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S. Gon et al.
Taken together, these data indicate that both in mouse and human, TCRαβ signaling network is largely disabled in PTEN-deficient TCRαβ+ T-ALLs.
Discussion
Here we undertook to investigate the impact of TCR signaling during T-ALL leukemogenesis. We show that in a mouse model of Pten loss-of-function, a frequent event among human TCRαβ+ T-ALLs (approx. 70%) (Online Supplementary Table S8), early counter-selection of fit TCRαβ+ thymocytes, occurs before the onset of leukemia development. Furthermore, we show that established TCRαβ+ T-ALLs are carrying TCRs which are unfit and/or impaired in signaling.
T-ALLs represent the malignant counterparts of most thymocyte stages of development. Analysis of T-ALL subtype distribution based on a human T-ALL cohort of 230 subjects25 showed that early immature (n=52), corti- cal T-ALL (n=103), mature TCRgδ+ T-ALL (n=36) and mature TCRαβ+ T-ALL (n=39) represent 22.6%, 44.8%, 15.6% and 17%, respectively, of T-ALL cases. Intriguingly, among the TCR+ T-ALL, TCRαβ+ T-ALLs were under-represented (52%) relative to TCRgδ+ (48%) compared to physiological counterparts in which TCRαβ largely dominate the fraction of TCR+ thymocytes (approx. 95% vs. approx. 5% TCRgδ).26 Unlike αβ T cells, gδ T cells are not restrained to MHC and do not undergo conventional MHC-mediated positive and negative selec- tions.26,27 Conversely to pre-tumoral αβ T cells, pre- tumoral gδ T cells might thus not be counter-selected, possibly explaining their over-representation compared to TCRαβ+ T-ALL.
Our in vivo data showed that pre-tumoral Ptendel cells with unfit TCR signaling (OT-II in I-Ab/d background) are positively selected for leukemogenesis, while thymocytes with fit TCR signaling (H-Y or OT-II in I-Ab/b background) are counter-selected and never developed leukemia. Thus this study points to a role of Pten during the positive selec- tion process. Yet the specific molecular mechanism allow- ing positive selection of pre-tumoral cells with unfit TCR (and counter-selection of cells with fit TCR) in the absence of Pten remains to be determined. A possible scenario would be that Pten loss merely shifts the window of pos- itive and negative selection thresholds (Figure 5). On one hand, PTEN loss might substitute for missing TCR signal- ing, allowing cells with no TCR or low affinity TCR to be rescued from death by neglect and bypass positive selec- tion. Accordingly, an increase in positively selected T cells was observed in mouse models in which AKT was hyper- active.28 Herein, we found that, following TCR-stimula- tion, AKT activation is similar in Pten-deficient T-ALL and in WT thymocytes. Therefore, while most of the TCR sig- naling network is disabled (Figure 4), AKT pathway appears ‘normal’ and, in the context of our scenario (Figure 5), is likely to be the main element contributing to the bypass of positive selection by Pten-deficient thymo- cytes harboring unfit/low TCR. On the other hand, inte- gration of signals resulting from both PTEN loss and a fit TCR (passing positive selection) might reach over-the- threshold signaling and trigger negative selection, elimi- nating thymocytes carrying fit TCRs even before malig- nant transformation. This would involve multiple path- ways downstream of PTEN loss,12,29 since a mere AKT
hyperactivation was insufficient to recapitulate the loss of fit H-Y+ thymocytes.28 A challenging perspective would be to decipher the molecular mechanism underlying the counter-selection of Pten-deficient fit αβ TCR+ thymo- cytes and then to assess the possibility of activating this apoptotic program in tumoral cells.
Our data indicate that TCRαβ signaling pathway is actively involved in T-ALL oncogenesis. We show that TCRαβ signaling can impede the development of Pten- deficient tumors and thus acts as a bona fide tumor sup- pressor. However, given the diametrically opposed effect of TCR activation on discrete stages of T-cell develop- ment, the TCR might also have pro-oncogenic effects in other contexts and/or developmental stages. For example, we show that in a Pten-proficient Cdkn2a–/– T-ALL model, TCRαβ+ tumors are sensitive to TCR activation. Likewise, thymocytes harboring fit H-Y TCR are not counter-selected in female TEL-JAK2 mouse model and develop leukemia.8 In the same line, Pten-deficient TCRα–/– or SLP76–/– mice, in which TCR signaling is abro- gated, display delayed tumor onset.17 Yet, and in contrast to the Ptendel model, TCRα–/– and SLP76–/– thymocytes are blocked before (and therefore not subjected to) positive selection.30,31 Pre-TCR might also be directly involved in oncogenesis. For instance, in dominant active NOTCH1 (ICN1) model, pre-TCR signaling is required for tumori- genesis.32 Conversely [Rag1–/– x Ptendel] thymocytes that are devoid of pre-TCR bypass β-selection and develop DP T-cell lymphoma in short latency (Figure 1); this is also in striking contrast to Pten-deficient TCRα–/– or SLP76–/– thy- mocytes (described above) that express a pre-TCR and for which leukemogenesis is impaired.17 In normal β-selected cells, the exit of proliferation is induced by pre- TCR signals that inhibit Notch1 pathway leading to Myc downregulation.33,34 In [Rag1–/– x Ptendel] DP lymphoma, Notch1 pathway is systematically activated leading to sustained expression of Myc (Figure 1C) it might be that pre-TCR signaling exerts a tumor suppressor role by shut- ting down Notch1 and Myc pathways.
A recent study indicated that TCRαβ+ T-ALL were prone to activation-induced cell death (AICD), and anti- CD3 stimulation of TCR signaling was proposed as a therapeutic strategy to eliminate leukemic blasts.8 By contrast, most of our TCRαβ+ T-ALL samples (4 of 5) were resistant to AICD (Figure 4E). This discrepancy could be due to the stage of arrest (before or after posi- tive/negative selections) of T-ALL samples, as in both studies sensitive TCRαβ+ samples (4 of 5 in the in vivo analysis of Trinquand et al.8 and 1 of 5 in our study) were CD1a+, consistent with an arrest at cortical DP stage, dur- ing which positive and negative selections occur.35 In con- trast, AICD-resistant samples in both studies represented true late-cortical CD1aneg SP T-ALLs. This cautions that anti-CD3 therapeutic strategies might be restrained to a subgroup of sensitive TCR+T-ALL (such as TCRαβ+CD1a+, eventually the rare TCRαβ+PTEN+ cases, or TCRgδ+ 8), and thus for mature TCRαβ+PTENneg T-ALL alternative options should be considered. Here we have showed that integration of Pten loss and fit TCR signaling promotes a deletional program. Thus, an attractive per- spective would be to decipher the mechanism underlying this apoptotic program in order to uncover an actionable target inducing cell-death, which might open new thera- peutic avenues for this poor prognosis PTEN-deficient TCRαβ+ subgroup.36
haematologica | 2018; 103(6)