Expression of a Fas-like proapoptotic molecule on the lymphocytes of Xenopus laevis

Ligation of the externally expressed Fas (APO(1)/CD95) molecule will initia te programmed cell death (apoptosis), in many mammalian developing and adul t cells. Fas-induced apoptosis has not been demonstrated with the cells of any non-mammalian vertebrate. We immunostained suspensions of splenocytes f rom adult Xenopus laevis, the South African clawed toad, with a polyclonal rabbit anti-human Fas antibody raised against the amino acid residues 321-3 35 of human Fas. The binding was specific, as it was dramatically reduced b y preincubation of the antibody with the Fas peptide used to make it, but n ot with a Fas-ligand (FasL) peptide. The binding was enhanced after in vitr o exposure of the splenocytes to phytahemagglutinin (PHA), a T cell mitogen and apoptogen in this species. Sections of developing Xenopus larval tissu e were also immunostained with the polyclonal rabbit anti-human Fas antibod y. Consistant binding of thymocytes and splenocytes was not observed until early metamorphosis in these immunological sites. A monoclonal mouse anti-h uman Fas antibody, previously used to stimulate apoptosis in mammalian cell s, induced significant levels of apoptosis in adult Xenopus splenocytes and additionally, bound specifically to a splenocyte extract, as assayed by EL ISA. Thus, a molecule on Xenopus splenocytes shares both structural and fun ctional homologies with human Fas, indicating the evolutionary conservation within vertebrates of this means of initiating apoptosis. (C) 1998 Elsevie r Science B.V. All rights reserved.