Physiologic Thymic Involution Underlies Age-Dependent Accumulation of Senescence-Associated CD4+ T Cells

Immune aging may underlie various aging-related disorders, including diminished resistance to infection, chronic inflammatory disorders, and autoimmunity. PD-1(+) and CD153(+) CD44(high) CD4(+) T cells with features of cellular senescence, termed senescenceassociated T (SA-T) cells, increasingly accumulate with age and may play a role in the immune aging phenotype. In this article, we demonstrate that, compared with young mice, the aged mouse environment is highly permissive for spontaneous proliferation of transferred naive CD4(+) T cells, and it drives their transition to PD-1(+) and CD153(+) CD44(high) CD4(+) T cells after extensive cell divisions. CD4(+) T cells with essentially the same features as SA-T cells in aged mice are also generated from naive CD4(+) T cells after extensive cell divisions under severe T-lymphopenic conditions by gamma irradiation or in developmental T cell defect, often in association with spontaneous germinal centers, as seen in aged mice. The increase in SA-T cells is significantly enhanced after thymectomy at the young adult stage, along with accelerated T cell homeostatic proliferation, whereas embryonic thymus implantation in the late adult stage markedly restricts the homeostatic proliferation of naive CD4(+) T cells in the host and delays the increase in SA-T cells. Our results suggest that reduced T cell output due to physiologic thymic involution underlies the agedependent accumulation of SA-T cells as a result of increasing homeostatic proliferation of naive CD4(+) T cells. SA-T cells may provide a suitable biomarker of immune aging, as well as a potential target for controlling aging-related disorders.