| dc.description.abstract | Inflammation is a necessary process in fighting infection or during injury. Subsequently, the active process of resolution of inflammation occurs, which assists in clearance of cell debris and dead cells and in tissue repair and regeneration to restore homeostasis in the tissue. Recently, it was determined that a novel family of lipid mediators, namely specialised pro-resolving mediators (SPMs), had a vital role in the activation of resolution. Macrophages are central in orchestrating the clearance of apoptotic cells and cellular debris during the resolution of inflammation. Here, the mechanism(s) regulating macrophage function, including efferocytosis, remain incompletely understood. Recent studies identified two novel mediator families, namely the n-3 docosapentaenoic acid (DPA)-derived protectins (PDn-3 DPA) and the maresin conjugates in tissue regeneration (MCTRs) that regulate the resolution of inflammation. My overarching hypothesis is that these mediators exert their pro-resolving activities, at least in part, via modulating macrophage phenotype and function. Therefore, I set out to detail the biology of these mediators in macrophages. Since the PDn-3 DPA biosynthetic pathway was unknown, I first studied the enzymes involved in the production of these mediators. In these studies, using in vitro cell systems, I established that, in the initial step in this pathway, 15-lipoxygenases (ALOX15) catalysed the conversion of n-3 DPA to the intermediate allylic epoxide, 16S, 17S-epoxy-7Z,10Z,12E,4E,19Z-docosapentaenoic acid (16S, 17S-ePDn-3 DPA). Thereafter, 16S, 17S-ePDn-3 DPA was enzymatically converted to the bioactive mediators in the PDn-3 DPA metabolome, PD1n-3 DPA and PD2n-3 DPA, by enzymes from the epoxide hydrolase family, where epoxide hydrolase 2 converted 16S,17S-ePDn-3 DPA to PD2n- 3DPA. In addition, the inhibition of ALOX15 inhibited PDn-3 DPA production and altered both macrophage phenotype and function. Yet, these biological activities were rescued when macrophages were incubated with either 16S,17S-ePDn-3 DPA or PD1n-3 DPA, which restored the expression of several lineage markers and macrophage efferocytosis and phagocytosis. Additionally, tissue protective macrophages produce, among other SPMs, maresin conjugates in tissue regeneration (MCTRs), which assist in promoting the resolution of inflammation to restore the host to homeostasis. However, if the resolution of inflammation fails to occur, inflammation persists and may lead to the development of chronic inflammation. Rheumatoid arthritis is an autoimmune chronic inflammatory disease that is associated with inflammation within the joints, which can lead to joint damage through cartilage degradation and bone erosion. Assessment of the biology of MCTR3 demonstrated that when monocytes from arthritic mice were incubated with MCTR3 ex vivo and then transferred back to arthritic mice, these reduced joint inflammation and increased cartilage integrity when compared with mice that were treated with monocytes incubated with vehicle alone. Macrophage efferocytosis was also enhanced in arthritic mice treated with MCTR3 reprogrammed monocytes. These data indicated that MCTR3 was able to reprogramme monocyte responses to exert anti-inflammatory and tissue protective activities. Evaluation of monocyte derived macrophages from arthritic mice treated with MCTR3 reprogrammed monocytes using transcriptomic profiling and flow cytometry indicated an upregulation of Arginase-1 (Arg1) in these cells. Furthermore, Arg1 inhibition reversed both the anti-arthritic and joint protective actions of MCTR3 reprogrammed monocytes. Taken together, these results establish a role for the PDn-3 DPA biosynthetic pathway and MCTR3 in regulating monocytes and macrophages and their role in resolution responses. | en_US |
