Immune-associated nucleotide-1 (IAN-1) is a thymic selection marker and defines a novel gene family conserved in plants

Positive selection of thymocytes is a complex and crucial event in T cell d evelopment that Is characterized by cell death rescue, commitment toward th e helper or cytotoxic lineage, and functional maturation of thymocytes bear ing an appropriate TCR, To search for novel genes involved in this process, we compared gene expression patterns in positively selected thymocytes and their immediate progenitors in mice using the differential display techniq ue. This approach lead to the identification of a novel gene, mIAN-1 (murin e immune-associated nucleotide-1), that is switched on upon positive select ion and predominantly expressed in the lymphoid system. We show that mIAN-1 encodes a 42-kDa protein sharing sequence homology with the pathogen-induc ed plant protein aig1 and that it defines a novel family of at least three putative GTP-binding proteins. Analysis of protein expression at various st ages of thymocyte development links mIAN-1 to CD3-mediated selection events , suggesting that it represents a key player of thymocyte development and t hat it participates to peripheral specific immune responses. The evolutiona ry conservation of the IAN family provides a unique example of a plant path ogen response gene conserved in animals.