Discovery of a VLRB-GPIbβ-GPⅨ Complex Illuminates Lineage-Specific Redeployment of a Platelet Module for VLR Signaling

Abstract Unlike jawed vertebrates that rely on immunoglobulin (Ig)-based adaptive immunity, jawless vertebrates utilize variable lymphocyte receptors (VLRs) for antigen recognition. Despite extensive studies into VLR diversity and antigen recognition, the molecular mechanism coupling VLR engagement to lymphocyte activation remains unexplored. By mining single-cell and full-length transcriptomes data of lamprey and hagfish along with functional studies, we successfully discovered a VLRB cell-specific tripartite complex comprising VLRB, GPIbβ, and GPⅨ that transduces activation signals via an immunoreceptor tyrosine-based activation motif (ITAM) within GPIbβ. Phylogenetic reconstruction revealed that this complex originated from a lineage-specific duplication of the ancestral platelet GPIbα-GPIbβ-GPⅨ complex in agnathans, yielding two functionally divergent paralogs, one retained in thrombocytes and the other co-opted by VLRB lymphocytes. Transcriptional profiling further demonstrated that BHLHE41 and FOS serve as lineage-restricted co-regulators that coordinately govern expression of the VLRB-GPIbβ-GPⅨ subunits. Analogous to mammalian B cells, scRNA transcriptome revealed that VLRB cells can transition from a pre-activated to a post-activated state characterized by highly proliferative, plasma-like features, and concomitant up-regulation of the VLRB-GPIbβ-GPⅨ complex. Collectively, a lineage-specific gene-duplication event in agnathan repurposed a platelet-adhesion module into a VLRB cell-specific signaling complex through lineage-specific redeployment, unveiling a key missing link in VLR-mediated adaptive immunity.