Table 1

Pathways of apoptotic death of mature CD4+ T lymphocytes. The comparative features of apoptotic pathways in mature CD4+ T lymphocytes are summarized. Note that these pathways of apoptosis may serve other functions in immature lymphocytes and in B cells, which are not included.

FeaturePassive cell death (“death by neglect”)Activation-induced cell death
InductionInadequate stimulation [absence of antigens or costimulators (or both)]Repeated antigenic stimulation
Sensitivity of naı̈ve and activated T cellsBoth sensitiveOnly activated cells sensitive
Role of IL-2ProtectsEnhances
Role of FasNoneObligatory
Role of Bcl-2 and Bcl-xProtectsNone
Physiological rolesElimination of mature cells that do not encounter antigens and costimulationElimination of mature self-reactive T cells (peripheral tolerance)
Homeostatic control of lymphocyte pool: loss of activated cells that do not receive continued stimulation or growth factorsHomeostatic control?