Teleost fish mount complex clonal IgM and IgT responses in spleen upon systemic viral infection.

PLoS Pathog. 2013 Jan;9(1):e1003098.

Castro R, Jouneau L, Pham HP, Bouchez O, Giudicelli V, Lefranc MP, Quillet E, Benmansour A, Cazals F, Six A, Fillatreau S, Sunyer O, Boudinot P.

Virologie et Immunologie Moléculaires, INRA, Jouy-en-Josas, France.

Abstract

Upon infection, B-lymphocytes expressing antibodies specific for the intruding pathogen develop clonal responses triggered by pathogen recognition via the B-cell receptor. The constant region of antibodies produced by such responding clones dictates their functional properties. In teleost fish, the clonal structure of B-cell responses and the respective contribution of the three isotypes IgM, IgD and IgT remain unknown. The expression of IgM and IgT are mutually exclusive, leading to the existence of two B-cell subsets expressing either both IgM and IgD or only IgT. Here, we undertook a comprehensive analysis of the variable heavy chain (VH) domain repertoires of the IgM, IgD and IgT in spleen of homozygous isogenic rainbow trout (Onchorhynchus mykiss) before, and after challenge with a rhabdovirus, the Viral Hemorrhagic Septicemia Virus (VHSV), using CDR3-length spectratyping and pyrosequencing of immunoglobulin (Ig) transcripts. In healthy fish, we observed distinct repertoires for IgM, IgD and IgT, respectively, with a few amplified μ and τ junctions, suggesting the presence of IgM- and IgT-secreting cells in the spleen. In infected animals, we detected complex and highly diverse IgM responses involving all VH subgroups, and dominated by a few large public and private clones. A lower number of robust clonal responses involving only a few VH were detected for the mucosal IgT, indicating that both IgM(+) and IgT(+) spleen B cells responded to systemic infection but at different degrees. In contrast, the IgD response to the infection was faint. Although fish IgD and IgT present different structural features and evolutionary origin compared to mammalian IgD and IgA, respectively, their implication in the B-cell response evokes these mouse and human counterparts. Thus, it appears that the general properties of antibody responses were already in place in common ancestors of fish and mammals, and were globally conserved during evolution with possible functional convergences.

PMID: 23326228

 

Supplement:

Upon infection Ag-specific B and T lymphocytes develop protective responses against the intruding pathogens, and after disease resolution they maintain an increased resistance against the eliciting agent through persistence of memory cells, which is the basis of vaccination.

Similarly to mouse or human B cells, fish B lymphocytes express immunoglobulin (Ig) on their surface and secrete antigen-specific antibodies in response to immune challenges. Three antibody classes are expressed in fish, namely IgM, IgD, and IgT, while IgG, IgA, and IgE are absent. IgM and IgD were found in all fish species analyzed, and constitute primordial antibody classes. IgM and IgD are generally co-expressed from the same VDJ rearrangement through alternative splicing, as in mammals. IgM (tetrameric) is the main antibody class found in serum. Another class of antibody, IgT, has been identified in several species. IgT has been found only in fish, and is particularly important for mucosal immunity. IgT are expressed from rearrangements using the same set of VH segments as IgM/IgD, but different sets of D and J segments. A B cell does not co-express IgM/IgD and IgT, the distinct expression of these isotypes indicates the existence of two distinct lineages of B cells in fish.

Pierre Boudinot-1

In fish, the clonal   structure of B-cell responses and the respective contribution of antibody   classes remain unknown.  This study is the first comprehensive   analysis of the impact of an infection on the variable heavy chain (VH)   domain repertoires of IgM, IgD and IgT in fish. The B cell repertoire of   the spleen of homozygous isogenic rainbow trout (Onchorhynchus mykiss)   was characterized before and after challenge with an important fish   rhabdovirus (the Viral Hemorrhagic Septicemia Virus, VHSV) using CDR3-length spectra-typing and pyrosequencing of immunoglobulin (Ig) transcripts.

In healthy fish, we observed distinct repertoires for IgM, IgD and IgT, respectively, with a few IgM- and IgT-secreting cells in the spleen. In infected animals, we detected complex and highly diverse IgM responses involving all VH subgroups, and dominated by a few large public and private clones. A lower number of robust clonal responses involving only a few VH were detected for the mucosal isotype IgT, indicating that both IgM+ and IgT+ spleen B cells responded to systemic infection but at different degrees. In contrast, the IgD response to the infection was faint. The implication of fish IgD and IgT in the B-cell response evokes their mouse and human counterparts, namely IgD and IgA. Thus, it appears that the general properties of B cell response in the common ancestor of mammals and fishes were globally conserved during the evolution of these two lineages

             The importance of this study is two-fold: (1) This study is a first comprehensive description of the Ig response to an infection in a lower vertebrate. It demonstrates that a secondary viral infection induces in spleen a strong IgM response with a high clonal complexity consisting of both public and private components, while the mucosal IgT is also involved to a lesser extent. (2) Our results provide a frame for further characterization and evaluation of responses induced by vaccines in aquaculture. Vaccines in aquaculture constitute a critical economical issue to minimize the utilization of antibiotics and to prevent viral diseases for which no effective treatment is available.

 

Acknowledgements

This work was supported by the French Institut National de la Recherche Agronomique, by the European Community’s Seventh Framework Program (LIFECYCLE), by Deutsche Forschungsgemeinschaft, the US National Science Foundation and the US National Institute of Health. Drs H. Rezaei, E. Duchaud, D. Thepot and J-P Levraud are gratefully acknowledged for insightful discussions.

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