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Platelet-rich plasma (PRP) is believed to induce regeneration of bone via the action of concentrated growth factors. The mechanism by which PRP affects the inflammatory response remains unclear.23 According to El Sharkawy et al.23 “Periodontal wound healing involves a series of well-orchestrated cell–cell interactions between gingival fibroblasts, epithelial cells, periodontal ligament fibroblasts, and osteoblasts, whereas disruption of the vasculature leads to fibrin formation, platelet aggregation, and release of several growth factors into the tissue from platelets.”
These factors are utilized in soft and mineralized tissue regeneration to regulate cellular events, such as proliferation, differentiation, chemotaxis, and morphogenesis of tissues and organs through auto-, para-, or endocrine mechanisms. Growth factors are delivered into the extracellular matrix where they are released during matrix degradation. They subsequently interact with surface receptors on the target cells which then stimulates intracellular signaling pathways to induce transcription of mRNA and proteins necessary for the regenerative process.23
Platelet-rich plasma (PRP) is a rich source of growth factors. Utilized by researchers in tissue engineering, it increases levels of growth factors by releasing them from their intracellular storage. A variety of growth factors are found in PRP, including platelet-derived growth factor (PDGF) and transforming growth factor (TGF)-β1, and both participate in regulation of bone regeneration. PRP can potentially serve as a convenient source for growth factors necessary for regenerative procedures, in addition to the growth factors released during periodontal wound healing. There are no concerns about immunogenic reactions and disease transmission between individuals because PRP is an autologous product.23
The results from animal studies and human trials indicate that PRP does demonstrate regenerative potential. Thus, tissue regeneration techniques may be used in conjunction with the administration of growth factors to improve the outcome in the repair of intrabony defects, furcations, and cyst cavities. Yet, the results that have been reported on the efficacy of this technique are both limited and controversial.23
When PRP was added to xenografts and synthetic graft materials, some recent clinical studies reported more rapid epithelization, denser and more mature bone with better organized trabeculae, and greater bone regeneration; however, some animal studies reported no beneficial additive effect of PRP. Although various clinical case reports supported the use of PRP, only a limited number of controlled studies have evaluated PRP, and they are still investigating the exact growth factor content in PRP and the biologic mechanisms mediated by PRP. Anecdotal observations have consistently reported the lack of post-surgical inflammation associated with the use of PRP.23
The potential for PRP to release growth factors has not been described well. In addition, whether PRP leads to any real inflammatory change is not yet known. For example, TGF-β1 is a major component of PRP. Its release from degranulating platelets or secretion by infiltrating macrophages and fibroblasts is critical to initiation or progression of tissue repair. Yet, it is believed that TGF-β1 may actually exacerbate inflammation and impede wound closure.23
Growth factors have been associated with growth of tumors. The actions of monocyte/macrophages and inflammatory processes could be involved in this mechanism. Evidence from in vitro and in vivo studies indicates that macrophage secretion of TGF-β1 is biphasic in activity, that is, in addition to its proinflammatory properties it also acts as an anti-inflammatory mediator. Scientists conclude that the reparative characteristics of TGF-β1 suggest a connection between mechanisms controlling resolution of inflammation and repair of damaged tissue.23
Endogenous lipid molecules derived from cellular arachidonic acid called lipoxins (LXs) are also natural mediators of inflammation resolution. Lipoxins (LXs) actively promote resolution by stimulating neutrophil apoptosis and inhibiting the entry of new neutrophils into sites of inflammation. Lipoxins are chemotactic for monocyte/macrophages that have a non-inflammatory phenotype. In addition, they phagocytise apoptotic neutrophils and stimulate healing and resolution of inflammation.23
Interaction of human neutrophils with platelets in the blood vessels is one of the major routes for biosynthesis of lipoxin A4 (LXA4). Cell-to-cell interactions stimulate hydroxylation of 12-hydroxyeicosatetraenoic acid (12-HETE) from platelets via 5-lipoxygenase (5-LO) derived from myeloid cells and produce 12-series LXs. This suggests that platelets may also participate actively in the resolution of inflammation.23
El Sharkawy et al.23 undertook this study to determine the content of growth factor in PRP, to measure the amount of cytokine/chemokines secreted by monocytes that had been treated with PRP, to evaluate how chemotactic properties of PRP influenced monocytes, and to investigate the amount and content of LXA4 in PRP. Their study provided evidence that PRP resulted in significantly higher levels of growth factors. In addition, it considerably suppressed inflammation by promoting secretion of LXA4.23
Previously described in platelet-rich plasma, PDGF-BB, TGF-β1 and IGF-I demonstrated strong mitogenic and anabolic properties. They promoted the growth of extracellular matrix formation, fibroblasts and periodontal ligament cells; stimulated collagen and total protein synthesis; and in addition, they stimulated the synthesis of hyaluronate from gingival fibroblasts.23
PDGF-BB was also reported to significantly stimulate adherence of periodontal ligament cells to root surfaces and decrease lipopolysaccharide inhibition of gingival fibroblast proliferation. TGF-β1 inhibits epithelial cell proliferation. IGF-I significantly affects mitogenesis of periodontal ligament fibroblasts and synthesis of protein in vitro. Osteoblasts synthesize and secrete high levels of IGF-I; and in an autocrine manner, they may regulate bone formation.23 In their study, El Sharkawy et al.23 discovered that IGF-I levels found in platelet-rich plasma (PRP) were not significantly different from those found in platelet-poor plasma (PPP), indicating that other cell types might be releasing IGF-I. The liver, smooth muscles, and placenta synthesize IGF-I. It is transported by plasma.23
Not only did El Sharkawy et al.23 confirm earlier findings on these growth factors, but they documented evidence that PRP is an important source of EGF, VEGF, and FGF-b. They reported that EGF has an important function in the regulation of the growth of many cells derived from both ectoderm and mesoderm. FGF-b demonstrated chemotactic and mitogenic properties for fibroblasts in the periodontal ligament. El Sharkawy et al.23 emphasized that these findings nullify the synthesis of type-I collagen, which is critical for calcified nodule formation and is one of the most common extracellular matrices found in the periodontium.
VEGF is now known to be critically important in forming new blood capillaries in healing wounds and is necessary for the integrity of the vasculature endothelial lining.23 So, not only did El Sharkawy et al.23 confirm earlier reports that PRP was potentially a good resource for growth factors, but that PRP collectively produces considerable quantities of growth factors critical to wound healing and regeneration.
Not only does PRP enhance delivery of growth factors to the wound area, but PRP also supports the regenerative processes through anti-inflammatory activity, confirmed by the absence of macrophage activation, inhibition of MCP-1, amplified delivery of RANTES, and release of LXA4. Essential for wound healing, the macrophage plays a key role in the inflammatory process and regulation of the regenerative repair process. During inflammation and wound healing, macrophages release many cytokines and chemokines. 23
El Sharkawy et al.23 reported that with the exception of RANTES and IL-12, monocyte cytokine and chemokine release did not increase in response to PRP, indicating that PRP stimulates the non-inflammatory activity of monocytes. Further investigation revealed that increased levels of RANTES in monocyte cultures treated with PRP were of platelet origin; this finding was supported by the considerable differences in RANTES in platelet-rich plasma compared to whole blood and platelet-poor plasma.23
Known as chemokine (C-C motif) ligand 5 (CCL5), RANTES is a chemokine that is regulated upon activation and may possibly play a dual role in inflammation. It is normally expressed in T cells. RANTES, a mediator of the inflammatory process, increases the adherence of monocytes to endothelial cells; this may potently chemo-attract and activate monocytes. Several cytokines can induce RANTES to inhibit the release of histamine from basophils, resulting in an inhibition of inflammatory processes and a modification in the wound environment toward repair and healing.23
Originating from the alpha granules in platelets, platelet-mediated RANTES could have beneficial effects in controlling the inflammatory processes; this concept is supported by studies in AIDS patients that revealed a diminished concentration of RANTES released from platelets.23 The findings of El Sharkawy et al.23 indicate that the release of RANTES mediated by monocytes/macrophages and platelets may act at different stages of activation or resolution of inflammation, in contrast to PRP that delivers considerable amounts of RANTES to wounds.
They designed an experiment to evaluate the chemotactic properties of PRP. The positive control was fMLP (N-formyl-methionyl-phenylalanine). They tested RANTES as a monocyte chemoattractant and they analyzed the chemotactic properties of PRP in a dose-response fashion.
Their results revealed that PRP, at 5% concentration and above, demonstrated a strong chemotactic effect on monocytes. RANTES had significant chemotactic effects on monocytes at concentrations equal to the concentration in PRP, indicating that RANTES, in part, is responsible for PRP-induced chemotaxis.23
Although, the effect of PRP is not attributable to RANTES alone, and it may possibly be associated with other factors requiring further evaluation. El Sharkawy et al.23 felt that LXA4 could be responsible for some of this effect that was described in previous studies; this strengthens the theory that increased monocyte/macrophage recruitment to the wound area could stimulate PRP-mediated healing and regeneration.
They also found an increase in another inflammatory mediator, IL-2, in monocyte cultures treated with PRP. Their analysis of the IL-12 concentration in PRP, WB, and PPP indicated that plasma was the source of IL-12, since no difference between PRP and PPP was evident. Thus, increased IL-12 generation could be due to multiple cells and processes in peripheral blood.23
PRP significantly inhibited the monocyte release of MCP-1. A member of the CC chemokine family, MCP-1 is produced by various cells in addition to monocyte/macrophages in response to proinflammatory stimuli. Chemotaxis, the respiratory burst, rapid induction of arachidonic acid release, and changes in Ca2+ concentration were all induced by MCP-1 In monocytes. Thus, PRP suppresses MCP-1 release by monocytes, in addition to the increased growth factor release. El Sharkawy et al.23 concluded while further investigation of these pathways is indicated and considering the limits of their study, their findings suggest that these properties of PRP additionally decrease monocyte-mediated inflammation and enhance wound healing.23
Based on years of research in wound healing and tissue regeneration, it is now an accepted fact that regeneration is inhibited by excess inflammation or chronic inflammation. Their findings confirmed the increased amount of platelet-derived LXA4, which leads to resolution of inflammation. In exudates that were resolving, LXA4 increased TGF-β1. LXA4 also stimulated the non-inflammatory infiltration of monocytes thought to be necessary for wound healing. In addition it promoted macrophage ingestion and clearing of apoptotic neutrophils.
Their results further suggested that increased amounts of LXA4 in PRP might promote the actions of the reparative properties of TGF-β1, thereby justifying the use of PRP in periodontal surgical procedures. It also accounts for the clinically observed beneficial healing after the application with PRP. In conclusion, through various mechanisms, PRP has the potential and capability to promote periodontal regeneration. It increases the concentrated delivery of various growth factors. In addition it acts as an anti-inflammatory agent through its concentration of LXA4, by producing RANTES, and by inhibiting MCP-1 release from monocytes. Collectively, this indicates that PRP promotes healing by regulating the local inflammatory response.23