Introduction

The thymus medulla epithelium provides a unique environment for newly generated T cells to acquire self-tolerance before their emigration to the circulation. A variety of medullary thymic epithelial cells (mTECs) contribute to displaying self-antigens by employing multiple mechanisms. Promiscuous gene expression by a subpopulation of mTECs that express the nuclear protein Aire allows chromatin remodeling and genome-wide ectopic gene transcription^1,2. Cellular mimesis in other mTEC subpopulations relies on a mosaic of tissue-specific transcription factor-dependent cells that individually resemble specialized cell types, including mucosal tuft cells and microfold cells^3,4. These mTEC subpopulations form a self-antigen-displaying mTEC subset that provides various self-components, including extrathymic tissue-specific molecules, in the thymus medulla, so that newly generated T cells have an opportunity to establish central tolerance to genome-wide and systemic self-antigens prior to export from the thymus.

In addition to the self-antigen-displaying mTEC subset, there exists a medullary epithelial subset that produces the C-C chemokine CCL21, which attracts developing thymocytes and dendritic cells to the medullary region⁵. Positive selection-inducing TCR signals in newly generated cortical thymocytes elevate the expression of CCR7, a receptor for CCL21, and positively selected cortical thymocytes are therefore specifically attracted to migrate to the medullary region through the CCL21-CCR7-mediated chemotactic signals^6,7. Two molecular species CCL21Ser and CCL21Leu with one amino acid difference are encoded in the mouse genome^8,9, and CCL21Ser encoded by Ccl21a locus is predominantly expressed in the thymus medulla¹⁰. In mice lacking either Ccr7 or Ccl21a, positively selected mature thymocytes fail to accumulate in the medulla, and therefore T cells fail to establish self-tolerance^10–12. Additional CCR7-ligand CCL19 has no appreciable role in the cortex-to-medulla migration of developing thymocytes^10,13. Thus, CCL21-expressing thymocyte-attracting mTECs represent a functional mTEC subset that is essential for the establishment of self-tolerance in T cells.

Aire-expressing mTECs primarily belong to MHC-II^high CD80^high mTEC subpopulation (mTEC^high)^14,15, whereas most thymic mimetic cells are detectable in MHC-II^low CD80^low mTEC subpopulation (mTEC^low), which are derived from Aire-expressing mTEC^high cells^4,16–18. The mTEC^low subpopulation is heterogeneous and contains immature mTEC progenitors that differentiate into the Aire-expressing mTEC^high subpopulation^19,20. Thus, developmental progression of the self-antigen-displaying mTEC subset occurs in a single stream as mTEC^low progenitors -> mTEC^high Aire-expressing cells -> mTEC^low mimetic cells.

CCL21-expressing mTECs are mostly included in the mTEC^low subpopulation²¹. However, how the CCL21-expressing mTEC subset is integrated with, and/or potentially diverted from, the developmental progression of the self-antigen-displaying mTEC subset remains unknown. A single cell RNA-sequencing-based cluster analysis reported that Ccl21a transcripts are detectable within a heterogeneous “intertypical” TEC subpopulation, which includes cells with gene expression profiles resembling immature mTEC progenitors²². However, an independent single cell RNA-sequencing-based study suggested that TECs expressing Ccl21a transcripts represent a terminal population derived from “transit-amplifying” mTEC progenitors, by branching out of the self-antigen-displaying mTEC lineage²³. Without experimental evaluation of developmental potential, it is controversial and unknown how CCL21-expressing mTECs are developmentally related to immature mTEC progenitors. It is even unclear how the Ccl21a-transcript-detectable TECs reported in these single cell transcriptomic studies are related to functionally relevant CCL21-protein-producing medullary TECs. More specifically, it remains unestablished whether the functionally relevant CCL21-expressing mTEC subset retains a developmental potential to give rise to the self-antigen-displaying mTEC subset.

The present study addresses whether the CCL21-expressing mTEC subset carries a developmental capability to differentiate into the self-antigen-displaying mTEC subset. We show that CCL21-expressing mTECs arise early during embryonic development in mice. By Cre-loxP-mediated fate-mapping analysis, we demonstrate that a vast majority of self-antigen-displaying mTEC subset, including Aire-expressing mTECs and thymic tuft cells, originates from CCL21-expressing cells. The developmental potential of CCL21-expressing embryonic mTECs to give rise to Aire-expressing mTECs is confirmed in reaggregate thymus experiments. These results indicate that functionally relevant CCL21-expressing mTEC subset contains a developmental capability to differentiate into self-antigen-displaying mTEC subset and that developmental conversion from thymocyte-attracting subset to self-antigen-displaying subset serves to assemble functional diversity in thymus medulla epithelium.

Results

CCL21-expressing mTECs arise in embryonic thymus

We began this study by examining embryonic appearance of CCL21-expressing mTECs during mouse ontogeny. To do so, we analyzed CCL21-expressing mTECs by detecting tandem dimer Tomato (tdTomato) fluorescence proteins in Ccl21a-tdTomato knock-in mice. In postnatal thymus of Ccl21a^tdTomato/+ mice, CCL21-expressing cells were detected in keratin 14 (Krt14)-expressing medullary TECs (Fig. 1A) and not in Ly51-expressing cortical TECs (Fig. 1B). Flow cytometric analysis confirmed that Ccl21a^tdTomato+ cells in the thymus are predominantly confined in EpCAM⁺ Ly51^- UEA1⁺ mTECs (Fig. 1C, D). In accordance with the medullary localization of CCL21-expressing mTECs, thymic medulla was abundant in CCL21 proteins (Fig. 1E), which contribute to attracting CCR7-expressing positively selected thymocytes from the cortex^7,10. Among mTECs, most CCL21-expressing mTECs were distinct from Aire-expressing mTECs and DCLK1⁺ thymic tuft cells (Fig. 1F). CCL21-expressing mTECs equally distributed throughout the medullary region and were enriched neither in the pericortical area, including the corticomedullary junction area, nor in the central area within the medulla (Fig. 1G, H, Fig. S1A).

Figure 1.

Analysis of embryonic thymus showed that CCL21-expressing cells in the central region of the thymus were prominent by embryonic day 15 (E15) (Fig. 2A) and detectable as early as E13 (Fig. 2B). The localization of CCL21-expressing cells in the central region of embryonic thymus lobes and their expression of Krt14 and Foxn1 ascertained that CCL21-expressing embryonic thymus cells represented mTECs (Fig. 2A-C), rather than neighboring CCL21⁺ parathyroid gland cells (Fig. 2B), which contribute to the attraction of hematopoietic cells to the pre-vascularized embryonic thymus²⁴. CCL21 proteins were detectable in the medullary region proximal to Ccl21a^tdTomato-expressing mTECs in the embryonic thymus by E15 (Fig. 2D), indicating that Ccl21a^tdTomato-expressing E15 embryonic mTECs represent functionally relevant CCL21-expressing mTECs. On the other hand, CCL21-expressing mTECs in the embryonic thymus were largely distinct from Aire-expressing mTECs and DCLK1⁺ thymic tuft cells (Fig. 2E). Similar to the postnatal thymus, CCL21-expressing mTECs in the embryonic thymus distributed throughout the medullary region and were not enriched in the pericortical areas including the corticomedullary junctions (Fig. 2F, G, Fig. S1B). These results indicate that CCL21-expressing mTECs arise by E15 in the central region of the embryonic thymus.

Figure 2.

Fate mapping of CCL21-expressing cells in thymus

To better understand whether CCL21-expressing mTECs have a developmental potential to give rise to the self-antigen-displaying mTEC subset, we engineered mice in which the P1 bacteriophage-derived Cre recombinase gene was knocked-in at Ccl21a locus (Fig. 3A). We bred Ccl21a^Cre/+ mice with mice that carried a ubiquitous CAG promoter-driven loxP-stop-loxP-EGFP (CAG-loxP-EGFP) transgene. In Ccl21a-Cre x CAG-loxP-EGFP mice, EGFP expression labeled cells that have transcribed Ccl21a previously and/or presently (Fig. 3A).

Figure 3.

In the postnatal thymus of Ccl21a-Cre x CAG-loxP-EGFP mice, EGFP signals in the thymus distributed prominently in the medullary region (Fig. 3B). EGFP⁺ cells in the medulla were detected in Krt14⁺ mTECs (Fig. 3B), including Aire⁺ mTECs and DCLK1⁺ thymic tuft cells (Fig. 3C). Flow cytometric analysis of enzyme-digested thymus cells showed that EGFP⁺ cells in the thymus were detected exclusively in EpCAM⁺CD45^- TECs and not in EpCAM^- CD45⁺ hematopoietic cells, including thymocytes and dendritic cells, or EpCAM^-CD45^- cells, including mesenchymal fibroblasts and endothelial cells (Fig. 3D, E). Within EpCAM⁺CD45^- TECs, approximately 95% of UEA1⁺Ly51^- mTECs were EGFP⁺, indicating that a vast majority of mTECs were derived from cells that previously transcribed Ccl21a (Fig. 3F, G). Flow cytometric analysis also confirmed the detection of Aire⁺ UEA1⁺Ly51^- cells and DCLK1⁺ UEA1⁺Ly51^- cells in EpCAM⁺CD45^- EGFP⁺ TECs (Fig. 3H). Thus, self-antigen-displaying mTECs, including Aire⁺ mTECs and DCLK1⁺ thymic tuft cells, which do not presently express CCL21 (Fig. 1F, 2E), originate from Ccl21a⁺ progenitors.

In the quantitative RT-PCR analysis of Cre, Ccl21a, and other TEC-associated genes, such as Aire, Tnfrsf11a, and Psmb11, Cre was detected specifically in the Ccl21a-expressing mTEC^low subpopulation, which includes the majority of CCL21-expressing mTECs²¹, and not in the Psmb11-expressing cTECs or the Aire- and Tnfrsf11a-expressing mTEC^high subpopulation (Fig. 4A). These results verify the lineage tracing potential of Ccl21a-Cre-expressing cells to differentiate into the mTEC^high subpopulation including Aire⁺ mTECs.

Figure 4.

In addition to mTECs, approximately 66% of UEA1^-Ly51⁺ cTECs in the postnatal thymus were EGFP⁺ in Ccl21a-Cre x CAG-loxP-EGFP mice (Fig. 3 F, G). Section analysis of the thymus verified the presence of EGFP⁺ cells in the cortex (Fig. 3B). Because cTECs do not presently express Ccl21a transcripts¹⁰ (also shown in Fig. 1B-D), these results indicate that approximately two-thirds of postnatal cTECs are derived from cells that previously transcribed Ccl21a (Fig. 3 F, G). To characterize cTECs that previously transcribed Ccl21a and those that did not, we isolated EGFP⁺ and EGFP^- cTECs from postnatal Ccl21a-Cre x CAG-loxP-EGFP mice (Fig. S2) and performed deep-sequencing transcriptomic analysis. The MA plot and the volcano plot of RNA-sequencing data demonstrated high similarity in gene expression profiles between EGFP⁺ and EGFP^- cTECs among 20,230 genes detected (Fig. 4B, C), unlike the pronounced difference in global gene expression profiles between cTECs and mTECs (Fig. S3A, B). The genes that are relevant for T cell-inducing and positive selection-inducing functions in cTECs, including Dll4, Il7, Cxcl12, Psmb11, Ctsl, Prss16, Cd83, H-2K, H-2D, and I-Aβ, were similarly abundant in EGFP⁺ and EGFP^- cTECs (Fig. S3C). These results suggest that cTECs derived from cells that either do or do not transcribe Ccl21a are functionally comparable to promote the generation and positive selection of cortical thymocytes.

Only 14 genes, including Ccl21a, showed significantly higher expression in EGFP⁺ cTECs than in EGFP^- cTECs, whereas only one gene, the parathyroid hormone-encoding Pth, showed significantly lower expression in EGFP⁺ cTECs than in EGFP^- cTECs (Fig. 4C, Fig. S3D). All the 14 genes that showed significantly higher expression in EGFP⁺ cTECs than in EGFP^- cTECs represented genes that showed higher expression in mTECs than in cTECs, and their expression, including Ccl21a expression, was much higher in mTECs than in EGFP⁺ cTECs (Fig. S3D). Interestingly, EGFP⁺ cTECs distributed unequally within the thymic cortex and were detected more frequently in the perimedullary cortical region than in the central cortical region or the subcapsular region of the cortex (Fig. 4D, E, Fig. S1C). In contrast, Ly51⁺ total cTECs did not show such an unequal distribution in the thymic cortex (Fig. S1D, E). Thus, the intrathymic proximity to the medullary region contributes to the previous expression of Ccl21a transcripts in cTECs, suggesting that approximately two-thirds of postnatal cTECs originate from Ccl21a⁺ cells generated in the thymic medulla.

Collectively, these fate-mapping experiments demonstrate that a vast majority of mTECs, including the self-antigen-displaying mTEC subset, and approximately two-thirds of cTECs, particularly cTECs in the perimedullary cortical region, are derived from progenitor cells that previously transcribed Ccl21a.

Ccl21a-expressing mTECs arise early in central region of thymus primordium

It is further interesting to note that EGFP⁺ cells within Foxn1⁺ mTECs were detected in the central region of the thymus primordium in Ccl21a-Cre x CAG-loxP-EGFP mice as early as E11 (Fig. 5A). The detection of EGFP⁺ cells occurred earlier in the ontogeny than the detection of Ccl21a^tdTomato+ mTECs by E13 (Fig. 2B) and CCL21-protein-expressing mTECs by E15 (Fig. 2D). These sequential detections suggested that low-level Ccl21a transcription initiated as early as E11 during thymus organogenesis was sufficient to produce functional Cre proteins to mediate the expression of CAG-promoter-dependent EGFP proteins, but failed to produce a detectable amount of tdTomato or CCL21 proteins. Subsequent elevation of Ccl21a transcription level resulted in the production of detectable amounts of tdTomato proteins followed by CCL21 proteins. Indeed, single-cell RNA-sequencing analysis of Ccl21a transcript counts demonstrated that Ccl21a expression levels within individual Epcam⁺ Foxn1⁺ TECs elevated during the embryogenesis from E12 to E14 (Fig. 5B). At E14, Ccl21a^low cells were detected at an equivalent frequency in β5t⁺ and β5t^- TEC populations (Fig. 5B). Ccl21a transcription in CCL21-expressing mTECs was further elevated from embryonic to postnatal development, as shown by single-cell RNA-sequencing analysis of Ccl21a transcript counts (Fig. 5B) and verified by flow cytometric analysis of Ccl21a-dependent tdTomato expression levels (Fig. 5C). Importantly, Ccl21a⁺ Foxn1⁺ mTECs detected during early thymus organogenesis were localized in the central region of the thymus primordium and were anatomically and transcriptomically distinguishable from Ccl21a⁺ Gcm2⁺ parathyroid epithelial cells (Fig. 2B, 5A, 5D). These results reveal that Ccl21a⁺ mTECs develop in the central region of the thymus primordium as early as E11 before the generation of detectably CCL21 protein-expressing mTECs in the embryonic thymus by E15.

Figure 5.

CCL21-expressing mTECs are distinct from RANK-expressing mTECs

Along with the fate-mapping detection of the developmental potential in Ccl21a⁺ cells, the early and temporal detection of Ccl21a⁺ mTECs and CCL21-expressing mTECs during embryogenesis tempted us to speculate that Ccl21a⁺ mTECs, and even CCL21-expressing mTECs, might possess mTEC progenitor activity. Previous studies reported that embryonic thymus develops RANK-expressing mTEC-restricted progenitors, which have a developmental capability to give rise to self-antigen-displaying mTECs, including Aire⁺ mTECs^25,26. RANK-expressing mTEC progenitors arise in the thymus during embryogenesis downstream of mTEC stem cells, which originate from cTEC-trait-expressing bipotent progenitors^5,25–28. We next examined whether CCL21-expressing mTECs might overlap with RANK-expressing mTEC progenitors in CCL21^tdTomato RANK^Venus embryonic thymus. Interestingly, however, we found that CCL21-expressing mTECs and RANK-expressing mTECs were largely distinct cell populations within the embryonic thymus (Fig. 6A, B), although they were closely localized within the medullary region (Fig. 6A). RNA-sequencing analysis of CCL21⁺RANK^- and CCL21^-RANK⁺ TECs highly purified from CCL21^tdTomato RANK^Venus embryonic thymus (Fig. S4A) demonstrated that CCL21-expressing mTECs were markedly different from RANK-expressing mTECs in global gene expression profiles (Fig. 6C). Gene ontology enrichment analysis ascertained the difference in global gene expression profiles in CCL21-expressing mTECs and RANK-expressing mTECs (Fig. 6D). Indeed, CCL21-expressing mTECs retained more cTEC-trait genes, including Psmb11, Cxcl12, CD83, and Ackr4, than RANK-expressing mTECs (Fig. 6E), suggesting that in comparison with RANK-expressing mTECs, CCL21-expressing mTECs are more recently derived from cTEC-trait-expressing bipotent TEC progenitors. However, similarly to RANK-expressing mTECs, CCL21-expressing mTECs expressed equivalent levels of some mTEC-trait transcripts, including Krt5 and Krt14 (Fig. 6E). Among the genes associated with TEC progenitor activity^29–34, Trp63 and Pax1 were predominant in CCL21-expressing mTECs, whereas Plet1, Krt19, Cldn3, Cldn4, and Ly6a were readily detectable in RANK-expressing mTECs (Fig. 6E). Thus, CCL21-expressing mTECs in embryonic thymus are largely distinct from RANK-expressing mTECs, whereas CCL21-expressing mTECs and RANK-expressing mTECs are proximally localized with each other within the thymus medulla.

Figure 6.

Nonetheless, it is important to point out that fate-mapping experiments using Ccl21a-Cre x CAG-loxP-EGFP mice were unable to distinguish whether TEC progenitor activity resides in CCL21-expressing mTECs generated within the thymus or in CCL21-expressing non-mTECs including neighboring parathyroid cells.

Developmental potential of CCL21-expressing mTECs

To directly examine whether CCL21-expressing mTECs have a developmental potential to give rise to self-antigen-displaying mTECs, we finally traced the fate of CCL21-expressing mTECs within the thymus microenvironment. To do so, we employed reaggregate thymus organ culture experiments^35,36 where RelB-deficient thymic stromal cells do not generate any functional mTECs^37,38. Reaggregate thymuses were transplanted under mouse kidney capsules for reconstitution with hematopoietic cells derived from recipient mice. In this experimental condition, RelB-deficient thymic stromal cells were reaggregated with tdTomato-expressing mTECs highly purified from Ccl21a^tdTomato E17 embryos (Fig. S4B). Ccl21a^tdTomato+ mTECs had already become functional CCL21-protein-expressing mTECs by E17 (Fig. 2), and the accumulation of CD4⁺CD8^- and CD4^-CD8⁺ thymocytes in E17 thymic medullary region was defective in mice deficient in CCL21 (Fig. 7A-C), so that the reaggregate thymus experiments would address whether functionally relevant CCL21-expressing mTECs give rise to other mTEC subpopulations including Aire⁺ mTECs.

Figure 7.

We found that Ccl21a^tdTomato-positive EpCAM⁺CD45^- mTECs isolated from Ccl21a^tdTomato/+ E17 embryos generated Aire⁺ Krt14⁺ mTECs in the thymus reaggregated with RelB-deficient thymic stromal cells (Fig. 8A). Aire⁺ Krt14⁺ mTECs also developed from an equal number of parallelly isolated Ccl21a^tdTomato-negative EpCAM⁺CD45^- TECs (Fig. 8B), which included cTEC-mTEC bipotent TEC progenitors^5,39. RelB-deficient thymic stromal reaggregates without additional TECs failed to generate Aire⁺ Krt14⁺ mTECs (Fig. 8C), whereas control organ culture using RelB-heterozygous thymus apparently developed medullary regions that contained Aire⁺ Krt14⁺ mTECs (Fig. 8D). Quantitative evaluation of Krt14⁺ medullary areas (Fig. 8E) within total thymus aggregates (Fig. 8F) demonstrated that Ccl21a⁺ mTECs have a developmental potential to give rise to Aire⁺ Krt14⁺ mTECs, equivalently to and not statistically different from Ccl21a-negative TECs, including bipotent TEC progenitors (Fig. 8G, H). Interestingly, Ccl21a⁺ mTECs were still detected in the Krt14⁺ medullary areas in reconstituted thymus reaggregated with either Ccl21a⁺ mTECs or Ccl21a-negative TECs (Fig. 8I, J), indicating that Ccl21a⁺ mTECs have a potential to maintain themselves in the thymus and that Ccl21a-negative TECs have a developmental potential to produce Ccl21a⁺ mTECs. Most importantly, these results directly indicate that functionally relevant CCL21-expressing mTECs have a developmental potential to give rise to Aire-expressing mTECs.

Figure 8.

In contrast to CCL21-expressing mTECs isolated from E17 embryos, Ccl21a^tdTomato-positive EpCAM⁺CD45^- mTECs isolated from 4-week-old Ccl21a^tdTomato/+ mice did not exhibit any signs of Aire⁺ Krt14⁺ mTEC generation in the thymus reaggregated with RelB-deficient thymic stromal cells (Fig. S5A-G), suggesting a decline in the developmental potential of CCL21⁺ mTECs during the ontogeny from embryonic to postnatal period. However, no detection of progeny could also result from technical caveats associated with reaggregate thymus organ culture conditions that were suitable for detection of the developmental potential of embryonic but not postnatal CCL21-expressing mTECs, or differences in the lifespan and/or turnover of embryonic and postnatal CCL21-expressing cells.

Throughout the development including E17 embryos, CCL21-expressing mTECs were localized specifically in the central region of the thymus and were Krt14⁺ (Fig. 1, 2, 6, 9A). However, single-cell RNA-sequencing analysis demonstrated that the global gene expression profiles were largely different between embryonic Ccl21a^low mTECs and postnatal Ccl21a^high mTECs (Fig. 9B). Indeed, the majority of CCL21-expressing mTECs at E17 but not the postnatal period simultaneously expressed cTEC-associated protein Ly51 and mTEC-associated UEA1-binding molecules on the cell surface (Fig. 9C). Ly51 expression was lower in embryonic TECs than postnatal TECs⁴⁰ and equivalently distributed in the thymus including the medullary region defined by Krt14-expressing mTECs centrally localized in E17 thymus (Fig. 9A). These results highlight the difference in molecular expression profiles, including Ly51 expression, between E17 CCL21-expressing mTECs and postnatal CCL21-expressing mTECs.

Figure 9.

Discussion

The present results demonstrate that CCL21-expressing mTECs arise early during embryonic thymus development. Ccl21a-Cre-mediated detection reveals that Ccl21a⁺ Foxn1⁺ mTECs are detectable as early as E11 in the central region of the embryonic thymus, shortly after the organogenesis of the thymus primordium, and this is followed by the detection of Ccl21a-tdTomato-expressing mTECs by E13 and CCL21 protein-expressing mTECs by E15. Ccl21a-Cre-mediated fate-mapping establishes that a vast majority of mTECs, including the self-antigen-displaying mTECs such as Aire⁺ mTECs and thymic tuft cells, and approximately two-thirds of cTECs originate from Ccl21a⁺ cells. Most importantly, our results demonstrate that Ccl21a-tdTomato-expressing and functional CCL21 protein-producing mTECs isolated from E17 embryonic thymus have a developmental potential to give rise to the self-antigen-displaying mTECs in the thymus microenvironment. These results demonstrate a previously unknown process by which the diversity in the thymus medulla epithelium is generated during embryogenesis by the developmental conversion of the CCL21-expressing thymocyte-attracting mTEC subset into the self-antigen-displaying mTEC subset, including Aire-expressing mTECs and thymic tuft cells.

Functional conversion of the thymocyte-attracting mTEC subset into the self-antigen-displaying mTEC subset offers an interesting implication that the thymus medulla is formed by initially generating CCL21-expressing mTECs and attracting CCR7-expressing positively selected thymocytes from the cortex even before the subsequent display of self-antigens mediated by Aire+ mTECs and thymic mimetic cells. Failure to establish self-tolerance by the loss of Ccr7 or Ccl21a in the thymus^10–12 underscored the importance of functional CCL21-expressing mTEC subset. Our present results showing developmental potential of functional CCL21-expressing mTECs to give rise to self-antigen-displaying mTECs reveals an additionally important function of CCL21-expressing mTECs in forming the diversity in thymus medulla epithelium, including the self-antigen-displaying mTEC subset. Stepwise progression in mTEC development from thymocyte-attracting function to self-antigen-displaying function fits stepwise requirement to initially recruit newly generated T cells from the thymic cortex before tolerizing T cells to self-antigens. Down-regulation of CCL21 in functionally converted self-antigen-displaying mTECs may further contribute to the prevention of excessive negative selection of newly generated T cells to self-antigens.

Recent studies demonstrated that the function of the thymus medulla is not limited to the establishment of self-tolerance in conventional αβ T cells but extends to the development of innate lymphocytes, including invariant NKT cells and γδ T cells^41,42. The developmental capability of CCL21-expressing mTECs may further extend to additional mTEC subpopulations that contribute to supporting the development of innate invariant lymphocytes. Thus, CCL21-expressing mTECs may represent a primordial mTECs which separate newly generated αβ T cells, γδ T cells and NKT cells from the thymic cortex to the medullary microenvironment for further selection and maturation.

In contrast to the robust generation of Aire⁺ mTECs from embryonic CCL21⁺ mTECs, our results show that the ability to give rise to Aire⁺ mTECs was not detectable in postnatal CCL21⁺ mTECs. The fact that no Aire⁺ mTECs were detected in reaggregate thymus organ culture may reflect a postnatal decline in the developmental potential of CCL21⁺ mTECs. However, it is also possible that the developmental kinetics and the requirements for postnatal CCL21⁺ mTECs may be different from those for E17 CCL21⁺ mTECs, so that the reaggregate thymus organ culture conditions for detecting the development of E17 CCL21⁺ mTECs may not be optimal for the detection of the survival and development of postnatal CCL21⁺ mTECs.

Nevertheless, it is important to notice the difference in the developmental potential between embryonic and postnatal CCL21⁺ mTECs in reaggregate thymus organ culture. Analysis of molecular expression profiles further highlighted the difference between embryonic and postnatal CCL21⁺ mTECs. We think that CCL21⁺ mTECs may be heterogenous in developmental capability. Embryonic CCL21⁺ mTECs, which give rise to self-antigen-displaying mTECs including Aire⁺ mTECs and mimetic mTECs, may also terminally differentiate into a subpopulation of CCL21⁺ mTECs, which no longer have further developmental potential, and which represent the majority of postnatal CCL21⁺ mTECs.

Interestingly, our results also reveal that approximately two-thirds of cTECs are derived from progenitor cells that previously transcribed Ccl21a. The previous expression of Ccl21a transcripts in the fraction of cTECs is influenced by the intrathymic proximity to the medullary region and is enriched in the perimedullary cortical region (Fig. 4E). Developmental capability to become cTECs is shown in Ccl21a gene-expressing cells, and it is unknown whether CCL21-protein-expressing functional mTECs retain the developmental potential to give rise to cTECs. In contrast, our results from reaggregate thymus experiments clarified that CCL21-protein-expressing functional mTECs have a developmental potential to convert into Aire-expressing mTECs. Consequently, we think that at least a fraction of Ccl21a transcript-expressing mTECs retain a bipotent developmental potential to give rise to both cTECs and mTECs. It is unclear whether most Ccl21a-expressing mTECs retain a bipotent developmental potential for cTECs and mTECs or Ccl21a-expressing mTECs represent a mixture of bipotent progenitors and lineage-committed mTEC progenitors. Because of the unequal distribution of EGFP-labeled cTECs within the thymic cortex, initial signals to promote bipotent progenitors to mTEC lineage may be provided by molecules restricted in the central region of the embryonic thymus, and those signals may promote the transcription of Ccl21a at an early stage of mTECs, which still retain the potential to become cTECs. Further examination of cTEC potential, for example, by using CCL21-tdTomato-expressing cells that constitutively express GFP, may lead to a better understanding of the cTEC-generating capability in embryonic and postnatal CCL21⁺ mTECs.

Further characterization of CCL21-expressing mTECs carrying a developmental potential to give rise to self-antigen-displaying mTEC subset is of great interest. Our results highlight the difference between CCL21-expressing mTECs and previously characterized RANK-expressing mTEC-restricted progenitors. CCL21-expressing mTECs and RANK-expressing mTECs are detectable in E13 embryonic thymus, are proximally localized within the medullary region in the thymus, and are capable of generating Aire-expressing mTECs. Unlike RANK-expressing mTECs, however, CCL21-expressing mTECs include developmental potential to give rise to cTECs. Gene expression profiles show that CCL21-expressing mTECs retained more cTEC-trait genes, including Psmb11, Cxcl12, CD83, and Ackr4, than RANK-expressing mTECs, suggesting that in comparison with RANK-expressing mTECs, CCL21-expressing mTECs are more closely related to and more recently derived from cTEC-trait-expressing bipotent TEC progenitors.

Furthermore, it is interesting to note that recently described Krt19⁺ progenitors that give rise to multiple mature mTEC populations including Aire⁺ mTECs, mimetic mTECs, and postnatal CCL21⁺ mTECs, lack expression of CCL21 and RANK³². An important aspect of future work will be to examine whether CCL21-expressing mTECs, RANK-expressing mTECs and Krt19-expressing mTECs are linked in upstream-downstream relationships in a single developmental pathway, or are developmentally distinct and parallel progenitors to give rise to mTECs. Similarly, it will be interesting to better clarify possible heterogeneity in CCL21-expressing mTECs in relation to their ability to give rise to cTECs as well as mTECs. Nevertheless, the finding in our study that CCL21⁺ embryonic mTECs can give rise to Aire⁺ mTECs reveals a new source of an important component of the thymus medulla.

Our results show that 95% of mTECs are labeled with EGFP fluorescence and therefore derived from Ccl21a⁺ progenitors, whereas 5% of mTECs are not labeled with EGFP fluorescence in Ccl21a-Cre x loxP-EGFP mice. Our previous study using β5t-driven Cre mice indicated fate-mapping of 95% EGFP⁺ mTECs in CAG-loxP-stop-loxP-EGFP-transgenic reporter mice used in this study and >99.5% ZsGreen⁺ mTECs in Rosa26-loxP-ZsGreen-knock-in reporter mice⁴³. Consequently, we think that 95% EGFP⁺ cells in mTECs detected in this study may indicate that virtually all (>99.5%) mTECs are derived from cells that once transcribed Ccl21a.

A previous study detected Ccl21a transcripts within a podoplanin-expressing TEC subpopulation that resembled immature TECs in postnatal thymus⁴⁴. It was reported that podoplanin-expressing mTEC progenitors, also known as junctional TECs (jTECs), were localized at the corticomedullary junction⁴⁴. However, our results show that CCL21-expressing mTECs in the embryonic and postnatal thymus distribute throughout the medullary region and are not enriched in the corticomedullary junctions (Fig. 1H, 2G). Indeed, the majority of CCL21-expressing mTECs in postnatal thymus did not express podoplanin (Fig. S5H). Moreover, podoplanin expression was broadly detected by the majority of embryonic E17 TECs, most of which were cTECs (Fig. S5H). Thus, neither podoplanin expression nor corticomedullary junctional distribution characterizes CCL21-expressing mTECs. More importantly, our results demonstrate developmental potential in thymocyte-attracting CCL21-expressing mTECs and identify developmental conversion between functionally distinct mTEC subsets, which serves to assemble a diverse medullary microenvironment in the thymus.

In conclusion, the present study establishes that CCL21-expressing functional mTEC subset carries a developmental potential to give rise to self-antigen-displaying mTECs, revealing that the sequential conversion of functionally distinct mTEC subsets from thymocyte-attracting cells into self-antigen-displaying cells contributes to the functional diversification of thymus medulla epithelium during embryonic development. The present results are likely to ignite many interesting questions, including the characterization of signals that drive the developmental conversion of mTECs and the elucidation of developmental pathways for diverse TEC subpopulations.

Materials and methods

Mice

C57BL/6 (B6) mice were obtained from The Jackson Laboratory (Bar Harbor, ME) and SLC (Shizuoka, Japan). Ccl21a-tdTomato knock-in mice¹⁰, RelB-knockout mice³⁸, RANK-Venus transgenic mice⁴⁵, β5t-Venus knock-in mice⁴⁶, CAG-loxP-stop-loxP-EGFP transgenic mice⁴⁷, and Foxn1-GFP transgenic mice⁴⁸ were described previously. All mouse experiments were performed with consent from the Animal Experimentation Committee of the University of Tokushima (T2022–50), the Animal Care and Use Committee of the National Cancer Institute (ASP 21–431, ASP 21-432, and EIB-076–3), the Birmingham Animal Welfare and Ethical Review Board and UK Home Office Animal Licences (PP7518148 and PP2990911), and the Institutional Animal Care and Use Committee of RIKEN Kobe Branch (A2001-03).

Generation of Ccl21a-Cre mice

The targeting vector was prepared by subcloning Ccl21a-containing mouse genomic BAC fragments (Advanced GenoTechs) and Cre-encoding cDNA into a plasmid containing a pgk-neo cassette (Gene Bridges). The linearized targeting vector was introduced into TT2 ES cells⁴⁹. Targeted knock-in alleles were screened by genomic PCR analysis and Southern blot analysis. The Ccl21a-Cre mice (https://large.riken.jp/distribution/mutant-list.html; Accession No. CDB1131K) are available to the scientific community and were backcrossed to B6 mouse strain for more than 5 generations. The knock-in allele is detected using PCR primers as follows; Cre-F, 5’-AGGTTCGTTCACTCATGGA-3’ and Cre-R, 5′-TCGACCAGTTTAGTTACCC-3′ (product size, 235bp). Wild-type allele is detected using PCR primers as follows; WT-F, 5′-CTGGTCTCATCCTCAACTCA-3′ and WT-R, 5′-TGTAACCCTAGGATTGTAGG-3′ (product size, 1,563bp).

Reaggregate thymus experiments

Fetal thymus lobes isolated from relB-KO mice at embryonic day (E) 15 were organ cultured for 5 days on sponge-supported Nucleopore filters (Whatman) placed on Dulbecco’s Modified Eagle Medium supplemented with 10% fetal bovine serum (FBS) and penicillin-streptomycin mixed solution. To eliminate endogenous hematopoietic cells from the lobes, 1.35 mM 2’-deoxyguanosine (dGuo) was added in the culture medium^35,36. dGuo-treated thymus lobes were digested with 0.25% trypsin supplemented with 0.45 mM EDTA and 0.02% DNase I for 15 min at 37℃ and reaggregated with tdTomato⁺ or tdTomato^- TECs isolated from Ccl21a^tdTomato/+ mice at E17 or 4 weeks old. After one day culture on Nucleopore filters, reaggregated thymuses were transplanted under kidney capsules of B6 mice. dGuo-treated fetal thymus lobes isolated from RelB-deficient and RelB-heterozygous mice were also transplanted under kidney capsules of B6 mice. Grafts were harvested 5 weeks post transplantation.

Multicolor immunofluorescence analysis

Paraformaldehyde-fixed frozen tissues embedded in OCT compound (Sakura Finetek) were sliced into 10-μm-thick sections. The sections were stained with anti-CCL21 (Bio-Rad), anti-Aire (eBioscience), anti-Ly51 (Biolegend), anti-Krt14 (Biolegend), anti-DCLK1 (abcam), anti-CD4 (Biolegend), anti-CD8 (Biolegend) antibodies, anti-Foxn1 antiserum⁵⁰, and UEA-1 (Vector Laboratories). Images were analyzed with a TCS SP8 confocal laser scanning microscope (Leica).

Fluorescence distribution analysis

The plot profile intensity for the fluorescence image of each channel in the regions of interest (ROI) was measured using ImageJ software (https://imagej.nih.gov/ij/index.html). The fluorescence intensity at each horizontal distance with the total intensity of the vertically averaged tdTomato, EGFP, or Ly51 in the ROI was calculated as 100. The sum of tdTomato or EGFP intensities for the medullary region defined by Krt14 expression or UEA1 reactivity and for the cortical region was calculated.

Multicolor flow cytometric analysis and cell sorting

For the analysis of thymic epithelial cells (TECs), minced thymuses were digested with 0.5 unit/ml Liberase (Roche) in the presence of 0.02% DNase I (Roche). Single-cell suspensions were stained with antibodies specific for EpCAM (BioLegend), CD45 (eBioscience), Ly51 (Biolegend), Podoplanin (Biolegend), I-A^b (BioLegend), and for the reactivity with UEA-1 (Vector Laboratories). For the analysis of Aire and DCLK1, surface-stained cells were fixed in 5% formaldehyde neutral buffer solution (Nacarai Tesque), permeabilized in 1x permeabilization buffer (eBioscience), and stained with anti-Aire (eBioscience) or anti-DCLK1 (abcam) antibody. For the isolation of TECs, CD45^- cells were enriched in magnetic bead conjugated anti-CD45 antibody (Miltenyi Biotec). Multicolor flow cytometry and cell sorting were performed on FACSVerse, FACSAria II, and LSRFortessa (BD Biosciences).

RNA-sequencing analysis

cDNAs were prepared by using SMART-Seq v4 Ultra Low Input RNA Kit, according to the manufacturer’s protocol (Clontech). Sequencing libraries were generated by using a Nextera XT DNA Library Prep Kit, according to the manufacturer’s protocol (Illumina). The concentration of libraries was measured by an ABI PRISM 7500 Real-time PCR system in combination with a Power SYBR Green PCR Master Mix (Thermo Fisher). Single-end sequencing of cDNA libraries with a read length of 51 was performed with HiSeq 1500 platform (Illumina). For RNA-sequencing analysis of CCL21⁺RANK^- and CCL21^-RANK⁺ TECs isolated from CCL21^tdTomato RANK^Venus embryonic E17 thymus, RNA was prepared using Qiagen RNAeasy Plus Micro Kit with genomic DNA eliminator columns. RNA-sequencing samples were sequenced on NovaSeq_SP flowcell using Illumina TruSeq Stranded Total RNA Library Prep with 2×100bp paired-end sequencing. Data were analyzed by using CLC Genomics Workbench 12.0 (Qiagen) with default parameters.

Single cell RNA-sequencing analysis

E12 embryonic pharyngeal cells, E14 embryonic CD45^-EpCAM⁺ β5t-Venus⁺ and β5t-Venus^- TECs, E16 embryonic CD45^-EpCAM⁺ β5t-Venus⁺ and β5t-Venus^- TECs isolated from β5t-Venus knock-in mice, and 4-week-old postnatal TEC-enriched thymus cells were employed for single-cell RNA sequencing. 3′ mRNA-seq library was generated as outlined in the Chromium Connect platform using 3’ v3.1 Single Cell RNA-Seq chemistry reagents from 10X Genomics. Reverse transcription and cDNA amplification were performed according to the manufacturer’s instructions. Libraries were sequenced on NextSeq 2000 and FASTQ files were generated from demultiplexes raw base call files using Cell Ranger 6.1.1 mkfastq. Alignment, filtering, barcode counting, and UMI counting were performed using Cell Ranger 6.1.1 count with mouse reference 3.0.0 (mm10 and Ensembl release 98). The resulting matrix was imported into Seurat (v4.3.0) in R v 4.3.0 to create a Seurat object. The object underwent normalization, filtering, scaling, and dimensionality reduction steps sequentially using Seurat build-in functions, with default parameters. To integrate all the filtered objects from each sample, the CCA algorithm was employed through the FindIntegrationAnchors and IntegrateData functions in Seurat. The Uniform Manifold Approximation and Projection (UMAP) plots were visualized using the DimPlot function of Seurat. The histograms were visualized using the ggplot2 package (v3.4.2).

Statistical analysis

Statistical significance was assessed using the two-tailed unpaired Student’s t test with Welch’s correction for unequal variances or one-way ANOVA with Tukey’s correction.

Data availability

RNA sequencing data are deposited in The DNA Data Bank of Japan (https://www.ddbj.nig.ac.jp) BioProject database with the accession number PRJDB15439 and NCBI Gene Expression Omnibus (GEO) (https://www.ncbi.nlm.nih.gov/geo) database with the accession number GSE239913. Single-cell RNA sequencing data are deposited in NCBI GEO with the accession numbers (deposition in progress).

Acknowledgements

We thank Drs. Takashi Amagai and Manami Itoi for providing anti-Foxn1 antibody, Dr. Thomas Boehm for providing Foxn1-GFP transgenic mice, Drs. Toyomasa Katagiri, Hitomi Kyuma, Eri Ootsu, Bao Tran, and Assiatu Crossman for technical assistance, and Drs. Alfred Singer, Richard Hodes, Mami Matsuda-Lennikov and Jie Li for reading the manuscript. This work was supported by the Intramural Research Program ZIA BC 011806 of the National Institutes of Health, the National Cancer Institute, and the Center for Cancer Research (Y.T.); by the JSPS KAKENHI 22K06900, the JSPS Bilateral Program 120219928, and the JST PRESTO Grant 22712940 (I.O.); and by the MRC Programme Grant MR/T029765/1 and the Wellcome Trust Collaborative Award SynThy 211944/Z/18/Z (G.A.).