B Cells and Antibodies in Jawless Vertebrates

Abstract An adaptive immune system (AIS) based on clonally diverse lymphocytes that can recognize specific pathogens and provide protective memory against a second encounter is found only in vertebrate species. Alternative AISs have now been defined in jawed and jawless vertebrates. Both of these systems use lymphocytes with a wide variety of anticipatory receptors, but they differ in that the lymphocytes in jawless vertebrates (lampreys and hagfish) use leucine-rich repeat (LRR)-based variable lymphocyte receptors (VLRs) for antigen recognition, whereas lymphocytes in vertebrates with jaws use immunoglobulin (Ig)-based receptors for the same purpose. There are three types of lamprey VLR genes, VLRA , VLRB , and VLRC , that are assembled and expressed in a clonally diverse manner by separate populations of lymphocytes that resemble our thymus-derived γδ and αβ T lymphocytes and bone marrow-derived B lymphocytes. The parallels between these lymphocyte lineages in jawless and jawed vertebrates indicate that the genetic programs for the major lymphocyte differentiation pathways evolved before the convergent evolution of the VLR and B cell receptor/T cell receptor types of antigen receptors. This chapter focuses on the current understanding of the lamprey B cells and their LRR-based antibody products.