High level expression of the Xlr nuclear protein in immature thymocytes and colocalization with the matrix-associated region-binding SATB1 protein.

The X-linked lymphocyte-regulated (Xlr) protein is a 30,000 Mr nuclear protein bearing homology with meiosis-specific proteins and expressed in late stage B lymphoid cell lines. In the present study we investigated its expression in the T lymphoid lineage. In adults, a high level of expression was detected in CD4-CD8- thymocytes. Most remarkably, the peak of Xlr expression occurred early during thymus cell ontogeny, precisely on days 14-15 of gestation, and was associated with the first wave of pre-T cell differentiation. Its onset preceded the rearrangement of TCR genes, as Xlr expression was conserved in thymus cells from RAG1(0/0) mice. The lower expression of Xlr on day 13 of fetal development, the bright Thy1+ phenotype of Xlr-positive cells, their large size, and their absence from subcapsular areas suggest that Xlr expression must be turned on within the thymus and not in prethymic precursors. From day 16 of gestation, Xlr expression decreased markedly. At birth and later, Xlr(high) cells were mostly large cells scattered throughout the cortical area. As shown by confocal microscopy, expression of Xlr closely overlapped that of SATB1, which binds special AT-rich DNA sequences associated with the nuclear matrix and plays an important regulatory role for many genes. The remarkably regulated expression of Xlr in the lymphoid cell lineage and of its homologue Xmr in the germ cell lineage suggests that they might play an important role in chromatin metabolism at critical stages of differentiation during which the genome undergoes irreversible rearrangements.