Abstract
Jawless vertebrates have an alternative adaptive immune system in which variable lymphocyte receptors (VLR) are somatically diversified through recombinatorial assembly of leucine-rich repeat cassettes during lymphocyte development. Three VLR loci (VLRA, VLRB, and VLRC) have been defined in lampreys and each is expressed by a distinct lymphocyte population. Lamprey VLRA and VLRC are expressed by T-like lineages of lymphocytes, whereas VLRB is expressed by a B-like lineage of lymphocytes, much like αβ T, γδ T and B cells in jawed vertebrates. Recently we have revised the nomenclature for the hagfish VLRs by defining a third VLR gene. Here, monoclonal and polyclonal antibodies were generated against invariant residues of hagfish VLRA, VLRB and VLRC and these were used to identify VLR-bearing lymphocytes and their products. We found that hagfish VLRA, VLRB and VLRC are expressed by three separate lymphocyte populations. Size-exclusion chromatography and western blot analysis indicated that multivalent VLRB proteins are released into the circulation, which suggests that hagfish VLRB+ cells are B-cell like. In contrast, neither VLRA nor VLRC was detectable in the plasma, supporting the hypothesis that these two lymphocyte populations are T-cell like. The demonstration of three lymphocyte populations in both lampreys and hagfish suggests that this tripartite lymphocytic differentiation program already existed in a common ancestor of the two cyclostome lineages around 480 Mya.
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