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Protective immunity towards intracellular pathogens
Protective immunity towards intracellular pathogens Katharina M Huster1,2, Christian Stemberger2 and Dirk H Busch1,2 Immunity towards intracellular pathogens is often dependent upon the generation of CD8+ memory T cells, which provide long-lasting and effective protection. Over the past few years, we have gained novel insights into the heterogeneity of CD8+ T cells, time points of lineage commitment, and lineage relationships between subpopulations. These studies suggest that memory CD8+ T cells progressively develop from naı¨ve cells early during the immune response and further differentiate unidirectionally into short-living effector cells. We have also learnt that different memory subsets play distinct roles in conferring protection: whereas effector memory T cells are able to provide immediate protection but are not necessarily maintained long-term, central memory T cells have the potential for constant self-renewal. Thus, neither subset really fulfills all criteria of memory. As protective effector memory cells can develop from central memory cells, vaccination strategies should focus on induction of a balanced ratio of the two memory T cell subsets. Addresses 1 Clinical Cooperation Group, Antigen Specific Immunotherapy, GSF, Institute of Health and Environment and Technical University of Munich, D-81675 Munich, Germany 2 Institute for Medical Microbiology, Immunology, and Hygiene, Technical University of Munich, Trogerstrasse 9, D-81675 Munich, Germany Corresponding author: Busch, Dirk H ([email protected])
Current Opinion in Immunology 2006, 18:458–464 This review comes from a themed issue on Host-pathogen interactions Edited by Stefen HE Kaufmann and Bruce D Walker Available online 12th June 2006 0952-7915/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. DOI 10.1016/j.coi.2006.05.008
Introduction CD8+ T cells provide immunity to viral, bacterial and protozoal infections. Naı¨ve CD8+ T cells reside mainly in lymphoid tissues; here they can encounter pathogenderived antigen together with appropriate co-stimulation, both of which are delivered by specialized antigen-presenting cells (APCs). This leads to proliferation of antigen-reactive naı¨ve cells, accompanied by phenotypical and functional changes such as acquisition of effector functions and of the ability to home to peripheral tissues. Clonal expansion is followed by substantial cell death, leaving only a small number of antigen-specific memory cells [1]. Memory cells are defined as cells that persist and Current Opinion in Immunology 2006, 18:458–464
rapidly regain effector function when re-exposed to antigen [2]. Induction of memory T cells that have fast protective effector functions represents the basic principle of T cell-based vaccination strategies. In this review, we discuss novel insights into the heterogeneity of memory cells, the time points of memory lineage commitment, and the lineage relationships between memory subpopulations. Most importantly, it has become obvious that different memory subsets play distinct roles in conferring protection.
Heterogeneity of memory T cells It is well accepted that memory T cells display diversity with respect to their effector functions, homing potential and proliferative capacity [3–5]. Short-living effector T cells (TEC) dominate the expansion phase, migrate to peripheral organs and display immediate effector function. Long-living memory cells can be assigned mainly to two classes: effector memory T cells (TEM) and central memory T cells (TCM). TEM preferentially home to peripheral tissues and respond to antigen encounter with immediate effector functions but only poor numeric expansion [4]. TCM home to lymphoid organs and can vigorously expand upon antigen reencounter. Most studies have characterized TCM as cell populations that produce large quantities of IL-2. However, acquisition of full effector functions requires further differentiation signals [4]. The functional diversity of CD8+ T cells complicates their analysis, and much effort has been made to correlate functional properties with phenotypical appearance. Sallusto et al. [3] described that loss of CCR7 expression is accompanied by gain of effector function by human CD8+ T cells. Unfortunately, expression of CCR7 in mice does not correlate similarly with functionally distinct CD8+ subsets [6–8]. Instead, CD62L, which like CCR7 is a molecule important for homing to lymph nodes, can be used to distinguish between TCM (CD62Lhigh) and TEM and TEC (CD62Llow) [9]. However, discrimination between long-living (TEM) and short-living (TEC) CD8+ T cells that have effector function is not possible just by determination of CD62L surface expression. A recent study linked the transient expression of CD8aa homodimer to the generation of memory: mice that lack expression of CD8aa were unable to generate memory to lymphocytic choriomeningitis virus (LCMV) infection [10]. CD8aa impedes MHC class Ia-driven activation by down-modulating T-cell receptor (TCR) signals — a function that might protect cells that have this receptor from activation-induced death. However, the importance www.sciencedirect.com
Protective immunity towards intracellular pathogens Huster, Stemberger and Busch 459
Figure 1
Correlation between the phenotype and function of antigen-specific CD8+ T cells. LLO91-99-specific splenic CD8+ T cells were stained with CD62L (y-axis) and with CD127 (x-axis) 12 days after primary infection with L. monocytogenes. Only CD8+ MHC Multimer (H2-Kd/LLO91–99)-positive cells are shown. T cell subsets were identified by expression of CD62L and CD127: TCM (CD127highCD62Lhigh), TEM (CD127highCD62Llow) and TEC (CD127lowCD62Llow).
of this finding has been questioned by more recent studies in which memory to LCMV [11] and influenza A virus [12] could be generated in animals impaired in CD8aa expression. The IL-7 receptor a-chain (CD127) might be the most reliable marker for the identification of memory cells in mice [13,14] and in humans [15–17] at the present time. IL-7 is needed for survival of memory cells [18]. Combination of CD127 and CD62L allows discrimination between CD8+ subsets that correlate with the functional properties reported for TCM (CD127highCD62Lhigh), TEM (CD127highCD62Llow) and TEC (CD127lowCD62Llow) [13,19] (Figure 1).
Time point of commitment to memory lineage Much effort has been applied to understand the differentiation pathway from naı¨ve to memory CD8+ T cells. The time point of memory induction is especially important for the identification of memory-determining conditions or factors. Activation and expansion of CD8+ cells can be initiated by brief exposure to antigen: in vitro, the presence of antigen for 2 hours is already sufficient to induce proliferation [20,21], whereas as few as 24 hours of antigen exposure in vivo are sufficient to induce proliferation and acquisition of effector [21] and memory characteristics of antigen-specific CD8+ populations [22,23]. www.sciencedirect.com
The first evidence for the early presence of memory cells during the initial priming period came from an activationdependent transgenic reporter gene expression model, in which a small number of reporter-positive T cells were present at the peak of the immune response and persisted over time as memory cells [24]. In accordance with these findings, stable numbers of memory cells were detected starting from day four after infection with Listeria monocytogenes or LCMV when using CD127 as a memory marker [13,14]. The first functional evidence for the early presence of memory cells also came from experiments in the Listeria infection model. Re-infection as early as five days after the primary infection resulted in typical memory expansion of antigen-specific T cells. These cells were as efficient in providing protection against subsequent challenge with Listeria as memory cells generated late after primary infection [25,26]. Although it is now widely accepted that T cells acquire a differentiation program shortly after activation, the program is not absolutely independent of extrinsic factors. The dose of antigen, for example, can influence the number of naı¨ve precursor T cells recruited into the immune response and can thus affect the overall burst size during the effector phase [23]. In addition, the degree and the quality of innate immune system stimulation can substantially affect T cell priming and differentiation Current Opinion in Immunology 2006, 18:458–464
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[27]. For example, CD4+ T cells provide help by activating dendritic cells. This help is required during primary infection for efficient memory generation [28–30]. Consistent with the observation of early programming of memory cells, CD4+ T cell help for efficient stimulation is rapid and transient, with a time frame of between 24 and 36 hours [31]. In conclusion, a brief encounter with antigen can be sufficient to induce a complex differentiation program. The developmental process continues without further antigen stimulation and is not interrupted by absence of antigen until memory is formed. The exact characteristics of the APC and other accessory cells necessary to initiate memory generation are not known.
Lineage relationships between memory cells The diversity of the CD8+ T cell subsets and the markers used for their discrimination complicates the analysis of lineage relationships between TCM, TEM and TEC. Two contradictory models have been proposed. According to the ‘linear differentiation model’, memory T cells develop from naı¨ve cells in a continuum from TEC ! TEM ! TCM [9]. According to the ‘progressive differentiation model’, differentiation depends upon applied signal strength early in the immune response. During the priming period, naı¨ve antigen-reactive CD8+ T cells do not receive stimuli of identical ‘strength’, a term subsuming a composition of TCR–MHC/peptide affinity, antigen concentration, and access to co-stimulatory molecules and cytokines. As a consequence, the cells reach different activation stages that define their phenotype and fate [32–34]. The linear differentiation model
The linear differentiation model relies mainly on data generated by adoptive transfer studies that use LCMVspecific TCR-transgenic CD8+ cells [9]. In this investigation, TEM converted over time to TCM. Unfortunately, the distinction of subsets was solely based on the presence or absence of CD62L, which did not allow discrimination between TEC and TEM. The same group used this experimental setting for gene expression analysis and, from these data, extrapolated support for the interpretation of late memory generation after the effector phase [35]. However, the analysis was limited to the spleen, and migration of distinct subsets to other organs was not controlled. In the non-infectious minor H antigen model only a fraction of TEM appeared to be capable of converting into TCM after transfer. This conversion was restricted to phenotypical (CD62L expression) properties; functional conversion could not be demonstrated [36]. A recent study on human peripheral-blood mononuclear cells claimed to have identified conversion of about 6% of the TEM to TCM after in vitro stimulation, on the basis of CCR7 expression and T cell function [37]. However, because unsorted peripheral-blood mononuclear cells consist of cell populations that have mixed Current Opinion in Immunology 2006, 18:458–464
antigen specificities and unknown activation histories, interpretations of such data are complex. The progressive differentiation model
In the Listeria and LCMV infection models, naı¨ve cells (CD127highCD62Lhigh) transiently down-regulate CD127 early after activation, thereby remaining CD62Lhigh. This intermediate subset (Tim) can directly give rise to memory cells, without detectable transition via other subsets such as TEC [19] (Busch and co-workers, unpublished; see Update). Prolonged presence of antigen stimulation resulted in development of TEC from Tim, whereas short antigen stimulation increased the fraction of memory cells [19]. It was proposed from data obtained in in vitro studies on human T cells that TCM are the source of TEM and TEC [3]. Analyses of endogenous CD8+ T cell responses to Listeria demonstrated extensive proliferation of TCM together with conversion to TEM and TEC after restimulation in vitro and in vivo; TEM proliferated poorly and became TEC but did not convert to TCM [13] (Busch and co-workers, unpublished; see Update). CD4+ T cell help, which is provided by CD40L–CD40 interactions [38], is important for memory generation [28–30]. Mice deficient in CD4+ T cells or in CD40L have a defect in generation of TEM [13]. The observation that TCM are found in normal numbers in these mice further indicates the unlikelihood that TCM are preferentially generated from TEM. In this context it is important to stress that the number of CD8+ TEC generated during primary immune responses does not necessarily correlate with the number of memory cells, indicating that generation of memory cells from TEC is not a physiological event. For example, mice treated with ampicillin early during Listeria infection showed reduced overall numbers of Listeria-specific cells during the effector phase; however, the number of memory cells generated (based on CD127 expression seven days after primary infection and bacterial clearance after secondary challenge) was nearly equal in antibiotic-treated and -untreated animals [39]. In a different set of experiments, induction of preferentially short-living TEC could be achieved by providing a strong adjuvant (CpG) together with Listeria infection. This protocol leads to impaired long-term protection against challenge with Listeria (Busch and co-workers, unpublished; see Update). Which model is correct?
The controversies regarding different models of T cell subset differentiation might be best explained by intrinsic experimental problems. The results of a recent study showed similarities to conversion of TEM to TCM only after transfer of unphysiologically high numbers of TCRtransgenic T cells; endogenous TEM or low numbers of transferred TEM demonstrated a stable phenotype [40]. www.sciencedirect.com
Protective immunity towards intracellular pathogens Huster, Stemberger and Busch 461
Figure 2
populations is crucial. TCM exhibit significant recall proliferative potential and are therefore seen by several investigators to be the most important population that confers long-lasting protective immunity against infection [9]. However, proliferative capacity does not necessarily correlate with protection: for example, after immunization with heat-killed Listeria, antigen-specific CD62Lhigh TCM-like cells are induced and expand vigorously when challenged with live Listeria, but they are unable to protect against the infection as determined by clearance of the pathogen [27]. Indirect evidence concerning the special importance of cells that have immediate effector function came from analyses of CD8+ T cell responses in CD4+ T cell helpdeficient mice, in which exclusively the number of TEM is reduced. The subpopulation of TCM is not impaired in these mice, but it is obviously not sufficient for protection against infection with rapidly replicating pathogens such as Listeria [13]. The defect in CD4+ T cell help-deficient mice can be restored by application of anti-CD40 stimulatory antibodies during the initial phase of primary infection (Busch and co-workers, unpublished). Interestingly, a recent study demonstrated that antigen-specific TCM can confer protection against LCMV infection but not against vaccinia virus. TEM, by contrast, protected against both types of virus [19].
Proposed lineage relationship of CD8+ T cell subsets. Dashed lines indicate potential differentiation pathways, which have so far not been elucidated.
Most studies in which the results support the linear differentiation model used relatively high numbers of TCR-transgenic T cells [9,35]. In addition, adoptive transfer experiments are further complicated by the fact that CD8+ T cell subsets rely on different conditions for survival. Survival of TCM after adoptive transfer is supported by their superior capacity for homeostatic proliferation. Thus, small contaminating numbers of TCM within sorted memory T cell subsets can disturb the result of experiments. TEM survive adoptive transfer poorly and, in addition, can persist as untouched cells for months to years in situ in peripheral organs; it is possible that adoptively transferred TEM do not reach their niches fast enough and are dependent upon survival factors not provided sufficiently in the blood. In conclusion, the more recent literature supports the progressive differentiation model as the preferred explanation of how and when memory subsets are generated early after priming (Figure 2).
Protective capacity of TCM and TEM For the generation of T cell-based vaccines, knowledge about the protective capacity of different memory www.sciencedirect.com
It is of note that LCMV is a pathogen that replicates rather slowly in lymphoid organs. This behavior is likely to provide TCM with the time and a suitable environment to develop into effector cells. By contrast, vaccinia virus replicates rapidly in peripheral organs (ovaries). In this situation, protection relies mostly on immediate effector functions, and the functional transition from TCM into protective TEM or TEC cannot occur quickly enough or is not possible at all outside of lymphatic tissues. Thus, immediate protection against rapidly replicating peripheral pathogens seems to crucially depend upon the presence of sufficient numbers of long-living effector cells, in particular those that reside at the specific entry site of the pathogen [13,41]. Protection against slowly replicating pathogens, however, can also be fulfilled by TCM [9,41]. Indeed, it was proposed in earlier studies that antigenspecific cells first proliferate in lymph nodes and subsequently migrate to the lung [42]. Animals that had CD62Lhigh memory cells did not reduce influenza virus titers on day four after challenge compared with naı¨ve animals upon primary virus challenge; they first proliferated in lymph nodes and then appeared as effector cells in the lung [43]. Another study even correlated protection against two viruses with the presence of TEM in lung: influenza-specific CD8+ T cells vanished within several months, which correlated with reduced protection. By contrast, Sendai virus-specific T cells persisted for at least 12 months, which correlates with maintenance of efficient protection [44]. Current Opinion in Immunology 2006, 18:458–464
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Figure 3
A model that illustrates the inverse correlation between longevity and protective capacity of antigen-specific CD8+ T cell subsets. Black arrows indicate pathways that have been experimentally proved. The dashed line indicates a potential additional differentiation pathway, which has so far not been elucidated.
TEM can survive and reside in peripheral organs of mice for as long as one year after infection [4]. However, in some tissues, the number of antigen-specific TEM has been shown to decline over a period of one year with only slow and insufficient replacement by proliferating cells [45]. Roberts et al. [46] used a double transfer system to demonstrate that TEM (CD62Llow) dominate during early time points after infection, whereas TCM (CD62Lhigh) dominate at later time points during the memory phase (20 months). This change in subpopulation prevalence resulted in substantially increased recall expansion, which correlates with the persistence of TCM. Unfortunately, protection against infection was not tested in this study. In humans, TEM appear to have an even shorter half-life of about 10 years, which is equivalent to 100–150 days in mice [47]. The specific factors required for long-term maintenance of TEM are not currently known, but TEM might depend not only on specific survival signals (e.g. anti-apoptotic molecules or response to survival cytokines) optimally provided in their resident niche but also on competition for space with other TEM. It is well known that consecutive infections with different pathogens can affect the quality of protection against infections encountered longer ago [48,49]. This phenomenon was originally explained by active deletion; however, competition for space and survival factors within the overall pool of TEM might represent an even more important regulator.
Conclusions In the past five years, substantial progress has been made in our understanding of the in vivo differentiation of CD8+ T cells and the specific roles of subsets in conferring protection. Lineage decisions take place primarily early during the immune response. TEC are the principal mediators of protection early during primary infection, but they are deleted rapidly from the repertoire. TEM are guardians that reside in peripheral organs, able to fight immediately Current Opinion in Immunology 2006, 18:458–464
against invading pathogens. Their longevity seems to be mostly dependent upon successful competition for space and for survival factors. Determination of the exact rules for survival hierarchies of TEM will be an important task for future research in this field. If the ‘firewall’ of TEM fails to combat a pathogen, antigen gets access to lymphoid organs in which TCM are waiting as a long-term reserve. TCM also represent the crucial resource for extensive antigen-specific recall expansion and development of novel effector cells. Critically viewed, neither TEM nor TCM fulfill both criteria of memory: longevity and rapid activation of effector functions (Figure 3). TEM, though required for protection, are not necessarily maintained over time and can become a short-living cell population. Although TCM seem to clearly represent a long-term persisting subpopulation, they can exert effector functions only after successful transition into other subsets and, therefore, primarily confer only limited or pathogen-dependent protective immunity. The heterogeneity of memory T cells helps to explain why several vaccination procedures, in particular the use of inactivated pathogens or split vaccines made from whole organisms or recombinant antigens, induce protection inefficiently despite clearly demonstrable generation of antigen-specific memory T cells. Therefore, our understanding of the exact mechanisms required for the in vivo generation of distinct memory T cell subtypes will become the crucial rationale for enhancing the efficiency and/or quality of vaccine-induced protective immunity.
Update The study cited in the main body of text as Busch and coworkers, unpublished, has now been accepted for publication [50]. www.sciencedirect.com
Protective immunity towards intracellular pathogens Huster, Stemberger and Busch 463
Acknowledgements This work was supported by the Sonderforschungsbereich (SFB) 576 (Teilprojekt A8) and a Gerhard Hess fellowship (DHB). KMH was supported by a Hochschul- und Wissenschaftsprogramm (HWPII).
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: of special interest of outstanding interest 1.
Busch DH, Pilip IM, Vijh S, Pamer EG: Coordinate regulation of complex T cell populations responding to bacterial infection. Immunity 1998, 8:353-362.
2.
Ahmed R, Gray D: Immunological memory and protective immunity: understanding their relation. Science 1996, 272:54-60.
3.
Sallusto F, Lenig D, Forster R, Lipp M, Lanzavecchia A: Two subsets of memory T lymphocytes with distinct homing potentials and effector functions. Nature 1999, 401:708-712.
4.
5.
Masopust D, Vezys V, Marzo AL, Lefrancois L: Preferential localization of effector memory cells in nonlymphoid tissue. Science 2001, 291:2413-2417. Willinger T, Freeman T, Hasegawa H, McMichael AJ, Callan MF: Molecular signatures distinguish human central memory from effector memory CD8 T cell subsets. J Immunol 2005, 175:5895-5903.
6.
Unsoeld H, Krautwald S, Voehringer D, Kunzendorf U, Pircher H: Cutting edge: CCR7+ and CCR7S memory T cells do not differ in immediate effector cell function. J Immunol 2002, 169:638-641.
7.
Unsoeld H, Pircher H: Complex memory T-cell phenotypes revealed by coexpression of CD62L and CCR7. J Virol 2005, 79:4510-4513.
8.
9.
Kursar M, Hopken UE, Koch M, Kohler A, Lipp M, Kaufmann SH, Mittrucker HW: Differential requirements for the chemokine receptor CCR7 in T cell activation during Listeria monocytogenes infection. J Exp Med 2005, 201:1447-1457. Wherry EJ, Teichgraber V, Becker TC, Masopust D, Kaech SM, Antia R, von Andrian UH, Ahmed R: Lineage relationship and protective immunity of memory CD8 T cell subsets. Nat Immunol 2003, 4:225-234.
expression distinguishes functional subsets of virus-specific human CD8+ T cells. Blood 2005, 106:2091-2098. 17. Fuller MJ, Hildeman DA, Sabbaj S, Gaddis DE, Tebo AE, Shang L, Goepfert PA, Zajac AJ: Cutting edge: emergence of CD127high functionally competent memory T cells is compromised by high viral loads and inadequate T cell help. J Immunol 2005, 174:5926-5930. 18. Prlic M, Lefrancois L, Jameson SC: Multiple choices: regulation of memory CD8 T cell generation and homeostasis by interleukin (IL)-7 and IL-15. J Exp Med 2002, 195:F49-F52. 19. Bachmann MF, Wolint P, Schwarz K, Jager P, Oxenius A: Functional properties and lineage relationship of CD8+ T cell subsets identified by expression of IL-7 receptor a and CD62L. J Immunol 2005, 175:4686-4696. Using CD127 and CD62L as T cell subset lineage markers, the authors deduced a clear hierarchical order of subset occurrence upon in vivo activation. 20. Wong P, Pamer EG: Cutting edge: antigen-independent CD8 T cell proliferation. J Immunol 2001, 166:5864-5868. 21. van Stipdonk MJ, Lemmens EE, Schoenberger SP: Naı¨ve CTLs require a single brief period of antigenic stimulation for clonal expansion and differentiation. Nat Immunol 2001, 2:423-429. 22. Mercado R, Vijh S, Allen SE, Kerksiek K, Pilip IM, Pamer EG: Early programming of T cell populations responding to bacterial infection. J Immunol 2000, 165:6833-6839. 23. Kaech SM, Ahmed R: Memory CD8+ T cell differentiation: initial antigen encounter triggers a developmental program in naı¨ve cells. Nat Immunol 2001, 2:415-422. 24. Jacob J, Baltimore D: Modelling T-cell memory by genetic marking of memory T cells in vivo. Nature 1999, 399:593-597. 25. Busch D, Kerksiek K, Pamer E: Differing roles of inflammation and antigen in T cell proliferation and memory generation. J Immunol 2000, 164:4063-4070. 26. Wong P, Lara-Tejero M, Ploss A, Leiner I, Pamer EG: Rapid development of T cell memory. J Immunol 2004, 172:7239-7245. 27. Lauvau G, Vijh S, Kong P, Horng T, Kerksiek K, Serbina N, Tuma RA, Pamer EG: Priming of memory but not effector CD8 T cells by a killed bacterial vaccine. Science 2001, 294:1735-1739. 28. Janssen EM, Lemmens EE, Wolfe T, Christen U, von Herrath MG, Schoenberger SP: CD4+ T cells are required for secondary expansion and memory in CD8+ T lymphocytes. Nature 2003, 421:852-856.
10. Madakamutil LT, Christen U, Lena CJ, Wang-Zhu Y, Attinger A, Sundarrajan M, Ellmeier W, von Herrath MG, Jensen P, Littman DR et al.: CD8aa-mediated survival and differentiation of CD8 memory T cell precursors. Science 2004, 304:590-593.
29. Sun JC, Bevan MJ: Defective CD8 T cell memory following acute infection without CD4 T cell help. Science 2003, 300:339342.
11. Chandele A, Kaech SM: Cutting edge: memory CD8 T cell maturation occurs independently of CD8aa. J Immunol 2005, 175:5619-5623.
30. Shedlock DJ, Shen H: Requirement for CD4 T cell help in generating functional CD8 T cell memory. Science 2003, 300:337-339.
12. Zhong W, Reinherz EL: CD8aa homodimer expression and role in CD8 T cell memory generation during influenza virus A infection in mice. Eur J Immunol 2005, 35:3103-3110.
31. Smith CM, Wilson NS, Waithman J, Villadangos JA, Carbone FR, Heath WR, Belz GT: Cognate CD4+ T cell licensing of dendritic cells in CD8+ T cell immunity. Nat Immunol 2004, 5:1143-1148.
13. Huster KM, Busch V, Schiemann M, Linkemann K, Kerksiek KM, Wagner H, Busch DH: Selective expression of IL-7 receptor on memory T cells identifies early CD40L-dependent generation of distinct CD8+ memory T cell subsets. Proc Natl Acad Sci USA 2004, 101:5610-5615.
32. Lang KS, Recher M, Navarini AA, Harris NL, Lohning M, Junt T, Probst HC, Hengartner H, Zinkernagel RM: Inverse correlation between IL-7 receptor expression and CD8 T cell exhaustion during persistent antigen stimulation. Eur J Immunol 2005, 35:738-745.
14. Kaech SM, Tan JT, Wherry EJ, Konieczny BT, Surh CD, Ahmed R: Selective expression of the interleukin 7 receptor identifies effector CD8 T cells that give rise to long-lived memory cells. Nat Immunol 2003, 4:1191-1198.
33. Lanzavecchia A, Sallusto F: Progressive differentiation and selection of the fittest in the immune response. Nat Rev Immunol 2002, 2:982-987.
15. Paiardini M, Cervasi B, Albrecht H, Muthukumar A, Dunham R, Gordon S, Radziewicz H, Piedimonte G, Magnani M, Montroni M et al.: Loss of CD127 expression defines an expansion of effector CD8+ T cells in HIV-infected individuals. J Immunol 2005, 174:2900-2909. 16. van Leeuwen EM, de Bree GJ, Remmerswaal EB, Yong SL, Tesselaar K, ten Berge IJ, van Lier RA: IL-7 receptor a chain www.sciencedirect.com
34. Gett AV, Sallusto F, Lanzavecchia A, Geginat J: T cell fitness determined by signal strength. Nat Immunol 2003, 4:355-360. 35. Kaech SM, Hemby S, Kersh E, Ahmed R: Molecular and functional profiling of memory CD8 T cell differentiation. Cell 2002, 111:837-851. 36. Bouneaud C, Garcia Z, Kourilsky P, Pannetier C: Lineage relationships, homeostasis, and recall capacities of Current Opinion in Immunology 2006, 18:458–464
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central- and effector-memory CD8 T cells in vivo. J Exp Med 2005, 201:579-590. 37. Schwendemann J, Choi C, Schirrmacher V, Beckhove P: Dynamic differentiation of activated human peripheral blood CD8+ and CD4+ effector memory T cells. J Immunol 2005, 175:1433-1439. 38. Schoenberger SP, Toes RE, van der Voort EI, Offringa R, Melief CJ: T-cell help for cytotoxic T lymphocytes is mediated by CD40–CD40L interactions. Nature 1998, 393:480-483. 39. Badovinac VP, Porter BB, Harty JT: CD8+ T cell contraction is controlled by early inflammation. Nat Immunol 2004, 5:809-817. 40. Marzo AL, Klonowski KD, Le Bon A, Borrow P, Tough DF, Lefrancois L: Initial T cell frequency dictates memory CD8+ T cell lineage commitment. Nat Immunol 2005, 6:793-799. This important study demonstrates experimental caveats when using adoptive transfer of TCR-transgenic T cells for T cell differentiation studies. 41. Bachmann MF, Wolint P, Schwarz K, Oxenius A: Recall proliferation potential of memory CD8+ T cells and antiviral protection. J Immunol 2005, 175:4677-4685. This report shows that, in most cases, TEM are required for efficient protective immunity. TCM only provide additional protection if the pathogen slowly replicates within lymphoid organs. 42. Flynn KJ, Belz GT, Altman JD, Ahmed R, Woodland DL, Doherty PC: Virus-specific CD8+ T cells in primary and secondary Influenza pneumonia. Immunity 1998, 8:683-691. 43. Cerwenka A, Morgan TM, Dutton RW: Naı¨ve, effector, and memory CD8 T cells in protection against pulmonary influenza virus infection: homing properties rather than initial frequencies are crucial. J Immunol 1999, 163:5535-5543.
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44. Hogan RJ, Usherwood EJ, Zhong W, Roberts AA, Dutton RW, Harmsen AG, Woodland DL: Activated antigen-specific CD8+ T cells persist in the lungs following recovery from respiratory virus infections. J Immunol 2001, 166:1813-1822. 45. Hogan RJ, Cauley LS, Ely KH, Cookenham T, Roberts AD, Brennan JW, Monard S, Woodland DL: Long-term maintenance of virus-specific effector memory CD8+ T cells in the lung airways depends on proliferation. J Immunol 2002, 169:4976-4981. 46. Roberts AD, Ely KH, Woodland DL: Differential contributions of central and effector memory T cells to recall responses. J Exp Med 2005, 202:123-133. 47. Hammarlund E, Lewis MW, Hansen SG, Strelow LI, Nelson JA, Sexton GJ, Hanifin JM, Slifka MK: Duration of antiviral immunity after smallpox vaccination. Nat Med 2003, 9:1131-1137. 48. Kim SK, Welsh RM: Comprehensive early and lasting loss of memory CD8 T cells and functional memory during acute and persistent viral infections. J Immunol 2004, 172:3139-3150. 49. Selin LK, Lin MY, Kraemer KA, Pardoll DM, Schneck JP, Varga SM, Santolucito PA, Pinto AK, Welsh RM: Attrition of T cell memory: selective loss of LCMV epitope-specific memory CD8 T cells following infections with heterologous viruses. Immunity 1999, 11:733-742. 50. Huster KM, Koffler M, Stemberger C, Schiemann M, Wagner H, Busch DH: Unidirectional development of CD8+ central memory T cells in protective Listeria-specific effector memory T cells. Eur J Immunol 2006. in press. Together with [19,41], this report demonstrates for the first time clear evidence for a linear and unidirectional differentiation pathway of memory T cell subsets, and indicates different requirements of memory T cell subsets for the quality of protective immunity.
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Current Opinion in Immunology 2006, 18:458–464 This review comes from a themed issue on Host-pathogen interactions Edited by Stefen HE Kaufmann and Bruce D Walker Available online 12th June 2006 0952-7915/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. DOI 10.1016/j.coi.2006.05.008
Introduction CD8+ T cells provide immunity to viral, bacterial and protozoal infections. Naı¨ve CD8+ T cells reside mainly in lymphoid tissues; here they can encounter pathogenderived antigen together with appropriate co-stimulation, both of which are delivered by specialized antigen-presenting cells (APCs). This leads to proliferation of antigen-reactive naı¨ve cells, accompanied by phenotypical and functional changes such as acquisition of effector functions and of the ability to home to peripheral tissues. Clonal expansion is followed by substantial cell death, leaving only a small number of antigen-specific memory cells [1]. Memory cells are defined as cells that persist and Current Opinion in Immunology 2006, 18:458–464
rapidly regain effector function when re-exposed to antigen [2]. Induction of memory T cells that have fast protective effector functions represents the basic principle of T cell-based vaccination strategies. In this review, we discuss novel insights into the heterogeneity of memory cells, the time points of memory lineage commitment, and the lineage relationships between memory subpopulations. Most importantly, it has become obvious that different memory subsets play distinct roles in conferring protection.
Heterogeneity of memory T cells It is well accepted that memory T cells display diversity with respect to their effector functions, homing potential and proliferative capacity [3–5]. Short-living effector T cells (TEC) dominate the expansion phase, migrate to peripheral organs and display immediate effector function. Long-living memory cells can be assigned mainly to two classes: effector memory T cells (TEM) and central memory T cells (TCM). TEM preferentially home to peripheral tissues and respond to antigen encounter with immediate effector functions but only poor numeric expansion [4]. TCM home to lymphoid organs and can vigorously expand upon antigen reencounter. Most studies have characterized TCM as cell populations that produce large quantities of IL-2. However, acquisition of full effector functions requires further differentiation signals [4]. The functional diversity of CD8+ T cells complicates their analysis, and much effort has been made to correlate functional properties with phenotypical appearance. Sallusto et al. [3] described that loss of CCR7 expression is accompanied by gain of effector function by human CD8+ T cells. Unfortunately, expression of CCR7 in mice does not correlate similarly with functionally distinct CD8+ subsets [6–8]. Instead, CD62L, which like CCR7 is a molecule important for homing to lymph nodes, can be used to distinguish between TCM (CD62Lhigh) and TEM and TEC (CD62Llow) [9]. However, discrimination between long-living (TEM) and short-living (TEC) CD8+ T cells that have effector function is not possible just by determination of CD62L surface expression. A recent study linked the transient expression of CD8aa homodimer to the generation of memory: mice that lack expression of CD8aa were unable to generate memory to lymphocytic choriomeningitis virus (LCMV) infection [10]. CD8aa impedes MHC class Ia-driven activation by down-modulating T-cell receptor (TCR) signals — a function that might protect cells that have this receptor from activation-induced death. However, the importance www.sciencedirect.com
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Figure 1
Correlation between the phenotype and function of antigen-specific CD8+ T cells. LLO91-99-specific splenic CD8+ T cells were stained with CD62L (y-axis) and with CD127 (x-axis) 12 days after primary infection with L. monocytogenes. Only CD8+ MHC Multimer (H2-Kd/LLO91–99)-positive cells are shown. T cell subsets were identified by expression of CD62L and CD127: TCM (CD127highCD62Lhigh), TEM (CD127highCD62Llow) and TEC (CD127lowCD62Llow).
of this finding has been questioned by more recent studies in which memory to LCMV [11] and influenza A virus [12] could be generated in animals impaired in CD8aa expression. The IL-7 receptor a-chain (CD127) might be the most reliable marker for the identification of memory cells in mice [13,14] and in humans [15–17] at the present time. IL-7 is needed for survival of memory cells [18]. Combination of CD127 and CD62L allows discrimination between CD8+ subsets that correlate with the functional properties reported for TCM (CD127highCD62Lhigh), TEM (CD127highCD62Llow) and TEC (CD127lowCD62Llow) [13,19] (Figure 1).
Time point of commitment to memory lineage Much effort has been applied to understand the differentiation pathway from naı¨ve to memory CD8+ T cells. The time point of memory induction is especially important for the identification of memory-determining conditions or factors. Activation and expansion of CD8+ cells can be initiated by brief exposure to antigen: in vitro, the presence of antigen for 2 hours is already sufficient to induce proliferation [20,21], whereas as few as 24 hours of antigen exposure in vivo are sufficient to induce proliferation and acquisition of effector [21] and memory characteristics of antigen-specific CD8+ populations [22,23]. www.sciencedirect.com
The first evidence for the early presence of memory cells during the initial priming period came from an activationdependent transgenic reporter gene expression model, in which a small number of reporter-positive T cells were present at the peak of the immune response and persisted over time as memory cells [24]. In accordance with these findings, stable numbers of memory cells were detected starting from day four after infection with Listeria monocytogenes or LCMV when using CD127 as a memory marker [13,14]. The first functional evidence for the early presence of memory cells also came from experiments in the Listeria infection model. Re-infection as early as five days after the primary infection resulted in typical memory expansion of antigen-specific T cells. These cells were as efficient in providing protection against subsequent challenge with Listeria as memory cells generated late after primary infection [25,26]. Although it is now widely accepted that T cells acquire a differentiation program shortly after activation, the program is not absolutely independent of extrinsic factors. The dose of antigen, for example, can influence the number of naı¨ve precursor T cells recruited into the immune response and can thus affect the overall burst size during the effector phase [23]. In addition, the degree and the quality of innate immune system stimulation can substantially affect T cell priming and differentiation Current Opinion in Immunology 2006, 18:458–464
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[27]. For example, CD4+ T cells provide help by activating dendritic cells. This help is required during primary infection for efficient memory generation [28–30]. Consistent with the observation of early programming of memory cells, CD4+ T cell help for efficient stimulation is rapid and transient, with a time frame of between 24 and 36 hours [31]. In conclusion, a brief encounter with antigen can be sufficient to induce a complex differentiation program. The developmental process continues without further antigen stimulation and is not interrupted by absence of antigen until memory is formed. The exact characteristics of the APC and other accessory cells necessary to initiate memory generation are not known.
Lineage relationships between memory cells The diversity of the CD8+ T cell subsets and the markers used for their discrimination complicates the analysis of lineage relationships between TCM, TEM and TEC. Two contradictory models have been proposed. According to the ‘linear differentiation model’, memory T cells develop from naı¨ve cells in a continuum from TEC ! TEM ! TCM [9]. According to the ‘progressive differentiation model’, differentiation depends upon applied signal strength early in the immune response. During the priming period, naı¨ve antigen-reactive CD8+ T cells do not receive stimuli of identical ‘strength’, a term subsuming a composition of TCR–MHC/peptide affinity, antigen concentration, and access to co-stimulatory molecules and cytokines. As a consequence, the cells reach different activation stages that define their phenotype and fate [32–34]. The linear differentiation model
The linear differentiation model relies mainly on data generated by adoptive transfer studies that use LCMVspecific TCR-transgenic CD8+ cells [9]. In this investigation, TEM converted over time to TCM. Unfortunately, the distinction of subsets was solely based on the presence or absence of CD62L, which did not allow discrimination between TEC and TEM. The same group used this experimental setting for gene expression analysis and, from these data, extrapolated support for the interpretation of late memory generation after the effector phase [35]. However, the analysis was limited to the spleen, and migration of distinct subsets to other organs was not controlled. In the non-infectious minor H antigen model only a fraction of TEM appeared to be capable of converting into TCM after transfer. This conversion was restricted to phenotypical (CD62L expression) properties; functional conversion could not be demonstrated [36]. A recent study on human peripheral-blood mononuclear cells claimed to have identified conversion of about 6% of the TEM to TCM after in vitro stimulation, on the basis of CCR7 expression and T cell function [37]. However, because unsorted peripheral-blood mononuclear cells consist of cell populations that have mixed Current Opinion in Immunology 2006, 18:458–464
antigen specificities and unknown activation histories, interpretations of such data are complex. The progressive differentiation model
In the Listeria and LCMV infection models, naı¨ve cells (CD127highCD62Lhigh) transiently down-regulate CD127 early after activation, thereby remaining CD62Lhigh. This intermediate subset (Tim) can directly give rise to memory cells, without detectable transition via other subsets such as TEC [19] (Busch and co-workers, unpublished; see Update). Prolonged presence of antigen stimulation resulted in development of TEC from Tim, whereas short antigen stimulation increased the fraction of memory cells [19]. It was proposed from data obtained in in vitro studies on human T cells that TCM are the source of TEM and TEC [3]. Analyses of endogenous CD8+ T cell responses to Listeria demonstrated extensive proliferation of TCM together with conversion to TEM and TEC after restimulation in vitro and in vivo; TEM proliferated poorly and became TEC but did not convert to TCM [13] (Busch and co-workers, unpublished; see Update). CD4+ T cell help, which is provided by CD40L–CD40 interactions [38], is important for memory generation [28–30]. Mice deficient in CD4+ T cells or in CD40L have a defect in generation of TEM [13]. The observation that TCM are found in normal numbers in these mice further indicates the unlikelihood that TCM are preferentially generated from TEM. In this context it is important to stress that the number of CD8+ TEC generated during primary immune responses does not necessarily correlate with the number of memory cells, indicating that generation of memory cells from TEC is not a physiological event. For example, mice treated with ampicillin early during Listeria infection showed reduced overall numbers of Listeria-specific cells during the effector phase; however, the number of memory cells generated (based on CD127 expression seven days after primary infection and bacterial clearance after secondary challenge) was nearly equal in antibiotic-treated and -untreated animals [39]. In a different set of experiments, induction of preferentially short-living TEC could be achieved by providing a strong adjuvant (CpG) together with Listeria infection. This protocol leads to impaired long-term protection against challenge with Listeria (Busch and co-workers, unpublished; see Update). Which model is correct?
The controversies regarding different models of T cell subset differentiation might be best explained by intrinsic experimental problems. The results of a recent study showed similarities to conversion of TEM to TCM only after transfer of unphysiologically high numbers of TCRtransgenic T cells; endogenous TEM or low numbers of transferred TEM demonstrated a stable phenotype [40]. www.sciencedirect.com
Protective immunity towards intracellular pathogens Huster, Stemberger and Busch 461
Figure 2
populations is crucial. TCM exhibit significant recall proliferative potential and are therefore seen by several investigators to be the most important population that confers long-lasting protective immunity against infection [9]. However, proliferative capacity does not necessarily correlate with protection: for example, after immunization with heat-killed Listeria, antigen-specific CD62Lhigh TCM-like cells are induced and expand vigorously when challenged with live Listeria, but they are unable to protect against the infection as determined by clearance of the pathogen [27]. Indirect evidence concerning the special importance of cells that have immediate effector function came from analyses of CD8+ T cell responses in CD4+ T cell helpdeficient mice, in which exclusively the number of TEM is reduced. The subpopulation of TCM is not impaired in these mice, but it is obviously not sufficient for protection against infection with rapidly replicating pathogens such as Listeria [13]. The defect in CD4+ T cell help-deficient mice can be restored by application of anti-CD40 stimulatory antibodies during the initial phase of primary infection (Busch and co-workers, unpublished). Interestingly, a recent study demonstrated that antigen-specific TCM can confer protection against LCMV infection but not against vaccinia virus. TEM, by contrast, protected against both types of virus [19].
Proposed lineage relationship of CD8+ T cell subsets. Dashed lines indicate potential differentiation pathways, which have so far not been elucidated.
Most studies in which the results support the linear differentiation model used relatively high numbers of TCR-transgenic T cells [9,35]. In addition, adoptive transfer experiments are further complicated by the fact that CD8+ T cell subsets rely on different conditions for survival. Survival of TCM after adoptive transfer is supported by their superior capacity for homeostatic proliferation. Thus, small contaminating numbers of TCM within sorted memory T cell subsets can disturb the result of experiments. TEM survive adoptive transfer poorly and, in addition, can persist as untouched cells for months to years in situ in peripheral organs; it is possible that adoptively transferred TEM do not reach their niches fast enough and are dependent upon survival factors not provided sufficiently in the blood. In conclusion, the more recent literature supports the progressive differentiation model as the preferred explanation of how and when memory subsets are generated early after priming (Figure 2).
Protective capacity of TCM and TEM For the generation of T cell-based vaccines, knowledge about the protective capacity of different memory www.sciencedirect.com
It is of note that LCMV is a pathogen that replicates rather slowly in lymphoid organs. This behavior is likely to provide TCM with the time and a suitable environment to develop into effector cells. By contrast, vaccinia virus replicates rapidly in peripheral organs (ovaries). In this situation, protection relies mostly on immediate effector functions, and the functional transition from TCM into protective TEM or TEC cannot occur quickly enough or is not possible at all outside of lymphatic tissues. Thus, immediate protection against rapidly replicating peripheral pathogens seems to crucially depend upon the presence of sufficient numbers of long-living effector cells, in particular those that reside at the specific entry site of the pathogen [13,41]. Protection against slowly replicating pathogens, however, can also be fulfilled by TCM [9,41]. Indeed, it was proposed in earlier studies that antigenspecific cells first proliferate in lymph nodes and subsequently migrate to the lung [42]. Animals that had CD62Lhigh memory cells did not reduce influenza virus titers on day four after challenge compared with naı¨ve animals upon primary virus challenge; they first proliferated in lymph nodes and then appeared as effector cells in the lung [43]. Another study even correlated protection against two viruses with the presence of TEM in lung: influenza-specific CD8+ T cells vanished within several months, which correlated with reduced protection. By contrast, Sendai virus-specific T cells persisted for at least 12 months, which correlates with maintenance of efficient protection [44]. Current Opinion in Immunology 2006, 18:458–464
462 Host-pathogen interactions
Figure 3
A model that illustrates the inverse correlation between longevity and protective capacity of antigen-specific CD8+ T cell subsets. Black arrows indicate pathways that have been experimentally proved. The dashed line indicates a potential additional differentiation pathway, which has so far not been elucidated.
TEM can survive and reside in peripheral organs of mice for as long as one year after infection [4]. However, in some tissues, the number of antigen-specific TEM has been shown to decline over a period of one year with only slow and insufficient replacement by proliferating cells [45]. Roberts et al. [46] used a double transfer system to demonstrate that TEM (CD62Llow) dominate during early time points after infection, whereas TCM (CD62Lhigh) dominate at later time points during the memory phase (20 months). This change in subpopulation prevalence resulted in substantially increased recall expansion, which correlates with the persistence of TCM. Unfortunately, protection against infection was not tested in this study. In humans, TEM appear to have an even shorter half-life of about 10 years, which is equivalent to 100–150 days in mice [47]. The specific factors required for long-term maintenance of TEM are not currently known, but TEM might depend not only on specific survival signals (e.g. anti-apoptotic molecules or response to survival cytokines) optimally provided in their resident niche but also on competition for space with other TEM. It is well known that consecutive infections with different pathogens can affect the quality of protection against infections encountered longer ago [48,49]. This phenomenon was originally explained by active deletion; however, competition for space and survival factors within the overall pool of TEM might represent an even more important regulator.
Conclusions In the past five years, substantial progress has been made in our understanding of the in vivo differentiation of CD8+ T cells and the specific roles of subsets in conferring protection. Lineage decisions take place primarily early during the immune response. TEC are the principal mediators of protection early during primary infection, but they are deleted rapidly from the repertoire. TEM are guardians that reside in peripheral organs, able to fight immediately Current Opinion in Immunology 2006, 18:458–464
against invading pathogens. Their longevity seems to be mostly dependent upon successful competition for space and for survival factors. Determination of the exact rules for survival hierarchies of TEM will be an important task for future research in this field. If the ‘firewall’ of TEM fails to combat a pathogen, antigen gets access to lymphoid organs in which TCM are waiting as a long-term reserve. TCM also represent the crucial resource for extensive antigen-specific recall expansion and development of novel effector cells. Critically viewed, neither TEM nor TCM fulfill both criteria of memory: longevity and rapid activation of effector functions (Figure 3). TEM, though required for protection, are not necessarily maintained over time and can become a short-living cell population. Although TCM seem to clearly represent a long-term persisting subpopulation, they can exert effector functions only after successful transition into other subsets and, therefore, primarily confer only limited or pathogen-dependent protective immunity. The heterogeneity of memory T cells helps to explain why several vaccination procedures, in particular the use of inactivated pathogens or split vaccines made from whole organisms or recombinant antigens, induce protection inefficiently despite clearly demonstrable generation of antigen-specific memory T cells. Therefore, our understanding of the exact mechanisms required for the in vivo generation of distinct memory T cell subtypes will become the crucial rationale for enhancing the efficiency and/or quality of vaccine-induced protective immunity.
Update The study cited in the main body of text as Busch and coworkers, unpublished, has now been accepted for publication [50]. www.sciencedirect.com
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Acknowledgements This work was supported by the Sonderforschungsbereich (SFB) 576 (Teilprojekt A8) and a Gerhard Hess fellowship (DHB). KMH was supported by a Hochschul- und Wissenschaftsprogramm (HWPII).
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: of special interest of outstanding interest 1.
Busch DH, Pilip IM, Vijh S, Pamer EG: Coordinate regulation of complex T cell populations responding to bacterial infection. Immunity 1998, 8:353-362.
2.
Ahmed R, Gray D: Immunological memory and protective immunity: understanding their relation. Science 1996, 272:54-60.
3.
Sallusto F, Lenig D, Forster R, Lipp M, Lanzavecchia A: Two subsets of memory T lymphocytes with distinct homing potentials and effector functions. Nature 1999, 401:708-712.
4.
5.
Masopust D, Vezys V, Marzo AL, Lefrancois L: Preferential localization of effector memory cells in nonlymphoid tissue. Science 2001, 291:2413-2417. Willinger T, Freeman T, Hasegawa H, McMichael AJ, Callan MF: Molecular signatures distinguish human central memory from effector memory CD8 T cell subsets. J Immunol 2005, 175:5895-5903.
6.
Unsoeld H, Krautwald S, Voehringer D, Kunzendorf U, Pircher H: Cutting edge: CCR7+ and CCR7S memory T cells do not differ in immediate effector cell function. J Immunol 2002, 169:638-641.
7.
Unsoeld H, Pircher H: Complex memory T-cell phenotypes revealed by coexpression of CD62L and CCR7. J Virol 2005, 79:4510-4513.
8.
9.
Kursar M, Hopken UE, Koch M, Kohler A, Lipp M, Kaufmann SH, Mittrucker HW: Differential requirements for the chemokine receptor CCR7 in T cell activation during Listeria monocytogenes infection. J Exp Med 2005, 201:1447-1457. Wherry EJ, Teichgraber V, Becker TC, Masopust D, Kaech SM, Antia R, von Andrian UH, Ahmed R: Lineage relationship and protective immunity of memory CD8 T cell subsets. Nat Immunol 2003, 4:225-234.
expression distinguishes functional subsets of virus-specific human CD8+ T cells. Blood 2005, 106:2091-2098. 17. Fuller MJ, Hildeman DA, Sabbaj S, Gaddis DE, Tebo AE, Shang L, Goepfert PA, Zajac AJ: Cutting edge: emergence of CD127high functionally competent memory T cells is compromised by high viral loads and inadequate T cell help. J Immunol 2005, 174:5926-5930. 18. Prlic M, Lefrancois L, Jameson SC: Multiple choices: regulation of memory CD8 T cell generation and homeostasis by interleukin (IL)-7 and IL-15. J Exp Med 2002, 195:F49-F52. 19. Bachmann MF, Wolint P, Schwarz K, Jager P, Oxenius A: Functional properties and lineage relationship of CD8+ T cell subsets identified by expression of IL-7 receptor a and CD62L. J Immunol 2005, 175:4686-4696. Using CD127 and CD62L as T cell subset lineage markers, the authors deduced a clear hierarchical order of subset occurrence upon in vivo activation. 20. Wong P, Pamer EG: Cutting edge: antigen-independent CD8 T cell proliferation. J Immunol 2001, 166:5864-5868. 21. van Stipdonk MJ, Lemmens EE, Schoenberger SP: Naı¨ve CTLs require a single brief period of antigenic stimulation for clonal expansion and differentiation. Nat Immunol 2001, 2:423-429. 22. Mercado R, Vijh S, Allen SE, Kerksiek K, Pilip IM, Pamer EG: Early programming of T cell populations responding to bacterial infection. J Immunol 2000, 165:6833-6839. 23. Kaech SM, Ahmed R: Memory CD8+ T cell differentiation: initial antigen encounter triggers a developmental program in naı¨ve cells. Nat Immunol 2001, 2:415-422. 24. Jacob J, Baltimore D: Modelling T-cell memory by genetic marking of memory T cells in vivo. Nature 1999, 399:593-597. 25. Busch D, Kerksiek K, Pamer E: Differing roles of inflammation and antigen in T cell proliferation and memory generation. J Immunol 2000, 164:4063-4070. 26. Wong P, Lara-Tejero M, Ploss A, Leiner I, Pamer EG: Rapid development of T cell memory. J Immunol 2004, 172:7239-7245. 27. Lauvau G, Vijh S, Kong P, Horng T, Kerksiek K, Serbina N, Tuma RA, Pamer EG: Priming of memory but not effector CD8 T cells by a killed bacterial vaccine. Science 2001, 294:1735-1739. 28. Janssen EM, Lemmens EE, Wolfe T, Christen U, von Herrath MG, Schoenberger SP: CD4+ T cells are required for secondary expansion and memory in CD8+ T lymphocytes. Nature 2003, 421:852-856.
10. Madakamutil LT, Christen U, Lena CJ, Wang-Zhu Y, Attinger A, Sundarrajan M, Ellmeier W, von Herrath MG, Jensen P, Littman DR et al.: CD8aa-mediated survival and differentiation of CD8 memory T cell precursors. Science 2004, 304:590-593.
29. Sun JC, Bevan MJ: Defective CD8 T cell memory following acute infection without CD4 T cell help. Science 2003, 300:339342.
11. Chandele A, Kaech SM: Cutting edge: memory CD8 T cell maturation occurs independently of CD8aa. J Immunol 2005, 175:5619-5623.
30. Shedlock DJ, Shen H: Requirement for CD4 T cell help in generating functional CD8 T cell memory. Science 2003, 300:337-339.
12. Zhong W, Reinherz EL: CD8aa homodimer expression and role in CD8 T cell memory generation during influenza virus A infection in mice. Eur J Immunol 2005, 35:3103-3110.
31. Smith CM, Wilson NS, Waithman J, Villadangos JA, Carbone FR, Heath WR, Belz GT: Cognate CD4+ T cell licensing of dendritic cells in CD8+ T cell immunity. Nat Immunol 2004, 5:1143-1148.
13. Huster KM, Busch V, Schiemann M, Linkemann K, Kerksiek KM, Wagner H, Busch DH: Selective expression of IL-7 receptor on memory T cells identifies early CD40L-dependent generation of distinct CD8+ memory T cell subsets. Proc Natl Acad Sci USA 2004, 101:5610-5615.
32. Lang KS, Recher M, Navarini AA, Harris NL, Lohning M, Junt T, Probst HC, Hengartner H, Zinkernagel RM: Inverse correlation between IL-7 receptor expression and CD8 T cell exhaustion during persistent antigen stimulation. Eur J Immunol 2005, 35:738-745.
14. Kaech SM, Tan JT, Wherry EJ, Konieczny BT, Surh CD, Ahmed R: Selective expression of the interleukin 7 receptor identifies effector CD8 T cells that give rise to long-lived memory cells. Nat Immunol 2003, 4:1191-1198.
33. Lanzavecchia A, Sallusto F: Progressive differentiation and selection of the fittest in the immune response. Nat Rev Immunol 2002, 2:982-987.
15. Paiardini M, Cervasi B, Albrecht H, Muthukumar A, Dunham R, Gordon S, Radziewicz H, Piedimonte G, Magnani M, Montroni M et al.: Loss of CD127 expression defines an expansion of effector CD8+ T cells in HIV-infected individuals. J Immunol 2005, 174:2900-2909. 16. van Leeuwen EM, de Bree GJ, Remmerswaal EB, Yong SL, Tesselaar K, ten Berge IJ, van Lier RA: IL-7 receptor a chain www.sciencedirect.com
34. Gett AV, Sallusto F, Lanzavecchia A, Geginat J: T cell fitness determined by signal strength. Nat Immunol 2003, 4:355-360. 35. Kaech SM, Hemby S, Kersh E, Ahmed R: Molecular and functional profiling of memory CD8 T cell differentiation. Cell 2002, 111:837-851. 36. Bouneaud C, Garcia Z, Kourilsky P, Pannetier C: Lineage relationships, homeostasis, and recall capacities of Current Opinion in Immunology 2006, 18:458–464
464 Host-pathogen interactions
central- and effector-memory CD8 T cells in vivo. J Exp Med 2005, 201:579-590. 37. Schwendemann J, Choi C, Schirrmacher V, Beckhove P: Dynamic differentiation of activated human peripheral blood CD8+ and CD4+ effector memory T cells. J Immunol 2005, 175:1433-1439. 38. Schoenberger SP, Toes RE, van der Voort EI, Offringa R, Melief CJ: T-cell help for cytotoxic T lymphocytes is mediated by CD40–CD40L interactions. Nature 1998, 393:480-483. 39. Badovinac VP, Porter BB, Harty JT: CD8+ T cell contraction is controlled by early inflammation. Nat Immunol 2004, 5:809-817. 40. Marzo AL, Klonowski KD, Le Bon A, Borrow P, Tough DF, Lefrancois L: Initial T cell frequency dictates memory CD8+ T cell lineage commitment. Nat Immunol 2005, 6:793-799. This important study demonstrates experimental caveats when using adoptive transfer of TCR-transgenic T cells for T cell differentiation studies. 41. Bachmann MF, Wolint P, Schwarz K, Oxenius A: Recall proliferation potential of memory CD8+ T cells and antiviral protection. J Immunol 2005, 175:4677-4685. This report shows that, in most cases, TEM are required for efficient protective immunity. TCM only provide additional protection if the pathogen slowly replicates within lymphoid organs. 42. Flynn KJ, Belz GT, Altman JD, Ahmed R, Woodland DL, Doherty PC: Virus-specific CD8+ T cells in primary and secondary Influenza pneumonia. Immunity 1998, 8:683-691. 43. Cerwenka A, Morgan TM, Dutton RW: Naı¨ve, effector, and memory CD8 T cells in protection against pulmonary influenza virus infection: homing properties rather than initial frequencies are crucial. J Immunol 1999, 163:5535-5543.
Current Opinion in Immunology 2006, 18:458–464
44. Hogan RJ, Usherwood EJ, Zhong W, Roberts AA, Dutton RW, Harmsen AG, Woodland DL: Activated antigen-specific CD8+ T cells persist in the lungs following recovery from respiratory virus infections. J Immunol 2001, 166:1813-1822. 45. Hogan RJ, Cauley LS, Ely KH, Cookenham T, Roberts AD, Brennan JW, Monard S, Woodland DL: Long-term maintenance of virus-specific effector memory CD8+ T cells in the lung airways depends on proliferation. J Immunol 2002, 169:4976-4981. 46. Roberts AD, Ely KH, Woodland DL: Differential contributions of central and effector memory T cells to recall responses. J Exp Med 2005, 202:123-133. 47. Hammarlund E, Lewis MW, Hansen SG, Strelow LI, Nelson JA, Sexton GJ, Hanifin JM, Slifka MK: Duration of antiviral immunity after smallpox vaccination. Nat Med 2003, 9:1131-1137. 48. Kim SK, Welsh RM: Comprehensive early and lasting loss of memory CD8 T cells and functional memory during acute and persistent viral infections. J Immunol 2004, 172:3139-3150. 49. Selin LK, Lin MY, Kraemer KA, Pardoll DM, Schneck JP, Varga SM, Santolucito PA, Pinto AK, Welsh RM: Attrition of T cell memory: selective loss of LCMV epitope-specific memory CD8 T cells following infections with heterologous viruses. Immunity 1999, 11:733-742. 50. Huster KM, Koffler M, Stemberger C, Schiemann M, Wagner H, Busch DH: Unidirectional development of CD8+ central memory T cells in protective Listeria-specific effector memory T cells. Eur J Immunol 2006. in press. Together with [19,41], this report demonstrates for the first time clear evidence for a linear and unidirectional differentiation pathway of memory T cell subsets, and indicates different requirements of memory T cell subsets for the quality of protective immunity.
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