Interleukin-23 is secreted by macrophages upon phagocytosis of apoptotic cells and influences the surrounding environment

Abstract

Macrophages play a crucial role in maintaining tissue homeostasis and orchestrating the immune responses through the phagocytosis of apoptotic cells. This process encompasses the clearance of dying cells not only under physiological conditions, but also during tissue injury. Macrophages express a diverse set of phagocytic receptors that enable the efficient engulfment of apoptotic cells. Upon the uptake of dying cells, macrophages modulate the immune environment, regulating inflammation and tissue repair, through tightly controlled expression of various cytokines. Consequently, a dysregulation in the phagocytic function of macrophages, due to e.g. altered expression of phagocytic receptors, can lead to inflammatory diseases. The aim of the present thesis was to deeper define the impact of macrophage-mediated apoptotic cell phagocytosis on the inflammatory response. Although, under homeostatic conditions, the uptake of apoptotic cells is known to occur as an “immunology silent” process, because it does not lead to any increase in typical proinflammatory cytokine secretion, the data obtained showed that phagocytosis leads to an increase in IL-23 expression by macrophages. In particular, the uptake of distinct apoptotic cells, such as apoptotic hepatocytes or apoptotic thymocytes, differentially regulates the amount of IL-23 secreted by phagocytic macrophages. This process appears to be dependent by the type of the phagocytic receptor engaged. In a model of S. mansoni infection, secretion of IL-23 seems to be triggered by AXL/MERTK-dependent phagocytosis. Similarly, in a murine model of cholangitis, IL-23 is highly expressed by a population of hepatic macrophages enriched in different phagocytic receptors. In both models the IL-23 levels expressed by macrophages correlate with the IL-17a expressed by hepatic CD4+ T cells. Additionally, in vitro, IL-23 secreted by phagocytic macrophages also impacts the fibroblast migration, as assessed via a series of wound healing assays. Even though the fibroblasts migratory capacity is modulated by IL-23, their transcriptomic profile was not significantly altered due to IL-23 presence, suggesting an alternative IL-23-mediated regulatory mechanism, such as metabolic reprogramming. To dissect the mechanism behind IL-23 expression in phagocytic macrophages, analysis of their transcriptomic profile was performed. Among the genes induced by the uptake of apoptotic cells, Irf7 appears to correlate with Il23 expression in phagocytic macrophages. Additionally, Irf7 binding to the Il23p19 promoter sequence was detected, thus suggesting a potential mechanism for Il23 transcription in macrophages upon uptake of apoptotic cells. Collectively, these findings define IL-23 as novel cytokine, secreted by phagocytic macrophages, which may impact the progression of inflammatory diseases.