Reconstitution of the subclass-specific expression of CD4 in thymocytes and peripheral T cells of transgenic mice: identification of a human CD4 enhancer.

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Abstract
During thymic maturation, CD4-CD8-TCR- immature thymocytes differentiate through a CD4+CD8+TCRlo intermediate into two functionally distinct mature T cell subsets: helper T cells expressing CD4 and a major histocompatibility complex (MHC) class II-restricted T cell receptor (TCR), and cytotoxic T cells expressing CD8 and and MHC class I-restricted TCR. The mutually exclusive expression of CD4 and CD8 is maintained in the periphery during expansion of these mature T cell subsets. To elucidate the mechanisms controlling CD4 and CD8 expression on differentiating thymocytes and mature peripheral T cells, we have examined the expression of human CD4 gene constructs in the lymphoid tissues of transgenic mice. Our analyses demonstrate that sequences contained within or closely linked to the human CD4 gene are sufficient to reconstitute the appropriate regulation of human CD4 expression on all thymocyte and mature peripheral T cell subsets. Specifically, appropriate developmental regulation was dependent on two sets of sequences, one contained within a 1.3-kb restriction fragment located 6.5 kb upstream of the human CD4 gene, and the other present within or immediately flanking the gene. Nucleotide sequence analysis identified the 1.3-kb restriction fragment as the likely human homologue of an enhancer found 13 kb upstream of the mouse CD4 transcription initiation site. The human CD4 transgenic mice provide a useful system for the identification and characterization of additional sequence elements that participate in human CD4 gene regulation and for the elucidation of regulatory mechanisms governing the developmental program mediating the maturation of the CD4+ and CD8+ peripheral T cell subsets.