Towards Understanding & Uncovering New Key Players in T-Cell Development upon Aging

Aging-associated immune remodeling (AAIR) leads to an impaired ability to respond to vaccination and combat infections, and is due to many factors acting in concert. Several studies have linked the T-cell decline that occurs with age to thymic involution. However, there is novel and mounting evidence that also aged lymphoid-primed multipotent progenitors (LMPPs) are immune system intrinsic players in AAIR. However, very little is known on molecular and cellular mechanisms by which aging LMPPs could drive this AAIR phenomenon. Deciphering the underlying mechanisms is of crucial importance for developing new therapies to attenuate AAIR. Here, we present new data demonstrating the dysregulated pathways associated with aged LMPPs and the cellular changes in early thymic differentiation events in driving AAIR.

To assess the T-lineage potential of aged LMPPs, we performed single cells ex-vivo OP9D assays using LMPPs (Lin^-cKit⁺Sca1⁺CD34^-Flt3^hi) from aged (18-20 month-old) and young (8-10 week-old) C57BL/6 animals as controls. The frequencies of T-cell lineage potential in aged LMPPs and young LMPPs at the single cell level were very similar. This result was also validated in vivo by transplantation assays where 5000 aged or young LMPPs were injected into sub-lethally irradiated young recipients followed by T-cell lineage (CD4⁺ & CD8⁺) analysis in the peripheral blood (PB) at 4 weeks post transplantation (Mean; Y:12,75 vs A:16:23 % of total, p=0.46). However, aged LMPPs were associated with a dramatic disadvantage of PB T-cells at 4 weeks post injections (Mean; Y:9 vs A:2.3 % of total, p<0.0001) and in the development of all thymic stages of thymocytes, from early double negative stage CD4^-CD8^-(DN1) (Mean; Y:30 vs A:8 % of parent, p<0.0001) to double positive stage CD4⁺CD8⁺(DP) (Mean; Y:53 vs A:15 % of parent, p<0.0001), as well as single positive (SP) CD4⁺(Mean; Y:54 vs A:13 % of parent, p<0.0001) and CD8⁺(Mean; Y:45 vs A:10 % of parent, p<0.0001) thymocytes when intravenously transplanted in combination with 1:1 ratio of young LMPPs into young recipients. To overcome a potential homing to the thymus bias of aged LMPPs in competitive transplants, we performed intra-thymic injections of young and aged LMPPs with identical ratios into sub-lethally irradiated young recipients. The analysis of PB at 4 weeks post injections show a dramatic reduction of PB T-cells derived from aged LMPPs in comparison with young LMPPs (Mean; Y:14.8 vs A:8 % of total, p<0.0001). There was primarily a strong disadvantage towards generating DP stage (Mean; Y:46 vs A:28 % of parent, p<0.0001), suggesting that the intra-thymic injections indeed alleviated the dramatic decrease in the early thymocyte stage DN1 (Mean; Y:31 vs A:22 % of parent, p<0.05). This suggests that aged LMPPs confer a T-cell differentiation and maybe an additional homing to the thymus defect.

We also performed RNA-Seq analyses on LMPPs from young and aged mice. Unsupervised hierarchical clustering of differentially expressed genes between young and aged LMPPs highlighted a clear dysregulation of only a few pathways that are involved in T-cell development such as Notch signaling. We next correlated our RNA-Seq data with other immunological signatures in attempt to look for more T-cell specific key factors that are differentially expressed between young and aged LMPPs. Importantly, the results show that the data from our RNA-Seq correlated with more than 400 immunological signatures among which 25 were most highly correlated. Interestingly, this correlation has allowed us to curate a list of the top 30 differentially expressed genes between young and aged LMPPs including T-cell specific transcription factors such as Satb1 and Foxo1.

Altogether, our findings reveal that the T-cell immune decline that occurs with age is already imprinted in LMPPs within the bone marrow and translates into a dysregulation of signaling pathways that are directly related to T-cell development. Targeting these pathways could open up new perspectives in attenuating AAIR.