Evolutionary implications of a third lymphocyte lineage in lampreys

Area of Science:

• Immunology
• Evolutionary Biology
• Vertebrate Zoology

Background:

• Jawed vertebrates (gnathostomes) possess adaptive immunity based on immunoglobulin superfamily receptors (T-cell and B-cell receptors).
• Jawless vertebrates (cyclostomes), like lampreys and hagfish, employ leucine-rich repeat proteins for variable lymphocyte receptors (VLRs), including VLRA and VLRB, in distinct lymphocyte populations.
• Previous research identified VLRA and VLRB, but the existence and function of other VLR-expressing lymphocytes remained unclear.

Purpose of the Study:

• To identify and characterize novel lymphocyte lineages in jawless vertebrates.
• To investigate the genetic and developmental pathways of these new lymphocytes.
• To understand the evolutionary origins of adaptive immune systems in vertebrates.

Main Methods:

• Identification and characterization of lymphocytes expressing the VLRC receptor in cyclostomes.
• Analysis of gene orthologues involved in lymphocyte differentiation in gnathostomes and cyclostomes.
• Investigation of VLR assembly and repertoire diversification in the 'thymoid' gill region.

Main Results:

• A new lineage of T-cell-like lymphocytes expressing VLRC receptors has been defined.
• Both VLRC(+) and VLRA(+) lymphocytes share gene orthologues with gnathostome T-cell differentiation pathways (γδ and αβ T cells).
• VLRC and VLRA receptors undergo assembly and diversification in the thymoid region and are expressed on the cell surface.

Conclusions:

• The genetic programs for two primordial T-cell lineages and a prototypic B-cell lineage likely existed in the last common vertebrate ancestor (~500 million years ago).
• Functional specialization of distinct T-cell-like lineages appears to be an ancient feature of the vertebrate immune system.
• This discovery reshapes our understanding of early adaptive immunity evolution.