Alvaro Fernández-Montero

RIG‐I and MDA5 are cytosolic RNA sensors that recognize distinct viral ligands in mammals, yet RIG‐I has been independently lost from multiple teleost lineages. How RIG‐I‐deficient vertebrates preserve recognition of RIG‐I‐associated viral RNA remains poorly understood. Here, we show that MDA5 from the large yellow croaker ( Larimichthys crocea , Lc ), a teleost species lacking RIG‐I, recognizes both 5′‐triphosphate dsRNA (5′ppp‐dsRNA), the canonical ligand of RIG‐I, and long dsRNA, the classical ligand of MDA5. Its C‐terminal domain (CTD) is essential for the recognition of both ligands, with residues specifically required for sensing the 5′ppp moiety. Structural modeling, molecular dynamics simulations, and mutational validation support a model in which a flexible RNA‐capping loop in the CTD confers Lc MDA5 a conformational plasticity, allowing the CTD to cap the 5′ppp‐dsRNA terminus for RIG‐I‐like recognition or orienting the CTD parallel to long‐dsRNA axis for canonical MDA5 recognition. Disruption of 5′ppp recognition impaired Lc MDA5‐mediated antiviral activity against a RIG‐I‐sensitive virus. Downstream, Lc MDA5 signals through an integrated MAVS–STING–TBK1/IRF3 axis to drive type I IFN responses to both RNA ligands. Our findings reveal an unexpectedly flexible architecture of vertebrate cytosolic RNA sensing in which a single RLR combines ligand‐recognition and signaling properties classically partitioned between RIG‐I and MDA5.

The coating of microbiota by secretory immunoglobulins (sIgs) determines which bacteria colonize the gut and influences bacterial metabolism. Previous work has identified sIgA and sIgT as mediators of gut homeostasis. However, sIgM coats a large proportion of the gut microbiota in humans and teleost fish, thus suggesting a conserved role of sIgM in microbiota homeostasis. Here, to investigate this hypothesis, we used the teleost rainbow trout as a model system. Depletion of IgM from trout resulted in severe microbiota-dependent gut tissue damage, body weight loss, bacterial translocation and gut dysbiosis. IgM depletion led also to alterations in microbiota-derived metabolites, including short-chain fatty acids and essential amino acids. Supporting a protective role for sIgM in the gut, high mortality of IgM-depleted fish occurred in an experimental colitis model as a result of severe systemic bacteraemia and septic shock. Our findings uncover sIgM as a previously unrecognized regulator of microbiota homeostasis and metabolism.

Germinal centers (GCs) or analogous secondary lymphoid microstructures (SLMs) are thought to have evolved in endothermic species. However, living representatives of their ectothermic ancestors can mount potent secondary antibody responses upon infection or immunization, despite the apparent lack of SLMs in these cold-blooded vertebrates. How and where adaptive immune responses are induced in ectothermic species in the absence of GCs or analogous SLMs remain poorly understood. Here, we infected a teleost fish (trout) with the parasite Ichthyophthirius multifiliis (Ich) and identified the formation of large aggregates of highly proliferating IgM ⁺ B cells and CD4 ⁺ T cells, contiguous to splenic melanomacrophage centers (MMCs). Most of these MMC-associated lymphoid aggregates (M-LAs) contained numerous antigen (Ag)–specific B cells. Analysis of the IgM heavy chain CDR3 repertoire of microdissected splenic M-LAs and non–M-LA areas revealed that the most frequent B cell clones induced after Ich infection were highly shared only within the M-LAs of infected animals. These M-LAs represented highly polyclonal SLMs in which Ag-specific B cell clonal expansion occurred. M-LA–associated B cells expressed high levels of activation-induced cytidine deaminase and underwent significant apoptosis, and somatic hypermutation of Igμ genes occurred prevalently in these cells. Our findings demonstrate that ectotherms evolved organized SLMs with GC-like roles. Moreover, our results also point to primordially conserved mechanisms by which M-LAs and mammalian polyclonal GCs develop and function.

The co-evolution between secretory immunoglobulins (sIgs) and microbiota began with the emergence of IgM over half a billion years ago. Yet, IgM function in vertebrates is mostly associated with systemic immunity against pathogens. sIgA and sIgT are the only sIgs known to be required in the control of microbiota homeostasis in warm- and cold-blooded vertebrates respectively. Recent studies have shown that sIgM coats a large proportion of the gut microbiota of humans and teleost fish, thus suggesting an ancient and conserved relationship between sIgM and microbiota early in vertebrate evolution. To test this hypothesis, we temporarily and selectively depleted IgM from rainbow trout, an old bony fish species. IgM depletion resulted in a drastic reduction in microbiota IgM coating levels and losses in gutassociated bacteria. These were accompanied by bacterial translocation, severe gut tissue damage, inflammation and dysbiosis predictive of metabolic shifts. Furthermore, depletion of IgM resulted in body weight loss and lethality in an experimental colitis model. Recovery of sIgM to physiological levels restores tissue barrier integrity, while microbiome homeostasis and their predictive metabolic capabilities are not fully restituted. Our findings uncover a previously unrecognized role of sIgM as an ancient master regulator of microbiota homeostasis and metabolism and challenge the current paradigm that sIgA and sIgT are the key vertebrate sIgs regulating microbiome homeostasis. One-Sentence Summary IgM, the most ancient and conserved immunoglobulin in jawed vertebrates, is required for successful symbiosis with the gut microbiota.

The aim of the present study is to evaluate the potential of two functional additives as gill endogenous antioxidant capacity boosters in European sea-bass juveniles fed low-FM/FO diets when challenged against physical and biological stressors. For that purpose, two isoenergetic and isonitrogenous diets with low FM (10%) and FO (6%) contents were supplemented with 5000 ppm plant-derived galactomannan–oligosaccharides (GMOS) or 200 ppm of a mixture of garlic and labiate plant essential oils (PHYTO). A control diet was void from supplementation. Fish were fed the experimental diet for nine weeks and subjected to a confinement stress challenge (C challenge) or a confinement stress challenge combined with an exposure to the pathogen Vibrio anguillarum (CI challenge). Both GMOS and PHYTO diets attenuated fish stress response, inducing lower circulating plasma cortisol and down-regulating nfκβ2 and gr relative gene-expression levels in the gill. This attenuated stress response was associated with a minor energetic metabolism response in relation to the down-regulation of nd5 and coxi gene expression.