The γδ T-Cell Population Changes with Age
The immune system is made up of many specialized populations of cells. Even within well recognized categories such as T cells of the adaptive immune system, there are numerous subcategories, defined by surface markers, that exhibit meaningfully different behaviors. The example for today is γδ T cells, known to be involved in the clearance of senescent cells. Unlike other T cells, γδ T cells behave more like innate immune cells, able to attack pathogens and potentially harmful cells without the need for other components of the adaptive immune system to process antigens for recognition. The γδ T cell population is relatively poorly understood, but like the rest of the immune system, it changes with age in ways that are likely in part dysfunctional, in part compensatory.
The transcription factors of the forkhead box O (Foxo) family, particularly Foxo1, play a pivotal role in regulating α/β T-cell key cellular processes. Interestingly, we recently found that the age-related decline in Foxo1 expression in mouse α/β T cells may drive the disruption of their peripheral homeostasis and contribute to the aging of this T-cell compartment. γ/δ T cells form a distinct subset of lymphocytes, differing from NK cells, B cells, and α/β T cells by combining adaptive properties with rapid, innate-like responses. Findings related to Foxo1 in α/β T cells prompted us to investigate how the functional capacities of γ/δ T cells are affected by aging, as well as whether Foxo1 expression could be modulated in this T-cell compartment with age.
In this study, we demonstrate that, as observed for α/β T cells, the homeostasis of the peripheral γ/δ T-cell compartment is markedly altered with age. Indeed, a comparison of the γ/δ T-cell compartment within the secondary lymphoid organs of old (18-month-old) and young (3-month-old) adult mice reveals that aging promotes the expansion of innate-like γ/δ T cells and enhances their capacity to produce IL-17. Notably, we found that these age-related changes were associated with the loss of Foxo1 expression within this T-cell compartment.
Finally, as observed in α/β T cells, our results indicate that the age-related decline in Foxo1 expression in γ/δ T cells is likely driven by a similar T cell-extrinsic factor. In this context, we identify type I IFNs as a key regulator that down-regulates Foxo1 in IL-17-producing γ/δ T cells with age and enhances the capacity of Ly-6C- CD44hi γ/δ T lymphocytes to mount a rapid in vivo response during aging.