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We have screened different mouse strains-including strains with enhanced susceptibility for autoimmune diseases-for deviations of Rt6 gene expression by RT-PCR. Most strains expressed varying amounts of Rt6-1 and Rt6-2. NZW mice, however, do not show any detectable Rt6-2 gene transcripts. BxSB mice show a near complete absence of Rt6-1 gene transcripts. Southern blot and sequence analyses revealed that NZW mice have suffered a deletion of the Rt6.2 gene while the Rt6-1 gene of BxSB mice has been inactivated by a premature stop codon. Thus, these mouse strains represent natural Rt6-2 and Rt6-1 single-gene 'knock-out's, respectively. Since the NZW mouse does not show any gross immunological abnormalities, loss of the Rt6-2 gene by itself is not associated with any obvious immunological phenotype. However, crosses between NZW and certain other mouse strains, e.g. (NZW x NWB)F1 and (NZW x SB)F1 animals, develop a systemic autoimmune disease reminiscent of human lupus erythematosus. Moreover, the BxSB mouse strain is considered to be an independent model for the same disease. It will be of interest to determine whether these spontaneous Rt6 gene defects constitute part of the polygenetic contribution to autoimmune disease in these animals.

RT6 is an enzymatically active GPI-anchored membrane protein that was originally discovered in the rat as a peripheral T cell alloantigen. It has attracted interest as an activation antigen and because defective RT6-expression coincides with increased susceptibility for autoimmune type I diabetes in the BB rat. Southern blot analyses indicate that the rat carries a single copy RT6 gene whereas the mouse carries a duplication of the homologous locus. We had previously cloned and sequenced a RT6-homologous cDNA from BALB/c mouse spleen. We now report the cloning and characterization of a second RT6-homologue from BALB/c and 129/Sv mice. The two mouse Rt6 genes (designated Rt6-1 and Rt6-2) encode similar open reading frames that are disrupted by conserved introns. The nucleotide sequences of the Rt6-1 and Rt6-2 coding regions show 87% sequence identity, the deduced amino acid sequences 79% identity. The amino acid sequences reveal significant similarity to recently cloned ADP-ribosylating ectoenzymes from rabbit and human skeletal muscle as well as chicken bone marrow cells. RT-PCR analyses reveal that the two Rt6 genes are differentially expressed in distinct inbred mouse strains and that their transcripts are properly processed. Western blot analyses demonstrate that the respective gene products are released from cells by treatment with PI-PLC. The results further show that both mouse Rt6 genes are translated into GPI-anchored cell surface molecules and that Rt6 gene expression is restricted to peripheral lymphoid tissues.