Skin Inflammation with a Focus on Wound Healing

Abstract

Significance:

The skin is the crucial first-line barrier against foreign pathogens. Compromise of this barrier presents in the context of inflammatory skin conditions and in chronic wounds. Skin conditions arising from dysfunctional inflammatory pathways severely compromise the quality of life of patients and have a high economic impact on the U.S. health care system. The development of a thorough understanding of the mechanisms that can disrupt skin inflammation is imperative to successfully modulate this inflammation with therapies.

Recent Advances:

Many advances in the understanding of skin inflammation have occurred during the past decade, including the development of multiple new pharmaceuticals. Mechanical force application has been greatly advanced clinically. Bioscaffolds also promote healing, while reducing scarring.

Critical Issues:

Various skin inflammatory conditions provide a framework for analysis of our understanding of the phases of successful wound healing. The large burden of chronic wounds on our society continues to focus attention on the chronic inflammatory state induced in many of these skin conditions.

Future Directions:

Better preclinical models of disease states such as chronic wounds, coupled with enhanced diagnostic abilities of human skin, will allow a better understanding of the mechanism of action. This will lead to improved treatments with biologics and other modalities such as the strategic application of mechanical forces and scaffolds, which ultimately results in better outcomes for our patients.

Dennis P. Orgill, MD

Adriana C. Panayi, MD

SCOPE

The skin serves as the crucial first-line barrier of the immune system to foreign pathogens. The different layers of the epidermis provide, in tandem, a physical barrier to water, foreign substances, and other microorganisms. Compromise of this barrier presents in the context of chronic wounds such as diabetic foot ulcers, venous stasis ulcers, and pressure ulcers. These pathologies are caused by hypoperfusion associated with diabetes, incompetent venous valves, and prolonged pressure, including sitting and bed rest.

When left untreated, these chronic wounds can develop infection and eventually lead to sepsis or amputation. Since chronic wounds often occur under chronic inflammatory conditions, the development of a thorough understanding of the conditions and drugs that alter skin inflammation is crucial.

TRANSLATIONAL RELEVANCE

As inflammation in skin conditions has become better understood, so too have the biological, pharmacological, and mechanical treatment options for chronic wounds. Mechanical force application, through vacuum assisted closure (VAC), for example, can stimulate the wound-healing process. Biodegradable scaffolds used on wounds can reduce scarring and wound contraction, while promoting re-epithelialization.¹ Wound treatments such as the delivery of exosomes (exos) and other cell-derived therapies show promise and/or have been demonstrated to accelerate wound healing and help treat various skin pathologies.

One limitation of this review is that some studies cited derive their data from rodent experimentation. Although wound-healing physiology is not homologous across rodents and humans, based on extensive prior research we can support the fact that the wound-healing processes are similar enough to make general conclusions. We hope that this review will contribute to a better understanding of the inflammatory processes involved in skin pathology and wound healing to inspire the development of more effective treatments.

CLINICAL RELEVANCE

This review outlines several inflammatory skin conditions arising from diverse inflammatory pathways, including psoriasis, hidradenitis suppurativa, contact dermatitis, toxic epidermal necrolysis (TEN), rosacea, and pyoderma gangrenosum. These inflammatory skin diseases are of high prevalence, placing a total cost in the United States' health care system in the range of billions of dollars (Table 1). Discussion of these conditions provides a framework for analysis of the human response to skin injury, the pathogenic inflammatory response to wound healing, and the failure to heal chronic wounds.

Table 1.

Economic cost and prevalence of inflammatory skin conditions

Disease	Annual U.S. Cost (2021 USD)	Prevalence, %
Psoriasis182,183	$128 billion	2
Hidradenitis suppurativa184,185	$2800 per patient	2
Contact dermatitis37,186	$1.8 billion	20
Pyoderma gangrenosum43,187	$9.5 million	0.006
Diabetic foot ulcer188,189	$21 billion	13
Venous stasis ulcer188,190	$1.7 billion	2
Pressure ulcer188,191	$25 billion	7
TEN192	$56000 per patient	1
Rosacea186,193	$243 million	5

TEN, toxic epidermal necrolysis.

DISCUSSION

Inflammation

Physiological inflammation

The normal inflammatory response to injury can be divided into two types: acute and chronic. Acute inflammation is an immediate response to injury characterized by innate immune mechanisms and it occurs in three stages. The first stage of acute inflammation classically results in skin redness, warmth, swelling, and pain—the Pillars rubor, calor, tumor, and dolor, as described by Celsus 2,000 years ago. This first stage of inflammation is initiated by sensory nociceptors densely distributed over the skin layers that on injury secrete neuropeptides to induce a local inflammatory response involving vasodilation and protein extravasation.²

Almost all skin cells express neuropeptide G-protein coupled receptors, which on activation trigger the secretion of further neuropeptides and neurotrophins. This exchange between skin cells and nerve cells acts as a positive bidirectional feedback loop that amplifies the inflammatory response.^3–6

In the dermis, neuropeptides released from local nerve fibers induce mast cell degranulation and pro-inflammatory histamine and prostaglandin release.^7,8 Histamine induces vasodilation and increased vascular permeability. This allows components of the innate immune system to reach the skin tissue and contributes to redness, warmth, and swelling.⁸ At this point, with help from the complement system, the regenerative stage of inflammation takes place with help from the complement system to facilitate further mast cell degranulation and neutrophil margination, adhesion, and chemotaxis.^9–11

Increased levels of neuropeptides promoted the expression of intercellular adhesion molecules (ICAM) and vascular endothelial cell adhesion molecules (VCAM),¹² as well as the release of vascular endothelial growth factor (VEGF) from mast cells and other immune cells.^13,14 VEGF induces local angiogenesis. Neutrophils marginate near the site of inflammation and adhere to the endothelial wall via the interactions of ICAM and VCAM, before undergoing chemotaxis toward interleukin (IL)-8, C5a, and LTB₄, released at the site of injury.

At the site of injury, neutrophils perform phagocytosis of microbes such as Staphylococcus aureus in the skin. Afterward, the neutrophils undergo apoptosis and the third stage of acute inflammation ensues. Macrophages then arrive at the site of injury and perform actions similar to those of neutrophils. Meanwhile, neuropeptides such as neuropeptide substance P (SP) and calcitonin gene related protein enhance the migration and antigen presentation of Langerhans cells, which promotes allergic sensitization.^15–17 This favors activation of the humoral immune system and chronic inflammatory response over acute inflammation associated with the cell-mediated immune system.¹⁸

In the epidermis, neuropeptide SP released from local nerve fibers stimulates keratinocytes to secrete proinflammatory cytokines.^19–22 In addition, both SP and other pro-inflammatory compounds such as interferon-gamma (IFN-γ) activate local fibroblasts to secrete SP, which further amplifies the inflammatory process in acute skin inflammation via a positive feedback loop.^23,24 Overexpression of SP and IFN-γ in the dermis has been shown to contribute to fibrosis in chronic skin inflammation such as in atopic dermatitis and psoriasis.^3,24

Chronic inflammation is characterized by lymphocytes in the tissue and more adaptive immune responses. Antigen-presenting cells of the skin present antigens on class II major histocompatibility complex (MHC) molecules to CD4⁺ helper T cells. After co-stimulation, CD4⁺ T helper 1 (Th1) cells further aid in chronic inflammation. Th1 cells release IFN-γ to induce B cell switching to IgG production for opsonization. Th1 cells additionally release IL-2 to activate CD8⁺ cytotoxic cells T. The Th2 subtype releases IL-4 and IL-5 to promote B cell switching to IgE production to promote eosinophil recruitment. These actions further help clear pathogens from the site of infection and remove infected tissue.

It is commonly accepted that inflammation is an active process mediated by proinflammatory mediators, whereas the downregulation of such mediators passively promotes tissue homeostasis and inflammation resolution. Serhan recently demonstrated that inflammation resolution can also present as an active process promoted by novel specialized pro-resolving mediators, and the disruption of this process may lead to chronic inflammation.^25,26 This is a paradigm shift in inflammation, one that may offer new opportunities for the treatment of inflammatory pathologies in the future.

Pathological inflammation

Pathological inflammation arises when physiological inflammation is interrupted. When macrophages fail to remove the inflammatory substance or pathogen, granulomas may form in which macrophages quarantine the microenvironment, preventing it from exerting more severe effects on the body.²⁷ Many factors and pathways contribute to a chronic inflammatory state in the skin that compromises this physical barrier. The following sections provide an overview of several inflammatory skin conditions ranging from one of the most common skin diseases, rosacea, to one of the most serious, TEN (Fig. 1). These conditions will serve as a framework for current and promising treatments of chronic skin conditions.

Figure 1.

Histological components of healthy skin and key inflammatory skin conditions. Healthy skin consists of a layer of epidermis covering a layer of dermis. The epidermis itself is composed of several different layers, from superficial to deep: stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. The dermis is an extracellular matrix predominantly comprising collagens, containing fibroblasts and the neurovascular network. In psoriasis, the stratum corneum becomes abnormal and is characterized by parakeratosis (retention of nuclei) and hyperkeratosis (thickening), manifesting clinically as scale formation. The epidermis is thickened, the rete ridges become elongated, and the vascular network becomes dilated. A lymphocytic infiltrate can be seen within the dermis, with a predominance of T cells and Langerhans cells. Munro's microabscesses, collections of polymorphonuclear leucocytes, can be seen within the stratum corneum. Hidradenitis suppurativa is characterized initially by keratin plug formation, subsequent follicular occlusion, and dilatation, followed by accumulation of cellular debris and cyst formation. This can result in extensive immune cell infiltration, including T cells, B cells, macrophages, and neutrophils. The follicle may eventually rupture and can lead to sinus tract formation. Contact dermatitis is triggered by exposure to contact allergens called haptens that induce a T cell-mediated inflammation. Epidermal edema leads to acanthosis (thickening of the epidermis) with hyperkeratosis and parakeratosis. The inflammatory infiltrate predominately comprises Langerhans cells and extensive cytokine release. In pyoderma gangrenosum, an initial intradermal abscess of a characteristic neutrophilic nature can be followed by epidermal ulceration and superficial dermal necrosis. Surrounding the ulcer is peripheral erythema, and the ulcer site has an undermining border and tenderness. Rosacea has been associated with physical triggers such as heat and certain foods, leading to increased activity of cathelicidin in its active form LL-37. Response is mediated by TRPA1, TRPV1, and TRPV4 channels leading to changes in neurovasculature that manifest as erythema and flushing. TEN is characterized by detachment of skin and mucosa, with an implication of CD8⁺ T cells that produce Granulysin, resulting in keratinocyte death. TEN, toxic epidermal necrolysis.

Psoriasis

Psoriasis is characterized by excessive growth of the epidermis from aberrant keratinocyte proliferation. Histologically, psoriasis presents as acanthosis (thickening of the stratum spinosum) with parakeratosis (retained nuclei in the stratum corneum). Keratinocytes in patients with psoriasis proliferate faster with shorter differentiation times compared with healthy skin.

Disease initiation and disease progression follow closely related, but different, immune mechanisms. After injury, keratinocytes release LL-37—a cationic antimicrobial peptide—which binds to self DNA or RNA fragments found in damaged skin cells. The resulting LL-37-nucleic acid complex activates plasmacytoid dendritic cells, causing them to release type I IFN.²⁸ These IFN go on to activate myeloid dermal dendritic cells, which migrate to draining lymph nodes to present novel antigens to naive T cells, producing T cells with autoimmune activity.²⁹

After T cell activation, a pro-inflammatory positive feedback IL cycle is created in the psoriatic skin. Dendritic cells and IL-23 further encourage type 17 helper cells (Th17) and cytotoxic (Tc17) functions. IL-23 has been demonstrated to maintain psoriasis-like skin inflammation in murine models.³⁰ Th17 and other specialized T cells serve as sources of IL-17A, IL-17F, and IL-22, which induce keratinocytes to recruit neutrophils, T cells, and LL-37, sustaining the pro-inflammatory loop in psoriasis.^31,32 IL-22 impairs keratinocyte differentiation, leading to the keratinocyte hyperplasia observed in psoriasis patients.³³

Hidradenitis suppurativa

Hidradenitis suppurativa, or acne inversa, is an inflammatory disease of apocrine gland-bearing skin situated at the axilla, inframammary folds, and groin. Although the main cause of hidradenitis suppurativa is still under investigation, several factors that are inflammatory, environmental, or genetic in nature have been implicated.

The inflammatory mechanism of hidradenitis suppurativa has been associated with increased synthesis and dysregulation of pro-inflammatory miRNA in skin leions.³⁴ Follicle inflammation causes production of cytokeratin 16, which promotes follicular hyperkeratinization and dilatation.³⁵ These processes activate the immune system in apocrine glands. After rupture of the follicle, molecules are exposed to a new dermal immune environment that induces further immune reactions via neutrophils, monocytes, and Th17 cytokines. This inflammation is maintained by repeated neutrophil infiltration via lipocalin-2.³⁶

Contact dermatitis

Contact dermatitis or allergic contact dermatitis is a delayed-type hypersensitivity reaction to environmental stimuli. Common environmental stimuli that trigger contact dermatitis include nickel, fragrances, chromium, and p-phenylenediamine.³⁷ The pathogenesis of allergic contact dermatitis begins with the innate immune system. The triggering irritant penetrates and forms a hapten with self-protein in the skin.³⁸ Studies have indicated that the hapten triggers toll-like receptors (TLR), TLR-2 and TLR-4, by introducing them to breakdown products of hyaluronic acid found in the skin.³⁹

In addition to the TLR-mediated mechanism of immune system activation, haptens can induce formation of the inflammasome complex, which includes NOD-like receptors. The downstream effects of this mechanism result in the release of inflammatory cytokines such as IL-1β.⁴⁰ These cytokines trigger the migration of dendritic cells to the microenvironment.

The arrival of dendritic cells marks the next phase of allergic contact dermatitis characterized by an adaptive immune response. Dendritic cells along with Langerhans cells present the allergen of interest on their MHC to naive T cells at their T cell receptors in draining lymph nodes. Sensitized effector T cells then migrate to the skin on further allergen exposure with the aid of homing receptors induced by dendritic cells.⁴¹

Cytotoxic T cells induce a cytotoxic inflammatory response to the antigen, creating the response seen on patient presentation. Deficiencies in human Treg cells may further exacerbate contact dermatitis, as patients with this condition are unable to desensitize the T cell response to allergens.⁴² Skin inflammation in contact dermatitis utilizes both innate and adaptive immune responses that may present opportunities for target therapies.

Pyoderma gangrenosum

Pyoderma gangrenosum is an inflammatory disease of a neutrophilic cause with average onset in the mid-40s to -50s and commonly manifested in the lower legs.⁴³ Although its etiology is still unknown, investigations have characterized pyoderma gangrenosum-affected skin as having abundant IL-8, matrix metalloproteinase (MMP)-9, MMP-10, IL-17, myeloperoxidase, and TNF-α with the absence of MMP-1 and MMP-26.^44,45

Further studies have found decreased Treg/Th17 ratios in pyoderma gangrenosum-affected skin.⁴⁶ In the blood of patients affected with pyoderma gangrenosum, CCR5 and CCR6 have been found to be elevated whereas CCR4 is downregulated.⁴⁷ These findings suggest a dysfunction of neutrophil migration and/or function; however, the exact mechanisms of action are still unclear.

Toxic epidermal necrolysis

The previously mentioned inflammatory dermatoses explain conditions that arise from aberrant physiological processes or contact exposures to foreign substances; however, adverse skin reactions can occur from drugs that are nontopical in nature. TEN is a skin reaction that is a more severe form of Stevens-Johnson Syndrome (SJS), involving more than 30% of the skin. Clinical presentations of SJS/TEN are characterized by detachment of skin and mucosa. Recent studies of the pathogenesis of TEN have implicated CD8⁺ T cells in TEN.

Granulysin produced by these cells are believed to be responsible for the keratinocyte death seen in this disease.⁴⁸ Although its pathogenesis remains to be explored, the triggers of TEN include medications such as lamotrigine, carbamazepine, sulfonamides, allopurinol, and nevirapine. Genetic factors further predispose to these drug-induced causes of TEN. HLA-B*1502 in those of Asian ancestry carry higher risks of experiencing SJS/TEN from carbamazepine than others.⁴⁹ The genetic variation causes immune cells to react strongly against these drugs to induce TEN.

Rosacea

Other noncontact causes of skin inflammation with environmental triggers sometimes result in a less severe course of progression. Rosacea is a long-term skin condition commonly affecting the face. Signs of rosacea include flushing, erythematous papules/pustules, and telangiectasia. The condition can be triggered by temperature extremes, exercise, stress, anxiety, caffeine, and spicy foods. Although triggers have been characterized, the understanding of the pathogenesis of rosacea is currently underway with several associations made with regard to its development.

For example, Demodex mites have been found to have a higher prevalence in rosacea lesions; however, treatments aimed at eradicating colonies showed no improvement of rosacea symptoms.⁵⁰ In terms of immunologic etiologies, patients with rosacea have also demonstrated increased activity of cathelicidin in its active form LL-37.⁵¹ The physical triggers such as heat and certain foods are mediated by TRPA1, TRPV1, and TRPV4 channels, leading to changes in neurovasculature that manifest as erythema and flushing seen in rosacea patients.^52,53

Wounds

Acute wound healing

Wound repair is a highly dynamic process that is regulated by complex interactions of extracellular matrix (ECM) molecules, soluble mediators, various resident cells, and infiltrating leukocyte subtypes. Normal wound repair proceeds through four overlapping sequential steps, as described in Figure 2: hemostasis, inflammation, proliferation, and finally maturation/remodeling.^54–57

Figure 2.

Phases of wound healing. There are four classic phases of wound healing: hemostasis, inflammation, proliferation, and resolution, with inflammation further separated into two sub-phases: early and late. During the first few hours of healing, the endothelial cells release anti-thrombotic agents to induce platelet aggregation. Factor X induces cleavage of fibrinogen to fibrin, which then cross-links and binds to the platelet aggregate to form a thrombus. The thrombus stops the bleeding and provides the preliminary matrix for healing. Histamine release from mast cells triggers a neutrophilic influx and the onset of inflammation. Through a series of immune responses, debris is removed to prevent infection. Inflammation can be further subdivided into early and late processes, distinguished primarily by their macrophage population. Early inflammation is characterized by a neutrophilic infiltrate and M1 macrophage population, and late inflammation comprising mainly M2 macrophages. During proliferation, an eschar (scab) covers the surface of the wound whereas keratinocytes migrate across the surface for wound closure. At the same time, fibroblasts promote the replacement of the initial fibrin clot with granulation tissue. This stage is also characterized by angiogenesis. The final stage is resolution, during which scar tissue is formed. This typically starts after 3 weeks and can last for longer than 2 years depending on the wound. The deposited matrix undergoes a fibroblast-based remodeling process, blood vessels regress, myofibroblasts are activated, and wound contraction ensues.

In response to an injury, the hemostasis phase of wound healing begins. This step spans seconds to hours and is initiated by endothelial ECM collagen exposure within the wound environment. Metalloproteinases in the wound site degrade endothelial collagen, which activates platelet activation and eventually forms fibrin clots.^54–57 The inflammation phase of the wound-healing process can take hours to days to complete, when degraded collagen fragments and local hypoxia cause nearby immune cells such as macrophages to migrate to the wound site.^54,58–60

During early inflammation, M1 macrophages activated by IFN-γ promote pro-inflammatory activity.⁶¹ During late inflammation, M2 macrophages secrete anti-inflammatory TGF-β1, IL-10, and VEGF to promote lymphatic vessel growth and skin antigen clearance.⁶² The proliferation step of wound healing can span days to weeks and involves the activation of profibrotic activity by fibroblasts and keratinocytes.^54,63

The final step of wound healing occurs when fibroblasts remodel the ECM by realigning and depositing collagen fiber such that collagen is matured into complex structures whose amount and organization determines the tensile strength of healed skin.^54–57 Collagen remodeling can occur for months after wound closure, and the tensile strength of repaired tissue can reach up to 85% of normal tissue if the wound healing process occurs without any perturbations.⁵⁴

Imbalances in enzyme activity during any stage can contribute to wound chronicity. For example, higher than normal levels of macrophages that release pro-inflammatory cytokines or lower levels of reducing enzymes such as Glutathione can promote wound chronicity by either producing or inhibiting the removal of reactive oxygen species (ROS), which consequently degrade the extracellular cell matrix.^55–57,62

Wound infection

Biofilm creation is one way in which pathogens impair the skin's wound healing response. S. aureus, one of the most prevalent pathogens of chronic wounds, impairs the wound-healing process by secreting a hyperglycosylated matrix that, when complexed with other bacteria, forms a biofilm that lowers the expression of type I collagen in wounds, impairing granulation tissue formation.⁶⁴ In addition to impairing type I collagen formation, S. aureus upregulates the synthesis of MMP-2, creating a collagenolytic environment in the skin that prevents biomechanical wound closure.⁶⁴

The secretion of a biofilm further promotes the progression of a chronic wound state. Pathogens can sometimes trigger lesions in inflammatory skin diseases listed in previous sections. Psoriasis can be exacerbated by the presence of S. aureus, Candida albicans, or Malassezia furfur.^65–67 In hidradenitis suppurativa, sweat glands can become infected with bacteria. Early stages of the disease are associated with anaerobic bacteria colonization, whereas later stages are associated with facultative anaerobic and aerobic bacteria. TEN is rarely caused by Mycoplasma pneumoniae infection. Lesions in rosacea are suspected to be caused by an immune response to the bacteria Bacillus oleronius found in Demodex mites.⁶⁸

Chronic wounds

Unlike acute wounds, chronic wounds do not progress through the normal four-phase wound-healing process. Instead, they are usually characterized by a longer inflammatory stage, and consequently increased amounts of proinflammatory cytokines, ROS, and proteases (Fig. 3).^69–72

Figure 3.

Chronic and acute wounds. Acute wounds typically progress through four phases of healing, with inflammation having two sub-phases, and are characterized by early re-epithelization. Signs of healing tend to occur within 4 weeks. In chronic wounds, hyperproliferation of inflammatory cells, particularly macrophages and neutrophils, results in an augmented and prolonged inflammatory phase. Further, angiogenesis is over-induced, resulting in numerous, but immature and friable microvessels. This establishes a microenvironment that is prone to bacterial infection and biofilm formation. Taken together, these factors limit and delay the proliferative phase and can lead to a lack of resolution even 2 years after wounding.

Chronic wounds have higher levels of leukocyte infiltration due to a heightened inflammatory state, which is often characterized by a variety of pathological changes such as an increased presence of macrophages and lymphocytes at the wound site.⁷³ Higher leukocyte infiltration ultimately prolongs oxidative stress and exacerbates tissue damage by increasing local ROS production.⁵⁴ Elevated ROS results in impaired wound healing by increasing cell apoptosis, senescence lipid peroxidation, protein modification, and DNA damage.⁷³

In fact, chronic wounds have been successfully induced in diabetic mice by administering an antioxidant enzyme inhibitor in the wound environment, which blocks antioxidant enzymes from converting ROS to nontoxic molecules.⁵⁴

Oxidative stress contributes to macrophage differentiation and polarization, which ultimately prolongs the inflammation stage of wound healing. Specifically, oxidative stress results in the downregulation of the genes responsible for hematopoietic stem cells differentiation to monocytes/macrophages, and it reduces macrophage infiltration, driving polarization toward M1 macrophages.^73–75 Higher M1 macrophage phenotypes can lead to an overproduction of the inflammatory cytokine TNF-α, which in abnormally high levels induces chronicity by the overproduction of ROS and degradation of the ECM via the activation of metalloproteases.^75–79

Overall, ROS plays a key role in chronic inflammation by promoting leukocyte adhesion and chemotaxis of proinflammatory factors into the wound, while inhibiting keratinocyte migration and re-epithelization.^80,81

A prolonged inflammatory stage in chronic wounds is also characterized by a longer period of angiogenesis. During normal wound healing, newly formed blood vessels are highly disorganized and poorly perfused during the inflammation stage, but a switch from proangiogenic to antiangiogenic factors during the proliferation and maturation phase leads to angiomaturation and apoptosis of immature vessels with restoration of a vascular environment.^73,82 Blood vessels in chronic wounds are prone to leakage and damage because they remain immature and poorly perfused, characterized by a lack of surrounding pericyte vascular smooth muscle cells.^73,83

Further, chronic wounds experience epidermal hyperproliferation of cell types such as keratinocytes, which clinically leads to overgrowth of the wound margin and raised borders.^73,84 Such pocket-like wounds are more difficult to clean and more prone to infection, contributing to wound chronicity.^54,85

Higher levels of bacterial colonies and biofilm production, specifically by species such as S. aureus and coagulase negative staphylococci, also contribute to wound chronicity by exacerbating leukocyte infiltration and subsequent ROS damage.^86,87 Further, although the production of type III collagen by fibroblasts is a characteristic of the late inflammation phase of normal wound healing, type III is commonly overproduced in chronic wounds.^54,73

Modulation of inflammation

Wound care clinicians have succeeded in treating several conditions by modulating the inflammatory response to skin irritation and trauma. Although the cascade of initiating and mediating factors of inflammation has not been fully elucidated, it is understood that the disruption of intact skin initiates an inflammatory response that helps to regenerate or repair the disturbed tissue.⁸⁸ If the stress is transient and the system in good health, growth factors and chemokines produced by macrophages in the skin stimulate granulation tissue formation and re-epithelialization and the tissue can return to homeostasis relatively quickly.

Under sustained insult, this inflammatory response can become maladaptive and contribute to pathological wound healing. However, when elements of the physiologic adaptive response are missing, as in some disease states, this process can be interrupted.⁸⁸ Clinicians have attempted to influence wound healing by modulating systemic and localized inflammatory processes.

Corticosteroids

Corticoids are one of the most widely used treatments for skin inflammation. The downstream effects of corticosteroids are varied but physiologically aligned, consisting of anti-inflammatory and immunosuppressive responses. Corticoids most commonly act by binding to intracellular glucocorticoid receptors (GR) that then either bind directly to other transcription factors or to glucocorticoid response elements (GRE) located in the DNA.

For example, by binding directly to NF-κb and AP-1 and inhibiting their activity, GR prevents both factors from activating DNA transcription of pro-inflammatory cytokines. In addition, GR can bind to certain GREs that promote the expression of mitogen-activated protein kinase (MAPK) phosphatase 1, which inhibits the pro-inflammatory MAPK pathway.⁸⁹ Corticoids also bind to certain GREs that suppress the expression of phospholipase A2, which decreases the synthesis of pro-inflammatory molecules such as prostaglandins and leukotrienes.⁹⁰

More generally, corticoids exert effects on various cells of the immune system. Corticoids induce apoptosis in T lymphocytes, decreasing local inflammatory responses.^91,92 Corticoids also affect macrophages by reducing their migratory functions and causing them to decrease secretions of collagenase, elastase, and plasminogen activators.^93,94

Corticosteroids have proven useful in treating many skin conditions. In psoriasis, topical corticosteroids can lower relapse and induce remission of disease for up to 6 months of treatment.⁹⁵ Anti-inflammatory properties of corticosteroids also reduce local inflammations in hidradenitis suppurativa.⁹⁶ For pyoderma gangrenosum, topical corticosteroids are first-line for small lesions with slow progression whereas larger, more rapidly progressive lesions are treated with systemic corticosteroids.⁹⁷ In TEN, corticosteroids are an ineffective monotherapy, but they have beneficial results in combination with intravenous immunoglobulin therapy.⁹⁸

Although acute treatment with corticosteroids has pro-healing anti-inflammatory effects, chronic steroid therapy can lead to delayed wound healing and a higher incidence of local infection and dehiscence.⁹⁹ In one study involving lagomorphs, vitamin A administration has been utilized to reverse some of the harmful effects by promoting epithelialization and collagen synthesis.¹⁰⁰

Antibiotics

Chronic inflammatory environments compromise the body's natural skin barrier to commensal bacteria. Pathogenic infiltration of normal skin microbiota can precipitate in the formation of ulcers and abscesses. Thus, antibiotics are often prescribed to prevent or treat infections occurring from these inflammatory skin conditions. However, in many skin conditions, antibiotics serve the dual role of combating infection as well as decreasing inflammation. Studies have found that macrolide antibiotic treatment successfully reduced plaque psoriasis severity and area in patients.¹⁰¹

Proposed mechanisms of actions for the efficacy of macrolides in psoriasis include the inhibition of pro-inflammatory cytokines and compounds that promote neutrophil chemotaxis.

Antibiotics are also often used in hidradenitis suppurativa depending on the severity of disease. Early hidradenitis is treated with topical antibiotics, with clindamycin being the most effective.¹⁰² Additional antibiotics used in inflammatory skin conditions include tetracyclines, which have been suggested for treatment in hidradenitis suppurativa¹⁰³ and pyoderma gangrenosum.¹⁰⁴ Doxycycline, a type of tetracycline, additionally shows great benefits in treating rosacea when used in a modified release form in combination with metronidazole or in combination with ivermectin.^105,106

Biologics

Biologics are complex molecules derived from natural sources and are a rapidly expanding class of drugs used for inflammatory skin conditions. Monoclonal antibodies have shown great efficacy in treating inflammatory dermatoses. Adalimumab, a fully human monoclonal antibody against TNF-α, has been studied for use in many skin conditions, including psoriasis,¹⁰⁷ hidradenitis suppurativa,¹⁰⁸ and pyoderma gangrenosum.¹⁰⁹ Adverse effects of these anti-TNF-α biologics are similar to those of immunosuppressants since TNF-α stimulates the immune response.¹¹⁰

A range of FDA-approved biologics used in the treatment of inflammatory dermatoses and common adverse reactions are outlined in Table 2. Additional information on biologics currently under FDA review is included in Supplementary Table S1. Until biosimilars become more widely available, costs of these biologic therapies will limit their use in the United States.

Table 2.

Available FDA-approved biologics used in the treatment of hidradenitis suppurativa, psoriasis, pyoderma gangrenosum, toxic epidermal necrolysis, rosacea, and allergic contact dermatitis

Disease	Target	Drug Name	ClinicalTrials.gov Identifier	Phase 4 Recruitment Status	Adverse Reactions
HS	TNF-α	Adalimumab	NCT02808975	Complete	Infection, URI, Antibody development, Injection site reaction, Positive ANA titer, Headache, Skin rash, Increased creatine phosphokinase
PP	IL-17A	Secukinumab	NCT02798211	Complete	Infection, URI
	IL-17A	Ixekizumab	NCT03151551	Complete	Infection, URI, Antibody development, Injection site reaction, Neutropenia
	IL-17 receptor A	Brodalumab	NCT03403036	Complete	Infection, URI
	IL-12/IL-23	Ustekinumab	NCT01511315	Complete	Infection, URI, Antibody development
	IL-23	Tildrakizumab	NCT04271540	Recruiting	Infection, URI
	IL-23	Risankizumab	NCT04340076	Recruiting	Infection, URI, Antibody development
	IL-23	Guselkumab	NCT03573323	Completed	Infection, URI
	TNF-α	Adalimumab	NCT01265823	Complete	Infection, URI, Antibody development, Injection site reaction, Positive ANA titer, Headache, Skin rash, Increased creatine phosphokinase
	TNF-α	Etanercept	NCT00967538	Complete	Infection, URI, Antibody development, Injection site reaction, Positive ANA titer, Skin rash, Diarrhea
	TNF-α	Infliximab	NCT00779675	Complete	Infection, URI, Antibody development, Positive ANA titer, double-stranded DNA antibody development, Infusion related reaction, Headache, Nausea, Abdominal pain, Abscess, Increased serum alanine aminotransferase
	TNF-α	Certolizumab pegol	NCT02326298	Complete	Infection, URI, Antibody development, Nausea
PG	No FDA approved biologics
TEN	No FDA approved biologics
RSA	No FDA approved biologics
ACD	No FDA approved biologics

Listed for each drug is the drug's target, identifier number, recruitment status as of October 2021, and common adverse reactions with incidences indicated in the most recent FDA label.

ACD, allergic contact dermatitis; ANA, antinuclear antibody; HS, hidradenitis suppurativa; IL, interleukin; PG, pyoderma gangrenosum; PP, plaque psoriasis; RSA, rosacea; TNF-α, tumour necrosis factor alpha; URI, upper respiratory infection.

Immunosuppressants

Immunosuppressants target immune cells that are responsible for releasing host immune cell secreted biological response modifiers. These drugs serve as additional treatment where corticosteroids may be contraindicated due to toxicity. In the realm of inflammatory modulation by immunosuppressants, methotrexate stands out as one of the most prominent immunosuppressants with anti-inflammatory properties. Methotrexate is one immunosuppressant that functions as a folic acid analogue that inhibits dihydrofolate reductase, which, in turn, impairs the production of thymidine, purines, and amino acids.

This effect culminates in decreased cytokine production in immune cells.¹¹¹ Specifically, methotrexate treatments promote accumulation of 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) and release of adenosine.¹¹² Adenosine interacts with receptors inhibiting the release of inflammatory TNF-α, IL-6, and IL-8 from monocytes.¹¹²

Mechanical forces

The discovery of reduced scarring from incisions made parallel to natural bands of tension in skin (Langer lines) first brought attention to the effects of forces on wound inflammation and healing.¹¹³ Within the first 2 weeks of injury, the epidermis may close but the wound healing process underneath the surface is not complete. If the wound continues to endure mechanical stress even from natural skin tension, inflammation may persist and ultimately lead to pathologic scarring¹¹⁴ such as hypertrophic and keloid scars.^115,116 The prevention of scar formation remains a difficult task, with clinicians currently using taping strategies, adjunctive chemotherapies ,and cryosurgery to further control the inflammatory process.^117–120

Mechanical forces stimulate mechanoreceptors to induce biochemical signaling that is involved in the life cycle, proliferation, and migration cellular processes in a process known as mechanotransduction.^121–124 Examples include mechano-regulated differentiation of human monocytes and their expression of proteolytic MMPs.¹²⁵ One clinical implication of mechanical forces include skin stretch with tissue-expanders, which can be utilized to increase the size of donor site tissue for the reconstruction of compromised tissue elsewhere on the patient's own integument.^126,127

The mechanisms that mechanical forces transduce biochemical signals have yet to be fully elucidated and findings depend on the model studied, type of mechanical stress induced, and tissue scaffolding selected.¹²⁸ However, mechanical forces generally alter dermal inflammation through downstream effects on cytokines, including IL-1α, IL-1β, IL-6, and TNF-α.^129,130 Skin stretch increases the quantity of local M2 macrophages, which promotes tissue proliferation and vascularization.¹³¹

However, Aragona et al. found that inflammation is not a key driver of skin-stretch induced tissue expansion. Using microarray analysis, they found that many of the genes upregulated by stretch expansion are also upregulated in wound healing, and that by knocking out the cytoskeleton remodeling genes Diaph3 and Myh9, they inhibited normal downstream effects of stretch on nuclear translocation of YAP1 and megakaryoblastic leukemia/myocardin-like 1 (MKL1), which led to barrier defects and complete inhibition of proliferation in response to stretch.¹³² A further study of these pathways may unlock more potential applications of skin stretch in wound care.

Negative pressure wound therapy

Negative pressure wound therapy (NPWT) is a mechanical technology used by clinicians to promote healing in slow-healing cutaneous wounds by optimizing local wound conditions.¹³³ Despite uncertain clinical efficacy, the use of NPWT has become prominent in the treatment of acute and chronic wounds since its introduction in 1997.¹³⁴ For example, NPWT has been identified as a safe and valuable component of care for hidradenitis suppurativa and pyoderma gangrenosum when added to standard of care surgical intervention and immunosuppression.^135,136

Falling under the umbrella of NPWT, VAC involves filling a wound with a porous material and attaching a drainage vacuum port to induce negative pressure.¹³³ The NPWT promotes healing through a number of mechanisms, including inflammation modulation, removal of harmful inflammatory byproducts, wound approximation, blood and lymphatic vessel growth stimulation, bacterial burden reduction, and exudate removal (Fig. 4).^137–140

Figure 4.

Effects of NPWT. NPWT exerts both macrodeformational and microdeformational forces, both of which are believed to induce angiogenesis and lymphangiogenesis. The mechanisms are multifactorial, but they include increased release of growth factors, edema-induced changes in hydrostatic compression and osmotic tension, and leucocyte elimination during exudate removal.¹³² In a recent murine experiment from our group (unpublished data), we showed that NPWT is able to stimulate lymphangiogenesis in diabetic wounds, leading to an overall promotion of healing. NPWT, negative pressure wound therapy.

In diabetic foot ulcers treated with NPWT, the inflammatory mediators IL-1β, TNF-α, and MMP-1 are downregulated whereas VEGF, TGF-β1, and TIMP metallopeptidase inhibitor 1 are upregulated.¹⁴¹ In 2019, Wang et al. found that IL-6, TNF-α, and inducible nitric oxide synthase were downregulated after NPWT via the MAPK-JNK pathway.¹⁴² More recently, Song et al. showed that NPWT reduced levels of proinflammatory cytokines and suppressed autophagy and overall macrophage inflammation to promote wound healing in diabetic mice.¹⁴³

In addition to the effects on inflammation, NPWT stimulates wound healing by reducing collagen turnover in the remodeling stage. Mous et al. found that the levels of MMP-9 enzyme and proenzyme, a collagenase believed to be partly responsible for poor wound closure, were significantly lower in wounds treated with NPWT.¹⁴⁴ Although there is a lack of preclinical basic science research that could elucidate the mechanisms of NPWT, its rapid adoption and prominent usage today in multimodal treatment of chronic wounds lends to the importance of further study.

Clinically available scaffolds

Clinically available biological scaffolds (bioscaffolds) provide surgeons with tools to reconstruct a wide array of structural defects. Bioscaffolds are dynamic structures that modulate the inflammation of chronic wounds to facilitate repair or regeneration of stromal tissues. Bioscaffolds are able to accomplish this because they possess physicochemical and structural properties that allow for both their passive and active interactions with host tissue cells.

They comprise a variety of bioactive components, such as adhesion ligands, that assist in establishing topography and promote correct cell deposition and alignment, and biological cues such as growth factors, nucleic acids, and cytokines that interact with the local ECM and promote tissue proliferation.^145,146

On implantation, multiple inflammation pathways are activated. Skin injury initiates an acute inflammatory response, causing secondary damage to the surrounding tissue that can hinder the integration of host cells into the scaffold.^147,148 Implantation can also intensify the skin's inflammatory response by triggering a foreign body reaction. For example, naturally derived templates may contain impurities and synthetic polymers may contain pro-inflammatory byproducts, all of which act as “non-self” signals that increase local inflammation.^149–152

The transition from an inflammatory M1 wound phenotype to an anti-inflammatory M2 phenotype is correlated with an increase in type 2 (Th2) cytokine secretion by CD4⁺ T (Th) cells, which ultimately leads to inflammation resolution.^153,154 A quicker resolution of inflammation increases the chance of implant acceptance and is ideally achieved within 2 weeks.

Bioscaffolds can modulate the inflammatory response and promote regenerative healing through a multifactorial process. Prior research has shown that scaffolds made of immune-inert biomaterials or hydrophilic and neutral materials can promote healing through a decrease in dendritic cell maturation, monocyte/macrophage differentiation, leukocyte migration, and pro-inflammatory cytokine production.^{143,155–158} Further, implantation of acellular scaffolds without chemical crosslinking and serum-coating can reduce the inflammatory response (Fig. 5).^159,160

Figure 5.

Wound-healing effects of scaffold application. The effects of scaffolds are multiple and varied but can be broadly classified under three main groups: surface adaptation, immune modulation, and structural modification. Superficially, scaffolds can help maintain a moist environment, promoting healing while inhibiting bacterial contamination and biofilm formation. At the immune level, scaffolds have been shown to be able to modify the macrophage population, to reduce cytokine production, and to increase leucocyte migration. Finally, structurally, scaffolds can promote re-epithelization, enhance granulation tissue formation, and improve angiogenesis. The three categories illustrated aim at providing an overview of the effects of scaffold application. Undoubtedly, the response to implanted material is complex and depends on multiple factors beyond the physico-chemical, biomechanical, and immunogenic properties of the scaffold.

Scaffolds with aligned fiber topography have been shown to reduce capsule formation whereas microporosity enables cell fusion and infiltration of the scaffold, promoting tissue granulation and angiogenesis, and reducing protein adsorption, monocyte adhesion, and pro-inflammatory cytokines.^161–163

Examples of biological scaffolds used to treat nonhealing wounds that are readily available on the market include synthetic (NovoSorb™ Biodegradable Temporising Matrix [BTM]), semisynthetic (Integra^®), and acellular (Alloderm^®) matrices. The BTM consists of a synthetic polyurethane polymerized matrix that eliminates any risk of cross-species residual antigenicity, reduces the chance of infection, and shows similar wound-healing effects compared with the Integra semisynthetic scaffold.^164,165

In a recently developed murine model of chronic wound healing, diabetic wounds treated with Integra Dermal Regeneration Template, a collagen-glycosaminoglycan (collagen-GAG) bioactive scaffold, exhibited a lower level of inflammation and a higher level of microvasculature maturation in comparison to untreated chronic wounds.⁵⁴

Wounds displayed faster wound healing due to enhanced cellular proliferation, collagen deposition, keratinocyte migration, and microvessel maturation. The Integra collagen-GAG matrix provided a favorable environment for keratinocyte survival via a decreased chronic inflammatory state and with migration occurring at the wound border and micrograft edges toward the center of the wound.⁵⁴

Acellular dermal matrices such as Alloderm act as 3D scaffolds that are infiltrated by host cells, including fibroblasts and endothelial cells, while resisting adhesion formation. They can degrade over time and are replaced by host ECM to promote healing.^166,167

Clinically available cell-based therapies

Many already established cell-based bioengineered skin products exist on the U.S. market that promote wound healing via a variety of processes, one of which includes the modification of cellular and molecular inflammatory signals. Examples include epithelial autografts (Epicel^® and ReCell^®), cellular dermal substitutes (Dermagraft^®), and bilayered skin substitutes (Apligraf^®). Epicel acts as an autologous skin product that involves sheets of keratinocytes attached to a supportive petrolatum gauze backing that is removed 1 week after direct treatment of difficult-to-heal skin burns.^168,169

This skin product is mainly used for patients who have extensive burns and lack enough donor skin for engraftment. ReCell is another autologous treatment commonly known as “Spray on Skin” that utilizes donor skin to produce a spray that once applied can regenerate a new outer layer of skin.¹⁷⁰ The major benefit of ReCell is that a large wound area can be treated with a relatively small amount of donor skin. Apligraf is a bi-layered living-skin construct that involves a dermis and well-differentiated epidermis.¹⁷¹

This skin construct has been valuable for treating chronic wounds such as diabetic foot ulcers and has been shown to decrease wound-healing time. Dermagraft is a dermal substitute that involves cryopreserved human neonatal fibroblasts attached to a bioabsorbable polyglactin mesh matrix.¹⁷² This dermal substitute has been used to accelerate chronic wound healing, such as full-thickness foot ulcers, by promoting re-epithelialization via activation of the patient's own keratinocytes.

New advances in tissue processing have also come from placenta-derived constructs. The ECM sheets derived from human placenta are rich in growth factors and promote full-thickness wound healing by providing a microenvironment favorable to both angiogenesis and the growth and differentiation of cells.¹⁷³ Further, acellular dermal matrices can also act as a scaffold to promote re-epithelialization and neoangiogenesis of the chronic wound bed. Multiple studies and reviews of chronic wounds have demonstrated improved healing times and wound area reductions.^174–176

In recent years, exosomes have garnered attention and shown promise as a potential treatment option for skin conditions involving chronic inflammation. Exosomes are small vesicles containing bioactive molecules such as lipids, proteins, mRNAs, and microRNAs that fuse with target cells to affect cellular functions.^177,178

For example, a recent study combined a decellularized small intestinal submucosa (SIS), mesoporous bioactive glass (MBG), and bone marrow derived mesenchymal stem cell (BMSC) exosomes to produce a hydrogel scaffold dressing (SIS/MBG@Exos), which allows for sustained release of bioactive BMSC exosomes. The scaffold promoted proliferation, migration, and angiogenesis, and it accelerated diabetic wound healing.¹⁷⁹

Other possible exosome treatments include mesenchymal stem cell-derived exos containing pioglitazone, which activates the PI3K/AKT/eNOS pathway and decreases oxidative stress.¹⁶² Further, exosomes derived from adipose-derived stem cells containing nuclear factor-erythroid factor 2-related factor 2 (Nrf2) protein promote the Nrf2/Keap1 pathway in rodent chronic wounds, decreasing oxidative stress.¹⁸⁰

In another study, exos carrying human beta-defensins, a group of antimicrobial peptides with pro- and anti-inflammatory functions that are usually released by resident cells of the epidermis to fight exogeneous pathogens, were used as a treatment of chronic wounds.¹⁸¹ Although not widely used clinically, exos demonstrate promise and deserve further attention.

SUMMARY

Skin inflammation can be characterized as a deviation from the normal physiological pathways of wound healing. It is important to recognize that any treatment used to resolve skin inflammation is an attempt to reorchestrate events at the molecular level, so it is essential to take into account the physiological underpinnings of all relevant processes. In comparison to acute wound healing, which progresses through a series of well-characterized phases, chronic wounds can stall at any phase of the wound-healing process and commonly exhibit a longer and altered inflammatory phase, leading to a variety of high-cost skin conditions such as psoriasis, hidradenitis, contact dermatitis, rosacea, TEN, and pyoderma gangrenosum.

Clinical modulation of chronic inflammatory states may involve therapeutics such as the administration of corticoids, biologics, antibiotics, mechanical forces, bioscaffolds, skin-based products, and exosomes. The goal of such treatments is to provide inflammation resolution, which can result in decreased scarring and increased scaffold engrafting.

TAKE HOME MESSAGES

• The normal physiology of wound healing involves a concerted inflammatory response by different types of cells involved in cell-mediated immunity.

• The altered healing of chronic wounds can be characterized by dysfunction in regulating the inflammatory phase of wound healing.

• Inflammatory skin conditions are of significant prevalence and high cost to the U.S. health care system.

• By modulating the cellular pathways underpinning chronic inflammation, quicker resolution and wound healing can be achieved.

• Targeting mechanotransduction pathways associated with inflammation such as the administration of VAC, NPWT, and skin stretch can lead to anti-inflammatory effects.

• Application of bioscaffolds and anti-inflammatory agents such as skin-cell products and bioactive compounds are established treatments for chronic wounds.

Abbreviations and Acronyms

ACD

allergic contact dermatitis

AICAR

5-aminoimidazole-4-carboxamide ribonucleotide

ANA

antinuclear antibody

AP-1

activator protein 1

BMSC

bone marrow derived mesenchymal stem cell

BTM

Biodegradable Temporising Matrix

C3, C4, C5

complement protein

CD4⁺, CD8⁺

cluster of differentiation

ECM

extracellular matrix

exos

exosomes

GAG

glycosaminoglycan

GR

glucocorticoid receptors

GRE

glucocorticoid response element

HS

hidradenitis suppurativa

ICAM

intercellular adhesion molecules

IFN-γ

interferon-gamma

IL

interleukin

JNK

c-Jun NH(2)-terminal kinases

LL-37

cathelicidin antimicrobial peptide (active form)

LTB₄

leukotriene B₄

MAPK

mitogen-activated protein kinase

MBG

mesoporous bioactive glass

MHC

major histocompatibility complex

MMP

matrix metalloproteinase

NF-κb

nuclear factor kappa-light-chain-enhancer of activated B cells

NPWT

negative pressure wound therapy

Nrf2

nuclear factor-erythroid factor 2-related factor 2

PG

pyoderma gangrenosum

PP

plaque psoriasis

ROS

reactive oxygen species

RSA

rosacea

SIS

small intestinal submucosa

SJS

Stevens-Johnson Syndrome

SP

substance P

TEN

toxic epidermal necrolysis

TGF-β1

transforming growth factor beta 1

Th1

type 1 helper T cell

Th2

type 2 helper T cell

TLR

toll-like receptors

TNF-α

tumour necrosis factor alpha

URI

upper respiratory infection

VAC

vacuum assisted closure

VCAM

vascular endothelial cell adhesion molecules

VEGF

vascular endothelial growth factor

AUTHORs' CONTRIBUTIONS

D.P.O. and A.C.P. planned, conceptualized, and orchestrated the writing; D.Y.M., B.N., and O.D. wrote the article; D.P.O. and A.C.P. verified the accuracy of key statements; A.C.P. and D.Y.M. made the figures and video; M.W., D.P.O., and A.C.P. critically edited the article; and the article was reviewed by all the authors.

ACKNOWLEDGMENTS AND FUNDING SOURCES

None declared. This work was funded by The Gillian Reny Stepping Strong Center for Trauma Innovation and is in part supported by NIH T35 HL110843 fellowship to Brian Ng.

AUTHOR DISCLOSURE AND GHOSTWRITING

The authors declare no competing financial interests.

ABOUT THE AUTHORS

Dany Y. Matar is a researcher at Brigham and Women's Hospital and an undergraduate student at Washington University in St. Louis studying Biochemistry and Molecular Biology.

Brian Ng, BA, is a medical student at Saint Louis University School of Medicine. He is a researcher at Brigham and Women's Hospital and a 2021 Summer Research Fellow in the Harvard-Longwood Short-Term Research Training in Vascular Surgery (NIH-T35) Program.

Oliver Darwish, BA, is a medical student at California Northstate University College of Medicine. He is a research collaborator at Brigham and Women's Hospital and a previous 2020 Summer Research Fellow in the Harvard-Longwood Short-Term Research Training in Vascular Surgery (NIH-T35) program.

Mengfan Wu, MD, PhD, is a postdoctoral research fellow in the Division of Plastic Surgery, Brigham and Women's Hospital, Harvard Medical School and a plastic surgeon in the Department of Plastic Surgery, Peking University, P.R. China.

Dennis P. Orgill, MD, PhD, directs the Wound Healing and Tissue Engineering Laboratory at Brigham and Women's Hospital, where he is a Plastic Surgeon. He is Professor of Surgery at Harvard Medical School.

Adriana C. Panayi, MD, is a Principal Investigator at Brigham and Women's Hospital and an instructor at Harvard Medical School. Dr. Panayi's research areas are wound healing, with a focus on chronic wounds.

SUPPLEMENTARY MATERIAL

Supplementary Table S1