Circadian clock regulates the host response to Salmonella

The diverse roles of the circadian clock in cancer

A growing part of the human population is affected by circadian misalignment caused by deregulated sleep, increased nighttime light exposure and erratic eating patterns. Thus, circadian rhythms are a key research area, with compelling links to cancer. Here, we review the circadian regulation of critical cellular processes, including immunity, metabolism, cell cycle control and DNA repair, under physiological homeostasis and in cancer. We discuss the divergent evidence indicating tissue-specific roles of the circadian clock in different cancer types and the potential link between circadian misalignment and early-onset cancers. Finally, we outline how understanding the circadian clock can improve cancer prevention and chronomedicine-based therapies.

A light-regulated circadian timer optimizes neutrophil bactericidal activity to boost daytime immunity

The immune response exhibits strong circadian rhythmicity, with enhanced bacterial clearance often synchronized with an organism's active phase. Despite providing the bulk of cellular antibacterial defense, the neutrophil clockwork is poorly understood. Here, we used larval zebrafish to explore the role of clock genes in neutrophils during infection. Per2 was required in neutrophils for reactive oxygen species (ROS) production and bacterial killing by enhancing infection-responsive expression of high-mobility group box 1a (hmgb1a). The Cry binding domain of Per2 was required for regulation of neutrophil bactericidal activity, and neutrophils lacking Cry1a had elevated bactericidal activity and infection-responsive hmgb1a expression. A conserved cis-regulatory element with BMAL1 and nuclear factor κB binding motifs gated infection-responsive hmgb1a expression to the light phase. Mutagenesis of the BMAL1 motif in neutrophils blunted the priming effect of light on bactericidal activity and hmgb1a expression. These findings identify a light-responsive cell-intrinsic timer that controls time-of-day variations in antibacterial activity.

The clockwork macrophage: timing in innate immunity

The circadian clock enables organisms to predict daily environmental changes and synchronize their physiology and behaviour accordingly. Macrophages, key sensor cells in the innate immune system, exhibit cell-autonomous circadian rhythmicity. This circadian rhythmic behaviour is synchronised to the central clock in the hypothalamus as a result of neural, and hormonal signals. Macrophage rhythms and responses involve sensing temporal cues, integrating information from tissue-specific environments, and initiating context-appropriate, time-gated responses. On a broader scale, monocytes and macrophages communicate and synchronize with other immune cells, migrate throughout the body, and infiltrate tissues, collectively contributing to circadian regulation in both health and disease. While the field of macrophage circadian biology is rapidly advancing, it is equally important to reflect on its historical development, which has been shaped by over two centuries of accumulating knowledge and technological progress. This review traces key milestones in macrophage and circadian research, examining how recent discoveries have refined our understanding of early foundational questions and setting the stage for future inquiries. Notably, many intriguing questions remain unresolved, including the circadian regulation of macrophage function under steady-state conditions, the tissue-specific heterogeneity of macrophage circadian rhythms, and the role of macrophage circadian clocks in disease pathogenesis and their potential clinical implications.

Circadian clock is critical for fungal pathogenesis by regulating zinc starvation response and secondary metabolism

Circadian clocks are known to modulate host immune responses to pathogen infections, yet their role in influencing pathogen pathogenesis remains unclear. Here, we investigated the role of circadian clocks in regulating the pathogenesis of the fungal pathogen Fusarium oxysporum, which has multiple genes homologous to the Neurospora crassa frq due to gene duplication events, with Fofrq1 being the primary circadian clock gene. The pathogenesis of F. oxysporum in plants is controlled by its circadian clock, with infections causing severe disease symptoms at dawn. Notably, disruption of clock genes dramatically reduces fungal pathogenicity. Circadian clocks regulate the rhythmic expression of several transcription factors, including FoZafA, which enables the pathogen to adapt to zinc starvation within the plant, and FoCzf1, which governs the production of the toxin fusaric acid. Together, our findings highlight the critical roles of circadian clocks in F. oxysporum pathogenicity by regulating zinc starvation response and secondary metabolite production.

The circadian rhythm as therapeutic target in inflammatory bowel disease

The primary objectives of the management of patients with inflammatory bowel disease (IBD) are to prevent IBD flares, prevent/delay disease progression and improve patients' quality of life. To this end, one needs to identify risk factor(s) associated with flare-ups and disease progression. We posit that disruption of circadian rhythms is one of the key factors that is associated with risk of flare-up and disease progression. This hypothesis is based on published studies that show: (1) The circadian rhythm regulates many biological processes including multiple IBD-relevant biological processes that are critical in inflammatory/immune processes such as environment/microbe interaction, microbe/host interaction, intestinal barrier integrity and mucosal immunity-all central in the pathogenesis of IBD, and (2) Circadian machinery is the primary tool for the host to interact with the environment. Circadian misalignment results in a loss of preparedness of the host to respond and adjust to the environmental changes that could make the host more vulnerable to IBD flare-ups. In this review, we first provide an overview of circadian rhythms and its role in healthy and disease states. Then we present data to support our hypothesis that: (1) IBD patients have disrupted circadian rhythms ("social jet lag") and (2) circadian misalignment and associated disrupted sleep decreases the resiliency of IBD patients resulting in microbiota dysbiosis, more disrupted intestinal barrier integrity and a more aggressive disease phenotype. We also show that circadian-directed interventions have a potential to mitigate the deleterious impact of disrupted circadian and improve IBD disease course.

Circadian metabolic adaptations to infections

Circadian clocks are biological oscillators that evolved to coordinate rhythms in behaviour and physiology around the 24-hour day. In mammalian tissues, circadian rhythms and metabolism are highly intertwined. The clock machinery controls rhythmic levels of circulating hormones and metabolites, as well as rate-limiting enzymes catalysing biosynthesis or degradation of macromolecules in metabolic tissues, such control being exerted both at the transcriptional and post-transcriptional level. During infections, major metabolic adaptation occurs in mammalian hosts, at the level of both the single immune cell and the whole organism. Under these circumstances, the rhythmic metabolic needs of the host intersect with those of two other players: the pathogen and the microbiota. These three components cooperate or compete to meet their own metabolic demands across the 24 hours. Here, we review findings describing the circadian regulation of the host response to infection, the circadian metabolic adaptations occurring during host-microbiota-pathogen interactions and how such regulation can influence the immune response of the host and, ultimately, its own survival.This article is part of the Theo Murphy meeting issue 'Circadian rhythms in infection and immunity'.

BioClocks UK: driving robust cycles of discovery to impact

Chronobiology is a multidisciplinary field that extends across the tree of life, transcends all scales of biological organization, and has huge translational potential. For the UK to harness the opportunities presented within applied chronobiology, we need to build our network outwards to reach stakeholders that can directly benefit from our discoveries. In this article, we discuss the importance of biological rhythms to our health, society, economy and environment, with a particular focus on circadian rhythms. We subsequently introduce the vision and objectives of BioClocks UK, a newly formed research network, whose mission is to stimulate researcher interactions and sustain discovery-impact cycles between chronobiologists, wider research communities and multiple industry sectors.This article is part of the Theo Murphy meeting issue 'Circadian rhythms in infection and immunity'.

Basic helix-loop-helix ARNT like 1 regulates the function of immune cells and participates in the development of immune-related diseases

The circadian clock is an internal timekeeper system that regulates biological processes through a central circadian clock and peripheral clocks controlling various genes. Basic helix-loop-helix ARNT-like 1 (BMAL1), also known as aryl hydrocarbon receptor nuclear translocator-like protein 1 (ARNTL1), is a key component of the circadian clock. The deletion of BMAL1 alone can abolish the circadian rhythms of the human body. BMAL1 plays a critical role in immune cell function. Dysregulation of BMAL1 is linked to immune-related diseases such as autoimmune diseases, infectious diseases, and cancer, and vice versa. This review highlights the significant role of BMAL1 in governing immune cells, including their development, differentiation, migration, homing, metabolism, and effector functions. This study also explores how dysregulation of BMAL1 can have far-reaching implications and potentially contribute to the onset of immune-related diseases such as autoimmune diseases, infectious diseases, cancer, sepsis, and trauma. Furthermore, this review discusses treatments for immune-related diseases that target BMAL1 disorders. Understanding the impact of BMAL1 on immune function can provide insights into the pathogenesis of immune-related diseases and help in the development of more effective treatment strategies. Targeting BMAL1 has been demonstrated to achieve good efficacy in immune-related diseases, indicating its promising potential as a targetable therapeutic target in these diseases.

Chemical interplay between gut microbiota and epigenetics: Implications in circadian biology

Circadian rhythms are intrinsic molecular mechanisms that synchronize biological functions with the day/night cycle. The mammalian gut is colonized by a myriad of microbes, collectively named the gut microbiota. The microbiota impacts host physiology via metabolites and structural components. A key mechanism is the modulation of host epigenetic pathways, especially histone modifications. An increasing number of studies indicate the role of the microbiota in regulating host circadian rhythms. However, the mechanisms remain largely unknown. Here, we summarize studies on microbial regulation of host circadian rhythms and epigenetic pathways, highlight recent findings on how the microbiota employs host epigenetic machinery to regulate circadian rhythms, and discuss its impacts on host physiology, particularly immune and metabolic functions. We further describe current challenges and resources that could facilitate research on microbiota-epigenetic-circadian rhythm interactions to advance our knowledge of circadian disorders and possible therapeutic avenues.

Heat-inactivated Streptococcus pneumoniae augments circadian clock gene expression in zebrafish cells

The circadian clock is a cell-autonomous process that regulates daily internal rhythms by interacting with environmental signals. Reports across species show that infection can alter the expression of circadian genes; however, in teleosts, these effects are influenced by light exposure. Currently, no reports analyze the direct effects of bacterial exposure on the zebrafish clock. Using zebrafish Z3 cells, we demonstrate that exposure to heat-killed Streptococcus pneumoniae (HK-Spn) augments the expression of core repressive factors in a light- and time-dependent manner. In constant darkness, HK-Spn highly upregulated cry1a, per3, and per1b expression. In the presence of light, HK-Spn exposure rapidly and strongly upregulated per2 and cry1a, and this was proportionally increased with light intensity. The combinatorial effect of light and HK-Spn on per2 and cry1a was not duplicated with H2O2, a known byproduct of light exposure. However, the ROS inhibitor N-acetyl cysteine was sufficient to block HK-Spn augmentation of per2, cry1a, and per3. These findings demonstrate that exposure to an inactive bacteria influences the expression of zebrafish clock genes under different light conditions.

Influence of Acute Inflammation on the Expression of Clock Genes in the Ovine Pars Tuberalis Under Different Photoperiodic Conditions

The pars tuberalis (PT) plays an important role in the photoperiodic regulation of the secretory activity of the pituitary gland. Additionally, PT secretory activity may be influenced by the animal's immune status. The melatonin signal processing in PT cells occurs through the presence of melatonin receptors and the expression of molecular clock genes. This study aimed to define the effects of acute inflammation induced by intravenous administration of lipopolysaccharide (LPS) on the expression of clock genes in the PT of ewes under different photoperiodic conditions. Two analogous experiments were conducted in different photoperiods: short-day and long-day. Both experiments included 24 sheep divided into two groups: day (n = 12) and night (n = 12), further subdivided into a control group (n = 6) and a group treated with LPS (n = 6) at a dose of 400 ng/kg. Under short-day conditions, the expression of clock circadian regulator, basic helix-loop-helix ARNT like 1, cryptochrome circadian regulator (CRY) 1, 2, and casein kinase 1 epsilon genes was lower during inflammation. LPS injection increased expression of the period circadian regulator 1 gene during the night. Under long-day conditions, CRY1 mRNA level was lower during the night, while diurnal CRY2 mRNA expression was decreased after LPS injection. Our results showed that inflammation disturbed the expression of molecular clock genes in the PT; however, this influence was partly dependent on photoperiod conditions.

Circadian gene CLOCK accelerates atherosclerosis by promoting endothelial autophagy

Atherosclerosis (AS) is a chronic inflammatory disease which gives rise to life-threatening complications like ischemic stroke. Rupture of carotid atherosclerotic plaque is the main cause of ischemic stroke. Emerging evidence has demonstrated that disturbed circadian rhythms could accelerate the progression of atherosclerosis by regulating endothelial function. Moreover, our previous study implicated the circadian gene circadian locomotor output cycles kaput (CLOCK) in the pathogenesis of unstable plaques. In this study, we explored the underlying mechanism that CLOCK mediates endothelial cell autophagy involved in the progression of AS. Circadian and autophagy gene expression was analyzed in the GSE41571 dataset and human carotid atherosclerotic plaque samples. Then we used ox-LDL to treat HUVECs, and analyzed CLOCK and autophagy gene in endothelial cells. Besides that, we comprehensively analyzed in vivo experiments to explore the function of CLOCK in autophagy and atherosclerosis using different staining including HE, MT and IF staining. In the dataset and patient samples, CLOCK expression and autophagy were decreased in the unstable plaque group compared with the stable group. Decreased Beclin1, ATG5, LC3, and CLOCK were also observed in HUVECs under oxidative stress condition which also enhances cell proliferation. In vivo, we also found decreasing level of CLOCK, Beclin1, LC3 and ATG5 in ApoE-/- mice compared with WT mice. Silencing of CLOCK in ApoE-/- mice may further aggravate atherosclerosis including decreased cap thickness and collagens. Our findings implicated that downregulation CLOCK would impair endothelial cell autophagy and accelerate atherosclerotic plaque, which provides a novel strategy for treatment of progression in AS.

Non-Hereditary Obesity Type Networks and New Drug Targets: An In Silico Approach

Obesity, a chronic, preventable disease, has significant comorbidities that are associated with a great human and financial cost for society. The aim of the present work is to reconstruct the interactomes of non-hereditary obesity to highlight recent advances of its pathogenesis, and discover potential therapeutic targets. Obesity and biological-clock-related genes and/or gene products were extracted from the biomedical literature databases PubMed, GeneCards and OMIM. Their interactions were investigated using STRING v11.0 (a database of known and predicted physical and indirect associations among genes/proteins), and a high confidence interaction score of >0.7 was set. We also applied virtual screening to discover natural compounds targeting obesity- and circadian-clock-associated proteins. Two updated and comprehensive interactomes, the (a) stress- and (b) inflammation-induced obesidomes involving 85 and 93 gene/gene products of known and/or predicted interactions with an average node degree of 9.41 and 10.8, respectively, were produced. Moreover, 15 of these were common between the two non-hereditary entities, namely, ADIPOQ, ADRB2/3, CCK, CRH, CXCL8, FOS, GCG, GNRH1, IGF1, INS, LEP, MC4R, NPY and POMC, while phelligridin E, a natural product, may function as a potent FOX1-DBD interaction blocker. Molecular networks may contribute to the understanding of the integrated regulation of energy balance/obesity pathogenesis and may associate chronopharmacology schemes with natural products.

Time of day and circadian disruption influence host response and parasite growth in a mouse model of cerebral malaria

Malaria is a disease caused by infection with parasite Plasmodium spp. We studied the circadian regulation of host responses to the parasite, in a mouse model of cerebral malaria. The course of the disease was markedly affected by time of infection, with decreased parasitemia and increased inflammation upon infection in the middle of the night. At this time, there were fewer reticulocytes, which are target cells of the parasites. We next investigated the effects of desynchronization of host clocks on the infection: after 10 weeks of recurrent jet lags, mice showed decreased parasite growth and lack of parasite load rhythmicity, paralleled by a loss of glucose rhythm. Accordingly, disrupting host metabolic rhythms impacted parasite load rhythmicity. In summary, our findings of a circadian modulation of malaria parasite growth and infection shed light on aspects of the disease relevant to human malaria and could contribute to new therapeutic or prophylactic measures.

Crosstalk between circadian clocks and pathogen niche

Circadian rhythms are intrinsic 24-hour oscillations found in nearly all life forms. They orchestrate key physiological and behavioral processes, allowing anticipation and response to daily environmental changes. These rhythms manifest across entire organisms, in various organs, and through intricate molecular feedback loops that govern cellular oscillations. Recent studies describe circadian regulation of pathogens, including parasites, bacteria, viruses, and fungi, some of which have their own circadian rhythms while others are influenced by the rhythmic environment of hosts. Pathogens target specific tissues and organs within the host to optimize their replication. Diverse cellular compositions and the interplay among various cell types create unique microenvironments in different tissues, and distinctive organs have unique circadian biology. Hence, residing pathogens are exposed to cyclic conditions, which can profoundly impact host-pathogen interactions. This review explores the influence of circadian rhythms and mammalian tissue-specific interactions on the dynamics of pathogen-host relationships. Overall, this demonstrates the intricate interplay between the body's internal timekeeping system and its susceptibility to pathogens, which has implications for the future of infectious disease research and treatment.

REV-ERBα negatively regulates NLRP6 transcription and reduces the severity of Salmonella infection in mice

Non-typhoidal Salmonella infection is among the most frequent foodborne diseases threatening human health worldwide. The host circadian clock orchestrates daily rhythms to adapt to environmental changes, including coordinating immune function in response to potential infections. However, the molecular mechanisms underlying the interplay between the circadian clock and the immune system in modulating infection processes are incompletely understood. Here, we demonstrate that NLRP6, a novel nucleotide-oligomerization domain (NOD)-like receptor (NLR) family member highly expressed in the intestine, is closely associated with the differential day-night response to Salmonella infection. The core clock component REV-ERBα negatively regulates NLRP6 transcription, leading to the rhythmic expression of NLRP6 and the secretion of IL-18 in intestinal epithelial cells, playing a crucial role in mediating the differential day-night response to Salmonella infection. Activating REV-ERBα with agonist SR9009 in wild-type mice attenuated the severity of infection by decreasing the NLRP6 level in intestinal epithelial cells. Our findings provide new insights into the association between the host circadian clock and the immune response to enteric infections by revealing the regulation of Salmonella infection via the inhibitory effect of REV-ERBα on NLRP6 transcription. Targeting REV-ERBα to modulate NLRP6 activation may be a potential therapeutic strategy for bacterial infections.

Autonomous circadian rhythms in the human hepatocyte regulate hepatic drug metabolism and inflammatory responses

Critical aspects of physiology and cell function exhibit self-sustained ~24-hour variations termed circadian rhythms. In the liver, circadian rhythms play fundamental roles in maintaining organ homeostasis. Here, we established and characterized an in vitro liver experimental system in which primary human hepatocytes display self-sustained oscillations. By generating gene expression profiles of these hepatocytes over time, we demonstrated that their transcriptional state is dynamic across 24 hours and identified a set of cycling genes with functions related to inflammation, drug metabolism, and energy homeostasis. We designed and tested a treatment protocol to minimize atorvastatin- and acetaminophen-induced hepatotoxicity. Last, we documented circadian-dependent induction of pro-inflammatory cytokines when triggered by LPS, IFN-β, or Plasmodium infection in human hepatocytes. Collectively, our findings emphasize that the phase of the circadian cycle has a robust impact on the efficacy and toxicity of drugs, and we provide a test bed to study the timing and magnitude of inflammatory responses over the course of infection in human liver.

Circadian rhythm regulation in the immune system

Circadian rhythms are a ubiquitous feature in nearly all living organisms, representing oscillatory patterns with a 24-h cycle that are widespread across various physiological processes. Circadian rhythms regulate a multitude of physiological systems, including the immune system. At the molecular level, most immune cells autonomously express clock-regulating genes, which play critical roles in regulating immune cell functions. These functions encompass migration, phagocytic activity, immune cell metabolism (such as mitochondrial structural function and metabolism), signalling pathway activation, inflammatory responses, innate immune recognition, and adaptive immune processes (including vaccine responses and pathogen clearance). The endogenous circadian clock orchestrates multifaceted rhythmicity within the immune system, optimizing immune surveillance and responsiveness; this bears significant implications for maintaining immune homeostasis and resilience against diseases. This work provides an overview of circadian rhythm regulation within the immune system.

Suppression of endoplasmic reticulum stress by 4-PBA enhanced atherosclerotic plaque stability via up-regulating CLOCK expression

Endoplasmic reticulum (ER) stress refers to a condition where the normal functioning of the ER is disrupted due to a variety of cellular stress factors. As a result, there is an accumulation of unfolded and misfolded proteins within the ER. Numerous studies have shown that ER stress can exacerbate inflammatory reactions and contribute to the development of various inflammatory diseases. However, the role of ER stress in the stability of atherosclerotic plaques remains poorly understood. In this study, we aimed to explore the potential impact of a specific ER stress inhibitor known as 4-phenyl butyric acid (4-PBA) on atherosclerosis in mice. The mice were fed a high-fat diet, and treatment with 4-PBA significantly improved the stability of the atherosclerotic plaques. This was evidenced by a reduction in oxidative stress and an increase in circadian locomotor output cycles kaput (CLOCK) protein and mRNA expression within the plaques. Additionally, 4-PBA reduced the expression of ER stress-related proteins and decreased apoptosis in the atherosclerotic plaques. In vitro investigation, we observed the effect of 4-PBA on vascular smooth muscle cells (VSMCs) that were exposed to oxidized low-density lipoprotein (ox-LDL), a significant contributor to the development of atherosclerosis. 4-PBA reduced reactive oxygen species (ROS) production and attenuated apoptosis, GRP78 and CHOP protein expression in ox-LDL-Induced VSMCs via up-regulating CLOCK expression. However, when the short hairpin RNA against CLOCK (sh-CLOCK) was introduced to the VSMCs, the protective effect of 4-PBA was abolished. This suggests that the up-regulation of CLOCK expression is crucial for the beneficial effects of 4-PBA on atherosclerotic plaque stability. This finding suggests that targeting ER stress and modulating CLOCK protein levels might be a promising way to enhance the stability of atherosclerotic plaques.

Using chronobiology-based second-generation artificial intelligence digital system for overcoming antimicrobial drug resistance in chronic infections

Antimicrobial resistance results from the widespread use of antimicrobial agents and is a significant obstacle to the effectiveness of these agents. Numerous methods are used to overcome this problem with moderate success. Besides efforts of antimicrobial stewards, several artificial intelligence (AI)-based technologies are being explored for preventing resistance development. These first-generation systems mainly focus on improving patients' adherence. Chronobiology is inherent in all biological systems. Host response to infections and pathogens activity are assumed to be affected by the circadian clock. This paper describes the problem of antimicrobial resistance and reviews some of the current AI technologies. We present the establishment of a second-generation AI chronobiology-based approach to help in preventing further resistance and possibly overcome existing resistance. An algorithm-controlled regimen that improves the long-term effectiveness of antimicrobial agents is being developed based on the implementation of variability in dosing and drug administration times. The method provides a means for ensuring a sustainable response and improved outcomes. Ongoing clinical trials determine the effectiveness of this second-generation system in chronic infections. Data from these studies are expected to shed light on a new aspect of resistance mechanisms and suggest methods for overcoming them.IMPORTANCE SECTIONThe paper presents the establishment of a second-generation AI chronobiology-based approach to help in preventing further resistance and possibly overcome existing resistance.Key messagesAntimicrobial resistance results from the widespread use of antimicrobial agents and is a significant obstacle to the effectiveness of these agents.We present the establishment of a second-generation AI chronobiology-based approach to help in preventing further resistance and possibly overcome existing resistance.

New Awareness of the Interplay Between the Gut Microbiota and Circadian Rhythms

Circadian rhythms influence various aspects of the biology and physiology of the host, such as food intake and sleep/wake cycles. In recent years, an increasing amount of genetic and epidemiological data has shown that the light/dark cycle is the main cue that regulates circadian rhythms. Other factors, including sleep/wake cycles and food intake, have necessary effects on the composition and rhythms of the gut microbiota. Interestingly, the gut microbiota can affect the circadian rhythm of hosts in turn through contact-dependent and contact-independent mechanisms. Furthermore, the gut microbiota has been shown to regulate the sleep/wake cycles through gut-brain-microbiota interaction. In addition to diabetes, the gut microbiota can also intervene in the progression of neuro- degenerative diseases through the gut-brain-microbiota interaction, and also in other diseases such as hypertension and rheumatoid arthritis, where it is thought to have a spare therapeutic potential. Even though fecal microbiota transplantation has good potential for treating many diseases, the risk of spreading intestinal pathogens should not be ignored.

Chikungunya Virus, Metabolism, and Circadian Rhythmicity Interplay in Phagocytic Cells

Chikungunya virus (CHIKV) is transmitted to humans by mosquitoes of the genus Aedes, causing the chikungunya fever disease, associated with inflammation and severe articular incapacitating pain. There has been a worldwide reemergence of chikungunya and the number of cases increased to 271,006 in 2022 in the Americas alone. The replication of CHIKV takes place in several cell types, including phagocytic cells. Monocytes and macrophages are susceptible to infection by CHIKV; at the same time, they provide protection as components of the innate immune system. However, in host-pathogen interactions, CHIKV might have the ability to alter the function of immune cells, partly by rewiring the tricarboxylic acid cycle. Some viral evasion mechanisms depend on the metabolic reprogramming of immune cells, and the cell metabolism is intertwined with circadian rhythmicity; thus, a circadian immunovirometabolism axis may influence viral pathogenicity. Therefore, analyzing the interplay between viral infection, circadian rhythmicity, and cellular metabolic reprogramming in human macrophages could shed some light on the new field of immunovirometabolism and eventually contribute to the development of novel drugs and therapeutic approaches based on circadian rhythmicity and metabolic reprogramming.

Acute HDM exposure shows time-of-day and sex-based differences in the severity of lung inflammation and circadian clock disruption

Background

Asthma is a chronic inflammatory disease that shows a time-of-day response to variations in symptoms/severity. However, how the lung circadian clock influences time-of-day response and sex-based differences in house dust mite (HDM)-induced airway inflammation and remodeling has not been thoroughly investigated.

Objective

We sought to determine whether acute HDM exposure in wild-type mice shows time-of-day response and sex-based differences in allergic airway inflammation and circadian clock disruption in the lungs.

Methods

Wild-type (C57BL/6J) and Rev-erbα knockout (KO) mice were exposed to either PBS or HDM (for 10 days) intranasally at Zeitgeber time (ZT0: 6 am; ZT12: 6 pm) and euthanized 48 hours after the last exposure. Acute HDM-induced time-of-day response and sex-based differences in lung inflammation, gated cytokines/chemokines, humoral and hormonal responses, and circadian clock gene expression were analyzed.

Results

Acute HDM-exposed mice showed a time-of-day response and sex-based differences in exaggerated lung inflammation (inflammatory eosinophils and interstitial macrophages) at ZT12 when compared with ZT0. HDM-exposed female mice showed increased inflammatory response at ZT12, but HDM-exposed male mice showed comparatively lower inflammation with no time-of-day response. HDM-exposed female and male mice showed augmented IgE levels at ZT12 when compared with ZT0. Myeloid innate immunity panel, cytokines/chemokines, and mucin genes showed a time-of-day gating response at ZT0 and ZT12 in the HDM group. In addition, HDM exposure altered the expression of circadian clock genes in the lung, which was evident in female mice at ZT12. Overall, female mice showed significant time-of-day responses to all these parameters compared with male mice. Rev-erbα KO mice exposed to acute HDM showed exaggerated lung inflammation associated with increased IgE and proinflammatory cytokines in bronchoalveolar lavage fluid. Interestingly, HDM exposure causes reduced expression of clock genes in flow-sorted resident eosinophils but not alveolar macrophages. Acute HDM exposure reduced the nocturnal locomotor activity in mice 5 days post-HDM exposure until day 10.

Conclusions

This study shows a time-of-day response to acute HDM exposure and sex-based differences in the severity of lung inflammation and humoral immune response associated with circadian clock disruption. Our findings support the use of separate female and male mice cohorts for preclinical studies to understand the molecular heterogeneity in asthma pathophysiology.

Effects of light and circadian clock on the antiviral immune response in zebrafish

The circadian clock mechanism, which is evolutionarily conserved across various organisms, plays a crucial role in synchronizing physiological responses to external conditions, primarily in response to light availability. By maintaining homeostasis of biological processes and behavior, the circadian clock serves as a key regulator. This biological mechanism also coordinates diurnal oscillations of the immune response during infections. However there is limited information available regarding the influence of circadian oscillation on immune regulation, especially in lower vertebrates like teleost fish. Therefore, the present study aimed to investigate the effects of light and the timing of infection induction on the antiviral immune response in zebrafish. To explore the relationship between the timing of infection and the response activated by viral pathogens, we used a zebrafish model infected with tilapia lake virus (TiLV). Our findings demonstrated that light availability significantly affects the antiviral immune response and the functioning of the molecular clock mechanism during TiLV infection. This is evident through alterations in the expression of major core clock genes and the regulation of TiLV replication and type I IFN pathway genes in the kidney of fish maintained under LD (light-dark) conditions compared to constant darkness (DD) conditions. Moreover, infection induced during the light phase of the LD cycle, in contrast to nocturnal infection, also exhibited similar effects on the expression of genes associated with the antiviral response. This study indicates a more effective mechanism of the zebrafish antiviral response during light exposure, which inherently involves modification of the expression of key components of the molecular circadian clock. It suggests that the zebrafish antiviral response to infection is regulated by both light and the circadian clock.

Chronodisruption of the acute inflammatory response by night lighting in rats

Daily oscillations are present in many aspects of the immune system, including responsiveness to infections, allowing temporal alignment of defence mechanisms with the external environment. Our study addresses whether compromised circadian timing function by dim artificial light at night (ALAN) impacts the time dependency of the acute inflammatory response in a rat model of lipopolysaccharide (LPS)-induced inflammation. After 2 weeks of exposure to low-intensity ALAN (~2 lx) or a standard light/dark cycle, male rats were challenged with LPS during either the day or the night. Dim ALAN attenuated the anorectic response when rats were stimulated during their early light phase. Next, ALAN suppressed daily variability in inflammatory changes in blood leukocyte numbers and increased the daytime sensitivity of neutrophils to the priming effects of LPS on oxidative burst. An altered renal inflammatory response in ALAN-exposed rats was manifested by stimulated T-cell infiltration into the kidney upon night-time LPS injection and the modified rhythmic response of genes involved in inflammatory pathways. Moreover, ALAN disturbed steady-state oscillations of the renal molecular clock and eliminated the inflammatory responsiveness of Rev-erbα. Altogether, dim ALAN impaired time-of-day-dependent sensitivity of inflammatory processes, pointing out a causal mechanism between light pollution and negative health effects.

Circadian Regulation of Macrophages and Osteoclasts in Rheumatoid Arthritis

Rheumatoid arthritis (RA) represents one of the best examples of circadian fluctuations in disease severity. Patients with RA experience stiffness, pain, and swelling in afflicted joints in the early morning, which tends to become milder toward the afternoon. This has been primarily explained by the higher blood levels of pro-inflammatory hormones and cytokines, such as melatonin, TNFα, IL-1, and IL-6, in the early morning than in the afternoon as well as insufficient levels of anti-inflammatory cortisol, which rises later in the morning. Clinical importance of the circadian regulation of RA symptoms has been demonstrated by the effectiveness of time-of-day-dependent delivery of therapeutic agents in chronotherapy. The primary inflammatory site in RA is the synovium, where increased macrophages, T cells, and synovial fibroblasts play central roles by secreting pro-inflammatory cytokines, chemokines, and enzymes to stimulate each other, additional immune cells, and osteoclasts, ultimately leading to cartilage and bone erosion. Among these central players, macrophages have been one of the prime targets for the study of the link between circadian rhythms and inflammatory activities. Gene knockout experiments of various core circadian regulators have established that disruption of any core circadian regulators results in hyper- or hypoactivation of inflammatory responses by macrophages when challenged by lipopolysaccharide and bacteria. Although these stimulations are not directly linked to RA etiology, these findings serve as a foundation for further study by providing proof of principle. On the other hand, circadian regulation of osteoclasts, downstream effectors of macrophages, remain under-explored. Nonetheless, circadian expression of the inducers of osteoclastogenesis, such as TNFα, IL-1, and IL-6, as well as the knockout phenotypes of circadian regulators in osteoclasts suggest the significance of the circadian control of osteoclast activity in the pathogenesis of RA. More detailed mechanistic understanding of the circadian regulation of macrophages and osteoclasts in the afflicted joints could add novel local therapeutic options for RA.

Murine Models of Salmonella Infection

The pathogen Salmonella enterica encompasses a range of bacterial serovars that cause intestinal inflammation and systemic infections in humans. Mice are a widely used infection model due to their relative simplicity and versatility. Here, we provide standardized protocols for culturing the prolific zoonotic pathogen S. enterica serovar Typhimurium for intragastric inoculation of mice to model colitis or systemic dissemination, along with techniques for direct extraintestinal infection. Furthermore, we present procedures for quantifying pathogen burden and for characterizing the immune response by analyzing tissue pathology, inflammatory markers, and immune cells from intestinal tissues. © 2023 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Murine colitis model utilizing oral streptomycin pretreatment and oral S. Typhimurium administration Basic Protocol 2: Intraperitoneal injection of S. Typhimurium for modeling extraintestinal infection Support Protocol 1: Preparation of S. Typhimurium inoculum Support Protocol 2: Preparation of mixed S. Typhimurium inoculum for competitive infection Basic Protocol 3: Assessment of S. Typhimurium burden Support Protocol 3: Preservation and pathological assessment of S. Typhimurium-infected tissues Support Protocol 4: Measurement of inflammatory marker expression in intestinal tissues by qPCR Support Protocol 5: Preparation of intestinal content for inflammatory marker quantification by ELISA Support Protocol 6: Immune cell isolation from Salmonella-infected intestinal tissues.

Genital tract microbiome dynamics are associated with time of Chlamydia infection in mice

We have previously shown that the time of Chlamydia infection was crucial in determining the chlamydial infectivity and pathogenesis. This study aims to determine whether the time of Chlamydia infection affects the genital tract microbiome. This study analyzed mice vaginal, uterine, and ovary/oviduct microbiome with and without Chlamydia infection. The mice were infected with Chlamydia at either 10:00 am (ZT3) or 10:00 pm (ZT15). The results showed that mice infected at ZT3 had higher Chlamydia infectivity than those infected at ZT15. There was more variation in the compositional complexity of the vaginal microbiome (alpha diversity) of mice infected at ZT3 than those mice infected at ZT15 throughout the infection within each treatment group, with both Shannon and Simpson diversity index values decreased over time. The analysis of samples collected four weeks post-infection showed that there were significant taxonomical differences (beta diversity) between different parts of the genital tract-vagina, uterus, and ovary/oviduct-and this difference was associated with the time of infection. Firmicutes and Proteobacteria were the most abundant phyla within the microbiome in all three genital tract regions for all the samples collected during this experiment. Additionally, Firmicutes was the dominant phylum in the uterine microbiome of ZT3 Chlamydia infected mice. The results show that the time of infection is associated with the microbial dynamics in the genital tract. And this association is more robust in the upper genital tract than in the vagina. This result implies that more emphasis should be placed on understanding the changes in the microbial dynamics of the upper genital tract over the course of infection.

Circadian mechanisms in adipose tissue bioenergetics and plasticity

The circadian clock plays an essential role in coordinating feeding and metabolic rhythms with the light/dark cycle. Disruption of clocks is associated with increased adiposity and metabolic disorders, whereas aligning feeding time with cell-autonomous rhythms in metabolism improves health. Here, we provide a comprehensive overview of recent literature in adipose tissue biology as well as our understanding of molecular mechanisms underlying the circadian regulation of transcription, metabolism, and inflammation in adipose tissue. We highlight recent efforts to uncover the mechanistic links between clocks and adipocyte metabolism, as well as its application to dietary and behavioral interventions to improve health and mitigate obesity.

Ventilator-induced Lung Injury Is Modulated by the Circadian Clock

Rationale: Mechanical ventilation (MV) is life-saving but may evoke ventilator-induced lung injury (VILI). Objectives: To explore how the circadian clock modulates severity of murine VILI via the core clock component BMAL1 (basic helix-loop-helix ARNT like 1) in myeloid cells. Methods: Myeloid cell BMAL1-deficient (LysM (lysozyme 2 promoter/enhancer driving cre recombinase expression)Bmal1-/-) or wild-type control (LysMBmal1+/+) mice were subjected to 4 hours MV (34 ml/kg body weight) to induce lung injury. Ventilation was initiated at dawn or dusk or in complete darkness (circadian time [CT] 0 or CT12) to determine diurnal and circadian effects. Lung injury was quantified by lung function, pulmonary permeability, blood gas analysis, neutrophil recruitment, inflammatory markers, and histology. Neutrophil activation and oxidative burst were analyzed ex vivo. Measurements and Main Results: In diurnal experiments, mice ventilated at dawn exhibited higher permeability and neutrophil recruitment compared with dusk. Experiments at CT showed deterioration of pulmonary function, worsening of oxygenation, and increased mortality at CT0 compared with CT12. Wild-type neutrophils isolated at dawn showed higher activation and reactive oxygen species production compared with dusk, whereas these day-night differences were dampened in LysMBmal1-/- neutrophils. In LysMBmal1-/- mice, circadian variations in VILI severity were dampened and VILI-induced mortality at CT0 was reduced compared with LysMBmal1+/+ mice. Conclusions: Inflammatory response and lung barrier dysfunction upon MV exhibit diurnal variations, regulated by the circadian clock. LysMBmal1-/- mice are less susceptible to ventilation-induced pathology and lack circadian variation of severity compared with LysMBmal1+/+ mice. Our data suggest that the internal clock in myeloid cells is an important modulator of VILI.

Circadian clocks in health and disease: Dissecting the roles of the biological pacemaker in cancer

In modern society, there is a growing population affected by circadian clock disruption through night shift work, artificial light-at-night exposure, and erratic eating patterns. Concurrently, the rate of cancer incidence in individuals under the age of 50 is increasing at an alarming rate, and though the precise risk factors remain undefined, the potential links between circadian clock deregulation and young-onset cancers is compelling. To explore the complex biological functions of the clock, this review will first provide a framework for the mammalian circadian clock in regulating critical cellular processes including cell cycle control, DNA damage response, DNA repair, and immunity under conditions of physiological homeostasis. Additionally, this review will deconvolute the role of the circadian clock in cancer, citing divergent evidence suggesting tissue-specific roles of the biological pacemaker in cancer types such as breast, lung, colorectal, and hepatocellular carcinoma. Recent evidence has emerged regarding the role of the clock in the intestinal epithelium, as well as new insights into how genetic and environmental disruption of the clock is linked with colorectal cancer, and the molecular underpinnings of these findings will be discussed. To place these findings within a context and framework that can be applied towards human health, a focus on how the circadian clock can be leveraged for cancer prevention and chronomedicine-based therapies will be outlined.

Microbial circadian clocks: host-microbe interplay in diel cycles

BACKGROUND

Circadian rhythms, observed across all domains of life, enable organisms to anticipate and prepare for diel changes in environmental conditions. In bacteria, a circadian clock mechanism has only been characterized in cyanobacteria to date. These clocks regulate cyclical patterns of gene expression and metabolism which contribute to the success of cyanobacteria in their natural environments. The potential impact of self-generated circadian rhythms in other bacterial and microbial populations has motivated extensive research to identify novel circadian clocks.

MAIN TEXT

Daily oscillations in microbial community composition and function have been observed in ocean ecosystems and in symbioses. These oscillations are influenced by abiotic factors such as light and the availability of nutrients. In the ocean ecosystems and in some marine symbioses, oscillations are largely controlled by light-dark cycles. In gut systems, the influx of nutrients after host feeding drastically alters the composition and function of the gut microbiota. Conversely, the gut microbiota can influence the host circadian rhythm by a variety of mechanisms including through interacting with the host immune system. The intricate and complex relationship between the microbiota and their host makes it challenging to disentangle host behaviors from bacterial circadian rhythms and clock mechanisms that might govern the daily oscillations observed in these microbial populations.

CONCLUSIONS

While the ability to anticipate the cyclical behaviors of their host would likely be enhanced by a self-sustained circadian rhythm, more evidence and further studies are needed to confirm whether host-associated heterotrophic bacteria possess such systems. In addition, the mechanisms by which heterotrophic bacteria might respond to diel cycles in environmental conditions has yet to be uncovered.

Circadian Disruption and Mental Health: The Chronotherapeutic Potential of Microbiome-Based and Dietary Strategies

Mental illness is alarmingly on the rise, and circadian disruptions linked to a modern lifestyle may largely explain this trend. Impaired circadian rhythms are associated with mental disorders. The evening chronotype, which is linked to circadian misalignment, is a risk factor for severe psychiatric symptoms and psychiatric metabolic comorbidities. Resynchronization of circadian rhythms commonly improves psychiatric symptoms. Furthermore, evidence indicates that preventing circadian misalignment may help reduce the risk of psychiatric disorders and the impact of neuro-immuno-metabolic disturbances in psychiatry. The gut microbiota exhibits diurnal rhythmicity, as largely governed by meal timing, which regulates the host's circadian rhythms. Temporal circadian regulation of feeding has emerged as a promising chronotherapeutic strategy to prevent and/or help with the treatment of mental illnesses, largely through the modulation of gut microbiota. Here, we provide an overview of the link between circadian disruption and mental illness. We summarize the connection between gut microbiota and circadian rhythms, supporting the idea that gut microbiota modulation may aid in preventing circadian misalignment and in the resynchronization of disrupted circadian rhythms. We describe diurnal microbiome rhythmicity and its related factors, highlighting the role of meal timing. Lastly, we emphasize the necessity and rationale for further research to develop effective and safe microbiome and dietary strategies based on chrononutrition to combat mental illness.

The circadian control of tryptophan metabolism regulates the host response to pulmonary fungal infections

The environmental light/dark cycle has left its mark on the body's physiological functions to condition not only our inner biology, but also the interaction with external cues. In this scenario, the circadian regulation of the immune response has emerged as a critical factor in defining the host-pathogen interaction and the identification of the underlying circuitry represents a prerequisite for the development of circadian-based therapeutic strategies. The possibility to track down the circadian regulation of the immune response to a metabolic pathway would represent a unique opportunity in this direction. Herein, we show that the metabolism of the essential amino acid tryptophan, involved in the regulation of fundamental processes in mammals, is regulated in a circadian manner in both murine and human cells and in mouse tissues. By resorting to a murine model of pulmonary infection with the opportunistic fungus Aspergillus fumigatus, we showed that the circadian oscillation in the lung of the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO)1, generating the immunoregulatory kynurenine, resulted in diurnal changes in the immune response and the outcome of fungal infection. In addition, the circadian regulation of IDO1 drives such diurnal changes in a pre-clinical model of cystic fibrosis (CF), an autosomal recessive disease characterized by progressive lung function decline and recurrent infections, thus acquiring considerable clinical relevance. Our results demonstrate that the circadian rhythm at the intersection between metabolism and immune response underlies the diurnal changes in host-fungal interaction, thus paving the way for a circadian-based antimicrobial therapy.

Regulatory Effects of Clock and Bmal1 on Circadian Rhythmic TLR Expression

Circadian locomotor output cycles kaput (Clock) and brain and muscle ARNT-like 1 (Bmal1) are two core circadian clock genes. They form a heterodimer that can bind to the E-box element in the promoters of Period circadian protein (Per) and Cryptochrome (Cry) genes, thereby inducing the rhythmic expression of circadian clock control genes. Toll-like receptors (TLRs) are type I transmembrane proteins belonging to the pattern recognition receptor (PRR) family. They can recognize a variety of pathogens and play an important role in innate immunity and adaptive immune responses. Recent studies have found that the circadian clock is closely associated with the immune system. TLRs have a certain correlation with the circadian rhythms; Bmal1 seems to be the central mediator connecting the circadian clock and the immune system. Research on Bmal1 and TLRs has made some progress, but the specific relationship between TLRs and Bmal1 remains unclear. Understanding the relationship between TLRs and Clock/Bmal1 genes is increasingly important for basic research and clinical treatment.

Circadian orchestration of host and gut microbiota in infection

Circadian rhythms are present in almost every organism and regulate multiple aspects of biological and physiological processes (e.g. metabolism, immune responses, and microbial exposure). There exists a bidirectional circadian interaction between the host and its gut microbiota, and potential circadian orchestration of both host and gut microbiota in response to invading pathogens. In this review, we summarize what is known about these intestinal microbial oscillations and the relationships between host circadian clocks and various infectious agents (bacteria, fungi, parasites, and viruses), and discuss how host circadian clocks prime the immune system to fight pathogen infections as well as the direct effects of circadian clocks on viral activity (e.g. SARS-CoV-2 entry and replication). Finally, we consider strategies employed to realign normal circadian rhythmicity for host health, such as chronotherapy, dietary intervention, good sleep hygiene, and gut microbiota-targeted therapy. We propose that targeting circadian rhythmicity may provide therapeutic opportunities for the treatment of infectious diseases.

Temporal transcriptome reveals that circadian clock is involved in the dynamic regulation of immune response to bacterial infection in Bombyx mori

The circadian clock plays a critical role in the regulation of host immune defense. However, the mechanistic basis for this regulation is largely unknown. Herein, the core clock gene cryptochrome1 (cry1) knockout line in Bombyx mori, an invertebrate animal model, was constructed to obtain the silkworm with dysfunctional molecular clock, and the dynamic regulation of the circadian clock on the immune responsiveness within 24 h of Staphylococcus aureus infection was analyzed. We found that deletion of cry1 decreased viability of silkworms and significantly reduced resistance of larvae to S. aureus. Time series RNA-seq analysis identified thousands of rhythmically expressed genes, including immune response genes, in the larval immune tissue, fat bodies. Uninfected cry1 knockout silkworms exhibited expression patterns of rhythmically expressed genes similar to wild-type (WT) silkworms infected with S. aureus. However, cry1 knockout silkworms exhibited a seriously weakened response to S. aureus infection. The immune response peaked at 6 and 24 h after infection, during which "transcription storms" occurred, and the expression levels of the immune response genes, PGRP and antimicrobial peptides (AMPs), were significantly upregulated in WT. In contrast, cry1 knockout did not effectively activate Toll, Imd, or NF-κB signaling pathways during the immune adjustment period from 12 to 18 h after infection, resulting in failure to initiate the immune responsiveness peak at 24 h after infection. This may be related to inhibited silkworm fat body energy metabolism. These results demonstrated the dynamic regulation of circadian clock on silkworm immune response to bacterial infection and provided important insights into host antimicrobial defense mechanisms.

Immunological and inflammatory effects of infectious diseases in circadian rhythm disruption and future therapeutic directions

BACKGROUND

Circadian rhythm is characterised by daily variations in biological activity to align with the light and dark cycle. These diurnal variations, in turn, influence physiological functions such as blood pressure, temperature, and sleep-wake cycle. Though it is well established that the circadian pathway is linked to pro-inflammatory responses and circulating immune cells, its association with infectious diseases is widely unknown.

OBJECTIVE

This comprehensive review aims to describe the association between circadian rhythm and host immune response to various kinds of infection.

METHODS

We conducted a literature search in databases Pubmed/Medline and Science direct. Our paper includes a comprehensive analysis of findings from articles in English which was related to our hypothesis.

FINDINGS

Molecular clocks determine circadian rhythm disruption in response to infection, influencing the host's response toward infection. Moreover, there is a complex interplay with intrinsic oscillators of pathogens and the influence of specific infectious processes on the CLOCK: BMAL1 pathway. Such mechanisms vary for bacterial and viral infections, both well studied in the literature. However, less is known about the association of parasitic infections and fungal pathogens with circadian rhythm modulation.

CONCLUSION

It is shown that bidirectional relationships exist between circadian rhythm disruption and infectious process, which contains interplay between the host's and pathogens' circadian oscillator, immune response, and the influence of specific infectious. Further studies exploring the modulations of circadian rhythm and immunity can offer novel explanations of different susceptibilities to infection and can lead to therapeutic avenues in circadian immune modulation of infectious diseases.

Ontogeny and function of the circadian clock in intestinal organoids

Circadian rhythms regulate diverse aspects of gastrointestinal physiology ranging from the composition of microbiota to motility. However, development of the intestinal circadian clock and detailed mechanisms regulating circadian physiology of the intestine remain largely unknown. In this report, we show that both pluripotent stem cell-derived human intestinal organoids engrafted into mice and patient-derived human intestinal enteroids possess circadian rhythms and demonstrate circadian phase-dependent necrotic cell death responses to Clostridium difficile toxin B (TcdB). Intriguingly, mouse and human enteroids demonstrate anti-phasic necrotic cell death responses to TcdB. RNA-Seq analysis shows that ~3-10% of the detectable transcripts are rhythmically expressed in mouse and human enteroids. Remarkably, we observe anti-phasic gene expression of Rac1, a small GTPase directly inactivated by TcdB, between mouse and human enteroids, and disruption of Rac1 abolishes clock-dependent necrotic cell death responses. Our findings uncover robust functions of circadian rhythms regulating clock-controlled genes in both mouse and human enteroids governing organism-specific, circadian phase-dependent necrotic cell death responses, and lay a foundation for human organ- and disease-specific investigation of clock functions using human organoids for translational applications.

Circadian Disruption and Consequences on Innate Immunity and Inflammatory Response

Circadian rhythms control almost all aspects of physiology and behavior, allowing temporal synchrony of these processes between each other, as well as with the external environment. In the immune system, daily rhythms of leukocyte functions can determine the strength of the immune response, thereby regulating the efficiency of defense mechanisms to cope with infections or tissue injury. The natural light/dark cycle is the prominent synchronizing agent perceived by the circadian clock, but this role of light is highly compromised by irregular working schedules and unintentional exposure to artificial light at night (ALAN). The primary concern is disrupted circadian control of important physiological processes, underlying potential links to adverse health effects. Here, we first discuss the immune consequences of genetic circadian disruption induced by mutation or deletion of specific clock genes. Next, we evaluate experimental research into the effects of disruptive light/dark regimes, particularly light-phase shifts, dim ALAN, and constant light on the innate immune mechanisms under steady state and acute inflammation, and in the pathogenesis of common lifestyle diseases. We suggest that a better understanding of the mechanisms by which circadian disruption influences immune status can be of importance in the search for strategies to minimize the negative consequences of chronodisruption on health.

Chronovaccination: Harnessing circadian rhythms to optimize immunisation strategies

Vaccination, as a public health measure, offers effective protection of populations against infectious diseases. Optimising vaccination efficacy, particularly for higher-risk individuals, like the elderly whose immunocompromised state can prevent the development of robust vaccine responses, is vital. It is now clear that 24-hour circadian rhythms, which govern virtually all aspects of physiology, can generate oscillations in immunological responses. Consequently, vaccine efficacy may depend critically on the time of day of administration(s), including for Covid-19, current vaccines, and any future diseases or pandemics. Published clinical vaccine trials exploring diurnal immune variations suggest this approach could represent a powerful adjunct strategy for optimising immunisation, but important questions remain to be addressed. This review explores the latest insights into diurnal immune variation and the outcomes of circadian timing of vaccination or 'chronovaccination'.

Circadian rhythms mediate infection risk in Daphnia dentifera

Biological rhythms mediate important within-host processes such as metabolism, immunity, and behavior which are often linked to combating disease exposure. For many hosts, exposure to pathogens occurs while feeding. However, the link between feeding rhythms and infection risk is unclear because feeding behavior is tightly coupled with immune and metabolic processes which may decrease susceptibility to infection. Here, we use the Daphnia dentifera-Metschnikowia bicuspidata host-pathogen system to determine how rhythms in feeding rate and immune function mediate infection risk. The host is known to have a nocturnal circadian rhythm in feeding rate, yet we found that they do not exhibit a circadian rhythm in phenoloxidase activity. We found that the time of day when individuals are exposed to pathogens affects the probability of infection with higher infection prevalence at night, indicating that infection risk is driven by a host's circadian rhythm in feeding behavior. These results suggest that the natural circadian rhythm of the host should be considered when addressing epidemiological dynamics.

New Insights into the Diurnal Rhythmicity of Gut Microbiota and Its Crosstalk with Host Circadian Rhythm

Unlike the strictly hierarchical organization in the circadian clock system, the gut microbiota rhythmicity has a more complex multilayer network of all taxonomic levels of microbial taxa and their metabolites. However, it is worth noting that the functionality of the gut microbiota rhythmicity is highly dependent on the host circadian clock and host physiological status. Here, we discussed the diurnal rhythmicity of the gut microbiota; its crucial role in host physiology, health, and metabolism; and the crosstalk between the gut microbial rhythmicity and host circadian rhythm. This knowledge lays the foundation for the development of chronotherapies targeting the gut microbiota. However, the formation mechanism, its beneficial effects on the host of gut microbial rhythmicity, and the dynamic microbial-host crosstalk are not yet clear and warrant further research.

Diurnal Differences in Intracellular Replication Within Splenic Macrophages Correlates With the Outcome of Pneumococcal Infection

Circadian rhythms affect the progression and severity of bacterial infections including those caused by Streptococcus pneumoniae, but the mechanisms responsible for this phenomenon remain largely elusive. Following advances in our understanding of the role of replication of S. pneumoniae within splenic macrophages, we sought to investigate whether events within the spleen correlate with differential outcomes of invasive pneumococcal infection. Utilising murine invasive pneumococcal disease (IPD) models, here we report that infection during the murine active phase (zeitgeber time 15; 15h after start of light cycle, 3h after start of dark cycle) resulted in significantly faster onset of septicaemia compared to rest phase (zeitgeber time 3; 3h after start of light cycle) infection. This correlated with significantly higher pneumococcal burden within the spleen of active phase-infected mice at early time points compared to rest phase-infected mice. Whole-section confocal microscopy analysis of these spleens revealed that the number of pneumococci is significantly higher exclusively within marginal zone metallophilic macrophages (MMMs) known to allow intracellular pneumococcal replication as a prerequisite step to the onset of septicaemia. Pneumococcal clusters within MMMs were more abundant and increased in size over time in active phase-infected mice compared to those in rest phase-infected mice which decreased in size and were present in a lower percentage of MMMs. This phenomenon preceded significantly higher levels of bacteraemia alongside serum IL-6 and TNF-α concentrations in active phase-infected mice following re-seeding of pneumococci into the blood. These data greatly advance our fundamental knowledge of pneumococcal infection by linking susceptibility to invasive pneumococcal infection to variation in the propensity of MMMs to allow persistence and replication of phagocytosed bacteria. These findings also outline a somewhat rare scenario whereby the active phase of an organism’s circadian cycle plays a seemingly counterproductive role in the control of invasive infection.

Macrophage circadian rhythms are differentially affected based on stimuli

Macrophages are white blood cells that play disparate roles in homeostasis and immune responses. They can reprogram their phenotypes to pro-inflammatory (M1) or anti-inflammatory (M2) states in response to their environment. About 8-15% of the macrophage transcriptome has circadian oscillations, including genes closely related to their functioning. As circadian rhythms are associated with cellular phenotypes, we hypothesized that polarization of macrophages to opposing subtypes might differently affect their circadian rhythms. We tracked circadian rhythms in RAW 264.7 macrophages using luminescent reporters. Cells were stably transfected with Bmal1:luc and Per2:luc reporters, representing positive and negative components of the molecular clock. Strength of rhythmicity, periods and amplitudes of time series were assessed using multiple approaches. M1 polarization decreased amplitudes and rhythmicities of Bmal1:luc and Per2:luc, but did not significantly affect periods, while M2 polarization increased periods but caused no substantial alterations to amplitudes or rhythmicity. As macrophage phenotypes are also altered in the presence of cancer cells, we tested circadian effects of conditioned media from mouse breast cancer cells. Media from highly aggressive 4T1 cells caused loss of rhythmicity, while media from less aggressive EMT6 cells yielded no changes. As macrophages play roles in tumors, and oncogenic features are associated with circadian rhythms, we tested whether conditioned media from macrophages could alter circadian rhythms of cancer cells. Conditioned media from RAW 264.7 cells resulted in lower rhythmicities and periods, but higher amplitudes in human osteosarcoma, U2OS-Per2:luc cells. We show that phenotypic changes in macrophages result in altered circadian characteristics and suggest that there is an association between circadian rhythms and macrophage polarization state. Additionally, our data demonstrate that macrophages treated with breast cancer-conditioned media have circadian phenotypes similar to those of the M1 subtype, and cancer cells treated with macrophage-conditioned media have circadian alterations, providing insight to another level of cross-talk between macrophages and cancer.

The circadian immune system

The immune system is highly time-of-day dependent. Pioneering studies in the 1960s were the first to identify immune responses to be under a circadian control. Only in the last decade, however, have the molecular factors governing circadian immune rhythms been identified. These studies have revealed a highly complex picture of the interconnectivity of rhythmicity within immune cells with that of their environment. Here, we provide a global overview of the circadian immune system, focusing on recent advances in the rapidly expanding field of circadian immunology.

CircadiOmics: circadian omic web portal

Circadian rhythms are a foundational aspect of biology. These rhythms are found at the molecular level in every cell of every living organism and they play a fundamental role in homeostasis and a variety of physiological processes. As a result, biomedical research of circadian rhythms continues to expand at a rapid pace. To support this research, CircadiOmics (http://circadiomics.igb.uci.edu/) is the largest annotated repository and analytic web server for high-throughput omic (e.g. transcriptomic, metabolomic, proteomic) circadian time series experimental data. CircadiOmics contains over 290 experiments and over 100 million individual measurements, across >20 unique tissues/organs, and 11 different species. Users are able to visualize and mine these datasets by deriving and comparing periodicity statistics for oscillating molecular species including: period, amplitude, phase, P-value and q-value. These statistics are obtained from BIO_CYCLE and JTK_CYCLE and are intuitively aggregated and displayed for comparison. CircadiOmics is the most up-to-date and cutting-edge web portal for searching and analyzing circadian omic data and is used by researchers around the world.

Deficiency of Lactoferrin aggravates lipopolysaccharide-induced acute inflammation via recruitment macrophage in mice

Lactoferrin (Lf), a multiple functional natural immune protein, is widely distributed in mammalian milk and glandular secretions (bile, saliva, tears and nasal mucosal secretions, etc.). In the previous study, we found that Lf plays an anti-inflammatory and anti-tumorigenesis role in AOM/DSS (azoxymethane/dextran sulfate sodium) induced mouse colitis-associated colon cancer model. Although we found that Lf has anti-inflammatory effects in chronic inflammation, its specific role and mechanisms in acute inflammation have not been clarified. Here, we reported that the expression levels of Lf were significantly increased when the organism was infected by Gram-negative bacteria. We then explored the role and potential mechanism of Lf in lipopolysaccharide (LPS)-induced acute inflammation. In the LPS-induced acute abdominal inflammation model, Lf deficiency aggravated inflammatory response and promoted macrophage chemotaxis to the inflammation site. Lf inhibited macrophage chemotaxis by suppressing the expression of macrophage-associated chemokines Ccl2 and Ccl5. Highly activated NF-κB signaling in Lf^−/− mice was responsible for the high expression of Ccl2 and Ccl5. Our results suggested that the anti-inflammatory effect of Lf offers a new potential treatment for acute inflammatory diseases.

Circadian rhythms in infectious diseases and symbiosis

Timing is everything. Many organisms across the tree of life have evolved timekeeping mechanisms that regulate numerous of their cellular functions to optimize timing by anticipating changes in the environment. The specific environmental changes that are sensed depends on the organism. For animals, plants, and free-living microbes, environmental cues include light/dark cycles, daily temperature fluctuations, among others. In contrast, for a microbe that is never free-living, its rhythmic environment is its host's rhythmic biology. Here, we describe recent research on the interactions between hosts and microbes, from the perspective both of symbiosis as well as infections. In addition to describing the biology of the microbes, we focus specifically on how circadian clocks modulate these host-microbe interactions.

The Oscillating Gut Microbiome and Its Effects on Host Circadian Biology

The microbial community colonizing the gastrointestinal tract, collectively termed the gut microbiota, is an important element of the host organism due to its impact on multiple aspects of health. The digestion of food, secretion of immunostimulatory molecules, performance of chemical reactions in the intestine, and production of metabolites by the microbiota contribute to host homeostasis and disease. Recent discoveries indicate that these major functions are not constantly performed over the course of a day, but rather undergo diurnal fluctuations due to compositional and biogeographical oscillations in the microbiota. Here, we summarize the characteristics and origins of diurnal microbiome rhythms as well as their functional consequences for the circadian biology of the host. We describe the major known pathways of circadian host-microbiome communication and discuss possible implications of altered diurnal microbiome rhythms for human disease. Expected final online publication date for the Annual Review of Nutrition, Volume 42 is August 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.