Thymic output and functionality of the IL-7/IL-7 receptor system in centenarians: implications for the neolymphogenesis at the limit of human life

During aging, the thymus undergoes a marked involution that is responsible for profound changes in the T‐cell compartment. To investigate the capacity of the thymus to produce new cells at the limit of human lifespan, we analyzed some basic mechanisms responsible for the renewal and maintenance of peripheral T lymphocytes in 44 centenarians. Thymic functionality was analyzed by the quantification of cells presenting the T‐cell receptor rearrangement excision circles (TREC). A new method based upon real‐time PCR was used, and we found that most centenarians (84%) had undetectable levels of TREC+ cells. Six‐color cytofluorimetric analysis revealed that centenarians had an extremely low number of naïve T cells; central memory and effector memory T cells were greatly increased, while terminally differentiated cells were as numerous as in young (aged 20–45) or middle‐aged (aged 58–62) donors. Interleukin (IL)‐7 and IL‐7 receptor α‐chain (CD127) levels were the same at all ages, as shown by ELISA, flow cytometry and real‐time PCR. However, IL‐7 plasma levels were higher in centenarian females than males. The presence of TREC+ cells and of very few naïve T lymphocytes suggests that in centenarians such cells could either derive from residues of thymic lymphopoietic islets, or even represent long‐living lymphocytes that have not yet encountered their antigen. IL‐7 could be one of the components responsible, among others, for the higher probability of reaching extreme ages typical of females.     

Reference ranges and age-related changes of peripheral blood lymphocyte subsets in Chinese healthy adults

This study was performed to build region-specific reference ranges of peripheral blood lymphocyte subsets for Chinese healthy adults from the young to the elderly and analyze the trends of changes in lymphocyte subsets for evaluating the impact of age on the values.151 healthy adults aged 19—86 were recruited based on the SENIEUR protocol.Three sets of reference ranges were finally built applicable for the healthy young(19—44 years),middle-aged(45—64 years) and elder adults(≥65).Comparisons in parameters am...     

CD127 and CD25 expression defines CD4+ T cell subsets that are differentially depleted during HIV infection

Decreased CD4(+) T cell counts are the best marker of disease progression during HIV infection. However, CD4(+) T cells are heterogeneous in phenotype and function, and it is unknown how preferential depletion of specific CD4(+) T cell subsets influences disease severity. CD4(+) T cells can be classified into three subsets by the expression of receptors for two T cell-tropic cytokines, IL-2 (CD25) and IL-7 (CD127). The CD127(+)CD25(low/-) subset includes IL-2-producing naive and central memory T cells; the CD127(-)CD25(-) subset includes mainly effector T cells expressing perforin and IFN-gamma; and the CD127(low)CD25(high) subset includes FoxP3-expressing regulatory T cells. Herein we investigated how the proportions of these T cell subsets are changed during HIV infection. When compared with healthy controls, HIV-infected patients show a relative increase in CD4(+)CD127(-)CD25(-) T cells that is related to an absolute decline of CD4(+)CD127(+)CD25(low/-) T cells. Interestingly, this expansion of CD4(+)CD127(-) T cells was not observed in naturally SIV-infected sooty mangabeys. The relative expansion of CD4(+)CD127(-)CD25(-) T cells correlated directly with the levels of total CD4(+) T cell depletion and immune activation. CD4(+)CD127(-)CD25(-) T cells were not selectively resistant to HIV infection as levels of cell-associated virus were similar in all non-naive CD4(+) T cell subsets. These data indicate that, during HIV infection, specific changes in the fraction of CD4(+) T cells expressing CD25 and/or CD127 are associated with disease progression. Further studies will determine whether monitoring the three subsets of CD4(+) T cells defined based on the expression of CD25 and CD127 should be used in the clinical management of HIV-infected individuals.     

Monocyte subsets in bone marrow grafts may contribute to a low incidence of acute graft‐vs‐host disease for young donors

Young donors are associated with a lower cumulative incidence of acute graft‐vs‐host disease (aGVHD) after allogenic haematopoietic stem cell transplantation (allo‐HSCT) than old donors. Although grafts are harvested from healthy donors, it is unclear whether donor age is associated with aGVHD occurrence owing to its effect on cell compositions in grafts. Moreover, the differences in monocyte subsets in grafts between young and old donors and the association between monocyte subsets in bone marrow (BM) grafts and aGVHD remain to be elucidated. In the current study, non‐classical monocytes and the CD4⁺/CD8⁺ T cell ratio were remarkably decreased in BM grafts in donors <30 years old. Multivariate analysis further revealed that the level of non‐classical monocytes in BM grafts (≥0.31 × 10⁶/kg) was an independent risk factor for the occurrence of II‐IV aGVHD. In summary, our data indicate that non‐classical monocytes in BM grafts may help identify patients at high risk for aGVHD after allo‐HSCT. Although further validation is required, our results suggest that the low level of non‐classical monocytes and a low ratio of CD4⁺/CD8⁺ T cell in BM grafts may be correlated with the lower incidence of aGVHD in young donors.     

Alteration of circadian rhythmicity of CD3+CD4+ lymphocyte subpopulation in healthy aging

The CD4+ T helper/inducer and the CD8+ T suppressor/cytotoxic are major lymphocyte subsets that play a key role in cell-mediated immunity. Aging-related changes of immune function have been demonstrated. The purpose of this study is to analyze the dynamics of variation of these specific lymphocyte subsets in the elderly. In our study cortisol and melatonin serum levels were measured and lymphocyte subpopulation analyses were performed on blood samples collected every four hours for 24 hours from fifteen healthy young middle-aged subjects (age range 36-55 years) and fifteen healthy elderly male subjects (age range 67-79 years). A clear circadian rhythm was validated for the time-qualified changes of CD3+ and CD4+ cells with acrophase at night and for the time-qualified changes of CD8+ cells with acrophase at noon in young middle-aged subjects and for the time-qualified changes of CD3+ cells with acrophase at night and for the time-qualified changes of CD8+ cells with acrophase at noon in elderly subjects. No clear circadian rhythm was validated for the time-qualified changes of CD4+ cells in elderly subjects. No statistically significant correlation among lymphocyte subsets was found in elderly subjects. In elderly subjects CD3+ lymphocyte percentage was higher in the photoperiod and in the scotoperiod and cortisol serum level were higher in the scotoperiod in respect to young middle-aged subjects. In the elderly there is an alteration of circadian rhythmicity of T helper/inducer lymphocytes and this phenomenon might contribute to the aging-related changes of immune responses.     

Age-related appearance of a CMV-specific high-avidity CD8<Superscript>+</Superscript> T cell clonotype which does not occur in young adults

Old age is associated with characteristic changes of the immune system contributing to higher incidence and severity of many infectious diseases. Particularly within the T cell compartment latent infection with human Cytomegalovirus (CMV) is contributing to and accelerating immunosenescence. However, latent CMV infection and reactivation usually does not cause overt symptoms in immunocompetent elderly persons indicating immunological control of disease. Little is still known about the clonal composition of CMV-specific T cell responses in donors of different age. We therefore analyzed CD8⁺ T cells specific for an immunodominant pp65-derived nonamer-peptide (NLVPMVATV; CMV_NLV) in different age-groups. Independent of donor age CMV_NLV-specific CD8⁺ T cells preferentially use the V beta family 8. This family has monoclonal expansions in the majority of donors after stimulation of CD8⁺ T cells with the peptide. By sequencing the CDR3 region of the T cell receptor we demonstrated that CMV_NLV-specific, BV8⁺ CD8⁺ T cells share the conserved CDR3-sequence motif SANYGYT in donors of all age groups. Interestingly, a second conserved clonotype with the CDR3-sequence motif SVNEAF appears in middle-aged and elderly donors. This clonotype is absent in young individuals. The age-related clonotype SVNEAF binds to the pMHC-complex with higher avidity than the clonotype SANYGYT, which is predominant in young adults. The dominance of this high avidity clonotype may explain the lack of overt CMV-disease in old age.     

Human CD8⁺ and CD4⁺ T cell memory to lymphocytic choriomeningitis virus infection

Although cellular immunity to acute lymphocytic choriomeningitis virus (LCMV) infection has been well characterized in experimental studies in mice, the T cell response to this virus in humans is incompletely understood. Thus, we analyzed the breadths, magnitudes, and differentiation phenotypes of memory LCMV-specific CD8(+) and CD4(+) T cells in three human donors displaying a variety of disease outcomes after accidental needle stick injury or exposure to LCMV. Although only a small cohort of donors was analyzed at a single time point postinfection, several interesting observations were made. First, we were able to detect LCMV-specific CD8(+) and CD4(+) T cell responses directly ex vivo at 4 to 8 years after exposure, demonstrating the longevity of T cell memory in humans. Second, unlike in murine models of LCMV infection, we found that the breadths of memory CD8(+) and CD4(+) T cell responses were not significantly different from one another. Third, it seemed that the overall CD8(+) T cell response was augmented with increasing severity of disease, while the LCMV-specific CD4(+) T cell response magnitude was highly variable between the three different donors. Next, we found that LCMV-specific CD8(+) T cells in the three donors analyzed seemed to undergo an effector memory differentiation program distinct from that of CD4(+) T cells. Finally, the levels of expression of memory, costimulatory, and inhibitory receptors on CD8(+) and CD4(+) T cell subsets, in some instances, correlated with disease outcome. These data demonstrate for the first time LCMV-specific CD8(+) and CD4(+) T cells in infected humans and begin to provide new insights into memory T cell responses following an acute virus infection.     

CD45 isoforms associated with distinct functions of CD4 cells derived from unusual healthy donors lacking CD45RA- T lymphocytes.

We now report two healthy individuals whose T lymphocytes were over 95% positive for CD45RA antigen expression. However, these donors normally expressed both the CD29 high (CD29+) and CD45RO high (CD45RO+) antigens on approximately 40 and 50% of their CD4 cells, respectively. Despite the strong CD45RA expression on the surface of almost all CD4 cells, the CD29 marker allowed T cells from these donors to be divided phenotypically into subsets having distinct in vitro function. CD4+CD29+ cells from these donors responded maximally to recall antigens such as TT and provided strong helper function for B cell Ig synthesis. In contrast, CD4+CD29- cells responded poorly to recall antigens and had poor helper function for B cell Ig synthesis, but had strong suppressor activity. Thus, CD29 antigen expression was still predictive of the in vitro functional activity as previously described for normal donors. Furthermore, biochemical analysis of the distribution of individual CD45 isoforms on the surface of these subsets of CD4 cells revealed distinct differences. The CD4+CD29 high (CD4+CD29+) subset of cells primarily expressed the 180-, 190-, and 205-kDa CD45 isoforms, while the CD4+CD29 low (CD4+CD29-) cells primarily expressed the 190-, 205-, and 220-kDa CD45 isoforms. These results suggest that despite the superficial phenotypic similarity of CD4 cells in these donors, distinctions in the distribution of both CD29 and the 180- and 220-kDa CD45 isoforms exist and might play a role in the different functions of freshly isolated CD4 lymphocytes.     

Selective identification of HLA-DP4 binding T cell epitopes encoded by the MAGE-A gene family

Because of the high frequency of HLA-DP4 in the Caucasian population, we have selectively delineated HLA-DP4 restricted T cell epitopes in the MAGE-A tumor antigens. We identified 12 good binders to HLA-DP4 and investigated the capacity of the seven best binders to induce in vitro specific CD4+ T cell lines from HLA-DP4 healthy donors. We found that the MAGE-A1 90–104 peptide exhibited a high and constant frequency of CD4+ T cell precursors in all the six tested donors. The MAGE-A1 268–282 peptide was found immunogenic in only two donors but with a high precursor frequency. The MAGE-A12 127–141 peptide was T cell stimulating in six different donors and induced fewer T cell lines. The peptide-specific T cell lines were stimulated by DC loaded with the lysates of cells transfected with MAGE-A1 or MAGE-A12, or loaded with the recombinant protein. We also show that the immunoreactivity of CD4+ T cell epitopes restricted to the same HLA II molecule may vary from one individual to another, as a result of inter-individual variations in the CD4+ T cell repertoire.     

Functional properties and lineage relationship of CD8+ T cell subsets identified by expression of IL-7 receptor alpha and CD62L

Three major subsets of Ag-experienced CD8+ T cells have been identified according to their expression of CD62L and CD127. These markers are associated with central memory T cells (CD62L+ CD127+), effector memory T cells (CD162L- CD127+), and effector T cells (CD62L- CD127-). In this study we characterized the development of these three populations during acute and chronic viral infections and after immunization with virus-like particles and determined their lineage relation and functional and protective properties. We found that the balance between the three subsets was critically regulated by the availability of Ag and time. After initial down-regulation of CD127, the responding CD8+ T cell population down-regulated CD62L and re-expressed CD127. Dependent on Ag availability, the cells then further differentiated into CD62L- CD127- effector cells or, in the absence of Ag, re-expressed CD62L to become central memory T cells. Although all three populations efficiently produced effector cytokines such as IFN-gamma, CD62L- CD127- effector cells exhibited the highest ex vivo lytic potential. In contrast, CD62L+ CD127+ central memory T cells most efficiently produced IL-2 and proliferated extensively in vitro and in vivo upon antigenic restimulation. Strikingly, only effector and effector memory, but not central memory, T cells were able to protect against peripheral infection with vaccinia virus, whereas central memory T cells were most potent at protecting against systemic infection with lymphocytic choriomeningitis virus, indicating that the antiviral protective capacities of specific CD8+ T cell subsets are closely related to the nature of the challenging pathogen.     

Regulatory and activated T cells in human Schistosoma haematobium infections

Acquired immunity against helminths is characterised by a complex interplay between the effector Th1 and Th2 immune responses and it slowly manifests with age as a result of cumulative exposure to parasite antigens. Data from experimental models suggest that immunity is also influenced by regulatory T cells (Treg), but as yet studies on Treg in human schistosome infections are limited. This study investigated the relationship between schistosome infection intensity and the two cell populations regulatory T cells (TREG: CD4(+(dim))CD25(+(high))FOXP3(+)CD127(low)), and activated (Tact: CD4(+)CD25(+)FOXP3(-)) T cells in Zimbabweans exposed to Schistosoma haematobium parasites. Participants were partitioned into two age groups, young children (8-13 years) in whom schistosome infection levels were rising to peak and older people (14+ years) with declining infection levels. The relationship between Tact proportions and schistosome infection intensity remained unchanged with age. However Treg proportions rose significantly with increasing infection in the younger age group. In contrast Treg were negatively correlated to infection intensity in the older age group. The relative proportions of regulatory T cells differ significantly between young individuals in whom high infection is associated with an enhanced regulatory phenotype and older infected patients in whom the regulatory response is attenuated. This may influence or reflect different stages of the development of protective schistosome acquired immunity and immunopathogenesis.     

Age restriction in antigen-specific immunosuppression

Age-related alterations of antigen-specific T cell-mediated suppression have been examined in the 4-hydroxy-3-nitrophenyl acetyl (NP) system. Inducer suppressor T cells (Tsi) were activated in mice at the age of 3 mo (young) or 18 mo (old) by i.v. injection of NP-conjugated syngeneic spleen cells (SC). Spleen cells from the NP-SC-injected mice were subcultured in vitro with spleen cells from normal young or old mice to generate transducer suppressor T cells (Tst). Four days later subcultured cells were added to responder cell cultures 1 day before the PFC assays to trigger effector suppressor T cells (Tse). Responder cell cultures, containing NP-conjugated horse red blood cells (HRBC) and spleen cells from HRBC-primed young or old mice, were assayed on day 4 for anti-NP and anti-HRBC PFC. Suppression was found to be antigen specific and age restricted. NP-specific suppressor cells are easily induced in subculture if the Tsi and Tst cell populations are both derived from young or old mice. Conversely, if Tsi cells from young or old mice are subcultured with Tst cells from mice of a different age, suppression of the anti-NP PFC response is hardly observed. Age restriction was also found to operate in the interactions between subcultured and responder cell populations, indicating that age-matching is required for effective triggering of Tse cells by Tst cells. These results altogether suggest that aging may affect the recognition repertoire expressed in suppressor T cell subsets. Moreover, the finding that suppression is less efficient when exerted on responder spleen cells from old than from young mice provides an explanation for the increased frequency of autoimmune disorders in aging.     

Age-related changes in mature CD4+ T cells: cell cycle analysis

T cell proliferative responses decrease with age, but the mechanisms responsible are unknown. We examined the impact of age on memory and naive CD4(+) T cell entry and progression through the cell cycle using acridine orange to identify cell cycle stage. For both subsets, fewer stimulated cells from old donors were able to enter and progress through the first cell cycle, with an increased number of cells arrested in G(0) and fewer cells in post G(0) phases. The number of dead cells as assessed by sub-G(0) DNA was also significantly greater in the old group. CD4(+) T cells from old mice also exhibited a significant reduction in clonal history as assessed by CFSE staining. This was associated with a significant decline in cyclin D2 mRNA and protein. We propose that decreases in cyclin D2 are at least partially responsible for the proliferative decline found in aged CD4(+) T cells.     

CD2 costimulation reveals defective activity by human CD4+CD25(hi) regulatory cells in patients with multiple sclerosis

Studying the activity of homogeneous regulatory T cell (Treg) populations will advance our understanding of their mechanisms of action and their role in human disease. Although isolating human Tregs exhibiting low expression of CD127 markedly increases purity, the resulting Treg populations are still heterogeneous. To examine the complexity of the Tregs defined by the CD127 phenotype in comparison with the previously described CD4(+)CD25(hi) subpopulations, we subdivided the CD25(hi) population of memory Tregs into subsets based on expression of CD127 and HLA-DR. These subsets exhibited differences in suppressive capacity, ability to secrete IL-10 and IL-17, Foxp3 gene methylation, cellular senescence, and frequency in neonatal and adult blood. The mature, short telomere, effector CD127(lo)HLA-DR(+) cells most strongly suppressed effector T cells within 48 h, whereas the less mature CD127(lo)HLA-DR(-) cells required 96 h to reach full suppressive capacity. In contrast, whereas the CD127(+)HLA-DR(-) cells also suppressed proliferation of effector cells, they could alternate between suppression or secretion of IL-17 depending upon the stimulation signals. When isolated from patients with multiple sclerosis, both the nonmature and the effector subsets of memory CD127(lo) Tregs exhibited kinetically distinct defects in suppression that were evident with CD2 costimulation. These data demonstrate that natural and not induced Tregs are less suppressive in patients with multiple sclerosis.     

Analysis of CD127 and KLRG1 expression on hepatitis C virus-specific CD8+ T cells reveals the existence of different memory T-cell subsets in the peripheral blood and liver

The differentiation and functional status of virus-specific CD8+ T cells is significantly influenced by specific and ongoing antigen recognition. Importantly, the expression profiles of the interleukin-7 receptor alpha chain (CD127) and the killer cell lectin-like receptor G1 (KLRG1) have been shown to be differentially influenced by repetitive T-cell receptor interactions. Indeed, antigen-specific CD8+ T cells targeting persistent viruses (e.g., human immunodeficiency virus and Epstein-Barr virus) have been shown to have low CD127 and high KLRG1 expressions, while CD8+ T cells targeting resolved viral antigens (e.g., FLU) typically display high CD127 and low KLRG1 expressions. Here, we analyzed the surface phenotype and function of hepatitis C virus (HCV)-specific CD8+ T cells. Surprisingly, despite viral persistence, we found that a large fraction of peripheral HCV-specific CD8+ T cells were CD127+ and KLRG1- and had good proliferative capacities, thus resembling memory cells that usually develop following acute resolving infection. Intrahepatic virus-specific CD8+ T cells displayed significantly reduced levels of CD127 expression but similar levels of KLRG1 expression compared to the peripheral blood. These results extend previous studies that demonstrated central memory (CCR7+) and early-differentiated phenotypes of HCV-specific CD8+ T cells and suggest that insufficient stimulation of virus-specific CD8+ T cells by viral antigen may be responsible for this alteration in HCV-specific CD8+ T-cell differentiation during chronic HCV infection.     

Differences in the expression profiles of CD45RB, Pgp-1, and 3G11 membrane antigens and in the patterns of lymphokine secretion by splenic CD4+ T cells from young and aged mice

Previous studies indicate that the 3G11, CD45RB, and Pgp-1 determinants are differentially expressed on CD4+ T cell subsets in the mouse. We used multicolor immunofluorescence staining and flow cytofluorometric analysis to examine the expression of each of these determinants on splenic CD4+ cells from young (age 3 to 6 mo) and aged (age 24 to 26 mo) C57BL/6 mice. The CD4+ pool from aged mice contained significantly reduced numbers of 3G11+ and CD45RBhi cells, but increased numbers of Pgp-1hi cells, in comparison with the young group. Analysis of the simultaneous expression of all three subset determinants on CD4+ cells revealed that, in young mice, the major fraction (greater than 50%) was 3G11+CD45RBhiPgp-1lo. Among the less prevalent cell phenotypes, reductions in 3G11 expression correlated with decreases in CD45RB levels and increases in Pgp-1 levels. The phenotype that dominated the young group (3G11+CD45RBhiPgp-1lo) was approximately fivefold less represented in the aged group. The CD4+ pool from aged mice was characterized by increases in the 3G11-CD45RBvariablePgp-1hi and the 3G11+CD45RBloPgp-1hi phenotypes. To evaluate possible age-associated differences in cytokine secretion patterns by splenic CD4+ cells, purified CD4+ cells from each age group were stimulated in vitro with immobilized anti-CD3 epsilon mAb and accessory cells. At various times thereafter, supernatants from cultures were tested for IL-2 and IL-4 content by using the CTLL.6 and 11.6 bioassays, respectively, and the CD4+ cells were assayed for [3H]TdR uptake. Cell cultures from the aged group exhibited similar peak IL-2 accumulation and lower peak [3H]TdR uptake, but greatly increased peak IL-4 accumulation, as compared with cell cultures from the young group. The expression patterns of subset determinants, in conjunction with cytokine secretion profiles, indicate that, in aged mice, marked alterations occur in the subset composition of the splenic CD4+ cell pool. These findings are discussed in the context of previous findings on changes in T cell reactivity with advancing donor age.     

Pathogen-induced inflammatory environment controls effector and memory CD8+ T cell differentiation

In response to infection, CD8(+) T cells integrate multiple signals and undergo an exponential increase in cell numbers. Simultaneously, a dynamic differentiation process occurs, resulting in the formation of short-lived effector cells (SLECs; CD127(low)KLRG1(high)) and memory precursor effector cells (CD127(high)KLRG1(low)) from an early effector cell that is CD127(low)KLRG1(low) in phenotype. CD8(+) T cell differentiation during vesicular stomatitis virus infection differed significantly than during Listeria monocytogenes infection with a substantial reduction in early effector cell differentiation into SLECs. SLEC generation was dependent on Ebi3 expression. Furthermore, SLEC differentiation during vesicular stomatitis virus infection was enhanced by administration of CpG-DNA, through an IL-12-dependent mechanism. Moreover, CpG-DNA treatment enhanced effector CD8(+) T cell functionality and memory subset distribution, but in an IL-12-independent manner. Population dynamics were dramatically different during secondary CD8(+) T cell responses, with a much greater accumulation of SLECs and the appearance of a significant number of CD127(high)KLRG1(high) memory cells, both of which were intrinsic to the memory CD8(+) T cell. These subsets persisted for several months but were less effective in recall than memory precursor effector cells. Thus, our data shed light on how varying the context of T cell priming alters downstream effector and memory CD8(+) T cell differentiation.     

Bcl-2 allows effector and memory CD8+ T cells to tolerate higher expression of Bim

As acute infections resolve, most effector CD8(+) T cells die, whereas some persist and become memory T cells. Recent work showed that subsets of effector CD8(+) T cells, identified by reciprocal expression of killer cell lectin-like receptor G1 (KLRG1) and CD127, have different lifespans. Similar to previous reports, we found that effector CD8(+) T cells reported to have a longer lifespan (i.e., KLRG1(low)CD127(high)) have increased levels of Bcl-2 compared with their shorter-lived KLRG1(high)CD127(low) counterparts. Surprisingly, we found that these effector KLRG1(low)CD127(high) CD8(+) T cells also had increased levels of Bim compared with KLRG1(high)CD127(low) cells. Similar effects were observed in memory cells, in which CD8(+) central memory T cells expressed higher levels of Bim and Bcl-2 than did CD8(+) effector memory T cells. Using both pharmacologic and genetic approaches, we found that survival of both subsets of effector and memory CD8(+) T cells required Bcl-2 to combat the proapoptotic activity of Bim. Interestingly, inhibition or absence of Bcl-2 led to significantly decreased expression of Bim in surviving effector and memory T cells. In addition, manipulation of Bcl-2 levels by IL-7 or IL-15 also affected expression of Bim in effector CD8(+) T cells. Finally, we found that Bim levels were significantly increased in effector CD8(+) T cells lacking Bax and Bak. Together, these data indicate that cells having the highest levels of Bim are selected against during contraction of the response and that Bcl-2 determines the level of Bim that effector and memory T cells can tolerate.