The polymeric immunoglobulin (Ig) receptor (pIgR) is an integral transmembrane glycoprotein that plays an important role in the mammalian immune response by transporting soluble polymeric Igs across mucosal epithelial cells. Single pIgR genes, which are expressed in lymphoid organs including mucosal tissues, have been identified in several teleost species. A single pigr gene has been identified on zebrafish chromosome 2 along with a large multigene family consisting of 29 pigr-like (PIGRL) genes. Full length transcripts from 10 different PIGRL genes that encode secreted and putative inhibitory membrane bound receptors have been characterized. Although PIGRL and pigr transcripts are detected in immune tissues, only PIGRL transcripts can be detected in lymphoid and myeloid cells. In contrast to pIgR which binds Igs, certain PIGRL proteins bind phospholipids. PIGRL transcript levels are increased after infection with Streptococcus iniae, suggesting a role for PIGRL genes during bacterial challenge. Transcript levels of PIGRL genes are decreased after infection with Snakehead rhabdovirus, suggesting that viral infection may suppress PIGRL function.

Genetics plays a central role in susceptibility to obesity and metabolic diseases. BALB/c mice are known to be resistant to high fat diet (HFD)-induced obesity, however the genetic cause remains unknown. We report that deletion of the innate immunity antibacterial gene Nod2 abolishes this resistance, as Nod2 −/− BALB/c mice developed HFD-dependent obesity and hallmark features of metabolic syndrome. Nod2 −/− HFD mice developed hyperlipidemia, hyperglycemia, glucose intolerance, increased adiposity, and steatosis, with large lipid droplets in their hepatocytes. These changes were accompanied by increased expression of immune genes in adipose tissue and differential expression of genes for lipid metabolism, signaling, stress, transport, cell cycle, and development in both adipose tissue and liver. Nod2 −/− HFD mice exhibited changes in the composition of the gut microbiota and long-term treatment with antibiotics abolished diet-dependent weight gain in Nod2 −/− mice, but not in wild type mice. Furthermore, microbiota from Nod2 −/− HFD mice transferred sensitivity to weight gain, steatosis, and hyperglycemia to wild type germ free mice. In summary, we have identified a novel role for Nod2 in obesity and demonstrate that Nod2 and Nod2-regulated microbiota protect BALB/c mice from diet-induced obesity and metabolic dysfunction.

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T he roles that mitochondria play in antiviral signaling, via mitochondrial antiviral-signaling protein and promotion of inflammation and apoptosis, are well established (1-3); however, their importance in innate immunity is only now becoming clear. While mitochondria can promote inflammation via NF-B signaling and NLRP3 inflammasome formation, inflammation can lead to mitochondrial dysfunction, which can compound the severity of exaggerated inflammatory conditions such as sepsis (4-10). As the site of the electron transport chain within cells, mitochondria are a major source of reactive oxygen species (ROS) (mainly superoxide anions). ROS play diverse roles in cellular and organismal health, especially in innate immunity and inflammation. While the use of ROS to clear infections is beneficial to the host, inappropriate ROS production or lack of ROS neutralization can damage host DNA, proteins, and cell membranes. ROS-induced cellular damage can contribute to the undesired side effects of infectious and inflammatory diseases. Mechanisms are in place in hosts, and even some pathogens, to chemically convert ROS into less toxic compounds; however, overproduction of ROS can overwhelm host antioxidants. Thus, a better understanding of the impact of mitochondria, ROS, and mechanisms for neutralization of ROS on innate immunity could lead to improved treatments for infectious diseases and inflammatory disorders.The cellular mechanisms to neutralize ROS include the glutathione system, catalases, and the superoxide dismutase (SOD) family of enzymes. As superoxide producers, mitochondria are equipped with nuclear-encoded, mitochondrially localized SODs (SOD2, MnSOD) that convert superoxide into hydrogen peroxide. The deleterious effects of mitochondrial superoxide are demonstrated by mutations in SOD2 being implicated in idiopathic cardiomyopathy, age-related macular degeneration, aberrant brain morphology, motor neuron disease, vascular complications of diabetes, and cancer, whereas overexpression of SOD2 increases the Drosophila life span (11,(69)(70)(71)(72). Despite the importance of SOD2 and the regulation of ROS for health, surprisingly little is known about the role of SOD2 in immunity. Numerous studies have implicated SOD2 in the immune response, but few have defined functional roles for SOD2 in immunity. SOD2 is upregulated in response to lipopolysaccharide (LPS), poly(I·C), beta-glucan, and numerous pathogens in multiple cell types and organisms (12-17). Functionally, SOD2 was found to be necessary for the phorbol myristate acetate-induced respiratory burst response and cell survival upon poly(I·C) exposure in vitro (16,17). In a mouse model with SOD2 deleted specifically from thymocytes, the animals did not mount an effective adaptive immune response to influenza virus infection because of disrupted T-cell

Five large multigene families encoding innate-type immune receptors that are comprised of immunoglobulin domains have been identified in bony fish, of which four do not possess definable mammalian orthologs. The members of some of the multigene families exhibit unusually extensive patterns of divergence and the individual family members demonstrate marked variation in interspecific comparisons. As a group, the gene families reveal striking differences in domain type and content, mechanisms of intracellular signaling, basic structural features, haplotype and allelic variation and ligand binding. The potential functional roles of these innate immune receptors, their relationships to immune genes in higher vertebrate species and the basis for their adaptive evolution are of broad interest. Ongoing investigations are expected to provide new insight into alternative mechanisms of immunity.