Metabolic mediators: How immunometabolism directs the immune response to infection

Immunometabolites in viral infections: Action mechanism and function

The interplay between viral pathogens and host metabolism plays a pivotal role in determining the outcome of viral infections. Upon viral detection, the metabolic landscape of the host cell undergoes significant changes, shifting from oxidative respiration via the tricarboxylic acid (TCA) cycle to increased aerobic glycolysis. This metabolic shift is accompanied by elevated nutrient accessibility, which is vital for cell function, development, and proliferation. Furthermore, depositing metabolites derived from fatty acids, TCA intermediates, and amino acid catabolism accelerates the immunometabolic transition, facilitating pro-inflammatory and antimicrobial responses. Immunometabolites refer to small molecules involved in cellular metabolism regulating the immune response. These molecules include nutrients, such as glucose and amino acids, along with metabolic intermediates and signaling molecules adenosine, lactate, itaconate, succinate, kynurenine, and prostaglandins. Emerging evidence suggests that immunometabolites released by immune cells establish a complex interaction network within local niches, orchestrating and fine-tuning immune responses during viral diseases. However, our current understanding of the immense capacity of metabolites to convey essential cell signals from one cell to another or within cellular compartments remains incomplete. Unraveling these complexities would be crucial for harnessing the potential of immunometabolites in therapeutic interventions. In this review, we discuss specific immunometabolites and their mechanisms of action in viral infections, emphasizing recent findings and future directions in this rapidly evolving field.

Integrated metabolism and epigenetic modifications in the macrophages of mice in responses to cold stress

The negative effects of low temperature can readily induce a variety of diseases. We sought to understand the reasons why cold stress induces disease by studying the mechanisms of fine-tuning in macrophages following cold exposure. We found that cold stress triggers increased macrophage activation accompanied by metabolic reprogramming of aerobic glycolysis. The discovery, by genome-wide RNA sequencing, of defective mitochondria in mice macrophages following cold exposure indicated that mitochondrial defects may contribute to this process. In addition, changes in metabolism drive the differentiation of macrophages by affecting histone modifications. Finally, we showed that histone acetylation and lactylation are modulators of macrophage differentiation following cold exposure. Collectively, metabolism-related epigenetic modifications are essential for the differentiation of macrophages in cold-stressed mice, and the regulation of metabolism may be crucial for alleviating the harm induced by cold stress.

Kynurenine-3-monooxygenase (KMO) broadly inhibits viral infections via triggering NMDAR/Ca2+ influx and CaMKII/ IRF3-mediated IFN-β production

Tryptophan (Trp) metabolism through the kynurenine pathway (KP) is well known to play a critical function in cancer, autoimmune and neurodegenerative diseases. However, its role in host-pathogen interactions has not been characterized yet. Herein, we identified that kynurenine-3-monooxygenase (KMO), a key rate-limiting enzyme in the KP, and quinolinic acid (QUIN), a key enzymatic product of KMO enzyme, exerted a novel antiviral function against a broad range of viruses. Mechanistically, QUIN induced the production of type I interferon (IFN-I) via activating the N-methyl-d-aspartate receptor (NMDAR) and Ca²⁺ influx to activate Calcium/calmodulin-dependent protein kinase II (CaMKII)/interferon regulatory factor 3 (IRF3). Importantly, QUIN treatment effectively inhibited viral infections and alleviated disease progression in mice. Furthermore, kmo^-/- mice were vulnerable to pathogenic viral challenge with severe clinical symptoms. Collectively, our results demonstrated that KMO and its enzymatic product QUIN were potential therapeutics against emerging pathogenic viruses.

The outbreaks of emerging infectious diseases have become a severe challenge worldwide, and therefore it is a public health priority to explore novel broad-spectrum antiviral agents with various mechanisms. This study reported that kynurenine-3-monooxygenase (KMO), a key rate-limiting enzyme during tryptophan metabolism, showed promise as a novel broad-spectrum antiviral factor against emerging pathogenic viruses. We further found that quinolinic acid (QUIN), an enzymatic product of KMO, could also act as a novel broad-spectrum antiviral agent. We then systematically studied the underlying mechanisms and broadly antiviral function of KMO and QUIN in vitro and in vivo. Our data highlight the importance of exploring novel antiviral targets from the key enzymes and their metabolites in tryptophan metabolism.

Sweet talk: Metabolic conversations between host and microbe during infection

In this issue, we introduce the second part of a series of reviews focusing on how immunometabolism influences host and pathogen interactions during infection. This part of the collection addresses the interface between metabolism and specific types of infection, including immunometabolism in macrophages during helminth infection, the role of metabolism in T-cell exhaustion during chronic viral infections and host immunometabolism in the defence against Mycobacterium tuberculosis infection. These reviews, together with the four articles published in part 1 of the series in November 2020, offer new insights into the complex interactions between mammalian hosts and microbial pathogens through the lens of cellular metabolic regulation.

A retrospective view of 2020: A unique year for Immunology

2020 was a year unlike any other for Immunology. Through the SARS-CoV-2 pandemic, with fantastic support from the global immunology community, we worked together to reach new heights. Here, we look back at some of the highlights for Immunology in a challenging and memorable year.