Increased Th17 differentiation in aged mice is significantly associated with high IL-1β level and low IL-2 expression

Objective: Aging has been reported to be associated with changes in immune function. Although frequent infection and the development of malignancy suggest the decline of immune function with aging, changes toward proinflammatory conditions also develop at the same time. Th17 cells are well known CD4⁺ T cell subpopulation closely linked to chronic inflammation and autoimmunity. In this study, changes in the Th17 population were investigated to elucidate a possible mechanism for this response with aging. Methods: Splenocytes were isolated from 2-month-old (young) and 20-month-old (aged) mice. CD4⁺CD44⁺ memory T cells and CD4⁺CD62L⁺ naïve T cells were isolated and sorted using magnetic beads and flow cytometry. The frequency of IL-17-producing cells was measured using flow cytometry. The expression of IL-17 and Th17-related factors at the mRNA level was measured with RT-PCR. IL-17 and Il-1β expression in spleen tissues was additionally assessed using confocal microscopy. Results: The proportion of IL-17-producing CD4⁺ T cells was higher in the splenocytes among the old mice than those of the young mice. When splenocytes were cultured in Th17 polarizing conditions, the proportion of IL-17 producing CD4⁺ T cells was higher in aged mice as well. This was consistently observed when naïve and memory cells were isolated and differentiated into Th17 respectively. In addition, the expression of retinoic acid receptor-related orphan nuclear receptor gamma t (RORγt) and other Th17-related factors (AhR, CCR6, and CCL20) increased in the splenocytes of aged mice compared to the young mice. The expression of IL-1β, showing to promote Th17 differentiation, was higher in the aged mice. Likewise, CD4⁺ T cell expression of IL-1R was higher in the aged mice, suggesting that the CD4⁺ T cells of the aged mice are readily prepared to differentiate into Th17 cells in response to IL-1β. Confocal microscopy showed that cells positive for IL-1R or IL-1β were more frequent in the spleens of the aged mice. When an anti-IL-2 antibody was applied, the proportion of IL-17-producing cells increased more prominently in the young mice. We observed that IL-2 production and IL-2R expression were reduced in the aged mice, respectively, explaining the blunted response to the anti-IL-2 antibody treatment and the consequent minimal change in the Th17 population. Conclusion: We demonstrated that the proportion of Th17 cells increased in the aged mice both in naïve and memory cell populations. Elevation of IL-1R and IL-1β expression and the reduction in IL-2 and IL-2R expression in aged mice seemed to promote Th17 differentiation. Our results suggest that enhanced Th17 differentiation in aging may have a pathogenic role in the development of Th17-mediated autoimmune diseases.