Interplay Between Nitric Oxide-Mediated Metabolic Rewiring And Immune Functions In Dendritic Cells

Recent years have highlighted the importance of metabolic processes in controlling immune responses at the cellular level. Studies have shown that metabolic changes during immune cell activation support energy needs and influence inflammation and cell function in both innate and adaptive immunity, strongly suggesting potential methods to regulate inflammation. Arginine metabolism is particularly crucial in the immune responses of myeloid cells, such as macrophages and dendritic cells (DCs). These cells produce two main arginine catabolizing enzymes, inducible nitric oxide synthase (iNOS) and arginases (ARG). iNOS utilizes L-arginine and oxygen to produce nitric oxide (NO), a vital component in fighting infections, but also harmful at high levels. Arginases convert L-arginine into L-ornithine and urea; ornithine builds polyamines for tissue repair and collagen, and can promote the expression of anti-inflammatory genes. DCs perform numerous functions, including phagocytosis, the production of chemokines and cytokines, the clearance of infections, and the activation of T cells to bridge innate and adaptive immunity. Our previous research demonstrated that: 1) DCs increase iNOS and NO production upon activation, shifting their metabolism toward glycolysis due to NO-mediated respiration inhibition; 2) a threshold of NO production exists — a concentration of NO that leads to a decline in mitochondrial respiratory function. Studies have observed ARG expression in DCs exhibiting tolerogenic features, suggesting that ARG may play a role in regulating iNOS via substrate competition. However, given the heterogeneity of DC subtypes, it remains unclear whether the known iNOS-producing DCs exhibit this ARG expression pattern, and whether competitive substrate usage by ARG affects iNOS activity, particularly in relation to the NO-respiration threshold. In this dissertation, using murine bone marrow-derived DCs (BMDCs) as a model, we examined whether arginase activity can limit iNOS function by decreasing arginine availability, thereby altering NO-driven immune functions in DCs. Our results showed that when arginase activity is blocked, NO levels increase, lowering the NO-respiration threshold. This shift in the threshold influences how long DCs survive after activation and how effectively they control the replication of pathogens, Listeria monocytogenes. Overall, our findings support the hypothesis that iNOS activity is partly regulated by arginases through substrate competition in DCs. By limiting NO production, arginase helps prevent mitochondrial damage and extends cell survival, although it may also reduce the cell’s ability to fight pathogens. This balance is crucial for proper immune function, underscoring the importance of arginine metabolism in regulating iNOS activity to mediate inflammation and facilitate effective immune responses.