We have previously described a model of chronic intraperitoneal inflammation based on a polyether-polyurethane sponge matrix implantation technique. The intraperitoneal implants were shown to be firmly adhered to surrounding organs (liver and/or intestines), highly vascularized, infiltrated with inflammatory cells, and producing pro-inflammatory and pro-angiogenic cytokines [9, 10].
Using this model, the data presented here demonstrate, for the first time, that the systemic treatment with water extract of propolis (WEP) reduced vascular permeability, angiogenesis, and fibrosis of intraperitoneal implants. Histological analysis of the tissue corroborated our biochemical findings indicating the anti-angiogenic and antifibrogenic effects of propolis. Conversely, propolis was shown to increase all the inflammatory parameters analyzed, neutrophil and macrophage recruitment, TNF-α production, and gene expression of classical and alternative macrophage activation. Several groups have reported that local or systemic administration of propolis extracts exert immunomodulatory, antimicrobial, anti-inflammatory, antioxidant, antiangiogenic, and pro-healing activity in various experimental models [5–8].
At least in part, this discrepancy may be attributed to a number of factors, such as animal models, type of injury, manner of administration, dosage, duration of the experiments and of the source and type of propolis sample. Indeed, variations in propolis formulation have been reported, related to the plants in the geographic region from which propolis has been collected . To some extent, our results are in agreement with the effects of propolis in other experimental models, for example its anti-angiogenic action. Interestingly, VEGF production was augmented in the propolis-treated group. This increase in pro-angiogenic cytokine may have been an attempt to compensate for decreased blood flow in the injury. It is intriguing that the reported anti-inflammatory effects of propolis are in contrast with the results presented here, particularly those from our group, which showed propolis to exert an anti-inflammatory effect on implants located subcutaneously in mice . We have previously demonstrated that the anatomical site markedly influences the host response to a synthetic matrix  and, herein, we extend this observation to the host response to a pharmacological compound. It is striking that by up-regulating pro-inflammatory pathways, propolis has down-regulated angiogenesis and fibrosis in the peritoneal implant. To some extent, our results challenge the notion of the co-dependence of angiogenesis and inflammation in the maintenance of fibroproliferative pathological processes . It may be that the co-dependence exists in many, but not in all, pathological processes, especially those in the peritoneal cavity. In this context, it is worth pointing out that the factors involved in healing peritoneal injuries are dissimilar to healing in other anatomical compartments. For instance, mesothelial cells are the specific cell type activated after injury to the peritoneum and responsible for the release of inflammatory mediators, chemokines, and cytokines that, in turn, recruit inflammatory cells. Activated mesothelial cells produce excessive plasminogen activator inhibitor-2 when compared with activated endothelial cells . Macrophages from the peritoneal cavity display a range of features that differs from macrophages in other sites . Likewise, fibroblasts in human tissue from the peritoneal cavity develop a specific phenotype expressing cyclooxygenase-2 . Furthermore, while many phases of wound repair and mechanisms that regulate this process are common to many types of wounds, there are differences between dermal and peritoneal healing. Dermal injuries heal inward from the edges and the rate of healing depends on the size of the lesion, whereas peritoneal wounds heal simultaneously throughout the lesion, and the rate is independent of the injury’s surface . Thus, it seems pertinent to attribute the different responses to the same compound to all these inherent dissimilarities between the cells and healing processes in injuries in different anatomical sites.
Another finding that has emerged from our study was the effect of propolis on macrophage activation states. Generically, these states are classified as inflammatory/classical (stimulated by TNF-α and IFN-γ) or alternative (expressing YM1 and FIZZ1), depending on a variety of environmental factors (cytokines, pattern recognition receptors, hormones) [14, 15]. Interestingly, propolis up-regulated both classical and alternative pathways, yielding approximately 23- and 7-fold increases in the NOS2/IFN-γ and 8- and 2-fold increases in the FIZZ1/YM1 cytokine/gene expression. By simultaneously activating the inflammatory and alternative macrophage states, propolis treatment decreased neovascularization and fibrosis (key components in the maintenance of chronic fibroproliferative processes) while, at the same time, increasing the pro-inflammatory markers and cytokine (TNF-α) involved in fibrinolytic activity. This phenotypic heterogeneity in macrophage responses to this compound is fully compatible with the opposing functions of this cell in repair processes (proinflammatory versus anti-inflammatory, tissue-repair versus tissue-destruction) [14, 27]. Therefore, it is likely that propolis exerted selective actions amplifying inflammatory signals that led to the increased production of “benign” pro-inflammatory molecules by the various cell types in the implant microenvironment. Conversely, the down-regulation of pro-fibrogenic signals (TGF-β1 production) resulted in decreased collagen deposition in the implant compartment. Negative modulation of these molecules is fully compatible with the notion that a decrease in both components results in fibrosis resolution . The net effect of propolis treatment was the attenuation of angiogenesis and fibrosis in intraperitoneal implant.