Thymocyte Maturation Is Regulated by the Activity of the Helix-Loop-Helix Protein, E47

The E2A- and E47-deficient mice have been described previously 4,15. The β2-microglobulin (β2M) mice were purchased from The Jackson Laboratory. All mice were analyzed between 4 and 7 wk of age.

Total thymocytes from E47- or E2A-deficient mice and wild-type littermates were dissected into RPMI medium containing 10% fetal bovine serum and 50 μM 2-ME and supplemented with glutamine, penicillin, and streptomycin. The cells were stained with antibodies to CD4 and CD8 or with propidium iodide (to determine viability) before culture (time = 0). The cells were then placed in culture at 37°C plus 5% CO₂ for 28 h before harvesting and staining with anti-CD4–PE and anti-CD8–FITC antibodies and propidium iodide to determine cell viability. The percentage of viable DP cells was determined by multiplying the number of viable cells at 28 h by the percentage of DP cells at 28 h and dividing this by the number of DP cells put into culture at time = 0.

Staining of the cells has been described previously 4. For two-color analysis, cells were stained with anti-CD4–PE (RM4-5; PharMingen), anti-CD8α–FITC (53-6.7; PharMingen), anti–TCR-α/β–FITC (H57-597; PharMingen), and anti-CD69–PE (H1.2F3; PharMingen). Hybridoma supernatant from the T3.70 line, provided by Dr. Ellen Robey (University of California, Berkeley, CA), was used for the Vα3 staining. For three-color analysis, cells were stained with anti-CD4–TriColor (CT-CD4; Caltag), anti-CD8α–PE (53-6.7; PharMingen), anti-CD69–FITC (H1.2F3), anti–heat stable antigen (HSA)–FITC (M1/69; PharMingen), and anti–TCR-α/β–FITC. Cells were analyzed on a FACScan™ (Becton Dickinson), and live cells were gated on the basis of forward and side scatter.

For the kinetic analysis of thymic development, mice were continuously exposed to the thymidine analogue bromodeoxyuridine (BrdU; 0.8 mg/ml) in their drinking water for the indicated times. Thymocytes were labeled with antibodies extracellularly as described above using biotinylated anti-CD4 or anti–TCR-α/β (PharMingen) and PE-labeled anti-CD8 or anti-CD69 (PharMingen). Cells were then fixed and stained with FITC-labeled anti-BrdU (Becton Dickinson) as described previously 16.

The 16610D9 cell line was derived from a thymoma that developed spontaneously in a p53-deficient mouse and was adapted to culture. The cells were subcloned to generate the 16610D9 line, which was then cultured at 37°C plus 5% CO₂ in Optimem (GIBCO BRL) containing 10% fetal bovine serum and 50 μM 2-ME and supplemented with glutamine, penicillin, and streptomycin. The retroviral vector LZRSpBMN-linker-IRES-EGFP (S-003) was obtained from Hergen Spits (The Netherlands Cancer Institute, Amsterdam, The Netherlands). The mouse Id3 cDNA that was cloned into the polylinker was generated by PCR of an Id3 cDNA with the following primers: Id3 forward, 5′-CCGGAATTCATGGACTACAAGGACGACGATGACAAGGGATCCATGAA-GGCGCTGAGCCCGGTG; and Id3 reverse, 5′-CCGGAATTCTCAGTGGCAAAAGCTCCTC, to generate FLAG-tagged Id3. The PCR product was cloned into pBSK, digested out of pBSK with EcoRI, and cloned into the EcoRI site in S-003. The φNX-eco packaging line 17 was transfected with the retroviral constructs by calcium phosphate precipitation. The transfected cells were switched to 16610D9 culture medium 24 h after transfection, and supernatants were harvested after an additional 24 h. Transduction of the 16610D9 cells was performed as described previously 18. Cells were harvested 48 h after infection and analyzed by flow cytometry as described above using PE-conjugated antibodies (with the exception of CD4, which was TriColor conjugated), or lysed to make whole cell extracts (WCEs). To prepare WCE, cells were pelleted, washed once in cold 1× PBS, and the cell pellet was then frozen on dry ice for ∼15 min. The pellet was thawed, resuspended in 15 μl/10⁶ cells cold buffer C (20 mM Hepes, pH 7.9, 0.4 M NaCl, 1 mM EDTA, 1 mM EGTA, 1 mM dithiothreitol, 1 mM PMSF plus protease inhibitors) containing 1% NP-40, and vortexed vigorously for 2 min at 4°C. Debris was pelleted by spinning at high speed in a microfuge at 4°C, and the supernatant was removed as the WCE. Protein concentrations were determined using Bio-Rad's protein assay reagent as described by the manufacturer. Double-stranded DNA probes were end-labeled using T4 polynucleotide kinase and purified over a G25 Sepharose column. 15 μg of WCE was used in a gel shift assay as described previously 19. The sequence of the μE5 oligo probe is as follows: 5′-TCGAAGAACACCTGCAGCAGCT-3′.

Previous studies have established that mutant mice lacking both E12 and E47 exhibit abnormalities in thymocyte development 7. To generate sufficient numbers of mice to further characterize the T cell phenotype, we analyzed E47-deficient mice, which have a significantly lower rate of postnatal lethality compared with the E2A^−/− mice 15. However, we note that the E47-deficient mice also express reduced levels of the E12 protein compared with control littermates 15. We analyzed thymocytes derived from 4–6-wk-old E2A- or E47-deficient mice for the expression of CD4 and CD8 by flow cytometry. Wild-type and E47 or E2A heterozygous mice had virtually identical thymic profiles (data not shown). However, E2A- and E47-deficient thymi showed significant decreases in the percentage of DP thymocytes compared with their heterozygous littermates (Fig. 1 A). In contrast, the proportions of both the CD4 and CD8 SP populations were increased an average of two- to threefold in the E2A- and E47-deficient thymi (Fig. 1A and Fig. B).

Within the thymus, an increase in the relative percentage of mature SP thymocytes can arise through several mechanisms. For example, either aberrant proliferation of the SP populations, increased frequency of apoptosis within the DP population, or increased maturation of the DP cells could explain the phenotypical changes observed in the E2A- and E47-deficient mice. To begin to address these possibilities, we analyzed thymi from E2A-deficient mice for incorporation of BrdU and survival in vitro. E2A-deficient mice and control littermates were injected with BrdU, and the thymic subpopulations were analyzed for BrdU incorporation 24 h after injection. We found no increase in the percentage of SP thymocytes from E2A-deficient mice staining positive for BrdU compared with the control littermates (data not shown). Thus, the increased percentage of SP cells is not a result of abnormal proliferation within this population. To determine whether the absence of E2A promotes DP cell death, we isolated total thymocytes from E47- or E2A-deficient mice and wild-type littermates and analyzed the viability of the cells after 28 h in culture. Whereas an average of 52% of the wild-type DP cells was alive after the culture period, only ∼17% of the E47- and E2A-deficient DP cells survived (Table, and data not shown). Similar percentages of wild-type or E2A-deficient CD4⁺ cells were alive after 28 h in culture, suggesting that the increase in cell death is specific to the DP population (Table). Thus, DP thymocytes lacking E2A activity display decreased survival in vitro.

To address the possibility of an increase in DP thymocyte maturation, we further characterized the changes in the T cell populations by analyzing thymocytes from E2A- and E47-deficient mice for the expression of TCR-α/β and CD69. During normal thymocyte development, expression of both TCR-α/β and CD69 increases upon positive selection 20,21,22,23,24. However, whereas SP thymocytes retain high levels of TCR-α/β expression, CD69 levels are subsequently downregulated upon completion of maturation 21,24,25. Thus, TCR^hiCD69⁺ cells represent those thymocytes in the process of positive selection, whereas TCR^hi CD69⁻ thymocytes have completed the maturation process. We find that thymocytes derived from E2A- or E47-deficient mice have a three- to fourfold increase in the proportion of TCR^hiCD69⁻ cells compared with their littermates (Fig. 2, and data not shown). The absolute number of mature TCR^hiCD69⁻ thymocytes is increased an average of 1.5–3-fold (data not shown). These data indicate that the absence of E47 leads to an increase in the proportion of SP cells that have completed thymocyte maturation, and suggest that E47 might have an additional role in regulating DP thymocyte differentiation.

To further explore whether the absence of E47 leads to an increase in thymocyte maturation, E47-deficient mice were crossed to AND TCR mice which express a class II–restricted TCR transgene. Maturation of CD4 SP thymocytes is strongly enhanced in H-2^b mice expressing the AND TCR 26. Thymocytes derived from E47-deficient mice and wild-type or heterozygous littermates carrying the AND transgene were analyzed by flow cytometry for the expression of CD4 and CD8. As expected, E47^+/+;AND mice contained a high fraction of mature CD4⁺ cells and a correspondingly lower fraction of immature DP thymocytes than nontransgenic thymi (Fig. 3, top panels). Thymic profiles from E47^+/−;AND mice were not significantly different from the E47^+/+;AND profiles (Fig. 3, top panels). However, the percentage of mature CD4 SP cells was significantly higher (87 vs. 73%) in thymocytes derived from E47^−/−;AND mice, and the proportion of DP cells was significantly lower (10 vs. 22%; Fig. 3, top panels). Importantly, we observed a similar trend in the peripheral T cell populations. The ratio of CD4⁺ to CD8⁺ splenic T cells increased from ∼22 in the wild-type or heterozygous mice expressing the AND TCR to 55 in the E47^−/−;AND mice (Fig. 3, middle panels). In the lymph nodes, the ratio of CD4⁺ to CD8⁺ cells increased from 11 in the E47^+/+; AND mice to 69 in the E47^−/−;AND mice (Fig. 3, bottom panels). The increase in total percentage of T cells in the E47-deficient lymph nodes (98 vs. 55%) is due to the complete absence of B cells in the E2A- and E47-deficient mice 4,15. Thus, there is a selective increase in only those SP cells expressing the relevant TCR, and this increase is reflected in the periphery. These data suggest that the absence of E47 can markedly enhance the positive selection of class II–restricted TCRs.

To determine whether the absence of E47 also promotes increased selection of class I–restricted TCRs, we analyzed E47-deficient mice expressing the H-Y TCR transgene, which specifies reactivity with the male H-Y antigen presented by the H-2D^b class I molecule 27,28. Expression of the H-Y TCR transgene in females of the H-2^b background leads to positive selection of CD8 SP thymocytes 27,28. In contrast, thymocytes in male mice expressing the H-Y TCR are deleted at the DP stage. Negative selection mediated by the H-Y TCR transgene was unaffected by the absence of E47 (data not shown). However, E47^−/−; H-Y female mice displayed an increased percentage of mature CD8 SP thymocytes compared with the E47 heterozygous littermates (56.6 vs. 33.6%; Fig. 4 A). This phenotype was accompanied by a significant decrease in the proportion of immature DP thymocytes (29.6 vs. 46%; Fig. 4 A). Because of continued rearrangement of endogenous α chain genes in TCR transgenic mice, a fraction of the thymocytes will express a TCR composed of the transgenic β chain and an endogenous α chain 25,27,29. To analyze the proportion of cells expressing the transgenic TCR-α/β, we stained total thymocytes with the T3.70 antibody, which reacts specifically with the transgenic α chain molecule 30. Thymocytes isolated from E47^−/−;H-Y mice contained almost twice the percentage of cells expressing high levels of both the transgenic α and β chains, consistent with a higher proportion of mature CD8 SP T cells (Fig. 4 A).

The increased percentage of mature CD8⁺ T cells observed in the E47^−/−;H-Y mice is also reflected in the periphery. The ratio of CD8⁺ to CD4⁺ T cells is increased from ∼0.4 in the E47^+/−;H-Y spleen to 1.2 in the E47^−/−;H-Y spleen (Fig. 4 B, top panels). A similar change in the CD8/CD4 ratio was observed in the lymph nodes (Fig. 4 B). Therefore, like the E47^−/−;AND mice, the E47^−/−;H-Y mice display an increase in only the percentage of cells expressing the relevant TCR, and the relative proportions of CD4 and CD8 lineage cells are altered in the peripheral lymphoid organs as well. These changes in peripheral CD8/CD4 ratios from E47-deficient/TCR transgenic mice would not be expected if the increase in proportion of thymic SP cells was entirely the result of increased death within the DP thymocyte population. In addition, a selective increase in only the population of cells expressing the relevant TCR is not consistent with a defect in emigration of mature SP thymocytes from the thymus.

Interestingly, in addition to the increase in CD8 SP cells, E47^−/−;H-Y thymi contained elevated percentages of DP cells that expressed CD69 and high levels of TCR-α/β (Fig. 4 C). Thus, a higher proportion of the DP population is undergoing positive selection in the absence of E47. Taken together, these data suggest that the absence of E47 results in increased selection to both class I– and class II–restricted T cell receptors.

As described above, heterozygosity at the E47 locus does not perturb maturation of SP thymocytes expressing class II–restricted TCR transgenes. In contrast, we consistently observed increases in the proportion of CD8⁺ cells in female H-Y transgenic mice that were heterozygous for E47 compared with wild-type littermates (Fig. 4 D). In most cases, the percentage of CD8⁺ cells was increased in the periphery of E47^+/−;H-Y mice as well (Fig. 4 D). Thus, maturation of CD8 SP cells in mice expressing the H-Y TCR transgene is influenced by the dosage of E47.

In addition to the increased production of CD8 SP cells, female E47^−/−;H-Y mice displayed an aberrant population of T cells in the peripheral lymphoid organs that expressed high levels of CD8 and intermediate to high levels of CD4 (Fig. 4 A and Fig. 5 A). To examine whether the peripheral DP (PDP) population expressed markers characteristic of mature T lineage cells, we analyzed this population for the expression of TCR-α/β and HSA. The PDP cells expressed HSA at a level similar to normal SP thymocytes, and significantly lower than the level of expression on thymic DP cells (Fig. 5 B). However, the expression of HSA on the PDP population is significantly higher than the level observed on normal SP peripheral T cells (Fig. 5 B, right). TCR-α/β expression on the PDP cells was identical to that of the mature SP T cells, with all of the cells expressing uniformly high levels of TCR (Fig. 5 C). Taken together, the data suggest that the PDP cells in the E47^−/−;H-Y mice are positively selected T lineage cells that fail to complete maturation before exiting to the periphery.

As described above, the absence of E47 promotes the development of SP thymocytes. To determine whether the absence of E47 allows the development of SP cells in the absence of an appropriate signal, we bred the E47-deficient mice to β2M–deficient mice, which virtually lack MHC class I expression and produce very few mature CD8 SP thymocytes 31,32. Although the E47 deficiency did not promote CD8 lineage development in the β2M-deficient background to wild-type levels, we consistently noted a small percentage of TCR^hi CD8 SP cells in the E47^−/−;β2M^−/− thymi that were absent in the E47^+/−;β2M^−/− thymi (Fig. 6 A, and data not shown). Furthermore, E47^−/−;β2M^−/− mice displayed a higher proportion of TCR^hi CD8 SP cells in the peripheral lymphoid organs compared with the control littermates (Fig. 6 B, and data not shown). The decreased CD4/CD8 ratio from 146 in the E47^+/−;β2M^−/− spleen to 37 in the E47^−/−;β2M^−/− spleen indicates that the absence of E47 allows for an increase in maturation of CD8⁺ cells even in the absence of the appropriate TCR–MHC interaction. Thus, the data suggest that the downregulation of E2A activity can promote thymocyte maturation.

The steady state size of the thymic subpopulations is determined by the rate at which cells are generated, the rate at which they die, and the rate at which they emigrate from the thymus. To more directly assess the effect of an E47 deficiency on the production of mature SP T cells, we measured the kinetics of appearance of mature thymocytes using the thymidine analogue BrdU. We exposed E47-deficient mice and their heterozygous littermates to BrdU in their drinking water and then analyzed for the presence of BrdU and the expression of TCR-α/β and CD69 (Fig. 7A and Fig. B). Because cells expressing medium to high levels of TCR are not dividing, there is a lag in the appearance of BrdU in these populations. As described above, both the E47-deficient and control mice display this lag in labeling of mature thymocytes, demonstrating that the absence of E47 does not lead to the aberrant proliferation of the SP cells. Because the TCR^med-hi populations are nondividing, the rate of appearance of BrdU in these populations reflects the rate at which these cells are produced. In mice deficient for E47, the rate of appearance of BrdU-labeled TCR^hiCD69⁻ mature thymocytes is significantly enhanced, suggesting that the relative proportion of cells that develop into mature CD69⁻ T cells is increased in the absence of E47 (Fig. 7 B). In contrast, E47-deficient mice display a decrease in the rate of appearance of BrdU in the TCR^med CD69⁺ population (Fig. 7 B). Taken together, these data suggest that the rate at which DP thymocytes mature into TCR^hiCD69⁻ SP T cells is enhanced in the absence of E47.

As shown in Table, E2A-deficient DP thymocytes show reduced survival in vitro. However, the data described above suggest that the absence of E47 also promotes thymocyte positive selection. To test directly whether lowering the activity of E47 promotes maturation, we used a retroviral transduction system to study the effects of inhibiting E2A activity in an immature DP T cell line. The mouse cDNA for Id3, a negative regulator of E2A binding activity, was cloned into the retroviral vector S-003 14. This vector allows translation of both Id3 and enhanced green fluorescent protein (EGFP) from one retroviral transcript. Thus, retroviral transductants can be identified by the expression of EGFP. The S-003/Id3 and S-003 empty vector constructs were transfected into the φNX-eco retroviral packaging line, and supernatants from these cells were used to transduce virus into the T cells. The 16610D9 T cell line used was derived from a thymoma that developed spontaneously in a p53-deficient mouse and was adapted to culture. This T cell line expresses characteristics typical of DP thymocytes, including high levels of HSA, intermediate levels of TCR, and low levels of CD5 and CD44 (Fig. 8 C). In addition, the 16610D9 cells express significant levels of E2A binding activity (Fig. 8 A).

Id3 was transduced into the 16610D9 line and analyzed 48 h after infection for the expression of HSA, CD69, CD44, CD5, TCR β chain, and CD4 and CD8. The early stages of positive selection are marked by increased expression of CD69, CD5, and TCR 20,22,24,25. During the later stages of positive selection, CD44 is upregulated whereas HSA is downregulated 33. In addition, T cells undergoing the initial stages of positive selection downmodulate both CD4 and CD8 expression 34. At the time of analysis, ∼85% of both the control and Id3-infected cells were EGFP⁺ (Fig. 8 B). The ability of Id3 to inhibit E47 binding activity was assayed by electrophoretic mobility shift assay using an E-box probe. E47 and HEB binding activity was easily detectable in the control transduced cells, but dropped to 10–20% of that level in the Id3 transductants (Fig. 8 A). Thus, overexpression of Id3 is sufficient to inhibit E47 binding activity within the 16610D9 cells.

The infected cells were then analyzed by flow cytometry for the expression of the indicated markers. Strikingly, the Id3 transductants displayed altered expression of all the markers analyzed. In particular, expression of both TCR-β and CD5 increased dramatically in the cells lacking E47 binding activity, but was unchanged in the nontransduced cells (Fig. 8 C). In addition, the Id3 transductants clearly upregulated CD69 and CD44 expression, as evidenced by an increase in both the mean linear value of the histogram and the median linear value of the histograms (Fig. 8 C). On the other hand, HSA expression was marginally downregulated in the Id3-expressing cells (Fig. 8 C). Furthermore, we detected a dramatic downmodulation of CD4 and CD8 coreceptor expression in Id3-transduced cells (Fig. 8 E). Thus, these data clearly show that Id3-transduced T cells acquire phenotypic characteristics of thymocytes undergoing positive selection. Taken together, these data demonstrate that lowering the level of E47 activity promotes T cell differentiation in vitro and are consistent with a key role for E47 in promoting DP thymocyte maturation in vivo.

Immature thymocytes have the ability to undergo two distinct fates. A DP thymocyte can be selected to mature into either a CD4⁺ or CD8⁺ T cell, or alternatively it may undergo cell death. Although a large number of signaling molecules have been implicated in thymic selection, their nuclear targets have remained largely unknown. Here, we show that the activity of one particular transcriptional regulator, E47, is important in regulating thymocyte positive selection. Our data indicate that the relative level of E47 can influence thymocyte selection to both the CD4 and CD8 lineages. In the absence of E47 activity, the rate of production of TCR^hiCD69⁻ thymocytes is enhanced, resulting in increased percentages of SP thymocytes. In a TCR transgenic background, a deficiency in E47 results in an increase in positive selection of only those T cells expressing the relevant TCR. In addition, inhibition of E47 activity within a DP T cell line induces the cells to differentiate. Based on these observations, we propose that the downregulation of E2A activity promotes thymocyte maturation.

Mice deficient for E2A display an increased percentage of mature SP thymocytes coupled with a decreased proportion of DP cells. Here we show that the E2A-deficient mice exhibit increases in DP cell death in vitro. Although it is likely that a decreased rate of DP survival could contribute to the observed phenotype, it is doubtful that the increase in proportion of SP cells is solely a result of increased death within the DP population. Our kinetic data demonstrate that the absence of E2A activity additionally promotes thymocyte maturation. Furthermore, E47-deficient TCR transgenic mice display a corresponding increase of SP T cells expressing only the relevant TCR in both the thymus and the peripheral lymphoid organs. The appearance of DP T cells expressing characteristics of positively selected cells in the periphery of E47^−/−;H-Y mice also indicates that the absence of E2A affects DP thymocyte maturation. Moreover, our data show that CD8⁺ T cells have the ability to mature in the absence of class I, albeit with low efficiency, when E47 is lacking. Finally, inhibition of E47 activity within a DP T cell line alters the expression of markers that correlate with DP maturation. Taken together, these data demonstrate that a downregulation of E47 activity promotes thymocyte maturation and that a combination of increased DP cell death and enhanced maturation contributes significantly to the phenotype observed in the E47-deficient mice.

This raises the question of how E47 is involved in positive selection. TCR–MHC interactions that induce positive selection must have two important effects. These signals must rescue the cells from apoptosis and promote maturation. Recent data indicate that the restoration of E47 activity in E2A-deficient DP thymoma cell lines leads to programmed cell death 18. It is possible that the continued presence of E2A activity in DP thymocytes that have received a signal through the TCR contributes to the induction of apoptosis. Thus, only those cells that downregulate E2A activity survive. We note that the thymic phenotype of E47-deficient mice shows similarities to the bcl-2 transgenic mice, including a higher percentage of CD8 SP cells and an alteration in the CD4/CD8 ratio 7,35,36,37,38. Like bcl-2 expression, decreased E47 activity might prolong the survival of the DP thymocytes, thus leading to enhanced production of CD8⁺ cells. However, as mentioned above, DP thymocytes lacking E2A show an increase in cell death in vitro, whereas bcl-2 expression prolongs DP thymocyte survival 36,38. Additionally, expression of bcl-2 promotes CD8⁺ thymocyte maturation in MHC class I–deficient mice and in class II–restricted TCR transgenic mice, whereas the absence of E47 does not (35; Fig. 3 and Fig. 6). Finally, in contrast to the E47-deficient mice, the excess CD8⁺ thymocytes that develop in the bcl-2 transgenic mice do not survive in the periphery 35,36.

Recently, activation of the Notch signaling pathway has been shown to upregulate several markers that correlate with DP maturation. In particular, a constitutively active fragment of Notch was capable of upregulating bcl-2 expression in DP thymomas and rendering these cells resistant to glucocorticoid-induced apoptosis 39. Interestingly, like the E47-deficient mice, activated Notch1 transgenics have increased percentages of CD8⁺ thymocytes 40. However, a significant population of CD8⁺ thymocytes can develop in Notch1 transgenic mice lacking expression of class I MHC, whereas only a small CD8⁺ population can develop in class I–deficient mice lacking E47 40. Nonetheless, it is possible that Notch signaling regulates E2A activity. In fact, others have shown that activated Notch1 and Notch2 effectively inhibit E47 activity in transient transfection assays 41.

It is conceivable that the lack of E47 activity lowers the threshold of avidity required for positive selection. This model has been proposed to explain the phenotype of the activated Notch1 transgenic mice 39. For example, the absence of E47 might allow for the development of T cells that would normally die by neglect because of an insufficient affinity for MHC. It is thought that CD8 lineage commitment requires weaker signals from the TCR compared with the strength of the signal required for CD4 lineage commitment 42,43. If less stringent signals are required for commitment to the CD8 lineage, then a slight lowering of the threshold affinity of positive selection may result in an even greater increase in CD8⁺ cells compared with CD4⁺ cells. Such a model might explain the decrease in the CD4⁺/CD8⁺ thymocyte ratio in E2A-deficient mice and the finding that more CD8⁺ T cells develop in MHC class I–deficient mice that also lack E47 expression. Thus, the absence of E47 might allow for the development of some T cells that would normally die by neglect.

The data described above suggest that E47 activity is downregulated during the final stages of thymocyte maturation. A key step during the transition of a DP to SP thymocyte is the termination of TCR-α rearrangements. Previous studies have suggested a link between E2A activity and RAG expression, which is also downregulated in immature thymocytes upon interaction of the TCR with the appropriate MHC–peptide complex 44. For example, expression of both RAG-1 and RAG-2 can be activated in a pre-T cell line by the ectopic expression of E47 9. Additionally, E2A and HEB have been implicated in regulating RAG transcription in committed T lineage cells 11. Consistent with this idea, E2A-deficient thymocytes show decreased levels of both RAG-1 and RAG-2 transcripts (Bain, G., unpublished observations). However, RAG levels are not completely abolished, and TCR V(D)J recombination proceeds in E2A-deficient mice, presumably due to the presence of an HEB homodimeric complex detectable in E2A-deficient thymocyte extracts 7. We would like to suggest that an interaction of a TCR with the appropriate peptide–MHC complex results in the downregulation of total HLH activity, which leads to the abatement of RAG expression 44. We also note that lowering the DNA binding activity of E47 in a DP cell line leads to the activation of CD5, CD69, and TCR expression and the downmodulation of CD4, CD8, and HSA levels. Thus, our observations indicate that E47 directly or indirectly regulates the expression of a wide variety of genes that are characteristic of thymocyte maturation.

We thank Leslie Sharp and Drs. Dawne Page, Eben Massari, and Isaac Engel for critical reading of the manuscript. The H-Y–specific antibody was kindly provided by Dr. Ellen Robey. We are indebted to Drs. Jack Bui and Barbara Kee for generously providing mice.

This work was supported by a Blood and Haemoglobin Training Grant (G. Bain) and grants from the National Institutes of Health (C. Murre).