VLRB

VLRB, or Variable Lymphocyte Receptor B, is an antigen receptor found in jawless vertebrates, notably lampreys. It is one of three VLR families expressed by lymphocytes, the others being VLRA and VLRC. In lampreys, VLRB-expressing lymphocytes are B-like and can produce membrane-bound receptors or secrete soluble VLRB molecules that participate in antigen recognition.

Structure and generation: VLRB proteins are built from leucine-rich repeats arranged in a single polypeptide; the mature receptor comprises an N-terminal LRR cap, a central array of LRRs, and a C-terminal LRR cap. Diversity is generated by somatic assembly of donor LRR cassettes into a germline VLRB locus, a process distinct from immunoglobulin V(D)J recombination. The resulting repertoire is extremely diverse and antigen-specific.

Expression and function: VLRB+ cells can be activated by pathogens such as bacteria and viruses; secreted VLRBs can form multimeric complexes, enabling soluble antigen binding. The VLRB system provides a humoral-like arm of adaptive immunity in jawless vertebrates, complementing cell-mediated VLRA- and VLRC-expressing lineages.

Significance: The VLR system illustrates convergent evolution of adaptive immunity, showing that highly specific antigen receptors can arise from different molecular architectures. Research on VLRBs informs understanding of receptor diversification mechanisms, the evolution of immune systems, and potential biotechnological applications.