CpG oligodeoxynucleotides stimulate immune cell proliferation but not specific antibody production in rainbow trout (Oncorhynchus mykiss)

The interactions between CpG oligodeoxynucleotides and Toll-like receptors in Pacific white shrimp Litopenaeus vannamei

CpG oligodeoxynucleotides (ODNs), as a novel type of adjuvant with immunomodulatory effects, are recognized by Toll-like receptors (TLRs) in Litopenaeus vannamei. In the present study, eleven LvTLRs-pCMV recombinants (rLvTLRs) were constructed to investigate the relationships between various CpG ODNs and different LvTLRs in human embryonic kidney 293T (HEK293T) cells, which was further confirmed by bio-layer interferometry (BLI) technique. The results of dual luciferase reporter assay showed that every LvTLR could activate multiple downstream genes, mainly including NF-κB, CREB, ISRE, IL-6-promoter, TNF-α-promoter and Myc, thereby inducing main signaling pathways in shrimps. Most CpG ODNs possessed affinities to more than one LvTLR, while each LvTLR could recognize multiple CpG ODNs, and the widely recognized ligands within CpG ODNs are A-class and B-class. Moreover, BLI analysis showed that CpG 2216, Cpg 2006, CpG 2143 and CpG 21425 exhibited dose-dependent affinity to the expressed TLR protein, which were consistent with the results in HEK293T cells. It suggested that the interactions of CpG ODNs with LvTLRs were indispensable for the immune regulation triggered by CpG ODNs, and these findings would lay foundations for studying the activations of LvTLRs to immune signaling pathways and shedding lights on the immune functions and mechanisms of CpG ODNs.

Effectiveness of dietary heat-killed Bacillus subtilis harboring plasmid containing 60 copies of CpG-ODN 1668 against Vibrio harveyi in Penaeus vannamei

The cost of the purification process hinders the extensive use of cytosine phosphate guanosine-oligodeoxynucleotides (CpG-ODNs) for shrimp culture. Therefore, this study used a shuttle vector plasmid to carry 60 copies of CpG-ODN 1668 (pAD43-25_60CpG), which can replicate in Escherichia coli and Bacillus subtilis strain RIK1285. The first experiment used a reverse gavage procedure to deliver a substance (PBS [CK], pAD43-25 [P0], and pAD43-25_60CpG [P60], respectively) directly into the anterior midgut of Penaeus vannamei and transcriptome sequence analysis with a reference genome was performed to examine the expression of well-known immune-related genes. The results showed that the expression levels of immune-related genes in P60 group were significantly increased, particularly those associated with AMPs. In addition, using RT‒qPCR, the expression levels of AMP genes (LvALF, LvPEN-2, and LvPEN-3) in the P60 group may vary depending on the tissue and time point. The second experiment used dietary supplementation with three kinds of heat-killed B. subtilis (HKBS, HKBS-P0, and HKBS-P60) in 28 days of feeding experiments. The results showed that dietary supplementation with HKBS-P60 did not significantly improve shrimp growth performance and survival. However, on days 14 and 28 of the feeding regimens, alkaline phosphatase (AKP) and acid phosphatase (ACP) activity were considerably higher than in other treatments. In addition, following infection with Vibrio harveyi, AKP and ACP activity in the HKBS-P60 group was significantly higher than in other treatments, particularly at the early stage of bacterial infection. Moreover, HKBS-P60 was found to be better protected against V. harveyi infection with lower cumulative mortality (60%) compared to HKBS (90%) and HKBS-P0 (100%) at 7 days after infection. Overall, these findings confirmed that P60 could increase immunological responses in the shrimp midgut, and HKBS-P60 could be used as an effective tool to enhance the immune response and disease resistance in shrimp.

CpG ODN 2102 promotes antibacterial immune responses and enhances vaccine-induced protection in golden pompano (Trachinotusovatus)

CpG oligodeoxynucleotides (ODNs) are oligodeoxynucleotides containing CpG motifs and can be recognized by toll-like receptor 9 (TLR9), activating the host's immune responses. In this study, ten different CpG ODNs were designed and synthesized to study the antibacterial immune responses of CpG ODNs in golden pompano (Trachinotus ovatus). Results showed that CpG ODN 2102 significantly improved the immunity of golden pompano against bacteria. Besides, CpG ODN 2102 promoted the proliferation of head kidney lymphocytes and activated the head kidney macrophages. When TLR9-specific small interfering RNA (siRNA) was used to interfere with TLR9 expression, the immune responses were decreased. Moreover, the expression levels of myeloid differentiation primary response 88 (Myd88), p65, tumor necrosis factor receptor-associated factor 6 (TRAF6), and tumor necrosis factor-alpha (TNF-α) in the TLR9-knockdown golden pompano kidney (GPK) cells were significantly reduced. The nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) promoter activity of the TLR9-knockdown GPK cells was also significantly reduced. In vivo, the antibacterial immune effects induced by CpG ODN 2102 in golden pompano were mostly abolished when TLR9 expression was knocked down. These results suggested that TLR9 was involved in the immune responses induced by CpG ODN 2102. CpG ODN 2102 also enhanced the protective effect of the Vibrio harveyi vaccine pCTssJ, where the survival rate of golden pompano was significantly improved by 20%. In addition, CpG ODN 2102 enhanced the messenger RNA (mRNA) expression levels of TLR9, Myxovirus resistance (Mx), interferon γ (IFN-γ), TNF-α, interleukin (IL)-1β, IL-8, major histocompatibility complex class (MHC) Iα, MHC IIα, Immunoglobulin D (IgD), and IgM. Therefore, TLR9 was involved in the antibacterial immune responses induced by CpG ODN 2102 and CpG ODN 2102 possessed adjuvant immune effects. These results enlarged our knowledge of the antibacterial immunity of fish TLRs signaling pathway and had important implications for exploring natural antibacterial molecules in fish and developing new vaccine adjuvants.

Comparative study of the impact of dietary supplementation with different types of CpG oligodeoxynucleotides (CpG ODNs) on enhancing intestinal microbiota diversity, antioxidant capacity, and immune-related gene expression profiles in Pacific white shrimp (Litopenaeus vannamei)

The CpG oligodeoxynucleotides (CpG ODNs) reportedly possess the capacity to strengthen immunity in mammals. This experiment was conducted to evaluate the impact of dietary supplementation with 17 types of CpG ODNs on intestinal microbiota diversity, antioxidant capacity, and immune-related gene expression profiles of the shrimp Litopenaeus vannamei. Diets including 50 mg kg^-1 CpG ODNs wrapped in egg whites were prepared and divided into 17 different groups, with 2 control groups (normal feed and feed with egg whites). These CpG ODNs supplemented diets and the control diets were fed to L. vannamei (5.15 ± 0.54 g) three times daily at 5%-8% shrimp body weight for three weeks. The results of consecutive detection of intestinal microbiota by 16S rDNA sequencing indicated that 11 of the 17 types of CpG ODNs significantly enhanced intestinal microbiota diversity, increased the populations of several probiotic bacteria, and activated possible mechanisms relevant to diseases. The immune-related genes expression and antioxidant capacity in hepatopancreas further demonstrated that the 11 types of CpG ODNs effectively improved the innate immunity of shrimp. Additionally, histology results showed that the CpG ODNs in the experiment did not damage the tissue structure of hepatopancreas. The results suggest that CpG ODNs could be used as a trace supplement to improve the intestinal health and immunity of shrimp.

Injection Vaccines Formulated with Nucleotide, Liposomal or Mineral Oil Adjuvants Induce Distinct Differences in Immunogenicity in Rainbow Trout

Protection facilitated by the widespread use of mineral oil adjuvanted injection vaccines in salmonid fish comes with adverse effects of varying severity. In this study, we characterized the immunological profiles of two alternative vaccine formulations, both with proven efficacy and an improved safety profile in rainbow trout. Experimental injection vaccines were prepared on an identical whole-cell Aeromonas salmonicida bacterin platform and were formulated with CpG oligodeoxynucleotides, a liposomal (CAF01) or a benchmark mineral oil adjuvant, respectively. A naïve group, as well as bacterin and saline-injected groups were also included. Following administration, antigen-specific serum antibody titers, the tissue distribution of immune cell markers, and the expression of immune-relevant genes following the in vitro antigenic restimulation of anterior kidney leukocytes was investigated. Immunohistochemical staining suggested prolonged antigen presentation for the particulate formulations and increased mucosal presence of antigen-presenting cells in all immunized fish. Unlike the other immunized groups, the CAF01 group only displayed a transient elevation in specific antibody titers and immunohistochemical observations, and the transcription data suggest an increased role of cell-mediated immunity for this group. Finally, the transcription profile of the CpG formulation approached that of a T_H1 profile. When compared to the benchmark formulation, CAF01 and CpG adjuvants induce slight, but distinct differences in the resulting protective immune responses. This is important, as it allows a broader immunological approach for the future development of safer vaccines.

Protective effects of CpG-ODN 2007 administration against Edwardsiella tarda infection in olive flounder (Paralichthys olivaceus)

In this study, we investigated the immunostimulatory and protective effects of CpG motif oligonucleotides (CpG-ODNs) against Edwardsiella tarda infection in olive flounder (Paralichthys olivaceus). Groups of fish injected with CpG-ODNs (1585, 1668, and 2007) or PBS (control) showed varying mortality rates in response to challenge with E. tarda. The survival rates of fish treated with CpG-ODN 1668 and 2007, which belonged to the same class type B, were 45% and 60%, respectively, with CpG-ODN 2007 showing the highest survival rate. Further analysis showed that the respiratory burst and bactericidal activities induced by CpG-ODN 2007 were higher than those in the control group (induced by non-CpG-ODNs) or in the group of fish induced by CpG-ODN 1585, which belonged to class type A. Additionally, the respiratory burst activity induced by CpG-ODN 2007 was higher than that induced by CpG-ODN 1668, despite similar bactericidal activity titers. In vivo experiments showed that CpG-ODN 2007 stimulation resulted in higher survival rates than CpG-ODN 1668 stimulation, possibly owing to differences in respiratory burst activity. In summary, we demonstrated that differences in CpG-motif or class type altered respiratory burst and bactericidal activities, resulting in differences in survival rates against E. tarda challenge in the olive flounder. Therefore, it is necessary to use CpG-ODNs optimized against E. tarda infection in olive flounder, because different CpG motifs belonging to the same class type have different effects.

Alternatives to mineral oil adjuvants in vaccines against Aeromonas salmonicida subsp. salmonicida in rainbow trout offer reductions in adverse effects

In an effort to reduce the frequency and severity of adverse reactions seen from the use of mineral oil adjuvants in salmonid fish, the effects of two alternative adjuvants were assessed, focusing on the induction of adverse effects as well as protection. Using rainbow trout (Oncorhynchus mykiss) as recipients, injection vaccines based on formalin-inactivated Aeromonas salmonicida subspecies salmonicida were formulated with CpG oligodeoxynucleotides, the liposomal cationic adjuvant formulation 01 (CAF01) or with Freund’s incomplete adjuvant and administered intraperitoneally. Control groups of unvaccinated, Tris-buffered saline-injected or bacterin-injected individuals were included, and each group included in the study held a total number of 240 individuals. Subsequently, individuals from each group were examined for differences in Fulton’s condition factor, macro- and microscopic pathological changes, as well as protection against experimental infection with A. salmonicida. While adverse effects were not eliminated, reductions in microscopic and macroscopic adverse effects, in particular, were seen for both the nucleotide- and liposome-based vaccine formulations. Furthermore, the induced protection appears similar to that of the benchmark formulation, thus introducing viable, potential alternative types of adjuvants for use in future fish vaccines.

Protective immunity against Megalocytivirus infection in rock bream (Oplegnathus fasciatus) following CpG ODN administration

Rock bream iridovirus (RBIV) disease in rock bream (Oplegnathus fasciatus) remains an unsolved problem in Korea aquaculture farms. CpG ODNs are known as immunostimulant, can improve the innate immune system of fish providing resistance to diseases. In this study, we evaluated the potential of CpG ODNs to induce anti-viral status protecting rock bream from different RBIV infection conditions. We found that, when administered into rock bream, CpG ODN 1668 induces better antiviral immune responses compared to other 5 CpG ODNs (2216, 1826, 2133, 2395 and 1720). All CpG ODN 1668 administered fish (1/5µg) at 2days before infection (1.1×10⁷) held at 26°C died even though mortality was delayed from 8days (1µg) and 4days (5µg). Similarly, CpG ODN 1668 administered (5µg) at 2days before infection (1.2×10⁶) held at 23/20°C had 100% mortality; the mortality was delayed from 9days (23°C) and 11days (20°C). Moreover, when CpG ODN 1668 administered (1/5/10µg) at 2/4/7days before infection or virus concentration was decreased to 1.1×10⁴ and held at 20°C had mortality rates of 20/60/30% (2days), 30/40/60% (4days) and 60/60/20% (7days), respectively, for the respective administration dose, through 100 dpi. To investigate the development of a protective immune response, survivors were re-infected with RBIV (1.1×10⁷) at 100 and 400 dpi, respectively. While 100% of the previously unexposed fish died, 100% of the previously infected fish survived. The high survival rate of fish following re-challenge with RBIV indicates that protective immunity was established in the surviving rock bream. Our results showed the possibility of developing preventive measures against RBIV using CpG ODN 1668 by reducing RBIV replication speed (i.e. water temperature of 20°C and infection dose of 1.1×10⁴).

CpG ODN mimicking CpG rich region of myxosporean Myxobolus supamattayai stimulates innate immunity in Asian sea bass (Lates calcarifer) and defense against Streptococcus iniae

Oligodeoxynucleotides (ODNs) containing unmethylated cytosine-phosphate-guanine CpG dinucleotides within specific sequence contexts (CpG motifs) have been reported as pathogen-associated molecular patterns (PAMPs). Its immunostimulatory effects have been demonstrated in diverse vertebrate models. CpG ODN is typically found in bacterial or viral genome and recognized by a non-self recognition receptor Toll-like receptor9 (TLR9). Here, a new CpG ODN 1013 which mimics sequence of SSU rDNA of early eukaryotic organism myxosporidia, Myxobolus supamattayai, was employed to stimulate the immune responses of Asian sea bass Lates calcarifer. Its immunostimulant potentiality was comparatively compared with that of CpG ODN 1668, a widely used as functional immunostimulant. Both unmethylated CpG ODNs with some modified phosphorothioated positions were intraperitoneally injection (5 μg/fish). Hematological examination, immunological assays and immune-related genes expression were evaluated 12 h, 1, 3 and 5 d after post CpG ODN challenge. The immunosimulatory effect of these CpG ODNs on fish immunity to protect the bacterial pathogen Streptococcus iniae was also determined. The results demonstrated that these two CpG ODNs could induce immune responses in Asian sea bass including the significant (P < 0.05) increase level of WBC, peroxidase activity and oxidative radicals in head kidney (HK) leukocyte, serum innate immune parameters and up-regulation of four immune responsive genes compared with the control group. Most of immune responses induced by ODN 1668 were strong within 1 d but lesser extended while ODN 1013 prolonged the stimulatory effects during the whole experimental period. After challenge with S. iniae, the survival proportion in ODN 1013-treated fish was apparently higher than that treated with ODN 1668 and PBS, respectively. The results together suggested that CpG ODN 1013 enhanced innate immune responses, including humoral and cellular responses, through TLR9 mediated signaling pathway which is mainly contribute to the protective immunity in Asian sea bass against S. iniae infection. These findings can lead to a new approach in immunostimulant development by using the novel CpG ODN originating from the parasite M. supamattayai, besides those from bacterial and viral genomes, for disease control in fish host.

B-cell activating CpG ODN 1668 enhance the immune response of Pacific red snapper (Lutjanus peru) exposed to Vibrio parahaemolitycus

B-class CpG ODN 1668 is known to possess clear immunostimulatory properties. In this study, we investigated the potential ability of CpG ODN 1668 to enhance the immune response of Pacific red snapper exposed to Vibrio parahaemolyticus. Four different treatments were evaluated in Pacific red snapper: (1) stimulatory CpG ODN 1668, (2) stimulatory CpG ODN 1668 and V. parahaemolyticus, (3) exposure only to V. parahaemolyticus and (4) PBS. Samples were taken at 24, 72, 168 and 240 h of stimulation/infection. The results show that intraperitoneal injection of CpG-ODN 1668 enhanced the anti-protease, superoxide dismutase and catalase activities in serum. CpG ODN 1668 upregulated TLR9 and IgM gene expression in head-kidney, intestine and skin, with higher expression in head-kidney. A higher correlation was observed between TLR9 and IgM in head-kidney and intestine. Finally, no histopathological damages were observed in fish stimulated with CpG ODN 1668. In contrast, melanomacrophages-like structures were present in higher numbers in infected fish. Taken together, these results indicate that CpG ODN 1668 activates innate immune response and upregulate the TLR9 and IgM-mediated immune response. These results may be exploited for the control of Vibriosis in farmed Pacific red snapper.

Whole-body transcriptome of selectively bred, resistant-, control-, and susceptible-line rainbow trout following experimental challenge with Flavobacterium psychrophilum

Genetic improvement for enhanced disease resistance in fish is an increasingly utilized approach to mitigate endemic infectious disease in aquaculture. In domesticated salmonid populations, large phenotypic variation in disease resistance has been identified but the genetic basis for altered responsiveness remains unclear. We previously reported three generations of selection and phenotypic validation of a bacterial cold water disease (BCWD) resistant line of rainbow trout, designated ARS-Fp-R. This line has higher survival after infection by either standardized laboratory challenge or natural challenge as compared to two reference lines, designated ARS-Fp-C (control) and ARS-Fp-S (susceptible). In this study, we utilized 1.1 g fry from the three genetic lines and performed RNA-seq to measure transcript abundance from the whole body of naive and Flavobacterium psychrophilum infected fish at day 1 (early time-point) and at day 5 post-challenge (onset of mortality). Sequences from 24 libraries were mapped onto the rainbow trout genome reference transcriptome of 46,585 predicted protein coding mRNAs that included 2633 putative immune-relevant gene transcripts. A total of 1884 genes (4.0% genome) exhibited differential transcript abundance between infected and mock-challenged fish (FDR < 0.05) that included chemokines, complement components, tnf receptor superfamily members, interleukins, nod-like receptor family members, and genes involved in metabolism and wound healing. The largest number of differentially expressed genes occurred on day 5 post-infection between naive and challenged ARS-Fp-S line fish correlating with high bacterial load. After excluding the effect of infection, we identified 21 differentially expressed genes between the three genetic lines. In summary, these data indicate global transcriptome differences between genetic lines of naive animals as well as differentially regulated transcriptional responses to infection.

Establishment and characterization of a new cell line (SSP-9) derived from Atlantic salmon Salmo salar that expresses type I ifn

In the present work, the establishment and biological characterization of a new cell line, SSP-9, derived from the pronephros of the Atlantic salmon Salmo salar, are reported. These cells grew well in Leibovitz's (L15) medium supplemented with 10% foetal calf serum at temperatures from 15 to 25° C, and they have been sub-cultured over 100 passages to produce a continuous cell line with an epithelial-like morphology. The SSP-9 cells attached and spread efficiently at different plating densities, retaining 80% of cell viability after storage in liquid nitrogen. When karyotyped, the cells had 40-52 chromosomes, with a modal number of 48. Viral susceptibility tests showed that SSP-9 cells were susceptible to infectious pancreatic necrosis virus and infectious haematopoietic necrosis virus, producing infectious virus and regular cytopathic effects. Moreover, these cells could be stimulated by poly I:C, showing significant up-regulation in the expression of the genes that regulate immune responses, such as ifn and mx-1. SSP-9 cells constitutively express genes characteristic of macrophages, such as major histocompatibility complex (mhc-II) and interleukin 12b (il-12b), and flow cytometry assays confirmed that SSP-9 cells can be permanently transfected with plasmids expressing a reporter gene. Accordingly, this new cell line is apparently suitable for transgenic manipulation, and to study host cell-virus interactions and immune processes.

C7: a CpG oligodeoxynucleotide that induces protective immune response against megalocytivirus in Japanese flounder (Paralichthys olivaceus) via Toll-like receptor 9-mediated signaling pathway

Megalocytivirus is the causative agent of severe disease outbreaks in farmed fish. Currently there is no effective control against megalocytivirus in aquaculture. Synthetic oligodeoxynucleotides (ODNs) containing unmethylated CpG motifs are known to possess marked immunostimulatory properties. In this study, we investigated the potentials of ten CpG ODNs as antiviral agents in a model of Japanese flounder (Paralichthys olivaceus). We found that, when administered into flounder, three of the ten CpG ODNs inhibited viral replication in kidney, spleen, and liver. ODN C7, which exhibited the strongest inhibitory activity, was able to promote proliferation of peripheral blood leukocytes and enhance activation of head kidney mononuclear adherent phagocytes. When the expression of toll-like receptor 9 (TLR9) was knocked down in vivo by small interfering RNA, C7-mediated immune response and antiviral activity were significantly blocked. Moreover, when C7 was co-administered with pCN86, a DNA vaccine against megalocytivirus, a significant increase in vaccine-induced protection was observed compared to administration with pCN86 alone. Further analysis showed that compared to fish immunized with pCN86, fish immunized with pCN86 plus C7 exhibited significantly upregulated expression of a wide range of genes involved in innate and adaptive immunity. Taken together, these results indicate that ODN C7 activates TLR9-mediated immune response and possesses antiviral and adjuvant potentials that may be exploited for the control of megalocytivirus infection in farmed flounder.

Poly D,L-lactide-co-glycolic acid-liposome encapsulated ODN on innate immunity in Epinephelus bruneus against Vibrio alginolyticus

The efficacy of poly D,L-lactide-co-glycolic acid (PLGA)-liposome (L) encapsulated oligodeoxynucleotides with unmethylated deoxycytidyl-deoxyguanosine motifs (CpG-ODNs) on innate and adaptive immune response and disease resistance in kelp grouper (Epinephelus bruneus) against Vibrio alginolyticus at weeks 1, 2, and 4 is reported. The superoxide dismutase (SOD), respiratory burst, and lysozyme activities significantly increased in E. bruneus when immunized with ODN, PLGA+ODN, L+ODN, and PLGA+L+ODN on weeks 2 and 4. The serum complement activity was significantly enhanced with L+ODN and PLGA+L+ODN on week 1 while it increased with PLGA+ODN, L+ODN, and PLGA+L+ODN on weeks 2 and 4. The antibody titre consistently was increased with PLGA or L encapsulated with ODN (PLGA+ODN, L+ODN, and PLGA+L+ODN) from weeks 1 to 4. The cumulative mortality was 20% each in PLGA+ODN administered groups and 15% each in ODN, L+ODN, and PLGA+L+ODN groups during a period of 30 days. The present study suggests that PLGA-liposome encapsulated ODN has the potential to modulate the immune system and can serve as a useful tool for further design of immunoprophylatic nano drug formulations against bacterial diseases.

The introduction of multi-copy CpG motifs into an antiviral DNA vaccine strongly up-regulates its immunogenicity in fish

The protection conferred by antiviral DNA vaccines in fish is known to rely greatly on innate immune responses. Since oligodeoxynucleotides (ODNs) containing unmethylated CpG dinucleotides (CpG motifs) have been shown to induce potential innate immune responses, we have introduced several copies (either two or four) of a fragment containing multiple CpG sequences of known immunostimulatory effects into a DNA vaccine against viral hemorrhagic septicemia virus (VHSV). We have determined the effects of this introduction on the vaccine immunogenicity, measured as immune gene induction, serum neutralizing activity and antigen-dependent proliferation. When comparing the effects of the vaccine containing 2 copies of this CpG fragment (pVHSV-2CpG) or that containing 4 copies of the fragment (pVHSV-4CpG) with the original VHSV DNA vaccine (pVHSV), we found that the levels of expression of type I interferon (IFN) were significantly up-regulated in muscle and spleen when the CpG fragments were introduced. An up-regulation in the levels of MHC-I expression in spleen were also observed in response to the modified vaccines, whereas, the levels of transcription of interleukin 1β (IL-1β) were strongly reduced in comparison to the original vaccine. Important but very variable differences were also observed concerning the vaccine induction of IFN-γ. Moreover, the serum neutralizing capacity was strongly increased as fish were vaccinated with plasmids containing more CpG fragments. Taken together, all these results demonstrate a strongly increased immunogenicity of the VHSV DNA vaccine, through the introduction of this multicopy CpG fragment.

In vitro analysis of the factors contributing to the antiviral state induced by a plasmid encoding the viral haemorrhagic septicaemia virus glycoprotein G in transfected trout cells

We have found out that transfection of the RTG-2 cell line with the viral haemorrhagic septicaemia virus (VHSV) glycoprotein G (G(VHSV))-coding plasmid induces an anti-VHSV state, similar to that induced by poly I:C. Taking the advantage of the constitutive expression of toll-like receptor 9 gene (tlr9) in RTG-2 cells, we have investigated whether this antiviral state was induced by the cytosine-phosphodiester-guanine (CpG) motifs present in the plasmid DNA, by the endogenous expression of G(VHSV) protein or by both elements. For that, we have analysed the expression profile of the rainbow trout tlr9 and several genes related to TLR9-mediated immune response in the absence or presence of a lysosomotropic drug that specifically blocks TLR9-CpG DNA interaction. The results suggested that the high levels of cell protection conferred by a plasmid encoding G(VHSV) gene are due to G(VHSV) rather than to the CpG motifs within plasmid DNA. Therefore, plasmid DNA might not play a key role in the immune response elicited by DNA vaccines or perhaps other receptors instead TLR9 could be implicated in CpG motifs recognition and signalling. In addition, since RTG-2 cells express tlr9 gene, this cell line could be a good tool for screening TLR9 agonists, such as the immunomodulatory oligonucleotides (IMOs), as fish DNA vaccine adjuvants.

Identification and analysis of a CpG motif that protects turbot (Scophthalmus maximus) against bacterial challenge and enhances vaccine-induced specific immunity

Oligodeoxynucleotides (ODNs) containing unmethylated CpG motifs in certain contexts are known to be immunostimulatory in vertebrate systems. CpG ODNs with immune effects have been identified for many fish species but, to our knowledge, not for turbot. In this study, a turbot-effective CpG ODN, ODN 205, was identified and a plasmid, pCN5, was constructed which contains the CpG motif of ODN 205. When administered into turbot via intraperitoneal (i.p.) injection, both ODN 205 and pCN5 could (i) inhibit bacterial dissemination in blood in dose and time dependent manners, and (ii) protect against lethal bacterial challenge. Immunological analyses showed that in vitro treatment with ODN 205 stimulated peripheral blood leukocyte proliferation, while i.p. injection with ODN 205 enhanced the respiratory burst activity, chemiluminescence response, and acid phosphatase activity of turbot head kidney macrophages. pCN5 treatment-induced immune responses similar to those induced by ODN 205 treatment except that pCN5 could also enhance serum bactericidal activity in a calcium-independent manner. To examine whether ODN 205 and pCN5 had any effect on specific immunity, ODN 205 and pCN5 were co-administered into turbot with a Vibrio harveyi subunit vaccine, DegQ. The results showed that pCN5, but not ODN 205, significantly increased the immunoprotective efficacy of DegQ and enhanced the production of specific serum antibodies in the vaccinated fish. Further analysis indicated that vaccination with DegQ in the presence of pCN5 upregulated the expression of the genes encoding MHC class IIalpha, IgM, Mx, and IL-8 receptor. Taken together, these results demonstrate that ODN 205 and pCN5 can stimulate the immune system of turbot and induce protection against bacterial challenge. In addition, pCN5 also possesses adjuvant property and can potentiate vaccine-induced specific immunity.

What happens to the DNA vaccine in fish? A review of current knowledge

The primary function of DNA vaccines, a bacterial plasmid DNA containing a construct for a given protective antigen, is to establish specific and long-lasting protective immunity against diseases where conventional vaccines fail to induce protection. It is acknowledged that less effort has been made to study the fate, in terms of cellular uptake, persistence and degradation, of DNA vaccines after in vivo administration. However, during the last year some papers have given new insights into the fate of DNA vaccines in fish. By comparing the newly acquired information in fish with similar knowledge from studies in mammals, similarities with regard to transport, blood clearance, cellular uptake and degradation of DNA vaccines have been found. But the amount of DNA vaccine redistributed from the administration site after intramuscular administration seems to differ between fish and mammals. This review presents up-to-date and in-depth knowledge concerning the fate of DNA vaccines with emphasis on tissue distribution, cellular uptake and uptake mechanism(s) before finally describing the intracellular hurdles that DNA vaccines need to overcome in order to produce their gene product.

Serum amyloid A: a typical acute-phase reactant in rainbow trout?

Acute serum amyloid A (A-SAA) has been considered a major acute-phase reactant and an effector of innate immunity in all vertebrates. The work presented here shows that the expression of A-SAA is strongly induced in a wide variety of immune-relevant tissues in rainbow trout, either naturally infected with Flavobacterium psychrophilum or challenged with lipopolysaccharide (LPS) or CpG oligonucleotides (CpG ODN). Nevertheless, A-SAA was undetectable by Western blot either in the plasma or in high-density lipoprotein (HDL) of infected or challenged fish, using either an anti-mouse SAA1 IgG or an anti-trout A-SAA peptide serum, which recognise both the intact recombinant trout A-SAA and fragments derived from it. However, the anti-peptide serum was the immunoreactive in all primary defence barriers and in mononuclear cells of head kidney, spleen and liver. These findings reveal that, unlike mammalian SAA, trout A-SAA does not increase significantly in the plasma of diseased fish, suggesting it is more likely to be involved in local defence.

CpG oligodeoxynucleotides up-regulate antibacterial systems and induce protection against bacterial challenge in rainbow trout (Oncorhynchus mykiss)

The effects of unmethylated CpG oligodeoxynucleotides (ODN) on the mammalian immune system are relatively well studied but much less is known of their effects on the immune systems of different fish species. Here we show that CpG ODNs significantly enhance the survival of rainbow trout (Oncorhynchus mykiss) following bacterial challenge when used both as stand-alone prophylactic agents, or as adjuvants to a commercially available vaccine. They are also capable of increasing serum lysozyme activity in vivo and stimulating the production of chemoattractant factors for rainbow trout head kidney (HK) leucocytes in vitro.

Construction of a recombinant plasmid containing multi-copy CpG motifs and its effects on the innate immune responses of aquatic animals

Bacterial DNA and synthetic oligodeoxynucleotides (ODNs) containing unmethylated CpG dinucleotides (CpG motifs) have been shown to induce potential immune responses. In this study, we designed a recombinant plasmid containing multi-copy CpG motifs, and observed its effects on innate immune responses of fish and prawn. The results showed that such plasmid DNA, compared to the vacant vector, can highly induce the activation of head kidney macrophages and the proliferation of peripheral blood leukocytes in Carassius auratus and Lateolabrax japonicus in vitro, as well as the activity of humoral defense proteins and the antibacterial activity of haemolymph in Litopenaeus vannamei in vivo. It implies that the multi-copy CpG motifs harboured in plasmid could contribute to these innate immunostimulatory effects. Therefore, the study suggested that the plasmid containing multi-copy CpG motifs might have its potential application in improving host resistance to pathogen insults in aquaculture, and have its notable advantages of high efficacy, economical cost and application to a broad range of aquatic species.

Characterization of three distinct CpG oligonucleotide classes which differ in ability to induce IFN alpha/beta activity and cell proliferation in Atlantic salmon (Salmo salar L.) leukocytes

Oligodeoxynucleotides (ODNs) containing unmethylated CpG dinucleotides within specific sequence contexts (CpG motifs) are in humans divided into three distinct classes (A, B and C). The CpG ODNs, like baterial DNA, are recognized by the vertebrate immune system and are known to stimulate immune responses. The characterization of the different classes is based on their structure and biological activity. In this study, we report the effects of these classes of CpG ODNs as they are defined in humans on IFN alpha/beta production and cell proliferation in Atlantic salmon spleen, head kidney and blood leukocytes. These studies revealed that CpG A together with CpG C are strong inducers of IFN alpha/beta in spleen and head kidney leukocytes, whilst CpG B and CpG C had the highest capacity to stimulate cell proliferation in all three cell populations. These findings are the first to establish that the effects of the different CpG classes are comparable between fish and man.

A review of CpGs and their relevance to aquaculture

CpG oligodeoxynucleotides (ODN) have been described as functioning as natural adjuvants because they promote professional antigen presenting cell (APC) function and co-stimulate lymphocytes. The majority of studies into the immune effects of CpG ODN to date have been carried out on mammals where they are proving very successful at stimulating innate and adaptive immune responses in a variety of species as well as protecting them from bacterial, viral and protozoan pathogens. Fish also possess the ability to raise both innate and adaptive immune responses to invading pathogens and interest in the effect of CpG ODN on the piscine immune system is growing. Various studies have now been carried out to elicit the effects of CpG ODN on diverse fish species showing that 31 different B-class CpG ODN exert various immune responses both in vivo and in vitro in salmonids, cyprinids and pleuronectiformes. These responses include activation of macrophages, proliferation of leucocytes and stimulation of cytokine expression. CpG ODN have also been shown to be protective against bacterial and viral challenge as well as against pathogenic amoebae. As would be expected these effects are all dependent on not only the ODN sequence and length but on the concentration and the species in which it is being used. This review provides the first comprehensive overview of all CpG ODN tested in fish to date and brings together all the work carried out in this field.

Stimulation of type I IFN activity in Atlantic salmon (Salmo salar L.) leukocytes: synergistic effects of cationic proteins and CpG ODN

Unmethylated CpG motifs in DNA are recognised by vertebrate immune cells as pathogen signatures. Consequently, oligodeoxynucleotides containing CpG motifs (CpG ODNs) are able to enhance and direct immune responses. Recent studies have demonstrated that CpG ODNs activate antiviral immune responses in Atlantic salmon (Salmo salar L.) leukocytes, and are therefore promising agents as vaccine adjuvants or immunostimulants in fish. In this work, we report synergy of CpG ODN and cationic proteins in the stimulation of type I IFN activity in Atlantic salmon leukocytes. Different fractions of cationic histone proteins derived from cod milt or poly-l-arginine and poly-l-lysine were screened for their ability to enhance CpG ODN induced type I IFN activity in Atlantic salmon leukocytes. Optimal ratio of histones to CpG ODN was identified, and effects on transcription of type I IFN and antiviral Mx genes were studied. Delivery of CpG ODN with cationic proteins enhanced the production of type I IFN and succeeding Mx transcripts after two and five days of stimulation at substimulatory concentrations of CpG ODN. These results indicate that co-delivery of CpG ODN and cationic proteins enhance antiviral mechanisms in Atlantic salmon leukocytes as compared to CpG ODN alone.