Measuring the diaspora for virus-specific CD8+ T cells

Complex memory T-cell phenotypes revealed by coexpression of CD62L and CCR7

Antigen-experienced T cells have been divided into CD62L+ CCR7+ central memory (TCM) and CD62L- CCR7- effector memory (TEM) cells. Here, we examined coexpression of CD62L and CCR7 in lymphocytic choriomeningitis virus-specific memory CD8 T cells from both lymphoid and nonlymphoid tissues. Three main points emerged: firstly, memory cells frequently expressed a mixed CD62L- CCR7+ phenotype that differed from the phenotypes of classical TEM and TCM cells; secondly, TCM cells were not restricted to lymphoid organs but were also present in significant numbers in nonlymphoid tissues; and thirdly, a major shift from a TCM to TEM phenotype was found in memory cells that had been stimulated repetitively with antigen.

Consequences of immunodominant epitope deletion for minor influenza virus-specific CD8+-T-cell responses

The extent to which CD8+ T cells specific for other antigens expand to compensate for the mutational loss of the prominent DbNP366 and DbPA224 epitopes has been investigated using H1N1 and H3N2 influenza A viruses modified by reverse genetics. Significantly increased numbers of CD8+ KbPB1(703)+, CD8+ KbNS2(114)+, and CD8+ DbPB1-F2(62)+ T cells were found in the spleen and in the inflammatory population recovered by bronchoalveolar lavage from mice that were first given the -NP-PA H1N1 virus intraperitoneally and then challenged intranasally with the homologous H3N2 virus. The effect was less consistent when this prime-boost protocol was reversed. Also, though the quality of the response measured by cytokine staining showed some evidence of modification when these minor CD8+-T-cell populations were forced to play a more prominent part, the effects were relatively small and no consistent pattern emerged. The magnitude of the enhanced clonal expansion following secondary challenge suggested that the prime-boost with the -NP-PA viruses gave a response overall that was little different in magnitude from that following comparable exposure to the unmanipulated viruses. This was indeed shown to be the case when the total response was measured by ELISPOT analysis with virus-infected cells as stimulators. More surprisingly, the same effect was seen following primary challenge, though individual analysis of the CD8+ KbPB1(703)+, CD8+ KbNS2(114)+, and CD8+ DbPB1-F2(62)+ sets gave no indication of compensatory expansion. A possible explanation is that novel, as yet undetected epitopes emerge following primary exposure to the -NP-PA deletion viruses. These findings have implications for both natural infections and vaccines.

Effector CD8+ T cells recovered from an influenza pneumonia differentiate to a state of focused gene expression

The restriction of influenza A virus replication to mouse respiratory epithelium means that this host response is anatomically compartmentalized, on the one hand, to sites of T cell stimulation and proliferation in the secondary lymphoid tissue and, on the other hand, to the site of effector T cell function and pathology in the pneumonic lung. Thus, it is hardly surprising that virus-specific CD8(+) T cells recovered by bronchoalveolar lavage (BAL) from the infected respiratory tract seem more "activated" in terms of both cytolytic activity and cytokine production than those cells isolated from the spleen. The present analysis uses Affymetrix microarray technology to compare profiles of gene expression in these two lineage-related, yet anatomically separate, lymphocyte populations. Ninety differentially expressed genes were identified for influenza-specific CD8(+)D(b)NP(366)(+) T cells obtained directly ex vivo by BAL or spleen disruption, with nine genes being further analyzed by quantitative, real-time PCR at the population level. Integrin alphaE, for example, was shown by Affymetrix and real-time mRNA analyses and then by single-cell PCR and protein staining to be present at a much higher prevalence on the BAL CD8(+)D(b)NP(366)(+) set. The unpredicted finding, however, was that mRNA expression for 75% of the 90 genes was lower in T cells from the BAL than from the spleen. Apparently, the localization of virus-specific CD8(+) T cells to the site of virus-induced pathology is associated with a narrowing, or "focusing," of gene expression that favors enhanced effector function in the damaged, "high-antigen load" environment of the pneumonic lung.

Bone marrow is a major reservoir and site of recruitment for central memory CD8+ T cells

Normal bone marrow (BM) contains T cells whose function and origin are poorly understood. We observed that CD8+ T cells in BM consist chiefly of CCR7+ L-selectin+ central memory cells (TCMs). Adoptively transferred TCMs accumulated more efficiently in the BM than naive and effector T cells. Intravital microscopy (IVM) showed that TCMs roll efficiently in BM microvessels via L-, P-, and E-selectin, whereas firm arrest required the VCAM-1/alpha4beta1 pathway. alpha4beta1 integrin activation did not depend on pertussis toxin (PTX)-sensitive Galphai proteins but was reduced by anti-CXCL12. In contrast, TCM diapedesis did not require CXCL12 but was blocked by PTX. After extravasation, TCMs displayed agile movement within BM cavities, remained viable, and mounted potent antigen-specific recall responses for at least two months. Thus, the BM functions as a major reservoir for TCMs by providing specific recruitment signals that act in sequence to mediate the constitutive recruitment of TCMs from the blood.

Bone marrow is a preferred site for homeostatic proliferation of memory CD8 T cells

Proliferative renewal of memory CD8 T cells is essential for maintaining long-term immunity. In this study, we examined the contributions that various tissue microenvironments make toward the homeostatic proliferation of Ag-specific memory CD8 T cells. We found that dividing memory T cells were present in both lymphoid and nonlymphoid tissues. However, the bone marrow was the preferred site for proliferation and contained a major pool of the most actively dividing memory CD8 T cells. Adoptive transfer studies indicated that memory cells migrated through the bone marrow and divided there preferentially. These results show that the bone marrow is not only the source of stem cells for generating naive T cells but also provides the necessary signals for the self-renewal of memory T cells.

Lack of prominent peptide-major histocompatibility complex features limits repertoire diversity in virus-specific CD8+ T cell populations

Using both 'reverse genetics' and structural analysis, we have examined the in vivo relationship between antigenicity and T cell receptor (TCR) repertoire diversity. Influenza A virus infection of C57BL/6 mice induces profoundly different TCR repertoires specific for the nucleoprotein peptide of amino acids 366-374 (NP366) and the acid polymerase peptide of amino acids 224-233 (PA224) presented by H-2D(b). Here we show the H-2D(b)-NP366 complex with a 'featureless' structure selected a limited TCR repertoire characterized by 'public' TCR usage. In contrast, the prominent H-2D(b)-PA224 complex selected diverse, individually 'private' TCR repertoires. Substitution of the arginine at position 7 of PA224 with an alanine reduced the accessible side chains of the epitope. Infection with an engineered virus containing a mutation at the site encoding the exposed arginine at position 7 of PA224 selected a restricted TCR repertoire similar in diversity to that of the H-2D(b)-NP366-specific response. Thus, the lack of prominent features in an antigenic complex of peptide and major histocompatibility complex class I is associated with a diminished spectrum of TCR usage.

The virus-immunity ecosystem

The ecology of pathogenic viruses can be considered both in the context of survival in the macro-environments of nature, the theme pursued generally by epidemiologists, and in the micro-environments of the infected host. The long-lived, complex, higher vertebrates have evolved specialized, adaptive immune systems designed to minimise the consequences of such parasitism. Through evolutionary time, the differential selective pressures exerted variously by the need for virus and host survival have shaped both the "one-host" viruses and vertebrate immunity. With the development of vaccines to protect us from many of our most familiar parasites, the most dangerous pathogens threatening us now tend to be those "emerging", or adventitious, infectious agents that sporadically enter human populations from avian or other wild-life reservoirs. Such incursions must, of course, have been happening through the millenia, and are likely to have led to the extraordinary diversity of recognition molecules, the breadth in effector functions, and the persistent memory that distinguishes the vertebrate, adaptive immune system from the innate response mechanisms that operate more widely through animal biology. Both are important to contemporary humans and, particularly in the period immediately following infection, we still rely heavily on an immediate response capacity, elements of which are shared with much simpler, and more primitive organisms. Perhaps we will now move forward to develop useful therapies that exploit, or mimic, such responses. At this stage, however, most of our hopes for minimizing the threat posed by viruses still focus on the manipulation of the more precisely targeted, adaptive immune system.

Anatomical features of anti-viral immunity in the respiratory tract

The mucosal surfaces of the lungs are a major portal of entry for virus infections and there are urgent needs for new vaccines that promote effective pulmonary immunity. However, we have only a rudimentary understanding of the requirements for effective cellular immunity in the respiratory tract. Recent studies have revealed that specialized cellular immune responses and lymphoid tissues are involved in the protection of distinct anatomical microenvironments of the respiratory tract, such as the large airways of the nose and the alveolar airspaces. This review discusses some of the anatomical features of anti-viral immunity in the respiratory tract including the role of local lymphoid tissues and the relationship between effector and memory T cells in the airways, the lung parenchyma, and lymphoid organs.

Characterization of CD8+ T cell repertoire diversity and persistence in the influenza A virus model of localized, transient infection

Influenza virus infection of C57BL/6 mice provides a well-characterized model for the study of acute CD8(+) T cell responses and for the analysis of memory in the absence of antigen persistence. The advent of tetramer reagents and intracellular cytokine staining, coupled with techniques such as single cell RT-PCR and influenza reverse genetics, has enabled the detailed molecular dissection of different epitope-specific primary, memory and secondary immune CD8(+) T cell responses. The approach offers novel insights into the factors determining the selection of immune repertoires, and their functional consequences for CD8(+) T cell-mediated immunity.

In vivo compartmentalization of functionally distinct, rapidly responsive antigen-specific T-cell populations in DNA-immunized or Salmonella enterica serovar Typhimurium-infected mice

The location and functional properties of antigen-specific memory T-cell populations in lymphoid and nonlymphoid compartments following DNA immunization or infection with Salmonella were investigated. Epitope-specific CD8+ -T-cell expansion and retention during the memory phase were analyzed for DNA-immunized mice by use of a 5-h peptide restimulation assay. These data revealed that epitope-specific gamma interferon (IFN-gamma)-positive CD8+ T cells occur at higher frequencies in the spleen, liver, and blood than in draining or peripheral lymph nodes during the expansion phase. Moreover, this distribution is maintained into long-term memory. The location and function of both CD4+ and CD8+ Salmonella-specific memory T cells in mice who were given a single dose of Salmonella enterica serovar Typhimurium was also quantitated by an ex vivo restimulation with bacterial lysate to detect the total Salmonella-specific memory pool. Mice immunized up to 6 months previously with S. enterica serovar Typhimurium had bacterium-specific CD4+ T cells that were capable of producing IFN-gamma or tumor necrosis factor alpha (TNF-alpha) at each site analyzed. Similar findings were observed for CD8+ T cells that were capable of producing IFN-gamma, while a much lower frequency and more restricted distribution were associated with TNF-alpha-producing CD8+ T cells. This study is the first to assess the frequencies, locations, and functions of both CD4+ and CD8+ memory T-cell populations in the same Salmonella-infected individuals and demonstrates the organ-specific functional compartmentalization of memory T cells after Salmonella infection.

Intravital microscopy: visualizing immunity in context

Recent advances in photonics, particularly multi-photon microscopy (MPM) and new molecular and genetic tools are empowering immunologists to answer longstanding unresolved questions in living animals. Using intravital microscopy (IVM) investigators are dissecting the cellular and molecular underpinnings controlling immune cell motility and interactions in tissues. Recent IVM work showed that T cell responses to antigen in lymph nodes are different from those observed in vitro and appear dictated by factors uniquely relevant to intact organs. Other IVM models, particularly in the bone marrow, reveal how different anatomic contexts regulate leukocyte development, immunity, and inflammation. This article will discuss the current state of the field and outline how IVM can generate new discoveries and serve as a "reality check" for areas of research that were formerly the exclusive domain of in vitro experimentation.

Duration of Infection and Antigen Display Have Minimal Influence on the Kinetics of the CD4+T Cell Response toListeria monocytogenesInfection

The T cell response to infection consists of clonal expansion of effector cells, followed by contraction to memory levels. It was previously thought that the duration of infection determines the magnitude and kinetics of the T cell response. However, recent analysis revealed that transition between the expansion and contraction phases of the Ag-specific CD8+ T cell response is not affected by experimental manipulation in the duration of infection or Ag display. We studied whether the duration of infection and Ag display influenced the kinetics of the Ag-specific CD4+ T cell response to Listeria monocytogenes (LM) infection. We found that truncating infection and Ag display with antibiotic treatment as early as 24 h postinfection had minimal impact on the expansion or contraction of CD4+ T cells; however, the magnitudes of the Ag-specific CD4+ and CD8+ T cell responses were differentially affected by the timing of antibiotic treatment. Treatment of LM-infected mice with antibiotics at 24 h postinfection did not prevent generation of detectable CD4+ and CD8+ memory T cells at 28 days after infection, vigorous secondary expansion of these memory T cells, or protection against a subsequent LM challenge. These results demonstrate that events within the first few days of infection stimulate CD4+ and CD8+ T cell responses that are capable of carrying out the full program of expansion and contraction to functional memory, independently of prolonged infection or Ag display.

Conditions for pathogen elimination by immune systems

A continuous harvest effort can lead a population to extinction. How an "unconscious" immune system would perpetrate such an effort in order to eliminate a self-replicating antigen (a pathogen) becomes an intriguing problem if the system responses are functions of the pathogen population: the responses cannot be a continuous effort as the pathogen vanishes. On theoretical grounds, we show some qualities an immune response must have to support pathogen elimination. Then, three specific mechanisms are addressed: a pathogen-independent positive feedback loop among the responding cells of the system (e.g., B-lymphocyte and T-helper); the persistence of antigen bound to presenting cells; and the programmed expansion/contraction of a pool of responding cells. The maintenance of responding cells due to these mechanisms is the essential feature to the effective clearance of self-replicating agents. Thus, evolutionarily, the primary function of a helper lymphocyte would be to amplify a response and the primary function of memory would be the very elimination of pathogens.

Activation, differentiation, and migration of naive virus-specific CD8+ T cells during pulmonary influenza virus infection

The low precursor frequency of individual virus-specific CD8(+) T cells in a naive host makes the early events of CD8(+) T cell activation, proliferation, and differentiation in response to viral infection a challenge to identify. We have therefore examined the response of naive CD8(+) T cells to pulmonary influenza virus infection with a murine adoptive transfer model using hemagglutinin-specific TCR transgenic CD8(+) T cells. Initial activation of CD8(+) T cells occurs during the first 3 days postinfection exclusively within the draining lymph nodes. Acquisition of CTL effector functions, including effector cytokine and granule-associated protease expression, occurs in the draining lymph nodes and differentially correlates with cell division. Division of activated CD8(+) T cells within the draining lymph nodes occurs in an asynchronous manner between days 3 and 4 postinfection. Despite the presence of Ag for several days within the draining lymph nodes, dividing T cells do not appear to maintain contact with residual Ag. After multiple cell divisions, CD8(+) T cells exit the draining lymph nodes and migrate to the infected lung. Activated CD8(+) T cells also disseminate throughout lymphoid tissue including the spleen and distal lymph nodes following their emigration from draining lymph nodes. These results demonstrate an important role for draining lymph nodes in orchestrating T cell responses during a local infection of a discrete organ to generate effector CD8(+) T cells capable of responding to infection and seeding peripheral lymphoid tissues.

Bcl6 acts as an amplifier for the generation and proliferative capacity of central memory CD8+ T cells

Central memory CD8(+) T cells (T(CM)) are considered to be more efficient than effector ones (T(EM)) for mediating protective immunity. The molecular mechanism involved in the generation of these cells remains elusive. Because Bcl6 plays a role in the generation and maintenance of memory CD8(+) T cells, we further examined this role in the process in relation to T(CM) and T(EM) subsets. In this study, we show that T(CM) and T(EM) were functionally identified in CD62L(+) and CD62L(-) memory (CD44(+)Ly6C(+)) CD8(+) T cell subsets, respectively. Although T(CM) produced similar amounts of IFN-gamma and IL-2 to T(EM) after anti-CD3 stimulation, the cell proliferation capacity after stimulation and tissue distribution profiles of T(CM) differed from those of T(EM). Numbers of T(CM) were greatly reduced and elevated in spleens of Bcl6-deficient and lck-Bcl6 transgenic mice, respectively, and those of T(EM) were constant in nonlymphoid organs of these same mice. The majority of Ag-specific memory CD8(+) T cells in spleens of these mice 10 wk after immunization were T(CM), and the number correlated with Bcl6 expression in T cells. The proliferation of Ag-specific memory CD8(+) T cells upon secondary stimulation was dramatically up-regulated in lck-Bcl6 transgenic mice, and the adoptive transfer experiments with Ag-specific naive CD8(+) T cells demonstrated that some of the up-regulation was due to the intrinsic effect of Bcl6 in the T cells. Thus, Bcl6 is apparently a crucial factor for the generation and secondary expansion of T(CM).

Hierarchies in cytokine expression profiles for acute and resolving influenza virus-specific CD8+ T cell responses: correlation of cytokine profile and TCR avidity

The development and resolution phases of influenza-specific CD8(+) T cell cytokine responses to epitopes derived from the viral nucleoprotein (D(b)NP(366)) and acid polymerase (D(b)PA(224)) were characterized in C57BL/6J mice for a range of anatomical compartments in the virus-infected lung and lymphoid tissue. Lymphocyte numbers were measured by IFN-gamma expression following stimulation with peptide, while the quality of the response was determined by the intensity of staining and the distribution of CD8(+) T cells producing TNF-alpha and IL-2. Both the levels of expression and the prevalence of TNF-alpha(+) and IL-2(+) cells reflected the likely Ag load, with clear differences being identified for populations from the alveolar space vs the lung parenchyma. Irrespective of the site or time of T cell recovery, IL-2(+) cells were consistently found to be a subset of the TNF-alpha(+) population which was, in turn, contained within the IFN-gamma(+) set. The capacity to produce IL-2 may thus be considered to reflect maximum functional differentiation. The hierarchy in cytokine expression throughout the acute phase of the primary and secondary response tended to be D(b)PA(224) > D(b)NP(366). Both elution studies with the cognate tetramers and experiments measuring CD8 beta coreceptor dependence for peptide stimulation demonstrated the same D(b)PA(224) > D(b)NP(366) profile for TCR avidity. Overall, the quality of any virus-specific CD8(+) T cell response appears variously determined by the avidity of the TCR-pMHC interaction, the duration and intensity of Ag stimulation characteristic of the particular tissue environment, and the availability of CD4(+) T help.

Lymphocyte activation gene-3 (CD223) regulates the size of the expanding T cell population following antigen activation in vivo

Lymphocyte activation gene-3 (LAG-3) is a CD4-related, activation-induced cell surface molecule that binds to MHC class II with high affinity. In this study, we used four experimental systems to reevaluate previous suggestions that LAG-3(-/-) mice had no T cell defect. First, LAG-3(-/-) T cells exhibited a delay in cell cycle arrest following in vivo stimulation with the superantigen staphylococcal enterotoxin B resulting in increased T cell expansion and splenomegaly. Second, increased T cell expansion was also observed in adoptive recipients of LAG-3(-/-) OT-II TCR transgenic T cells following in vivo Ag stimulation. Third, infection of LAG-3(-/-) mice with Sendai virus resulted in increased numbers of memory CD4(+) and CD8(+) T cells. Fourth, CD4(+) T cells exhibited a delayed expansion in LAG-3(-/-) mice infected with murine gammaherpesvirus. In summary, these data suggest that LAG-3 negatively regulates T cell expansion and controls the size of the memory T cell pool.

Specifically activated memory T cell subsets from cancer patients recognize and reject xenotransplanted autologous tumors

Bone marrow of breast cancer patients was found to contain CD8(+) T cells specific for peptides derived from breast cancer-associated proteins MUC1 and Her-2/neu. Most of these cells had a central or effector memory phenotype (CD45RA(-)CD62L(+) or CD45RA(-)CD62L(-), respectively). To test their in vivo function, we separated bone marrow-derived CD45RA(+) naive or CD45RA(-)CD45RO(+) memory T cells, stimulated them with autologous dendritic cells pulsed with tumor lysate, and transferred them into NOD/SCID mice bearing autologous breast tumors and normal skin transplants. CD45RA(-) memory but not CD45RA(+) naive T cells infiltrated autologous tumor but not skin tissues after the transfer. These tumor-infiltrating cells had a central or effector memory phenotype and produced perforin. Many of them expressed the P-selectin glycoprotein ligand 1 and were found around P-selectin(+) tumor endothelium. Tumor infiltration included cluster formation in tumor tissue by memory T cells with cotransferred dendritic cells. It was associated with the induction of tumor cell apoptosis and significant tumor reduction. We thus demonstrate selective homing of memory T cells to human tumors and suggest that tumor rejection is based on the recognition of tumor-associated antigens on tumor cells and dendritic cells by autologous specifically activated central and effector memory T cells.

Immunological memory to viral infections

The purpose of immunological memory is to protect the host from reinfection, to control persistent infections, and, through maternal antibody, to protect the host's immunologically immature offspring from primary infections. Immunological memory is an exclusive property of the acquired immune system, where in the presence of CD4 T cell help, T cells and B cells clonally expand and differentiate to provide effector systems that protect the host from pathogens. Here we describe how T and B cell memory is generated in response to virus infections and how these cells respond when the host is infected again by similar or different viruses.

Maintenance, loss, and resurgence of T cell responses during acute, protracted, and chronic viral infections

The acute phase of many viral infections is associated with the induction of a pronounced CD8 T cell response which plays a principle role in clearing the infection. By contrast, certain infections are not as readily controlled. In this study, we have used the well-defined system of lymphocytic choriomeningitis virus (LCMV) infection of mice to determine quantitative and qualitative changes in virus-specific CD8 T cell responses that rapidly resolve acute infections, more slowly control protracted infections, or fail to clear chronic infections. Acute LCMV infection elicits potent, functional, multi-epitope-specific CD8 T cell responses. Virus-specific CD8 T cells also expand, albeit to a lesser extent, during protracted LCMV infection. Under these conditions, there is a progressive diminution in the capacity to produce IL-2, TNF-alpha, and IFN-gamma. Changes in cytotoxic activities are also detectable but differ depending upon the specificity of the responding cells. As the infection is slowly resolved, a resurgence of cytokine production by virus-specific CD8 T cells is observed. CD4-deficient mice cannot control infection with certain strains of LCMV, but do mount multi-epitope-specific CD8 T cell responses that also lose effector capabilities; however, they are not maintained indefinitely in an unresponsive state as these cells become deleted over time. Overall, our findings suggest that constant high viral loads result in the progressive diminution of T cell effector functions and subsequent physical loss of the responding cells, whereas if the viral load is brought under control a partial restoration of CD8 T cell functions can occur.

Activated primary and memory CD8 T cells migrate to nonlymphoid tissues regardless of site of activation or tissue of origin

Following activation within secondary lymphoid tissue, CD8 T cells must migrate to targets, such as infected self tissue, allografts, and tumors, to mediate contact-dependent effector functions. To test whether the pattern of migration of activated CD8 T cells was dependent on the site of Ag encounter, we examined the distribution of mouse Ag-specific CD8 T cells following local challenges. Our findings indicated that activated CD8 T cells migrated pervasively to all nonlymphoid organs irrespective of the site of initial Ag engagement. Using an adoptive transfer system, migration of nonlymphoid memory cells was also examined. Although some limited preference for the tissue of origin was noted, transferred CD8 memory T cells from various nonlymphoid tissues migrated promiscuously, except to the intestinal mucosa, supporting the concept that distinct memory pools may exist. However, regardless of the tissue of origin, reactivation of transferred memory cells resulted in widespread dissemination of new effector cells. These data indicated that recently activated primary or memory CD8 T cells were transiently endowed with the ability to traffic to all nonlymphoid organs, while memory cell trafficking was more restricted. These observations will help refine our understanding of effector and memory CD8 T cell migration patterns.

Cutting Edge: Effector Memory CD8+T Cells Play a Prominent Role in Recall Responses to Secondary Viral Infection in the Lung

The relative contributions of CD62L(high) (central) memory and CD62L(low) (effector) memory T cell populations to recall responses are poorly understood, especially in the respiratory tract. In this study, we took advantage of a dual-adoptive transfer system in the mouse to simultaneously follow the recall response of effector and central memory subpopulations to intranasal parainfluenza virus infection. Using MHC class I and class II multimers, we tracked the responses of Ag-specific CD8(+) and CD4(+) memory T cells in the same animals. The data show that effector memory T cells mounted recall responses that were equal to, or greater than, those mounted by central memory T cells. Moreover, effector memory T cells were more efficient at subsequently establishing a second generation of memory T cells. These data contrast with other studies indicating that central memory CD8(+) T cells are the prominent contributors to systemic virus infections.

Differential tissue-specific regulation of antiviral CD8+ T-cell immune responses during chronic viral infection

The hallmarks of the immune response to viral infections are the expansion of antigen-specific CD8(+) cytotoxic T lymphocytes (CTLs) after they encounter antigen-presenting cells in the lymphoid tissues and their subsequent redistribution to nonlymphoid tissues to deal with the pathogen. Control mechanisms exist within CTL activation pathways to prevent inappropriate CTL responses against disseminating infections with a broad distribution of pathogen in host tissues. This is demonstrated during overwhelming infection with the noncytolytic murine lymphocytic choriomeningitis virus, in which clonal exhaustion (anergy and/or deletion) of CTLs prevents immune-mediated pathology but allows persistence of the virus. The mechanism by which the immune system determines whether or not to mount a full response to such infections is unknown. Here we present data showing that the initial encounter of specific CTLs with infected cells in lymphoid tissues is critical for this decision. Whether the course of the viral infection is acute or persistent for life primarily depends on the degree and kinetics of CTL exhaustion in infected lymphoid tissues. Virus-driven CTL expansion in lymphoid tissues resulted in the migration of large quantities of CTLs to nonlymphoid tissues, where they persisted at stable levels. Surprisingly, although virus-specific CTLs were rapidly clonally exhausted in lymphoid tissues under conditions of chronic infection, a substantial number of them migrated to nonlymphoid tissues, where they retained an effector phenotype for a long time. However, these cells were unable to control the infection and progressively lost their antiviral capacities (cytotoxicity and cytokine secretion) in a hierarchical manner before their eventual physical elimination. These results illustrate the differential tissue-specific regulation of antiviral T-cell responses during chronic infections and may help us to understand the dynamic relationship between antigen and T-cell populations in many persistent infections in humans.

Conserved T cell receptor usage in primary and recall responses to an immunodominant influenza virus nucleoprotein epitope

The CD8+ T cell response to the immunodominant DbNP366 epitope has been analyzed sequentially to determine the prevalence and persistence of different T cell antigen receptor (TCR)Vbeta8.3 clonotypes after primary and secondary influenza virus challenge. Based on the length and amino acid sequences of the complementarity-determining region 3 of TCRbeta (CDR3beta) loop and associated Jbeta usage, the same dominant TCRbeta signatures were found in the blood, the spleen, and the site of virus-induced pathology in the infected respiratory tract. Longitudinal analysis demonstrated that TCRbeta prominent in the antigen-driven phase of response persisted into memory and were again expanded after secondary challenge. A proportion of these high-frequency TCRbeta expressed "public" CDR3beta sequences that were detected in every mouse sampled, whereas others were found more than once but were not invariably present. Analysis of N-region nucleotide diversity established that as many as 10 different nucleic acid sequences (maximum of four "nucleotypes" in any one mouse) could encode a single public TCRbeta amino acid sequence. Conversely, whereas some of the unique, "private" TCRbeta achieved a substantial clone size, they were always specified by a single nucleotype. Although there is a strong stochastic element in this response, the public TCRbeta seem to represent a "best fit" for this immunodominant epitope, are selected preferentially from the naive TCR repertoire, and assume even greater prominence after secondary challenge.

Memories of virus-specific CD8+ T cells

This brief review focuses on the way that our understanding of virus-specific CD8(+) T-cell-mediated immunity evolved, giving particular attention to the early impact of the program at the Australian National University. The story developed through a sequence of distinct eras, each of which can be defined in the context of the technologies available at that time. The progress has been enormous, but there is a great deal still to be learned. A particular challenge is to use what we know for human benefit.

The collagen binding alpha1beta1 integrin VLA-1 regulates CD8 T cell-mediated immune protection against heterologous influenza infection

A common feature of many infections is that many pathogen-specific memory T cells become established in diverse nonlymphoid tissues. A mechanism that promotes the retention and survival of the memory T cells in diverse tissues has not been described. Our studies show that the collagen binding alpha1beta1 integrin, VLA-1, is expressed by the majority of influenza-specific CD8 T cells recovered from nonlymphoid tissues during both the acute and memory phases of the response. Antibody treatment or genetic deficiency of VLA-1 decreased virus-specific CTL in the lung and other nonlymphoid tissues, and increased them in the spleen. In spite of the increase in the spleen, secondary heterosubtypic immunity against flu was compromised. This suggests that VLA-1 is responsible for retaining protective memory CD8 T cells in the lung and other tissues via attachment to the extracellular matrix.

Expansion of protective CD8+ T-cell responses driven by recombinant cytomegaloviruses

CD8(+) T cells are critical for the control of many persistent viral infections, such as human immunodeficiency virus, hepatitis C virus, Epstein-Barr virus, and cytomegalovirus (CMV). In most infections, large CD8(+)-T-cell populations are induced early but then contract and are maintained thereafter at lower levels. In contrast, CD8(+) T cells specific for murine CMV (MCMV) have been shown to gradually accumulate after resolution of primary infection. This unique behavior is restricted to certain epitopes, including an immunodominant epitope derived from the immediate-early 1 (IE1) gene product. To explore the mechanism behind this further, we measured CD8(+)-T-cell-mediated immunity induced by recombinant MCMV-expressing epitopes derived from influenza A virus or lymphocytic choriomeningitis virus placed under the control of an IE promoter. We observed that virus-specific CD8(+)-T-cell populations were induced and that these expanded gradually over time. Importantly, these CD8(+) T cells provided long-term protection against challenge without boosting. These results demonstrate a unique pattern of accumulating T cells, which provide long-lasting immune protection, that is independent of the initial immunodominance of the epitope and indicates the potential of T-cell-inducing vaccines based on persistent vectors.

Differential tumor necrosis factor receptor 2-mediated editing of virus-specific CD8+ effector T cells

Much of the CD8(+) T cell response in H2(b) mice with influenza pneumonia is directed at the nucleoprotein(366-374) (NP(366)) and acid polymerase(224-233) (PA(224)) peptides presented by the H2D(b) MHC class I glycoprotein. These D(b)NP(366)- and D(b)PA(224)-specific T cell populations are readily analyzed by staining with tetrameric complexes of MHC(+) peptide (tetramers) or by cytokine production subsequent to in vitro stimulation with the cognate peptides. The D(b)PA(224)-specific CD8(+) effector T cells make more tumor necrosis factor (TNF) alpha than the comparable CD8(+)D(b)NP(366)(+) set, a difference reflected in the greater sensitivity of the CD8(+)D(b)PA(224)(+) population to TNF receptor (TNFR) 2-mediated apoptosis under conditions of in vitro culture. Freshly isolated CD8(+)D(b)NP(366)(+) and CD8(+)D(b)PA(224)(+) T cells from influenza-infected TNFR2(-/-) mice produce higher levels of IFN-gamma and TNF-alpha after in vitro stimulation with peptide, although the avidity of the T cell receptor-epitope interaction does not change. Increased numbers of both CD8(+)D(b)PA(224)(+) and CD8(+)D(b)NP(366)(+) T cells were recovered from the lungs (but not the spleens) of secondarily challenged TNFR2(-/-) mice, a pattern that correlates with the profiles of TNFR expression in the TNFR2(+/+) controls. Thus, it seems that TNFR2-mediated editing of influenza-specific CD8(+) T cells functions to limit the numbers of effectors that have localized to the site of pathology in the lung but does not modify the size of the less activated responder T cell populations in the spleen. Therefore, the massive difference in magnitude for the secondary, although not the primary, response to these D(b)NP(366) and D(b)PA(224) epitopes cannot be considered to reflect differential TNFR2-mediated T cell editing.

Epstein-Barr virus (EBV) reactivation in allogeneic stem-cell transplantation: relationship between viral load, EBV-specific T-cell reconstitution and rituximab therapy

BACKGROUND

Monitoring of Epstein-Barr virus (EBV) reactivation after allogeneic hematopoietic stem-cell transplantation markedly improved with quantitative real-time polymerase chain reaction amplification of EBV DNA and visualization of EBV-specific CD8+ T cells with peptide-human leukocyte antigen (HLA) class I tetramers. We decided to combine these methods to evaluate posttransplant EBV reactivation and rituximab therapy.

METHODS

We followed 56 patients treated with an HLA-genoidentical sibling (n=32), an HLA-matched unrelated donor (MUD, n=19), or an unrelated cord-blood transplant (n=5). EBV DNA was quantified in plasma and in peripheral blood mononuclear cells (PBMC). Patient CD8+ T cells were stained with a panel of eight tetramers.

RESULTS

EBV DNA was detected in half of the patients, mainly in the MUD group (17/19). In 19 patients, viral DNA was detected only in the cellular compartment. All patients who controlled reactivation without rituximab and despite a viral load of greater than 500 genome equivalents (gEq)/150,000 PBMC mounted an EBV-specific CD8+ T-cell response in greater than 1.4% of CD3+CD8+ T cells. Plasmatic EBV genome was found in nine patients preceded by a high cellular viral load. Three of these patients controlled the reactivation before or without the introduction of rituximab, and they all developed a significant and increasing EBV-specific T-cell response. Patients with EBV-specific T cells at the onset of reactivation controlled viral reactivation without rituximab.

CONCLUSION

This study emphasizes the benefit of an early and close monitoring of EBV reactivation and CD8+-specific immune responses to initiate rituximab only when necessary and before the immune response becomes overwhelmed by the viral burden.

Memory CD8+ T cells provide innate immune protection against Listeria monocytogenes in the absence of cognate antigen

Interferon (IFN)-γ plays an important role in the innate immune response against intracellular bacterial pathogens. It is commonly thought that natural killer cells are the primary source of this cytokine that is involved in activating antibacterial effects in infected cells and polarizing CD4⁺ T cells toward the Th1 subset. However, here we show that both effector and memory CD8⁺ T cells have the potential to secrete IFN-γ in response to interleukin (IL)-12 and IL-18 in the absence of cognate antigen. We demonstrate that memory CD8⁺ T cells specific for the ovalbumin protein secrete IFN-γ rapidly after infection with wild-type Listeria monocytogenes (LM). Furthermore, small numbers of ovalbumin-specific, memory CD8⁺ T cells can reduce spleen and liver bacterial counts in IFN-γ–deficient mice 3 d after LM infection. Up-regulation of the receptors for IL-12 and IL-18 provides a mechanism for the ability of memory CD8⁺ T cells to respond in this antigen nonspecific manner. Thus, CD8⁺ T cells play an important role in the innate immune response against intracellular pathogens by rapidly secreting IFN-γ in response to IL-12 and IL-18.

Plasticity of T cell memory responses to viruses

Virus-specific memory T cell populations demonstrate plasticity in antigenic and functional phenotype, in recognition of antigen, and in their ability to accommodate new memory T cell populations. The adaptability of complex antigen-specific T cell repertoires allows the host to respond to a diverse array of pathogens and accommodate memory pools to many pathogens in a finite immune system. This is in part accounted for by crossreactive memory T cells, which can be employed in immune responses and mediate protective immunity or life-threatening immunopathology.

Recall helper T cell response: T helper 1 cell-resistant allergic susceptibility without biasing uncommitted CD4 T cells

Effector and memory T lymphocytes differ significantly, and there is no experimental evidence that memory cells are sufficient to render an otherwise normal individual susceptible to localized allergic inflammation. Furthermore, nothing is known about the kinetics of memory responses after inhalation of antigen or interplay between an allergen-specific memory helper T (Th) cell Th2 population and uncommitted or competing Th1 cells. To study these processes, T cell receptor-transgenic CD4(+) effector cells were generated in vitro, transferred into naive recipients, and allowed to resume a quiescent state. Inhalation of protein antigen reactivated these Ag-specific Th2 donor cells, leading to allergic pulmonary inflammation and airway hyperreactivity. Susceptibility was correlated with the size of the input Th2 population, but Th1 cells neither enhanced nor reduced inflammation in this model. Importantly, the reactivation of these antigen-experienced cells by inhaled antigen did not skew the cytokine balance of recipient-derived T cells recruited to the lung nor did it inhibit the development of donor-derived Th1 cells from uncommitted antigen-experienced cells that form a normal part of immune responses. These data indicate that a quiescent memory Th2-cell population can create susceptibility to allergic pulmonary inflammation in a manner refractory to inhibition by Th1 cells or endogenous inhibitory mechanisms.

IMMUNOPATHOLOGIC EFFECTS ASSOCIATED WITH SARCOCYSTIS NEURONA–INFECTED INTERFERON-GAMMA KNOCKOUT MICE

Interferon-gamma knockout (IFN-gamma KO) mice were infected with Sarcocystis neurona merozoites to characterize the immunopathology associated with infection. By day 14 postinfection (PI), mice developed splenomegaly and lymphadenopathy, characterized by marked lymphoid hyperplasia with increased numbers of germinal centers. Additional histopathologic changes included increased extramedullary hematopoiesis, multifocal mixed inflammatory infiltrates in the liver, perivascular infiltrate of the liver and lung, and interstitial pneumonia. The total number of B-cell splenocytes (P < 0.05) and the percentage of B-cells increased on day 14 PI in the spleen and on day 28 PI in the lymph nodes (P < 0.05). By day 28 PI, the number of B-cell splenocytes decreased significantly. A non-subset-specific decrease in percentages of CD4 lymphocytes throughout all lymphoid organs was observed on day 14 PI. However, total CD4 and CD44/CD4 splenocytes increased significantly by day 28 PI. Early-activation CD8 lymphocytes were reduced in the blood and spleen, whereas memory CD8 lymphocyte percentages and total numbers were significantly increased. On the basis of the results, we propose that S. neurona-infected IFN-gamma KO mice are immunocompromised and unable to clear the infection. Thus, they develop B-cell exhaustion and a delayed, but sustained, increased number of memory CD4 and CD8 lymphocytes due to chronic antigen stimulation.

PROTECTIVE IMMUNE RESPONSE TO EXPERIMENTAL INFECTION WITH SARCOCYSTIS NEURONA IN C57BL/6 MICE

Immunocompetent C57BL/6 mice were infected with Sarcocystis neurona merozoites to assess the protective immune response to active infection. Using a direct agglutination test, all infected mice seroconverted to S. neurona merozoite antigens by day 14 postinfection (PI). Further, mice developed splenomegaly and bilateral symmetrical lymphadenopathy by day 14 PI, which appeared to be resolving by day 28 PI. Histologic analysis revealed a marked increase in germinal center formation in the spleen and lymph nodes by day 14 PI. Corresponding to gross and histopathological changes, the percentage of B-cells decreased significantly by day 14 PI but then increased significantly and persisted at day 28 PI in the blood, spleen, and multiple lymph nodes. There was a sharp nonspecific significant decrease in CD4 percentages by day 14 PI in the blood, spleen, and lymph nodes. Early-activation CD8 lymphocytes (CD62/CD8) were significantly down-regulated coinciding with a significant compensatory up-regulation of memory (CD44/CD8) lymphocytes in multiple organs. We propose that the protective cell-mediated immune response to S. neurona involves both CD4 and CD8 cells, with CD8 lymphocytes appearing to play a more critical role.

Bone marrow as a priming site for T-cell responses to blood-borne antigen

Although bone marrow is known as a primary lymphoid organ, its potential to serve as a secondary immune organ has hardly been explored. Here we demonstrate that naive, antigen-specific T cells home to bone marrow, where they can be primed. Antigen presentation to T cells in bone marrow is mediated via resident CD11c+ dendritic cells. They are highly efficient in taking up exogenous blood-borne antigen and processing it via major histocompatibility complex class I and class II pathways. T-cell activation correlates with dendritic cell-T cell clustering in bone marrow stroma. Primary CD4+ and CD8+ T-cell responses generated in bone marrow occur in the absence of secondary lymphoid organs. The responses are not tolerogenic and result in generation of cytotoxic T cells, protective anti-tumor immunity and immunological memory. These findings highlight the uniqueness of bone marrow as an organ important for hemato- and lymphopoiesis and for systemic T cell-mediated immunity.

CD8 T cell responses to infectious pathogens

CD8 T cells respond to viral infections but also participate in defense against bacterial and protozoal infections. In the last few years, as new methods to accurately quantify and characterize pathogen-specific CD8 T cells have become available, our understanding of in vivo T cell responses has increased dramatically. Pathogen-specific T cells, once thought to be quite rare following infection, are now known to be present at very high frequencies, particularly in peripheral, nonlymphoid tissues. With the ability to visualize in vivo CD8 T cell responses has come the recognition that T cell expansion is programmed and, to a great extent, independent of antigen concentrations. Comparison of CD8 T cell responses to different pathogens also highlights the intricate relationship between microbially induced innate inflammatory responses and the kinetics, magnitude, and character of long-term T cell responses. This review describes recent progress in some of the major murine models of CD8 T cell-mediated immunity to viral, bacterial, and protozoal infection.

Quantitative studies of CD8+ T-cell responses during microbial infection

MHC class I tetramer staining, intracellular cytokine staining and ELISPOT assays have made it possible to quantify CD8(+) T-cell responses precisely during and following viral and bacterial infection. Although these quantitative methods are by now familiar and trusted components of the immunologist's toolbox, their application to models of microbial infection continues to provide surprising insights into mammalian adaptive immunity. In the past year there have been many exciting new findings on CD8(+) T-cell priming, expansion and memory formation in response to microbial infection.

Cell-mediated immunity to respiratory virus infections

The mucosal surfaces of the lungs pose tremendous problems for an immune system charged with maintaining a sterile pulmonary environment. Despite these problems, the immune system is effective at controlling most pulmonary infections. Over the past few years significant progress has been made in our understanding of how adaptive (humoral and cellular) immunity is able to control infections in the respiratory tract. Recent advances include the identification of effector memory T-cell populations in the lungs and an appreciation for the role of cytokines in regulating memory T-cell pools.

Quantitative analysis of long-term virus-specific CD8+-T-cell memory in mice challenged with unrelated pathogens

The consequences for the long-term maintenance of virus-specific CD8+-T-cell memory have been analyzed experimentally for sequential respiratory infections with readily eliminated (influenza virus) and persistent (gammaherpesvirus 68 [gammaHV68]) pathogens. Sampling a broad range of tissue sites established that the numbers of CD8+ T cells specific for the prominent influenza virus D(b)NP(366) epitope were reduced by about half in mice that had been challenged 100 days previously with gammaHV68, though the prior presence of a large CD8+ D(b)NP366+ population caused no selective defect in the gammaHV68-specific CD8+ K(b)p79+ response. Conversely, mice that had been primed and boosted to generate substantial gammaHV68-specific CD8+ D(b)p56+ populations did not show any decrease in prevalence for this set of CD8+ memory cytotoxic T lymphocytes (CTL) at 200 days after respiratory exposure to an influenza A virus. However, in both experiments, the total magnitude of the CD8+-T-cell pool was significantly diminished in those that had been infected with gammaHV68 and the influenza A virus. The broader implications of these findings, especially under conditions of repeated exposure to unrelated pathogens, are explored with a mathematical model which emphasizes that the immune effector and memory "phenome" is a function of the overall infection experience of the individual.

The evolution of antigen-specific CD8+ T cell responses after natural primary infection of humans with Epstein-Barr virus

Epstein-Barr virus (EBV) is a persistent, gamma-herpes virus that infects 90% of the human population. Primary infection, particularly if it is delayed until adolescence or beyond, may cause acute infectious mononucleosis and persistent infection may be associated with the development of several malignancies. CD8(+) T cells play a critical role in controlling both the primary and persistent phases of infection. This review summarises work that has been done characterising the primary immune responses to EBV. It goes on to describe the down regulation of the primary immune response and to discuss some of the factors that may be involved in determining the death or survival of populations of antigen-specific CD8(+) T cells. Finally it describes features of the populations of memory cells that mediate the long-term control of EBV in healthy seropositive individuals. The studies show differences in the responses to epitopes from lytic cycle versus latent proteins and highlight the complexity of naturally occurring, in vivo, immune responses. A clear understanding of the means by which CD8(+) T cells control EBV is important if we are to successfully develop vaccines and other forms of immunotherapy for the virus and its related malignancies.

Migration and T-lymphocyte effector function

The development of immunity depends upon the capacity of responding T cells to become mobilized from lymphoid tissues where they are primed to sites of antigen exposure, wherever they occur in the body. Activation-induced alterations in the ability of T cells to migrate signify a fundamental change in biological function. Considerable attention is now focused on identifying mechanisms that regulate the migration and persistence of T cells that disseminate to non-lymphoid compartments as effector cells, and those that are retained in the lymphoid compartment. There are many unanswered questions about the developmental relationships and roles in protective immunity of antigen-experienced T cells that partition in different tissues.

Renewal of peripheral CD8+ memory T cells during secondary viral infection of antibody-sufficient mice

Kinetic studies and short pulses of injected 5-bromo-2-deoxyuridine have been used to analyze the development and renewal of peripheral CD8(+) memory T cells in the lungs during primary and secondary respiratory virus infections. We show that developing peripheral CD8(+) memory T cells proliferate during acute viral infection with kinetics that are indistinguishable from those of lymphoid CD8(+) memory T cells. Secondary exposure to the same virus induces a new round of T cell proliferation and extensive renewal of the peripheral and lymphoid CD8(+) memory T cell pools in both B cell-deficient mice and mice with immune Abs. In mice with virus-specific Abs, CD8(+) T cell proliferation takes place with minimal inflammation or effector cell recruitment to the lungs. The delayed arrival of CD8(+) memory T cells to the lungs of these animals suggests that developing memory cells do not require the same inflammatory signals as effector cells to reach the lung airways. These studies provide important new insight into mechanisms that control the maintenance and renewal of peripheral memory T cell populations during natural infections.

Regulation of CD8+ T cells undergoing primary and secondary responses to infection in the same host

Naive Ag-specific CD8(+) T cells expand, contract, and become memory cells after infection and/or vaccination. Memory CD8(+) T cells provide faster, more effective secondary responses against repeated exposure to the same pathogen. Using an adoptive transfer system with low numbers of trackable nontransgenic memory CD8(+) T cells, we showed that secondary responses can be comprised of both primary (naive) and secondary (memory) CD8(+) T cells after bacterial (Listeria monocytogenes) and/or viral (lymphocytic choriomeningitis virus) infections. The level of memory CD8(+) T cells present at the time of infection inversely correlated with the magnitude of primary CD8(+) T cell responses against the same epitope but directly correlated with the level of protection against infection. However, similar numbers of Ag-specific CD8(+) T cells were found 8 days postinfection no matter how many memory cells were present at the time of infection. Rapid contraction of primary CD8(+) T cell responses was not influenced by the presence of memory CD8(+) T cells. However, contraction of secondary CD8(+) T cell responses was markedly prolonged compared with primary responses in the same host mice. This situation occurred in response to lymphocytic choriomeningitis virus or L. monocytogenes infection and for CD8(+) T cell responses against multiple epitopes. The delayed contraction of secondary CD8(+) T cells was also observed after immunization with peptide-coated dendritic cells. Together, the results show that the level of memory CD8(+) T cells influences protective immunity and activation of naive precursors specific for the same epitope but has little impact on the magnitude or program of the CD8(+) T cell response.

Herpes simplex virus-specific memory CD8+ T cells are selectively activated and retained in latently infected sensory ganglia

This study challenges the concept that herpes simplex virus type 1 (HSV-1) latency represents a silent infection that is ignored by the host immune system, and suggests antigen-directed retention of memory CD8(+) T cells. CD8(+) T cells specific for the immunodominant gB(498-505) HSV-1 epitope are selectively retained in the ophthalmic branch of the latently infected trigeminal ganglion, where they acquire and maintain an activation phenotype and the capacity to produce IFN-gamma. Some CD8(+) T cells showed TCR polarization to junctions with neurons. A gB(498-505) peptide-specific CD8(+) T cell clone can block HSV-1 reactivation from latency in ex vivo trigeminal ganglion cultures. We conclude that CD8(+) T cells provide active surveillance of HSV-1 gene expression in latently infected sensory neurons.

CD4+ cell memory: the enigma of Th1 cells

Despite a wealth of information pertaining to functional and phenotypic attributes of memory CD4(+) cells, the mechanisms that underlie the generation and persistence of memory in this subset are largely unknown. Recent work suggests that the development of memory might be differently regulated in T-helper-1 and T-helper-2 cells, owing to differences in their susceptibility to cell death. These studies support a new paradigm, in which memory T cells are heterogeneous in terms of their stage of maturation and function as well as mechanisms of homeostatic control.