Regulation of expression of TL genes of the mouse Mhc

Expression profiling of a transformed thymocyte cell line undergoing maturation in vitro identifies multiple genes involved in positive selection

Biochemical and genetic studies of thymocyte maturation would be facilitated by the development of cultured cell lines that reflect stages of positive selection. We have derived a CD4(+)CD8(+)TCR(+) T-lymphoid cell line (M20) from a murine thymic tumor induced by a retrovirus carrying the v-myc oncogene (M-MuLV(myc)). M20 subclones undergo several aspects of positive selection in response to co-culture with a thymic stromal cell line (St3), including down-regulation of CD4 and CD8, and up-regulation of CD5 and TCR. M20 possesses a functional TCR complex, and ligation of this complex produces changes similar to co-culture with St3 stroma. Expression profiling of M20 cells in this system identified 23 genes previously shown to be important in thymocyte maturation, as well as several novel candidate genes. This system provides a new model to elucidate the molecular mechanisms of thymocyte maturation and TCR-mediated cell signaling in double-positive thymocytes.

Characterization of mature and immature RadLV-induced thymic T-cell lines for tumorigenesis and MHC-class-I gene expression

Class-I-MHC molecules are divided into class-Ia molecules, which play major roles in recognition of virus-infected cells, graft rejection and immune responses against tumors, and class-Ib molecules, which are less polymorphic and may be responsible for presenting unique classes of peptides. Our report characterizes RadLV-induced thymic T-cell lines that differ both in their tumorigenic potential and in the level of protein for class-Ia and TL genes. The PD1.1 cell line is CD4-CD8+ and expresses relatively high levels of class-I as compared with the CD4+CD8+ PD1.2 cell line. These class-I-expression levels correlate with thymocytes and splenic T cells of the same phenotype, except that normal cells fail to express TL3b. Interferon-treated PD1.2 cells demonstrate significantly lower levels of class-I expression than do interferon-treated PD1.1 cells, and were shown to contain large amounts of degraded class-I mRNA, at least some of which was TL in origin. These RNA products were not detected in PD1.1 cells, suggesting the existence of a mechanism controlling cell-specific and gene-specific mRNA stability. Such RadLV-induced cell lines provide a means for obtaining stage-specific T cells, which can be used for studying the regulation of class-I gene expression during T-cell differentiation, as well as factors that differentially regulate class-Ia and class-Ib expression and are potentially useful for studying T-cell differentiation in general.

Genomic DNA sequence and organization of a TL-like gene in the grc-G/C region of the rat

Genes in the grc-G/C region, which is linked to the rat major histocompatibility complex, influence the control of growth, development, and susceptibility to chemical carcinogens. As an initial approach to analyzing the structure and organization of these genes, a class I hybridizing fragment designated RT(5.8) was isolated from an R21 genomic DNA library and sequenced from overlapping restriction enzyme fragments. The RT(5.8) clone has 5788 base pairs and contains the eight exons characteristic of a class I gene. There are CAAT and TATA boxes upstream of the signal peptide, and the recognition sequence that precedes the site of polyadenylation is located downstream from the third cytoplasmic domain. Comparison of the RT(5.8) gene with representative class I genes from the rat and other species shows that the nucleotide sequences of RT(5.8) have a high level of similarity to those of TL region genes of several strains of mice. The peptide sequence deduced from the RT(5.8) clone is distinct from all previously published class I gene sequences, and at many positions there are amino acid residues that are unique to the RT(5.8) sequence. Probes have been isolated from the third exon and from the 5' and 3' flanking regions of the RT(5.8) clone, and Southern blot analysis with genomic DNA of various rat strains shows that these probes are specific for the RT(5.8) fragment. Northern blot analysis shows that the gene is transcribed in the thymus but not in the liver or spleen. The RT(5.8) sequence is more similar to some mouse TL genes (especially in the alpha 2 and cytoplasmic domains and in the 5' and 3' untranslated regions) than it is to other rat class I genes. Hence, TL-like genes are not restricted to the mouse.

A role for the interferon response DNA sequence in directing transcription of the T18d Tla gene

T18d of BALB/c mice is a member of the Tla category of class I genes of the major histocompatibility complex of the mouse and is highly restricted in expression. Deletion analysis implies that an element essential to T18d expression resides within the region -4 to +54. The homologous region of T3d, a Tla gene which normally is not expressed in BALB/c mice, also has promoter activity. Thus the expressibility of T18d vs T3d is unlikely to be due to sequence differences in this region. A DNA-binding protein, factor VI, was found to bind to the region -33 to +54. DNase I footprinting analysis indicated that the DNA fragment 5'-ACTATAGTTTCACTTTTT-3' (+3 to +20) was protected by factor VI. This region includes the interferon response sequence (IRS). Homologous DNA segments of other class I genes, Ld and Dd, competed for factor VI in DNA-protein binding assay with lower affinity as compared with T18d. In mutation analysis, the 3' portion of the IRS is more important than the 5' portion with respect to binding affinity of factor VI and to transcriptional activity in transfected cells. This result signifies a role of IRS in T18d transcription and suggests that the mechanism of T18d transcription might be unusual.