not receiving HRT (P < 0.01). Furthermore, the numbers of dermal dendritic cells (DCs), macrophages, and CD11c+CD206- and CD68+CD206- subsets were found to be significantly (P < 0.05) higher in those taking HRT following SLS challenge.

CONCLUSIONS

Individuals receiving HRT displayed enhanced skin barrier response to SLS challenge with thicker filaggrin and increased K10+ epidermal cell layers. Following challenge, HRT users exhibited elevated LC, inflammatory DC and macrophage counts in the dermis. These may render skin both more prone to inflammation and more capable of resolving it, while also promoting skin repair.

Mannose and Lactobionic Acid in Nasal Vaccination: Enhancing Antigen Delivery via C-Type Lectin Receptors

BACKGROUND/OBJECTIVES

Nasal vaccines are a promising strategy for enhancing mucosal immune responses and preventing diseases at mucosal sites by stimulating the secretion of secretory IgA, which is crucial for early pathogen neutralization. However, designing effective nasal vaccines is challenging due to the complex immunological mechanisms in the nasal mucosa, which must balance protection and tolerance against constant exposure to inhaled pathogens. The nasal route also presents unique formulation and delivery hurdles, such as the mucous layer hindering antigen penetration and immune cell access.

METHODS

This review focuses on cutting-edge approaches to enhance nasal vaccine delivery, particularly those targeting C-type lectin receptors (CLRs) like the mannose receptor and macrophage galactose-type lectin (MGL) receptor. It elucidates the roles of these receptors in antigen recognition and uptake by antigen-presenting cells (APCs), providing insights into optimizing vaccine delivery.

RESULTS

While a comprehensive examination of targeted glycoconjugate vaccine development is outside the scope of this study, we provide key examples of glycan-based ligands, such as lactobionic acid and mannose, which can selectively target CLRs in the nasal mucosa.

CONCLUSIONS

With the rise of new viral infections, this review aims to facilitate the design of innovative vaccines and equip researchers, clinicians, and vaccine developers with the knowledge to enhance immune defenses against respiratory pathogens, ultimately protecting public health.

Resilience of dermis resident macrophages to inflammatory challenges

The skin serves as a complex barrier organ populated by tissue-resident macrophages (TRMs), which play critical roles in defense, homeostasis, and tissue repair. This review examines the functions of dermis resident TRMs in different inflammatory settings, their embryonic origins, and their long-term self-renewal capabilities. We highlight the M2-like phenotype of dermal TRMs and their specialized functions in perivascular and perineuronal niches. Their interactions with type 2 immune cells, autocrine cytokines such as IL-10, and their phagocytic clearance of apoptotic cells have been explored as mechanisms for M2-like dermal TRM self-maintenance and function. In conclusion, we address the need to bridge murine models with human studies, with the possibility of targeting TRMs to promote skin immunity or restrain cutaneous pathology.

The role of the epithelial sentinels, Langerhans cells and γδT cells, in oral squamous cell carcinoma

Oral squamous cell carcinoma (OSCC) arises in the oral epithelium, a tissue in which immune surveillance is mediated by its primary resident leukocytes, Langerhans cells (LCs), and γδT cells. Under steady-state conditions, LCs and γδT cells play a critical role in maintaining oral mucosal homeostasis. As antigen-presenting cells of stratified epithelia, LCs respond to various challenges faced by the epithelium, orchestrating innate, and adaptive immune responses in order to resolve them. γδT cells also sense diverse epithelial insults and react rapidly through cytokine production and cytolytic activity. These epithelial sentinels are also considered to be the first leukocytes in the oral epithelium to encounter early carcinogenic events that have the potential of becoming OSCC. As evident in many malignancies, leukocyte populations help prevent cancer development although they also promote tumor progression. OSCC is no exception, as studies have reported both anti- and pro-tumor roles of LCs and γδT cells. In this review, we summarize the ontogeny of LCs and γδT cells in the oral epithelium and discuss their role in OSCC.

Langerhans cells and skin immune diseases

Langerhans cells (LCs) are the key antigen-presenting cells in the epidermis in normal conditions and respond differentially to environmental and/or endogenous stimuli, exerting either proinflammatory or anti-inflammatory effects. Current knowledge about LCs mainly originates from studies utilizing mouse models, whereas with the development of single-cell techniques, there has been significant progress for human LCs, which has updated our understanding of the phenotype, ontogeny, differentiation regulation, and function of LCs. In this review, we delineated the progress of human LCs and summarized LCs' function in inflammatory skin diseases, providing new ideas for precise regulation of LC function in the prevention and treatment of skin diseases.

Calcium dynamics of skin-resident macrophages during homeostasis and tissue injury

Immune cells depend on rapid changes in intracellular calcium activity to modulate cell function. Skin contains diverse immune cell types and is critically dependent on calcium signaling for homeostasis and repair, yet the dynamics and functions of calcium in skin immune cells remain poorly understood. Here, we characterize calcium activity in Langerhans cells, skin-resident macrophages responsible for surveillance and clearance of cellular debris after tissue damage. Langerhans cells reside in the epidermis and extend dynamic dendrites in close proximity to adjacent keratinocytes and somatosensory peripheral axons. We find that homeostatic Langerhans cells exhibit spontaneous and transient changes in calcium activity, with calcium flux occurring primarily in the cell body and rarely in the dendrites. Triggering somatosensory axon degeneration increases the frequency of calcium activity in Langerhans cell dendrites. By contrast, we show that Langerhans cells exhibit a sustained increase in intracellular calcium following engulfment of damaged keratinocytes. Altering intracellular calcium activity leads to a decrease in engulfment efficiency of keratinocyte debris. Our findings demonstrate that Langerhans cells exhibit context-specific changes in calcium activity and highlight the utility of skin as an accessible model for imaging calcium dynamics in tissue-resident macrophages.

Comparative analysis of antigen-presenting cells in gingival tissues in healthy and periodontitis patients

AIMS

Microbial flora of dental plaque trigger innate and adaptive immune responses. The function of antigen-presenting cells (APCs) is to bridge the innate and adaptive immune systems. The human immune system contains three main types of APCs: dendritic cells (DC) (Langerhans cells (LCs) and interstitial DCs, IDCs), macrophages and B lymphocytes. In this study, the distribution and density of all APCs in healthy and inflamed human gingival tissue were comparatively analysed.

METHODS

Research was conducted on gingival biopsy specimens obtained from 55 patients and classified in three groups: healthy gingiva (control group, n=10), moderate periodontal disease (PD) (n=21) and severe PD (n=24). For APCs' identification antibodies raised against CD1a (for LCs), S100 protein (for iDCs), CD68 (for macrophages) and CD20 (for B lymphocytes) were used.

RESULTS

Increased density of IDCs, macrophages and B lymphocytes in lamina propria and reduced density of LCs in the gingival epithelium were found in patients with periodontitis. Simultaneously, it was noticed an increased concentration of macrophages and B cells in the gingival epithelium in patients with PD. No statistically significant difference in the distribution and density of APC was found among patients with moderate and advanced periodontitis.

CONCLUSIONS

It was hypothesised that in the periodontitis the role of antigen presentation was largely taken from LCs by the DCs, macrophages and B cells. These APCs are thought to have less protective and tolerogenic potential than LCs and this is a significant reason for alveolar bone destruction in periodontitis.

Leveraging current insights on IL-10-producing dendritic cells for developing effective immunotherapeutic approaches

Dendritic cells (DC) are professional antigen-presenting cells involved in promoting and controlling immune responses. Different subsets of DC, named tolerogenic (tol)DC, play a critical role in the maintenance of tissue homeostasis and in fostering tolerance. These unique skills make tolDC especially attractive for strategies aimed at re-establishing/inducing tolerance in immune-mediated conditions. The generation of potent tolDC in vitro from peripheral blood monocytes has seen remarkable advancements. TolDC modulate T cell dynamics by favoring regulatory T cells (Tregs) and curbing effector/pathogenic T cells. Among the several methods developed for in vitro tolDC generation, IL-10 conditioning has been proven to be the most efficient, as IL-10-modulated tolDC were demonstrated to promote Tregs with the strongest suppressive activities. Investigating the molecular, metabolic, and functional profiles of tolDC uncovers essential pathways that facilitate their immunoregulatory functions. This Review provides an overview of current knowledge on the role of tolDC in health and disease, focusing on IL-10 production, functional characterization of in vitro generated tolDC, molecular and metabolic changes occurring in tolDC induced by tolerogenic agents, clinical applications of tolDC-based therapy, and finally new perspectives in the generation of effective tolDC.

Langerhans cells regulate immunity in adulthood by regulating postnatal dermal lymphatic development

The communication between skin and draining lymph nodes is crucial for well-regulated immune responses to skin insults. The skin sends antigen and other signals via lymphatic vessels to regulate lymph node activity, and regulating dermal lymphatic function is another means to control immunity. Here, we show that Langerhans cells (LCs), epidermis-derived antigen-presenting cells, mediate dermal lymphatic expansion and phenotype acquisition postnatally, a function is independent of LC entry into lymphatic vessels. This postnatal LC-lymphatic axis serves in part to control inflammatory systemic T cell responses in adulthood. Our data provide a tissue-based mechanism by which LCs regulate T cells remotely across time and space and raise the possibility that immune diseases in adulthood could reflect compromise of the LC-lymphatic axis in childhood.

Single-cell resolution characterization of myeloid-derived cell states with implication in cancer outcome

Tumor-associated myeloid-derived cells (MDCs) significantly impact cancer prognosis and treatment responses due to their remarkable plasticity and tumorigenic behaviors. Here, we integrate single-cell RNA-sequencing data from different cancer types, identifying 29 MDC subpopulations within the tumor microenvironment. Our analysis reveals abnormally expanded MDC subpopulations across various tumors and distinguishes cell states that have often been grouped together, such as TREM2+ and FOLR2+ subpopulations. Using deconvolution approaches, we identify five subpopulations as independent prognostic markers, including states co-expressing TREM2 and PD-1, and FOLR2 and PDL-2. Additionally, TREM2 alone does not reliably predict cancer prognosis, as other TREM2+ macrophages show varied associations with prognosis depending on local cues. Validation in independent cohorts confirms that FOLR2-expressing macrophages correlate with poor clinical outcomes in ovarian and triple-negative breast cancers. This comprehensive MDC atlas offers valuable insights and a foundation for futher analyses, advancing strategies for treating solid cancers.

The Role of PGC-1α in Aging Skin Barrier Function

Skin provides a physical and immune barrier to protect the body from foreign substances, microbial invasion, and desiccation. Aging reduces the barrier function of skin and its rate of repair. Aged skin exhibits decreased mitochondrial function and prolonged low-level inflammation that can be seen in other organs with aging. Peroxisome proliferator-activated receptor (PPAR)-γ coactivator-1α (PGC-1α), an important transcriptional coactivator, plays a central role in modulating mitochondrial function and antioxidant production. Mitochondrial function and inflammation have been linked to epidermal function, but the mechanisms are unclear. The aim of this review is to discuss the mechanisms by which PGC-1α might exert a positive effect on aged skin barrier function. Initially, we provide an overview of the function of skin under physiological and aging conditions, focusing on the epidermis. We then discuss mitochondrial function, oxidative stress, cellular senescence, and inflamm-aging, the chronic low-level inflammation observed in aging individuals. Finally, we discuss the effects of PGC-1α on mitochondrial function, as well as the regulation and role of PGC-1α in the aging epidermis.

Knowledge on Human Papillomavirus Infections, Cancer Biology, Immune Interactions, Vaccination Coverage and Common Treatments: A Comprehensive Review

Human papillomavirus (HPV) is a circular, double-stranded DNA virus and recognized as the most prevalent sexually transmitted infectious agent worldwide. The HPV life cycle encompasses three primary stages. First, the virus infiltrates the basal cells of the stratified epidermis. Second, there is a low-level expression of viral genes and preservation of the viral genome in the basal layer. Lastly, productive replication of HPV occurs in differentiated cells. An effective immune response, involving various immune cells, including innate immunity, keratinocytes, dendritic cells, and natural killer T cells, is instrumental in clearing HPV infection and thwarting the development of HPV-associated tumors. Vaccines have demonstrated their efficacy in preventing genital warts, high-grade precancerous lesions, and cancers in females. In males, the vaccines can also aid in preventing genital warts, anal precancerous lesions, and cancer. This comprehensive review aims to provide a thorough and detailed exploration of HPV infections, delving into its genetic characteristics, life cycle, pathogenesis, and the role of high-risk and low-risk HPV strains. In addition, this review seeks to elucidate the intricate immune interactions that govern HPV infections, spanning from innate immunity to adaptive immune responses, as well as examining the evasion mechanisms used by the virus. Furthermore, the article discusses the current landscape of HPV vaccines and common treatments, contributing to a holistic understanding of HPV and its associated diseases.

Advancing immunotherapy using biomaterials to control tissue, cellular, and molecular level immune signaling in skin

Immunotherapies have been transformative in many areas, including cancer treatments, allergies, and autoimmune diseases. However, significant challenges persist in extending the reach of these technologies to new indications and patients. Some of the major hurdles include narrow applicability to patient groups, transient efficacy, high cost burdens, poor immunogenicity, and side effects or off-target toxicity that results from lack of disease-specificity and inefficient delivery. Thus, there is a significant need for strategies that control immune responses generated by immunotherapies while targeting infection, cancer, allergy, and autoimmunity. Being the outermost barrier of the body and the first line of host defense, the skin presents a unique immunological interface to achieve these goals. The skin contains a high concentration of specialized immune cells, such as antigen-presenting cells and tissue-resident memory T cells. These cells feature diverse and potent combinations of immune receptors, providing access to cellular and molecular level control to modulate immune responses. Thus, skin provides accessible tissue, cellular, and molecular level controls that can be harnessed to improve immunotherapies. Biomaterial platforms - microneedles, nano- and micro-particles, scaffolds, and other technologies - are uniquely capable of modulating the specialized immunological niche in skin by targeting these distinct biological levels of control. This review highlights recent pre-clinical and clinical advances in biomaterial-based approaches to target and modulate immune signaling in the skin at the tissue, cellular, and molecular levels for immunotherapeutic applications. We begin by discussing skin cytoarchitecture and resident immune cells to establish the biological rationale for skin-targeting immunotherapies. This is followed by a critical presentation of biomaterial-based pre-clinical and clinical studies aimed at controlling the immune response in the skin for immunotherapy and therapeutic vaccine applications in cancer, allergy, and autoimmunity.

Rho-associated kinase regulates Langerhans cell morphology and responsiveness to tissue damage

Skin damage requires efficient immune cell responses to restore organ function. Epidermal-resident immune cells known as Langerhans cells use dendritic protrusions to surveil the skin microenvironment, which contains keratinocytes and peripheral axons. The mechanisms governing Langerhans cell dendrite dynamics and responses to tissue damage are poorly understood. Using skin explants from adult zebrafish, we show that Langerhans cells maintain normal surveillance following axonal degeneration and use their dendrites to engulf small axonal debris. By contrast, a ramified-to-rounded shape transition accommodates the engulfment of larger keratinocyte debris. We find that Langerhans cell dendrites are populated with actin and sensitive to a broad-spectrum actin inhibitor. We show that Rho-associated kinase (ROCK) inhibition leads to elongated dendrites, perturbed clearance of large debris, and reduced Langerhans cell migration to epidermal wounds. Our work describes the dynamics of Langerhans cells and involvement of the ROCK pathway in immune cell responses.

Macrophage Functions in Psoriasis: Lessons from Mouse Models

Psoriasis is a systemic autoimmune/autoinflammatory disease that can be well studied in established mouse models. Skin-resident macrophages are classified into epidermal Langerhans cells and dermal macrophages and are involved in innate immunity, orchestration of adaptive immunity, and maintenance of tissue homeostasis due to their ability to constantly shift their phenotype and adapt to the current microenvironment. Consequently, both macrophage populations play dual roles in psoriasis. In some circumstances, pro-inflammatory activated macrophages and Langerhans cells trigger psoriatic inflammation, while in other cases their anti-inflammatory stimulation results in amelioration of the disease. These features make macrophages interesting candidates for modern therapeutic strategies. Owing to the significant progress in knowledge, our review article summarizes current achievements and indicates future research directions to better understand the function of macrophages in psoriasis.

rWTC-MBTA Vaccine Induces Potent Adaptive Immune Responses Against Glioblastomas via Dynamic Activation of Dendritic Cells

Despite strides in immunotherapy, glioblastoma multiforme (GBM) remains challenging due to low inherent immunogenicity and suppressive tumor microenvironment. Converting "cold" GBMs to "hot" is crucial for immune activation and improved outcomes. This study comprehensively characterized a therapeutic vaccination strategy for preclinical GBM models. The vaccine consists of Mannan-BAM-anchored irradiated whole tumor cells, Toll-like receptor ligands [lipoteichoic acid (LTA), polyinosinic-polycytidylic acid (Poly (I:C)), and resiquimod (R-848)], and anti-CD40 agonistic antibody (rWTC-MBTA). Intracranial GBM models (GL261, SB28 cells) are used to evaluate the vaccine efficacy. A substantial number of vaccinated mice exhibited complete regression of GBM tumors in a T-cell-dependent manner, with no significant toxicity. Long-term tumor-specific immune memory is confirmed upon tumor rechallenge. In the vaccine-draining lymph nodes of the SB28 model, rWTC-MBTA vaccination triggered a major rise in conventional dendritic cell type 1 (cDC1) 12 h post-treatment, followed by an increase in conventional dendritic cell type 2 (cDC2), monocyte-derived dendritic cell (moDC), and plasmacytoid dendritic cell (pDC) on Day 5 and Day 13. Enhanced cytotoxicity of CD4+ and CD8+ T cells in vaccinated mice is verified in co-culture with tumor cells. Analyses of immunosuppressive signals (T-cell exhaustion, myeloid-derived suppressor cells (MDSC), M2 macrophages) in the GBM microenvironment suggest potential combinations with other immunotherapies for enhanced efficacy. In conclusion, the authors findings demonstrate that rWTC-MBTA induces potent and long-term adaptive immune responses against GBM.

Etiopathogenesis of Psoriasis: Integration of Proposed Theories

Psoriasis is a chronic inflammatory disease characterized by squamous and erythematous plaques on the skin and the involvement of the immune system. Global prevalence for psoriasis has been reported around 1-3% with a higher incidence in adults and similar proportions between men and women. The risk factors associated with psoriasis are both extrinsic and intrinsic, out of which a polygenic predisposition is a highlight out of the latter. Psoriasis etiology is not yet fully described, but several hypothesis have been proposed: 1) the autoimmunity hypothesis is based on the over-expression of antimicrobial peptides such as LL-37, the proteins ADAMTSL5, K17, and hsp27, or lipids synthesized by the PLA2G4D enzyme, all of which may serve as autoantigens to promote the differentiation of autoreactive lymphocytes T and unleash a chronic inflammatory response; 2) dysbiosis of skin microbiota hypothesis in psoriasis has gained relevance due to the observations of a loss of diversity and the participation of pathogenic bacteria such as Streptococcus spp. or Staphylococcus spp. the fungi Malassezia spp. or Candida spp. and the virus HPV, HCV, or HIV in psoriatic plaques; 3) the oxidative stress hypothesis, the most recent one, describes that the cell injury and the release of proinflammatory mediators and antimicrobial peptides that leads to activate of the Th1/Th17 axis observed in psoriasis is caused by a higher release of reactive oxygen species and the imbalance between oxidant and antioxidant mechanisms. This review aims to describe the mechanisms involved in the three hypotheses on the etiopathogeneses of psoriasis.

Dendritic cells overcome Cre/Lox induced gene deficiency by siphoning cytosolic material from surrounding cells

In a previous report, keratinocytes were shown to share their gene expression profile with surrounding Langerhans cells (LCs), influencing LC biology. Here, we investigated whether transferred material could substitute for lost gene products in cells subjected to Cre/Lox conditional gene deletion. We found that in human Langerin-Cre mice, epidermal LCs and CD11b+CD103+ mesenteric DCs overcome gene deletion if the deleted gene was expressed by neighboring cells. The mechanism of material transfer differed from traditional antigen uptake routes, relying on calcium and PI3K, being susceptible to polyguanylic acid inhibition, and remaining unaffected by inflammation. Termed intracellular monitoring, this process was specific to DCs, occurring in all murine DC subsets tested and human monocyte-derived DCs. The transferred material was presented on MHC-I and MHC-II, suggesting a role in regulating immune responses.

Border-associated macrophages in the central nervous system

Tissue-resident macrophages play an important role in the local maintenance of homeostasis and immune surveillance. In the central nervous system (CNS), brain macrophages are anatomically divided into parenchymal microglia and non-parenchymal border-associated macrophages (BAMs). Among these immune cell populations, microglia have been well-studied for their roles during development as well as in health and disease. BAMs, mostly located in the choroid plexus, meningeal and perivascular spaces, are now gaining increased attention due to advancements in multi-omics technologies and genetic methodologies. Research on BAMs over the past decade has focused on their ontogeny, immunophenotypes, involvement in various CNS diseases, and potential as therapeutic targets. Unlike microglia, BAMs display mixed origins and distinct self-renewal capacity. BAMs are believed to regulate neuroimmune responses associated with brain barriers and contribute to immune-mediated neuropathology. Notably, BAMs have been observed to function in diverse cerebral pathologies, including Alzheimer's disease, Parkinson's disease, multiple sclerosis, ischemic stroke, and gliomas. The elucidation of the heterogeneity and diverse functions of BAMs during homeostasis and neuroinflammation is mesmerizing, since it may shed light on the precision medicine that emphasizes deep insights into programming cues in the unique brain immune microenvironment. In this review, we delve into the latest findings on BAMs, covering aspects like their origins, self-renewal capacity, adaptability, and implications in different brain disorders.

Major Histocompatibility Complex II Expression on Oral Langerhans Cells Differentially Regulates Mucosal CD4 and CD8 T Cells

In murine periodontitis, the T helper (Th)17 response against Porphyromonas gingivalis in cervical lymph node is abrogated by diphtheria toxin-driven depletion of Langerhans cells (LCs). We determined the impact of major histocompatibility complex class II (MHC-II) presentation in LCs on Th17 cells in the oral mucosa of mice. Using an established human-Langerin promoter-Cre mouse model, we generated LC-specific deletion of the H2-Ab1 (MHC-II) gene. MHC-II expression was ablated in 81.2% of oral-resident LCs compared with >99% of skin-resident LCs. MHC-II (LCΔMHC-II) depletion did not reduce the number of CD4 T cells nor the frequency of Th17 cells compared with that in wild-type mice. However, the frequencies of Th1 cells decreased, and Helios+ T-regulatory cells increased. In ligature-induced periodontitis, the numbers of CD4 T cells and Th17 cells were similar in LCΔMHC-II and wild-type mice. Normal numbers of Th17 cells can therefore be sustained by as little as 18.8% of MHC-II-expressing LCs in oral mucosa. Unexpectedly, oral mucosa CD8 T cells increased >25-fold in LCΔMHC-II mice. Hence, these residual MHC-II-expressing LCs appear unable to suppress the local expansion of CD8 T cells while sufficient to sustain a homeostatic CD4 T-cell response. Reducing the expression of MHC-II on specific LC subpopulations may ultimately boost CD8-mediated intraepithelial surveillance at mucosal surfaces.

Differential Activation of Splenic cDC1 and cDC2 Cell Subsets following Poxvirus Infection of BALB/c and C57BL/6 Mice

Conventional dendritic cells (cDCs) are innate immune cells that play a pivotal role in inducing antiviral adaptive immune responses due to their extraordinary ability to prime and polarize naïve T cells into different effector T helper (Th) subsets. The two major subpopulations of cDCs, cDC1 (CD8α+ in mice and CD141+ in human) and cDC2 (CD11b+ in mice and CD1c+ in human), can preferentially polarize T cells toward a Th1 and Th2 phenotype, respectively. During infection with ectromelia virus (ECTV), an orthopoxvirus from the Poxviridae family, the timing and activation of an appropriate Th immune response contributes to the resistance (Th1) or susceptibility (Th2) of inbred mouse strains to the lethal form of mousepox. Due to the high plasticity and diverse properties of cDC subpopulations in regulating the quality of a specific immune response, in the present study we compared the ability of splenic cDC1 and cDC2 originating from different ECTV-infected mouse strains to mature, activate, and polarize the Th immune response during mousepox. Our results demonstrated that during early stages of mousepox, both cDC subsets from resistant C57BL/6 and susceptible BALB/c mice were activated upon in vivo ECTV infection. These cells exhibited elevated levels of surface MHC class I and II, and co-stimulatory molecules and showed enhanced potential to produce cytokines. However, both cDC subsets from BALB/c mice displayed a higher maturation status than that of their counterparts from C57BL/6 mice. Despite their higher activation status, cDC1 and cDC2 from susceptible mice produced low amounts of Th1-polarizing cytokines, including IL-12 and IFN-γ, and the ability of these cells to stimulate the proliferation and Th1 polarization of allogeneic CD4+ T cells was severely compromised. In contrast, both cDC subsets from resistant mice produced significant amounts of Th1-polarizing cytokines and demonstrated greater capability in differentiating allogeneic T cells into Th1 cells compared to cDCs from BALB/c mice. Collectively, our results indicate that in the early stages of mousepox, splenic cDC subpopulations from the resistant mouse strain can better elicit a Th1 cell-mediated response than the susceptible strain can, probably contributing to the induction of the protective immune responses necessary for the control of virus dissemination and for survival from ECTV challenge.

Immunomodulatory functions of microorganisms in tissue regenerative healing

External pathogenic microorganisms and commensal microorganisms in the body have either harmful or beneficial impacts on the regenerative repair of tissues, and the immune system plays a crucial regulatory role in this process. This review summarises our current understanding of microorganism-immune system interactions, with a focus on how these interactions impact the renewal and repair ability of tissues, including skin, bone, gut, liver, and nerves. This review concludes with a discussion of the mechanisms by which microbes act on various types of immune cells to affect tissue regeneration, offers potential strategies for using microbial therapies to enhance the regenerative repair function of tissues, and suggest novel therapeutic approaches for regenerative medicine. STATEMENT OF SIGNIFICANCE: Microbiological communities have crucial impacts on human health and illness by participating in energy collection and storage and performing various metabolic processes. External pathogenic microorganisms and commensal microorganisms in the body have either harmful or beneficial impacts on the regenerative repair of tissues, and the immune system plays a critical regulatory role in this process. This study reviews the important correlation between microorganisms and the immune system and investigates the mechanism of various microorganism that participate in the regeneration and repair of tissues and organs by modulating immune system.

Tissue-specific antigen-presenting cells contribute to distinct phenotypes of allergy

Antigen-presenting cells (APCs) are critical cells bridging innate and adaptive immune responses by taking up, processing, and presenting antigens to naïve T cells. At steady state, APCs thus control both tissue homeostasis and the induction of tolerance. In allergies however, APCs drive a Th2-biased immune response that is directed against otherwise harmless antigens from the environment. The main types of APCs involved in the induction of allergy are dendritic cells, monocytes, and macrophages. However, these cell types can be further divided into local, tissue-specific populations that differ in their phenotype, migratory capacity, T-cell activating potential, and production of effector molecules. Understanding if distinct populations of APCs contribute to either tissue-specific immune tolerance, allergen sensitization, or allergic inflammation will allow us to better understand disease pathology and develop targeted treatment options for different stages of allergic disease. Therefore, this review describes the main characteristics, phenotypes, and effector molecules of the APCs involved in the induction of allergen-specific Th2 responses in affected barrier sites, such as the skin, nose, lung, and gastrointestinal tract. Furthermore, we highlight open questions that remain to be addressed to fully understand the contribution of different APCs to allergic disease.

Proliferation of monocytes and macrophages in homeostasis, infection, injury, and disease

Monocytes (Mo) and macrophages (Mφ) play important roles in the function of tissues, organs, and systems of all animals during homeostasis, infection, injury, and disease. For decades, conventional wisdom has dictated that Mo and Mφ are end-stage cells that do not proliferate and that Mφ accumulation in tissues is the result of infiltration of Mo from the blood and subsequent differentiation to Mφ. However, reports from the early 1900s to the present describe evidence of Mo and Mφ proliferation in different tissues and contexts. The purpose of this review is to summarize both historical and current evidence for the contribution of Mφ proliferation to their accumulation in different tissues during homeostasis, infection, injury, and disease. Mφ proliferate in different organs and tissues, including skin, peritoneum, lung, heart, aorta, kidney, liver, pancreas, brain, spinal cord, eye, adipose tissue, and uterus, and in different species including mouse, rat, rabbit, and human. Mφ can proliferate at different stages of differentiation with infiltrating Mo-like cells proliferating in certain inflammatory contexts (e.g. skin wounding, kidney injury, bladder and liver infection) and mature resident Mφ proliferating in other inflammatory contexts (e.g. nematode infection, acetaminophen liver injury) and during homeostasis. The pathways involved in stimulating Mφ proliferation also may be context dependent, with different cytokines and transcription factors implicated in different studies. Although Mφ are known to proliferate in health, injury, and disease, much remains to be learned about the regulation of Mφ proliferation in different contexts and its impact on the homeostasis, injury, and repair of different organs and tissues.

Recent advances in epicutaneous immunotherapy and potential applications in food allergy

Given the potent immunological properties of the skin, epicutaneous immunotherapy (EPIT) emerges as a promising treatment approach for inducing immune tolerance, particularly for food allergies. Targeting the highly immunocompetent, non-vascularized epidermis allows for the application of microgram amounts of allergen while significantly reducing the risk of allergen passage into the bloodstream, thus limiting systemic allergen exposure and distribution. This makes EPIT highly suitable for the treatment of potentially life-threatening allergies such as food allergies. Multiple approaches to EPIT are currently under investigation for the treatment of food allergy, and these include the use of allergen-coated microneedles, application of allergen on the skin pretreated by tape stripping, abrasion or laser-mediated microperforation, or the application of allergen on the intact skin using an occlusive epicutaneous system. To date, the most clinically advanced approach to EPIT is the Viaskin technology platform. Viaskin is an occlusive epicutaneous system (patch) containing dried native allergen extracts, without adjuvants, which relies on frequent application for the progressive passage of small amounts of allergen to the epidermis through occlusion of the intact skin. Numerous preclinical studies of Viaskin have demonstrated that this particular approach to EPIT can induce potent and long-lasting T-regulatory cells with broad homing capabilities, which can exert their suppressive effects in multiple organs and ameliorate immune responses from different routes of allergen exposure. Clinical trials of the Viaskin patch have studied the efficacy and safety for the treatment of life-threatening allergies in younger patients, at an age when allergic diseases start to occur. Moreover, this treatment approach is designed to provide a non-invasive therapy with no restrictions on daily activities. Taken together, the preclinical and clinical data on the use of EPIT support the continued investigation of this therapeutic approach to provide improved treatment options for patients with allergic disorders in the near future.

Controlling Antigen Fate in Therapeutic Cancer Vaccines by Targeting Dendritic Cell Receptors

Antigen-presenting cells (APCs) orchestrate immune responses and are therefore of interest for the targeted delivery of therapeutic vaccines. Dendritic cells (DCs) are professional APCs that excel in presentation of exogenous antigens toward CD4+ T helper cells, as well as cytotoxic CD8+ T cells. DCs are highly heterogeneous and can be divided into subpopulations that differ in abundance, function, and phenotype, such as differential expression of endocytic receptor molecules. It is firmly established that targeting antigens to DC receptors enhances the efficacy of therapeutic vaccines. While most studies emphasize the importance of targeting a specific DC subset, we argue that the differential intracellular routing downstream of the targeted receptors within the DC subset should also be considered. Here, we review the mouse and human receptors studied as target for therapeutic vaccines, focusing on antibody and ligand conjugates and how their targeting affects antigen presentation. We aim to delineate how targeting distinct receptors affects antigen presentation and vaccine efficacy, which will guide target selection for future therapeutic vaccine development.

Structural Changes of Cutaneous Immune Cells in Patients With Type 1 Diabetes and Their Relationship With Diabetic Polyneuropathy

BACKGROUND AND OBJECTIVES

Diabetic polyneuropathy (DPN) is a complication of diabetes characterized by pain or lack of peripheral sensation, but the underlying mechanisms are not yet fully understood. Recent evidence showed increased cutaneous macrophage infiltration in patients with type 2 diabetes and painful DPN, and this study aimed to understand whether the same applies to type 1 diabetes.

METHODS

The study included 104 participants: 26 healthy controls and 78 participants with type 1 diabetes (participants without DPN [n = 24], participants with painless DPN [n = 29], and participants with painful DPN [n = 25]). Two immune cells, dermal IBA1+ macrophages and epidermal Langerhans cells (LCs, CD207+), were visualized and quantified using immunohistological labeling and stereological counting methods on skin biopsies from the participants. The IBA1+ macrophage infiltration, LC number density, LC soma cross-sectional area, and LC processes were measured in this study.

RESULTS

Significant difference in IBA1+ macrophage expression was seen between the groups (p = 0.003), with lower expression of IBA1 in participants with DPN. No differences in LC morphologies (LC number density, soma cross-sectional area, and process level) were found between the groups (all p > 0.05). In addition, IBA1+ macrophages, but not LCs, correlated with intraepidermal nerve fiber density, Michigan neuropathy symptom inventory, (questionnaire and total score), severity of neuropathy as assessed by the Toronto clinical neuropathy score, and vibration detection threshold in the whole study cohort.

DISCUSSION

This study showed expressional differences of cutaneous IBA1+ macrophages but not LC in participants with type 1 diabetes-induced DPN compared with those in controls. The study suggests that a reduction in macrophages may play a role in the development and progression of autoimmune-induced diabetic neuropathy.

Innovative delivery systems for epicutaneous immunotherapy

Allergen-specific immunotherapy (AIT) describes the establishment of peripheral tolerance through repeated allergen exposure, which qualifies as the only curative treatment for allergic diseases. Although conventional subcutaneous immunotherapy (SCIT) and sublingual immunotherapy (SLIT) have been approved to treat respiratory allergies clinically, the progress made is far from satisfactory. Epicutaneous immunotherapy (EPIT) exploits the skin's immune properties to modulate immunological response, which is emerging as a promising alternative and has shown effectiveness in many preclinical and clinical studies for both respiratory and food allergies. It is worth noting that the stratum corneum (SC) barrier impedes the effective delivery of allergens, while disrupting the SC layer excessively often triggers unexpected Th2 immune responses. This work aims to comprehend the immunological mechanisms of EPIT, and summarize the innovative system for sufficient delivery of allergens as well as tolerogenic adjuvants. Finally, the safety, acceptability, and cost-effectiveness of these innovative delivery systems are discussed, which directs the development of future immunotherapies with all desirable characteristics.

M1/M2 macrophages and their overlaps - myth or reality?

Macrophages represent heterogeneous cell population with important roles in defence mechanisms and in homoeostasis. Tissue macrophages from diverse anatomical locations adopt distinct activation states. M1 and M2 macrophages are two polarized forms of mononuclear phagocyte in vitro differentiation with distinct phenotypic patterns and functional properties, but in vivo, there is a wide range of different macrophage phenotypes in between depending on the microenvironment and natural signals they receive. In human infections, pathogens use different strategies to combat macrophages and these strategies include shaping the macrophage polarization towards one or another phenotype. Macrophages infiltrating the tumours can affect the patient's prognosis. M2 macrophages have been shown to promote tumour growth, while M1 macrophages provide both tumour-promoting and anti-tumour properties. In autoimmune diseases, both prolonged M1 activation, as well as altered M2 function can contribute to their onset and activity. In human atherosclerotic lesions, macrophages expressing both M1 and M2 profiles have been detected as one of the potential factors affecting occurrence of cardiovascular diseases. In allergic inflammation, T2 cytokines drive macrophage polarization towards M2 profiles, which promote airway inflammation and remodelling. M1 macrophages in transplantations seem to contribute to acute rejection, while M2 macrophages promote the fibrosis of the graft. The view of pro-inflammatory M1 macrophages and M2 macrophages suppressing inflammation seems to be an oversimplification because these cells exploit very high level of plasticity and represent a large scale of different immunophenotypes with overlapping properties. In this respect, it would be more precise to describe macrophages as M1-like and M2-like.

Rho-associated kinase regulates Langerhans cell morphology and responsiveness to tissue damage

Skin is often the first physical barrier to encounter invading pathogens and physical damage. Damage to the skin must be resolved quickly and efficiently to maintain organ homeostasis. Epidermal-resident immune cells known as Langerhans cells use dendritic protrusions to dynamically surveil the skin microenvironment, which contains epithelial keratinocytes and somatosensory peripheral axons. The mechanisms governing Langerhans cell dendrite dynamics and responses to tissue damage are not well understood. Using skin explants from adult zebrafish, we show that Langerhans cells maintain normal surveillance activity following axonal degeneration and use their dynamic dendrites to engulf small axonal debris. By contrast, a ramified-to-rounded shape transition accommodates the engulfment of larger keratinocyte debris. We find that Langerhans cell dendrites are richly populated with actin and sensitive to a broad spectrum actin inhibitor. We further show that Rho-associated kinase (ROCK) inhibition leads to elongated dendrites, perturbed clearance of large debris, and reduced Langerhans cell migration to tissue-scale wounds. Altogether, our work describes the unique dynamics of Langerhans cells and involvement of the ROCK pathway in immune cell responses to damage of varying magnitude.

Transcriptional regulation of dendritic cell development and function

Dendritic cells (DCs) are sentinel immune cells that form a critical bridge linking the innate and adaptive immune systems. Extensive research addressing the cellular origin and heterogeneity of the DC network has revealed the essential role played by the spatiotemporal activity of key transcription factors. In response to environmental signals DC mature but it is only following the sensing of environmental signals that DC can induce an antigen specific T cell response. Thus, whilst the coordinate action of transcription factors governs DC differentiation, sensing of environmental signals by DC is instrumental in shaping their functional properties. In this review, we provide an overview that focuses on recent advances in understanding the transcriptional networks that regulate the development of the reported DC subsets, shedding light on the function of different DC subsets. Specifically, we discuss the emerging knowledge on the heterogeneity of cDC2s, the ontogeny of pDCs, and the newly described DC subset, DC3. Additionally, we examine critical transcription factors such as IRF8, PU.1, and E2-2 and their regulatory mechanisms and downstream targets. We highlight the complex interplay between these transcription factors, which shape the DC transcriptome and influence their function in response to environmental stimuli. The information presented in this review provides essential insights into the regulation of DC development and function, which might have implications for developing novel therapeutic strategies for immune-related diseases.

Macrophage-mediated extracellular matrix remodeling controls host Staphylococcus aureus susceptibility in the skin

Hypodermis is the predominant site of Staphylococcus aureus infections that cause cellulitis. Given the importance of macrophages in tissue remodeling, we examined the hypodermal macrophages (HDMs) and their impact on host susceptibility to infection. Bulk and single-cell transcriptomics uncovered HDM subsets with CCR2-dichotomy. HDM homeostasis required the fibroblast-derived growth factor CSF1, ablation of which abrogated HDMs from the hypodermal adventitia. Loss of CCR2- HDMs resulted in accumulation of the extracellular matrix component, hyaluronic acid (HA). HDM-mediated HA clearance required sensing by the HA receptor, LYVE-1. Cell-autonomous IGF1 was required for accessibility of AP-1 transcription factor motifs that controlled LYVE-1 expression. Remarkably, loss of HDMs or IGF1 limited Staphylococcus aureus expansion via HA and conferred protection against cellulitis. Our findings reveal a function for macrophages in the regulation of HA with an impact on infection outcomes, which may be harnessed to limit the establishment of infection in the hypodermal niche.