Chronic inflammatory diseases are triggered by causal stimuli that might occur long before the appearance of the symptoms. Increasing evidence suggests that these stimuli are necessary but not always sufficient to induce the diseases. The murine model of type II collagen emulsified in Freund's incomplete adjuvant (collagen‐induced arthritis) to induce rheumatoid arthritis (RA) follows this pattern as some animals do not develop the chronically inflamed phenotype. Considering that in the immune–pineal axis (IPA) theory adrenal–pineal cross‐talk adjusts early phases of inflammatory processes, we investigated whether differences in IPA activation could explain why some animals are resistant (RES) while others develop RA. We observed a similar increase in 6‐sulfatoxymelatonin (aMT6s) excretion from day 3 to 13 in both RES and RA animals, followed by a significant decrease in RA animals. This pattern of aMT6s excretion positively correlated with plasma corticosterone (CORT) in RES animals. Additionally, RA animals presented a lower aMT6s/CORT ratio than saline‐injected or RES animals. Plasmatic levels of tumour necrosis factor were similar in both groups, but interleukin (IL)‐1β and monocyte chemotactic protein 1 (MCP‐1) levels were lower in RES compared to RA animals. IL‐2 and IL‐4 were decreased in RES animals compared to saline‐injected animals. The aMT6s/CORT ratio inversely correlated with the paw thickness and the inflammatory score (levels of IL‐1β, MCP‐1, IL‐2 and IL‐4 combined). Thus, adrenocortical–pineal positive interaction is an early defence mechanism for avoiding inflammatory chronification. Key points Immune–pineal axis imbalance is observed in early‐phase rheumatoid arthritis development. Only resistant animals present a positive association between adrenal and pineal hormones. The 6‐sulfatoxymelatonin/corticosterone ratio is decreased in animals that develop rheumatoid arthritis. The inflammatory score combining the levels of nocturnal interleukin (IL)‐1β, monocyte chemotactic protein 1, IL‐2 and IL‐4 presents a very strong positive correlation with the size of inflammatory lesion. The 6‐sulfatoxymelatonin/corticosterone ratio presents a strong negative correlation with the inflammatory score and paw oedema size.

Daily oscillation of the immune system follows the central biological clock outputs control such as melatonin produced by the pineal gland. Despite the literature showing that melatonin is also synthesized by macrophages and t lymphocytes, no information is available regarding the temporal profile of the melatonergic system of immune cells and organs in steady-state. Here, the expression of the enzymes arylalkylamine-n-acetyltransferase (AA-nAt), its phosphorylated form (p-AA-nAt) and acetylserotonin-O-methyltransferase (ASMT) were evaluated in phagocytes and T cells of the bone marrow (BM) and spleen. We also determined how the melatonergic system of these cells is modulated by LPS and the cytokine IL-10. The expression of the melatonergic enzymes showed daily rhythms in BM and spleen cells. Melatonin rhythm in the BM, but not in the spleen, follows P-AA-NAT daily variation. In BM cells, LPS and IL10 induced an increase in melatonin levels associated with the increased expressions of P-AA-NAT and ASMT. In spleen cells, LPS induced an increase in the expression of P-AA-NAT but not of melatonin. Conversely, IL10 induced a significant increase in melatonin production associated with increased AA-NAT/P-AA-NAT expressions. In conclusion, BM and spleen cells present different profiles of circadian production of local melatonin and responses to immune signals.Organs and cells of the immune system present daily variations regulated by oscillators present in each cell 1-6 . The intrinsic circadian clock of most of the immune cells imposes circadian expression of downstream genes and functions 4 . This is the case for the expression of pattern-recognition receptors and cytokines, the recruitment to tissues and the phagocytic activity of monocytes, macrophages and microglia 7-10 . Clock genes are also circadian expressed in mouse lymph nodes 10,11 and in B splenic cells 12 , where they control the activity of the cells 4,11 . Besides the intrinsic rhythmicity of cells and organs, there is a central synchronization that relies on neural and hormonal signaling controlled by the central clock in the suprachiasmatic nuclei 13,14 . After a sympathetic input, the darkness hormone melatonin, prolactin and glucocorticoids impose, for example, a daily rhythm in the migration of leukocytes to peripheral tissues 6 .In vertebrates, melatonin is known to be produced in a rhythmic manner by the pineal gland and retina, constitutively by the gastrointestinal tract and on demand by some immunocompetent cells 15,16 . Activated monocytes/macrophages/microglia and T lymphocytes expressed the enzymes arylalkylamine-N-acetyltransferase (AA-NAT), its active phosphorylated form (P-AA-NAT) and acetylserotonin-O-methyltransferase (ASMT) and melatonin 17-21 . In the spleen and in the bone marrow (BM), some works have shown the expression and activity of the melatonergic enzymes 22-24 , however, whether the immune cells of these organs also present www.nature.com/scientificreports www.nature.com/scientificreports/ circadian variations of the melaton...

Sepsis constitutes one of the major causes of death in intensive care units. In sepsis induced by Gram-negative, while LPS initially induces an exacerbated secretion of proinflammatory cytokines leading to endotoxic shock and death resembling a septic shock, it is also capable of inducing refractoriness to subsequent challenge with LPS, a state known as endotoxin tolerance, which is considered the initial step of the immunosuppression found in septic patients. Since we previously demonstrated the importance of glucocorticoids in endotoxin tolerance, the aim of this study was to evaluate the contribution of IL-10 both in the endotoxic shock and in the development of the tolerance and its relationship with glucocorticoids. Our results show that, upon LPS challenge, IL-10 KO mice had an enhanced LPS sensitivity, along with elevated levels of proinflammatory cytokines as TNF-α, IL-12 and IFN-γ, and enhanced tissue damage, despite the high levels of glucocorticoids. This effect may be due, in part, to the higher expression of TNFRs in IL-10 KO mice. Further, the injection of dexamethasone did not protect IL-10 KO mice from a LPS lethal challenge. While tolerance was achieved in the absence of IL-10, it was weaker and the elevated levels of glucocorticoids were not able to reverse the high sensitivity of IL-10 KO mice to LPS. Nevertheless, glucocorticoids would play a pivotal role in the establishment and maintenance of this partial tolerance in IL-10KO mice. Finally, our results show that IL-10 and glucocorticoids could act in a bidirectional way influencing the inflammatory and anti-inflammatory periods.

The pandemic dissemination of the SARS-CoV-2 led, on the one hand, to a worldwide effort to develop mechanistic-based therapeutics and vaccines, and on the other hand, the searching for determining the spreaders and the mechanisms of transmission. Melatonin, a multitask molecule, orchestrates defense responses by allowing the proper mounting, duration, and magnitude of innate immune responses. Melatonin is synthesized on demand by immune-competent cells and constitutively by resident macrophages such as alveolar macrophages. Here we investigated whether the expression of genes relevant to virus invasion and infection varies according to a genic index (MEL-Index) that estimates the capacity of the lung to synthesize melatonin. A COVID-19-Signature composed of 455 genes of 288 human lungs (GTEX, UCSD) was correlated with MEL-Index by Pearson correlation test, gene-set enrichment analysis, and networking tool that integrates the connectivity between the most expressed genes, allowing us to compare the same set of genes under different states. The three independent procedures point to a negative relationship between MEL-Index and SARS-CoV-2 infection. The entry in epithelial AT2 cells should be hampered by a positive correlation TMRPSS2  and a negative correlation with the coding gene for furin, suggesting dysfunctional processing in virus spike. Moreover, MEL-Index also negatively correlates with the genes that codify the proteins of multi-molecular receptor complex CD147, the gateway in macrophages, and other immune cells. In summary, the perspective that lung and respiratory tract melatonin could be a natural protective factor opens new epidemiological and pharmacological perspectives, as high MEL-Index scores could be predictive of asymptomatic carriers, and nasal administrated melatonin could prevent evolution of presymptomatic carriers.