Because the inflammatory response results in the production of a radical, hostile
environment in response to injury or infection, coordination of resolution of the inflammatory response is key to limiting excess damage to the host. Although much is known about the onset of inflammation, and the characteristics of the tissue repair process, the mechanisms governing transition from active inflammation to the resolution phase of the immune response are
incompletely understood. The general consensus regarding the transition to the resolution phase of inflammation is that the wound must first inhibit the influx of inflammatory cells, and then must orchestrate the generation of a localized area of immune suppression [3]. Induction of the immunosuppressive pathways is an absolutely crucial component of the wound healing program. As with all mechanisms that govern homeostasis in the body, the inflammatory response has methods to both promote the immune response when necessary, and of course, to inhibit the immune response when appropriate. Many types of immune cells are used to orchestrate
immunosuppression, therefore it can be said that immune cells have dual functionality. Immune cells can either enhance or inhibit the inflammatory response, through the generation of specific cellular subtypes. These subtypes are created through polarization events, which are primarily driven through cytokine signals. The immunosuppressive effector cell subtypes use many different methods to control the immune responses, including cell-cell contact through counter receptor ligation, and secretion of soluble factors, such as lipid mediators and cytokines [3]. Like the onset of inflammation, suppressive functions of the immune system are frequently
orchestrated by secreted cytokines. Of all the soluble mediators that regulate inflammation, the three major cytokine contributors to resolution of inflammation are IL-4, IL-10, and TGF-β [35, 38, 52]. The following section will discuss, in detail, the major functions of these
immunosuppressive cytokines, and their role in establishing localized immune suppression that supports tissue repair during inflammation resolution, as the third function of the inflammatory response.
1.9.2.1 IL-10 coordinates the inhibition of active inflammation in the wound site
IL-10 is powerful anti-inflammatory cytokine that plays a vital role in the tissue repair process [38]. IL-10 is secreted by many cells in response to inflammatory stimuli[36]. This cytokine works through many different functions to quell the active inflammatory response in favor of promoting the tissue repair process. The first method in which IL-10 suppresses inflammation in favor of tissue repair is by limiting the production of inflammatory cytokines produced by various cells in the insulted tissue. IL-10 has been show to reduce the production of many inflammatory cytokines, including but limited to TNFα, IL-1β, IL-6, and IL-12 [38]. A second function of IL-10-mediated inflammation inhibition is the ability for this cytokine to drastically reduce the production and secretion of chemoattractants, such as IL-8 and MCP-1 [38]. This reduction enhances the inhibition of leukocyte recruitment process that has already been established by lipid mediator secretion by the infiltrating neutrophils. Thus, IL-10 helps stem the flood of inflammatory leukocytes into the insulted tissue, which allows for the transition of the tissue out of the inflammatory condition, and into the tissue repair process. Furthermore, IL-10 conditions the microenvironment toward a tissue repair-promoting milieu, and in so doing, induces the skewing of macrophage and T cell polarization toward immunosuppressive M2 and Treg phenotypes [5]. Once polarization of these immune subtypes is induced, they continue to promote immunosuppressive conditioning of the microenvironment through secretion of immunosuppressive cytokines, including IL-4, IL-13, TGF-β, and of course, additional IL-10 [34]. IL-10 is also a powerful inducer of STAT3, which is a major transcription factor responsible
for driving tissue repair processes, such as cell survival, growth and division, and angiogenesis [53]. Through induction of these tissue repair pathways, and by inhibiting the further
accumulation of inflammatory cells and cytokines within the insulted tissue, IL-10 is a potent inhibitor of inflammation in the wound site.
1.9.2.2 IL-4 and IL-13 coordinate tissue repair processes
Like IL-10, IL-4 is a pleiotropic cytokine show to have a profound role in orchestrating the tissue repair response [54]. Secreted mainly by M2 macrophages and Th2 polarized T cells, IL-4
functions to inhibit the inflammatory response by inhibiting the polarization of classically
activated M1 macrophages in favor of tissue repair-supportive M2 cells [5]. Because IL-4 and IL- 13 are secreted by M2 and Th2 cells, and induce further M2 and Th2 skewing, it can be said that M2 and Th2 cells promote further generation of their own phenotype. Furthermore, IL-4 and IL- 13, both considered signature cytokines secreted by M2 and Th2 cells, go on to promote the expression of tissue repair genes, such as MMP12, EGF, and IL-10, and Arg1 [55]. MMP12, also known as macrophage elastase, promotes wound healing by enabling the degradation of
extracellular matrix components to stimulate tissue remodeling after inflammatory insult.
Epidermal growth factor (EGF) is a growth factor necessary for promoting many functions of the wound healing response. For example, EGF promotes the deposition of fibronectin, induces the synthesis of various angiogenic factors, and increases the activity of collagenase [56]. EGF uses these pro-repair functions to actively coordinate tissue repair and remodeling of wounded tissue. Furthermore, increased secretion of IL-10 works in concert with IL-4 and IL-13 to maintain the generation of suppressive phenotype immune cells within the injured tissue. Additionally, it has been shown, through the use of various tissue repair animal models, that ablation of the IL-4Rα
of tissue repair, but it is also able in inhibit the proinflammatory effects of classically activated immune cells [55]. IL-4 potently suppresses the generation of Th1 and Th17 inflammatory
responses by increases the activity of transcriptional repressors of IFNγ and IL-17, both cytokines that stimulate the inflammatory response [6]. Furthermore, IL-4 plays and indispensible role in limiting the inflammatory response by enhancing the production of arginase, which depletes arginine in the local environment [35]. Depletion of this essential amino acid potently impairs the activation of proliferation in responding T cells, thereby limiting the immune response in favor of promotion of tissue repair. Because IL-4, and its shared cytokine signaling partner IL-13, exert both tissue repair stimulating activities, and suppress proinflammatory immune cell activities, these cytokines are absolutely essential to the coordination of the tissue repair process[35].
1.9.2.3 TGF-β facilitates return to homeostasis
Transforming growth factor beta, or TGF-β is another cytokine/growth factor that is key to the wound healing process. TGF-β is secreted by many cells in the insulted tissue
microenvironment, including platelets, and macrophages [52]. Its secretion occurs throughout the inflammatory and tissue repair processes, and coordinates many different aspects of the healing process. The first major goal of TGF-β is to stimulate the formation of the granulation tissue (a temporary layer of extracellular matrix components that acts to plug the wound to keep out infection) [52]. TGF-β does this by stimulating the production of connective tissue factors, including collagen and fibronectin [52]. At the same time, TGF-β acts to inhibit the production of MMP molecules that contribute to digestion of the ECM [52]. Next, TGF-β, along with other growth factors, acts to stimulate fibroblast proliferation and migration, and stimulates activation of angiogenesis to increase oxygenation to support the new tissue growth [33, 52]. As mentioned previously, TGF-β, in conjunction with IL-6, and IL-10 works to polarize T cells toward a
regulatory subset (Tregs and Th2 subsets) [6]. These cells actively secrete more TGF-β and IL-10, which pushes the microenvironment toward an immunosuppressive phenotype by maintaining the M2 polarization of the local macrophages [34]. TGF-β has been shown to have pathogenic
functions as well. During periods of sustained tissue repair signaling, such as chronic
inflammation and tumorigenesis, excess production TGF-β leads to generation of tissue fibrosis. TGF-β has also been demonstrated to support growth and development of tumors [52].