Peripheral selection rather than thymic involution explains sudden contraction in naive CD4 T-cell diversity with age

Area of Science:

• Immunology
• Gerontology
• Computational Biology

Background:

• A diverse T-cell repertoire is essential for effective immune defense against pathogens.
• Naive and memory CD4 T-cell diversity peaks in early adulthood and is maintained until approximately age 70.
• A sudden and significant loss of T-cell diversity is observed around age 70.

Purpose of the Study:

• To investigate the mechanisms underlying the abrupt decline in T-cell repertoire diversity in older individuals.
• To evaluate prevailing hypotheses regarding age-related T-cell diversity loss.

Main Methods:

• Utilized mathematical modeling to simulate and test different hypotheses for T-cell diversity loss.
• Compared model predictions with observed patterns of T-cell repertoire changes in aging humans.

Main Results:

• Mathematical models rejected hypotheses based on decreased thymus emigration or overall cell number contraction.
• These gradual mechanisms failed to replicate the observed sudden and profound decrease in T-cell diversity.
• The study proposes a novel mechanism involving the accumulation of advantageous mutations in T-cell clones.

Conclusions:

• An abrupt loss of T-cell repertoire diversity in aging is unlikely due to thymus function decline or cell count reduction.
• A plausible explanation is the accumulation of cell-intrinsic mutations conferring a short-term fitness advantage to specific T-cell clones.
• This clonal expansion, while beneficial in the short term, leads to a long-term reduction in immune defense capability.