Introduction
The intestine not only serves as a barrier against external harmful agents and pathogen infection, but also interacts with commensal microbiome and food antigens. Therefore, the intestinal immune system reacts to both inner and external environments to maintain homeostasis. When this balance is disrupted, chronic inflammatory disorders can occur in the intestine, leading to inflammatory bowel disease (IBD) (Maloy & Powrie, 2011). There are two main forms of IBD: Crohn’s disease, which includes inflammatory disorder throughout the gastrointestinal tract, and ulcerative colitis, which is restricted in the colon (Kaser, Zeissig, & Blumberg, 2010). Causes of IBD is complicated. It involves multiple factors such as genetic factors, failure of the host immune system, and disruption of gut microbiota (Hill & Artis, 2010; Podolsky, 1991).
Macrophages are innate immune cells well distributed in almost all tissues and are highly heterogenous to achieve niche-specific functions. Gut macrophages not only can maintain immune homeostasis by driving clearance of infection, but also can control oral tolerance and tissue repair (Mazzini, Massimiliano, Penna, & Rescigno, 2014; Murray & Wynn, 2011). Gut macrophages initiate inflammation in response to stimuli such as pathogen or cell damage. After an inflammatory response takes place, complete resolution must take place to maintain homeostasis. Gut macrophages are key players initiating inflammation to stimuli such as infection, but also in resolution of inflammation afterwards. Dysfunction of macrophages may result in failure of resolution, leading to chronic inflammation (Na, Stakenborg, Seok, & Matteoli, 2019). Alternatively activated macrophages can produce anti-inflammatory cytokines IL-10 and growth factors such as TGF-β, prostaglandin E2 (PGE2), bone morphogenetic protein 2 (BMP2), and WNT ligands (Lin et al., 2010; Wynn & Vannella, 2016). IL-10 production is known to promote immunosuppressive CD4+ regulatory T cells (Treg).
Gut macrophages contain self-maintaining resident macrophages derived from embryonic birth and bone marrow derived macrophages matured from circulating monocytes (Viola & Boeckxstaens, 2021). Freshly supplied bone marrow derived Ly6Chi monocytes undergo differentiation into CX3CR1+macrophages in the lamina propria (Bain et al., 2014). They rapidly upregulate major histocompatibility complex type II (MHCII) and downregulate Ly6C. During colitis, CX3CR1int expressing monocytes are recruited to the gut. They show a pro-inflammatory phenotype. Mature CX3CR1hi macrophages are rather anti-inflammatory. They can maintain homeostasis (Diehl et al., 2013; Geissmann, Jung, & Littman, 2003; Viola & Boeckxstaens, 2021).
IBD patients show altered differentiation of macrophages compared to healthy individuals. Since functional macrophage defect can lead to chronic inflammatory disorders, macrophages can be considered a potential target for IBD treatment. Previous studies have shown that macrophages have distinct phenotypes through metabolic regulation. Inflammatory macrophages show enhanced glycolysis while anti-inflammatory macrophages show enhanced oxidative phosphorylation (Russell, Huang, & VanderVen, 2019; Wculek et al., 2023). Distinct pathways of arginine metabolism determine macrophage characters. Inflammatory macrophages express nitric oxide synthase which leads to production of nitric oxide and citrulline, while anti-inflammatory macrophages express arginase which leads to production of urea and ornithine (Rath, Muller, Kropf, Closs, & Munder, 2014). Also, inflammatory macrophages can be induced by accumulation of citrate or succinate due to impairment of the TCA cycle (Harber et al., 2020; Palmieri et al., 2020). Thus, utilizing metabolic modulation to induce macrophage polarization can be a therapeutic approach to treat IBD.
In this study, we introduce LMT503, an IBD therapeutic candidate, which act as a substrate for NADH quinone oxidoreductase 1 (NQO1), a multi-functional protein that can increase cellular NAD+, drive oxidation-reduction reaction, and stabilize proteins such as p53 (Nebert, Roe, Vandale, Bingham, & Oakley, 2002; Ross & Siegel, 2021). Increased NAD+ by NQO1 can activate sirtuins and drive metabolic modulation of macrophages to have an anti-inflammatory character (S. Y. Park et al., 2017). The objective of this study was to determine whether LMT503 could induce polarization of macrophages to an immune-suppressive type to alleviate colitis in a DSS-induced murine model.