The B7 family of immunoregulatory receptors: a comparative and evolutionary perspective.

In mammals, T cell activation requires specific recognition of the peptide-MHC complex by the TcR and co-stimulatory signals. Important co-stimulatory receptors expressed by T cells are the molecules of the CD28 family, that regulate T cell activation, proliferation and tolerance. These receptors recognize B7s and B7-homologous (B7H) molecules that are typically expressed by the antigen presenting cells. In teleost fish, typical T cell responses have been described and the TcR, MHC and CD28/CTLA4 genes have been characterized. In contrast, the members of the B7 gene family have only been described in mammals and birds and have yet to be addressed in lower vertebrates. To learn more about the evolution of components guiding T cell activation in vertebrates, we performed a systematic genomic survey for the B7 co-stimulatory and co-inhibitory IgSF receptors in lower vertebrates with an emphasis on teleost fish. Our search identified fish sequences that are orthologous to B7, B7-H1/B7-DC, B7-H3 and B7-H4 as defined by sequence identity, phylogeny and combinations of short or long-range syntenic relationships. However, we were unable to identify clear orthologs for B7-H2 (CD275, ICOS ligand) in bony fish, which correlates with our prior inability to find ICOS in fish. Interestingly, our results indicate that teleost fish possess a single B7.1/B7.2 (CD80/86) molecule that likely interacts with CD28/CTLA4 as the ligand-binding regions seem to be conserved in both partners. Overall, our analyses implies that gene duplication (and loss) have shaped a molecular repertoire of B7-like molecules that was recruited for the refinement of T cell activation during the evolution of the vertebrates.