The Origin and Early Evolution of Adaptive Immune Systems

Abstract

Two forms of recombinatorial adaptive immune systems arose in vertebrates about 480 million years ago. The repertoire of immunoglobulin domain-based T and B cell antigen receptors in jawed vertebrates is diversified primarily through the rearrangement of V(D)J (variable, diversity, and/or joining) gene segments and somatic hypermutation, but none of the major recognition elements in jawed vertebrates have been found in jawless vertebrates. Instead, the adaptive immune systems of jawless vertebrates are based on variable lymphocyte receptors (VLRs) that are generated through recombinatorial usage of a large panel of greatly diverse leucine rich repeat sequences. Whereas the emergence of transposon-like, recombination-activating genes (RAGs) contributed uniquely to the origin of the adaptive immune systems in jawed vertebrates, the use of activation-induced cytidine deaminase (AICDA) for receptor diversification is common to both the jawed and jawless vertebrates. Despite these differences in anticipatory receptor structure, the basic design of adaptive immune system featuring two cooperating T and B lymphocyte arms apparently evolved in an ancestor of jawed and jawless vertebrates within the context of established innate immunity and has been sustained due to powerful and durable selection, mostly for pathogen defense commitments.