Stable T3- and T3+ subclones derived from the mosaic human T cell leukemia cell line, CEM

An unexplained feature of the human T lymphoblastic leukemia cell line, CEM, is a heterogeneous expression of surface membrane T3-TCR complex. CEM cells constitutively maintain two distinct populations whereby a minority of cells (5 to 16%) express T3 but the remainder do not. From the parental cell line, we have derived stable subclones in which all cells express surface T3 and companion subclones that lack surface T3. Inasmuch as surface T3 expression is contingent on assembly and coexpression of the multichain T3 protein complex with the TCR, we compared the surface T3+ and T3- subclones for their rearrangement and expression of genes of the T3-TCR complex. Restricted DNA of both subclone types showed the identical monoclonal pattern of TCR gene rearrangement seen in the mosaic parental CEM line. Northern blot analysis revealed mRNA transcripts of TCR beta subunit and T3 delta in both the surface T3+ and T3- subclones. However, surface T3+ subclones transcribed abundant full length (1.5-kb) TCR alpha mRNA in contrast to the surface T3- subclones in which TCR alpha transcripts were undetectable. Lack of TCR alpha gene transcription was not attributable to either unrearranged or aberrantly rearranged TCR alpha genes because exposure of surface T3- cells to phorbol ester induced a rapid (15-min) and progressive accumulation of full length (1.5-kb) TCR alpha mRNA. This was followed by surface membrane T3 expression in 24 to 72 h. We conclude that the unusual phenotypic mosaicism of CEM cells is attributable to a reversible lack of transcription of the TCR alpha gene.