The macrophage is a prominent inflammatory cell in wounds, but its role in healing remains incompletely understood. Macrophages have been described to have many functions in wounds, including host defense, the promotion and resolution of inflammation, the removal of apoptotic cells, and the support of cell proliferation and tissue restoration following injury. Recent studies suggest that macrophages exist in several different phenotypic states within the healing wound, and that the influence of these cells on each stage of repair varies with the specific phenotypes. While the macrophage is beneficial to the repair of normally healing wounds, this pleotropic cell type may promote excessive inflammation and/or fibrosis in certain circumstances. Emerging evidence suggests that macrophage dysfunction is a component of the pathogenesis of non-healing and poorly healing wounds. Due to advances in the understanding of this multi-functional cell, the macrophage continues to be an attractive therapeutic target both to reduce fibrosis and scarring, and to improve healing of chronic wounds.

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-␣ and reduced levels of transforming growth factor-␤1 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.

Mp are crucial for tissue repair and regeneration but can also contribute to tissue damage and fibrosis. Mp can adopt a variety of functional phenotypes in response to different stimuli; two of the best-characterized in vitro phenotypes are a proinflammatory "M1" phenotype, produced by exposure to IFN-γ and TNF-α, and an anti-inflammatory "M2a" phenotype, produced by IL-4 or IL-13. M2a Mp are frequently termed "wound healing" Mp, as they express factors that are important for tissue repair. This review will summarize current knowledge of Mp phenotypes during tissue repair and will argue that these in vivo Mp populations are heterogeneous and temporally regulated and do not conform to existing, in vitro-defined M1 or M2 phenotypes. Mp during the early stages of tissue repair exhibit a more proinflammatory phenotype than their later counterparts, which in turn may exhibit some M2a-associated characteristics. However, phenotypic markers that appear to be coregulated in cultured Mp can be expressed independently of each other in vivo. Additionally, M1- and M2-associated markers may be expressed simultaneously by actual tissue-repair Mp. Improved understanding of Mp phenotypes and their regulation may assist in generation of novel therapies based on manipulating Mp function to improve healing.

Macrophages exhibit phenotypic diversity permitting wide-ranging roles in maintaining physiologic homeostasis. Hyaluronic acid, a major glycosaminoglycan of the extracellular matrix, has been shown to have differential signaling based on its molecular weight. With this in mind, the main objective of this study was to elucidate the role of hyaluronic acid molecular weight on macrophage activation and reprogramming. Changes in macrophage activation were assessed by activation state selective marker measurement, specifically quantitative real time polymerase chain reaction, and cytokine enzyme-linked immunoassays, after macrophage treatment with differing molecular weights of hyaluronic acid under four conditions: the resting state, concurrent with classical activation, and following inflammation involving either classically or alternatively activated macrophages. Regardless of initial polarization state, low molecular weight hyaluronic acid induced a classically activated-like state, confirmed by up-regulation of pro-inflammatory genes, including nos2, tnf, il12b, and cd80, and enhanced secretion of nitric oxide and TNF-α. High molecular weight hyaluronic acid promoted an alternatively activated-like state, confirmed by up regulation of pro-resolving gene transcription, including arg1, il10, and mrc1, and enhanced arginase activity. Overall, our observations suggest that macrophages undergo phenotypic changes dependent on molecular weight of hyaluronan that correspond to either (1) pro-inflammatory response for low molecular weight HA or (2) pro-resolving response for high molecular weight HA. These observations bring significant further understanding of the influence of extracellular matrix polymers, hyaluronic acid in particular, on regulating the inflammatory response of macrophages. This knowledge can be used to guide the design of HA-containing biomaterials to better utilize the natural response to HAs.

Diabetes is associated with persistent inflammation and defective tissue repair responses. The hypothesis of this study was that interleukin (IL)-1β is part of a proinflammatory positive feedback loop that sustains a persistent proinflammatory wound macrophage phenotype that contributes to impaired healing in diabetes. Macrophages isolated from wounds in diabetic humans and mice exhibited a proinflammatory phenotype, including expression and secretion of IL-1β. The diabetic wound environment appears to be sufficient to induce these inflammatory phenomena because in vitro studies demonstrated that conditioned medium of both mouse and human wounds upregulates expression of proinflammatory genes and downregulates expression of prohealing factors in cultured macrophages. Furthermore, inhibiting the IL-1β pathway using a neutralizing antibody and macrophages from IL-1 receptor knockout mice blocked the conditioned medium–induced upregulation of proinflammatory genes and downregulation of prohealing factors. Importantly, inhibiting the IL-1β pathway in wounds of diabetic mice using a neutralizing antibody induced a switch from proinflammatory to healing-associated macrophage phenotypes, increased levels of wound growth factors, and improved healing of these wounds. Our findings indicate that targeting the IL-1β pathway represents a new therapeutic approach for improving the healing of diabetic wounds.

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Macrophages are essential for the efficient healing of numerous tissues, and they contribute to impaired healing and fibrosis. Tissue repair proceeds through overlapping phases of inflammation, proliferation, and remodeling, and macrophages are present throughout this progression. Macrophages exhibit transitions in phenotype and function as tissue repair progresses, although the precise factors regulating these transitions remain poorly defined. In efficiently healing injuries, macrophages present during a given stage of repair appear to orchestrate transition into the next phase and, in turn, can promote debridement of the injury site, cell proliferation and angiogenesis, collagen deposition, and matrix remodeling. However, dysregulated macrophage function can contribute to failure to heal or fibrosis in several pathological situations. This review will address current knowledge of the origins and functions of macrophages during the progression of tissue repair, with emphasis on skin and skeletal muscle. Dysregulation of macrophages in disease states and therapies targeting macrophage activation to promote tissue repair are also discussed.

The hypothesis of this study was that sustained activity of the Nod-like receptor protein (NLRP)-3 inflammasome in wounds of diabetic humans and mice contributes to the persistent inflammatory response and impaired healing characteristic of these wounds. Macrophages (Mp) isolated from wounds on diabetic humans and db/db mice exhibited sustained inflammasome activity associated with low level of expression of endogenous inflammasome inhibitors. Soluble factors in the biochemical milieu of these wounds are sufficient to activate the inflammasome, as wound-conditioned medium activates caspase-1 and induces release of interleukin (IL)-1β and IL-18 in cultured Mp via a reactive oxygen species–mediated pathway. Importantly, inhibiting inflammasome activity in wounds of db/db mice using topical application of pharmacological inhibitors improved healing of these wounds, induced a switch from proinflammatory to healing-associated Mp phenotypes, and increased levels of prohealing growth factors. Furthermore, data generated from bone marrow–transfer experiments from NLRP-3 or caspase-1 knockout to db/db mice indicated that blocking inflammasome activity in bone marrow cells is sufficient to improve healing. Our findings indicate that sustained inflammasome activity in wound Mp contributes to impaired early healing responses of diabetic wounds and that the inflammasome may represent a new therapeutic target for improving healing in diabetic individuals.