Table I.

Some of the known or suspected defects in T cell senescence

Affected cell/process	Defect (reference)
Common lymphoid precursor (CLP)	Decreased efficacy of differentiation (4)
Early T cell progenitor (ETP)	Decreased efficacy of differentiation; decreased migration into the thymus? (4)
Intrathymic maturation	Decreased IL-7 and c-kit production?; reduced VDJ recombination?;   disorganized epithelial architecture; altered selection? (1, 4)
Recent thymic emigrant production,   migration, and/or peripheral seeding	Reduced generation of new T cells; reduced ability of new T cells to populate periphery? (13)
Naive T cell priming	Dendritic cell defects (impaired migration into draining lymph   nodes; impaired antigen uptake and processing?; impaired maturation?)   T cell defects (deficient synapse formation; diminished signaling along TCR and costimulatory pathways;  reduced IL-2 production) (1, 2, 4)
T cell homeostasis	Decrease in CD4/CD8 and naive/memory T cell ratios;   accumulation of dysfunctional memory cells including T cell clonal expansions (1, 2, 4, 16)