Age-related dysregulation in CD8 T cell homeostasis: kinetics of a diversity loss.

Relative diversity and representation of peripheral T cells bearing different TCR Vbeta families are remarkably tightly regulated between birth and advanced adulthood. By contrast, individual elderly humans and C3H.SW and B10.BR aged mice display drastic disruption in such regulation. It was suggested that the alterations in the murine aged T cell compartment were due to age-related clonal T cell expansions (TCE). Here, we studied the kinetics of homeostatic dysregulation of T cell populations in aged C57BL/6 (B6) mice. Using mAb staining, we show that the percentages of alphabeta+CD8+ or CD4+ T cells bearing different TCRVbeta elements remain virtually constant in mice up to 12 mo of age. In 22-mo-old mice, however, there is a dramatic disturbance of this pattern owing to the emergence of CD8+ TCE. Expanded T cells did not show any obvious bias in Vbeta usage and were derived in all cases examined thus far from a single clone. TCE appeared later in life, compared with B cell clonal expansions. However, and in contrast to those detected in humans, TCE were frequently unstable disappearing within 2-4 mo, with other TCE appearing within the same time frame. Additional studies carried on thymic T cells, thymectomized mice, and young T transferred cells into Rag1-/- mice suggest that the clonal expansions occur in the periphery and that their onset is accelerated by decreased thymic output and/or function(s).