Heterogeneity of the memory CD4 T cell response: persisting effectors and resting memory T cells

Defining the cellular composition of the memory T cell pool has been complicated by an inability to distinguish effector and memory T cells. We present here an activation profile assay, using anti-CD3 and antigenic stimuli, that clearly distinguishes effector and memory CD4 T cells and defines subsets of long-lived memory CD4 T cells based on CD62 ligand (CD62L) expression. The CD62L(low) memory subset functionally resembles effector cells, exhibiting hyper-responsiveness to antigenic and anti-CD3 mediated stimuli, high proliferative capacity, and rapid activation kinetics. The CD62L(high) memory subset functionally resembles resting memory cells, exhibiting hyporesponsiveness to anti-CD3 stimuli, lower proliferative capacity, and slower activation kinetics. Our results indicate that the memory CD4 T cell pool is heterogeneous, consisting of persisting effectors and resting memory T cells.     

Development and homeostasis of T cell memory in rhesus macaque.

The rhesus macaque (RM) is a critical animal model for studies of viral pathogenesis and immunity, yet fundamental aspects of their cellular immune response remain poorly defined. One such deficiency is the lack of validated phenotypic signatures for their naive and memory T cell subsets, and the resultant unavailability of accurate information on their memory T cell development, homeostasis, and function. In this study, we report a phenotypic paradigm allowing definitive characterization of these subsets and their comprehensive functional analysis. Naive T cells are optimally delineated by their homogeneous CD95(low)CD28(high)beta(7) integrin(int) (CD4+) or CD95(low)CD28(int)CD11a(low) (CD8+) phenotypes. This subset 1) was present in blood and secondary lymph tissues, but not effector sites; 2) vastly predominated in the fetal/neonatal immune system, but rapidly diminished with postnatal age; 3) lacked IFN-gamma production capability, and specific responses to RM CMV; and 4) demonstrated low in vivo proliferative activity. CD4+ and CD8+ memory subsets were CD95(high), but otherwise phenotypically heterogeneous and included all IFN-gamma production, RM CMV-specific responses, effector site T cells, and demonstrated high in vivo proliferative activity ( approximately 10 times the naive subset). These analyses also revealed the RM "effector memory" subset within the overall memory population. This population, best defined by lack of CD28 expression, contained the majority of RM CMV-specific cells, was highly enriched in extralymphoid effector sites, and comprised an increasing proportion of total memory cells with age. The effector memory subset demonstrated similar in vivo proliferative activity and survival as CD28+ "central memory" T cells, consistent with independent homeostatic regulation.     

Differential SLP-76 expression and TCR-mediated signaling in effector and memory CD4 T cells

We present in this study novel findings on TCR-mediated signaling in naive, effector, and memory CD4 T cells that identify critical biochemical markers to distinguish these subsets. We demonstrate that relative to naive CD4 T cells, memory CD4 T cells exhibit a profound decrease in expression of the linker/adapter molecule SLP-76, while effector T cells express normal to elevated levels of SLP-76. The reduced level of SLP-76 is memory CD4 T cells is coincident with reduced phosphorylation overall, yet the residual SLP-76 couples to a subset of TCR-associated linker molecules, leading to downstream mitogen-activated protein (MAP) kinase activation. By contrast, effector CD4 T cells strongly phosphorylate SLP-76, linker for activation of T cells, and additional Grb2-coupled proteins, exhibit increased associations of SLP-76 to phosphorylated linkers, and hyperphosphorylate downstream Erk1/2 MAP kinases. Our results suggest distinct coupling of signaling intermediates to the TCR in naive, effector, and memory CD4 T cells. Whereas effector CD4 T cells amplify existing TCR signaling events accounting for rapid effector responses, memory T cells engage fewer signaling intermediates to efficiently link TCR triggering directly to downstream MAP kinase activation.     

Differential in vivo persistence of two subsets of memory phenotype CD8 T cells defined by CD44 and CD122 expression levels

The existence of distinct subsets of memory CD8 T cells with different characteristics is now well established. In this work, we describe two subsets of mouse CD8 T cells with memory characteristics that coexist in primed thymectomized TCR-transgenic F5 mice and that share some properties with the human central and effector memory cells. The first subset corresponds to CD8 T cells generated following nucleoprotein 68 peptide priming which are CD44(int)CD122(-)nucleoprotein 68/H-2D(b) tetramer(+) and express high levels of CCR7 mRNA. In contrast, CD8 T cells in the second subset are CD44(high)CD122(+), are heterogeneous in terms of Ag specificity, and express low levels of CCR7 mRNA. We have studied the functional characteristics and the persistence of these two subsets in thymectomized mice. CD44(int) CD8 T cells persist like naive cells; i.e., they are slowly lost with time. However, surviving cells maintain their phenotype and memory characteristics for the entire life span of the animal. In contrast, CD44(high) CD8 T cells are persistent and accumulate in thymectomized but not euthymic mice. This is correlated with an increased in vivo proliferative and survival potential of these cells. These results show that acquisition of enhanced functional characteristics and long-term persistence by memory T cells are independent. This may have important consequences for the design of specific vaccine.     

Withdrawal of stimulation may initiate the transition of effector to memory CD4 cells.

The initial steps that determine development of memory in CD4 cells are unknown. To distinguish an intrinsic capacity of effectors to become memory cells from contributions of as yet undefined survival factors, we analyzed the effects of withdrawal of signals via TCR, costimulation, and cytokines from Th1 or Th2 primary effectors induced in vitro from TCR-transgenic CD4 cells. Withdrawal of stimulation caused the transition of effectors to resting populations with a memory phenotype that did not undergo division following transfer to normal syngeneic recipients. The return of effectors to rest was accompanied by acquisition of the capacity to function as memory cells in vivo as defined by extended persistence and a more rapid response to Ag in vivo than naive cells in adoptive hosts. Upon challenge with Ag, these in vitro-rested Th1 and Th2 cells were similar to long-term in vivo-rested memory cells, but distinct from in vitro-generated primary effectors and in vivo-restimulated memory effectors by their ability to resist apoptosis. Cessation of stimulation may occur when activated CD4 cells exit lymphoid tissues after priming and transition to memory may be initiated if effectors either fail to gain access to Ag in peripheral tissues where restimulation can lead to activation-induced cell death or do not receive sufficient stimuli to continue a response. Our results suggest that the first stage leading to stable CD4 memory could occur stochastically and independently of instructional processes and as such, the development of memory may be a default pathway when signals that direct responses are not received.     

Flt3 ligand-generated murine plasmacytoid and conventional dendritic cells differ in their capacity to prime naive CD8 T cells and to generate memory cells in vivo

Mature dendritic cells (DCs) have the capacity to induce efficient primary T cell response and effector cell differentiation. Thus, these cells are a major tool in the design of various immunotherapeutic protocols. We have tested the capacity of different subsets of matured DCs pulsed with a peptide to induce the differentiation of naive CD8 T cells into memory cells in vivo. Flt3 ligand (FL) induces the differentiation of conventional DCs (cDCs) and plasmacytoid DCs (PDCs) from murine bone marrow precursors in vitro. After maturation, both subsets become strong stimulators of Ag-specific T cell responses in vitro. However, the in vivo T cell stimulatory capacity of these DC subsets has not been studied in detail. In the present study, we demonstrate that mature FL-generated DCs induce efficient peptide-specific CD8 T cell response and memory cell differentiation in vivo. This is mainly due to the cDC subset because the PDC subset induced only a negligible primary CD8 response without detectable levels of memory CD8 T cell differentiation. Thus, in vitro FL-generated mature cDCs, but not PDCs, are potent stimulators of peptide-specific CD8 T cell responses and memory generation in vivo.     

Unique ability of activated CD4+ T cells but not rested effectors to migrate to non-lymphoid sites in the absence of inflammation

Recent studies suggest that effector T cells generated by immune responses migrate to multiple non-lymphoid sites, even those without apparent expression of antigen or inflammation. To investigate the ability of distinct CD4(+) T lymphocyte subsets to enter and persist in non-lymphoid, noninflamed compartments, we examined the migration and persistence of na?ve, effector, and rested effector CD4(+) T cells generated in vitro following transfer to nonimmunized adoptive hosts. Th1 and Th2 effectors migrated to both lymphoid and non-lymphoid organs (peritoneum, fat pads, and lung). In contrast, rested effectors and na?ve cells migrated only to lymphoid areas. Adhesion molecule expression, but not chemokine receptor expression, correlated with the ability to enter non-lymphoid sites. Donor cells persisted longer in lymphoid than in non-lymphoid sites. When hosts with na?ve and memory donor cells were challenged with antigen, effectors developed in situ, which also migrated to non-lymphoid sites. Memory cells showed an accelerated shift to non-lymphoid migration, in keeping with memory effector formation. These results suggest that only recently activated effector T cells can disperse to non-lymphoid sites in the absence of antigen and inflammation, and as effectors return to rest, they lose this ability. These data also argue that memory cells in lymphoid sites are longer lived and not in equilibrium with those in non-lymphoid sites.     

A biochemical signature for rapid recall of memory CD4 T cells

Mechanisms for the rapid recall response mediated by memory T cells remain unknown. In this study, we present a novel, multiparameter analysis of TCR-coupled signaling and function in resting and activated naive and memory CD4 T cells, revealing a biochemical basis for immunological recall. We identify a striking elevation in expression of the proximal tyrosine kinase Zap70 in resting Ag-specific and polyclonal mouse memory vs naive CD4 T cells that is stably maintained independent of protein synthesis. Elevated Zap70 protein levels control effector function as IFN-gamma production occurs exclusively from the Zap70(high) fraction of activated T cells in vitro and in vivo, and specific down-modulation of Zap70 expression in memory CD4 T cells by small interfering RNA or protein inhibition significantly reduces rapid IFN-gamma production. Downstream of Zap70, we show quantitative differences in distal phosphorylation associated with effector function in naive and memory subsets, with low accumulation of phosphorylation in memory T cells producing IFN-gamma at early time points, contrasting extensive phosphorylation associated with IFN-gamma production following sustained activation of naive T cells. Our results reveal a novel biochemical signature imparted to memory CD4 T cells enabling efficacious responses through increased Zap70 expression and reduced accumulation of downstream signaling events.     

Divergent generation of heterogeneous memory CD4 T cells

Mechanisms for the generation of memory CD4 T cells and their delineation into diverse subsets remain largely unknown. In this study, we demonstrate in two Ag systems, divergent generation of heterogeneous memory CD4 T cells from activated precursors in distinct differentiation stages. Specifically, we show that influenza hemagglutinin- and OVA-specific CD4 T cells activated for 1, 2, and 3 days, respectively, exhibit gradations of differentiation by cell surface phenotype, IFN-gamma production, and proliferation, yet all serve as direct precursors for functional memory CD4 T cells when transferred in vivo into Ag-free mouse hosts. Using a conversion assay to track the immediate fate of activated precursors in vivo, we show that day 1- to 3-activated cells all rapidly convert from an activated phenotype (CD25(high)IL-7R(low)CD44(high)) to a resting memory phenotype (IL-7R(high)CD25(low)CD44(high)) 1 day after antigenic withdrawal. Paradoxically, stable memory subset delineation from undifferentiated (day 1- to 2-activated) precursors was predominantly an effector memory (CD62L(low)) profile, with an increased proportion of central memory (CD62L(high)) T cells arising from more differentiated (day 3-activated) precursors. Our findings support a divergent model for generation of memory CD4 T cells directly from activated precursors in multiple differentiation states, with subset heterogeneity maximized by increased activation and differentiation during priming.     

Progressive loss of memory T cell potential and commitment to exhaustion during chronic viral infection

T cell exhaustion and loss of memory potential occur during many chronic viral infections and cancer. We investigated when during chronic viral infection virus-specific CD8 T cells lose the potential to form memory. Virus-specific CD8 T cells from established chronic infection were unable to become memory CD8 T cells if removed from infection. However, at earlier stages of chronic infection, these virus-specific CD8 T cells retained the potential to partially or fully revert to a memory differentiation program after transfer to infection-free mice. Conversely, effector CD8 T cells primed during acute infection were not protected from exhaustion if transferred to a chronic infection. We also tested whether memory and exhausted CD8 T cells arose from different subpopulations of effector CD8 T cells and found that only the KLRG1(lo) memory precursor subset gave rise to exhausted CD8 T cells. Together, these studies demonstrate that CD8 T cell exhaustion is a progressive developmental process. Early during chronic infection, the fate of virus-specific CD8 T cells remains plastic, while later, exhausted CD8 T cells become fixed in their differentiation state. Moreover, exhausted CD8 T cells arise from the memory precursor and not the terminally differentiated subset of effector CD8 T cells. These studies have implications for our understanding of senescence versus exhaustion and for therapeutic interventions during chronic infection.     

CD7 is a differentiation marker that identifies multiple CD8 T cell effector subsets

The adaptive immune response of human CD8 T cells to invading pathogens involves the differentiation of naive cells into memory and effector cells. However, the lineage relationship between memory and effector cells and the differentiation of CD8 T cells into distinct subsets of effector cell subpopulations are subjects of considerable debate. CD7 identifies three populations of CD8 T cells: CD7 high (CD7(high)), low (CD7(low)), and negative (CD7(neg)) that translate into subsets with distinct functional properties. The CD7(high) subset contains naive and memory cells and the CD7(low) and CD7(neg) subsets contain effector cells. The effector cells can functionally be divided into cytokine-secreting effector CD8 T cells and lytic effector CD8 T cells. These data provide a model of human CD8 T cell differentiation in which specialized distinct subpopulations can be identified by expression of CD7.     

Expression of CD161 (NKR-P1A) defines subsets of human CD4 and CD8 T cells with different functional activities

A subset of T cells in human peripheral blood expresses CD161 (NKR-P1A) receptors that are primarily associated with NK cells. In the current study we isolated blood T cell subsets according to the expression of CD161 and examined their contents of naive, central memory, and effector memory cells and their capacities for proliferation, cytokine secretion, and natural cytolysis. We found that CD4+CD161- and CD8+CD161- subsets contained predominantly naive T cells that secreted high levels of IL-2 after in vitro stimulation, and CD4+CD161int and CD8+CD161int subsets contained predominantly effector and central memory T cells that secreted high levels of IFN-gamma and TNF-alpha. All of these subsets showed vigorous proliferation after stimulation in vitro, but none had NK lytic activity. Unexpectedly, the CD8+CD161+ cells contained an anergic CD8alpha+CD8betalow/-CD161high T cell subset that failed to proliferate, secrete cytokines, or mediate NK lytic activity.     

CD4+ T cell subsets. Lymphokine secretion of memory cells and of effector cells that develop from precursors in vitro

We have studied the properties of several developmentally defined subpopulations of CD4+ T cells from normal animals which can be stimulated to secrete lymphokines. We find that the Th cells responsible for direct secretion of lymphokines after stimulation are from a resting, very long lived subpopulation of CD4+ T cells which persists for over 25 wk after adult thymectomy. These T cells are depleted by in vivo administration of antithymocyte serum and they are enriched among T cells which express high levels of Pgp-1. This phenotype suggests that the T cells responsible are most likely memory T cells which have resulted from antigen exposure in vivo. T cells in this subset secrete predominantly IL-2 with small quantities of IL-3, granulocyte/macrophage CSF, and IFN-gamma. In contrast, the CD4+ T cells which require in vitro culture and restimulation before they develop into an effector population with the ability to secrete lymphokines after restimulation, differ dramatically by most of these criteria. The precursors we study are resting Th cells which are considerably shorter lived after adult thymectomy (5 to 10 wk) and resistant to the same doses of antithymocyte serum which deplete the putative memory population. We hypothesize that this precursor population represents naive helper cells which have not yet encountered Ag. The effectors derived from such precursors can be stimulated to secrete high levels of both Th cell types 1 and 2 lymphokines (IFN-gamma, IL-4, IL-5, granulocyte/macrophage CSF, and IL-3). Generation of effectors requires proliferation and differentiation events which occur during a mandatory culture with lymphokines and antigen presenting cells for 3 to 4 days. We discuss the striking phenotypic and functional differences among these subpopulations of helper cells--the precursor population and the two types--memory and cultured effector Th which secrete lymphokines. We also discuss the relationship of these populations to CD4+ T cell subsets defined by other studies of patterns of lymphokine secretion and by cell surface phenotype.     

Roles of CD4+ and CD8+ T cells in murine contact sensitivity revealed by in vivo monoclonal antibody depletion

mAb specific for murine CD4+ and CD8+ T cell subsets were utilized to determine the populations participating in delayed-in-time, cutaneous hypersensitivity responses in BALB/c mice. In vivo depletions of these T cell phenotypes revealed that delayed-type hypersensitivity to cellular and protein Ag were mediated by CD4+ effector cells, whereas CD8+ cells down-regulated such responses. Similar depletions in mice prior to sensitization with the hapten 1-fluoro-2,4-dinitrobenzene demonstrated a more complex pattern of cell participation in contact sensitivity (CS) responses. Depletion of CD4+ cells resulted in strikingly enhanced ear swelling, indicating not only an important effector role for CD8+ cells but also a down-regulatory role for some CD4+ cells; depletion of CD8+ cells revealed that some CD4+ cells also act as CS effectors. In vitro depletion of immune lymph node cells with the same mAb before adoptive transfer confirmed CS effector roles for both subsets, and also suggested that at least some CD4+ suppressors act on the efferent limb of the CS response, perhaps by down-regulating the activity of CD8+ effector cells. Partial in vivo depletion with small amounts anti-CD4 mAb and subsequent flow cytometric analysis of residual CD4+ cells was consistent with the hypothesis that CD4+ CS effector cells express a higher density of the CD4 antigen than do CD4+ suppressor cells, raising the possibility that these two functionally distinct CD4+ populations might be separable on the basis of their surface expression of CD4.     

Peptide antigen priming of naive,but not memory,CD8 T cells requires a third signal that can be provided by IL-12

Challenge with peptide Ag in the absence of adjuvant results in tolerance of CD8 T cells specific for the Ag. In contrast, administration of IL-12 along with peptide results in massive clonal expansion, development of effector function, and establishment of a long-lived memory population. Using adoptive transfer of TCR-transgenic CD8 T cells, this effect of IL-12 is shown to be independent of CD4 T cells and to require costimulation provided by CD28 and possibly LFA-1. IL-12 supports responses when IL-12Rbeta1-deficient mice are used as recipients for the adoptively transferred CD8 T cells, demonstrating that the IL-12 is acting directly on the T cells rather than on host APC. These results provide strong support for a three-signal model for in vivo activation of naive CD8 T cells by peptide Ag, in which the presence or absence of the third signal determines whether tolerance or activation occurs. In contrast, memory CD8 T cells are effectively activated by peptide Ag in the absence of IL-12 or adjuvant.     

Rapid turnover of the CD8(+)CD28(-) T-cell subset of effector cells in the circulation of patients with head and neck cancer

CD8⁺ T cells in the circulation of patients with head and neck cancer (HNC) were previously shown to be significantly more sensitive to, and preferentially targeted for, apoptosis than CD4⁺ T cells (Hoffmann et al., Clin Cancer Res, 8:2553–2562, 2002). To distinguish global from CD8⁺ subset-specific apoptosis, we studied Annexin-binding to naïve, memory, and effector subsets of CD8⁺ cells by multicolor flow cytometry. Age-related changes in naïve and effector CD8⁺ cell subsets were observed in patients and normal controls (NC). The frequencies of naïve (CD28⁺CD45RO^-) CD8⁺ T cells were lower and those of memory (CD28⁺CD45RO⁺) and effector (CD28^-) CD8⁺ T cells significantly higher in the circulation of HNC patients relative to age-matched NC. Among CD8⁺ T cells, the CD28^- effector cell subset contained the highest proportion of Annexin-binding cells, while the naïve CD28⁺CD45RO^- subset contained the lowest. This suggested a high turnover rate of the CD8⁺CD28^- effector cell subset in patients with HNC, which was being compensated by a rapid transition of naïve CD8⁺ T cells to the effector cell pool. Following tumor resection, the frequency of CD8⁺CD28^- T cells normalized in the patients, an indication that the presence of tumor had an influence on the size of CD8⁺CD28^- T-cell pool. Ex vivo, in mixed lymphocyte-tumor cultures (MLTC) with semiallogeneic T cells as responders, CD8⁺CD28^- T cells could be generated from CD8⁺CD28⁺ cells by repeated stimulations with tumor cells. These CD8⁺CD28^- effector cells lysed the tumor, produced IFN- in response to the tumor, and strongly expressed granzyme B. Thus, the high rate of their apoptosis in the circulation of patients with HNC might be expected to contribute to tumor progression. However, the ex vivo generation of this cell subset was suppressed by strong CD28/B7 ligation or by overexpresson of MHC molecules on tumor cells, suggesting that adequate costimulation is necessary for protection from apoptosis. It appears that interactions of immune and tumor cells might determine the fate of this terminally differentiated effector cell subset.Supported in part by NIH grants: PO-1 DE 12321 and RO-1 CA 82016 to Theresa L. Whiteside.     

Disparate primary and secondary allospecific CD8+ T cell cytolytic effector function in the presence or absence of host CD4+ T cells

The role of CD4+ T cells in promoting CD8+ T cell effector activity in response to transplant Ags in vivo has not been reported. We used a hepatocellular allograft model known to initiate both CD4-dependent and CD4-independent rejection responses to investigate the contribution of CD4+ T cells to the development, function, and persistence of allospecific CD8+ T cell effectors in vivo. Complete MHC-mismatched hepatocellular allografts were transplanted into C57BL/6 (CD4-sufficient) or CD4 knockout (CD4-deficient) hosts. The development of in vivo allospecific cytotoxicity was determined by clearance of CFSE-labeled target cells. CD8+ T cell cytotoxic effector activity was enhanced in response to allogeneic hepatocellular grafts with a greater magnitude of allocytotoxicity and a prolonged persistence of CTL effector activity in CD4-sufficient hosts compared with CD4-deficient hosts. Cytolytic activity was mediated by CD8+ T cells in both recipient groups. In response to a second hepatocyte transplant, rejection kinetics were enhanced in both CD4-sufficient and CD4-deficient hepatocyte recipients. However, only CD4-sufficient hosts developed recall CTL responses with an augmented magnitude and persistence of allocytotoxicity in comparison with primary CTL responses. These studies show important functional differences between alloreactive CD8+ T cell cytolytic effectors that mature in vivo in the presence or absence of CD4+ T cells.     

In vivo differentiated cytokine-producing CD4(+) T cells express functional CCR7

Chemokines and their receptors fulfill specialized roles in inflammation and under homeostatic conditions. CCR7 and its ligands, CCL19 and CCL21, are involved in lymphocyte recirculation through secondary lymphoid organs and additionally navigate lymphocytes into distinct tissue compartments. The role of CCR7 in the migration of polarized T effector/memory cell subsets in vivo is still poorly understood. We therefore analyzed murine and human CD4(+) cytokine-producing cells developed in vivo for their chemotactic reactivity to CCR7 ligands. The responses of cells producing cytokines, such as IFN-gamma, IL-4, and IL-10, as well as of subsets defined by memory or activation markers were comparable to that of naive CD4(+) cells, with slightly lower reactivity in cells expressing IL-10 or CD69. This indicates that CCR7 ligands are able to attract naive as well as the vast majority of activated and effector/memory T cell stages. Chemotactic reactivity of these cells toward CCL21 was absent in CCR7-deficient cells, proving that effector cells do not use alternative receptors for this chemokine. Th1 cells generated from CCR7(-/-) mice failed to enter lymph nodes and Peyer's patches, but did enter a site of inflammation. These findings indicate that CD4(+) cells producing effector cytokines upon stimulation retain the capacity to recirculate through lymphoid tissues via CCR7.     

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.