Conservation of structural and functional features in a primordial CD80/86 molecule from rainbow trout (Oncorhynchus mykiss), a primitive teleost fish.

In mammals, interaction of CD28 with CD80 or CD86 molecules provides costimulatory signals for T cell activation that leads to increased IL-2 gene and protein expression by activated T cells. Thus far, CD80 and CD86 have been cloned and functionally characterized only in mammals and birds. To shed light into the evolution of CD80 and CD86, we have cloned and functionally characterized a rainbow trout (rt) molecule (rtCD80/86) that shows the highest degree of sequence conservation and phylogenetic relationship with CD80 and CD86 molecules. Moreover, its genomic organization was almost identical to that of human CD86. Rainbow trout possess one membrane-bound and two soluble CD80/86 transcripts, all of which are derived from the same rtCD80/86 gene. The membrane-bound form exhibited its highest degree of expression in lymphoid tissues, particularly on B cells. Incubation of trout leukocytes with LPS and bacteria leads to up-regulation of rtCD80/86 gene expression. Importantly, we show that trout and other teleost fish contain a single CD80/86 gene, thus suggesting that this gene may represent the ancestor from which CD80 and CD86 arose by gene duplication in more evolved species. To gain further insights into the function of rtCD80/86, we have identified and cloned trout IL-2 and have shown that recombinantly produced trout CD80/86 up-regulates the expression of IL-2 in trout blood leukocytes. Significantly, this finding indicates that the capacity to modulate IL-2 expression is a primordial function that has been conserved both in fish and mammalian CD80/CD86 molecules throughout 350 million years of evolution.