The deregulated expression of c-Myc protein is associated with the non-random locus-specific amplification of the dihydrofolate reductase (DHFR) gene. This study was performed to determine whether additional chromosomal aberrations occur when c-Myc protein levels are up-regulated for prolonged periods. To this end, we have used Rat1A-MycER cells, which allow the experimental regulation of Myc protein levels. We examined the genomic stability of Rat1A-MycER cells cultivated in either the absence or the presence of estrogen, which reportedly activates the chimeric MycER protein in these cells. Following prolonged periods of MycER activation, Rat1A-Mycer cells exhibited irreversible chromosomal aberrations. The aberrations included numerical changes, chromosome breakage, the formation of circular chromosomal structures, chromosome fusions, and extrachromosomal elements.

The chicken T cell receptor CD3epsilon gene was isolated using a degenerate polymerase chain reaction. The 1883 bp long cDNA encoded a transmembrane protein of 16.9 kDa lacking N-linked glycosylation sites. Comparison of the chicken and mammalian CD3epsilon proteins revealed low homology in the extracellular domain with clusters of similarities located around the N-terminal cysteine residue and proximal to the transmembrane region. The high conservation of the cytoplasmic domain included motifs important for signal transduction. The alignment of all CD3gamma, CD3delta and CD3epsilon proteins allowed the identification of highly conserved residues and motifs. Southern blot analysis indicated the presence of a single copy CD3epsilon gene. The expression of the CD3epsilon transcript was limited to T cells and natural killer cells. A recessive mutation of the CD3epsilon gene in the CB chicken strain enabled the mapping of the epitope recognized by the CT3 monoclonal antibody. This analysis of the first non-mammalian CD3epsilon gene provides novel information about evolutionary conserved structural features and its expression in natural killer cells.

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Ag-specific as well as Ia-restricted killing of certain APC by CD4+ T cells was investigated. The CD4-mediated killing is not only a characteristic of in vitro long term cultured T cell lines or clones, but is also manifest after in vivo priming. Thus, CD4+ killer T cells are generated in vivo as well. CD4+ killer T cells are detected in the Th1, but not in the Th2 subset, and they do not appear to lyse Ia+ APC or bystander cells by a pathway mediated by secreted T cell factors. The latter observation is demonstrated by cold target inhibition experiments as well as by the failure of puromycin to inhibit killing, if applied in doses which completely block lymphokine secretion. Ia+ APC differ in their susceptibility to lysis. Transformed APC are usually better lysed than nontransformed APC. Unstimulated B cells are not killed, while LPS-stimulated B cell blasts are killed. The results of cold target inhibition and bystander killing experiments suggest that CD4+ killer T cells are activated by the common pathway, i.e., by Ag presented in the context of Ia, but killing requires the recognition of additional determinant(s) on APC. It is proposed that these killing-inducing determinants are continuously expressed on most transformed Ia+ cells and on nontransformed but stimulated APC.