The expression of thec-kitreceptor by epidermal melanocytes may be reduced in vitiligo

Emerging Role of Fibroblasts in Vitiligo: A Formerly Underestimated Rising Star

Vitiligo is a disfiguring depigmentation disorder characterized by loss of melanocytes. Although numerous studies have been conducted on the pathogenesis of vitiligo, the underlying mechanisms remain unclear. Although most studies have focused on melanocytes and keratinocytes, growing evidence suggests the involvement of dermal fibroblasts, residing deeper in the skin. This review aims to elucidate the role of fibroblasts in both the physiological regulation of skin pigmentation and their pathological contribution to depigmentation, with the goal of shedding light on the involvement of fibroblasts in vitiligo. The topics covered in this review include alterations in the secretome, premature senescence, autophagy dysfunction, abnormal extracellular matrix, autoimmunity, and metabolic changes.

Discovery of an agonistic Siglec-6 antibody that inhibits and reduces human mast cells

Mast cells (MC) are key drivers of allergic and inflammatory diseases. Sialic acid-binding immunoglobulin-like lectin (Siglec)-6 is an immunoregulatory receptor found on MCs. While it is recognized that engaging Siglecs with antibodies mediates inhibition across immune cells, the mechanisms that govern this agonism are not understood. Here we generated Siglec-6 mAb clones (AK01 to AK18) to better understand Siglec-6-mediated agonism. Siglec-6 mAbs displayed epitope-dependent receptor internalization and inhibitory activity. We identified a Siglec-6 mAb (AK04) that required Fc-mediated interaction for receptor internalization and induced inhibition and antibody-dependent cellular phagocytosis against MCs. AK04-mediated MC inhibition required Siglec-6 immunoreceptor tyrosine-based inhibitory motif (ITIM) and ITIM-like domains and was associated with receptor cluster formation containing inhibitory phosphatases. Treatment of humanized mice with AK04 inhibited systemic anaphylaxis with a single dose and reduced MCs with chronic dosing. Our findings suggest Siglec-6 activity is epitope dependent and highlight an agonistic Siglec-6 mAb as a potential therapeutic approach in allergic disease.

Quantification of Immunohistochemically Stained Cells in Skin Biopsies

Immunohistochemical quantification of inflammatory cells in skin biopsies is a valuable tool for diagnosing skin diseases and assessing treatment response. The quantification of individual cells in biopsies is time-consuming, tedious, and difficult. In this study, we presented and compared two methods for the quantification of CD8⁺ T cells in skin biopsies from patients with psoriasis using both commercial software (Adobe Photoshop) and open-source software (Qupath). In addition, we provided a detailed, step-by-step description of both methods. The methods are scalable by replacing the CD8 antibody with other antibodies to target different cells. Moreover, we investigated the correlation between quantifying CD8⁺ cells normalized to area or epidermal length and cell classifications, compared cell classifications in QuPath with threshold classifications in Photoshop, and analyzed the impact of data normalization to epidermal length or area on inflammatory cell densities in skin biopsies from patients with psoriasis. We found a satisfactory correlation between normalizing data to epidermal length and area for psoriasis skin. However, when non-lesional and lesional skin samples were compared, a significant underestimation of inflammatory cell density was found when data were normalized to area instead of epidermal length. Finally, Bland–Altman plots comparing Qupath and Photoshop to quantify inflammatory cell density demonstrated a good agreement between the two methods.

Characterization of a melanocyte progenitor population in human interfollicular epidermis

It is still unknown whether the human interfollicular epidermis harbors a reservoir of melanocyte precursor cells. Here, we clearly distinguish between three distinct types of melanocytes in human interfollicular epidermis: (1) cKit⁺CD90^-, (2) cKit⁺CD90⁺, and (3) cKit^-CD90⁺. Importantly, we identify the Kit tyrosine kinase receptor (cKit) as a marker expressed specifically in mature, melanin-producing melanocytes. Thus, both cKit⁺CD90^- and cKit⁺CD90⁺ cells represent polydendritic, pigmented mature melanocytes, whereas cKit^-CD90⁺ cells display bipolar, non-dendritic morphology with reduced melanin content. Additionally, using tissue-engineered pigmented dermo-epidermal skin substitutes (melDESSs), we reveal that the cKit expression also plays an important role during melanogenesis in melDESS in vivo. Interestingly, cKit^-CD90⁺ cells lack the expression of markers such as HMB45, TYR, and TRP1 in vitro and in vivo. However, they co-express neural-crest progenitor markers and demonstrate multilineage differentiation potential in vitro. Hence, we propose that cKit^-CD90⁺ cells constitute the precursor melanocyte reservoir in human interfollicular epidermis.

Human melanocyte development and melanoma dedifferentiation at single-cell resolution

In humans, epidermal melanocytes are responsible for skin pigmentation, defence against ultraviolet radiation and the deadliest common skin cancer, melanoma. Although there is substantial overlap in melanocyte development pathways between different model organisms, species-dependent differences are frequent and the conservation of these processes in human skin remains unresolved. Here, we used a single-cell enrichment and RNA-sequencing pipeline to study human epidermal melanocytes directly from the skin, capturing transcriptomes across different anatomical sites, developmental age, sexes and multiple skin tones. We uncovered subpopulations of melanocytes that exhibit anatomical site-specific enrichment that occurs during gestation and persists through adulthood. The transcriptional signature of the volar-enriched subpopulation is retained in acral melanomas. Furthermore, we identified human melanocyte differentiation transcriptional programs that are distinct from gene signatures generated from model systems. Finally, we used these programs to define patterns of dedifferentiation that are predictive of melanoma prognosis and response to immune checkpoint inhibitor therapy.

KIT is involved in melanocyte proliferation, apoptosis and melanogenesis in the Rex Rabbit

BACKGROUND

Melanocytes play an extremely important role in the process of skin and coat colors in mammals which is regulated by melanin-related genes. Previous studies have demonstrated that KIT is implicated in the process of determining the color of the coat in Rex rabbits. However, the effect of KIT on the proliferation and apoptosis of melanocytes and melanogenesis has not been clarified.

METHODS

The mRNA and protein expression levels of KIT were quantified in different coat colored rabbits by qRT-PCR and a Wes assay. To identify whether KIT functions by regulating of melanogenesis, KIT overexpression and knockdown was conducted in melanocytes, and KIT mRNA expression and melanin-related genes TYR, MITF, PMEL and DCT were quantified by qRT-PCR. To further confirm whether KIT influences melanogenesis in melanocytes, melanin content was quantified using NaOH lysis after overexpression and knockdown of KIT. Melanocyte proliferation was estimated using a CCK-8 assay at 0, 24, 48 and 72 h after transfection, and the rate of apoptosis of melanocytes was measured by fluorescence-activated cell sorting.

RESULTS

KITmRNA and protein expression levels were significantly different in the skin of Rex rabbits with different color coats (P < 0.05), the greatest levels observed in those with black skin. The mRNA expression levels of KIT significantly affected the mRNA expression of the pigmentation-related genes TYR, MITF, PMEL and DCT (P < 0.01). Melanin content was evidently regulated by the change in expression patterns of KIT (P < 0.01). In addition, KIT clearly promoted melanocyte proliferation, but inhibited apoptosis.

CONCLUSIONS

Our results reveal that KIT is a critical gene in the regulation of melanogenesis, controlling proliferation and apoptosis in melanocytes, providing additional evidence for the mechanism of pigmentation of animal fur.

Expression of Epidermal c-Kit+ of Vitiligo Lesions Is Related to Responses to Excimer Laser

Background

The survival and growth of melanocytes are controlled by the binding of stem cell factor to its cell surface receptor c-kit+ (CD117). We have observed that c-kit+ melanocytes existed in some lesions of vitiligo, while Melan A+ cells were absent.

Objective

To verify possible relation between c-kit+ expression and treatment response in non-segmental vitiligo lesions

Methods

Skin biopsies were done from the center of the 47 lesions from the 47 patients with non-segmental vitiligo. Expression of c-kit+ and Melan A, and amounts of melanin in the epidermis were assessed in each lesion, and treatment responses to excimer laser were evaluated.

Results

Thirty-five of the 47 lesions (74.5%) had c-kit+ phenotypes. There was significant difference of c-kit staining value between good responders in 3 months of excimer laser treatment (average of 24 sessions) and the others.

Conclusion

c-Kit expression in vitiliginous epidermis may be related to better treatment responses to excimer laser.

Evidence that geniposide abrogates norepinephrine-induced hypopigmentation by the activation of GLP-1R-dependent c-kit receptor signaling in melanocyte

Geniposide (GP) as an agonist of glucagon-like peptide-1 receptor (GLP-1R) is an iridoid glycoside from the fruit of Gardenia jasminoides Ellis used as a Chinese traditional medicine for treatment of vitiligo vulgaris. Interaction of c-kit receptor with its ligand-SCF potent enhances the melanocytic melanogenesis, which can be repressed by norepinephrine (NE). To discover economic and efficient drug against vitiligo vulgaris, this paper addresses the action and mechanism of GP abrogating the NE-induced hypopigmentation in melanocyte. Flow cytometry exhibited the up-regulation effect of GP on NE-suppressed production of c-kit by normal human epidermal melanocyte (HEMn) in a concentration-dependent manner, and exendin-(9-39) (selective GLP-1R antagonist) appeared to alleviate the GP-stimulated expression of c-kit. However, neither NE nor GP affected the production of SCF by normal human epidermal keratinocyte (HEKn) assessed by cellular enzyme-linked immunosorbent assay. Spectrophotometry documented that GP abrogated the repression effect of NE on tyrosinase activity and melanin production in HEMn in the presence of recombination SCF significantly. The response of melanocytic melanogenesis to GP was blocked by exendin-(9-39) or K44.2 antibody (c-kit inhibitory antibody). Data from this paper provide the evidence that GP abrogates the NE-induced hypopigmentation by the activation of GLP-1R-dependent c-kit receptor signaling in which c-kit expression is augmented in HEMn.

Physiopathology and genetics of vitiligo

Generalized vitiligo is an acquired disorder in which white patches of skin and overlying hair result from autoimmune loss of melanocytes from involved areas. The autoimmune pathogenesis of vitiligo has become a rapidly evolving field of research. A humoral immune reaction has been implicated through the detection of circulating antibodies. However, recent research focuses on a melanocyte-specific cytotoxic-T-cell immune reaction in the melanocyte destruction. Several candidate genes have been proposed for vitiligo susceptibility. They include genes important for melanin biosynthesis, response to oxidative stress and/or regulation of autoimmunity. A recent genome-wide scan performed on families with numerous members presenting vitiligo has clearly revealed linkage of susceptibility loci.

Melanin pigmentation in mammalian skin and its hormonal regulation

Cutaneous melanin pigment plays a critical role in camouflage, mimicry, social communication, and protection against harmful effects of solar radiation. Melanogenesis is under complex regulatory control by multiple agents interacting via pathways activated by receptor-dependent and -independent mechanisms, in hormonal, auto-, para-, or intracrine fashion. Because of the multidirectional nature and heterogeneous character of the melanogenesis modifying agents, its controlling factors are not organized into simple linear sequences, but they interphase instead in a multidimensional network, with extensive functional overlapping with connections arranged both in series and in parallel. The most important positive regulator of melanogenesis is the MC1 receptor with its ligands melanocortins and ACTH, whereas among the negative regulators agouti protein stands out, determining intensity of melanogenesis and also the type of melanin synthesized. Within the context of the skin as a stress organ, melanogenic activity serves as a unique molecular sensor and transducer of noxious signals and as regulator of local homeostasis. In keeping with these multiple roles, melanogenesis is controlled by a highly structured system, active since early embryogenesis and capable of superselective functional regulation that may reach down to the cellular level represented by single melanocytes. Indeed, the significance of melanogenesis extends beyond the mere assignment of a color trait.

Value of histopathology in vitiligo

BACKGROUND

Histopathologic studies of vitiligo have been rather limited in number, thus the microscopic features of this common disorder are not very well known.

SUBJECTS AND METHODS

Skin specimens from 20 human vitiligo patients and skin specimens from five equine vitiligo patients were studied by light and transmission electron microscopy.

RESULTS AND DISCUSSION

Absence of melanocytes, increased number of Langerhans' cells, epidermal vacuolization, thickening of the basement membrane, T-cell inflammatory infiltrate, and neural alterations were noted in the vitiligo lesions. These results may explain the development of depigmentation, but do not yet fully explain why and how this occurs.

Possible mechanisms of hypopigmentation in lichen sclerosus

Lichen sclerosus (LS) shares with vitiligo a milky-white appearance. By biopsy, pathognomonic dermal sclerosis readily distinguishes LS from vitiligo and other causes of leukoderma. To determine what the mechanism of hypopigmentation is in LS, we examined samples from LS cases for alterations in melanin content (Fontana-Masson stain) and melanocyte number (HMB-45 [PMEL-17/gp100], Mel-5 [TRP-1], Mart-1 [Melan A]) and compared these findings with those in controls of normal skin, acute scars, vitiligo, and lichen planus (LP; a common inflammatory cause of hyperpigmentation). The degree and extent of melanization found in LS overlapped with that in acute scars showing predominantly hypomelanized keratinocytes, with that in LP containing regions with numerous melanophages, and with that in vitiligo exhibiting focal regions of keratinocytes devoid of melanin pigment. By hematoxylin-eosin staining and immunocytochemistry for Mel-5 and Mart-1, LS had a lower mean count of melanocytes than acute scars, LP, and normal skin per 200 basal keratinocytes. In addition, a few LS cases had a significant loss of melanocytes comparable to that of vitiligo. Surprisingly, Mart-1 identified rare melanocytes in 67% of vitiligo cases and a significantly larger pool of melanocytes in LS and controls other than those labeled by Mel-5. Furthermore, LP and evolving lesions of LS contained the highest Mart-1 counts. HMB-45-immunoreactive melanocytes were found in the majority of acute scars and in LP and late-stage LS lesions at significantly lower levels than Mel-5- and Mart-1- labeled melanocytes, but they were not found in vitiligo or normal skin. We propose that several mechanisms may play a role in the production of leucoderma in LS: 1) decreased melanin production; 2) block in transfer of melanosomes to keratinocytes; and 3) melanocyte loss. The latter finding may be the pathogenic connection (lichenoid dermatitis of LS triggering an autoimmune reaction to melanocytes) that underlies the documented association of LS with vitiligo.

New insights into the pathogenesis of vitiligo: imbalance of epidermal cytokines at sites of lesions

Vitiligo is a skin disease that is caused by selective destruction of melanocytes and is characterized by white spots. Melanocytes and keratinocytes seem to exhibit a functional close relationship, mediated at least in part by keratinocyte-derived cytokines, which seem important for survival and activity of melanocytic cells. We wanted to investigate the hypothesis that in vitiligo the expression of epidermal cytokines may be modified compared with normal skin. In 15 patients with active, non-segmental vitiligo, biopsies were obtained from lesional, perilesional and non-lesional skin; normal skin from five healthy donors was also tested. Tissue sections were tested using immunohistochemistry for the expression of keratinocyte-derived cytokines with stimulating activity, such as granulocyte-monocyte colony stimulating factor (GM-CSF), basic fibroblastic growth factor (bFGF), and stem cell factor (SCF) or with inhibiting activity, such as interleukin 6 (IL-6) and tumour necrosis factor alpha (TNF-alpha) on melanocytes. Cytokine receptors and specific melanocytic markers were also investigated. No melanocyte was identified in lesional skin by means of specific markers or c-kit receptor, whereas in perilesional, non-lesional and healthy skin, melanocytes were found in similar number. In vitiligo skin a significantly lower expression of GM-CSF, bFGF and SCF was found, and a significantly higher expression of IL-6 and TNF-alpha was detected, compared with perilesional, non-lesional and healthy skin. In conclusion, we provided evidence that a significant change of epidermal cytokines exists in vitiligo skin compared with perilesional, non-lesional and healthy skin, suggesting that the cytokine production of epidermal microenvironment may be involved in vitiligo.

Increased sensitivity to peroxidative agents as a possible pathogenic factor of melanocyte damage in vitiligo

To examine the sensitivity of vitiligo melanocytes to external oxidative stress, we studied enzymatic and non-enzymatic anti-oxidants in cultured melanocytes of normal subjects (n = 20) and melanocytes from apparently normal skin of vitiligo patients (n = 10). The activity of superoxide dismutase and catalase and the intracellular concentrations of vitamin E and ubiquinone were evaluated in cultures at the fourth or fifth passage. In addition, cells were exposed to various concentrations of a peroxidizing agent, cumene hydroperoxide (CUH, 0.66-20 microM), for 1 and 24 h. Compared to normal melanocytes, vitiligo melanocytes showed normal superoxide dismutase and significantly lower catalase activities and higher vitamin E and lower ubiquinone levels. At the concentration used, CUH did not significantly affect cell number or viability of melanocytes after either period of culture. On the contrary, vitiligo melanocytes were susceptible to the toxic effect of CUH after 24 h of continuous treatment at concentrations greater than 6.6 microM. The degree of CUH toxicity correlated strictly with the anti-oxidant pattern, defined as the ratio between vitamin E concentration and catalase activity, suggesting that the alteration in the antioxidants was the basis for sensitivity to the external oxidative stress. Our results demonstrate the presence of an imbalance in the anti-oxidant system in vitiligo melanocytes and provide further support for a free radical-mediated damage as an initial pathogenic event in melanocyte degeneration in vitiligo.