Age-Related Decrease of Human CD8⁺ T-Cell Mir-92a Level Correlated with Reduction of naïve T Lymphocyte

MicroRNAs (miRNAs) consist of short noncoding RNA molecules of approximately 18–22 nucleotide that regulate post-transcriptional gene expression by degradation or repression of mRNA molecules. The miR-17-92a cluster is known as a regulator of the immune system and is critical for lymphoid cellular development and tumorigenesis in lymphoid tissue. Most knowledge of the miR-17-92 cluster in normal and abnormal conditions of the lymphoid system is based on mouse experiments. It is suggested that the accumulation of activated CD4⁺ T cells by higher mir-17-92 expression leads to a breakdown of T-cell tolerance in the periphery and may promote B-cell activation, germinal center reaction and autoantibody generation. However, only limited reports on in vivo human lymphocyte senescence exist. We therefore set out to determine miR-92a levels in circulating lymphocytes obtained from healthy participants to ascertain the possible association between immunological condition and the expression level of miR-17-92.

We separated lymphocytes from 21 healthy volunteers, aged 23 to 58 years (13 men and 8 women), for surface marker and miR-92a level analyses. The CD4⁺ or CD8⁺ T-cell fractions were separated with an isolation kit for humans (Miltenyi Biotec, Bergisch Gladbach, Germany) and AutoMACS Pro Separator (Miltenyi Biotec), according to the supplier's instruction, and stored at −80°C until utilization. After separation of CD4⁺ or CD8⁺ cells, the miR-92a levels were measure, as reported previously (PLoS ONE. 2011, 24;6(2);e16408). Immunophenotyping was done with flow cytometry.

The miR-92a of separated CD8⁺ lymphocytes decreased significantly with age (P = 0.0002), and miR-92a in CD4⁺ cells tended to decrease with age (P = 0.0635). We found a positive correlation between CD8⁺ miR-92a expression level and the percentage of naive CD8⁺ T cells (RO⁻CD8⁺CD27⁺ cells (P = 0.0046)) with L-selectin antigen (CD3⁺CD8⁺CD62L⁺ (P = 0.0011)) in healthy subjects. This suggests that the miR-92a of a majority of CD8⁺ T is derived from naive CD8⁺ T cells with L-selectin antigen, and CD8⁺ miR-92a expression level declines progressively with age (P < 0.0001 and P < 0.0001, respectively). In CD4⁺ cells, we observed a trend of decreasing CD4⁺ miR-92a level with age, while no significant difference was notable with lymphocyte subset fraction as far as we tested. The index of CD8⁺ miR-92a values (CD8⁺ miR-92a×number of CD8 cells) was positively correlated with the index of CD4⁺ miR-92a values (P = 0.0101).

These results indicate an age-related reduction of naive T cells may link to miR-92a of T-lymphocytes and may influence immune dysfuction with age. In conclusion, our results suggest that the miR-92a level may represent attrition of naïve CD8⁺ T cells, possibly due to apoptosis of naïve T cells. Additionally down-regulation of the miR-92a level in individuals older than 40 years may indicate impairment or exhaustion of naïve T-cells linked to immune dysfunction and contributed disease states.

No relevant conflicts of interest to declare.