title: Ontogeny of Xenopus NK cells in the absence of MHC class I antigens.
abstract: This paper explores the ontogeny of NK cells in control and early-thymectomized (Tx) Xenopus laevis through phenotypic analysis of cells expressing the NK cell antigen 1F8 and by performing in vitro cytotoxic assays. Dual color flow cytometry reveals that a few 1F8positive splenocytes first emerge in late larval life, at approximately 7-weeks post-fertilization. This is about 2-weeks after the time when surface MHC class Ia expression can first be detected. The proportion of splenocytes expressing 1F8 remains very low in 3-4 month-old froglets, but by 1 year there is a sizeable 1F8positive population, which is proportionally elevated in Tx frogs. The ontogeny of NK cell function is monitored by a 5 h DNA fragmentation (JAM) assay. Control and Tx larval splenocytes (from either 5- or 7-week-old tadpoles) fail to kill MHC-deficient thymus-derived tumor cell targets. Such in vitro killing is still relatively poor in 3-4 month froglets, compared with high levels of tumor cell cytotoxicity mediated by splenocytes from older frogs. Immunoprecipitation studies identify that the major ligand for the 1F8 mAb is a 55 kDa polypeptide. Finally, further evidence is provided that 1F8positive lymphocytes are indeed bona fide NK cells, distinct from T cells, since purified 1F8positive splenocytes from Tx Xenopus fail to express fully rearranged TCRbeta V region transcripts. We conclude that NK cells fail to develop prior to MHC class I protein expression and, therefore, do not contribute to the larval immune system, whereas they do provide an important backup for T cells in the adult frog by contributing to anti-tumor immunity.
authors.affiliation: School of Biological and Biomedical Sciences, University of Durham, Durham DH1 3LE, UK.
authors.forename: Trudy L
authors.initials: TL
authors.lastname: Horton
authors.forename: Rebecca
authors.initials: R
authors.lastname: Stewart
authors.forename: Nicholas
authors.initials: N
authors.lastname: Cohen
authors.forename: Laura
authors.initials: L
authors.lastname: Rau
authors.forename: Pamela
authors.initials: P
authors.lastname: Ritchie
authors.forename: Martin D
authors.initials: MD
authors.lastname: Watson
authors.forename: Jacques
authors.initials: J
authors.lastname: Robert
authors.forename: John D
authors.initials: JD
authors.lastname: Horton
mesh_terms: D000818:Animals; D002460:Cell Line; D003602:Cytotoxicity, Immunologic; D005434:Flow Cytometry; D015395:Histocompatibility Antigens Class I / Q000276:immunology*; D007694:Killer Cells, Natural / Q000276:immunology*; D007814:Larva / Q000276:immunology; D010641:Phenotype; D011233:Precipitin Tests; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013934:Thymectomy; D014982:Xenopus laevis / Q000254:growth & development* / Q000276:immunology*
chemical_list: D015395:Histocompatibility Antigens Class I