Mast cells contribute to scar formation during fetal wound healing

Scar formation is a potentially detrimental process of tissue restoration in adults, affecting organ form and function. During fetal development, cutaneous wounds heal without inflammation or scarring at early stages of development, but begin to heal with significant inflammation and scarring as the skin becomes more mature. One possible cell type that could regulate the change from scarless to fibrotic healing is the mast cell. We show here that dermal mast cells in scarless wounds generated at embryonic day 15 (E15) are fewer in number, less mature and do not degranulate in response to wounding as effectively as mast cells of fibrotic wounds made at embryonic day 18 (E18). Differences were also observed between cultured mast cells from E15 and E18 skin with regard to degranulation and preformed cytokine levels. Injection of mast cell lysates into E15 wounds disrupted scarless healing, suggesting that mast cells interfere with scarless repair. Finally, wounds produced at E18, which normally heal with a scar, healed with significantly smaller scars in mast cell-deficient Kit^W/W-v mice compared to Kit^+/+ littermates. Together, these data suggest that mast cells enhance scar formation, and that these cells may mediate the transition from scarless to fibrotic healing during fetal development.

Epithelial regulation of mesenchymal tissue behavior

Fibroproliferative scars are an important clinical problem, and yet the mechanisms that regulate scar formation remain poorly understood. This study explored the hypothesis that the epithelium has a critical role in dictating scar formation, and that these interactions differ in skin and mucosa. Paired skin and vaginal mucosal wounds on New Zealand white (NZW) rabbits diverged significantly; the cutaneous epithelium exhibited a greater and prolonged response to injury when compared with the mucosa. Microarray analysis of the injured epithelium was performed, and numerous factors were identified that were more strongly upregulated in skin, including several proinflammatory cytokines and profibrotic growth factors. Analysis of the underlying mesenchymal tissue demonstrated a fibrotic response in the dermis of the skin but not the mucosal lamina propria, in the absence of a connective tissue injury. To determine if the proinflammatory factors produced by the epidermis may have a role in dermal fibrosis, an IL-1 receptor antagonist was administered locally to healing skin wounds. In the NZW rabbit model, blockade of IL-1 signaling was effective in preventing hypertrophic scar formation. These results support the idea that soluble factors produced by the epithelium in response to injury may influence fibroblast behavior and regulate scar formation in vivo.

Sprouty2 downregulates angiogenesis during mouse skin wound healing

Angiogenesis is regulated by signals received by receptor tyrosine kinases such as vascular endothelial growth factor receptors. Mammalian Sprouty (Spry) proteins are known to function by specifically antagonizing the activation of the mitogen-activated protein kinase signaling pathway by receptor tyrosine kinases, a pathway known to promote angiogenesis. To examine the role of Spry2 in the regulation of angiogenesis during wound repair, we used a model of murine dermal wound healing. Full-thickness excisional wounds (3 mm) were made on the dorsum of anesthetized adult female FVB mice. Samples were harvested at multiple time points postwounding and analyzed using real-time RT-PCR, Western blot analysis, and immunofluorescent histochemistry. Spry2 mRNA and protein levels in the wound bed increased significantly during the resolving phases of healing, coincident with the onset of vascular regression in this wound model. In another experiment, intracellular levels of Spry2 or its dominant-negative mutant (Y55F) were elevated by a topical application to the wounds of controlled-release gel containing cell permeable, transactivator of transcription-tagged Spry2, Spry2Y55F, or green fluorescent protein (as control). Wound samples were analyzed for vascularity using CD31 immunofluorescent histochemistry as well as for total and phospho-Erk1/2 protein content. The treatment of wounds with Spry2 resulted in a significant decrease in vascularity and a reduced abundance of phospho-Erk1/2 compared with wounds treated with the green fluorescent protein control. In contrast, the wounds treated with the dominant-negative Spry2Y55F exhibited a moderate increase in vascularity and elevated phospho-Erk1/2 content. These results indicate that endogenous Spry2 functions to downregulate angiogenesis in the healing murine skin wound, potentially by inhibiting the mitogen-activated protein kinase signaling pathway.

TLR2 expression and signaling-dependent inflammation impair wound healing in diabetic mice

Toll-like receptor-2 (TLR2) is a pivotal pathogen recognition receptor that has a key role in inflammation, diabetes, and injury. Hyperglycemia, inflammation, and oxidative stress induce TLR2-myeloid differentiation factor-88 (MyD88) expression and signaling, and are major pathophysiological mechanisms in the impaired diabetic wound-healing process. The aim of the study was to examine the contribution of TLR2-MyD88 expression and signaling to the prolonged inflammation observed in diabetic wounds. Diabetes was induced in male C57BL/6J and TLR2(-/-) mice using streptozotocin (STZ) with matching nondiabetic mice as control. In addition, nonobese diabetic (NOD) mice were used to represent the spontaneous type 1 diabetes condition. After 2 weeks of persistent hyperglycemia in the mice, full-thickness excision wounds were made on the backs aseptically. Total RNA and protein were subjected to real-time PCR and western blot analyses. Wound sizes were measured using digital planimetry. TLR2 mRNA and protein expression increased significantly in wounds of C57BL/6J+STZ and NOD mice (P<0.05) compared with nondiabetic C57BL/6J mice. MyD88 expression, interleukin receptor-associated kinase-1 phosphorylation, and nuclear factor-κ B (NF-κB) activation were increased in diabetic wounds compared with nondiabetic wounds. Wounds of TLR2(-/-)+STZ mice showed less oxidative stress, decreased MyD88 signaling, NF-κB activation, and cytokine secretion. The wound closure was significant in TLR2(-/-)+ STZ mice compared with C57BL/6J+STZ mice. Collectively, our findings show that increased TLR2 mRNA and protein expression, signaling, and activation contribute to the prolonged inflammation in the diabetic wounds and that absence of TLR2 may result in decreased inflammation and improved wound healing.

Fibroblast function and wound breaking strength is impaired by acute ethanol intoxication

BACKGROUND

Alcohol intoxication occurs in nearly half of all trauma patients and increases the morbidity, mortality, and healing complications of these patients. Prior studies in our laboratory and elsewhere have demonstrated impairments in re-epithelialization, angiogenesis, and inflammation in wounds following acute ethanol exposure. Clinically, acute ethanol exposure has been shown to cause an increased breakdown of wounds. To date, the mechanisms by which acute ethanol exposure modifies wound strength have received little experimental attention.

METHODS

To examine how ethanol influences functions critical to the development of wound strength, the effect of ethanol exposure on fibroblast proliferation and extracellular matrix production was examined. Normal human dermal fibroblasts (NHDF) were exposed to ethanol (100 mg/dl) and then examined for proliferative capacity and mRNA production of collagen I, collagen III, and lysyl oxidase (LOX). In in vivo studies, the wound breaking strength, LOX activity, collagen, and hyaluronic acid (HA) contents of wounds of ethanol-exposed (100 mg/dl) mice were examined.

RESULTS

At 24, 48, and 72 hours after acute ethanol exposure (8 hours duration), NHDF displayed a significant impairment in proliferative capacity (up to 50% at 24 hours p < 0.001). After ethanol exposure, NHDF produced less collagen I and LOX mRNA, but more collagen III mRNA than control fibroblasts (p < 0.05). Ethanol exposure in vivo caused a reduction in wound breaking strength of up to 40% when compared to control mice (p < 0.01). LOX activity, collagen, and HA contents in the wounds of ethanol-exposed mice were significantly reduced (p < 0.01).

CONCLUSIONS

These studies reveal that a single exposure to ethanol prior to injury can cause a significant decrease in wound breaking strength. Our studies suggest that ethanol directly impairs fibroblast function, leading to decreased collagen production. The results provide a possible explanation for how acute ethanol exposure might increase in wound complications and wound failure.

Importance of housekeeping gene selection for accurate reverse transcription-quantitative polymerase chain reaction in a wound healing model

Studies in the field of wound healing have utilized a variety of different housekeeping genes for reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analysis. However, nearly all of these studies assume that the selected normalization gene is stably expressed throughout the course of the repair process. The purpose of our current investigation was to identify the most stable housekeeping genes for studying gene expression in mouse wound healing using RT-qPCR. To identify which housekeeping genes are optimal for studying gene expression in wound healing, we examined all articles published in Wound Repair and Regeneration that cited RT-qPCR during the period of January/February 2008 until July/August 2009. We determined that ACTβ, GAPDH, 18S, and β2M were the most frequently used housekeeping genes in human, mouse, and pig studies. We also investigated nine commonly used housekeeping genes that are not generally used in wound healing models: GUS, TBP, RPLP2, ATP5B, SDHA, UBC, CANX, CYC1, and YWHAZ. We observed that wounded and unwounded tissues have contrasting housekeeping gene expression stability. The results demonstrate that commonly used housekeeping genes must be validated as accurate normalizing genes for each individual experimental condition.

Positional differences in the wound transcriptome of skin and oral mucosa

Background

When compared to skin, oral mucosal wounds heal rapidly and with reduced scar formation. Recent studies suggest that intrinsic differences in inflammation, growth factor production, levels of stem cells, and cellular proliferation capacity may underlie the exceptional healing that occurs in oral mucosa. The current study was designed to compare the transcriptomes of oral mucosal and skin wounds in order to identify critical differences in the healing response at these two sites using an unbiased approach.

Results

Using microarray analysis, we explored the differences in gene expression in skin and oral mucosal wound healing in a murine model of paired equivalent sized wounds. Samples were examined from days 0 to 10 and spanned all stages of the wound healing process. Using unwounded matched tissue as a control, filtering identified 1,479 probe sets in skin wounds yet only 502 probe sets in mucosal wounds that were significantly differentially expressed over time. Clusters of genes that showed similar patterns of expression were also identified in each wound type. Analysis of functionally related gene expression demonstrated dramatically different reactions to injury between skin and mucosal wounds. To explore whether site-specific differences might be derived from intrinsic differences in cellular responses at each site, we compared the response of isolated epithelial cells from skin and oral mucosa to a defined in vitro stimulus. When cytokine levels were measured, epithelial cells from skin produced significantly higher amounts of proinflammatory cytokines than cells from oral mucosa.

Conclusions

The results provide the first detailed molecular profile of the site-specific differences in the genetic response to injury in mucosa and skin, and suggest the divergent reactions to injury may derive from intrinsic differences in the cellular responses at each site.

Selective and specific macrophage ablation is detrimental to wound healing in mice

Macrophages are thought to play important roles during wound healing, but definition of these roles has been hampered by our technical inability to specifically eliminate macrophages during wound repair. The purpose of this study was to test the hypothesis that specific depletion of macrophages after excisional skin wounding would detrimentally affect healing by reducing the production of growth factors important in the repair process. We used transgenic mice that express the human diphtheria toxin (DT) receptor under the control of the CD11b promoter (DTR mice) to specifically ablate macrophages during wound healing. Mice without the transgene are relatively insensitive to DT, and administration of DT to wild-type mice does not alter macrophage or other inflammatory cell accumulation after injury and does not influence wound healing. In contrast, treatment of DTR mice with DT prevented macrophage accumulation in healing wounds but did not affect the accumulation of neutrophils or monocytes. Such macrophage depletion resulted in delayed re-epithelialization, reduced collagen deposition, impaired angiogenesis, and decreased cell proliferation in the healing wounds. These adverse changes were associated with increased levels of tumor necrosis factor-alpha and reduced levels of transforming growth factor-beta1 and vascular endothelial growth factor in the wound. In summary, macrophages seem to promote both wound closure and dermal healing, in part by regulating the cytokine environment of the healing wound.

Brewing complications: the effect of acute ethanol exposure on wound healing

Ethanol consumption is linked to a higher incidence of traumatic wounds and increases the risk for morbidity and mortality following surgical or traumatic injury. One of the most profound effects of acute ethanol exposure on wound healing occurs during the inflammatory response, and altered cytokine production is a primary component. Acute ethanol exposure also impairs the proliferative response during healing, causing delays in epithelial coverage, collagen synthesis, and blood vessel regrowth. The accumulated data support the paradigm that acute ethanol intoxication prior to injury significantly diminishes a patient's ability to heal efficiently.

Microgravity and the implications for wound healing

Wound healing is a sophisticated response ubiquitous to various traumatic stimuli leading to an anatomical/functional disruption. The aim of present article was to review the current evidence regarding the effects of microgravity on wound healing dynamics. Modulation of haemostatic phase because of alteration of platelet quantity and function seems probable. Furthermore, production of growth factors that are released from activated platelets and infiltration/function of inflammatory cells seem to be impaired by microgravity. Proliferation of damaged structures is dependent on orchestrated function of various growth factors, for example transforming growth factors, platelet-derived growth factor and epidermal growth factor, all of which are affected by microgravitational status. Moreover, gravity-induced alterations of gap junction, neural inputs, and cell populations have been reported. It may be concluded that different cellular and extracellular element involved in the healing response are modified through effect of microgravity which may lead to impairment in healing dynamics.

Regulation of scar formation by vascular endothelial growth factor

Vascular endothelial growth factor (VEGF-A) is known for its effects on endothelial cells and as a positive mediator of angiogenesis. VEGF is thought to promote repair of cutaneous wounds due to its proangiogenic properties, but its ability to regulate other aspects of wound repair, such as the generation of scar tissue, has not been studied well. We examined the role of VEGF in scar tissue production using models of scarless and fibrotic repair. Scarless fetal wounds had lower levels of VEGF and were less vascular than fibrotic fetal wounds, and the scarless phenotype could be converted to a scar-forming phenotype by adding exogenous VEGF. Similarly, neutralization of VEGF reduced vascularity and decreased scar formation in adult wounds. These results show that VEGF levels have a strong influence on scar tissue formation. Our data suggest that VEGF may not simply function as a mediator of wound angiogenesis, but instead may play a more diverse role in the wound repair process.

Acute ethanol exposure disrupts VEGF receptor cell signaling in endothelial cells

Physiological angiogenesis is regulated by various factors, including signaling through vascular endothelial growth factor (VEGF) receptors. We previously reported that a single dose of ethanol (1.4 g/kg), yielding a blood alcohol concentration of 100 mg/dl, significantly impairs angiogenesis in murine wounds, despite adequate levels of VEGF, suggesting direct effects of ethanol on endothelial cell signaling (40). To examine the mechanism by which ethanol influences angiogenesis in wounds, we employed two different in vitro angiogenesis assays to determine whether acute ethanol exposure (100 mg/dl) would have long-lasting effects on VEGF-induced capillary network formation. Ethanol exposure resulted in reduced VEGF-induced cord formation on collagen and reduced capillary network structure on Matrigel in vitro. In addition, ethanol exposure decreased expression of endothelial VEGF receptor-2, as well as VEGF receptor-2 phosphorylation in vitro. Inhibition of ethanol metabolism by 4-methylpyrazole partially abrogated the effect of ethanol on endothelial cell cord formation. However, mice treated with t-butanol, an alcohol not metabolized by alcohol dehydrogenase, exhibited no change in wound vascularity. These results suggest that products of ethanol metabolism are important factors in the development of ethanol-induced changes in endothelial cell responsiveness to VEGF. In vivo, ethanol exposure caused both decreased angiogenesis and increased hypoxia in wounds. Moreover, in vitro experiments demonstrated a direct effect of ethanol on the response to hypoxia in endothelial cells, as ethanol diminished nuclear hypoxia-inducible factor-1alpha protein levels. Together, the data establish that acute ethanol exposure significantly impairs angiogenesis and suggest that this effect is mediated by changes in endothelial cell responsiveness to both VEGF and hypoxia.

Mechanical unloading impairs keratinocyte migration and angiogenesis during cutaneous wound healing

Although initially thought to improve an individual's ability to heal, mechanical unloading promoted by extended periods of bed rest has emerged as a contributing factor to delayed or aberrant tissue repair. Using a rat hindlimb unloading (HLU) model of hypogravity, we mimicked some aspects of physical inactivity by removing weight-bearing loads from the hindlimbs and producing a systemic cephalic fluid shift. This model simulates bed rest in that the animal undergoes physiological adaptations, resulting in a reduction in exercise capability, increased frequency of orthostatic intolerance, and a reduction in plasma volume. To investigate whether changes associated with prior prolonged bed rest correlate with impaired cutaneous wound healing, we examined wound closure, angiogenesis, and collagen content in day 2 to day 21 wounds from rats exposed to HLU 2 wk before excisional wounding. Wound closure was delayed in day 2 wounds from HLU rats compared with ambulatory controls. Although the levels of proangiogenic growth factors, fibroblast growth factor-2 (FGF-2), and vascular endothelial growth factor (VEGF) were similar between the two groups, wound vascularity was significantly reduced in day 7 wounds from HLU animals. To further examine this disparity, total collagen content was assessed but found to be similar between the two groups. Taken together, these results suggest that keratinocyte and endothelial cell function may be impaired during the wound healing process under periods of prolonged inactivity or bed rest.

Site-specific production of TGF-beta in oral mucosal and cutaneous wounds

Wound healing in the oral mucosa is clinically distinguished by rapid healing and lack of scar formation compared with dermal wounds. Mechanisms of favorable mucosal healing are yet to be elucidated. Utilizing a murine model of equivalent-size mucosal and skin wounds, we verified the rapid reepithelializaton and reduction in scarring of oral wounds reported in humans. Collagen fibrillar structure in oral wounds rapidly approached the size of normal collagen fibrils, while the collagen ultrastructure in skin remained immature through the later phases of healing. To determine whether the transforming growth factor-beta (TGF-beta) contributes to the lack of scar formation in oral mucosa, we compared the expression and production in oral and skin wounds. The RNase protection assay demonstrated significantly lower levels of TGF-beta1 expression in oral wounds compared with dermal wounds, and no changes were observed in the expression levels of TGF-beta2 or TGF-beta 3. ELISA analysis confirmed that oral wounds contained lower levels of TGF-beta1 levels compared with dermal wounds, along with a significant increase in the ratio of TGF-beta 3 to -beta1. These findings showed reduced scarring in oral wounds at the ultrastructural level, and provide evidence that site-specific differences in TGF-beta production contributes to the superior healing of oral wounds.

Exercise accelerates cutaneous wound healing and decreases wound inflammation in aged mice

The purpose of this study was to determine the effect of exercise on wound healing and inflammation in young (3 mo) and old (18 mo) female BALB/cByJ mice. Mice were assigned to either exercise or sedentary control (control) groups. The exercise group mice were run on a motorized treadmill at a moderate intensity 30 min/day for 8 days. All mice were given four full-thickness dermal wounds, and the rate of wound closure was assessed daily for 10 days. Four months later, the aged mice were rerandomized to treatment, wounded again in different locations, and wounds were harvested at 1, 3, or 5 days postwounding. Wound tissue was analyzed for IL-1beta, IL-6, keratinocyte chemoattractant (KC), monocyte chemoattractant protein-1 (MCP-1), and TNF-alpha protein. Myeloperoxidase (MPO) activity and F4/80 mRNA were assessed as an indirect measure of neutrophil and macrophage content, respectively. There was a trend (P = 0.10) for exercise to reduce wound size in young mice, and exercise significantly (P < 0.05) decreased wound size in old mice. TNF-alpha, KC, and MCP-1 were significantly (P < 0.05) lower in wounds from exercised old mice compared with control. No group differences were found for wound IL-1beta or IL-6, MPO activity, or F4/80 mRNA. Our data suggest that exercise accelerates the wound healing process in old mice. This improved healing response in the old mice may be the result of an exercise-induced anti-inflammatory response in the wound.

Norepinephrine suppresses wound macrophage phagocytic efficiency through alpha- and beta-adrenoreceptor dependent pathways

BACKGROUND

The systemic response to injury is characterized by massive release of norepinephrine (NE) into the circulation as a result of global sympathetic activation. We have recently demonstrated that NE modulates the recruitment of macrophages to the cutaneous wound. We hypothesized that NE suppresses wound macrophage phagocytic function through canonical adrenergic signaling pathways.

METHODS

Murine wound macrophages were harvested at 5 days after injury and treated with physiologic and pharmacologic dose norepinephrine. Phagocytosis of green fluorescent protein-labeled Escherichia coli was assayed by flow cytometry. The signaling pathways mediating NE modulation of wound macrophage phagocytosis were interrogated by pharmacologic manipulation of alpha- and beta-adrenoreceptors (ARs), intracellular cyclic adenosine monophosphate (cAMP), and protein kinase A (PKA). Tissue specificity was determined by comparison of wound macrophages to splenic macrophages.

RESULTS

Both physiologic and pharmacologic dose NE suppressed wound macrophage phagocytic efficiency. This effect was mediated by alpha- and beta-ARs in a dose-dependent fashion. Direct stimulation of cAMP-suppressed phagocytic efficiency and blockade of PKA signaling prevented NE-mediated suppression of phagocytic efficiency. Splenic macrophage phagocytic efficiency was less than that of wound macrophages and was not altered by NE.

CONCLUSIONS

NE has a profound immunosuppressive effect on wound macrophage function that is tissue specific and appears to be mediated through adrenergic receptors and their canonical downstream signaling pathway. Attenuation of post-injury immunosuppression represents another potential mechanism by which beta-AR blockade may reduce morbidity and mortality after severe injury.

Matrix proteolytic activity during wound healing: modulation by acute ethanol exposure

BACKGROUND

Clinical studies demonstrate that intoxicated patients exhibit an increased incidence of wound healing complications. Previous studies in a murine excisional wound model revealed that acute ethanol exposure impairs the wound healing response, causing decreased angiogenesis and a significant reduction in wound collagen content.

METHODS

Using the same murine model of excisional wounding, we examined the effect of a single dose of ethanol on the overall collagen content and collagen type I and type III mRNA expression, transforming growth factor-beta (TGF-beta) production, and levels of several components of the extracellular matrix proteolytic cascade.

RESULTS

Wounds from ethanol-treated mice exhibited a significant decrease in collagen and in the production of collagen type I mRNA compared with saline controls. Exposure to ethanol also caused significant increase in wound TGF-beta by day 2 after injury (1.69 +/- 0.29 vs 12.34 +/- 3.97 pg/microg protein, p<0.01). In addition, wounds from mice exposed to ethanol had significantly increased levels of active urokinase plasminogen activator at day 7, (205.10 +/- 48.79 vs 642.70 +/- 159.80 pg/microg protein, p<0.001). The level of matrix metalloproteinase-8, a collagen type I proteinase, was 2.2-fold higher in wounds of ethanol-treated mice compared with control at day 7 (p<0.05).

CONCLUSIONS

These studies demonstrate that a single dose of ethanol decreases collagen production, increases the production of TGF-beta and increases levels of matrix degrading enzymes. This alteration in protease balance may partially explain the impaired wound healing that follows acute alcohol intoxication.

Effects of acute ethanol exposure on the early inflammatory response after excisional injury

BACKGROUND

Alcohol consumption is involved in over half of all trauma-related injuries. These patients are known to exhibit a higher incidence of mortality and morbidity following injury compared with patients not exposed to ethanol. As studies from our laboratory demonstrated that ethanol exposure impairs re-epithelialization and angiogenesis after dermal wounding and because the earlier inflammatory phase of wound healing is likely to influence later responses, we chose to examine neutrophil infiltration and chemokine and proinflammatory cytokine levels in the skin following administration of a dermal excisional wound.

METHODS

BALB/c mice were given ethanol at a dose designed to increase blood alcohol concentration to 100 to 120 mg/dL at 30 minutes after treatment. Mice were then subjected to a full-thickness excisional wound. Wounds from ethanol and saline-treated mice were collected within the first 24 hours postinjury to assess neutrophil infiltration and myeloperoxidase (MPO) activity, neutrophil chemoattractant macrophage inflammatory protein-2 (MIP-2) and KC levels, and proinflammatory cytokine interleukin-1 beta (IL-1beta) and tumor necrosis factor alpha (TNFalpha) levels.

RESULTS

At 12 and 24 hours after injury, MPO in wounds from ethanol-exposed mice was significantly reduced compared with wounds from vehicle-treated animals. Despite this, histological examination of wounds did not reveal a difference in neutrophil infiltration between the 2 groups. Peak levels of MIP-2 and KC observed at 12 hours postinjury were decreased in wounds from ethanol-treated mice by 32 and 45%, respectively, relative to wounds from control mice. Levels of TNFalpha and IL-1beta (potent inducers of MIP-2 and KC, as well as neutrophil activation) were also assessed. Levels of TNFalpha were not elevated in either group after injury. However, IL-1beta demonstrated significantly lower peak levels at 6 hours postinjury in wounds from ethanol-treated mice, 58% less than wounds from controls.

CONCLUSION

These studies reveal that early dermal inflammatory responses including MPO activity, production of MIP-2, KC, and IL-1beta are impaired in mice given ethanol before injury, which may also have detrimental affects on later stages of wound healing.

Lessons learned from psoriatic plaques concerning mechanisms of tissue repair, remodeling, and inflammation

Following injury, skin establishes a balance between too little inflammation increasing risk of infection, and excessive inflammation contributing to delayed wound healing and scarring. Mounting evidence indicates both initiation and termination of inflammation involve active mechanisms. Not only does inflammation itself seem to be a paradox because inflammatory responses are both essential and potentially detrimental, but one chronic inflammatory skin disease (e.g. psoriasis) presents additional paradoxes. While plaques share several factors with wound healing, two understudied and puzzling aspects include why do not inflamed plaques more frequently transform?; and why do not plaques result in scarring? To get at these questions, we review responses involved in wound repair. Oral mucosa was probed because, like fetal skin, wound repair is characterized by its rapidity, low inflammation, and scarless resolution. Active roles for macrophages as both initiators and terminators of inflammation are highlighted. Therapeutic implications are discussed regarding psoriasis and pyoderma gangrenosum. Based on biochemical and immunohistochemical considerations linking psoriatic plaques to hard palate, a novel metaplastic model is presented. We hypothesize saliva and chronic trauma contribute to a constitutive epithelial program where keratinocyte proliferation is more intense prior to differentiation, accompanied by keratin 16 expression in hard palate, thereby resembling plaques. Rather than viewing psoriasis as a nonspecific response to inflammation, we postulate a metaplastic switch by which prepsoriatic skin is converted to a distinct adult tissue type resembling hard palate. In summary, many lessons can be learned by focusing on complex processes involved in regulation of inflammation, tissue repair, and remodeling.

Exogenous pro-angiogenic stimuli cannot prevent physiologic vessel regression

BACKGROUND

In healing wounds, rising levels of vascular endothelial growth factor (VEGF) induce a period of robust angiogenesis. The levels of pro-angiogenic factors in the wound begin to decline just before a period of vascular regression, suggesting that these mediators are necessary to sustain vessel density. The purpose of this study was to determine if the maintenance of pro-angiogenic stimuli in the wound would prevent physiological vessel regression.

MATERIALS AND METHODS

A standard subcutaneous sponge wound model was modified by the addition of a mini-osmotic pump, allowing manipulation of the wound milieu by the addition of exogenous growth factors. After initial characterization of this model, exogenous VEGF (10 microg/mL), FGF (10 microg/mL), PDGF (10 microg/mL), or VEGF (10 microg/mL) plus FGF (10 microg/mL) were delivered to wounds and blood vessel density analyzed by immunohistochemistry.

RESULTS

VEGF administration resulted in a transient increase in wound vessel density (P < 0.05). None of the pro-angiogenic growth factors (VEGF, FGF, PDGF, VEGF/FGF) were able to prevent vascular regression (P = NS).

CONCLUSIONS

These findings suggest that the anti-angiogenic signals that mediate physiological vascular regression in wounds are strongly dominant over pro-angiogenic stimuli during the later phases of wound healing. Clinical manipulation of anti-angiogenic signals in addition to the currently used pro-angiogenic targets may be needed to achieve therapeutic modulation of blood vessel density.

Diminished induction of skin fibrosis in mice with MCP-1 deficiency

Scar and fibrosis are often the end result of mechanical injury and inflammatory diseases. One chemokine that is repeatedly linked to fibrotic responses is monocyte chemoattractant protein-1 (MCP-1). We utilized a murine fibrosis model that produces dermal lesions similar to scleroderma to evaluate collagen fibrillogenesis in the absence of MCP-1. Dermal fibrosis was induced by subcutaneous injection of bleomycin into the dorsal skin of MCP-1-/- and wild-type C57BL/6 mice. After 4 weeks of daily injections, bleomycin treatment led to thickened collagen bundles with robust inflammation in the lesional dermis of wild-type mice. In contrast, the lesional skin of MCP-1-/- mice exhibited a dermal architecture similar to phosphate-buffered saline (PBS)-injected control and normal skin, with few inflammatory cells. Ultrastructural analysis of the lesional dermis from bleomycin-injected wild-type mice revealed markedly abnormal arrangement of collagen fibrils, with normal large diameter collagen fibrils replaced by small collagen fibrils of 41.5 nm. In comparison, the dermis of bleomycin-injected MCP-1-/- mice displayed a uniform pattern of fibril diameters that was similar to normal skin (average diameter 76.7 nm). The findings implicate MCP-1 as a key determinant in the development of skin fibrosis induced by bleomycin, and suggest that MCP-1 may influence collagen fiber formation in vivo.