Trends in Cell Biology
Inflammatory cells during wound repair: the good, the bad and the ugly
Introduction
Wounding of adult tissues generally leads to rapid conversion of blood-borne, discus-shaped platelets into spidery shaped cells that degranulate, releasing a plethora of wound-active signals, and aid in plugging the defect with a fibrin-rich clot or scab. Subsequently, after a delay period of several hours, the epidermal layer begins to repair by migration of keratinocytes from the cut edges of the wound and from amputated stumps of any damaged appendages, such as hairs or sweat glands, where there are new free epidermal edges. From all of these epidermal fronts, sheets of keratinocytes sweep forward at interfaces between the wound dermis and the fibrin clot. Cells within the front few rows extend lamellipodia and alter their expression of integrin adhesion molecules, so that the epidermal sheet can attach down and drag itself forward over the wound substratum 1, 2, 3. The deeper connective tissue is replaced by activated fibroblasts at the wound edge that proliferate and then migrate into the wound bed to form a granulation tissue (so named because of its granular appearance owing to massive angiogenic invasion by a network of capillary blood vessels, which supply the metabolically demanding wound tissues with nutrients and oxygen). Some of the fibroblasts within this granulation tissue transform into specialist contractile myofibroblasts [4], which might contribute to the wound being drawn closed. After repair is complete, the blood vessels regress and the myofibroblasts die, but an inevitable consequence of tissue repair, at least in adults, is fibrosis and scarring within the remodeling connective tissue of the wound site (Figure 1; Table 1). Strikingly, in embryos up until the late organ-forming stages of fetal development, repair of tissue damage is rapid and perfect without any sign of the inevitable scarring of repairing adult tissues.
While there are small numbers of the various leukocyte lineages present in resting tissues, these numbers are massively augmented by recruitment from the circulation in response to inflammatory cues. Platelets are the first cells recruited at sites of injury, as a result of the coagulation process. Platelets aggregate at the ends of damaged blood vessels, convert fibrinogen to fibrin and prevent loss of blood from damaged vessels. Platelets have generally been believed to be important in the wound-healing cascade, both as initiators of coagulation and through the release of growth factors such as platelet-derived growth factor (PDGF) and transforming growth factor β (TGFβ) at the site of injury, thus initiating activation of fibroblasts and other mesenchymal cells. Neutrophils arrive on the scene very early after any tissue damage. Their prime role appears to be to kill microbes. This is usually achieved in phagolysosomes, but often results in neutrophils blitzing their environs with free radicals that kill many otherwise-healthy host cells as well as the target infectious agents. This is particularly apparent in chronic wound situations, and might well underlie the persistent tissue-destroying nature of such wounds. New studies have shown that another tactic for killing by neutrophils involves trapping microbes in extruded nets of histones and DNA [5]. Macrophages, which arrive a little later, operate as voracious phagocytes, clearing the wound of all matrix and cell debris, including fibrin and spent neutrophils [6]. Macrophages also produce numerous cytokines, growth and angiogenic factors that are believed to play important roles in the regulation of fibro-proliferation and angiogenesis 7, 8. Another key leukocyte lineage, the mast cell, derived from circulating basophils, might be part of the initial activation response at the wound site but is subsequently also recruited with a somewhat later time course than for neutrophils and macrophages, and, in this regard, has been postulated to act during the later post-inflammatory phases of repair. Each of these inflammatory cell types releases overlapping cocktails of growth factors and cytokines that are presumed to function as tissue repair signals, directing the various behaviors of the host cells as they draw the wound closed, but they also act to amplify the inflammatory signal, so drawing in yet more neutrophils, macrophages and mast cells. Only when infection has been countered and repair completed will these inflammatory cells disperse from the wound site. Throughout the repair process, inflammatory cells are clearly an abundant and active component of the healing response, but how essential are they, and might the harm they do sometimes outweigh the good? A classic series of experiments in the 1970s attempted to directly test neutrophil and macrophage function by depleting them in a guinea pig wound model. These experiments showed that, so long as conditions were kept sterile, antisera depletion of neutrophils surprisingly seemed not to perturb tissue repair, but depletion of macrophages with antisera and steroids resulted in a failure of debridement – the clearance of dead and damaged cells, fibrin and tissue debris from the wound – and, as a result, seriously disturbed the healing process 6, 9. Because of these data, it has largely been considered taboo to tinker too heavily with the inflammatory response at a wound site for fear of aborting the repair process.
Section snippets
New wound-healing studies in leukocyte-deficient mice
A more recent series of depletion studies utilizing knockout and other approaches has allowed more precise tests of function for each of the inflammatory cell lineages at the wound site. These experiments have thrown up several surprises and opened the door to novel inflammation-blocking therapies for improving healing. It seems that no one cell lineage is absolutely necessary for directing repair, and, when some of them are absent, wounds might close faster and with less scarring.
In recent
Tinkering with the chemotactic signals
The damage signals that lead to recruitment of inflammatory cells from surrounding tissues and the circulation to wounds change over the time-course of the inflammatory response. The earliest signals are small molecules such as ATP, adenosine, uric acid and arachidonic-acid-derived and other bioactive lipids leaking from damaged cells at the wound site. These early signals are rapidly followed by growth factors released from degranulating platelets wherever blood vessels have been nicked and by
Interfering with trans-endothelial migration of leukocytes
The vast majority of inflammatory cells emigrating to the wound site derive from circulating leukocytes that diapedese through blood vessels adjacent to the wound that have become transiently leaky in response to signals released upon injury and infection. Activated endothelial cells lining these regions of vessels cause them to become ‘sticky’ such that circulating leukocytes now become lightly tethered and roll along this region of the endothelial wall [28] (Figure 2). The molecular basis of
Tinkering with the leukocyte motility machinery
Once freed from the confines of a blood vessel, leukocytes actively crawl towards the wound target, presumably using a battery of small molecules released by damaged cells, constituents of bacterial cell walls and other chemotactic substances to navigate their way. The best-characterized in vitro studies of macrophage migration have analysed their chemotactic response to colony stimulating factor 1 (CSF-1), and these data show that the small GTPase molecular switches Rac, Rho and Cdc42 each
Altering the activation state of leukocytes at the wound site
Simply exiting from a blood vessel dramatically alters the potential and fate of leukocytes called to a site of inflammation. The longevity of neutrophils immediately extends from hours while in circulation, to tens of hours once drawn to a wound. Monocytes transform into macrophages, which is significantly more than a change in name. Macrophages do not constitutively express high levels of cytokines and growth factors in their non-active state, but they are exquisitely sensitive to their
Blocking the signals released by leukocytes
It is clear that the various leukocyte lineages express and release into the wound milieu many cytokine and growth factor signals. The expression portfolios of these cells are partially redundant, and many of the factors they express are also upregulated by epithelial cells and fibroblasts at the wound site. An early bolus of some of these signals is released by degranulating platelets, although it appears that this contribution is non-essential, at least for VEGF and TGFβ1, as mice with no
Sending inflammatory cells home early
The last step in any inflammatory response is one of resolution, whereby all those leukocytes that have been recruited either disperse or die. Neutrophils generally self destruct by apoptosis once they have performed their duties and then are rapidly recognized and cleared by macrophages 62, 63. The fate of macrophages and mast cells at the wound site is less clear, although probably many eventually die, whereas others might disperse into surrounding tissues and/or escape via the lymphatic
Concluding remarks
So where do these various knockout and knockdown studies in mice leave us? They tell us that no one cell type appears absolutely essential for efficient healing (Figure 3; Table 2), and indeed many of the data suggest inflammation is bad for repair, either slowing its progress or inducing excessive fibrosis. Undoubtedly, there will turn out to be subtle functions of leukocytes at the wound site, beyond dealing with infective agents, that are not immediately obvious in the first trawl of
Acknowledgements
We are grateful to Iwan Evans (University of Bristol, UK) for assistance in preparing the figures.
References (70)
The effect of thrombocytopenia on dermal wound healing
J. Invest. Dermatol.
(2003)Wound healing in the PU.1 null mouse – Tissue repair is not dependent on inflammatory cells
Curr. Biol.
(2003)Transforming growth factor-beta (TGF-β) is chemotactic for human monocytes and induces their expression of angiogenic activity
Biochem. Biophys. Res. Commun.
(1988)Of man and mouse: Leukocyte and endothelial adhesion molecule deficiencies
Blood
(1999)Platelet-endothelial cell adhesion molecule-1 (PECAM-1)-deficient mice demonstrate a transient and cytokine-specific role for PECAM-1 in leukocyte migration through the perivascular basement membrane
Blood
(2001)Lack of platelet endothelial cell adhesion molecule-1 attenuates foreign body inflammation because of decreased angiogenesis
Am. J. Pathol.
(2003)- et al.
The function of scavenger receptors expressed by macrophages and their role in the regulation of inflammation
Microbes Infect.
(2001) Synergistic up-regulation of vascular endothelial growth factor expression in murine macrophages by adenosine A2A receptor agonists and endotoxin
Am. J. Pathol.
(2002)- et al.
Multiple signals converging on NF-kappaB
Curr. Opin. Cell Biol.
(1999) An angiogenic switch in macrophages involving synergy between Toll-like receptors 2, 4, 7 and 9 and adenosine A2A receptors
Am. J. Pathol.
(2003)