Chemokines as Therapeutic Targets to Improve Healing Efficiency of Chronic Wounds

Abstract

Significance: Impaired wound healing leading to chronic wounds is an important clinical problem that needs immediate attention to develop new effective therapies. Members of the chemokine family seem to be attractive and amenable to stimulate the healing process in chronic wounds. Targeting specific chemokines and/or their receptors has the potential to modify chronic inflammation to acute inflammation, which will hasten the healing process.

Recent Advances: Over the years, expression levels of various chemokines and their receptors have been identified as key players in the inflammatory phase of wound healing. In addition, they contribute to regulating other phases of wound healing making them key targets for novel therapies. Understanding the signaling pathways of these chemokines will provide valuable clues for modulating their function to enhance the wound healing process.

Critical Issues: Inflammation, an important first-stage process in wound healing, is dysregulated in chronic wounds; emerging studies show that chemokines play a crucial role in regulating inflammation. The knowledge gained so far is still limited in understanding the enormous complexity of the chemokine network during inflammation not just in chronic wounds but also in acute (normal) wounds. A much better understanding of the individual chemokines will pave the way for better targets and therapies to improve the healing efficiency of chronic wounds.

Future Directions: Effective understanding of the interaction of chemokines and their receptors during chronic wound healing would facilitate the design of novel therapeutic drugs. Development of chemokine-based drugs targeting specific inflammatory cells will be invaluable in the treatment of chronic wounds, in which inflammation plays a major role.

Latha Satish, MSc, MPhil, PhD

Scope and Significance

Chronic wounds are difficult-to-heal wounds and contribute to significant healthcare expenditures. Unlike normal wounds, chronic wounds usually do not follow the conventional stages of healing and may remain open for many years. Several events that occur in a coordinated manner in healing normal wounds do not happen in nonhealing chronic wounds. For instance, inflammation appears to be prolonged for an extended period leading to distraught production of pro- and anti-inflammatory cytokines. This results in an impaired angiogenic response, a decreased granulation tissue formation, and a reduced keratinocyte and fibroblast migration and proliferation.^1,2 Over the years, several clinically approved evidence-based approaches have been utilized as treatment strategies to heal chronic wounds utilizing growth factors and biological dressings, but with limited success.^3,4 Chemokines, a family of small chemotactic cytokines seem to be attractive candidates to target chronic wounds as the members of the chemokine family, tightly regulate the process of inflammation and angiogenesis. This review will provide a brief overview on the structure and classification of chemokines and its function in acute wound healing. Furthermore, an in-depth analysis on the chemokines that could be used as targets to improve healing efficiency of chronic wounds will be discussed.

Translational Relevance

To overcome the current failure of healing chronic wounds using the available strategies targeting chemokines seems conceivable as major adjuvants to stimulate wound healing. Increasing or decreasing the expression of chemokines in a chronic wound seems attainable, which can modulate the hostile nonhealing environment for successful closure of wounds.

Clinical Relevance

It has been estimated that between 3 and 6 million people in the United States suffer from one of the three main types of chronic wounds—diabetic, venous, and pressure ulcers (PUs);⁵ these are responsible for significant healthcare expenditures. Although there have been extensive investigations into the mechanisms responsible for chronic wound formation especially in diabetics, effective clinical therapies are yet to be developed. Chemokines that belong to a small family of cytokines have been the focus for many years now to understand the functional significance in wound healing. Over the years, a greater understanding on the function of chemokines has been achieved through in vitro studies and preclinical models. In the clinical realm, targeting chemokines holds a great promise as chemokines can be easily manipulated to tailor the needs of the patients to improve healing.

Background

Chemokines and chemokine receptors

The chemokines or chemotactic cytokines are an important family of signaling molecules. Chemokines were first discovered as substances that assist in the recruitment of leukocytes to sites of injury or infection and, thereby, modulate immune and inflammatory responses.⁶ However, now, it is very apparent that chemokines contribute to a wide variety of processes, such as hematopoiesis, angiogenesis, and tissue growth and repair. To date, ∼50 human chemokines have been identified, and they are classified into four subgroups according to the polypeptide chain cysteine location: C, CC, CXC, and CX3C.⁷ Most of the known chemokines belong to the CC and CXC families. Chemokines interact with cells through receptors belonging to the superfamily of rhodopsin-like, G protein-coupled seven-transmembrane receptors.⁸ Some receptors bind with multiple chemokines, and various chemokines can interact with several different receptors.⁹

Most chemokines produced in the human body are inflammatory chemokines. The monocyte chemoattractant protein (MCP-1; CCL2), regulated on activation, normal T-cell expressed and secreted (RANTES) (CCL5), and eotaxin (CCL11) are representatives of this group. Inflammatory chemokines are produced in an inducible manner by various tissues and infiltrating leukocytes in response to bacterial toxins. This is followed by the activation of the key proinflammatory cytokines, such as interleukin-1 (IL-1), tumor necrosis factor (TNF), and interferons (IFNs). The primary task of inflammatory chemokines is to regulate leukocyte recruitment toward the site of inflammation or infection to provide the host defense. CXC chemokines primarily attract neutrophils and lymphocytes and are believed to orchestrate the early phases of wound healing.⁷ CXC chemokines are further subdivided based on the presence or absence of a glutamic acid–leucine–arginine (ELR) motif immediately before the first cysteine residue. In most cases, the ELR+ chemokines promote angiogenesis and the ELR− chemokines are angiostatic. During normal wound healing, neutrophil infiltration induced by CXC chemokines is followed by mononuclear cell recruitment in response to CC chemokines. Thus, the coordinated expression of chemokines orchestrates a well-ordered pattern of inflammatory cell infiltration and aids in effective wound healing.^10,11

It has been documented extensively in the literature that ligands for the chemokine receptors CCR1, CCR2, CCR5, and CXCR3 are ubiquitous in chronic inflammation, whereas in acute inflammation, CXCR1 and CXCR2 dominate. Activated neutrophils and T lymphocytes have high expression of CXCR1 and CXCR2, and the Th1 lymphocytes also express CXCR3. Monocytes, eosinophils, and basophils express CCR1 and CCR2, and monocytes also express CCR5.^12,13 For an extensive discussion on the role of chemokines and chemokine receptors in normal wound healing, refer to the review by Martins-Green et al.¹⁴ Roy et al.¹⁵ also provide a detailed description on chemokines and chemokine receptors and explored the possibility of utilizing chemokines as a potential pharmaceutical target for various malignancies. The role of chemokines, in both acute and chronic wound healing, is growing, and several chemokines and their receptors are reported as significant players, as discussed below in detail.

Chronic nonhealing wounds

The body is naturally programmed for self-repair, and most wounds proceed toward healing in a timely manner. In chronic metabolic disease, diabetes in particular, the healing process is defective, leading to what is termed a “chronic wound.”⁵ Over 29.1 million people or 9.3% of the US population suffer from diabetes, costing 176 billion dollars in direct medical costs (American Diabetes Association, 2014). Diabetic complications plague macro- and microvascular structures causing stroke, atherosclerosis, impaired wound healing, retinopathy, neuropathy, and nephropathy.¹⁶ Age-related decline in repair and regeneration potential in the skin might be a cause of nonhealing in wounds, such as chronic venous leg ulcers, PUs, and diabetic foot ulcers. Indeed, a steady increase in their incidence in individuals older than 65 years has been reported¹⁷ with a longer hospital stay along with diminished quality of life.

Although there is no single unifying theory as to why chronic wounds fail to heal, chronic inflammation^18,19 and bacterial infection^20,21 may be the two major factors contributing to the persistence of the wound chronicity. In addition, three cell populations that are key players in wound healing, namely fibroblasts, keratinocytes, and endothelial cells, are also found to be functionally compromised in chronic wounds. Decreased proliferative efficiency has been described for fibroblasts isolated from the chronic wound site compared with normal nonwound skin fibroblasts.^22–24 Several studies also point toward decreased migration ability of fibroblasts derived from chronic wounds in comparison to healthy fibroblasts. It has been shown that increased alpha-smooth muscle actin expression tends to slow the process of migration.^25–27 Keratinocytes at the nonhealing edge of the chronic wounds have a very different phenotype and biological function when compared with a healing acute wound. Various signaling molecules and activation pathways have been identified for nonmigratory hyperproliferative epidermis of chronic wounds that fail to reepithelialize and restore the epidermal barrier for successful healing (see review by Pastar et al.).²⁸ It has been well established in chronic wounds associated with diabetes that the process of angiogenesis is impaired.²

Growth factors secreted by all these cell populations contribute extensively toward the final outcome of wound healing. Growth factors have been extensively studied due to their influence in various phases of healing. In vitro, growth factors, such as epidermal growth factor (EGF), vascular endothelial growth factor (VEGF), insulin growth factor, nerve-derived growth factor, and platelet-derived growth factor (PDGF), have been shown to promote cell migration, proliferation, and synthesis of extracellular matrix (ECM) proteins.²⁹ Cytokines are important in regulating the intensity and duration of the inflammatory response. In general, the balance between pro- and anti-inflammatory cytokines is lost in chronic wounds, which leads to an extended inflammatory response leading to nonhealing wounds. In particular, the levels of interleukin-6 (IL-6), interleukin-8 (IL-8), interleukin-1 (IL-1), and TNF-α are high, which might be the leading cause for persistent inflammation seen in chronic wounds.³⁰ Interleukin-10 (IL-10), a potential anti-inflammatory protein has been researched extensively and has been shown that elevated levels of IL-10 in wounds recapitulated fetal-like scarless wound healing in postnatal tissues (refer to a review article by King et al.).³¹ The significance of increased expression of IL-10 seen in chronic venous insufficiency ulcers needs to be studied.³² In general, more studies are needed to explore the functional importance of IL-10 in chronic wounds. PDGF is the only recombinant growth factor approved by the U.S. Food and Drug Administration to promote wound closure through topical application. Recent conflicting reports detailing the efficacy of PDGF for the treatment of chronic wounds have been reported from within the wound healing community (refer to Park et al. for more references on this topic).³³ A recent study by Barrientos et al.³⁴ discusses the literature-based evidence on effective growth factors and cytokines in the management of nonhealing chronic wounds. The author concludes that PDGF, VEGF, basic fibroblast growth factor, and cytokine granulocyte–macrophage colony-stimulating factor show great promises in enhancing the healing of chronic wounds in small randomized control trials, and long-term controlled trials are required to deliver long-term outcomes.

There is still a pressing need to develop new therapeutic regimens to heal chronic wounds for which chemokines seem to be the best option as their small size makes them ideal candidates for treating the wounds exogenously. By targeting chemokines through genetic and molecular biology tools, one might alter signaling pathways that directly affect cellular migration/proliferation, angiogenesis, and epithelialization; this may serve as a useful approach in healing chronic wounds. Characteristic differences between acute and chronic wounds are summarized in Table 1.

Table 1.

Characteristic differences observed between acute and chronic wounds

Stages in Wound Healing	Acute Skin Wounds	Chronic Skin Wounds
Inflammation2	Robust	Prolonged
Neutrophils2	Recruited immediately and disappear by apoptosis after macrophage recruitment	Neutrophils stay around for a longer period of time than required
Macrophages2	Recruited once neutrophils disappear	Recruitment delayed
Growth factor/cytokine expression
EGF57	Increased	Low to none
PDGF2	Increased	Decreased
TGF-β158	Increased	Increased
VEGF2	Increased	Decreased
bFGF2	Increased	Decreased
GM-CSF34	Increased	Decreased
IFN-γ58	Expressed	Increased
MCP-137	Expressed	Increased
IL-159	Expressed	400-fold increase
IL-660	Increased	Increased
IL-837	Increased	20-fold increase
IL-1032	Expressed	5-fold increase
Formation of granulation tissue2	Yes	No
Fibroblast morphology25	Compact and spindle-shaped	Larger and polygonal
Angiogenesis61	Good	Poor
Re-epithelialization62	Yes	No
MMP activity63	Low	Very high

bFGF, basic fibroblast growth factor; EGF, epidermal growth factor; GM-CSF, granulocyte–macrophage colony-stimulating factor; IFN, interferon; IL, interleukin; MCP-1, monocyte chemoattractant protein-1; MMP, matrix metalloproteinase; PDGF, platelet-derived growth factor; VEGF, vascular endothelial growth factor.

Discussion of Findings and Literature

Chemokines identified in normal wound healing as targets to treat chronic wounds

Inflammatory phase

Following hemostasis, inflammation, the first stage of wound healing, is initiated by platelet granule release. It begins within minutes to hours of wounding and typically lasts for 4 days. The inflammatory process sets in with the infiltration of polymorphonuclear neutrophils followed by the invasion of monocytes and macrophages into the wound that are key players during this phase.

Inflammatory cells

Chemokines produced at the injured site promote the directional migration of leukocytes (neutrophils and monocytes/macrophages). In wounds that have demonstrated delayed or decreased repair capacity as seen in chronic wounds, the main cause is attributed to impairment of inflammatory cell infiltration and activation in the early stages of wound healing. Multiple chemoattractants regulate neutrophil trafficking that includes CXCL1, 4, 5, 6, 7, and 8. In humans, CXCL8 is a key potent chemoattractant for neutrophils, although both the CXCR1 and CXCR2 receptors and newly arrived neutrophils also secrete CXCL8, increasing the neutrophil numbers.³⁵ A recent study by Hasegawa et al.³⁶ showed that by topically applying recombinant human dermokine-β (shown to be abundant in stratified epithelia and differentiating keratinocytes) to excisional wounds in mice reduced the expression of CXCL1 and CXCL5; both of which are chemoattractants for neutrophils into wounds. This resulted in reduced influx of neutrophils and macrophages, inhibition of angiogenesis, decrease in the number of myofibroblasts into the wounds, and eventually leading to delay in early skin wound healing. Patreaca et al.³⁷ showed that deletion of TNF superfamily member 14 gene in mice leads to impaired wounds (mirrors human chronic wound); these mice showed excessive production of chemokines CXCL8, CXCL10, and CCL2 very early after wounding, which may have led to abnormal initiation and resolution of inflammation. In contrast, a diabetic rabbit ear wound model showed an increase in baseline gene expression of IL-6, IL-8, CXCR1, and CXCR2 postinjury and the expression was significantly less in diabetic wounds.³⁸ The differences observed between the two studies could be argued due to the use of different animal models and need a much better understanding of CXCL8 and its receptors CXCR1 and CXCR2 in chronic wounds.

A previous study by Devalaraja et al.³⁹ has shown that CXCR2 knockout mice showed delayed healing as the wounds exhibited defective neutrophil recruitment, altered pattern of monocyte recruitment, delay in epithelialization, and decreased neovascularization. Significant lower expression of CXCR2 is also reported postinjury in diabetic wounds³⁸ highlighting the importance of specific chemokines required precisely during the early stages of healing for recruiting appropriate inflammatory cells. Neutrophils disappear from the wound site through apoptosis, and it has been shown that apoptotic neutrophils increase their CCR5 receptor expression on the surface, which binds and sequesters chemokines and also eventually leads to neutrophil apoptosis.

Once the neutrophils disappear, monocytes arrive at the wound site through chemokines, such as CCL2, and differentiate into macrophages that are considered to play a central role in wound repair by producing a battery of growth factors and inflammatory cytokines.⁴⁰ Macrophages facilitate the phagocytosis of apoptotic neutrophils and other dead cells at the wound site to clear the debris. At this stage, a large number of monocyte/macrophage-attractant chemokines, namely RANTES (CCL5), macrophage inflammatory protein (MIP-1; CCL4), and monocyte chemoattractant protein (MCP-1 [ CCL2] and MCP-3 [CCL7]), are exclusively found to be expressed during the first week after wounding.¹⁰ At this stage, keratinocytes also contribute to the inflammatory network by expressing MCP-1 in the basal keratinocytes at the wound edge. It should also be noted that the MCP-1 not only attracts monocytes but also at later time points attracts mast cells.⁴¹ In early stages of diabetic chronic wounds, it has been noted that there is a damaging delay in essential macrophage response, and it was primarily due to insufficient chemokine expression. A one-time treatment with CCL2 in wounds of diabetic mice significantly stimulated healing in diabetic wounds by restoring the macrophage response.⁴² Comparing fibroblasts derived from properly healing and chronic human wounds showed increased expression of MCP-1 in chronic wounds implying that constitutive release of MCP-1 becomes disadvantageous for the healing process.²⁶ Once macrophage inflammation resolves, lymphocytes appear as the last cell type to arrive at the wound site. Chemokines that are spatially associated with lymphocyte accumulation are MCP-1, IFN-γ–inducible-protein-10 (IP-10; CXCL10), monokine-induced by IFN-γ (Mig-9; CXCL9), and macrophage-derived chemokine (MDC; CCL22). Lymphocyte inflammation is resolved by apoptosis when there is a major shift in the cytokine profile from TNF-α/IL-1 to IFN-γ.

Proliferative phase

This phase is marked by the formation of granulation tissue, a well-vascularized connective tissue containing macrophages and fibroblasts that replace the fibrin clot, a characteristic feature of the proliferative phase of normal wound healing. The presence of granulation tissue in an open wound also allows the process of reepithelialization to begin, as epithelial cells migrate across the new tissue to form a barrier between the wound and the environment.

Angiogenesis and reepithelialization

In normal wounds, endothelial cells promote healing by mediating and regulating the recruitment of inflammatory cells into sites of injury and by the formation of new vessels from the preexisting neovasculature, a process named as angiogenesis. Chemokines play a crucial role in the processes of angiogenesis and reepithelialization.⁴³ Endothelial cells comprise a broad repertoire of chemokines, which may be expressed after appropriate stimulation. These include CC chemokines, such as MCP-1 and RANTES as well as CXC family members like IL-8, GRO-alpha, IP-10, Mig, and others. IL-8 may furthermore be presented on the surface of endothelial cells to allow interaction with leukocytes to promote recruitment from the intraluminal compartment. MCP-1 contributes to the establishment of a chemokine gradient, which allows subset-specific recruitment of leukocytes to sites of inflammation. Conflicting results exist on endothelial cell expression of CXCR2, as some groups identified its expression in cultured endothelial cells while others could not.⁴⁴ However, CXCR-2 (ligand CXCL2 also referred as MIP2-alpha; macrophage inhibitory protein-2) knockout mice showed a significant delay in neovascularization, and in the reepithelialization process, this was postulated that this would be a result of the diminished angiogenic response toward MIP-1, the functional homologues of IL-8 in the murine system. Examining MIP-1 alpha (−/−) and MCP-1 (−/−) mice showed that only MCP-1 (−/−) displayed a significant delay in angiogenesis, reepithelialization, and collagen synthesis but not the MIP-1 alpha (−/−) mice.⁴⁵ Interestingly, in MCP-1 (−/−) mice, there was no change in the recruitment of wound macrophages implying that recruitment of monocyte is independent of this chemokine. Low-intensity vibration has shown to improve the delayed wound healing in diabetic mice by increasing the expression of prohealing growth factors and chemokines, namely MCP-1.⁴⁶

On the other hand, CXCR3 is shown to be expressed on vessels at days 7–21 postwounding, and treatment of endothelial cords with CXCR3 ligand IP-10 during the resolving phase of wounds caused dissociation of blood vessels even in the presence of angiogenic growth factors.⁴⁷ Along the same lines, mice lacking CXCR3 receptor showed delayed reepithelialization and basement membrane regeneration in excisional wounds.⁴⁸ We are still yet to understand the functions of CXCR3 and its ligands in the biology of chronic wounds. Please refer to the article titled “Chemokine Regulation of Angiogenesis During Wound Healing” by Dr. Richard Bodnar in this issue for a detailed review on the role played by chemokines in angiogenesis during wound healing.

Stromal cell-derived factor-1 (SDF-1/CXCL12), a potent chemokine that express CXCR4 receptor for SDF-1, is considered to play an important role in postinjury leukocyte chemotaxis, migration, and homing of stem cells. A recent study by Su et al.⁴⁹ showed that SDF-1/CXCR4 signaling pathway facilitates wound healing through augmenting bone marrow-derived stromal cells recruitment to wound tissues and promoted neovascularization. Recent evidence shows that increasing the expression of CXCL12 in wounds of diabetic mice increased stem cell recruitment to the wound and improved wound healing outcome.⁵⁰ SDF-1 alpha has also been shown to have a decreased expression in diabetic skin wounds. An interesting study by Castilla et al.⁵¹ showed that priming bone marrow-derived stem cells with SDF-1 alpha and injecting subcutaneously into skin wounds of diabetic mice promoted wound healing by improving neovascularization and endothelial progenitor cell recruitment. Similarly, inhibition of SDF-1 alpha in diabetic skin wounds decreased the rate of healing.⁵² In contrast, a study by Nishimura et al.⁵³ showed that one-time application of CXCR4 antagonist AMD3100 to the skin wounds of diabetic mice promote wound healing by increasing cytokine production, mobilizing bone marrow EPCs, and by enhancing angiogenesis and vasculogenesis. These results confound each other on the function of CXCR4/SDF-1 axes in wound healing but clearly implies that enhancement of CXCL12 (SDF-1) or CCL2 (MCP-1) expression would be a novel therapeutic strategy to promote healing in chronic wounds.

Remodeling phase

The remodeling phase can last for few months to a year depending on the size of the wound, wherein there is a continuous synthesis and degradation of collagen and other ECM proteins resulting in a mature scar.

Extracellular matrix

The matrix metalloproteinases (MMPs) play an important role in remodeling the ECM. MMPs are involved in tissue repair and remodeling process, such as inflammation, angiogenesis, and re-epithelialization. MMPs, such as MMP-1, −2, and-3, are primarily responsible for degradation and turnover of ECM. In addition, MMPs regulate inflammatory cell function by cleaving MCPs and SDF-1 to reduce cell function. MMP-2 (gelatinase A), MMP-3 (stromelysin-1), and MMP-9 (gelatinase B) cleave MCP to generate CCR-1, 2, and 3 antagonist inhibiting leukocyte function and cleaves SDF-1.⁵⁴ TGF-β1 promotes matrix deposition and stabilization by repressing MMPs and stimulating tissue inhibitor of metalloproteinases expression to orchestrate the final remodeling of the ECM.⁵⁵ In addition, expression of membrane-type matrix metalloproteinases (MT-MMP) has been found to be important for cell migration in collagen and the remodeling phase of wound healing. MT-MMP activates MMP-2, which has been found to have anti-inflammatory functions and promote collagen reorganization. Nonhealing chronic wounds, in particular diabetic wounds, exhibit impairments in ECM production and increased degradation owing to elevated concentrations of proteolytic MMPs. The decreased tensile strength and the nonability to withstand trauma in chronic wounds are mainly because there is a decreased amount of collagen and increased MMP production. Chronic wound fluid has shown to impair cell proliferation and angiogenesis and also showed increased levels of MMPs.⁵⁶ Chemokines are responsible for both production and degradation of ECM. Many of the chemokines that were discussed above have either a direct chemokine-induced MMP production by resident cells (fibroblasts, keratinocytes, and endothelial) within the wound or through sustained inflammatory cell infiltrate. Increasing or decreasing the expression of appropriate chemokines would benefit the wounds to lay down an appropriate matrix for a favorable healing outcome. More extensive research is required to understand the specific functions of chemokines in ECM production in both acute and chronic wounds. Table 2 summarizes the select chemokines function in acute and chronic wound healing. Table 3 summarizes the current preclinical and clinical studies showing the changes in the expression levels of certain chemokines through various treatment modalities.

Table 2.

Select chemokines function during wound healing

		Functions
Ligand	Receptors	Normal (Acute) Skin Wound Healing	Chronic Skin Wound Healing
CXCL2/MIP2-alpha39	CXCR2	Chemoattractants for polymorphonuclear leukocytes and hematopoietic stem cells	Not studied
CXCL8/IL-814	CXCR1/CXCR2	Chemoattractants for neutrophils	Excessive production leads to abnormal initiation and resolution of inflammation.37
			Significantly decreased expression is also noted in rabbit diabetic ear wound model.38
CXCL9/IP-948	CXCR3	Resolution of inflammatory phase	Not studied
CXCL10/IP-1047	CXCR3	Resolution of inflammatory phase	Not studied
CXCL12/SDF149	CXCR4	Migration and homing of stem cells	Decreased expression in diabetic skin wounds.50
CCL2/MCP-110	CCR2	Chemoattractants for monocytes/macrophages	Increased expression promotes wound healing in diabetic wounds by restoring macrophage response.42
CCL7/MCP-310	CCR1/CCR2	Chemoattractants for monocytes/macrophages	Not studied
CCL4/MIP-110	CCR5	Chemoattractants for monocytes/macrophages	Not studied
CCL5/RANTES10	CCR5/CCR3/CCR1	Chemoattractants for monocytes/macrophages	Not studied

MIP-1, macrophage inflammatory protein-1; RANTES, regulated on activation, normal T-cell expressed and secreted; SDF-1, stromal cell-derived factor-1.

Table 3.

Synopsis of current preclinical and clinical studies showing promise in healing chronic wounds through changes in the expression of chemokines

Study Title	Study Design	Chemokines Implicated	Outcome
Weinheimer-Haus et al.46	Application of low-intensity vibration (diabetic mouse skin wound)	Increased expression levels of MCP-1	Enhanced healing in diabetic wounds
Stenstresser et al.64	Skin electroporation of a plasmid encoding hCAP-18/LL-37 host defense peptide (diabetic mouse skin wound)	Increased expression levels of SDF-1 alpha and CXCR4 receptor	Promoted healing in diabetic wounds
Castilla et al.51	Ex vivo priming of BMDSCs with SDF-1 alpha (diabetic mouse skin wound)	Increased expression levels of SDF-1 alpha in wounds	Promoted healing in diabetic wounds
Lima et al.65	Topical application of insulin (preclinical model; diabetic mouse skin wound and clinical trial; diabetic ulcer patients)	Increased expression levels of SDF-1 alpha	Improved healing in diabetic wounds
Schǖrmann et al.66	Oral administration of linagliptin (diabetic mouse skin wound)	Decreased expression levels of MIP-2	Beneficial in healing of diabetic wounds
Nishimura et al.53	Application of CXCR4 antagonist AMD3100 (diabetic mouse skin wound)	Increased expression levels of SDF-1 alpha	Promoted healing in diabetic wounds
Bermudez et al.52	Inhibition of SDF-1 alpha levels in a diabetic mouse skin wound	Inhibition of SDF-1 alpha levels	Decreased the rate of healing in diabetic wounds
Nguyen et al.67	Topical silencing of p53 in diabetic mouse skin wound	Increased expression levels of SDF-1 alpha	Improved healing in diabetic wounds
Liu et al.68	Topical injection of SDF-1 alpha engineered bone marrow-derived fibroblasts in diabetic mouse skin wound	Increased expression levels of SDF-1 alpha	Improved healing in diabetic wounds
Restivo et al.50	Topical application of CXCL12 expression plasmid in diabetic mouse skin wound	Increased expression levels of CXCL12	Improved healing in diabetic wounds
de Leon et al.69	Treating chronic wounds with platelet-rich plasma (PRP) gel (clinical trial)	Increase in the levels of chemokines reported	PRP can restart the healing process in chronic wounds
Badillo et al.70	Treatment with lentiviral construct containing SDF-1 alpha gene (diabetic mouse skin wound)	Overproduction of SDF-1 alpha levels	Improved healing in diabetic wounds

Summary

The potential of the chemokine system as drug targets in chronic wounds is achievable but requires more in-depth understanding of the chemokine interactions and its receptors in both acute and chronic wound settings. To date, only a few of the ∼50 human chemokines' functions have been explored. This may be a possible reason for the present stalemate and inability to achieve a successful outcome for healing chronic wounds. More importantly, chemokines can be considered as adjuvants to stimulate a nonhealing wound environment provided that the timely and spatially different expression pattern of chemokines is captured as detected in a physiological wound milieu.

Abbreviations and Acronyms

bFGF

basic fibroblast growth factor

ECM

extracellular matrix

EGF

epidermal growth factor

GM-CSF

granulocyte–macrophage colony-stimulating factor

IFN

interferon

IL

interleukin

MCP-1

monocyte chemoattractant protein-1

MIP-1

macrophage inflammatory protein-1

MMPs

matrix metalloproteinases

MT-MMP

membrane-type matrix metalloproteinases

PDGF

platelet-derived growth factor

RANTES

regulated on activation, normal T-cell expressed and secreted

SDF-1

stromal cell-derived factor-1

TNF

tumor necrosis factor

VEGF

vascular endothelial growth factor

Acknowledgments and Funding Sources

The author expresses her thanks to Dr. Richard Bodnar, Department of Pathology, VA Pittsburgh Health Care System, Pittsburgh, for his time and help in critically reviewing the article. She also expresses thanks to the Department of Plastic Surgery for providing the resources to complete this review article.

Author Disclosure and Ghost Writing

The author has no disclosed commercial associations that could lead to a conflict of interest. The author is solely responsible for writing the article and did not use ghost writers.

About the Author

Latha Satish, MSc, PhD, joined the Department of Pathology as a postdoctoral research fellow at the University of Pittsburgh after completion of the PhD program at the University of Madras, Chennai, India. She then joined the Allegheny Research Institute, Pittsburgh, as Research Associate and then was promoted as Research Assistant Professor in the same institute. Dr. Satish joined the Department of Plastic Surgery as Research Assistant Professor at the University of Pittsburgh in 2011. Her long-term research interest has been to uncover the genes and signaling pathways responsible for skin fibrosis and scarless wound healing along with studying chronic wounds and biofilms. Her interest is also to uncover the biological mechanisms leading to Dupuytren's contracture (palmar fascia fibrosis).

Take Home Messages.

• • The best approach to modulate inflammatory responses in a nonhealing wound can be achieved through targeting chemokines.

• • Chemokines have the ability to convert chronic inflammation to acute inflammation.

• • Various chemokines are expressed throughout the process of wound healing, and one-time application of a candidate chemokine can stimulate the nonhealing environment by producing factors necessary for healing.

• • A much better understanding of chemokine function in the chronic wound biology is required to identify the candidate drug.

• • Chemokines that currently show promising evidence for future clinical trials in treating chronic wounds are CXCL12 and CCL2.