Age-related Dynamics of CD8⁺ regulatory T cells: Implications of NKG2D in Immune Aging

Aging is a complex biological process that impacts various physiological functions, including the immune system. Our study investigated the impact of aging on CD8⁺ regulatory T cells(CD8⁺Treg), which showed a pattern of initial increase up to 12 months, then a decline by 24 months in a C57BL/6 mouse model, unlike memory T cells, which consistently increase with age. Functionally, CD8⁺Treg, irrespective of age, did not produce cytokines in response to TCR stimulation, highlighting a stark functional contrast with memory cells. However, upon IL-15 stimulation, CD8⁺Tregs, unlike their memory cell counterparts, demonstrated enhanced cytokine production. Transcriptomic analysis revealed an abundance of Klrk1 (killer cell lectin-like receptor subfamily K member 1)gene transcripts in aging CD8⁺Tregs. Klrk1 gene encodes Natural Killer Group2 Member D (NKG2D), an activating receptor NK cells express. This upregulation was specific to CD8⁺CD122^hiLy49⁺ and absent in CD8⁺CD122^hiLy49^-. To explore this correlation further, we utilized a Klrk1^-/- mouse and observed an increased CD8⁺Treg population, suggesting a potential negative regulatory role of NKG2D in CD8⁺Treg homeostasis. Transcriptome analysis of Klrk1^-/- CD8⁺Tregs showed enrichment of aging-related gene signatures, further corroborating the link between NKG2D and the aging process. Our findings suggest NKG2D's involvement in aging and immune dysfunction, providing avenues for intervention in the elderly.