RECONSTITUTION OF THE SUBCLASS-SPECIFIC EXPRESSION OF CD4 IN THYMOCYTES AND PERIPHERAL T-CELLS OF TRANSGENIC MICE - IDENTIFICATION OF A HUMAN CD4 ENHANCER

During thymic maturation, CD4-CD8-TCR- immature thymocytes differentia te through a CD4+CD8+TCR(lo) intermediate into two functionally distin ct mature T cell subsets: helper T cells expressing CD4 and a major hi stocompatibility complex (MHC) class II-restricted T cell receptor (TC R), and cytotoxic T cells expressing CD8 and an MHC class I-restricted TCR. The mutually exclusive expression of CD4 and CD8 is maintained i n the periphery during expansion of these mature T cell subsets. To el ucidate the mechanisms controlling CD4 and CD8 expression on different iating thymocytes and mature peripheral T cells, we have examined the expression of human CD4 gene constructs in the lymphoid tissues of tra nsgenic mice. Our analyses demonstrate that sequences contained within or closely linked to the human CD4 gene are sufficient to reconstitut e the appropriate regulation of human CD4 expression on all thymocyte and mature peripheral T cell subsets. Specifically, appropriate develo pmental regulation was dependent on two sets of sequences, one contain ed 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 restricti on fragment as the likely human homologue of an enhancer found 13-kb u pstream of the mouse CD4 transcription initiation site. The human CD4 transgenic mice provide a useful system for the identification and cha racterization of additional sequence elements that participate in huma n CD4 gene regulation and for the elucidation of regulatory mechanisms governing the developmental program mediating the maturation of the C D4+ and CD8+ peripheral T cell subsets.