4. Conclusion

assessment of IL-21R expression with aging was conducted to discard the possibility that IL-21R expression declines progressively akin to the GRLR situation [134]. Analysis performed on total and fractionated thymocytes (DN, DP, and SPs) revealed that IL-21R expression profiles are comparable between young and aged mice (unpublished data). Next, wild type-aged C57Bl/6 mice (14–18 months old) received three IP injections of IL-21 prior to their analysis on the following day. This treatment led to increased thymic size, weight, and cellularity. Analysis of thymic subsets showed a drastic increase in the proportion of DN thymocytes as it reached 15% in contrast to 3% in PBS control mice. Likewise, a notable increase in the frequency of CD4 and CD8 SP T cells was also observed in the IL-21 group. This outcome was further reflected in the overall absolute number of DN, DP, and SP thymocytes. As one of the predominant deficiencies occurring with chronic thymic atrophy is decreased migration, proliferation, and survival of ETPs, we assessed the effect of IL-21 administration on the scarcity of this central progenitor population in the thymus. Flow-cytometry analysis of ETPs showed a significant increase in their frequencies within the IL-21 group suggesting preferential expansion of ETPs in response to IL-21 administration. As the aim of using IL-21 focuses on rejuvenating T-cell immunity of aged mice, T-cell output was analyzed weekly in PBS- vs. IL-21-treated aged mice over a total period of 3 weeks. Interestingly, aged mice undergoing IL-21 treatment showed distinctively improved T-cell responses [18]. In addition, while IL-21 administration had no impact on the absolute number of all peripheral lymphocytes, the frequency of peripheral GFP+ T-cells increased substantially in comparison with control mice [18]. This can also explain the improved anti-tumoral response observed in IL-21-pre-conditionned aged mice prior to cancer vaccination [18]. The sum of these results serves as the basis to investigate the use of IL-21 as an elderly pre-conditioning therapy to rejuvenate immunity, and thus, improve T-cell responsive-

The above findings provided the impetus to investigate the use of IL-21 in T-cell reconstitution post-BMT. Although this medical procedure is adopted as a life-saving procedure for specific malignant and non-malignant conditions [161], it remains unfortunately associated with dangerous life-threatening complications. The high morbidity and mortality associated with BMT persist as a major clinical problem associated with increased risks of: (i) relapse or development of secondary malignancies [162, 163], (ii) infection [162, 164, 165], and (iii) reduced responsiveness to vaccination [166]. The primary factors associated with these complications are the significant delay and improper reconstitution of T-cells post-BMT [165, 167] mostly due to the significant damage inflicted to TECs [168–171]. Therefore, the ability of IL-21 to improve de novo T-cell reconstitution post-BMT was investigated. For this purpose, T-cell-depleted

ent animals followed by IL-21 administration. In this experiment, GFP expression driven by the Rag2 promoter allows direct assessment of de novo peripheral T-cell reconstitution [172]. Analysis of transplanted mice revealed that IL-21: (i) accelerates lymphocyte reconstitution including T-cells (also observed in NOD scid gamma (NSG) mice receiving human IL-21 following cord-blood transplantation), (ii) slightly improved TECs recovery, (iii) regenerates a naïve peripheral T-cell pool with a diverse TCR repertoire, (iv) enhances regulatory B-cell development, and (v) protects from GHVD while retaining the graft-versus-tumor effect. Of note, IL-21 was originally believed to specifically affect the thymus [154]. In line with a

) were transplanted into irradiated LP/J (H2-Kb/c) recipi-

ness to vaccination.

50 Gerontology

RAG2p-GFP-derived BM cells (H2-Kb

So far, the use of IL-21 in the clinic remains limited to cancer immunotherapy (IL-21-based stimulation) or in the setting of autoimmune diseases (IL-21 inhibition). The latter clinical objective is particularly important as it raises major concerns related to the use of IL-21 in patients prone to develop autoimmune ailments. Therefore, additional studies are warranted to specifically address the clinical use of IL-21 in indications such as immunotherapies. Alternatively, all pre-clinical observations related to IL-21 strongly suggest that this cytokine could be exploited either as a monotherapy or in combination with other standards of care. This would ensure, at least in the context of thymopoiesis, improved de novo T-cell development in aged subjects as a mean to reverse thymic involution or post-BMT to accelerate the regeneration of naive T cells.
