Primary stimulation of T cells occurs upon prolonged encounter with antigen-presenting cells (APCs). Optimal T-cell activation is achieved via recognition of antigen (Ag)-loaded MHC complexes through a specific T cell receptor (TCR), and concomitant engagement of CD28, inducible costimulator (ICOS), and/or other costimulatory molecules on the APC surface. This interaction triggers blastogenesis, rapid production of interleukin 2 (IL-2), and upregulation of the high affinity IL-2 receptor CD25. IL-2 is a key growth and survival cytokine for activated T cells. Its uptake via CD25 binding comprises an autocrine signaling loop that drives responding T cells into cycle for several rounds of proliferation. Other cytokines and signaling receptors help to program specific effector functions for these responding T cells. However, it is the relative abundance of Ag and IL-2 in the surrounding milieu that ultimately dictates how and when the vast majority of effector T cells will proliferate or succumb to apoptosis (2
) (). Indeed, emerging evidence suggests that relative sensitivity to IL-2 delineates CD25hi
‘terminally differentiated’ effector T cells that are more prone to apoptosis from CD25lo
cells that give rise to long-lived memory T cells (3
Enforcement of T-cell homeostasis via apoptosis
When Ag and IL-2 are abundant, TCR re-engagement of an activated, cycling T cell can trigger an autoregulatory form of apoptosis we refer to herein as ‘restimulation-induced cell death’ (RICD). This phenomenon of ‘cell suicide’ was first described over 20 years ago in murine T-cell hybridomas and clones upon exposure to Ag-loaded APCs or agonistic anti-TCR Abs (4
). After subsequent work had demonstrated that TCR ligation was only connected to this built-in ‘death program’ in activated T cells that were proliferating after recent primary Ag stimulation (6
), this pathway became commonly referred to as ‘activation-induced cell death’ (AICD), a mellifluous but confusing term. As recent reviews have pointed out, AICD can encompass cell death triggered by antigen in any context (e.g. immature thymocyte deletion in the thymus), thus obscuring mechanistic differences (8
). Moreover, the term AICD superficially and perhaps unintentionally equates the process of naive T-cell ‘activation’, synonymous with expansion and differentiation of apoptosis-resistant naive T cells, with TCR restimulation of activated, cycling T cells in the presence of IL-2 that promotes T-cell death. Because it is difficult to understand how an immune response could be launched if activation generally induced cell death, we demarcate these two processes and use RICD in reference to TCR-mediated apoptosis of mature T cells. In retrospect, it is now clear how a defined window of apoptosis susceptibility for non-transformed T cells, coupled to cell cycle progression, could explain why immortalized, continually cycling T-cell hybridomas are exquisitely sensitive to RICD without IL-2 (11
For primary T cells, competency to die through TCR restimulation is conferred by the presence of IL-2, although additional cytokines (e.g. IL-4) can participate by driving T cells into cycle (13
). These requirements permit the conceptualization of RICD as a self-limiting negative feedback mechanism for controlling T-cell expansion during an ongoing immune response (2
). The paradoxical buildup of activated T cells and autoantibodies in mice lacking IL-2 or IL-2 receptor subunits reflects the loss of this clonal deletion mechanism (14
), which is exacerbated by a failure of polyclonal deletion by Treg cells (21
). Through Ag-specific deletion in the periphery, RICD helps to limit immunopathologic or autoimmune manifestations that may arise from excessive effector T-cell expansion. This process also maintains adequate ‘space’ for other T-cell clones to respond to different antigenic challenges. Thus, RICD sets the upper boundary for T-cell expansion in the presence of persistent Ag and is therefore considered a ‘propriocidal’ form of cell death that constrains the size of the effector T-cell pool (2
Historically, RICD has primarily been studied in well-defined experimental systems, usually in vitro.
Understanding its physiological relevance in vivo
has required increasingly more sophisticated experimental approaches. There is also a paradox inherent in destroying effector T cells through Ag restimulation during an ongoing immune response. However, we posit that RICD establishes a propriocidal threshold for responding T cells, based on the quantity and quality of the antigenic stimulus, growth cytokines, and other environmental signals. The rapid reproduction of most microbial pathogens requires an explosive lymphocyte response which requires tempering. Self-regulation by feedback could minimize immune-mediated tissue damage during the unpredictable evolution of an infection. This balance between pathogen clearance and T-cell regulation is perturbed if normal RICD is impaired. For instance, a profound RICD defect in patients with X-linked lymphoproliferative disease (XLP) explains the acute bouts of lymphocyte hyperproliferation spurred by recurrent Ag encounters associated with persistent viral infections (17
). As discussed later, XLP reveals the clinical significance of RICD in T-cell homeostasis.
Most effector T cells reacting to an invading pathogen are purged as the infection is cleared and IL-2 levels dwindle at the conclusion of a successful immune response. During this contraction phase, an intrinsic pathway of apoptosis is triggered by withdrawal of IL-2 and other cytokines, i.e. cytokine withdrawal-induced death (CWID). CWID culls the vast majority of effector T cells, save a select few that survive as memory T cells. Like RICD, the connection between apoptosis induction and IL-2 withdrawal was first established in mouse T-cell lines (18
). Kuroda and colleagues (20
) demonstrated the physiological relevance of CWID in observing prolonged survival of staphylococcal enterotoxin A (SEA)-activated T cells in vivo
in mice implanted with slow-release IL-2 osmotic pumps. Indeed, the decline of effector T-cell numbers under normal physiological conditions correlates well with waning IL-2 concentration in the surrounding environment. Consequently, CD25 expressed on the cell surface also decreases. This relationship is also exploited by IL-2-dependent, FoxP3+
regulatory T cells (Tregs) that can ‘steal’ survival cytokines from competing conventional T cells, ostensibly through constitutive expression of CD25 (21
). Hence Tregs can suppress T-cell responses by accelerating the natural process of CWID via IL-2 consumption, examined in detail in the next section.
CWID and RICD operate at different phases of the immune response as hard-wired feedback response programs, influenced by the dynamic localization of cells, antigen, and cytokine. Both processes are exquisitely regulated by the availability of Ag and IL-2 as well as other growth/survival cytokines. Mechanistically, the prevailing dogma suggests that these two processes eliminate T cells through distinct biochemical mechanisms of apoptosis, known as the intrinsic and extrinsic pathways (2
). The intrinsic pathway is controlled by relative expression of Bcl-2 family proteins that regulate mitochondrial outer membrane potential (MOMP). When mitochondria are depolarized, cytochrome c release catalyzes the cleavage and activation of procaspase 9. Extrinsic apoptosis is signaled principally through death receptors (DRs) of the tumor necrosis factor receptor (TNFR) superfamily, such as Fas. Ligand binding triggers the nucleation of death-inducing signaling complexes (DISCs) at the DR cytoplasmic tail, which serve as platforms for recruitment and autocatalytic cleavage and activation of procaspases 8 and 10. Both pathways converge at the activation of downstream caspases (i.e. caspase 3/6/7). In certain contexts, extrinsic apoptosis signals also trigger the intrinsic pathway. For example, cleavage of Bid by caspase 8 generates an active, truncated form (tBid) capable of inducing mitochondrial depolarization and magnifying downstream caspase activity (22
). Dependence on this amplification loop for Fas-induced apoptosis distinguishes Type II from Type I cells; indeed, IL-2 may facilitate RICD in part by converting activated T cells to a Type I phenotype (23
CWID induces intrinsic apoptosis. Withdrawal of IL-2 or other γ-chain cytokines specifically upregulates and activates Bim, a key pro-apoptotic protein that antagonizes the function of anti-apoptotic Bcl-2 family proteins (e.g. Bcl-2, Bcl-xL, and Mcl-1) and activates Bax, which causes mitochondrial permeabilization (25
). RICD is often wholly attributed to an extrinsic apoptosis signal through Fas, which may be stimulated in cis
or in trans
by membrane-anchored FasL exposed on the surface of restimulated T cells. The involvement of Bim and Fas in CWID and RICD, respectively, was unveiled in studies of mutant and knockout mouse models. Subsequently, genetic studies demonstrated the importance of Bim- and Fas-mediated apoptosis in human diseases (8
). However, as we discuss later, growing evidence forces us to reconsider how Fas and Bim may actually cooperate in governing T-cell homeostasis.