Identification of two FoxP3 genes in rainbow trout (Oncorhynchus mykiss) with differential induction patterns

FoxP3 is a master transcription factor for the development and function of regulatory T cells in mammals, but little is known about this molecule in fish. Two paralogues of mammalian FoxP3 that share 83.9% identity at the amino acid level have been identified in rainbow trout (Oncorhynchus mykiss). The C-terminal region containing a Zn_C2H2 domain, a leucine zipper-like domain and a forkhead (FH) domain important for dimerization, nuclear translocation, and DNA binding, is well conserved between fish and other vertebrate FoxP3. However, the N-terminal of FoxP3 that is required for FoxP3-mediated repression of transcription is greatly diverged between fish, amphibians and monotreme mammals compared to eutherian mammals, suggesting that FoxP3 in fish, frog and platypus may have a different role to the human and mouse counterpart that defines the Treg cellular lineage and mediates the immune regulatory function. The expression of both trout (t) FoxP3a and tFoxP3b are detectable in all the 14 tissues examined without any significant difference except in muscle in which the expression of tFoxP3a was higher. Both tFoxP3a and tFoxP3b are highly expressed in thymus and in immune related organs including the spleen, kidney, gills and intestine, and are up-regulated by phytohaemagglutinin (PHA) in splenocytes and thymocytes. Whilst the up-regulated tFoxP3b expression induced by PHA was dose-dependent it required a higher PHA concentration to achieve maximal expression relative to tFoxP3a where the highest expression level was seen using 1 μg/ml PHA with higher concentrations having no further effects. In addition, the tFoxP3b expression increased during development from eyed eggs to fry, when it reached a comparable level to that of tFoxP3a. In contrast, tFoxP3a expression was at a high and almost constant level over all of the developmental stages examined. The high level of tFoxP3a expression in early development may be related to the relatively high constitutive level of tFoxP3a expression seen in muscle, perhaps suggesting novel roles of tFoxP3 in fish muscle. The structural and expression analysis suggests that the tFoxP3a and tFoxP3b are subject to differential modulation of expression and may have evolved novel functions. The identification of the two trout FoxP3 paralogues will help to clarify the existence of Treg cells and to dissect the T cell differentiation pathways in fish.