Phenotypic and functional similarity of gut intraepithelial and systemic T cells in a teleost fish

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 Peyer's 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 is an element of, TCR zeta, TCR gamma, and TCR beta 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 TCRP repertoire of rainbow trout IELs was performed in both naive and virus-infected animals. TCR beta 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 beta repertoire were observed after a systemic infection with a fish rhabdovirus and were especially marked for V beta 4-bearing receptors as previously reported for spleen cells. Thus, we could not find any specific properties of the trout IEL TCRP repertoire compared with the spleen and pronephros TCR beta repertoire, which questions the reality of a distinct IEL compartment in teleosts. Our findings suggest that a highly diversified alpha beta TCR repertoire is maintained in fish IELs in the absence of Peyer's patches and mesenteric lymph nodes, whereas the restricted diversity of mouse alpha beta IELs is attributed to multiple cycles of activation and recirculation, allowing a progressive narrowing of the repertoire.