Recent advances in understanding of radiation-induced skin tissue reactions with respect to acute tissue injury and late adverse effect

Abstract

Skin is a tissue vulnerable to radiation exposure, which causes acute tissue reactions, including erythema, edema, desquamation, ulceration and late effects, such as skin cancers. As the effects of radiation exposure on the skin tissue are easily evaluated by visual examination, much information on radiation-induced skin reactions has been available from the clinical observation of people exposed to ionizing radiation, such as cancer patients receiving radiotherapy, although the mechanisms underlying skin reactions have not yet been fully understood. Recent advances in tissue biology at the molecular level have provided insights into the mechanisms of skin tissue reactions from the stem cell points of view. For example, our understanding of epidermal regeneration by epidermal stem cells as well as cells from the bulge in humans and the sebaceous gland in mouse, descriptions of the role of skin immune cells on inflammatory response and maintenance of genome integrity by epidermal stem cell competition, have greatly improved in the last decade with the identification of several key molecules. Thus, this review will provide an overview of the current status toward the comprehensive understanding of the mechanisms of adverse skin tissue reactions, with respect to mitigation of acute skin injuries as well as late carcinogenesis in response to ionizing radiation. In particular, the pleiotropic features of various types of cells consisting of skin tissue and their roles in securing skin functional homeostasis will be discussed.

INTRODUCTION

As the largest organ of the body, the skin plays a crucial role as a physical barrier against pathogens, thermoregulation, water transpiration, immune surveillance and perception. The skin is also essential for ensuring smooth movement of the body and exerts endocrine and exocrine activities [1–4]. The skin is composed of three major layers, which are epidermis, dermis and subcutaneous tissue. Epidermis has four functional layers, including basal, spinous, granular and cornified layers. Dermis connects epidermis and subcutaneous tissue, and skin appendages, such as hair follicles, sebaceous glands and sweat glands, are embedded in the epidermis [3–8].

The skin, as the outermost organ, is exposed to various types of physical, chemical and biological stresses, so that there are sophisticated systems to maintain organizational homeostasis and integrity [2–4, 7, 8]. Skin tissue response so far has been extensively studied after exposure to ionizing radiation, which is one of such stressors. Radiation causes acute skin tissue reactions as well as late skin cancers [9]. As these are mostly an epidermal origin, previous studies have primarily focused on the responses of the epidermis. Since the discovery of X-rays, skin complications were the first medical symptom to be recorded. Therefore, historically, it has been described that the skin is one of the most sensitive organs to radiation [9, 10]. From a radiation protection point of view, the threshold doses were assigned. For example, the threshold dose for early transient erythema, which is due to vascular dilation, is estimated to be 2 Gy. A main erythema reaction, which is caused by the death of basal epithelial cells followed by inflammation, will be manifested with ~6 Gy or more. In contrast to such lower doses, higher dose exposure may cause serious outcomes, such as dry and moist desquamation and ulceration, although severer effects need longer time of onset. The matrix cells at the bottom of hair follicle are also sensitive to radiation exposure, whose death gives rise to epilation and alopecia [9, 11–16].

The late effect of radiation exposure is characterized by the induction of skin cancer, most of which is also of epidermal origin. Non-melanoma skin cancers are the predominant cancer type in human skin, and basal cell carcinoma (BCC) is more frequent than squamous cell carcinoma (SCC) in response to radiation exposure. A dose-dependent and age-at-exposure-dependent risk was described in detail among populations receiving ionizing radiation [17–19].

While previous information described above seems to be sufficient for understanding the effects of radiation on skin tissue, most of them were descriptive and obtained from a clinical point of view. Thus, the knowledge is still insufficient in terms of mechanistic insights in view of recent advances in the skin biology, which is essential for management and mitigation of the adverse effects of skin tissue reactions caused by radiation exposure. Therefore, the present review will provide an overview of the pleiotropic aspects of skin tissue reaction to ionizing radiation obtained recently, with an emphasis on the mechanistic perspective. In this review, unless otherwise stated, all descriptions relate to human skin.

STRUCTURE OF THE SKIN

Epidermis

The uppermost layer of the skin is the epidermis, which forms the waterproof barrier (Fig. 1). It consists of four layers, the stratum corneum (SC), stratum granulosum, stratum spinosum and the basal layer, in which the bottom basal layer contains clonogenic cells. The epidermis is largely composed of keratinocytes, together with melanocytes, Langerhans cells and other immune cells. The renewal of the epidermis is dependent upon growth of the keratinocytes at the basal layer, and the turnover of epidermis takes ~39–56 days in human [20–23]. The turnover of the skin has been demonstrated to decrease as skin ages. In studies, it was reported that 30–50% decrease between third and eighth decades [24–29]. Accordingly, the increase of transepidermal water loss was observed age-dependently [24].

Fig. 1.

Structure of the epidermis. The epidermis consists of four layers. The bottommost layer named basal cell layer consists of keratinocytes with epidermal stem cells. The differentiating cells leave the basal layer, and migrate to the epidermal surface, through stratum spinosum, stratum granulosum, and finally, they are shed periodically from SC. The epidermis also contains other types of cells, including melanocytes and Langerhans cells.

Epidermal stem cells are located in the basal layer, and it has been reported that about 7% of cells in the basal layer could be stem and its progenitor cells. In another study, using stem- and progenitor-specific membrane markers demonstrated that less than 1% and 10% of total cell population are stem and progenitor cells, respectively [30, 31]. More recently, a technique defining long-term repopulating cells was introduced [32], and the frequency of stem cells was estimated to be 0.003% of total epidermal cells, which corresponded to 0.01% of basal cells [32].

While the concept named the epidermal proliferating unit (EPU) was proposed by Potten over 50 years ago [33, 34], the stem cell population in the interfollicular epidermis and its hierarchy within the EPU have not been sufficiently determined yet. Recent studies using lineage-tracing technique and single cell transcriptomics have suggested that stochastic asymmetric division of stem cells within the EPU serves a progenitor cell, and they are doomed to give rise to cornified layers with multiple cell divisions [35–37]. Other studies pointed out that stem cell-derived progenitors from multiple units likely contribute to form EPUs [38]. Furthermore, a single-cell RNA sequencing has uncovered heterogeneity of interfollicular stem cells and demonstrated that four spatially distinct stem cells were identified [39]. Thus, further studies are needed to draw an entire view of the stem cells in epidermis.

Hair follicle and other epidermal appendages

Hair follicles play indispensable roles in the function and regeneration of the skin. The outermost region of the hair follicle is called the connective tissue sheath including inner root sheath and outer root sheath, whose components are provided by the dermis. The lower half of hair follicle contains dermal papilla at the bottom, surrounded by the hair matrix and melanocytes (Fig. 2). The dermal papilla is rich in capillaries and has stem cells. The hair matrix cells form medulla of the shaft and inner root sheath with the aid of dermal papilla. The outer root sheath is provided by epidermis. The upper half contains the bulge, sebaceous gland and arrector pili muscle [40–42]. The hair cycle is divided into three stages, such as anagen, catagen and telogen. The anagen stage is the growing stage of hair cycle, which lasts for 2–8 years, followed by the catagen phase of ~2 weeks. During catagen, matrix cells terminate their growth, and lower follicles regress to move condensed dermal papilla upward toward the bulge region. When regression completes, the follicle enters to telogen, which is the resting phase of hair cycle that lasts for 2–3 months [41–43].

Fig. 2.

Structure of the skin appendages. The skin appendages include hairs, sweat glands and sebaceous glands. The hair structure divides into the hair shaft and hair follicle, the latter contains the hair bulb. The bottom part of the hair follicle is called the dermal papilla, which is an invagination of vascularized connective tissues. Around the papilla is the hair matrix, containing melanocytes. The upper part of hair bulb continues to a root sheath composed of an outer and inner root sheath, and the inner root sheath forms cuticle that constitutes the outermost layer of the hair shaft. Cutaneous microvasculature is organized in the dermis, and there are two layers of horizontal plexuses.

In addition to hair follicles, the epidermal appendages include sebaceous glands and sweat glands, in which the latter has two different types called eccrine and apocrine glands [44, 45] (Fig. 2).

Dermis

The dermis is located beneath the epidermis, and is composed of fibrous proteins, including collagen and elastic fibers, which supports the epidermis. The dermis also plays essential roles in thermoregulation, water transpiration, immune surveillance and perception, as the vasculature, nerves and glands are located within the dermis [46, 47]. The primary cells in the dermis are fibroblasts, and several other types of cells, like immune cells, also exist. The dermis is also a space where cutaneous microvasculature is organized. There are two layers of horizontal plexuses, in which one is beneath the epidermis and the other is above the dermal-subcutaneous junction [48–50].

Subcutaneous tissue

The subcutaneous tissue is the lowermost layer of the skin, which plays a major storage site of fat. The subcutaneous tissue also contains fibroblasts and macrophages, as well as large blood and lymphatic vessels [51–54]. Beneath the subcutaneous layer is the ‘panniculus carnosus’, which is a striated muscular layer with fascia [55–57]. In mouse back skin, the panniculus carnosus is accompanied by the perifascial areolar tissue, which is rich in macrophages and fibroblasts [58].

FUNCTION OF THE SKIN

Barrier

The defensive function is the indispensable role of skin tissue, which serves as the physical barrier against pathogens, limits passive water transpiration and reduces absorption of environmental toxic materials. The outermost physical barrier is secured by SC, which is formed by terminal differentiation of keratinocytes named keratinization. The second biological barrier is ensured by cell-to-cell connections through tight junctions (TJ), adherence junctions (AJ), gap junctions (GJ) and desmosomes. TJ, which constitute the second barrier after SC, function as a water flux regulator. It also plays a crucial role in the epithelial cell polarization. AJ and desmosome are involved in strong cell-to-cell adhesion, while GJ forms a cell-to-cell communication mediating the exchange of ions and second messengers through connexin channels [5–8, 59].

In addition to the physical barrier, chemical barrier, which serves acidic pH condition, and biological barrier through the release of antimicrobial peptides (AMPs), such as defensins, cathelicidins and dermcidin exist [60–64]. The maintenance of acidic pH is managed by several mechanisms, including weakly acidic natural-moisturizing factors derived from epidermal filaggrin, and secreted factors from the sebaceous gland, such as triglycerides, fatty acids, wax esters and squalene [65–67]. For example, triglycerides are hydrolyzed to free fatty acids by epidermal microorganisms, such as bacteria and yeasts, contributing the acidification of the skin [60–64].

Immunity

Cutaneous immune surveillance is an active defence mechanism, by which the host is securely protected from the insults of microbial pathogens. Immune response is highly sophisticated mechanism orchestrated by the interaction of innate and adaptive immune systems [68]. Invasion of microorganisms is attentively watched by the sentinel cells resident in the epidermis and dermis, such as keratinocytes, Langerhans cells (LCs), dendritic cells (DCs), macrophages and mast cells [68–70] (Table 1). Keratinocytes play the primary roles in sensing the pathogens and mediating immune responses. They present receptors recognizing pathogen-associated molecular patterns, among which the Toll-like receptors (TLRs) are the best studied [69]. Keratinocytes express several TLRs, and upon activation of TLRs, they produce AMPs as well as a variety of cytokines, including tumor necrosis factor (TNF), and the IL-33 and IL-1 families, which stimulate innate immune system. AMPs production from keratinocytes is potentiated by IL-17 and IL-22 secreted from TH₁₇ cells, which is activated by IL-1. IL-1 also stimulates DCs and macrophages to execute local inflammation. Keratinocytes also secrete several chemokines, such as CCL20, CXCL9, CXCL10 and CXCL11, thereby activating skin-resident immune cells and recruiting immune cells to the skin [71].

Table 1.

Immune cells in the skin and their radiation sensitivity

	Human	Mouse	Radiation sensitivity
Epidermis	Langerhans	Langerhans	Resistant
	CD8+ TRM	CD8+ TRM	Slightly resistant
		DETC	Sensitive
Dermis	cDC1	cDC1	Resistant
	cDC2	cDC2	Resistant
	Macrophage	Macrophage	Resistant
	Treg	Treg	Slightly resistant
	CD4+ TRM	CD4+ TRM	Slightly resistant
	γδT cells	γδT cells	Sensitive/resistant
	ILC2	ILC2	Sensitive
	NKT	NKT	Slightly resistant

For more information, see Ref. [16].

Once epidermal LCs and dermal DCs are activated as professional antigen presenting cells (APCs), they present antigens and promote maturation of naive T cells [72–77]. Together with CD8+ T cells in the epidermis, functionally activated T cells constitute one branch of adaptive immune systems [78]. In the dermis, B cells also constitute another branch of adaptive immune systems. Like T cells, B cells are migrated to the site of inflammation, which is mediated by selectins, chemokines receptors and integrins [79, 80].

Accumulating evidences have demonstrated that other types of immune cells exist in the skin [81, 82]. Those include γδT (dendritic epidermal T cells: DETCs in mice) cells and CD8⁺ resident memory cells in the epidermis, and CD4⁺ and CD8⁺ T cells, NK cells, NKT cells and innate lymphoid cells in the dermis [83–93] (Table 1). Several reports have indicated that they also play a role in orchestrating the skin immunity, however, further studies will be needed to understand complex and sophisticated crosstalk between these cells.

MECHANISMS OF SKIN TISSUE REACTION TO RADIATION

Radiation exposure induces various types of DNA damage, among which DNA double-strand breaks (DSBs) are the most harmful [10]. Mammalian cells have multiple sophisticated DSB repair systems, such as non-homologous end-joining and homologous recombination, however, as the dose increases the fraction of DSBs that are unable to repair increase. Such irreparable DSBs trigger cell death, giving rise to tissue reaction [9–11].

So far, it has been described that radiation exposure executes several cell death pathways including apoptosis, necrosis, autophagy and premature senescence. Manifestation of cell death modes is dependent on tissue types. For example, hematopoietic tissues such as bone marrow, thymus and spleen prefer to induce apoptosis, while many epithelial tissues, such as lung, liver, thyroid gland and mammary gland, primarily exhibit premature senescence. Cells comprising connective tissues, such as fibroblasts, endothelial cells and macrophages also induce premature senescence. If the microvasculature is disabled by radiation exposure, necrosis is finally triggered [94].

In the skin, epidermal keratinocytes mostly induce premature senescence in response to ionizing radiation, while they induce apoptosis after ultraviolet light irradiation [95–97]. Apoptosis could be the major mode of cell death in hair follicles [15, 98, 99], sebaceous glands and sweat glands, although the final conclusion still awaits further confirmation [100–104]. Whereas keratinocytes and epithelial cells of sebaceous and sweat glands are radiation sensitive, dermal fibroblasts are more resistant. While T cells in epidermis are radiation sensitive, LC in the epidermis, DC and macrophages in the dermis are resistant [16] (Table 1).

Acute radiation syndrome

The skin is one of the most responsive tissues to radiation exposure. In general, as the dose increases, the number of dead cells also increases, giving rise to severe symptoms (Table 2). In other words, even if cell death is induced, functional failure may not be visible, if the amount of dead cells is marginal. Thus, even if the skin tissue is exposed to radiation, it may not always manifest tissue reaction. This is the fundamental mechanism defining the threshold dose. Therefore, over the threshold dose, several adverse reactions are induced. For example, several waves of erythema are well documented [9, 10]. Epilation, dry and moist desquamation and ulceration become detectable depending on the dose [9, 10] (Table 2).

Table 2.

Approximate threshold single dose for the reaction of human skin tissue

Effect	Approximate threshold dose (Gy)	Time of onset
Early transient erythema	2	2–24 h
Main erythema reaction	6	~1.5 weeks
Temporary epilation	3	~3 weeks
Permanent epilation	7	~3 weeks
Dry desquamation	14	~4–6 weeks
Moist desquamation	18	~4 weeks
Secondary ulceration	24	>6 weeks
Late erythema	15	8–10 weeks
Ischaemic dermal necrosis	18	>10 weeks
Dermal atrophy (first phase)	10	>52 weeks
Telangiectasia	10	>52 weeks
Dermal necrosis (late phase)	>15?	>52 weeks

From ICRP publication 18, 2012 (Ref. [10]).

Since acute radiation dermatitis is one critical adverse effect of radiotherapy, particularly in the patients with the radiotherapy for breast, and head and neck cancers, scoring systems for grading acute radiation dermatitis caused by radiotherapy have been established by several organizations (Table 3), which are useful for preventing and management of acute radiation dermatitis.

Table 3.

Scoring criteria used for grading acute radiation dermatitis

		Grade/Score 1		Grade/Score 2	Grade/Score 3	Grade/Score 4
CTCAE (5.0)		Faint erythema or dry desquamation		Moderate to brisk erythema patchy moist desquamation mostly confined to skin folds and creases; moderate edema	Moist desquamation in areas other than skin folds and creases; bleeding induced by minor trauma or abrasion	Life-threatening consequences; skin necrosis or ulceration of full thickness dermis; spontaneous bleeding from involved site; skin graft indicated
RTOG		Erythema, epilation dry desquamation		Bright erythema, edema moist desquamation,	Confluent moist desquamation, pitting edema	Ulceration, hemorrhage necrosis
	Score 1.0	Score 1.5	Score 2.0	Score 2.5	Score 3.0	Score 3.5
ONS	Faint or dull erythema	Bright erythema	Dry desquamation with or without erythema	Small to moderate amount of moist desquamation	Confluent moist desquamation	Ulceration, hemorrhage or necrosis

CTCAE = Common terminology criteria for adverse events, RTOG = Radiation Therapy Oncology Group, ONS = Oncology Nursing Society.

Ryan et al. [105].

The uniqueness of the skin tissue reaction to radiation is that the manifestation of radiation effect shows an area-size dependency. For example, small area of skin is more resistant to higher doses than large area, indicating efficient repair of epidermal injuries, which is confirmed by the evidence that epidermis at the edge of an irradiated area is highly proliferative and mobile [9, 16, 106–108].

Another uniqueness is radiation quality dependence. As radiation dermatitis is well documented in relation to radiotherapy, X-rays or γ-rays, which can penetrate deep into the skin tissue, are the subject of consideration, and therefore, radiation dermatitis is commonly observed. In contrast, α-rays are unable to penetrate even the outermost keratin layer of skin, so that the skin injury, called alpha burns, is only limited to mild reddening of the outmost layers [109, 110]. β-rays are able to penetrate into the skin layer, although their penetrability is limited to ~1 cm, and so-called beta burns, which are superficial injuries like sunburn, are caused [109, 110]. Accordingly, different threshold dose for the acute effects from β-rays was documented [111]. For example, the threshold dose for early erythema was ~6 Gy for β-rays, which is much higher than that reported after radiotherapy (Table 2).

Multiple mechanisms are involved in the manifestation of radiation-induced dermatitis [16]. Upon irradiation, keratinocytes halt cell cycle progression, and if DNA damage is irreparable, permanent cell cycle arrest gives rise to the induction of premature senescence [112]. Senesced keratinocytes secrete soluble factors that regulate inflammatory response as described below. They also result in dysregulation of proteins involved in TJ formation [113], thereby, eradicates permeability barrier. These are the mechanisms underlying the manifestation of erythema, desquamation and ulcer. Epilation is another deleterious outcome caused by radiation exposure. Cells consisting hair follicles, including epithelial cells and melanocytes are sensitive to radiation, which die by apoptosis [100, 114]. Proliferating hair matrix keratinocytes are the most sensitive, while quiescent hair follicle stem cells in bulge and melanocyte stem cells are rather resistant [115]. Cells in dermal papilla seem to be the most resistant [100]. Since matrix keratinocytes are essential for regulating hair cycle, their death is directly related to the manifestation of alopecia [104, 115]. Also, the crosstalk between matrix cells and melanocytes in hair bulb is critical for hair pigmentation, so matrix cell death causes gray hair as well [116, 117].

In order to mitigate the manifestation of acute radiation syndrome, single-cell RNA sequencing as well as multi-omics analysis were subjected to identify molecular mechanisms contributing to radiation dermatitis [97, 118, 119]. For example, a study using C57BL/6 mice exposed to 20 Gy of X-rays demonstrated that cell cluster-dependent changes in gene expression in response to radiation exposure. The gene-set enrichment analysis revealed that the BMP signaling pathway, which could activate the WNT and SMAD pathways, was enriched in proliferating keratinocytes. Papillary fibroblasts and lymphatic endothelial cells activated pathways attracting immune cells and migration of leukocytes [118]. Another study described alterations in fatty acid metabolism involved in radiation-induced skin damage [97]. Although the datasets are insufficient at present, integrated analyses in the future are expected to extract critical pathway(s) regulating radiation dermatitis.

Inflammatory response

Excess radiation damage to keratinocytes in the epidermal basal layer triggers premature senescence induction, which gives rise to the activation of inflammatory response. Senescent cells have been proven to show secretory phenotype, known as senescence-associated secretory phenotype (SASP) [120–125], and secrete soluble factors including IL-1β, IL-6, IL-8, CCL20, CCL5 and TNF, all of which attract immune cells, stimulating their invasion to the damaged sites as well as their activity (Table 4). Senesced fibroblasts also coordinate with this process by secreting IL-6 and TNF. Cytokines and chemokines secreted by damaged keratinocytes and activated DCs in the dermis promote differentiation of naive T cells to helper T cells, such as TH₁₇ cells. Then, TH₁₇ cells secrete IL-17 and IL-22 to stimulate keratinocyte growth [126–131]. Fibroblasts in the dermis, which are exposed to TNF, IL-1β and TGF-β from senesced keratinocytes, result in release of KGF, EGF and TGF-β, and this crosstalk stimulates proliferation of keratinocytes, contributing to the recovery of the epidermis (Table 4) [108, 132, 133].

Table 4.

Soluble factors secreted by various types of cells in epidermis of the skin

Cell source	Factors	Functions
Keratinocytes	IL-1α/β	Initiate rapid immune response. Stimulate IL-6, IL-8 and TNF-α expression
	IL-6	Promote keratinocyte proliferation
	IL-7	Stimulate T cell growth
	IL-8	Attracts neutrophils
	IL-15	Stimulate T cell growth
	IL-18	Potentiates inflammation
	CCL5	Stimulate LC immigration
	CCL20/MIP-3α	Attracts immune cells. Stimulates LC immigration
	TGF-β	Controls keratinocyte proliferation and differentiation
	TNF-α	Regulate various effects, including proinflammatory cytokines, and chemokines
	TSLP	Promote Th2 cell differentiation
Langerhans cells	IL-6	Promote keratinocyte proliferation
	IL-15	Stimulates Th17 and Th22 to secrete IL-17 and IL-22
	IL-23	Stimulates Th17 and Th22 to secrete IL-17 and IL-22
	CXCL9	Recruitment of T cells
	CXCL10	Recruitment of T cells
	CCL20/MIP-3α	Attracts immune cells
DETC	IL-2	Facilitate pre-DETC expansion and survival
	IL-13	Regulate homeostatic proliferation of keratinocyte
	IL-15	Stimulates Th17 and Th22 to secrete IL-17 and IL-22
	IL-17A	Stimulate keratinocyte proliferation and potentiate anti-microviral barrier
	CCL3/MIP-1α	Promotes immune cell migration
	CCL4/MIP-1β	Promotes immune cell migration
	CCL5	Attracts and promotes immune cell migration
	CCL20/MIP-3α	Attracts immune cells
	MCP-1	Attracts immune cells
	XCL1	Attracts immune cells
	KGF-1/2	Promotes keratinocyte proliferation and maturation
	IGF-1	Stimulate keratinocyte growth
	TNF-α	Regulate various effects, including proinflammatory cytokines and chemokines
	GM-CSF	Promotes differentiation and activation of LC

Since senescent cells express NKG2D, which is a ligand recognized by NKG2D receptor on NK cells, they are hypothesized to induce apoptosis in senesced keratinocytes. This promotes the clearance of the dead cells, which facilitates the regeneration of the epidermis [92, 93, 134–138].

Apart from the cell death in keratinocytes, the inflammatory response is also activated by pathogen invasion caused by the breakdown of the epidermal barrier [69, 70]. Although γδT cells are disappeared soon after irradiation, LCs remain in the epidermis functionally as the sentinels for securing immune homeostasis [139]. In the dermis, DCs and macrophages are also resistant to radiation exposure and assist in tissue clearance and the presentation of pathological antigens if any [140, 141]. Epidermal γδT cells are more abundant in mice, which consist of more than 90% of T cells in the epidermis, and are named dendritic epidermal T cells [142].

Both LC and dendritic epidermal T cells (DETC), resident in the basal layer of the epidermis, elongate dendrites upwards at the TJs formed between differentiated keratinocytes. DETC secrete several soluble factors that promote keratinocytes survival and proliferation (Table 4), while keratinocytes secrete IL-15 that promote DETC survival and renewal. Once the epidermis is injured, keratinocytes present DETC-specific antigen, which activates DETC and promotes secretion of soluble factors, such as IL-2. IL-13, IGF-1 and KGFs, resulting in the modulation of the functions of neighboring cells (Table 4) [143, 144].

We have discovered that coordinated pattern of DETC is disrupted even 3 weeks after the completion of 40 Gy of X-rays (unpublished data), indicating that they are quite immobile. While both LCs and DETCs are the epidermis-resident APCs, their roles are somewhat different. LCs are the positive regulator of the immune response, whereas DETC seem to negatively regulate inflammation, suggesting that disappearance of DETC after radiation exposure stimulates an inflammatory response that facilitates epidermal regeneration.

Besides γδT cells, other T cell compartments in the epidermis include regulatory T cell (Treg) and tissue-resident memory CD8+ T cell (T_RM). Treg is known to be radiation sensitive, and reduction of Treg is reported to cause unwanted enhancement of skin immune function. Upon recognition of cognate antigens, T_RM is reactivated and differentiated into cytotoxic effector cells, which secrete Interferon-γ (IFN-γ), potentiating anti-viral activity. Recently, it has been shown that reactivation of T_RM requires cross presentation of keratinocyte-expressed antigens on LCs. As such, not only LCs and DETCs, but also keratinocytes are members coordinating the immune response [85, 145, 146].

Regeneration of the skin

Regeneration of the epidermis is carried out by proliferation of epidermal stem cells, which reside in the basal layer of the epidermis and in the bulge region of the hair follicle. The epidermal stem cells provide progenitor cells, which divide symmetrically to supply cells to be differentiated. Recently, it was reported that skin homeostasis is regulated through cell competition between epidermal stem cells in the basal layer [147]. Differential expression of collagen XVII (COL17A1) in basal keratinocytes, which is a component of hemidesmosome, was involved in cell competition. In fact, damage-induced proteolysis of COL17A1 resulted in low expression of COL17A1, and stem cells with low level of COL17A1 will be eliminated by the COL17A1-high, undamaged stem cells through cell competition. Cell competition was also reported to be the fundamental mechanism by which stem cells with DSBs were selectively eliminated. It was shown that selective elimination was coupled with clonal expansion of intact stem cells, keeping the genomic integrity of the epidermis [148].

In conclusion, skin tissue reactions to radiation exposure are manifested by diverse cell types, originally resident or infiltrating, which orchestrate inflammation. Faithful recovery of the epidermis is critical to maintain the homeostasis of the skin. It should be mentioned that understanding of the process of injury and regeneration of skin appendages in response to radiation exposure is less examined compared with the regeneration of the epidermis. As skin appendages are indispensable for the function of the skin, further studies are needed [149–151].

Skin cancer

A proven link between exposure to ionizing radiation and the development of skin cancer was documented in a number of studies, including epidemiological studies among the atomic bomb (A-bomb) survivors, patients receiving radiotherapy and radiologists [9, 17–19, 152–155]. The two major types of skin cancers include BCC and SCC, in which the former accounts for ~90% of all skin cancers, so that BCC is the more frequent skin cancer induced by radiation exposure [9, 152, 156]. An excess relative risk (ERR) for BCC showed dose dependency in the LSS study among A-bomb survivors, and for doses less than 1 Gy, ERR per Gy is 0.48, while it is 2.64 per Gy for doses greater than 1 Gy [18, 19]. Age at exposure is another factor modifying risk, and the estimated ERR per Gy for age at exposure of 0–9, 10–19, 20–39, and over 40 years were 15, 5.7, 1.3, and 0.9, respectively [18, 19]. The ERR for SCC was estimated to be 0.71 per Gy. The dose−response of radiation risk for BCC has been reported to best fit a linear model with a threshold at 0.63 Gy, whose dose–response was not reported in other solid cancer cases [18]. A similar nonlinear dose–response was observed experimentally in an ICR/CRJ mice skin cancer model with fractionated beta-ray exposure, and three-times-weekly exposure through the life span of the mice to 0.5 Gy of beta-rays per fraction (the maximum total surface dose was 195 Gy) from a ⁹⁰Sr-⁹⁰Y source did not develop skin cancer [157–159], confirming the presence of a threshold dose.

As skin tumors related to radiation exposure are very similar clinically to sporadic skin tumors [9], IR may result in an earlier spontaneous onset of BCC and SCC, a possibility supported by the molecular mechanisms underlying BCC induction. It is well-described that mutations in the Ptch1 and the Smoothened genes, which result in dysregulated signaling of SHH pathway, leading to aberrant activation of the GLI transcription factor, are driver mutations associated with BCC, causing an inappropriate growth of basal cells [160–162]. Since mutations in those genes were caused predominantly by C > T transition, they seem not be induced by radiation exposure.

Since the mutational signatures in skin cancers induced by ionizing radiation have not yet been determined, we should be cautious to make a final conclusion. But, it is possible to hypothesize that radiation exposure gives rise to a circumstance, where basal cells with spontaneous mutations initiate clonal growth; otherwise, it will be eliminated by cell competition with neighboring normal cells. In fact, recent papers have suggested that radiation exposure may bring early onset of aging [163]. As skin cancers are apparently associated with skin aging, this could be the underlying mechanism, by which skin cancers become manifested among people receiving ionizing radiation.

It should be emphasized that epidemiological studies among A-bomb survivors [164, 165] and skin cancer patients who experienced radiation therapy [17] demonstrated people in an earlier age showed more susceptibility to skin cancer induction by radiation exposure. Also, it was shown that a linear decrease in the relative risk of head and neck skin cancer with increasing age at exposure to therapeutic radiation [165]. Thus, aging may not be a cause of the initiation of skin cancer but plays a role in the manifestation of the initiated cells that do develop into cancers.

SKIN CARE FOR MITIGATION OF RADIATION-INDUCED SKIN COMPLICATION

Acute radiation dermatitis, the representative adverse effect of radiotherapy, has been well documented in people with breast, and head and neck cancers [16]. Although there are several trials for preventing and management of acute radiation dermatitis, it is still a highly prevalent issue, causing impaired health-related quality of life, discomfort, pain, and reduction of treatment efficacy and poorer outcomes. This is due to a lack of a ‘gold standard’ for the prevention and management of radiation dermatitis, and we are still urged to establish particularly an international standard remedy for acute radiation dermatitis prevention and management.

Clinical practice guidelines on acute radiation dermatitis have been published by several organizations [166–171], such as Oncology Nursing Society, the Multinational Association of Supportive Care in Cancer (MASCC), British Columbia Cancer Agency, Cancer Care Manitoba, Society and College of Radiographers, and International Society of Nurses in Cancer Care, however, there are diverse differences in the recommendations, most likely due to the different approaches used in developing recommendations.

Recently, a unique attempt has been made and published by MASCC, which conducts a Delphi consensus-based method with compiling opinions and establishing a possible consensus based upon the evidence from the existing literatures [170]. Different therapeutic interventions have been discussed extensively, and six interventions, including photobiomodulation therapy and Mepitel® film for the patients with breast cancer treatment, Hydrofilm, mometasone, betamethasone and olive oil, are advocated as evidence-based skin-care recommendations.

Photobiomodulation therapy, also known as low-level laser therapy, uses low-power light sources in the visible and infra-red spectrum. Although the mechanism has not been fully understood yet, it is able to promote wound healing and alleviate inflammation [172]. To protect vulnerable skin, Mepitel® Film, a soft silicone transparent film dressing, is used for superficial lacerations. Radiotherapy for breast cancer tends to be more prone to radiation dermatitis than any other form of radiotherapy, but the use of Mepitel® Film has been reported to reduce the incidence of Grade 2 and Grade 3 radiation dermatitis. In addition, it has been shown to significantly reduce total Radiation-Induced Skin Reaction Assessment Scale (RISRAS) scores, which assess skin symptoms by both patients and healthcare professionals [173, 174]. A lower incidence of Grade 2 and Grade 3 radiodermatitis and lower mean RISRAS scores have also been reported in the case of head and neck cancer radiotherapy, but this differs from breast cancer radiotherapy, where poor adherence to medication and scratching have been identified as problems [175, 176].

Hydrofilm is a transparent film dressing, which has a semi-permeable hypoallergenic polyurethane polyacrylate top layer and a hypoallergenic, polyacrylate adhesive layer [177]. As shown with Mepitel® film, it also efficiently reduces the severity of radiation dermatitis. The use of topical corticosteroids is quite common for the treatment of radiation dermatitis, and it now turns out to be clear that mometasone furoate as well as betamethasone are effective for the prevention of severe radiation dermatitis, among which the latter is more effective [178]. Other types of intervention include the usage of topical Hirudoid®, a heparinoid-containing product, and wash with soap, both of which were reported to alleviate some of the symptoms. It is worth noticing that a recent paper comparing radiation dermatitis clinical practice guidelines demonstrated that the use of topical corticosteroids, and washing with water and soap, were consistently supported by all recommendations [179]. Thus, although there is still discordance among guideline recommendations, further comprehensive researches are expected to establish ideal and optimal recommendations for patients and frontline healthcare professionals.

SUMMARY AND FUTURE STUDIES

The skin cancer is one of the malignant adverse effects caused by radiation exposure. Ionizing radiation executes cell death in not only basal cells that are required for epidermis renewal, but also dermal and subcutaneous cells that support the epidermal homeostasis. It is quite unique to ionizing radiation that cell death mode manifested after exposure is primarily premature senescence, triggering a unique inflammatory immune response. Hence, cell death-initiating tissue reaction gives rise to visible clinical symptoms, which was extensively discussed in the previous studies. Understanding such tissue reactions more in detail, particularly the dynamics of stem cells and their competition in the basal layer, should be critical for avoiding unwanted adverse effects and assuring the quality of life of the patients receiving radiotherapy. It is also essential for proper estimation of the risk of skin cancer attributable to radiation exposure.

CONFLICT OF INTEREST

All authors declare that there are no conflict of interest.