Specificity and complexity of the Caenorhabditis elegans innate immune response

In response to infection, Caenorhabditis elegans produces an array of antimicrobial proteins. To understand the C. elegans immune response, we have investigated the regulation of a large, representative sample of candidate antimicrobial genes. We found that all these putative antimicrobial genes are expressed in tissues exposed to the environment, a position from which they can ward off infection. Using RNA interference to inhibit the function of immune signaling pathways in C. elegans, we found that different immune response pathways regulate expression of distinct but overlapping sets of antimicrobial genes. We also show that different bacterial pathogens regulate distinct but overlapping sets of antimicrobial genes. The patterns of genes induced by pathogens do not coincide with any single immune signaling pathway. Thus, even in this simple model system for innate immunity, striking specificity and complexity exist in the immune response. The unique patterns of antimicrobial gene expression observed when C. elegans is exposed to different pathogens or when different immune signaling pathways are perturbed suggest that a large set of yet to be identified pathogen recognition receptors (PRRs) exist in the nematode. These PRRs must interact in a complicated fashion to induce a unique set of antimicrobial genes. We also propose the existence of an "antimicrobial fingerprint," which will aid in assigning newly identified C. elegans innate immunity genes to known immune signaling pathways.     

Inducible antibacterial defense system in C. elegans

The term innate immunity refers to a number of evolutionary ancient mechanisms that serve to defend animals and plants against infection. Genetically tractable model organisms, especially Drosophila, have contributed greatly to advances in our understanding of mammalian innate immunity. Essentially, nothing is known about immune responses in the nematode Caenorhabditis elegans. Using high-density cDNA microarrays, we show here that infection of C. elegans by the Gram-negative bacterium Serratia marcescens provokes a marked upregulation of the expression of many genes. Among the most robustly induced are genes encoding lectins and lysozymes, known to be involved in immune responses in other organisms. Certain infection-inducible genes are under the control of the DBL-1/TGFbeta pathway. We found that dbl-1 mutants exhibit increased susceptibility to infection. Conversely, overexpression of the lysozyme gene lys-1 augments the resistance of C. elegans to S. marcescens. These results constitute the first demonstration of inducible antibacterial defenses in C. elegans and open new avenues for the investigation of evolutionary conserved mechanisms of innate immunity.     

A reverse genetic analysis of components of the Toll signaling pathway in Caenorhabditis elegans

BACKGROUND: Both animals and plants respond rapidly to pathogens by inducing the expression of defense-related genes. Whether such an inducible system of innate immunity is present in the model nematode Caenorhabditis elegans is currently an open question. Among conserved signaling pathways important for innate immunity, the Toll pathway is the best characterized. In Drosophila, this pathway also has an essential developmental role. C. elegans possesses structural homologs of components of this pathway, and this observation raises the possibility that a Toll pathway might also function in nematodes to trigger defense mechanisms or to control development. RESULTS: We have generated and characterized deletion mutants for four genes supposed to function in a nematode Toll signaling pathway. These genes are tol-1, trf-1, pik-1, and ikb-1 and are homologous to the Drosophila melanogaster Toll, dTraf, pelle, and cactus genes, respectively. Of these four genes, only tol-1 is required for nematode development. None of them are important for the resistance of C. elegans to a number of pathogens. On the other hand, C. elegans is capable of distinguishing different bacterial species and has a tendency to avoid certain pathogens, including Serratia marcescens. The tol-1 mutants are defective in their avoidance of pathogenic S. marcescens, although other chemosensory behaviors are wild type. CONCLUSIONS: In C. elegans, tol-1 is important for development and pathogen recognition, as is Toll in Drosophila, but remarkably for the latter r?le, it functions in the context of a behavioral mechanism that keeps worms away from potential danger.     

Cyclin D involvement demarcates a late transition in C. elegans embryogenesis

During development, progression through the cell cycle must be coordinately regulated with cellular differentiation. Despite significant progress in identifying genes required independently for each of these processes, the molecules which facilitate this cross talk have for the most part been elusive. Using the six macrophage-like coelomocytes of the nematode Caenorhabditis elegans as a model system to gain insight into the mesodermal differentiation pathway, we have isolated a set of mutants that alter coelomocyte numbers. One of these mutations, cc600, apparently results from a partial loss-of-function in the C. elegans cyclin D gene, cyd-1. The mutant has coelomocyte-specific defects without changes in other lineages. The mutants show that cell growth, terminal differentiation and cellular function proceed in the absence of cyd-1 activity and cell division. The results suggest that certain mesodermal lineages may be uniquely affected by changes in cyd-1 activity.     

A decline in p38 MAPK signaling underlies immunosenescence in Caenorhabditis elegans

The decline in immune function with aging, known as immunosenescence, has been implicated in evolutionarily diverse species, but the underlying molecular mechanisms are not understood. During aging in Caenorhabditis elegans, intestinal tissue deterioration and the increased intestinal proliferation of bacteria are observed, but how innate immunity changes during C. elegans aging has not been defined. Here we show that C. elegans exhibits increased susceptibility to bacterial infection with age, and we establish that aging is associated with a decline in the activity of the conserved PMK-1 p38 mitogen-activated protein kinase pathway, which regulates innate immunity in C. elegans. Our data define the phenomenon of innate immunosenescence in C. elegans in terms of the age-dependent dynamics of the PMK-1 innate immune signaling pathway, and they suggest that a cycle of intestinal tissue aging, immunosenescence, and bacterial proliferation leads to death in aging C. elegans.     

Staphylococcal biofilm exopolysaccharide protects against Caenorhabditis elegans immune defenses

Staphylococcus epidermidis and Staphylococcus aureus are leading causes of hospital-acquired infections that have become increasingly difficult to treat due to the prevalence of antibiotic resistance in these organisms. The ability of staphylococci to produce biofilm is an important virulence mechanism that allows bacteria both to adhere to living and artificial surfaces and to resist host immune factors and antibiotics. Here, we show that the icaADBC locus, which synthesizes the biofilm-associated polysaccharide intercellular adhesin (PIA) in staphylococci, is required for the formation of a lethal S. epidermidis infection in the intestine of the model nematode Caenorhabditis elegans. Susceptibility to S. epidermidis infection is influenced by mutation of the C. elegans PMK-1 p38 mitogen-activated protein (MAP) kinase or DAF-2 insulin-signaling pathways. Loss of PIA production abrogates nematocidal activity and leads to reduced bacterial accumulation in the C. elegans intestine, while overexpression of the icaADBC locus in S. aureus augments virulence towards nematodes. PIA-producing S. epidermidis has a significant survival advantage over ica-deficient S. epidermidis within the intestinal tract of wild-type C. elegans, but not in immunocompromised nematodes harboring a loss-of-function mutation in the p38 MAP kinase pathway gene sek-1. Moreover, sek-1 and pmk-1 mutants are equally sensitive to wild-type and icaADBC-deficient S. epidermidis. These results suggest that biofilm exopolysaccharide enhances virulence by playing an immunoprotective role during colonization of the C. elegans intestine. These studies demonstrate that C. elegans can serve as a simple animal model for studying host-pathogen interactions involving staphylococcal biofilm exopolysaccharide and suggest that the protective activity of biofilm matrix represents an ancient conserved function for resisting predation.     

Lack of the RNA chaperone Hfq attenuates pathogenicity of several Escherichia coli pathotypes towards Caenorhabditis elegans

Escherichia coli is an important agent of Gram-negative bacterial infections worldwide, being one of the leading causes of diarrhoea and urinary tract infections. Strategies to understand pathogenesis and develop therapeutic compounds include the use of the nematode Caenorhabditis elegans as a model for virulence characterization and screening for novel antimicrobial entities. Several E. coli human pathotypes are also pathogenic towards C. elegans, and we show here that lack of the RNA chaperone Hfq significantly reduces pathogenicity of VTEC, EAEC, and UPEC in the nematode model. Thus, Hfq is intrinsically essential to pathogenic E. coli for survival and virulence exerted in the C. elegans host.     

Characterization of Burkholderia pseudomallei infection and identification of novel virulence factors using a Caenorhabditis elegans host system

The environmental saphrophyte Burkholderia pseudomallei is the causative agent of melioidosis, a systemic, potentially life-threatening condition endemic to many parts of south-east Asia and northern Australia. We have used the soil nematode Caenorhabditis elegans as a model host to characterize the mechanisms by which this bacterium mounts a successful infection. We find that C. elegans is susceptible to a broad range of Burkholderia species, and that the virulence mechanisms used by this pathogen to kill nematodes may be similar to those used to infect mammals. We also find that the specific dynamics of the C. elegans-B. pseudomallei host-pathogen interaction can be highly influenced by environmental factors, and that nematode killing results at least in part from the presence of a diffusible toxin. Finally, by screening for bacterial mutants attenuated in their ability to kill C. elegans, we genetically identify several new potential virulence factors in B. pseudomallei. The use of C. elegans as a model host should greatly facilitate future investigations into how B. pseudomallei can interact with host organisms.     

Pharyngeal polysaccharide deacetylases affect development in the nematode C. elegans and deacetylate chitin in vitro

Chitin (β-1,4-linked-N-acetylglucosamine) provides structural integrity to the nematode eggshell and pharyngeal lining. Chitin is synthesized in nematodes, but not in plants and vertebrates, which are often hosts to parasitic roundworms; hence, the chitin metabolism pathway is considered a potential target for selective interventions. Polysaccharide deacetylases (PDAs), including those that convert chitin to chitosan, have been previously demonstrated in protists, fungi and insects. We show that genes encoding PDAs are distributed throughout the phylum Nematoda, with the two paralogs F48E3.8 and C54G7.3 found in C. elegans. We confirm that the genes are somatically expressed and show that RNAi knockdown of these genes retards C. elegans development. Additionally, we show that proteins from the nematode deacetylate chitin in vitro, we quantify the substrate available in vivo as targets of these enzymes, and we show that Eosin Y (which specifically stains chitosan in fungal cells walls) stains the C. elegans pharynx. Our results suggest that one function of PDAs in nematodes may be deacetylation of the chitinous pharyngeal lining.     

Every sperm is sacred: fertilization in Caenorhabditis elegans

The nematode Caenorhabditis elegans is an attractive model system for the study of fertilization. C. elegans exists as a self-fertilizing hermaphrodite or as a male. This unusual situation provides an excellent opportunity to identify and maintain sterile mutants that affect sperm and no other cells. Analysis of these mutants can identify genes that encode proteins required for gamete recognition, adhesion, signaling, fusion, and/or activation at fertilization. These genes can also provide a starting point for the identification of additional molecules required for fertility. This review describes progress in the genetic and molecular dissection of fertilization in C. elegans and related studies on sperm competition.