Sharks Provide Evidence for a Highly Complex TNFSF Repertoire in the Jawed Vertebrate Ancestor

Cytokines of the TNF superfamily (TNFSF) control many immunological processes and are implicated in the etiology of many immune disorders and diseases. Despite their obvious biological importance, the TNFSF repertoires of many species remain poorly characterized. In this study, we perform detailed bioinformatic, phylogenetic, and syntenic analyses of five cartilaginous fish genomes to identify their TNFSF repertoires. Strikingly, we find that shark genomes harbor ∼30 TNFSF genes, more than any other vertebrate examined to date and substantially more than humans. This is due to better retention of the ancestral jawed vertebrate TNFSF repertoire than any other jawed vertebrate lineage, combined with lineage-specific gene family expansions. All human TNFSFs appear in shark genomes, except for lymphotoxin-α (LTA; TNFSF1) and TNF (TNFSF2), and CD70 (TNFSF7) and 4-1BBL (TNFSF9), which diverged by tandem duplications early in tetrapod and mammalian evolution, respectively. Although lacking one-to-one LTA and TNF orthologs, sharks have evolved lineage-specific clusters of LTA/TNF co-orthologs. Other key findings include the presence of two BAFF (TNFSF13B) genes along with orthologs of APRIL (TNFSF13) and BALM (TNFSF13C) in sharks, and that all cartilaginous fish genomes harbor an ∼400-million-year-old cluster of multiple FASLG (TNFSF6) orthologs. Finally, sharks have retained seven ancestral jawed vertebrate TNFSF genes lost in humans. Taken together, our data indicate that the jawed vertebrate ancestor possessed a much larger and diverse TNFSF repertoire than previously hypothesized and oppose the idea that the cartilaginous fish immune system is "primitive" compared with that of mammals.