Nuclear receptor Nr4a1 modulates both regulatory T-cell (Treg) differentiation and clonal deletion

Abstract

Immature thymocytes expressing autoreactive T-cell receptors (TCR) can adopt differing cell fates: clonal deletion by apoptosis or deviation into alternative lineages such as FoxP3 + regulatory T cells (Treg). We revisited the role of the transcription factor Nr4a1 (Nur77), an immediate-early response gene induced by TCR engagement. Nr4a1KO mice show clear quantitative defects in antigen-induced clonal deletion. The impact of the Nr4a1 deletion is not enhanced by deletion of the proapoptotic factor Bim. In addition, Nr4a1 curtails initial differentiation into the Treg lineage in TCR transgenic mice and in nontransgenic mice. Transcriptional profiling of Nr4a1KO thymocytes under selection conditions reveals that Nr4a1 activates the transcription of several targets, consistent with these diverse actions: (i) Nr4a1 partakes in the induction of Bim after TCR triggering; (ii) perhaps paradoxically, Nr4a1 positively controls several transcripts of the Treg signature, in particular Ikzf2 and Tnfrsf9; (iii) consistent with its prosurvival and metabolic role in the liver, Nr4a1 is also required for the induction by TCR of a coordinated set of enzymes of the glycolytic and Krebs cycle pathways, which we propose may antagonize Treg selection as does activation of mTOR/Akt. Thus, Nr4a1 appears to act as a balancing molecule in fate determination at a critical juncture of T-cell differentiation.glycolysis | immune tolerance | thymus | nuclear receptor A hallmark of the adaptive immune system is its ability to respond to the infinite repertoire of potential pathogens to which it might someday be exposed, but the undirected process of antigen receptor rearrangement in thymocytes generates receptors that are reactive to self and hence are potentially dangerous. Fledgling T cells carrying a nascent T-cell receptor (TCR) must undergo a rigorous process of negative selection, during which cells expressing a self-reactive TCR are eliminated by induced apoptosis or deviated to alternative differentiation pathways in which self-reactivity is defused [NKT, CD8αα, or FoxP3 + Triggering of mitochondrial apoptosis by signaling cascades downstream from the TCR is clearly established. Bim and Bax/ Bak, proapoptotic members of the mitochondrial apoptosis pathway, are essential for efficient negative selection (2, 3), but this may not be the sole mechanism. Early experiments involving blockade of RNA or protein synthesis suggested that negative selection is both transcription and translation dependent (4). Indeed, the transcription factor Nr4a1 has been implicated in negative selection (5, 6). An orphan member of the nuclear receptor superfamily and the Nr4a1/Nr4a2/Nr4a3 subfamily, Nr4a1 is a transcriptional activator, but can also promote mitochondrial apoptosis. In both tumor cell lines and thymocytes undergoing negative selection, Nr4a1 undergoes phosphorylation-dependent export from the nucleus to the mitochondria, where it conformationally alters Bcl-2 to promote apoptosis (7-10). Nr4a1 is associated with apoptosis in a number of phys...

Differential Roles for Bim and Nur77 in Thymocyte Clonal Deletion Induced by Ubiquitous Self-Antigen

“…Previous studies have concluded that deficiency in Nur77 alone does not impair UbA-mediated clonal deletion, perhaps due to redundant functions of its family member Nor-1 (also known as Nr4a3) (23,28,29). However, recent data suggest that Nur77 deficiency is sufficient to impair TRA-mediated clonal deletion (27), arguing against complete redundancy by Nor-1 in some contexts.…”

“…Furthermore, overexpression of Nur77 results in dramatic decreases in thymocyte numbers and an increased frequency of thymocytes with DNA fragmentation (23). Two mechanisms of Nur77-mediated death have suggested: induction of proapoptotic genes through its role as a transcription factor in the nucleus and conversion of Bcl-2 to a proapoptotic form via exposure of its Bcl-2 homology domain 3 (BH3) at mitochondria (24)(25)(26)(27). Previous studies have concluded that deficiency in Nur77 alone does not impair UbA-mediated clonal deletion, perhaps due to redundant functions of its family member Nor-1 (also known as Nr4a3) (23,28,29).…”

Expression of Twist2 is controlled by T-cell receptor signaling and determines the survival and death of thymocytes

“…33 Unlike the dramatic reduction of both total thymic cellularity and the percentage of the CD24 lo Vα2 hi population in non-transgenic littermate control (NLC) mice, negative selection was impaired in Twist2-Tg mice (Figures 5b-d).…”

“…Nur77 is required for the negative selection in OT2-TCR × mOVA-Tg mice system. 33,34 To determine whether Twist2 regulates Nur77 induction in this system, we measured Nur77 levels in the CD24 lo Vα2 hi CD4 + population (Figure 6a). In OT2-TCR × mOVA-Tg mice, both the percentage of Nur77 hi cells and the expression of Nur77 were significantly increased compared with OT2-TCR-Tg mice (Figures 6a-c).…”

Programmed Cell Death in T Cell Development

“…Nevertheless, little is known about Nor-1 compared to Nur77. Recently, Nur77 deficiency alone has also been shown to impair negative selection (Fassett et al 2012). The MEK5-ERK5 pathway has been reported to induce Nur77 expression in thymocytes and DO11.10 cells (Kasler et al 2000;Sohn et al 2008).…”