The early growth response (Egr) family of transcriptional regulators consists of four proteins that share highly conserved DNAbinding domains. In many cell types, they are coexpressed and appear to have cooperative roles in regulating gene expression during growth and differentiation. Three Egr proteins, Egr1, Egr2, and Egr3, are induced during thymocyte differentiation in response to pre-TCR signaling, suggesting they may be critical for some aspects of pre-TCR-mediated differentiation. Indeed, enforced expression of Egr proteins in developing thymocytes can recapitulate some aspects of pre-TCR signaling, but the mechanisms by which they contribute to ␤-selection are still poorly understood. Egr3 stimulates proliferation of ␤-selected thymocytes, and Egr3-deficient mice have hypocellular thymuses, defects in proliferation, and impaired progression from double-negative 3 to double-negative 4. Surprisingly, Egr1-deficient mice exhibit normal ␤-selection, indicating that the functions of Egr1 during ␤-selection are likely compensated by other Egr proteins. In this study, we show that mice lacking both Egr1 and Egr3 exhibit a more severe thymic atrophy and impairment of thymocyte differentiation than mice lacking either Egr1 or Egr3. This is due to a proliferation defect and cell-autonomous increase in apoptosis, indicating that Egr1 and Egr3 cooperate to promote thymocyte survival. Microarray analysis of deregulated gene expression in immature thymocytes lacking both Egr1 and Egr3 revealed a previously unknown role for Egr proteins in the maintenance of cellular metabolism, providing new insight into the function of these molecules during T cell development. The Journal of Immunology, 2007, 178: 6796 -6805.T cell development in the thymus progresses through welldefined stages characterized by the expression of the CD4 and CD8 cell surface markers. The earliest T cell precursors express neither CD4 nor CD8 (double-negative (DN) 3 thymocytes), which progress through a series of maturational steps to form double-positive (DP) thymocytes that express both CD4 and CD8, and then finally to mature single-positive (SP) thymocytes that express either CD4 or CD8. DN thymocytes can be further subdivided into several substages of maturation on the basis of CD25 and CD44 expression, namely DN1 (CD44 ϩ and CD25 Ϫ ), DN2 (CD44 ϩ and CD25 ϩ ), DN3 (CD44 Ϫ and CD25 ϩ ), and DN4 (CD44 Ϫ and CD25 Ϫ ) thymocytes (1). As part of this complex maturational process, rearrangement of the TCR␤ gene occurs at the DN3 stage during which cells express the successfully rearranged TCR␤ gene along with the pre-TCR␣ (pT␣) chain to form the functional pre-TCR receptor complex. Signaling through the pre-TCR receptor involves a variety of cellular responses, including robust proliferation, enhanced cell survival, down-regulation of the Rag1 and Rag2 recombinase genes, reduced CD25 expression, decreased pT␣ expression, induction of the TCR␣ chain, and allelic exclusion of the nonrearranged TCR␤ locus (2). Differentiation through DN3 to DN4 is known a...

Early growth response (Egr) proteins comprise a family of transcriptional regulators (Egr1–4) that modulate gene expression involved in the growth and differentiation of many cell types. In particular, Egr1 is widely believed to have an essential role in regulating monocyte/macrophage differentiation. However, Egr1-deficient mice have normal numbers of functional macrophages, an observation that has led to the hypothesis that other Egr proteins may compensate for Egr1 function in vivo. We examined whether other Egr transcription factors have a functionally redundant role in monocyte/macrophage differentiation. Egr1 and Egr3 expression was found to be induced in myeloid cells when they were differentiated into macrophages by treatment with M-CSF, whereas Egr2 was minimally induced and Egr4 was not detected. In either Egr1/Egr3 or Egr1/Egr2 double homozygous mutant mice, macrophage differentiation and function remained unimpaired. Additionally, the expression of molecules that broadly inhibit Egr function failed to block commitment to the monocytic lineage or inhibit the maturation of monocyte precursors. Finally, several hemopoietic growth factors were found to induce Egr gene expression, indicating that Egr gene expression is not cell lineage specific. Taken together, these results demonstrate that Egr transcription factors are neither essential for nor specific to monocyte/macrophage differentiation.