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37 p.-8 fig.-1 tab. ; There are still many details of how intestinal immunity is regulated that remain unsolved in teleost.Although leukocytes are present all along the digestive tract, most immunological studies have focused on the posterior segments and the importance of each gut segment in terms of immunity has barely been addressed. In the current work, we have studied the regulation of several immune genes along five segments of the rainbow trout (Oncorhynchus mykiss) digestive tract, comparing the effects observed in response to an infectious pancreatic necrosis virus (IPNV) infection to those elicited by oral vaccination with a plasmid coding for viral VP2. We have focused on the regulation of several mucosal chemokines,chemokine receptors, the major histocompatibility complex II (MHC-II) and tumor necrosis factor a (TNFa).Furthermore, the recruitment of IgM+ cells and CD3+ cells was evaluated along the different segments in response to IPNV by immunohistochemical techniques. Our results provide evidences that there is a differential regulation of these immune genes in response to both stimuli along the gut segments. Along with this chemokine and chemokine receptor induction, IPNV provoked a mobilization of IgM+ and IgT+ cells to the foregut and pyloric caeca region, and CD3+ cells to the pyloric caeca and midgut/hindgut regions. Our results will contribute to a better understanding of how mucosal immunity is orchestrated in the different gut segments of teleost. ; This work was supported by the European Commission under the 7th Framework Programme for Research and Technological Development (FP7) of the European Union (Grant Agreement 311993 TARGETFISH), by projects AGL2011-29676 and AGL2010-18454 from the Spanish Ministry of Economy and Competitiveness (MINECO) and project 201020E084 from the Consejo Superior de Investigaciones Científicas (CSIC). ; Peer reviewed

ACKNOWLEDGMENTS The authors want to thank Dr. Oriol Sunyer for the anti-IgT and Dr. Uwe Fischer for the anti-CD8 antibodies used in this study. We also want to acknowledge Lucía González Torres for technical assistance. GRANT SUPPORT This work was supported by the European Research Council (ERC Starting Grant 2011 280469), by the European Commission under the 7th Framework Programme for Research and Technological Development (FP7) of the European Union (Grant Agreement 311993 TARGETFISH) and by project AGL2011-29676 from the Spanish Ministry of Economy and Competitiveness (MINECO). C. Aquilino was supported by a MINECO PhD student fellowship ; Peer reviewed ; Publisher PDF

Passive immunization constitutes an emerging field of interest in aquaculture, particularly with the restrictions for antibiotic use. Enteromyxum leei is a myxozoan intestinal parasite that invades the paracellular space of the intestinal epithelium, producing a slow-progressing disease, leading to anorexia, cachexia and mortalities. We have previously demonstrated that gilthead sea bream (GSB, Sparus aurata) that survive E. leei infection become resistant upon re-exposure, and this resistance is directly related to the presence of high levels of specific IgM in serum. Thus, the current work was aimed to determine if passive immunization could help to prevent enteromyxosis in GSB and to study in detail the nature of these protective antibodies. Serum from a pool of resistant (SUR) or naïve (NAI) animals was intracoelomically injected 24 h prior to the E. leei-effluent challenge and at 9 days post-challenge (dpc). Effluent challenge lasted for 23 days, and then the injected groups were allocated in separate tanks with clean water. A non-lethal parasite diagnosis was performed at 56 dpc. At the final sampling (100 dpc), blood, serum and tissues were collected for histology, molecular diagnosis and the detection of circulating antibodies. In parallel, we performed an immunoglobulin repertoire analysis of the fish generating SUR and NAI sera. The results showed that, fish injected with parasite-specific antibodies (spAbs) became infected with the parasite, but showed lower disease signs and intensity of infection than the other groups, indicating a later establishment of the parasite. Repertoire analysis revealed that E. leei induced a polyclonal expansion of diverse IgM and IgT subsets that could be in part an evasion strategy of the parasite. Nonetheless, GSB was able to produce sufficient levels of parasite-spAbs to avoid re-infection of surviving animals and confer certain degree of protection upon passive transfer of antibodies. These results highlight the crucial role of spAb responses against E. leei and set the basis for the development of effective treatment or prophylactic methods for aquaculture. ; This work was funded by EU H2020 program through ParaFishControl Project (634429) and by the European Research Council (ERC Consolidator Grant 2016 725061 TEMUBLYM). This publication reflects only the authors' view and the European Union cannot be held responsible for any use that may be made of the information contained herein. MP was funded by a Ramón y Cajal Postdoctoral Research Fellowship (RYC2018-024049-I/AEI/10.13039/501100011033 & ESF), IE under APOSTD/2016/037 grant by the "Generalitat Valenciana" and RP was contracted under the PTA-Program from the Spanish Ministry of Science, Innovation and Universities (PTA2018-015315-I). We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).

We would like to thank Lucia Gonzalez and Maria Sanz for technical assistance. Professor Øystein Evensen is also acknowledged for providing us with the inactivated IPNV. This work was supported by the European Research Council (ERC Starting Grant 2011 280469) and by the European Commission under the 7th Framework Programme for Research and Technological Development (FP7) of the European Union (Grant Agreement 311993 TARGETFISH). T. W. received funding from the MASTS pooling initiative (The Marine Alliance for Science and Technology for Scotland). MASTS is funded by the Scottish Funding Council (grant reference HR09011). ; Peer reviewed ; Publisher PDF

ACKNOWLEDGMENTS The VHSV-infected samples were generated within the Scottish Government funded research project FC1996 and kindly provided by Marine Scotland staff. Thanks to ELANCO for providing the A. davidanieli (Renogen). FINANCIAL SUPPORT T. W. received funding from the MASTS pooling initiative (The Marine Alliance for Science and Technology for Scotland). MASTS is funded by the Scottish Funding Council (grant reference HR09011) and contributing institutions. Y.J., W.H. and Q.X. were supported financially by the National Scholarship Council of China. Z.Q. was supported by grants from the National Natural Science Foundation of China (31302221) and the overseas training plan for young and middle-aged teachers and principals of colleges and universities in Jiangsu Province, China. M.M.C. was funded by an Ángeles Alvariño postdoctoral contract from the Consejo Superior de Investigaciones Científicas and the Xunta de Galicia. P.D.-R. was funded by a European Commission (EC) Marie Curie Intra European Fellowship (FP7). J.W.H. was funded by the Biotechnology and Biological Sciences Research Council (BB/K009125/1). This work was also supported financially by the EC, under contract Nos. 222719 (LIFECYCLE) and 311993 (TargetFish), and by the European Research Council Starting Grant 2011 (contract No. 280469). ; Peer reviewed ; Publisher PDF