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* Institut National de la Recherche Agronomique, Unité de Virologie et Immunologie Moléculaires, Jouy-en-Josas, France;
Unité dImmunophysiopathologie Infectieuse, Institut Pasteur, Paris, France;
Institut National de la Recherche Agronomique, Unité de Recherches Laitières et de Génétique Appliquée, Jouy-en-Josas, France; and
Institut National de la Recherche Agronomique, Laboratoire de Génétique des Poissons, Jouy-en-Josas, France
Gut-associated lymphocytes were described in fish, but their involvement in immune responses is still unknown. In rainbow trout, intraepithelial lymphocytes (IELs) are scattered between gut epithelial cells, but neither Peyers patches nor mesenteric lymph nodes were identified. Rainbow trout IELs contain mainly T cells, because they expressed transcripts of T cell marker homologs of CD8, CD4, CD28, CD3
, TCR
, TCR
, and TCR
and lacked IgM. However, trout IELs did not show specific homing to the gut mucosa, which in mammals defines IELs as a distinctive mucosal population. A detailed analysis of the TCR
repertoire of rainbow trout IELs was performed in both naive and virus-infected animals. TCR
transcripts of rainbow trout IELs were highly diverse and polyclonal in adult naive individuals, in sharp contrast with the restricted diversity of IEL oligoclonal repertoires described in birds and mammals. Significant modifications of the trout IEL TCR
repertoire were observed after a systemic infection with a fish rhabdovirus and were especially marked for V
4-bearing receptors as previously reported for spleen cells. Thus, we could not find any specific properties of the trout IEL TCR
repertoire compared with the spleen and pronephros TCR
repertoire, which questions the reality of a distinct IEL compartment in teleosts. Our findings suggest that a highly diversified 
TCR repertoire is maintained in fish IELs in the absence of Peyers patches and mesenteric lymph nodes, whereas the restricted diversity of mouse 
IELs is attributed to multiple cycles of activation and recirculation, allowing a progressive narrowing of the repertoire.
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