Two types of recombinatorial adaptive immune systems appeared at the dawn of vertebrate radiation ~500 million years ago. Jawed vertebrates generate a diverse repertoire of B and T cell antigen receptors through V(D)J rearrangement of immunoglobulin (Ig) gene fragments. In sharp contrast, jawless vertebrates such as the sea lamprey, assemble their variable lymphocyte receptors (VLR) through recombinatorial insertion of diverse leucine-rich repeat (LRR) modular units, from flanking cassettes into the germline VLR gene. The combinatorial diversity of the repertoire is about 1014 unique antigen receptors, both for the LRR and the Ig-based adaptive immune systems. This is intriguing when considering that jawless and jawed vertebrates are the only two vertebrate taxons that survived from the Cambrian period. What was the selective pressure to evolve not one but two different types of rearranging lymphocyte antigen receptors? Did VLRs and antibodies evolve by chance and necessity to recognize similar, or perhaps different determinants of the antigenic world? The role of VLRs in lamprey immunity will be explored in this project. Antigen recognition by VLR will be studied in animals immunostimulated with various types of antigens, including foreign proteins, microbial cell wall components, and microbial DNA and RNA. Libraries of VLRs will be constructed from lymphocytes of immunized animals, to select those receptors that can bind the antigens with high specificity. Selected VLRs bound to their cognate ligand will then be crystallized, to determine the three-dimensional structure of the complexes, and to identify contact residues between the VLR and its ligand. Functional and structural characterization of the VLR may help elucidate the origin of adaptive immunity, in particular since LRR modules are also the building blocks of our own cardinal innate microbial detection receptors, such as the Toll-like receptors and the cytoplasmic NBS-LRR family members, and the LRR-containing Disease Resistance genes of plants. This project will involve the training of several students at varying academic backgrounds. In addition, Dr. Pancer will incorporate his research results in teaching a course in comparative and evolutionary immunobiology.
