Applications and future directions for optical coherence tomography in dermatology.

Optical coherence tomography (OCT) is a noninvasive optical imaging method that can generate high-resolution en face and cross-sectional images of the skin in vivo to a maximum depth of 2 mm. While OCT holds considerable potential for noninvasive diagnosis and disease monitoring, it is poorly understood by many dermatologists. Here we aim to equip the practising dermatologist with an understanding of the principles of skin OCT and the potential clinical indications. We begin with an introduction to the technology and discuss the different modalities of OCT including angiographic (dynamic) OCT, which can image cutaneous blood vessels at high resolution. Next we review clinical applications. OCT has been most extensively investigated in the diagnosis of keratinocyte carcinomas, particularly basal cell carcinoma. To date, OCT has not proven sufficiently accurate for the robust diagnosis of malignant melanoma; however, the evaluation of abnormal vasculature with angiographic OCT is an area of active investigation. OCT, and in particular angiographic OCT, also shows promise in monitoring the response to therapy of inflammatory dermatoses, such as psoriasis and connective tissues disease. We additionally discuss a potential role for artificial intelligence in improving the accuracy of interpretation of OCT imaging data.

What is AI? Applications of artificial intelligence to dermatology.

In the past, the skills required to make an accurate dermatological diagnosis have required exposure to thousands of patients over many years. However, in recent years, artificial intelligence (AI) has made enormous advances, particularly in the area of image classification. This has led computer scientists to apply these techniques to develop algorithms that are able to recognize skin lesions, particularly melanoma. Since 2017, there have been numerous studies assessing the accuracy of algorithms, with some reporting that the accuracy matches or surpasses that of a dermatologist. While the principles underlying these methods are relatively straightforward, it can be challenging for the practising dermatologist to make sense of a plethora of unfamiliar terms in this domain. Here we explain the concepts of AI, machine learning, neural networks and deep learning, and explore the principles of how these tasks are accomplished. We critically evaluate the studies that have assessed the efficacy of these methods and discuss limitations and potential ethical issues. The burden of skin cancer is growing within the Western world, with major implications for both population skin health and the provision of dermatology services. AI has the potential to assist in the diagnosis of skin lesions and may have particular value at the interface between primary and secondary care. The emerging technology represents an exciting opportunity for dermatologists, who are the individuals best informed to explore the utility of this powerful novel diagnostic tool, and facilitate its safe and ethical implementation within healthcare systems.

Spatial constraints govern competition of mutant clones in human epidermis.

Deep sequencing can detect somatic DNA mutations in tissues permitting inference of clonal relationships. This has been applied to human epidermis, where sun exposure leads to the accumulation of mutations and an increased risk of skin cancer. However, previous studies have yielded conflicting conclusions about the relative importance of positive selection and neutral drift in clonal evolution. Here, we sequenced larger areas of skin than previously, focusing on cancer-prone skin spanning five decades of life. The mutant clones identified were too large to be accounted for solely by neutral drift. Rather, using mathematical modelling and computational lattice-based simulations, we show that observed clone size distributions can be explained by a combination of neutral drift and stochastic nucleation of mutations at the boundary of expanding mutant clones that have a competitive advantage. These findings demonstrate that spatial context and cell competition cooperate to determine the fate of a mutant stem cell.

Expression of a Chimeric Antigen Receptor Specific for Donor HLA Class I Enhances the Potency of Human Regulatory T Cells in Preventing Human Skin Transplant Rejection.

Regulatory T cell (Treg) therapy using recipient-derived Tregs expanded ex vivo is currently being investigated clinically by us and others as a means of reducing allograft rejection following organ transplantation. Data from animal models has demonstrated that adoptive transfer of allospecific Tregs offers greater protection from graft rejection compared to polyclonal Tregs. Chimeric antigen receptors (CAR) are clinically translatable synthetic fusion proteins that can redirect the specificity of T cells toward designated antigens. We used CAR technology to redirect human polyclonal Tregs toward donor-MHC class I molecules, which are ubiquitously expressed in allografts. Two novel HLA-A2-specific CARs were engineered: one comprising a CD28-CD3ζ signaling domain (CAR) and one lacking an intracellular signaling domain (ΔCAR). CAR Tregs were specifically activated and significantly more suppressive than polyclonal or ΔCAR Tregs in the presence of HLA-A2, without eliciting cytotoxic activity. Furthermore, CAR and ΔCAR Tregs preferentially transmigrated across HLA-A2-expressing endothelial cell monolayers. In a human skin xenograft transplant model, adoptive transfer of CAR Tregs alleviated the alloimmune-mediated skin injury caused by transferring allogeneic peripheral blood mononuclear cells more effectively than polyclonal Tregs. Our results demonstrated that the use of CAR technology is a clinically applicable refinement of Treg therapy for organ transplantation.

Upregulation of CD26 expression in epithelial cells and stromal cells during wound-induced skin tumour formation.

We have previously described InvEE transgenic mice in which non-dividing, differentiating epidermal cells express oncogenically activated MAPK kinase 1 (MEK1). Skin wounding triggers tumour formation in InvEE mice via a mechanism that involves epidermal release of IL-1α and attraction of a pro-tumorigenic inflammatory infiltrate. To look for potential effects on the underlying connective tissue, we screened InvEE and wild-type epidermis for differential expression of cytokines and immune modulators. We identified a single protein, CD26 (dipeptidyl peptidase-4). CD26 serum levels were not increased in InvEE mice. In contrast, CD26 was upregulated in keratinocytes expressing mutant MEK1 and in the epithelial compartment of InvEE tumours, where it accumulated at cell-cell borders. CD26 expression was increased in dermal fibroblasts following skin wounding but was downregulated in tumour stroma. CD26 activity was stimulated by calcium-induced intercellular adhesion in keratinocytes, suggesting that the upregulation of CD26 in InvEE epidermis is due to expansion of the differentiated cell layers. IL-1α treatment of dermal fibroblasts stimulated CD26 activity, and therefore epidermal IL-1α release may contribute to the upregulation of CD26 expression in wounded dermis. Pharmacological blockade of CD26, via Sitagliptin, reduced growth of InvEE tumours, while combined inhibition of IL-1α and CD26 delayed tumour onset and reduced tumour incidence. Our results demonstrate that inappropriate activation of MEK1 in the epidermis leads to changes in dermal fibroblasts that, like the skin inflammatory infiltrate, contribute to tumour formation.

Role of beta-catenin in epidermal stem cell expansion, lineage selection, and cancer.

The mammalian epidermis is an excellent model with which to analyze the factors that regulate adult stem cell renewal, lineage selection, and tumor formation. One of the key regulators of all three processes is beta-catenin, the main cytoplasmic effector of the canonical Wnt signaling pathway. In this chapter, we review some of the ways in which beta-catenin exerts its effects on cultured human epidermal cells and in genetically modified mice. We highlight the importance of the timing and level of activation and discuss some of the pathways activated downstream from beta-catenin. Finally, we demonstrate the importance of Lef/Tcf-independent beta-catenin signaling through interaction with the vitamin D receptor.

Indian hedgehog and beta-catenin signaling: role in the sebaceous lineage of normal and neoplastic mammalian epidermis.

In mammalian epidermis, the level of beta-catenin signaling regulates lineage selection by stem cell progeny. High levels of beta-catenin stimulate formation of hair follicles, whereas low levels favor differentiation into interfollicular epidermis and sebocytes. In transgenic mouse epidermis, overexpression of beta-catenin leads to formation of hair follicle tumors, whereas overexpression of N-terminally truncated Lef1, which blocks beta-catenin signaling, results in spontaneous sebaceous tumors. Accompanying overexpression of beta-catenin is up-regulation of Sonic hedgehog (SHH) and its receptor, Patched (PTCH/Ptch). In DeltaNLef1 tumors Ptch mRNA is up-regulated in the absence of SHH. We now show that PTCH is up-regulated in both human and mouse sebaceous tumors and is accompanied by overexpression of Indian hedgehog (IHH). In normal sebaceous glands IHH is expressed in differentiated sebocytes and the transcription factor GLI1 is activated in sebocyte progenitors, suggesting a paracrine signaling mechanism. PTCH1 and IHH are up-regulated during human sebocyte differentiation in vitro and inhibition of hedgehog signaling inhibits growth and stimulates differentiation. Overexpression of DeltaNLef1 up-regulates IHH and stimulates proliferation of undifferentiated sebocytes. We present a model of the interactions between beta-catenin and hedgehog signaling in the epidermis in which SHH promotes proliferation of progenitors of the hair lineages whereas IHH stimulates proliferation of sebocyte precursors.

Gene targeting of envoplakin, a cytoskeletal linker protein and precursor of the epidermal cornified envelope.

Envoplakin, a member of the plakin family of cytoskeletal linker proteins, is localized in desmosomes of stratified epithelial cells and is a component of the epidermal cornified envelope. Gene targeting in mouse embryonic stem cells was used to generate a null allele of envoplakin. No envoplakin transcripts from the targeted allele could be detected in the skin of newborn mice. Mice homozygous for the targeted allele were born in the normal Mendelian ratio and were fertile. They did not develop any discernible pathological phenotype up to the age of 1 year. The ultrastructural appearance of cornified envelopes from adult epidermis was indistinguishable between wild-type and knockout mice, and there was no evidence that the absence of envoplakin affected the subcellular distribution of periplakin or desmoplakin, two other plakins found in desmosomes. The proportion of immature cornified envelopes in the epidermis of newborn mice was greater in envoplakin-null animals than in heterozygous littermates or wild-type mice, and the envelopes had a larger surface area. This correlated with a slight delay in barrier acquisition during embryonic development. We conclude that although envoplakin is part of the scaffolding on which the cornified envelope is assembled, it is not essential for envelope formation or epidermal barrier function.

Cutting edge: amelioration of kidney disease in a transgenic mouse model of lupus nephritis by administration of the caspase inhibitor carbobenzoxy-valyl-alanyl-aspartyl-(beta-o-methyl)-fluoromethylketone.

Systemic lupus erythematosus (SLE) is a common, potentially fatal, non-organ-specific autoimmune disorder. Immune complex-mediated kidney disease is the major cause of mortality. Apoptotic cells in the epidermis are a possible source of self Ags, and apoptosis of endothelial cells and lymphocytes is thought to contribute to end-organ damage. We have previously shown that female transgenic mice expressing IFN-gamma in the epidermis develop inflammatory skin disease and features of SLE that have striking parallels with the human condition. We have now tested the effects of a pan-caspase inhibitor, carbobenzoxy-valyl-alanyl-aspartyl-(beta-o-methyl)-fluoromethylketone, on disease progression. Daily s.c. administration of carbobenzoxy-valyl-alanyl-aspartyl-(beta-o-methyl)-fluoromethylketone to female transgenic mice over a 3-wk period resulted in significant amelioration of both glomerular and interstitial renal damage, independent of the effects on autoantibody levels or skin inflammation. We propose that apoptosis inhibitors could be beneficial in the treatment of human SLE.

Delta regulates keratinocyte spreading and motility independently of differentiation.

In human interfollicular epidermis stem cells lie in clusters surrounded by their differentiated daughters, transit amplifying cells, an arrangement that reflects differences in cell cohesiveness and motility. Keratinocytes expressing a dominant negative Delta1 mutant, Delta(T), lacking most of the cytoplasmic domain, acquired the motile behaviour of transit cells while retaining their stem cell identity. Conversely, overexpression of Delta1 promoted keratinocyte cohesiveness. The adhesive effects of Delta1 and Delta(T) were independent of SuH-dependent Notch signalling. Delta(T) increased motility and spreading of individual keratinocytes and stimulated lamellipodia formation. Delta and Delta(T) colocalised with cortical actin and redistributed on Latrunculin treatment. We propose that Delta promotes keratinocyte cohesiveness by restricting motility and discuss potential mechanisms by which Delta could interact with the actin cytoskeleton.

A role for mitogen-activated protein kinase activation by integrins in the pathogenesis of psoriasis.

In normal epidermis, beta1 integrin expression is confined to the basal layer, whereas in hyperproliferative epidermis, integrins are also expressed in the suprabasal layers. Transgenic mice in which integrins are expressed suprabasally via the involucrin promoter have a sporadic psoriatic phenotype; however, the mechanism by which integrins contribute to the pathogenesis of psoriasis is unknown. We observed activation of mitogen-activated protein kinase (MAPK) in basal and suprabasal keratinocytes of human and transgenic mouse psoriatic lesions and healing mouse skin wounds, correlating in each case with suprabasal integrin expression. Phenotypically normal human and transgenic mouse epidermis did not contain activated MAPK. Transgene-positive keratinocytes produced more IL-1alpha than controls did, and keratinocyte MAPK could be activated by ligation of suprabasal integrins or treatment with IL-1alpha. Constitutive activation of MAPK increased the growth rate of human keratinocytes and delayed the onset of terminal differentiation, recreating many of the histological features of psoriatic epidermis. We propose that activation of MAPK by integrins, either directly or through increased IL-1alpha production, is responsible for epidermal hyperproliferation in psoriasis and wound healing, and that the sporadic phenotype of the transgenic mice may reflect the complex mechanisms by which IL-1 release and responsiveness are controlled in skin.

Stem cell fate and patterning in mammalian epidermis.

Recent studies highlight characteristics of epidermal stem cells that were not fully appreciated before. Stem cells are multipotential and signals exchanged with their neighbours help to regulate exit from the stem cell compartment and differentiation along specific lineages. Stem cells exhibit a high degree of spatial organisation, and cell clustering and motility contribute to the assembly and maintenance of the epidermis.

Influence of beta1 integrins on epidermal squamous cell carcinoma formation in a transgenic mouse model: alpha3beta1, but not alpha2beta1, suppresses malignant conversion.

Although aberrant integrin expression has been documented in many epithelial tumors, little is known about how integrins influence neoplastic progression. To examine this issue, transgenic mice in which the alpha2beta1 or alpha3beta1 integrin was expressed in the suprabasal epidermal layers via the involucrin promoter were subjected to skin carcinogenesis. Equal numbers of benign squamous papillomas were observed in transgenic and wild-type animals. However, the frequency of conversion of papillomas to malignant squamous cell carcinomas was much lower in alpha3beta1 transgenic than in alpha2beta1 transgenic and wild-type mice. No differences were observed in apoptosis or in the expression of endogenous integrins in transgenic and wild-type papillomas. However, alpha3beta1 transgenic papillomas displayed a diminished proliferative capacity and were more highly differentiated as judged by BrdUrd incorporation and keratin 10 expression, respectively, than alpha2beta1 transgenic and wild-type papillomas. Two proteins that associate with alpha3beta1 and not alpha2beta1 are extracellular matrix metalloproteinase inducer and CD81. Extracellular matrix metalloproteinase inducer expression correlated inversely with the degree of differentiation in normal epidermis and in transgenic and wild-type papillomas. Up-regulation of CD81 was observed in 100% of wild-type and 88% of alpha2beta1 transgenic papillomas but in only 25% of alpha3beta1 transgenic papillomas. CD81 was undetectable in untreated epidermis and strongly expressed in all transgenic and wild-type squamous cell carcinomas. Our results demonstrate that the alpha3beta1 integrin can suppress malignant conversion, and that the mechanism may involve CD81.

c-Myc activation in transgenic mouse epidermis results in mobilization of stem cells and differentiation of their progeny.

BACKGROUND: The epidermis is maintained throughout adult life by pluripotential stem cells that give rise, via daughter cells of restricted self-renewal capacity and high differentiation probability (transit-amplifying cells), to interfollicular epidermis, hair follicles, and sebaceous glands. In vivo, transit-amplifying cells are actively cycling, whereas stem cells divide infrequently. Experiments with cultured human keratinocytes suggest that c-Myc promotes epidermal-stem cell differentiation. However, Myc is a potent oncogene that suppresses differentiation and causes reversible neoplasia when expressed in the differentiating epidermal layers of transgenic mice. To investigate the effects of c-Myc on the stem cell compartment in vivo, we targetted c-MycER to the basal layer of transgenic mouse epidermis. RESULTS: The activation of c-Myc by the application of 4-hydroxy-tamoxifen caused progressive and irreversible changes in adult epidermis. Proliferation was stimulated, but interfollicular keratinocytes still underwent normal terminal differentiation. Hair follicles were abnormal, and sebaceous differentiation was stimulated at the expense of hair differentiation. The activation of c-Myc by a single application of 4-hydroxy-tamoxifen was as effective as continuous treatment in stimulating proliferation and sebocyte differentiation, and the c-Myc-induced phenotype continued to develop even after the grafting of treated skin to an untreated recipient. CONCLUSIONS: We propose that transient activation of c-Myc drives keratinocytes from the stem to the transit-amplifying compartment and thereby stimulates proliferation and differentiation along the epidermal and sebaceous lineages. The ability, demonstrated here for the first time, to manipulate exit from the stem cell compartment in vivo will facilitate further investigations of the relationship between stem cells and cancer.

Paraneoplastic pemphigus sera react strongly with multiple epitopes on the various regions of envoplakin and periplakin, except for the c-terminal homologous domain of periplakin.

Paraneoplastic pemphigus sera react with multiple plakin family proteins, among which only envoplakin and periplakin are constantly detected by immunoblotting using normal human epidermal extracts. Using bacterial expression vectors containing polymerase chain reaction-amplified cDNA, we have prepared variously truncated recombinant glutathione-S-transferase-fusion proteins of envoplakin and periplakin, which presented N-terminal, central and C-terminal domains of each protein, as well as the so-called C-terminal homologous domain of envoplakin and the junctional regions of these domains. By immunoblotting using these 11 recombinant proteins, we demonstrated that most of the 26 paraneoplastic pemphigus sera reacted very strongly with multiple recombinant proteins of envoplakin and periplakin, except for the C-terminal homologous domain of periplakin. We also examined the reactivity with these recombinant proteins of other blistering diseases, including pemphigus vulgaris, pemphigus foliaceus, and bullous pemphigoid, and found that a few nonparaneoplastic pemphigus sera showed a weak reactivity with some of the recombinant proteins. Interestingly, some sera showed relatively strong reactivity with the C-terminal homologous domain of periplakin to which paraneoplastic pemphigus sera reacted less frequently. These results indicate that, although nonparaneoplastic pemphigus sera occasionally show a weak reactivity with envoplakin and periplakin, the pathogenicity and the mechanism of antibody production in these cases may be different from those in paraneoplastic pemphigus.

p19ARF-independent induction of p53 and cell cycle arrest by Raf in murine keratinocytes.

In tumorigenesis of the skin, activated Ras co-operates with mutations that inactivate the tumour suppressor p53, but the molecular basis for this co-operation remains unresolved. Here we show that activation of the Raf/MAP kinase pathway in primary mouse keratinocytes leads to a p53 and p21Cip1-dependent cycle arrest and to terminal differentiation. Raf activation in keratinocytes lacking p53 or p21Cip1 genes leads to expression of differentiation markers, but the cells do not cease to proliferate. Thus, loss of p53 or p21Cip1 function is necessary to disable growth-inhibitory Raf/MAP kinase signalling. Activation of oncogenes, including Ras, has been reported to stabilize and activate p53 via induction of the tumour suppressor p19ARF. However, the response to Raf in p19ARFI-/- keratinocytes was indistinguishable from wild-type controls. Thus, p19ARF is not essential for Raf-induced p53 induction and cell cycle arrest in keratinocytes, indicating that oncogenes engage p53 activity via multiple mechanisms.

Dermal fibroblast-derived growth factors restore the ability of beta(1) integrin-deficient embryonal stem cells to differentiate into keratinocytes.

Embryonal stem (ES) cells that are homozygous null for the beta(1) integrin subunit fail to differentiate into keratinocytes in vitro but do differentiate in teratomas and wild-type/beta(1)-null chimeric mice. The failure of beta(1)-null ES cells to differentiate in culture might be the result of defective extracellular matrix assembly or reduced sensitivity to soluble inducing factors. By culturing embryoid bodies on dead, deepidermized human dermis (DED) we showed that epidermal basement membrane did not induce beta(1)-null ES cells to undergo keratinocyte differentiation and did not stimulate the differentiation of wild-type ES cells. Coculture with epidermal keratinocytes also had no effect. However, when human dermal fibroblasts were incorporated into DED, the number of epidermal cysts formed by wild-type ES cells increased dramatically, and small groups of keratin 14-positive cells differentiated from beta(1)-null ES cells. Fibroblast-conditioned medium stimulated differentiation of K14-positive cells in wild-type and beta(1)-null embryoid bodies. Of a range of growth factors tested, KGF, FGF10, and TGFalpha all stimulated differentiation of keratin 14-positive beta(1)-null cells, and KGF and FGF10 were shown to be produced by the fibroblasts used in coculture experiments. The effects of the growth factors on wild-type ES cells were much less pronounced, suggesting that the concentrations of inducing factors already present in the medium were not limiting for wild-type cells. We conclude that the lack of beta(1) integrins decreases the sensitivity of ES cells to soluble factors that induce keratinocyte differentiation.

Epithelial cell differentiation pathways in the human prostate: identification of intermediate phenotypes by keratin expression.

The prostate grows slowly throughout adult life, leading to benign prostatic hyperplasia (BPH), which often results in urethral obstruction in later years. The symptoms of BPH are the second most common reason for surgery in men over 65. The aim of this study was to determine the relationship between cell proliferation and cell differentiation in BPH tissue. Using multiple antibodies, simultaneously detected with different fluorophore-conjugated secondary antibodies, several subpopulations of epithelial cells were detected. In addition to K14, basal cells also expressed keratins 15, 17, and 19 in various combinations, and some of the luminal cells also expressed K19 together with K8 and K18. Co-staining for cytokeratins and Ki-67 indicated that 44% of proliferative cells expressed K14 and 36% K19, although the difference was not statistically significant. This report provides a detailed description of the relationship between keratin expression and cell proliferation in the prostate and indicates that K19-positive cells form the link between the basal and luminal layers of the epithelium. (J Histochem Cytochem 49:271-278, 2001)

PA-FABP, a novel marker of human epidermal transit amplifying cells revealed by 2D protein gel electrophoresis and cDNA array hybridisation.

Human epidermal stem cells express higher levels of beta1 integrins than their more differentiated daughters, transit amplifying cells. In a search for additional stem and transit cell markers we used proteomics and differential cDNA hybridisation to compare keratinocytes fractionated on the basis of beta1 integrin expression. There were remarkably few differences between the two populations and none of the RNAs differed in abundance by more than 2-fold. Nevertheless, proteomics revealed upregulated expression of epidermal fatty acid binding protein (PA-FABP, also known as E-FABP), Annexin II and two keratin related proteins in the transit population. An unknown high molecular mass protein was upregulated in the stem cell population. The upregulation of PA-FABP was confirmed by Northern blotting and conventional and whole mount labelling of human epidermis. We conclude that PA-FABP is a novel marker of epidermal transit amplifying cells.