Maternal transmission of immunity to white spot syndrome associated virus (WSSV) in shrimp (Penaeus monodon)

Analysis of the VP19 and VP28 genes of white spot syndrome virus in Korea and comparison with strains from other countries

The results of our DNA analysis showed that there was 100% homology of the VP28 and VP19 genes of white spot syndrome virus (WSSV) among the samples collected from different shrimp farms in Korea. Comparing with an earlier isolated Korean strain, Korea01, the genes nucleotide sequences had only a single base difference which was observed in both VP19 and VP28. This resulted in a single amino acid substitution at position 40 of the latter. This implies that a single genetic strain of WSSV has been circulating in Korea and that its mutation rate is very low. In comparison with known sequences of VP19 and VP28 genes of WSSV isolates from other countries, the Korean strains had more than 99% sequence homology with those in gene and protein. Based on our sequence analysis of VP19 and VP28 of WSSV from various shrimp farms in Korea, the WSSV strains circulating in the region were genetically identical and similar to the strain identified two years ago. In addition, the Korean strain had close genetic identity with strains circulating in other Asian countries as well as other continents.

Role of maternally derived immunity in fish

Maternal immunity is of paramount importance for protection of young ones at early stage of life since the immune factors of an immunocompetent female are transferred transplacentally or through colostrum, milk or yolk to an immunologically naive neonate. Both innate and adaptive type of immunity are transferred of from mother to offspring in fishes. These factors include immunoglobulin (Ig)/antibody, complement factors, lysozymes, protease inhibitors like alpha macroglobulin, different types of lectins and serine proteases like molecules. Among different types of Ig viz. IgM, IgD, IgT/IgZ and IgM-IgZ chimera types, IgM is present in most of the teleostean fishes. In teleosts, IgM either as a reduced/breakdown product or monomeric form is usually transferred to the offsprings. The maternally derived IgM usually persists for a limited duration, exhausts within the completion of yolk absorption process, and completely disappears thereafter during larval stages. Maternal transfer of immunity which provides defense to embryo and larvae depends upon the health as well as the immune status of brood fish. The overall health status of brood fish can affect breeding performances, quality seed production and protection of offsprings. However, factors such as age, maturation, reproductive behaviour and nutrition (micro and macro-nutrients) may affect the immunity in brood fishes. Besides these, seasonal changes such as photoperiods, temperature, adverse environmental conditions, and stress conditions like handling, crowding, and water pollution/contamination can also affect the immunity of brood fishes. The maintenance of the brood stock immunity at high level during vitellogenesis and oogenesis, is utmost important for reducing mortalities at larval/post larval stages through maximum/optimum transfer of maternal immunity. Brood stock immunization prior to breeding as well as selective breeding among the disease resistant families might be the ideal criteria for producing quality seed.

Antiviral immunity in crustaceans

Viral diseases of shrimp have caused negative effects on the economy in several countries in Asia, South America and America, where they have numerous shrimp culture industries. The studies on the immunity of shrimp and other crustaceans have mainly focused on general aspects of immunity and as a consequence little is known about the antiviral responses in crustaceans. The aim of this review is to update recent knowledge of innate immunity against viral infections in crustaceans. Several antiviral molecules have been isolated and characterized recently from decapods. Characterization and identification of these molecules might provide a promising strategy for protection and treatment of these viral diseases. In addition dsRNA-induced antiviral immunity is also included.

Detection of phenoloxidase activity in early stages of the Pacific oyster Crassostrea gigas (Thunberg)

The presence of phenoloxidase (PO) activity was detected in different developmental stages of the Pacific oyster, Crassostrea gigas. A significant reduction in PO activity was observed from the 6h embryo stage to the day 11 larvae by spectrophotometry. A progressive increase was also observed from the day 13 larvae right through to the juvenile stage. The microscopy studies with '6h embryo' and adult samples confirmed the presence of PO activity. Various modulators of PO activity were used to study the triggering of pro-phenoloxidase (proPO) activating system of C. gigas but also to confirm the exact nature of the monitored activity. The enzyme activation mechanisms appear to differ with the developmental stage: bacterial lipopolysaccharides constitute an early elicitor of the proPO-PO system, whereas a purified trypsin triggers proPO-PO system in C. gigas spat. Phenoloxidase activity was totally suppressed by PO-specific inhibitors such as beta-2-mercaptoethanol, sodium diethyldithiocarbonate and tropolone. This study demonstrated the selective response of PO-like activity by different elicitors and suggested that proPO-PO activating system, which is supposed to play an important function in non-self recognition and host immune reactions in oyster, is expressed early in the Pacific oyster, C. gigas.

Alternative complement activity in the egg cytosol of amphioxus Branchiostoma belcheri: evidence for the defense role of maternal complement components

Background

The eggs in most invertebrates are fertilized externally, and therefore their resulting embryos are exposed to an environment full of microbes, many of which are pathogens capable of killing other organisms. How the developing embryos of invertebrates defend themselves against pathogenic attacks is an intriguing question to biologists, and remains largely unknown.

Methodology/Principal Findings

Here we clearly demonstrated that the egg cytosol prepared from the newly fertilized eggs of amphioxus Branchiostoma belcheri, an invertebrate chordate, was able to inhibit the growth of both the Gram-negative bacterium Vibrio anguillarum and the Gram-positive bacterium Staphylococcus aureus. All findings point to that it is the complement system operating via the alternative pathway that is attributable to the bacteriostatic activity.

Conclusions/Significance

This appears to be the first report providing the evidence for the functional role of the maternal complement components in the eggs of invertebrate species, paving the way for the study of maternal immunity in other invertebrate organisms whose eggs are fertilized in vitro. It also supports the notion that the early developing embryos share some defense mechanisms common with the adult species.

Beta-glucans as conductors of immune symphonies

The use of immunostimulants has received increased attention due to the discovery of Toll-like receptors (TLR) or/and pattern recognition receptors (PRR). These receptors have been found to bind molecules from a range of pathogens including self-molecules. When cell damage has occurred many of the released molecular structures act as so-called "danger" signals possessing pathogen-associated molecular patterns (PAMP). These danger signals often consist of repeating molecular moieties yielding high molecular weight compounds. Examples are beta-glucans and CpG containing DNA, but some danger signals possess low molecular weight structures. It has been found that the PRR bind unit structures of PAMP, and that PAMP-binding involves several other humoral and cell membrane proteins, exemplified by the more or less simultaneous LPS recognition displayed by MD-2, CD-14 and TLR4 on the cell membrane. Also, the binding of beta-glucans has been shown to include several different cell membrane receptors. Several immunostimulants are commercially exploited in aquaculture as feed additives. This applies to beta-glucans, alginates and nucleotides. Despite their use as feed additives no targeted approach has been conducted to include PAMP as adjuvants in fish vaccines. Interestingly, most of the PAMP studied activate antigen-presenting cells together with naïve T cells into dendritic cells and Th1 or Th2 cells [1]. In turn, this may activate Th1 and Th2 immune responses with production of Th1 or Th2 signature molecules such as IFN-gamma and IL-4, respectively [2-4]. This review will mainly focus on binding characteristics of beta-glucans, their effects on T helper cell differentiation, effects on functional levels, gene expression profiles and application of the commonly used ss-glucan in the aquaculture sector. In addition, ss-glucans show promises in shrimp aquaculture by inducing disease resistance, this review will also highlight the use and the effects of beta-glucans in experimental models.

Trans-generational immune priming in a social insect

Detecting functional homology between invertebrate and vertebrate immunity is of interest in terms of understanding the dynamics and evolution of immune systems. Trans-generational effects on immunity are well known from vertebrates, but their existence in invertebrates remains controversial. Earlier work on invertebrates has interpreted increased offspring resistance to pathogens as trans-generational immune priming. However, interpretation of these earlier studies involves some caveats and thus full evidence for a direct effect of maternal immune experience on offspring immunity is still lacking in invertebrates. Here we show that induced levels of antibacterial activity are higher in the worker offspring of the bumblebee, Bombus terrestris L. when their mother queen received a corresponding immune challenge prior to colony founding. This shows trans-generational immune priming in an insect, with ramifications for the evolution of sociality.

Double-stranded RNA and antiviral immunity in marine shrimp: inducible host mechanisms and evidence for the evolution of viral counter-responses

Double-stranded RNA (dsRNA) is a common virus-associated molecular pattern and a potent inducer of antiviral responses in many organisms. While it is clear that the specific RNA interference (RNAi) response, a phenomenon triggered by dsRNA, serves antiviral functions in invertebrates, innate (non-specific) antiviral immune reactions induced by dsRNA (e.g. the Interferon response) have long been thought to be restricted to vertebrates. Recent work in an underappreciated experimental model, the penaeid shrimp, is challenging these traditional distinctions, by demonstrating the existence of both innate (non sequence-specific) and RNAi-related (sequence-specific) antiviral phenomena in crustacea. Here we discuss the evidence for this bivalent role of dsRNA in the initiation of antiviral responses in shrimp, and present new data that suggest that the antiviral functions of the shrimp RNAi machinery have imposed selective pressures on an evolving viral pathogen. These findings open the door for the discovery of novel mechanisms of innate immunity, and provide a basis for the future development of strategies to control viral diseases in the commercially important penaeid shrimp.

Invertebrate immunity and the limits of mechanistic immunology

Rapid progress is being made in elucidating the molecular mechanisms involved in invertebrate immunity. This search for molecules runs the risk of missing important phenomena. In vertebrates, acquired protection and pathogen-specific responses were demonstrated experimentally long before the mechanisms responsible were elucidated. Without analogous experiments, mechanism-driven work may not demonstrate the full richness of invertebrate immunity.

Ostreid herpesvirus 1 (OsHV-1) detection among three successive generations of Pacific oysters (Crassostrea gigas)

Ostreid Herpesvirus 1 (OsHV-1) was likely detected in Pacific oysters, Crassostrea gigas, at different stages of development. Viral infections were associated with high mortality rates in the spat and larvae. Furthermore, the persistance of OsHV-1 in asymptomatic adults was demonstrated by detection of viral DNA and proteins. In the present study, three successive generations of C. gigas (G0 and G1 parental oysters, G1 and G2 larvae) were screened for OsHV-1 by PCR. Viral DNA was detected in 2-day-old larvae, indicating that infection may take place at very early stages. Although results strengthen the hypothesis of a vertical transmission, it was not possible to predict the issue of a particular type of cross. Indeed, the detection of viral DNA in parental oysters did not systematically correspond to a productive infection or result in a successful transmission to the progeny. However, the infective status of the parents appeared to have an influence on both the infection and the survival rates of the progeny. Crosses involving an OsHV-1 infected male and a non-infected female resulted in hatching and larval survival rates statistically lower than those observed in the other types of cross. These results suggest that OsHV-1-infected females may transmit to their offspring some kind of protection or resistance against viral infection.

Response of crayfish, Procambarus clarkii, haemocytes infected by white spot syndrome virus

White spot syndrome virus (WSSV) is a serious pathogen of aquatic crustaceans. Little is known about its transmission in vivo and the immune reaction of its hosts. In this study, the circulating haemocytes of crayfish, Procambarus clarkii, infected by WSSV, and primary haemocyte cultures inoculated with WSSV, were collected and observed by transmission electron microscopy and light microscopy following in situ hybridization. In ultra-thin sections of infected haemocytes, the enveloped virions were seen to be phagocytosed in the cytoplasm and no viral particles were observed in the nuclei. In situ hybridization with WSSV-specific probes also demonstrated that there were no specific positive signals present in the haemocytes. Conversely, strong specific positive signals showed that WSSV replicated in the nuclei of gill cells. As a control, the lymphoid organ of shrimp, Penaeus monodon, infected by WSSV was examined by in situ hybridization which showed that WSSV did not replicate within the tubules of the lymphoid organ. In contrast to previous studies, it is concluded that neither shrimp nor crayfish haemocytes support WSSV replication.

Induction of antiviral immunity by double-stranded RNA in a marine invertebrate

Vertebrates mount a strong innate immune response against viruses, largely by activating the interferon system. Double-stranded RNA (dsRNA), a common intermediate formed during the life cycle of many viruses, is a potent trigger of this response. In contrast, no general inducible antiviral defense mechanism has been reported in any invertebrate. Here we show that dsRNA induces antiviral protection in the marine crustacean Litopenaeus vannamei. When treated with dsRNA, shrimp showed increased resistance to infection by two unrelated viruses, white spot syndrome virus and Taura syndrome virus. Induction of this antiviral state is independent of the sequence of the dsRNA used and therefore distinct from the sequence-specific dsRNA-mediated genetic interference phenomenon. This demonstrates for the first time that an invertebrate immune system, like its vertebrate counterparts, can recognize dsRNA as a virus-associated molecular pattern, resulting in the activation of an innate antiviral response.

Adaptive innate immunity? Responsive-mode prophylaxis in the mealworm beetle, Tenebrio molitor

A primary infection by a parasite may indicate a higher risk of being reinfected in the near future (since infection may indicate that enemies are becoming more abundant). Acquired immunity does not exist in invertebrates despite the fact that they also face increased risks of reinfection following primary exposure. However, when subjected to immune insult, insects can produce immune responses that persist for long enough to provide prophylaxis. Because these immune responses are costly, persistence must be maintained through a selective advantage. We tested for the possibility that these long-lasting immune responses provided increased resistance to later infections by experimentally mimicking a primary immune insult (pre-challenge) in larvae of the mealworm beetle, Tenebrio molitor, with lipopolysaccharides (LPS) prior to early or late exposure to spores of the entomopathogenic fungus Metarhizium anisopliae. We found that pre-challenged larvae produced a long-lasting antimicrobial response, which provided a survival benefit when the larvae were exposed to fungal infection. These results suggest that the observed response is functionally "adaptive".

Immune function across generations: integrating mechanism and evolutionary process in maternal antibody transmission

The past 30 years of immunological research have revealed much about the proximate mechanisms of maternal antibody transmission and utilization, but have not adequately addressed how these issues are related to evolutionary and ecological theory. Much remains to be learned about individual differences within a species in maternal antibody transmission as well as differences among species in transmission or utilization of antibodies. Similarly, maternal-effects theory has generally neglected the mechanisms by which mothers influence offspring phenotype. Although the environmental cues that generate maternal effects and the consequent effects for offspring phenotype are often well characterized, the intermediary physiological and developmental steps through which the maternal effect is transmitted are generally unknown. Integration of the proximate mechanisms of maternal antibody transmission with evolutionary theory on maternal effects affords an important opportunity to unite mechanism and process by focusing on the links between genetics, environment and physiology, with the ultimate goal of explaining differences among individuals and species in the transfer of immune function from one generation to the next.

Immunostimulation in crustaceans: does it really protect against infection?

There is a growing need to control, prevent or minimise the devastating effects of disease in crustacean culture without recourse to toxic chemicals or antibiotics. In keeping with approaches to disease control in fish and higher mammals, interest is developing in compounds that confer protection and/or enhance immune reactivity to likely pathogens in shellfish (sometimes, erroneously, referred to as as "shellfish vaccines"). The agents currently under scrutiny for crustaceans include glucans, lipopolysaccharides and killed bacterial cells. They are thought to act as "immuno-stimulants" because of their known effects on the crustacean immune system in vitro. A number of papers are now appearing in the literature claiming to demonstrate their positive impact on immunity and immunity and disease resistance. This review article considers the problem of disease and its control in crustacean farming, describing the types of capability in cultured species. Analysis of the validity of the results of many of the published studies raises questions about the value of these compounds for cost-effective control of infection in aquaculture, especially for long lasting protection in both adults and juveniles. This review further discusses the potential risks to the wellbeing of the stock animals from repeated use of these agents and makes the case for rigorous testing of putative stimulants, at the gene, protein and functional levels, as well as for the need to consider alternative strategies and approaches to disease control.