Immune cells and associated molecular markers in dermal fibrosis with focus on raised cutaneous scars

Abstract

Raised dermal scars including hypertrophic, and keloid scars as well as scalp‐associated fibrosing Folliculitis Keloidalis Nuchae (FKN) are a group of fibrotic raised dermal lesions that mostly occur following cutaneous injury. They are characterized by increased extracellular matrix (ECM) deposition, primarily excessive collagen type 1 production by hyperproliferative fibroblasts. The extent of ECM deposition is thought to be proportional to the severity of local skin inflammation leading to excessive fibrosis of the dermis. Due to a lack of suitable study models, therapy for raised dermal scars remains ill‐defined. Immune cells and their associated markers have been strongly associated with dermal fibrosis. Therefore, modulation of the immune system and use of anti‐inflammatory cytokines are of potential interest in the management of dermal fibrosis. In this review, we will discuss the importance of immune factors in the pathogenesis of raised dermal scarring. The aim here is to provide an up‐to‐date comprehensive review of the literature, from PubMed, Scopus, and other relevant search engines in order to describe the known immunological factors associated with raised dermal scarring. The importance of immune cells including mast cells, macrophages, lymphocytes, and relevant molecules such as cytokines, chemokines, and growth factors, antibodies, transcription factors, and other immune‐associated molecules as well as tissue lymphoid aggregates identified within raised dermal scars will be presented. A growing body of evidence points to a shift from proinflammatory Th1 response to regulatory/anti‐inflammatory Th2 response being associated with the development of fibrogenesis in raised dermal scarring. In summary, a better understanding of immune cells and associated molecular markers in dermal fibrosis will likely enable future development of potential immune‐modulated therapeutic, diagnostic, and theranostic targets in raised dermal scarring.

1. INTRODUCTION

Fibrosis was first described in the 19th century and the medical term “fibrosis” was derived from the Latin word “fibra” meaning fibre and the Greek/Latin suffix “oasis” which describes a physiological state or process usually being abnormal or diseased. ¹ The extent and intensity of extracellular matrix (ECM) deposition by fibroblasts beyond normotrophic requirements is considered to be a characteristic hallmark of fibrotic tissue formation and is proportional to the severity of inflammation leading to the onset and subsequent development of skin fibrotic disorders. Fibrosis equally plays a pivotal role in the development of chronic diseases, and several fibrotic conditions have been described as affecting the kidney, liver, lung, eye, pancreas, intestine, brain, and joints. ² , ³ , ⁴ , ⁵ Cutaneous fibroproliferative disorders characterized by excessive scarring can cause a visual aesthetic disfigurement as well as physcial symptoms which may lead to psychosocial issues. ⁶ , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹

Although they manifest differently, hypertrophic scars (HTS), keloid disease (KD), and fibrotic scalp Folliculitis Keloidalis Nuchae (FKN) have several clinical aspects that appear to be similar, as they are all raised cutaneous fibrotic disorders with common dysregulation of connective tissue pathobiology and metabolism leading to excessive collagen and ECM deposition (Figure 1). The intensity of ECM deposition (the hallmark of fibrosis) in raised dermal scars is proportional to the severity of fibrosis and has therefore been used for their characterization. ¹² , ¹³

FIGURE 1.

The spectrum of wound healing, from normal scars (includes fine line flat and stretched scars) to raised dermal scars. Raised dermal scars comprise of hypertrophic scars and keloids, which are characterized by growth within and beyond wound margins, respectively.

Key aspects of the wound healing process play a critical role in development of fibrosis and particularly in formation of raised dermal scarring. ¹⁴ , ¹⁵ Raised dermal scarring often manifests from trauma or an injury to the deep connective tissue layer of the skin and could be considered as a complication or a pathologically abnormal form of the wound healing process. ⁸⁹ Keloid disease together with other fibrotic conditions including HTS ⁶⁷ and FKN ¹⁰ , ¹¹ are the end result of uncontrolled inflammation leading to excessive collagen deposition and increased ECM production. ¹² , ¹³ However, Keloid disease is a heterogeneous lesion, also recognized as abnormal scar tissue that manifests with uncontrolled fibroproliferative growth beyond the boundaries of the original lesions that is recalcitrant to conventional scar therapy. KD also presents with excessive dysregulated collagen deposition, leading to a progressively invasive dermal disorder with an expansile and exophytic growth pattern as well as an aggressive clinical behaviour. ¹⁶ , ¹⁷

Keloid disease prevalence is higher in individuals with pigmented skin including, African, Hispanic and Asian ethnic skins but low in the Caucasian population. ¹⁸ , ¹⁹ , ²⁰ , ²¹ , ²² , ²³ In a study carried out in Nigeria, patients were reported to have a family history with known registered KD symptoms, ²⁰ and additionally a high occurrence of KD has been reported in rural black South African populations. ²³ , ²⁴ It is not clear why keloid lesions develop in some individuals and not in others, and consensus on a definitive therapy is yet to be agreed upon.

Keloid therapy most often targets hyperactive fibroblast proliferation to halt increased amount of collagen deposition, ²⁴ , ²⁶ and to induce shrinkage of the keloid lesion. ²⁷ , ²⁸ , ²⁹ The effect of therapeutic compounds against HTS, KD, and FKN dermal scars can be investigated using several models and assays including investigating keloid derived fibroblast proliferation, migration and invasion, in order to evaluate the effect of potential anti‐fibrotic candidates. ³⁰ , ³¹ The high recurrence rate that often occurs post‐conventional therapies is a clinical challege and a major cause to intensify research for identifying better therapies with minimal side effects and reduced recurrence rates. Current treatment approaches include topical anti‐scarring application, surgical excision, cryotherapy, pressure dressing, laser therapy, intralesional corticosteroid injections, and antimetabolites. ³² , ³³ , ³⁴ , ³⁵ , ³⁶ , ³⁷ , ³⁸ , ³⁹ , ⁴⁰ , ⁴¹ , ⁴² , ⁴³ , ⁴⁴ , ⁴⁵ , ⁴⁶ , ⁴⁷ , ⁴⁸

The aim of this review is to briefly describe cutaneous wound healing processes and the participating immune cells, then provide a comprehensive overview of the immune and associated molecular targets involved in raised dermal scarring conditions.

2. CUTANEOUS WOUND HEALING

Cutaneous wound healing in human skin is a complex process that requires a well‐orchestrated series of events for the successful restoration of the structural barrier and partial return of the complete physiological, sensory and mechanical functions of the skin. ⁴⁹ The process of wound healing can be divided into four distinct but overlapping steps: hemostasis, inflammation, proliferation, and remodelling. ⁵⁰ , ⁵¹ , ⁵² Studies have demonstrated the indispensable role of the immune system in wound healing. Hence, in this section of the review, we will evaluate the contribution of various cells of the immune system and associated chemical effectors (cytokines) involved in and facilitating cutaneous wound healing (Figure 2).

FIGURE 2.

Phases of wound healing; inflammation, proliferation, and tissue remodelling. Key immunologic phenotype (immune cells) and key chemical messengers (cytokines and chemokines) for each phase of wound healing. The inflammation phase is characterized by neutrophil recruitment, as well as pro‐inflammatory cytokine release by macrophages. Proliferation also comprises macrophage recruitment by neutrophils and mast cells, which in turn initiate resident fibroblast proliferation. Later, anti‐inflammatory cascade by macrophages is evident. During the last stage of wound healing, re‐epithelialization continues, as wound granulation tissue forms, and collagen deposition increases, due to increased cell proliferation, while T‐cells recruit late‐stage (pro‐resolving) macrophages, as well as formation of cell–cell interaction with skin cells. TGF‐β, transforming growth factor beta; VEGF, vascular endothelial growth factor; CXCR1, C‐X‐C Motif Chemokine Receptor 1; IL6, interleukin 6; INF‐γ, interferon‐gamma; IGF, insulin‐like growth factor‐1; CD4, helper T cells; bFGF, basic fibroblast growth factor; EGF, epidermal growth factor; PDGF, platelet‐derived growth factor; TNF‐α, tumor necrosis factor alpha; MMP‐2, matrix metalloproteinase‐2; TIMP‐1, tissue inhibitor of metalloproteinases 1.

2.1. Immune cells involved in would healing

2.1.1. Macrophages

Macrophages are one of the most important cells of the innate immune system that play a critical role in wound healing. ⁵³ , ⁵⁴ , ⁵⁵ , ⁵⁶ Macrophages are derived from monocytes that are recruited from peripheral blood to the site of injury and are essential for initiating the healing cascade. ⁵⁷ , ⁵⁸ The phenotype of monocyte‐derived macrophages during wound healing is influenced by the local microenvironment at the site of injury. ⁵⁹ , ⁶⁰ , ⁶¹ During the inflammatory phase, macrophages play a key role in driving the inflammatory response that is characterized by clearing debris and secretion of pro‐inflammatory cytokines. ⁵¹ , ⁵³ However, at a later stage, they contribute to wound repair and control of inflammation. ⁵³ This plasticity of macrophages is crucial for the wound healing process.

The role of monocyte‐derived macrophages during wound healing has been elucidated using C‐C chemokine receptor type 2 (CCR2) deficient mice. ⁵⁴ This study showed that CCR2 knock‐out mice had impaired wound healing at all times‐points post‐injury, had reduced frequency of Ly6C^hi monocytes/macrophages, and reduced production of IL‐1β plus tumor necrosis factor (TNF)‐α. ⁵⁴ Finally, this study showed that the adoptive transfer of wild‐type monocytes into CCR2^−/− mice restored wound healing, ⁵⁴ demonstrating the importance of macrophages in wound healing. The Ly6C^hi cells have been shown to produce pro‐inflammatory cytokines, including interleukin (IL)‐1β, TNF‐α, IL‐6, ⁶² and are indispensable for the initiation of inflammation that is required for the transition to the reparative phase of healing. ⁵³ In fact, the absence of initial inflammation has been shown to impede the wound‐healing cascade resulting in failure of wound healing. ⁵³ In contrast, the Ly6C^lo cells have been shown to appear at the later stages of wound healing, displaying anti‐inflammatory characteristics, as well as promoting tissue remodelling, fibrosis, and secretion of transforming growth factor (TGF)‐β and IL‐10. ⁶³ , ⁶⁴ , ⁶⁵

2.1.2. Neutrophils

Neutrophils are amongst the first inflammatory cells to arrive at the site of injury and they play a key role in removing debris and preventing infection by phagocytosing and killing pathogens through the release of reactive oxygen species, proteases, antimicrobial peptides, and neutrophil extracellular traps (NETs). ⁶⁶ , ⁶⁷ The recruitment and activation of neutrophils is driven by the chemical messenger chemokine ligand CXCL‐8, also known as IL‐8, that is secreted by activated tissue‐resident macrophages and fibroblasts, ⁶⁸ , ⁶⁹ and other danger associated molecular patterns (DAMPS) such as hydrogen peroxide and leukotriene B₄ that are released by necrotic cells in the wound bed. ⁷⁰ , ⁷¹ CXCL8 binds to CXC receptor‐1 (CXCR‐1) and CXCR2 on the surface of neutrophils, leading to their recruitment to the site of inflammation. Once neutrophils arrive in the wound, they are able to secrete CXCL8, thus, amplifying neutrophil recruitment via a pro‐inflammatory feedback loop. ⁷²

A study by Simpson and Ross showed that neutrophils are dispensable during wound healing in guinea pigs that were depleted of neutrophils after administration of anti‐neutrophil serum. ⁷³ This study showed that there was no difference between neutropenic and control wounds in the total wound volume occupied by mononuclear leukocytes and fibrin using histological analysis. ⁷³ Importantly, there was no difference in the rate of wound debridement, cellularity, and degree of connective tissue formation between the two groups of animals, demonstrating that neutrophils are not essential for wound healing. ⁷³ However, neutrophils have been shown to play an active role in secreting cytokines, chemokines, and growth factors such as TNF‐α, IL‐6, IL‐1β, CXCL8, CXCL2, monocyte chemoattractant factor‐1 (MCP‐1), vascular endothelial growth factor (VEGF), laminin 5 β‐3, and urokinase‐type plasminogen activator that are crucial for the recruitment of macrophages and T‐cells to the site of injury. ⁷² , ⁷⁴ , ⁷⁵ These molecules are also essential for angiogenesis, the proliferation of keratinocytes and fibroblasts, adhesion of keratinocytes to the dermal layer, and tissue remodelling.

2.1.3. Mast cells

Mast cells (MCs) are specialized innate immune cells that are abundant in the dermis and play a varied role across the different stages of wound healing. ⁷⁶ , ⁷⁷ , ⁷⁸ A study by Younan and colleagues, investigated the requirement of MCs in wound healing using MCs‐deficient mice (WWv mice) that had uniform full‐thickness wound surface micro‐deformations that were induced with suction combined with open‐pore polyurethane foam. ⁷⁹ This study unequivocally demonstrated that MCs are required for wound tissue granulation, cell proliferation, blood vessel sprouting, and collagen deposition during the proliferation and remodelling stages of healing. ⁷⁹ This was further corroborated by subsequent studies that showed MCs activation and histamine release are essential for cutaneous wound healing in mice. ⁸⁰

The contribution of MCs in wound healing also involves their ability to secrete various key factors that are essential for coordinating wound healing. For instance, MCs secrete histamine and VEGF that mediate vascular permeability and vasodilation during injury. ⁷⁶ , ⁷⁹ Moreover, they also secrete TNF‐α, Macrophage Inflammatory Protein 2 (MIP‐2), and IL‐8 which are involved in the recruitment of neutrophils to the site of injury. ⁸¹ , ⁸² They also secrete proteases such as tryptase that induces vasodilation that further enhances the trafficking of neutrophils and other inflammatory mediators to the injury site. ⁸³ , ⁸⁴ Finally, MCs contribute to angiogenesis by secreting platelet‐derived growth factor (PDGF), chymase, angiopoietin 1, and fibroblast growth factor 2 (FGF‐2). ⁸² , ⁸⁵

2.1.4. Innate lymphoid cells

The contribution of innate lymphoid cells (ILC) in immunity, inflammation, and homeostasis is well established in the literature. ⁸⁶ , ⁸⁷ However, a few studies have explored their role during cutaneous wound healing. Three subsets of ILC have been defined and designated into groups 1, 2, and 3 based on their transcriptional landscape and effector functions. ⁸⁸ , ⁸⁹ , ⁹⁰ Interestingly, several studies have identified the presence of ILC2s in human and murine skin, and revealed that these cells are elevated after cutaneous inflammation. ⁹¹ , ⁹² , ⁹³ Importantly, skin ILC2s have been shown to respond to IL‐25, IL‐33, and TSLP and express type‐2 associated cytokines IL‐5 and IL‐13. ⁹³ A series of studies by Yin and colleagues demonstrated that IL‐33, an alarmin that promotes ILC2 differentiation facilitated wound healing in an excisional wound healing model ⁹⁴ and Staphylococcus aureus incision model. ⁹⁵ A study by Rak and colleagues investigated the direct role of IL‐33‐dependent ILC2 responses in cutaneous wound healing after full‐thickness dermal injury in mice. ⁹⁶ This study found that IL‐33 and ILC2s are elevated following cutaneous wounding in mice and humans. ⁹⁶ Interestingly, depletion of ILC2 using anti‐CD90.2 monoclonal antibody delayed wound closure and impaired re‐epithelialization, a key process during wound healing. ⁹⁶

2.1.5. Regulatory T‐cells

T regulatory cells (Tregs) are abundant in the tissues such as the skin and play a key role in homeostasis and regulating inflammation. ⁹⁷ , ⁹⁸ A study by Nosbaum and colleagues investigated the role of FoxP3‐expressing Tregs in excisional wound healing using FoxP3 express knocked‐in human diphtheria toxin receptor (FoxP3‐DTR) mice that allowed for inducible ablation of Tregs after the administration of the diphtheria toxin. ⁹⁹ Depletion of Tregs delayed the kinetics of wound healing and impaired wound re‐epithelialization early after wounding, however, no significant differences were observed late after wounding. ⁹⁹ Moreover, this study showed that activated Tregs characterized by expression of CD25, cytotoxic T‐lymphocyte‐associated protein 4 (CTLA‐4), and inducible T‐cell co‐stimulator (ICOS) accumulated at the site of injury, and suppressed the production of interferon (IFN)‐γ and recruitment of pro‐inflammatory macrophages. ⁹⁹ Finally, it was demonstrated that conditional deletion of the epidermal growth factor receptor (EGFR) on Tregs using Foxp3^creEGFR^fl/fl mice delayed wound healing, showing the requirement of EGFR‐mediated Tregs function during wound healing. ⁹⁹ However, Tregs have also been shown previously to negatively affect wound healing in diabetic mice. ¹⁰⁰

2.2. Cytokines involved in wound healing

2.2.1. Tumor necrosis factor‐alpha

Tumor necrosis factor‐alpha (TNF‐α) is a pleiotropic cytokine that is secreted by keratinocytes, macrophages, MCs, and T cells. The contribution of TNF‐α during wound healing is concentration dependent – elevated levels of TNF‐α lead to a decrease in granulation tissue while low levels of TNF‐α promote collagen deposition. ¹⁰¹ A study by Mori and colleagues showed that mice deficient in TNF‐Rp55 had accelerated wound healing due to enhanced angiogenesis, collagen accumulation, re‐epithelialization, and leukocyte infiltration, suggesting a negative role of TNF‐α in wound healing. ¹⁰²

2.2.2. IL‐17

IL‐17 is a pro‐inflammatory cytokine that has been implicated in the pathogenesis of inflammatory diseases such as psoriasis and rheumatoid arthritis. ¹⁰³ , ¹⁰⁴ bleomycin and IL‐1β‐induced pulmonary fibrosis, ¹⁰⁵ skin sclerosis, ¹⁰⁶ and liver fibrosis. ¹⁰⁷ IL‐17 was demonstrated to be essential for wound closure in IL‐17^−/− mice after incisional wound injury. ¹⁰⁸ IL‐17 mediates its effects by enhancing the skin's antimicrobial barrier and regulating epidermal regeneration and differentiation. ¹⁰⁸ , ¹⁰⁹

2.2.3. IL‐33

IL‐33 is an alarmin that plays a central role in wound healing by facilitating the differentiation of ILC2s. ⁹⁴ , ⁹⁶ Administration of IL‐33 accelerated healing of incisional wounds in mice and was associated with improved re‐epithelialization, collagen deposition, expression of genes associated with the extracellular matrix, and development of alternatively activated macrophage. ⁹⁴

3. ABERRANT WOUND HEALING

Aberrant wound healing is a manifestation of immune‐related perturbations in molecular and cellular processes that guide normal wound healing. ¹¹⁰ , ¹¹¹ , ¹¹² Such disruptions may occur at any of the stages of wound healing as outlined above. A transition, delay, or acceleration of any of these processes may result in the formation of either chronic (non‐healing wounds) or fibrotic disorders such as HTS and KD or the scalp‐associated FKN. ¹¹¹ In this section of the review, we will focus on the major immune initiators and chemical mediators (cytokines and chemokines) that result in impaired healing such as prolonged inflammation, dysregulated re‐epithelialization processes, excessive ECM deposition, as well as atypical ECM remodelling (Figure 3). ⁶ , ¹¹³ , ¹¹⁴ , ¹¹⁵

FIGURE 3.

Wound healing phenotypic differences between hypertrophic scars and keloids. The 3 phases of wound healing (inflammation, proliferation, and tissue remodelling), with their associated upregulated or downregulated functional markers. Inflammation in HTS is characterized by the early recruitment of CD4+ T cells at the site of injury, and their subsequent polarization to Th‐2 phenotype, with the production of cytokines such as TNF‐α and IL‐1. VEGF, PDGF, FGF, IGF, TGF‐β, and CCL2 are cytokines and chemokines upregulated during the proliferation phase that is characterized by the increased proportion and longevity of pro‐fibrotic M2 macrophages, as well as the persistence of CD4+ T cells thereby allowing for increased fibroblast proliferation, myofibroblast differentiation, and increased collagen deposition. Chemical messengers such as IL‐4, IL‐5, IL‐10, IL‐13, and MMP‐2 are produced during the late stages of wound healing (the matrix formation or tissue remodelling). Th‐2, Th‐2 lymphocytes; TGF‐β, transforming growth factor beta; VEGF, vascular endothelial growth factor; HIF‐1α, hypoxia‐inducible factor 1‐alpha; CCL2, C–C motif chemokine ligand 2; IL6, interleukin 6; INF‐γ, interferon‐gamma; IGF, insulin‐like growth factor‐1; CD4, helper T cells; FGF‐1, fibroblast growth factor 1; EGF, epidermal growth factor; PDGF, platelet‐derived growth factor; TNF‐α, tumor necrosis factor alpha; MMP‐2, matrix metalloproteinase‐2; TIMP‐2, tissue inhibitor of metalloproteinases 2.

3.1. Hypertrophic scars

Hypertrophic scars (HTS) are a result of aberrant wound healing processes and are characterized by raised stiff scar tissue accompanied by redness, pain, and itch. ⁶ They also present with a lack of skin appendages such as hair follicles and sebaceous glands, with a consequent loss of skin stem cells, which play an essential role in replenishing cells in the skin. ¹¹⁶ This type of scarring mostly results from burn injuries and can be debilitating to patients. ¹¹⁷ In this section of the review, we look closely at the cellular (immune) processes and resulting molecular culprits associated with impaired homeostasis (prolonged inflammation, dysregulated re‐epithelialization, and excessive ECM deposition) in HTS. Furthermore, possible immune‐derived molecular targets are suggested with the aim to prevent HTS formation.

3.1.1. Prolonged inflammation

Multiple studies have implicated prolonged inflammation as a key player in the development of hypertrophic scarring post‐wounding. ¹¹⁰ , ¹¹⁸ , ¹¹⁹ , ¹²⁰ Herein platelets, neutrophils, macrophages, MCs, and CD4⁺ T lymphocytes have been shown to produce proinflammatory cytokines such as IL‐1, ¹¹⁴ , ¹²¹ IL‐6, ¹²² IL‐10, ¹¹⁵ and TNF‐α, ¹²¹ as well as growth factors such as TGF‐β1 ¹¹⁵ , ¹²¹ , ¹²³ , ¹²⁴ and PDGF, ¹¹⁴ , ¹²¹ with concomitant CCL2 chemokine production. ¹²² Persistent production of such molecules often result in hypertrophic scarring during the tissue remodelling stage of skin wound healing. Therefore, anti‐scarring therapies could be aimed at ameliorating this pro‐inflammatory immune phenotype in HTS. ¹¹⁵

A longitudinal study by Tredget et al. ¹²⁵ showed that a polarized Th2 response to injury leads to increased Th2 fibrogenic cytokines (IL‐4 and IL‐10) and growth factors (TGF‐β), which in turn results in the development of fibrogenesis in HTS. Several studies have also shown increases in Th2 cytokines such as IL‐2 and IL‐4, as well as regulatory cytokines IL‐10 and TGF‐β in fibrotic conditions such as HTS. ¹¹⁵ , ¹²⁶ , ¹²⁷ Therefore, inducing a polarized Th1 cell‐mediated response, which leads to the production of pro‐apoptotic cytokines seems a plausible phenomenon in the treatment of HTS. Furthermore, since HTS fibroblasts have been shown to have low collagenase activity, coupled with nitric oxide synthase (NOS) activity, ¹²⁸ increased collagenase activity, and matrix remodelling could be achieved via NOS activation by the Th1 cytokines. However, care must be taken when manipulating such cell‐mediated immune responses as a prolonged Th1 cell response (inflammation) might prove futile.

3.1.2. Dysregulated re‐epithelialization

Major players in dysregulated epithelialization resulting in hypertrophic scarring are skin resident immune cells; Langerhans cells, which produce growth factors such as insulin‐like growth factor one (IGF‐1) and fibroblast growth factor 1 (FGF‐1) ⁵⁶ responsible for fibroblast proliferation and activation, as well as monocyte‐derived macrophages recruited to the site of injury. Alterations in macrophage number and phenotype can disrupt re‐epithelialization process and could dictate the level of scar formation. ⁵³ , ⁵⁶ , ¹²⁹ Depletion of macrophages restricted to the early stage of the repair response (inflammatory phase) significantly reduced the formation of vascularized granulation tissue, impaired epithelialization, and resulted in minimized scar formation. ⁵³

CXC chemokine receptor 2 (CXCR2) also known as interleukin 8 receptor beta (IL8Rb) is an essential mediator of neutrophil chemotaxis via the release of numerous keratinocyte‐derived chemokine ligands. ¹³⁰ Its knockdown in wound healing mouse models has been implicated with a lack of monocyte infiltration, thereby resulting in reduced keratinocyte migration and proliferation (markers of re‐epithelialization) during wound healing. Chemotactic perturbation during the epidermal proliferation stages of wound healing therefore has the potential as a therapeutic anti‐scarring agent. A study by Dovi et al. (2003) suggested that re‐epithelialization might be improved in wounds of neutrophil‐depleted mice. ¹³¹ The investigators performed a time course neutrophil depletion by rabbit anti‐mouse neutrophil‐serum injection into mice, followed by wounding. Wound histology and re‐epithelialization protocols showed that neutropenic mice exhibited accelerated wound healing (closure), even in diabetic mouse models. Therefore, regarding excessive healing conditions such as keloid and HTS, neutrophil enrichment studies may be beneficial, with the aim of slowing the rate of re‐epithelialization, thus making neutrophils a potential therapeutic target for such raised dermal scarring conditions. ¹¹⁹

Taken together, these studies highlight the possibility of manipulating early immune processes such as cells (neutrophils and macrophages), as well as their associated chemokines and/or receptors in limiting the excessive epidermal proliferation and migration that happens during HTS formation.

3.1.3. Excessive ECM deposition and remodelling

The secretion of cytokines such as IL‐1, ¹¹⁵ IL‐4, ¹¹⁵ , ¹²⁵ , ¹²⁷ and TNF‐α ¹¹⁵ as well as growth factors including VEGF, TGF‐β1, and FGF ⁵⁶ by macrophages, CD4⁺ T cells, resident Langerhans and MCs have been implicated in excessive matrix deposition, with aberrant ECM remodelling. M2 macrophages secrete factors such as TGF‐β1 that induce the proliferation of fibroblasts and their differentiation into myofibroblasts. ¹³² Furthermore, the development of a Th2 response has been associated with fibrogenesis. ¹¹⁵ , ¹²⁵ , ¹²⁶ For instance, a study by Wang and colleagues showed that fibroblasts cultured with CD4⁺ T cells that were purified from peripheral blood mononuclear‐cells (PBMCs) from burn patients exhibited increased cell proliferation, collagen synthesis, and alpha‐smooth muscle actin (α‐SMA), as well as a significant up‐regulation of TGF‐β, which inhibits Th1 cytokine expression and finally contributes to HTS formation. ¹²³ These studies also suggest the hypothesis that prolonging polarized Th1 cell‐mediated response, which in turn results in the synthesis of anti‐fibrotic processes such as increased collagenase activity and matrix remodelling as well as manipulating macrophage phenotypic characteristics (shift from M2 to M1) could be a potential therapeutic target for HTS.

Chemokines and their receptors may also be potential targets for hypertrophic scarring. Ishida et al. ⁵⁶ showed that CX3CR1 mediates the direct recruitment of bone marrow‐derived monocytes/macrophages which release profibrotic and angiogenic mediators. In their study, a CX3CR1 knockout mouse model of excisional skin wound healing resulted in the amelioration of a pro‐fibrotic macrophage phenotype, leading to a decreased expression of αSMA and collagen deposition. This renders the chemokine receptor CX3CR1 a potential immune target for HTS.

IL‐1α and PDGF derived from Langerhans cells, macrophages, and platelets have been identified as some of the molecular targets in fibrotic conditions such as wound healing. Another study ¹¹⁴ showed low expression of IL‐1α and increased expression of PDGF in biopsies in both the epidermis and dermis of individuals who developed HTS after undergoing breast reduction surgery. ¹¹⁴

3.2. Keloid disease

Keloid disease (KD) is a complex fibroproliferative scarring disorder that even though it is non‐malignant, behaves in an aggressive, irreversible and progressive quasi‐neoplastic manner, and many studies have indicated that the immune system may play a role in keloid formation. ¹³³ , ¹³⁴ KD is characterized by an exaggerated exophytic growth and excessive deposition of collagen in areas of previous trauma or cutaneous injury. ²⁷ , ¹³⁵ KD is unique to humans since no animal model exists. ¹³⁶ , ¹³⁷ , ¹³⁸ , ¹³⁹ Keloid is thought to develop after aberrant wound healing due to alterations in the stages of normal wound healing as outlined in the sections above, and it is known to have a high recurrence rate upon resection. ²⁵ Wound healing process after trauma or cutaneous injury could become keloidal, possibly due to a reduction in the apoptosis of fibroblasts or an imbalance between collagen production and degradation. ¹⁴⁰ The development of keloids has been linked to the downregulation of apoptosis‐related genes including p53, ¹⁴⁰ TGF‐β₁/Mothers Against Decapentaplegic Homolog (Smad) signal transduction mediated by mitogen‐activated protein kinases (MAPKs), ¹⁴¹ the extracellular signal‐regulated protein kinase (ERK), c‐Jun N‐terminal kinase (JNK), and p38 pathways. ¹⁴¹ , ¹⁴² These might be considered specific targets of drug therapy for keloids. It is not clear why keloids develop in some individuals and not in others, and consensus on a definitive therapy is yet to be achieved.

The primary treatment target would lead to a decrease in collagen deposition and activation of fibroblasts. When keloid‐derived fibroblasts, which are the major cell type in KD, are compared with fibroblasts derived from normal skin or HTS, the keloid‐derived fibroblasts present several abnormal changes such as excessive extracellular matrix production and proliferation, altered apoptosis, growth factor response, and cytokine production. ¹⁴⁰ Currently, there is no consensus on a definitive therapeutic target and therapy. Several studies have been carried out providing evidence of the effective use of corticosteroids including Dexamethasone, and Triamcinolone post‐surgical excision. ³⁵ , ³⁶ , ³⁷ , ³⁸ , ³⁹ , ⁴⁰ , ⁴¹ , ⁴² , ⁴³ , ⁴⁴ , ⁴⁵ , ⁴⁶ , ⁴⁷ , ⁴⁸ However, the problem of recurrence of keloids after excision is of primary concern and therefore, there is an urgent need for more research on alternative therapeutic options. There is also a need to identify potential biomarkers and targets that would aid in the diagnosis and treatment of dermal scars.

It is important to note that several study models have been investigated for functional testing of therapeutic anti‐scarring drug targets in keloids by employing key assays and techniques. ³⁰ , ³¹ Our group has looked at multidimensional models of KD, ³¹ and are currently developing new models for functional testing of therapeutic anti‐scarring drug targets in keloids. ³⁰ Going forward, understanding immune and associated molecular target candidate regulators would be significantly helpful. In addition, immune‐associated molecular targets are vital, as they may contribute either by regulating or are regulated in inflammatory molecular events involved in fibroblast activation leading to fibrosis. ¹⁴³ These may consist of alterations in fibroblast and keratinocyte involvement in raised dermal scarring conditions, as well as alterations in growth factor expression. Moreover, it is vital to understand the role of bioenergetic changes that may be involved in regulating immunometabolism in skin fibrotic disorders.

In this section, we will look at Immune and associated immune targets involved in KD. Several immune cells and molecules that mediate their activity either directly or indirectly in KD have been reported (Figure 3). Here, we will discuss the various cellular as well as molecular targets associated with KD, which could be important to guide and inform future studies on possible immune‐diagnostic, −therapeutic, and ‐theranostic strategies against keloid tissue formation. Some important potential immune‐associated therapeutic targets in hypertrophic and keloid scars discussed in this review have been summarized in Figure 4. ¹⁴⁴ , ¹⁴⁵ , ¹⁴⁶ , ¹⁴⁷ , ¹⁴⁸ , ¹⁴⁹ , ¹⁵⁰ , ¹⁵¹ , ¹⁵² , ¹⁵³ , ¹⁵⁴ , ¹⁵⁵ , ¹⁵⁶ , ¹⁵⁷ , ¹⁵⁸ , ¹⁵⁹ , ¹⁶⁰ , ¹⁶¹ , ¹⁶² , ¹⁶³ , ¹⁶⁴ , ¹⁶⁵ , ¹⁶⁶ , ¹⁶⁷ , ¹⁶⁸ , ¹⁶⁹ , ¹⁷⁰ , ¹⁷¹ , ¹⁷² , ¹⁷³ , ¹⁷⁴ , ¹⁷⁵ Also, we will have a comprehensive look at the involvement of keloid‐associated lymphoid tissues on the pathophysiology of keloid tissue formation. ¹³³

FIGURE 4.

Immune‐associated therapeutic targets in hypertrophic scars and keloids. Potential targets for raised dermal scarring therapeutics include Immune (KALT, neutrophils, macrophages, Langerhans cells, FDCs, T‐cells, B‐cells, PCs, MCs) and non‐immune (fibroblasts and keratinocytes) cells, and their associated matrices such as collagens, fibrin, fibronectin, elastin, HA, laminins, and tenascin. Several genes and molecules secreted by immune cells activate fibroblasts, myofibroblasts, keratinocytes, and enhanced by EMT for matrix production. Matrix production is mediated through several pathways that could be potential targets as therapeutic candidates. FDC, follicular dendritic cells; T‐cells, T lymphocytes; B‐cells, B lymphocytes; PCs, plasma cells; MCs, mast cells; HA, hyaluronidase; KALT, keloid associated lymphoid tissues; TGF‐β, transforming growth factor beta; CTGF, connective tissue growth factor; HIF‐1α, hypoxia‐inducible factor 1‐alpha; VEGF, vascular endothelial growth factor; HSP27, heat shock protein 27; α2β1, collagen‐binding Integrin α2β1; PAI‐1/2, plasminogen activator inhibitor‐1/2; CD138, cluster of differentiation 138 (syndecan‐1); SiRNAs, small interfering RNAs; p53, p53 gene; NEDD4, neuronal precursor cell‐expressed developmentally downregulated 4; CCL2, C–C motif chemokine ligand 2; IL6, interleukin 6; S100A, S100 calcium‐binding protein A1; IGF, insulin‐like growth factor‐1; CD3, T lymphocytes; CD4, helper T cells; CD8, cytotoxic T cells; CD45RO, memory T cells; CD20, B‐lymphocyte antigen; CD79a, B‐cell antigen receptor complex‐associated protein alpha chain and MB‐1 membrane glycoprotein; CD68, transmembrane glycoprotein; HLA‐DR, human leukocyte antigen – DR isotype; CD36, platelet glycoprotein 4, fatty acid translocase; ICAM‐1, intercellular adhesion molecule 1; CD31, platelet endothelial cell adhesion molecule; OX40L/CD134, tumor necrosis factor (ligand) superfamily, member 4; OCT4, octamer‐binding transcription factor 4; SOX2, sex determining region Y‐box 2; NANONG, homeobox protein NANOG; INF‐γ, interferon gamma; CX3CR1, CX3C chemokine receptor 1; IGF‐1, insulin‐like growth factor 1; FGF‐1, fibroblast growth factor 1; EGF, epidermal growth factor; PDGF, platelet‐derived growth factor; TNF‐α, tumor necrosis factor alpha; JAK, janus kinase; STAT, signal transducers and activators of transcription; SMAD‐3, Mothers against decapentaplegic homologue 3; MAPKs, mitogen‐activated protein kinases; ERK, extracellular signal‐regulated kinase; PI3K, phosphatidylinositol 3‐kinase.

3.2.1. Keloid‐associated lymphoid tissue (KALT)

Studies have investigated the role of immune cells in keloid tissue and have reported a range of immune markers. One of the key studies by Bagabir et al. showed that in intralesional and perilesional keloid tissue samples, there was a presence of distinctive lymphoid aggregates (LA) in the dermis, such as those observed in tertiary lymphoid tissues (TLT) ¹⁷⁶ , ¹⁷⁷ and mucosa‐associated lymphoid tissue (MALT), ¹⁷⁸ which they termed keloid‐associated lymphoid tissue (KALT). ¹³³ In addition, Bagabir et al. demonstrated that degranulated MCs and mature MCs (co‐expressing OX40L) were significantly increased in keloid lesional sites compared to the extra‐lesional, normal scar, and normal skin tissue. ¹³³ Also, M2 macrophages a component of KALT, were found to be present with fibroblasts and activated T‐cells [T helper type 2 (Th2) and Th17] which further promote wound healing and fibrogenesis. ¹⁷⁹ , ¹⁸⁰ Grant et al. in a recent report showed that embryonic stem cell (ESC) markers were expressed in KALT including octamer‐binding transcription factor 4 (OCT4), SRY homology‐box 2 (SOX2), phospho‐signal transducer and activator of transcription 3 (pSTAT3), and homeobox protein North American Network Operators' Group (NANOG), and are located at the endothelium of the microvessels within KALT. ¹⁸¹ They may be responsible for the production of aberrant fibroblasts and myofibroblasts through a mesenchymal stem cell (MSC) intermediate in an endothelial‐to‐mesenchymal transition (Endo‐MT) process induced by cytokines such as TGF‐β1 thereby promoting keloid tissue formation. ¹⁸¹ , ¹⁸² , ¹⁸³

The presence of KALT in organized TLT structures in KD is indicative of a chronic inflammatory milieu which may be important for KD fibrogenesis and may be exploited as a target for immunotherapy. Since there is a milieu of immune cells within KALT, it provides the opportunity for further research to tease out the various cells and networks to identify the possible options for immune‐targeted therapy.

3.2.2. Mast cells

Mast cells have been shown to be activated and increased in the dermis of the keloid lesional site. ¹³³ , ¹⁸⁴ A study by Bagabir et al. demonstrated a significant increase in the frequency of MCs degranulation compared to non‐degranulated MCs, and mature MCs in KD than in control normal skin tissue, indicating that the number and degranulation status of MCs are important factors in KD development. ¹³³ Moreover, they found a significantly increased frequency of c‐kit⁺ MCs in the KD dermis and mature β‐tryptase MCs co‐expressing OX40L in intralesional and perilesional KD sites compared to normal skin control samples. ¹³³ Their findings are in line with an earlier study that performed a comparative proteomic analysis between normal skin and keloid scar and found that MCs β‐tryptase protein was significantly upregulated in KD compared to normal skin. ¹⁸⁵ Other studies have reported that MCs enhance scar formation and may mediate the transition from scarless to fibrotic healing. ¹⁸⁶

There is evidence that MCs promote ECM production in in vitro co‐culture of KD fibroblasts and MCs, ¹⁸⁷ which is reported to occur through their direct and immediate cell‐to‐cell contact with fibroblasts a phenomenon termed “cell talk.” ¹⁸⁷ , ¹⁸⁸ Cell talk has been shown to facilitate expression of VEGF and hypoxia‐inducible factor‐1α (HIF‐1α) which promote aberrant fibrogenesis in KD. ¹⁸⁷ , ¹⁸⁸ The involvement of the extracellular signal‐regulated kinase 1 and 2 (ERK1/2) cascade and phosphatidylinositol‐3‐kinase (PI3K)/Akt signalling pathways have been reported to promote the accumulation of HIF‐1α in co‐culture of keloid fibroblasts and MCs. ¹⁸⁷ There are reports associating hypoxia environment with keloid fibrogenesis ¹⁸⁷ , ¹⁸⁸ , ¹⁸⁹ , ¹⁹⁰ which could be beneficial for the survival of keloid lesions. Keloid fibroblasts have been shown to exhibit the Warburg effect which is the preference of glycolysis and dependence on glucose for their metabolism, ¹⁹¹ thought to be enhanced by glycolytic enzymes activated by HIF‐1α. ¹⁹² , ¹⁹³ , ¹⁹⁴

3.2.3. Macrophages

Macrophages have been implicated in many fibrotic disorders, and are thought to be a contributor to keloid fibrogenesis. ¹⁸⁸ , ¹⁹⁵ Bagabir et al. ¹³³ showed that the expression of major histocompatibility complex (MHC) II in both M1 and M2 macrophages was altered in KD compared to normal tissue, ¹³³ which confirms findings of previous studies that reported low expression of MHC II by M2 and tumor associated macrophages (TAM) in patients with atherosclerosis a fibrotic disease as well as in cancers such as pancreatic cancer. ¹⁹⁶ , ¹⁹⁷ , ¹⁹⁸ In addition, Bagabir et al. reported a significantly higher frequency of M2 macrophages at intralesional and perilesional sites in keloid tissue compared to normal control skin. ¹³³ This finding has been supported by other studies that have reported the presence of macrophages in KD. ¹⁹⁵ , ¹⁹⁹ , ²⁰⁰ Li et al. in their study investigating the status of M1 and M2 macrophages in KD, consistently demonstrated that M2 infiltration outnumbered M1 macrophages in the dermis of keloid compared with normal control tissues, a finding similar to Bagabir's. ¹³³ , ²⁰⁰ Several human leucocyte antigen (HLA) could act as potential biomarkers for KD. ²⁰¹ , ²⁰² , ²⁰³ High frequency of HLA‐DRB1*15 phenotype has been reported in Caucasian KD patients compared to controls. ²⁰¹ Lu et al. showed a positive association between HLA‐DQA1 and ‐DQB1 genotypes and the development of KD in a Chinese cohort. ²⁰² Rossi and colleagues reported an association between HLA class 1 antigen (HLA‐B21) and class 2 antigens (HLA‐DR5 and HLA‐DQw3), with keloids in their Italian cohort. ²⁰³

At the cellular level in an in vitro co‐culture experiment, there is evidence that monocytes differentiate into M2 macrophages when in an early phase of wound regeneration and repair in the presence of either keloid or normal tissue, indicating that this would occur irrespective of the individual being prone to normal or keloid scar formation. ²⁰⁴ M2 macrophages play a critical part in fibrogenesis by secreting growth factors that play a vital role in KD pathogenesis, as elevated levels of TGF‐β, VEGF, connective tissue growth factor (CTGF), and PDGF are reportedly involved in stimulating collagen secretion, growth, and promote keloid fibroblast proliferation as well as keloid tissue expansion. ²⁰⁵ , ²⁰⁶ , ²⁰⁷

3.2.4. T lymphocytes mediated cytokine responses

T helper type 1 (Th1) immune responses linked with pro‐inflammation have been shown to be stimulated in normal wound healing, while Th2 responses have been shown to be stimulated in keloid formation. ¹⁷⁹ , ¹⁸⁰ IFN‐α, IFN‐γ, and TNF‐β which promote Th1 differentiation and with proinflammatory capacity have been shown to inhibit fibroblast proliferation in keloid cells, while IL‐4, IL‐6, IL‐10, and IL‐13 which promote Th2 differentiation have been linked to fibrogenesis in keloids. ¹⁸⁶ , ²⁰⁸ , ²⁰⁹ , ²¹⁰ There is evidence suggesting that the type of immune response in addition to the severity of inflammation is a vital factor that predisposes individuals to hypertrophic and keloid scarring. ²¹¹

In a study investigating various cytokines in PBMCs of black KD individuals by McCauley et al., IFN‐α, INF‐γ, and TNF‐β were reported to be highly suppressed in keloid patients compared to their normal control counterparts. ²¹⁰ On the contrary to the expected reduction of Th1 cytokine production by keloid individuals, McCauley et al. showed that expression of IFN‐β, and TNF‐α production was markedly higher in PBMCs of KD individuals. ²¹⁰ There was increased expression of IL‐6 a Th2 cytokine in PBMCs from individuals with KD compared to control PBMCs from individuals without keloids. ²¹⁰ IL‐6 and its mediated response through the Janus kinase (JAK)/Stat3 transcription factor signalling pathway, ²¹² , ²¹³ , ²¹⁴ have been shown to promote keloid fibroblast proliferation and ECM synthesis. ²⁰⁸

A study by da Silva et al. reported significantly lower mRNA expressions of IFN‐γ, IFN‐γR1, and IL‐10 in keloid tissues than in normal control tissue, but no significant difference in TNF‐α expression. ²¹⁵ Interestingly, they found a negative correlation between collagen type III and expression of IFN‐γ, IFN‐γR1, and IL‐10 in keloid biopsies. ²¹⁵ The predominant expression of collagen type III in keloids compared to the controls, is a reflection of keloid as an immature lesion. ²¹⁵ Further, increased IL‐4/IL‐13 signalling and responses have been reported in keloid lesions, with IL‐13 significantly increased in lesional and non‐lesional keloid tissue than in normal control tissue. ²¹⁶ In addition, a study by Oriente et al. demonstrated that post IL‐13 stimulation of keloid fibroblasts, there was high expression of the procollagen 1α1 gene compared to normal skin fibroblasts. ²¹⁷ Also, the ability of IL‐13 to induce gene expression for procollagen 3α1 in keloid fibroblasts was unique compared to IL‐4 or TGF‐β. ²¹⁷

Zhang et al. in their study investigating alteration in stem cell niche in keloid, demonstrated a robust increase in IL‐6 and IL‐17 expression in keloid stem cells. ¹⁸³ Employing sub‐clonal assay, flow cytometry, and multipotent differentiation analyses, they reported that keloid lesions have a new population of stem cells, named keloid‐derived precursor cells, which exhibit clonogenicity, self‐renewal, distinct embryonic and MSC surface markers, and multipotent differentiation. ¹⁸³ Their findings suggested that the altered biological functions in KD are highly regulated by the inflammatory niche mediated by an autocrine/paracrine cytokine IL‐17/IL‐6 axis. ¹⁸³

3.2.5. Memory T cells

There is evidence linking memory T‐cells to the regulation of inflammation in KD. Chen et al. characterized memory T‐cells expressing CD45RO in primary cutaneous cells in the tissue and PBMCs from patients with KD. ²¹⁸ They found an increased frequency of CD8⁺CD45RO⁺ memory T‐cells in KD tissue compared to normal tissue. ²¹⁸ Similarly, a study by Santucci et al. identified CD45RO⁺ memory T cells in keloid tissues from Caucasian patients compared to individuals with HTS in their Italian cohort. ²¹⁹ Also, Chen et al. found no significant difference in CCR7 expression between CD8⁺CD45RO⁺ memory T cells derived from healthy and KD tissue. ²¹⁸ This was further corroborated by the observation that the CD45RA‐CD45RO+ memory T cells while poorly expressing CCR7 were significantly expressing CD103 compared to control normal cells, indicating that these memory T cells were functionally not recirculating. ²¹⁸ These findings by Chen et al. show abnormalities of CD45RO+ memory T cells in KD and indicate that a disruptive T‐cell response contributes to KD progression. ²¹⁸

3.2.6. Regulatory T cells

Chen et al. found a reduced frequency of CD4⁺CD25⁺FOXP3⁺ regulatory T‐cells (Tregs) in PBMCs of patience with multiple keloids. In cells derived from KD tissue expressing CD4⁺FoxP3⁺CD45RO⁺ memory T‐cells, there was reduced expression of CD25 and cytotoxic T‐lymphocyte associated antigen 4 (CTLA‐4), and IL‐10 production. ²¹⁸ This indicates a lack of suppression of the inflammatory machinery of memory T‐cells in KD. Tregs can suppress effector CD4⁺ T‐cells and modulate their responses. It has been shown by Murao et al. that the Tregs:CD4⁺ T‐cells ratio is lower in KD, which may be beneficial to keloid lesions. ²²⁰

3.2.7. Keratinocytes

Conventionally, keloids are considered a raised dermal scarring disorder involving mainly abnormal fibroblasts, however, recent studies have demonstrated the involvement of potentially abnormal keratinocytes and other cell types as well. ²²¹ , ²²² Keratinocytes are the main cell type of the epidermis which are tightly held together by cell junctions and can trigger immune response via secretion of various chemokines, cytokines, cytokine receptors, and various antimicrobial peptides. ²²³ Keratinocytes have been shown to enhance proliferation and reduce apoptosis of underlying fibroblasts. ²²⁴ Bagabir et al. suggested that epidermal keratinocytes communicate with fibroblasts, and this intercommunication was shown to play a significant role in keloid formation. ¹³³ Gene expression profile of keloid keratinocytes have reported expression of numerous genes involved in epithelial‐mesenchymal transition (EMT), suggesting the involvement of EMT in adhesion abnormalities that may be beneficial to KD. ²²⁵ This has been supported by other studies that have shown that keloid keratinocytes exhibit an EMT‐like state similar to activated keratinocytes during wound healing. ¹⁵⁸ Interestingly, TGF‐β1 has been shown to decrease expression of EMT‐related genes including IL‐6, vimentin and cadherin‐11, and keloid keratinocyte migration through canonical and non‐canonical signalling pathways. ¹⁵⁸ , ²²⁶ Hence, TGF‐β1 can be explored as a potential therapy for KD.

3.3. Folliculitis keloidalis nuchae

Folliculitis keloidalis nuchae (FKN), also known as acne keloidalis nuchae, is a chronic type of folliculitis that typically involves the back of the neck and is commonly seen in young men of African descent. The lesion is neither a keloid nor is it associated with acne vulgaris. It is often pustular in its early stages and can later coalesce to form scarred keloidal plaques. ²²⁷ Advanced lesions may present purulent discharges and abscesses when secondarily infected. It is characterized histopathologically by deep scarring folliculitis, naked hair shafts, upper dermal inflammation, and occasional plaque or pustulopapular plaque lesions in the peri‐inflammation area. Aggregates of red blood cells, bacterial colonies, and neutrophils can be seen in the superficial epithelial layers. There can also be completely/partially ruptured hair follicles in the mid dermis. Partially ruptured hair follicles may contain basophilic debris, fragmented hair shafts, neutrophils, abundant keratins, and bacterial colonies. ²²⁷

The perifollicular region may be infiltrated by lymphocytes, histiocytes, plasma cells, and eosinophils. Completely ruptured hair follicles may show infiltration of lymphocytes, histiocytes, and plasma cells, as well as a neutrophilic aggregation that replaces the original follicular structures. ²²⁷ An important aetiological factor is suspected to be inflammation/infection. Inflammation in FKN could potentially be stimulated by chronic mechanical irritation/trauma and friction to the occipital, nuchal and occipital area during frequent haircuts, when wearing a helmet or from shirt collars. ²²⁷ Other speculated etiological factors that have been implicated in some patients include the use of anti‐epileptic agents, immunosuppressive agents, metabolic syndrome, and chronic infection. ²²⁷ , ²²⁸ , ²²⁹ , ²³⁰ , ²³¹ , ²³² A few other suggested inciting agents are androgens, genetics, and in‐growing hairs. ¹⁰ As in pseudo folliculitis barbae, shaving‐induced trauma may result in abnormal keratin expression in FKN. ²³³ Chronic low‐grade infection (e.g. Malassezia or S. Aureus), use of medication (e.g. cyclosporine, carbamazepine, and diphenylhydantoin), and autoimmunity have been implicated in a few individuals. ²²⁸ , ²³⁴

Although inflammation is canonically central to the pathogenesis of FKN, it is unclear whether it is causal or consequent. ¹⁰ Hence, FKN is described as primary cicatricial alopecia in the absence of overwhelming evidence of causative agents. Previous histological studies have alluded to possible pathogenesis from inflammation of follicular units, leading to the attraction of autogenous antigenic targets by host inflammatory cells and the destruction of follicular or pilosebaceous units. ²³⁵ , ²³⁶ One of the questions that still needs to be resolved is whether the sebaceous gland is a direct target of the inflammatory process or gets destroyed in the process of inflammation. ²³⁷ Although the reasons for nuchal localization of FKN are unclear, abundant MCs and dermal papillary dilatation in this region have been speculated as responsible. ²³⁸ Further, obesity and friction in the scalp skin fold in this region may be responsible. In fact, a study has alluded to FKN being a cutaneous feature of a metabolic syndrome, albeit with very little proof. ²³⁰

At the molecular level, very few studies have specifically investigated the direct immune targets of FKN. A recent case report detected the expression of IL‐6 in FKN tissue using immunohistochemistry and confocal microscopy. ²³⁹ Even though, the keloidal collagen that is typically found in keloids is absent in FKN, ²⁴⁰ the authors argue that molecules involved in the pathogenesis of keloids (e.g. CKLF‐1, IL‐6, IL‐24, IL‐18, TGF‐β1, STAT3, JAK1, ELK1, and RAF1), ²⁰⁸ , ²⁴¹ may be targeted for FKN. ²³⁹

4. CONCLUDING REMARKS

We have highlighted in this review, various immunological targets identified in raised dermal scars with a focus on hypertrophic, keloid, and FKN scars. Key pathways/interactions that are perturbed during dermal fibrogenesis have been discussed in detail and essential pathological deviations that result in raised dermal fibrogenic phenotypes were also discussed. The key interplay between traditional immune cells and non‐immune cell capable of producing chemical mediators of aberrant inflammation were highlighted. Understanding the immune regulators and role of immune cells, non‐immune cells, chemical modulators, genetic alteration, and “fibroblast‐to‐myofibroblast” switch in the excessive deposition of extracellular matrix molecules that always characterize raised dermal scars is key for the development of effective targeted therapies, that are highly desired for the management of raised dermal scars.

AUTHOR CONTRIBUTIONS

Elvis Banboye Kidzeru, Jyoti Rajan Sharma, Maribanyana Lebeko, Lucia Nkengazong, enry Ademola Adeola, Hlumani Ndlovu, Nonhlanhla Khumalo, and Ardeshir Bayat participated in drafting, editing, discussion, and revising the paper, and have all read and approved the final manuscript for submission.

CONFLICT OF INTEREST

None of the authors have a conflict of interest to declare.

Kidzeru EB, Lebeko M, Sharma JR, et al. Immune cells and associated molecular markers in dermal fibrosis with focus on raised cutaneous scars. Exp Dermatol. 2023;32:570‐587. doi: 10.1111/exd.14734

DATA AVAILABILITY STATEMENT

Data sharing not applicable to this article as no datasets were generated or analysed during the current study.