Cell generation dynamics underlying naïve T cell homeostasis in adult humans

ABSTRACT

Thymic involution and proliferation of naive T cells both contribute to shaping the naive T cell repertoire as humans age, but a clear understanding of the roles of each throughout a human lifespan has been difficult to determine. By measuring nuclear bomb test-derived ¹⁴C in genomic DNA we determined the turnover rates of CD4⁺ and CD8⁺ naïve T cell populations and defined their dynamics in healthy individuals ranging from 20-65 years of age. We demonstrate that naïve T cell generation decreases with age, and that this could be explained by a combination of declining cell loss, peripheral division and thymic production during adulthood. We investigated putative mechanisms underlying age-related changes in homeostatic regulation of naïve T cell turnover using mass cytometry to profile candidate signaling pathways involved in T cell activation and proliferation in CD4⁺ naive T cells relative to CD31 expression, a marker of thymic proximity. We show that basal NF-κB phosphorylation inversely correlated with CD31 expression and thus is decreased in peripherally expanded naive T cell clones. Functionally we found that NF-κB signaling was essential for naive T cell proliferation to the homeostatic growth factor IL-7, and reduced NF-κB phosphorylation in CD4⁺CD31⁻ naive T cells is linked to reduced homeostatic proliferation potential. Our results reveal an age-related decline in naïve T cell turnover as a putative regulator of naïve T cell diversity and identify a molecular pathway that restricts proliferation of peripherally expanded naive T cell clones that accumulate with age.