Autoreactive blood cells and programmed cell death in growth and development of protochordates

The tunicate Botryllus is a marine protochordate whose clonal colonies undergo regulated natural transplantations when they come into contact in nature. The outcome of these transplantations (fusion or rejection) is controlled by genes of a highly polymorphic histocompatibility system that resembles in many respects the mammalian major histocompatibility complex (MHC). While fusion or rejection reactions are often completed within 24 hr after transplantation, resorption of one partner of a pair of fused semiallogeneic colonies may occur days to weeks after initial contact. The latter process is similar to the degeneration of old individuals, or zooids, that precedes maturation of each new generation of asexual buds. Here we describe comparisons of in vitro reactions of a) mixtures of cells from allogeneic animals and b) cells taken from animals at the zooid-resorption ("takeover") stage of colony development. In vitro autoreactivity of cells from resorbing colonies may reflect in vivo responses to senescent cells, which in turn may be related to allorecognition events that govern fusion or rejection between colonies.