Regulation of expression of TL genes of the mouse Mhc

The expression of thymus leukemia (TL) antigens and genes in thymocytes and activated T cells was examined by immunoprecipitation, flow cytometric, northern, and nuclear run-off transcription analyses. Cell surface forms of TL were detectable by immunoprecipitation on activated peripheral T cells from Tla haplotypes except Tla(b), in agreement with expression observed on thymocytes. Approximately 40%-50% of concanavalin A (Con A) or anti-CD3-activated T cells were TL+, with expression detected on both the CD4 and CD8 subsets by dual-color analysis. Activated T cells expressed detectable levels of TL mRNA 48 h after stimulation, but no TL transcripts were detectable in unstimulated splenocytes. However, TL mRNA expression in mature activated T cells did not precisely mimic thymocyte expression: the level of expression was considerably lower in activated T cells, and in most haplotypes the transcripts produced in activated T cells appeared to represent a subset of the transcripts produced in thymocytes. By run-off transcription assays in isolated nuclei, TL gene expression was detected in activated but not resting T cells indicating that lack of expression of TL in resting T cells is not due to message instability. These data demonstrate that TL genes are inducible and transcriptionally regulated.