An Early Sign of Adaptive Immunity

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Science  07 Feb 2003:
Vol. 299, Issue 5608, pp. 785
DOI: 10.1126/science.299.5608.785c

The urochordates represent an early step in the evolutionary lineage of vertebrates. Urochordate larvae develop a notochord and nervous system, chordate features that are then lost in the adult phase of the organism, as observed in the sessile, and sometimes colonial, sea squirt. Khalturin et al. show that the adults may share other features with higher vertebrates.

On physical contact, two Botryllus colonies either fuse into one or reject each other in a process determined by a single, highly polymorphic genetic locus reminiscent of the vertebrate major histocompatibility (MHC) locus. By screening for changes in gene expression during the process of acceptance or rejection, they identified a gene, BsCD94-1, that encodes a membrane protein similar to the CD94 class of natural killer (NK) receptors. This protein is found on the surface of a group of granulocyte-like Botryllus blood cells. In mammals, NK lymphocytes distinguish between normal cells (self) and foreign cells (nonself). Thus, rejection of incompatible transplants in vertebrates may function via a process with evolutionary roots in sea squirts. — PJH

Proc. Natl. Acad. Sci. U.S.A. 100, 622 (2003).

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