Evolutionary implications of a third lymphocyte lineage in lampreys

Jawed vertebrates (gnathostomes) and jawless vertebrates (cyclostomes) have different adaptive immune systems. Gnathostomes use T- and B-cell antigen receptors belonging to the immunoglobulin superfamily. Cyclostomes, the lampreys and hagfish, instead use leucine-rich repeat proteins to construct variable lymphocyte receptors (VLRs), two types of which, VLRA and VLRB, are reciprocally expressed by lymphocytes resembling gnathostome T and B cells. Here we define another lineage of T-cell-like lymphocytes that express the recently identified VLRC receptors. Both VLRC⁺ and VLRA⁺ lymphocytes express orthologues of genes that gnathostome γδ and αβ T cells use for their differentiation, undergo VLRC and VLRA assembly and repertoire diversification in the `thymoid' gill region, and express their VLRs solely as cell-surface proteins. Our findings suggest that the genetic programmes for two primordial T-cell lineages and a prototypic B-cell lineage were already present in the last common vertebrate ancestor approximately 500 million years ago. We propose that functional specialization of distinct T-cell-like lineages was an ancient feature of a primordial immune system.