The human IL-3/granulocyte-macrophage colony-stimulating factor locus is epigenetically silent in immature thymocytes and is progressively activated during T cell development.

The closely linked IL-3 and GM-CSF genes are located within a cluster of cytokine genes co-expressed in activated T cells. Their activation in response to TCR signaling pathways is controlled by specific, inducible upstream enhancers. To study the developmental regulation of this locus in T lineage cells, we created a transgenic mouse model encompassing the human IL-3 and GM-CSF genes plus the known enhancers. We demonstrated that the IL-3/GM-CSF locus undergoes progressive stages of activation, with stepwise increases in active modifications and the proportion of cytokine-expressing cells, throughout the course of T cell differentiation. Looking first at immature cells, we found that the IL-3/GM-CSF locus was epigenetically silent in CD4/CD8 double positive thymocytes, thereby minimizing the potential for inappropriate activation during the course of TCR selection. Furthermore, we demonstrated that the locus did not reach its maximal transcriptional potential until after T cells had undergone blast cell transformation to become fully activated proliferating T cells. Inducible locus activation in mature T cells was accompanied by noncoding transcription initiating within the enhancer elements. Significantly, we also found that memory CD4 positive T cells, but not naive T cells, maintain a remodeled chromatin structure resembling that seen in T blast cells.