VLRAexpressing

VLRAexpressing, often written as VLRA-expressing, refers to a lineage of lymphocytes in jawless vertebrates that express the VLRA receptor, a member of the variable lymphocyte receptor (VLR) family. In lampreys and hagfish, the adaptive immune system relies on VLRs rather than immunoglobulins. Three major VLR lineages define the lymphocyte repertoire: VLRA-expressing, VLRC-expressing, and VLRB-expressing cells. VLRA-expressing and VLRC-expressing cells are regarded as T-like, whereas VLRB-expressing cells act in ways comparable to B cells.

Diversity of VLRs is achieved by somatic rearrangement and gene conversion of leucine-rich repeat (LRR) modules within VLR gene cassettes, generating a highly diverse set of receptors without V(D)J recombination. In individual lymphocytes, expression tends to be restricted to a single VLR type, producing clonal populations with defined antigen specificities.

Activation and function: VLRA-expressing cells respond to antigen exposure and cytokine signals, undergoing clonal expansion that constitutes cell-mediated-like responses. The VLRC and VLRB lineages exhibit distinct effector programs; together they mediate the lamprey's adaptive immune responses. The existence of memory-like states in VLR-based immunity is an area of ongoing research.

Significance: The VLRA-expressing lineage exemplifies convergent evolution of adaptive immunity in jawless vertebrates, showing how somatic diversification of receptor repertoires can support antigen-specific responses without antibodies. Ongoing research aims to clarify lineage relationships, development, and the full spectrum of functions of VLRA-expressing cells.