Structure and Specificity of Lamprey Monoclonal Antibodies

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The adaptive immune system of the extant agnathans (lamprey and hagfish) is comprised of clonally diverse lymphocytes that express variable lymphocyte receptors (VLRs) created by the combinatorial assembly of leucine‐rich repeat (LRR) gene segments. During the development of lymphocyte lineage cells, components of the flanking LRR modular units are sequentially inserted into the incomplete germline VLR gene via a gene conversion mechanism to create a potential repertoire of > 1014distinct VLR‐B antibodies. To generate VLR‐B antibody‐secreting cells, we developed a heterologous expression system consisting of HEK‐293T cells transfected with VLR‐B cDNAs. Antigen‐specific VLR‐B antibody clones were isolated by screening VLR‐B cDNA libraries, derived from the lymphocytes of immunized lamprey, for antigen binding. The recombinant VLR‐B antibodies consisted of 8 – 10 uniform subunits arranged into disulfide‐linked tetramers and pentamers of dimers that collectively bind antigen with high avidity. Sequence analysis, mutagenesis, and modeling of antigen‐specific VLR‐B antibodies demonstrated that the primary antigen‐binding site resides in the β ‐sheets of the LRR subunits lining the VLR‐B concave surface. The remarkable antigen‐binding specificity, avidity, and stability of these unusual LRR‐based monoclonal antibodies suggest they will find many biomedical uses.