Concurrent naive and memory CD8(+) T cell responses to an influenza A virus

'Rinse and Replace': Boosting T Cell Turnover To Reduce HIV-1 Reservoirs

Latent HIV-1 persists indefinitely during antiretroviral therapy (ART) as an integrated silent genome in long-lived memory CD4+ T cells. In untreated infections, immune activation increases the turnover of intrinsically long-lived provirus-containing CD4+ T cells. Those are 'washed out' as a result of their activation, which when coupled to viral protein expression can facilitate local inflammation and recruitment of uninfected cells to activation sites, causing latently infected cells to compete for survival. De novo infection can counter this washout. During ART, inflammation and CD4+ T cell activation wane, resulting in reduced cell turnover and a persistent reservoir. We propose accelerating reservoir washout during ART by triggering sequential waves of polyclonal CD4+ T cell activation while simultaneously enhancing virus protein expression. Reservoir reduction as an adjunct to other therapies might achieve lifelong viral control.

Development of a BiTE®-mediated CD8+ cytotoxic T-lymphocyte activity assay to assess immunomodulatory potential of drug candidates in Cynomolgus macaque

The immunotoxic potential of drug candidates is assessed through the examination of results from a variety of studies and endpoints. While the functional assessment of CD8⁺ cytotoxic T-lymphocytes (CTL) is well-characterized in the clinic, the lack of a robust macaque CTL functional assay has been an important hurdle in evaluating and accurately quantifying cell-mediated CD8⁺ T-cell effector responses in the nonclinical setting. This paper describes the development of an assay to measure CTL activity in peripheral blood mononuclear cells (PBMC) isolated from Cynomolgus macaques. A human EGFR/CD3 Bispecific T-cell Engager (BiTE^®) was used to mount a robust CD8⁺ T-cell response in the presence of target-expressing cells. Upon target engagement, degranulation of CD107a and production of interferon (IFN)-γ both reliably indicated a robust functional response in CD8⁺ T-cells. The BiTE^®-mediated stimulation method proved to be favorable when compared to other methods of stimulation in the absence of target cells. These studies demonstrated acceptable longitudinal variability of the functional assay and sensitivity to dexamethasone-mediated immunosuppression. Taken together, the results indicated an assay leveraging CD3-bispecific antibodies and target-expressing cells can provide a robust approach to the in vitro or ex vivo assessment of CTL function in Cynomolgus macaques. Because the impairment of CTL activity by immunomodulators is recognized to be an important contributor to decreased antiviral defense and increased carcinogenicity risk, we believe that this novel assay to be a valuable addition to the immunotoxicology assessment of therapeutic drug candidates.

Progesterone impairs antigen-non-specific immune protection by CD8 T memory cells via interferon-γ gene hypermethylation

Pregnant women and animals have increased susceptibility to a variety of intracellular pathogens including Listeria monocytogenes (LM), which has been associated with significantly increased level of sex hormones such as progesterone. CD8 T memory(Tm) cell-mediated antigen-non-specific IFN-γ responses are critically required in the host defense against LM. However, whether and how increased progesterone during pregnancy modulates CD8 Tm cell-mediated antigen-non-specific IFN-γ production and immune protection against LM remain poorly understood. Here we show in pregnant women that increased serum progesterone levels are associated with DNA hypermethylation of IFN-γ gene promoter region and decreased IFN-γ production in CD8 Tm cells upon antigen-non-specific stimulation ex vivo. Moreover, IFN-γ gene hypermethylation and significantly reduced IFN-γ production post LM infection in antigen-non-specific CD8 Tm cells are also observed in pregnant mice or progesterone treated non-pregnant female mice, which is a reversible phenotype following demethylation treatment. Importantly, antigen-non-specific CD8 Tm cells from progesterone treated mice have impaired anti-LM protection when adoptive transferred in either pregnant wild type mice or IFN-γ-deficient mice, and demethylation treatment rescues the adoptive protection of such CD8 Tm cells. These data demonstrate that increased progesterone impairs immune protective functions of antigen-non-specific CD8 Tm cells via inducing IFN-γ gene hypermethylation. Our findings thus provide insights into a new mechanism through which increased female sex hormone regulate CD8 Tm cell functions during pregnancy.

Increased female sex hormones during pregnancy generate a temporary immune suppression status in the pregnant that protect the developing fetus from maternal rejection but renders the pregnant highly susceptible to various pathogens. However, molecular mechanisms underlying such an increased maternal susceptibility to pathogens during pregnancy remain to be further understood. Here we show in pregnant women that increased progesterone levels are associated with IFN-γ gene hypermethylation and reduced IFN-γ production in peripheral CD8 Tm cells. By using murine models of LM infection, for the first time we show a causal relationship between increased level of progesterone, a characteristic female sex hormone of pregnancy, and increased susceptibility to Listeria monocytogenes, an intracellular bacterium that endangers both the pregnant and the fetus. Such an impact on anti-listeria host defense is mediated through progesterone-induced IFN-γ gene hypermethylation in CD8 Tm cells, resulting in impaired IFN-γ production and reduced immune protection by antigen-non-specific CD8 Tm cells. This study provides new insights into molecular mechanisms underlying the increased susceptibility to intracellular pathogens during pregnancy.

Recent trends on hydrogel based drug delivery systems for infectious diseases

Since centuries, the rapid spread and cure of infectious diseases have been a major concern to the progress and survival of humans. These diseases are a global burden and the prominent cause for worldwide deaths and disabilities. Nanomedicine has emerged as the most excellent tool to eradicate and halt their spread. Various nanoformulations (NFs) using advanced nanotechnology are in demand. Recently, hydrogel and nanogel based drug delivery devices have posed new prospects to simulate the natural intelligence of various biological systems. Owing to their unique porous interpenetrating network design, hydrophobic drug incorporation and stimulus sensitivity hydrogels owe excellent potential as targeted drug delivery systems. The present review is an attempt to highlight the recent trends of hydrogel based drug delivery systems for the delivery of therapeutic agents and diagnostics for major infectious diseases including acquired immune deficiency syndrome (AIDS), malaria, tuberculosis, influenza and ebola. Future prospects and challenges are also described.

Differential Immune Profiles in Two Pandemic Influenza A(H1N1)pdm09 Virus Waves at Pandemic Epicenter

Background and Aims

Severe influenza A(H1N1)pdm2009 virus infection cases are characterized by sustained immune activation during influenza pandemics. Seasonal flu data suggest that immune mediators could be modified by wave-related changes. Our aim was to determine the behavior of soluble and cell-related mediators in two waves at the epicenter of the 2009 influenza pandemic.

Methods

Leukocyte surface activation markers were studied in serum from peripheral blood samples, collected from the 1^st (April–May, 2009) and 2^nd (October 2009–February 2010) pandemic waves. Patients with confirmed influenza A(H1N1)pdm2009 virus infection (H1N1), influenza-like illness (ILI) or healthy donors (H) were analyzed.

Results

Serum IL-6, IL-4 and IL-10 levels were elevated in H1N1 patients from the 2^nd pandemic wave. Additionally, the frequency of helper and cytotoxic T cells was reduced during the 1^st wave, whereas CD69 expression in helper T cells was increased in the 2^nd wave for both H1N1 and ILI patients. In contrast, CD62L expression in granulocytes from the ILI group was increased in both waves but in monocytes only in the 2^nd wave. Triggering Receptor Expressed on Myeloid cells (TREM)-1 expression was elevated only in H1N1 patients at the 1^st wave.

Conclusions

Our results show that during the 2009 influenza pandemic a T cell activation phenotype is observed in a wave-dependent fashion, with an expanded activation in the 2^nd wave, compared to the 1^st wave. Conversely, granulocyte and monocyte activation is infection-dependent. This evidence collected at the pandemic epicenter in 2009 could help us understand the differences in the underlying cellular mechanisms that drive the wave-related immune profile behaviors that occur against influenza viruses during pandemics.

Memory CD8⁺ T cells can outsource IFN-γ production but not cytolytic killing for antiviral protection

Immunization with vaccinia virus (VACV), the virus comprising the smallpox vaccine, induces memory CD8(+) T cells that protect from subsequent infections with smallpox in humans or the related ectromelia virus (ECTV) in mice. Memory CD8(+) T cells largely mediate these effects by expanding into secondary effectors that secrete the antiviral cytokine interferon-γ (IFN-γ) and induce cytolysis via releasing factors such as perforin, which permeabilizes target cells. We show that protection from ECTV infection after VACV immunization depends on the initial memory cell frequency and ability of expanded secondary effectors to kill infected targets in a perforin-dependent manner. Although IFN-γ is essential for antiviral protection, it can be produced by either secondary effectors or concomitant primary effector CD8(+) T cells recruited to the response. Thus, during lethal virus challenge, memory CD8(+) T cells are required for cytolytic killing of infected cells, but primary effectors can play important roles by producing IFN-γ.

Cutting edge: Heterosubtypic influenza infection antagonizes elicitation of immunological reactivity to hemagglutinin

Influenza-specific immunity in humans is unique because there are repeated exposures to viral strains containing genetically conserved epitopes recruiting memory CD4 T cells and novel epitopes stimulating naive CD4 T cells, possibly resulting in competition between memory and naive lymphocytes. In this study, we evaluated the effect of this competition on CD4 T cell and B cell response specificity using a murine model of sequential influenza infection. We found striking and selective decreases in CD4 T cell reactivity to nonconserved hemagglutinin (HA) epitopes following secondary influenza infection. Surprisingly, this shift in CD4 T cell specificity was associated with dramatic decreases in HA-specific Ab. These results suggest that repeated exposure to influenza viruses and vaccines containing conserved internal proteins may have unintended and negative consequences on the ability to induce HA-specific Ab to novel pandemic strains of influenza. These finding could have important implications on pandemic influenza preparedness strategies.

Subsisting H1N1 influenza memory responses are insufficient to protect from pandemic H1N1 influenza challenge in C57BL/6 mice

The 2009 swine-origin pandemic H1N1 (pH1N1) influenza virus transmitted and caused disease in many individuals immune to pre-2009 H1N1 influenza virus. Whilst extensive studies on antibody-mediated pH1N1 cross-reactivity have been described, few studies have focused on influenza-specific memory T-cells. To address this, the immune response in pre-2009 H1N1 influenza-immune mice was evaluated after pH1N1 challenge and disease pathogenesis was determined. The results show that despite homology shared between pre-2009 H1N1 and pH1N1 strains, the effector memory T-cell response to pre-2009 H1N1 was generally ineffective, a finding that correlated with lung virus persistence. Additionally, pH1N1 challenge generated T-cells reactive to new pH1N1 epitopes. These studies highlight the importance of vaccinating against immunodominant T-cell epitopes to provide for a more effective strategy to control influenza virus through heterosubtypic immunity.

Biopolymer encapsulated live influenza virus as a universal CD8+ T cell vaccine against influenza virus

Current influenza virus vaccines primarily elicit antibodies and can be rendered ineffective by antigenic drift and shift. Vaccines that elicit CD8+ T cell responses targeting less variable proteins may function as universal vaccines that have broad reactivity against different influenza virus strains. To generate such a universal vaccine, we encapsulated live influenza virus in a biopolymer and delivered it to mice subcutaneously. This vaccine was safe, induced potent CD8+ T cell immunity and protected mice against heterosubtypic lethal challenge. Safety of subcutaneous (SQ) vaccination was tested in Rag-/-γc-/- double knockout mice which we show cannot control intranasal infection. Biopolymer encapsulation of live influenza virus could be used to develop universal CD8+ T cell vaccines against heterosubtypic and pandemic strains.

Homogenization of TCR repertoires within secondary CD62Lhigh and CD62Llow virus-specific CD8+ T cell populations

Influenza virus-specific CD8(+) T cell clonotypes generated and maintained in C57BL/6J mice after respiratory challenge were found previously to distribute unequally between the CD62L(low) "effector" (T(EM)) and CD62L(high) "central" (T(CM)) memory subsets. Defined by the CDR3beta sequence, most of the prominent TCRs were represented in both the CD62L(high) and CD62L(low) subsets, but there was also a substantial number of diverse, but generally small, CD62L(high)-only clonotypes. The question asked here is how secondary challenge influences both the diversity and the continuity of TCR representation in the T(CM) and T(EM) subsets generated following primary exposure. The experiments use single-cell RT-PCR to correlate clonotypic composition with CD62L phenotype for secondary influenza-specific CD8(+) T cell responses directed at the prominent D(b)NP(366) and D(b)PA(224) epitopes. In both the acute and long-term memory phases of the recall responses to these epitopes, we found evidence of a convergence of TCR repertoire expression for the CD62L(low) and CD62L(high) populations. In fact, unlike the primary response, there were no significant differences in clonotypic diversity between the CD62L(low) and CD62L(high) subsets. This "TCR homogenization" for the CD62L(high) and CD62L(low) CD8(+) populations recalled after secondary challenge indicates common origin, most likely from the high prevalence populations in the CD62L(high) central memory set. Our study thus provides key insights into the TCR diversity spectrum for CD62L(high) and CD62L(low) T cells generated from a normal, unmanipulated T cell repertoire following secondary challenge. A better understanding of TCR selection and maintenance has implications for improved vaccine and immunotherapy protocols.

The chemokine receptor CCR5 plays a key role in the early memory CD8+ T cell response to respiratory virus infections

Innate recognition of invading pathogens in peripheral tissues results in the recruitment of circulating memory CD8(+) T cells to sites of localized inflammation during the early phase of a recall response. However, the mechanisms that control the rapid recruitment of these cells to peripheral sites are poorly understood, particularly in relation to influenza and parainfluenza infections of the respiratory tract. In this study, we demonstrate a crucial role for C-C chemokine receptor 5 (CCR5) in the accelerated recruitment of memory CD8(+) T cells to the lung airways during virus challenge. Most importantly, CCR5 deficiency resulted in decreased recruitment of memory T cells expressing key effector molecules and impaired control of virus replication during the initial stages of a secondary response. These data highlight the critical importance of early memory T cell recruitment for the efficacy of cellular immunity in the lung.

Tentative T cells: memory cells are quick to respond, but slow to divide

T cell memory is a cornerstone of protective immunity, and is the key element in successful vaccination. Upon encountering the relevant pathogen, memory T cells are thought to initiate cell division much more rapidly than their naïve counterparts, and this is thought to confer a significant biological advantage upon an immune host. Here, we use traceable TCR-transgenic T cells to evaluate this proposed characteristic in CD4+ and CD8+ memory T cells. We find that, even in the presence of abundant antigen that was sufficient to induce in vivo IFNgamma production by memory T cells, both memory and naïve T cells show an extended, and indistinguishable, delay in the onset of proliferation. Although memory cells can detect, and respond to, virus infection within a few hours, their proliferation did not begin until approximately 3 days after infection, and occurred simultaneously in all anatomical compartments. Thereafter, cell division was extraordinarily rapid for both naïve and memory cells, with the latter showing a somewhat accelerated accumulation. We propose that, by permitting memory T cells to rapidly exert their effector functions while delaying the onset of their proliferation, evolution has provided a safeguard that balances the risk of infection against the consequences of severe T cell-mediated immunopathology.

A unified framework of immunological and epidemiological dynamics for the spread of viral infections in a simple network-based population

BACKGROUND

The desire to better understand the immuno-biology of infectious diseases as a broader ecological system has motivated the explicit representation of epidemiological processes as a function of immune system dynamics. While several recent and innovative contributions have explored unified models across cellular and organismal domains, and appear well-suited to describing particular aspects of intracellular pathogen infections, these existing immuno-epidemiological models lack representation of certain cellular components and immunological processes needed to adequately characterize the dynamics of some important epidemiological contexts. Here, we complement existing models by presenting an alternate framework of anti-viral immune responses within individual hosts and infection spread across a simple network-based population.

RESULTS

Our compartmental formulation parsimoniously demonstrates a correlation between immune responsiveness, network connectivity, and the natural history of infection in a population. It suggests that an increased disparity between people's ability to respond to an infection, while maintaining an average immune responsiveness rate, may worsen the overall impact of an outbreak within a population. Additionally, varying an individual's network connectivity affects the rate with which the population-wide viral load accumulates, but has little impact on the asymptotic limit in which it approaches. Whilst the clearance of a pathogen in a population will lower viral loads in the short-term, the longer the time until re-infection, the more severe an outbreak is likely to be. Given the eventual likelihood of reinfection, the resulting long-run viral burden after elimination of an infection is negligible compared to the situation in which infection is persistent.

CONCLUSION

Future infectious disease research would benefit by striving to not only continue to understand the properties of an invading microbe, or the body's response to infections, but how these properties, jointly, affect the propagation of an infection throughout a population. These initial results offer a refinement to current immuno-epidemiological modelling methodology, and reinforce how coupling principles of immunology with epidemiology can provide insight into a multi-scaled description of an ecological system. Overall, we anticipate these results to as a further step towards articulating an integrated, more refined epidemiological theory of the reciprocal influences between host-pathogen interactions, epidemiological mixing, and disease spread.

Minimal activation of memory CD8+ T cell by tissue-derived dendritic cells favors the stimulation of naive CD8+ T cells

Of the many dendritic cell (DC) subsets, DCs expressing the monomorphic coreceptor CD8 alpha-chain (CD8alpha) are localized permanently in lymphoid organs, whereas 'tissue-derived DCs' remain in nonlymphoid tissues until they 'capture' antigen and then move to local lymph nodes. Here we show that after lung infection, both naive and memory CD8+ 'killer' T cells responded to influenza virus antigens presented by lymph node-resident CD8alpha+ DCs, but only naive cells responded to antigens presented by lung-derived DCs. This difference provides a mechanism for priming naive T cell responses in conditions in which robust memory predominates. Our findings have implications for immunity to pathogens that can mutate their T cell epitopes, such as influenza virus and human immunodeficiency virus, and challenge the long-held view that memory T cells have less-stringent requirements for activation than naive T cells have.

Antigen-specific immune responses to influenza vaccine in utero

Initial immune responses to allergens may occur before birth, thereby modulating the subsequent development of atopy. This paradigm remains controversial, however, due to the inability to identify antigen-specific T cells in cord blood. The advent of MHC tetramers has revolutionized the detection of antigen-specific T cells. Tetramer staining of cord blood after CMV infection has demonstrated that effective CD8(+) antigen-specific immune responses can follow intrauterine viral infections. We hypothesized that sensitization to antigens occurs in utero in humans. We studied cord blood B and T cell immune responses following vaccination against influenza during pregnancy. Anti-Fluzone and anti-matrix protein IgM antibodies were detected in 38.5% (27 of 70) and 40.0% (28 of 70), respectively, of cord blood specimens. Using MHC tetramers, HA-specific CD4(+) T cells were detected among 25.0% (3 of 12) and 42.9% (6 of 14) of cord blood specimens possessing DRB1*0101 and DRB1*0401 HLA types, respectively, and were detected even when the DRB1 HLA type was inherited from the father. Matrix protein-specific CD8(+) T cells were detected among 10.0% (2 of 20) of HLA-A*0201(+) newborns. These results suggest that B and T cell immune responses occur in the fetus following vaccination against influenza and have important implications for determining when immune responses to environmental exposures begin.

Mutant influenza A virus nucleoprotein is preferentially localized in the cytoplasm and its immunization in mice shows higher immunogenicity and cross-reactivity

Many influenza vaccines targeted to hemagglutinin (HA) show efficient immunogenicity for protecting subjects against influenza virus infection. Major antigenic changes to HA molecules can help influenza virus to develop resistance against HA-targeted vaccines. DNA vaccines encoding conserved antigens protect animals against diverse subtypes, but their potency requires further improvement. We generated a DNA-based nucleoprotein (NP)-targeted vaccine using an N-terminal mutant of NP (NPm) that efficiently localized in the cytoplasm, and examined the immune responses in mice immunized with NPm or wild-type (WT) NP DNA vaccine. Importantly, the NPm vaccine showed 1.5-2-fold higher immunogenicity than the WT NP vaccine in mice. Furthermore, NPm vaccination efficiently protected the mice against lethal challenge with influenza viruses and showed cross-reactivity toward heterologous viruses. Therefore, DNA-based vaccination with NPm may contribute to the development of protective immunity against diverse influenza virus through its ability to stimulate cellular immunity.

Lack of intrinsic CTLA-4 expression has minimal effect on regulation of antiviral T-cell immunity

CTLA-4 is considered one of the most potent negative regulators of T-cell activation. To circumvent experimental limitations due to fatal lymphoproliferative disease associated with genetic ablation of CTLA-4, we have used radiation chimeras reconstituted with a mixture of CTLA-4+/+ and CTLA-4-/- bone marrow that retain a normal phenotype and allow the evaluation of long-term T-cell immunity under conditions of intrinsic CTLA-4 deficiency. Following virus infection, we profiled primary, memory, and secondary CD8+ and CD4+ T-cell responses directed against eight different viral epitopes. Our data demonstrate unaltered antigen-driven proliferation, acquisition of effector functions, distribution of epitope hierarchies, T-cell receptor repertoire selection, functional avidities, and long-term memory maintenance in the absence of CTLA-4. Moreover, regulation of memory T-cell survival and homeostatic proliferation, as well as secondary responses, was equivalent in virus-specific CTLA4+/+ and CTL-A-4-/- T-cell populations. Thus, lack of CTLA-4 expression by antigen-specific T cells can be compensated for by extrinsic factors in the presence of CTLA-4 expression by other cells. These findings have implications for the physiologic, pathological, and therapeutic regulation of costimulation.

Humoral immunity is not critical for protection against experimental infection with Sarcocystis neurona in B-cell-deficient mice

Immunodeficient B-cell-deficient mice (mmuMT) were infected with Sarcocystis neurona merozoites to determine the role of B cells and the humoral immune response in protective immunity. As expected, the mice did not seroconvert based on a direct agglutination test. Infected mice did not have significant changes in gross pathology at the time points examined. Histologic changes included mild perivascular and peribronchial infiltrate in the lungs; perivascular infiltrate and mild inflammatory sinusoidal foci in the liver; prominent high endothelial venules in the lymph nodes; and moderate cellular expansion of the periarteriolar sheaths (PALS) in the spleen. Changes resolved by day 60 postinfection. Mice developed significant CD4 and CD8 responses in lymphoid organs, including significant effector (CD45RB(high)) and memory (CD44(high)) CD4 and CD8 responses. Flow cytometry confirmed the lack of B cells. Overall, these data suggest that B cells are not critical to the protective immune response to SN infection.

Immunologic homeostasis during infection: coexistence of strong pulmonary cell-mediated immunity to secondary Cryptococcus neoformans infection while the primary infection still persists at low levels in the lungs

Maintenance of immunity to persistent pathogens is poorly understood. In this study, we used a murine model of persistent pulmonary fungal infection to study the ongoing cell-mediated immune response. CBA/J mice with low-level persistent Cryptococcus neoformans infection had CD4+ T cells of effector memory phenotype present in their lungs. Although unable to eliminate the primary infection to sterility, these mice displayed hallmarks of immunologic memory in response to rechallenge with C. neoformans: 1) the secondary cryptococcal challenge was controlled much more rapidly, 2) the inflammatory response developed and resolved more rapidly, 3) CD4+ T and CD8+ T cell responses were higher in magnitude, and 4) effector cytokine production by T cells was greatly enhanced. Depletion of CD4+ T cells at the time of secondary challenge adversely affected clearance of C. neoformans from the lungs. These results demonstrate that persistent low-level infection with C. neoformans does not impair the cell-mediated response to the fungus. Although they are relatively free of overt disease, these mice can respond with a rapid secondary immune response if the burden of C. neoformans increases. These data support the concept that immunologically healthy individuals can maintain low numbers of cryptococci that can become a nidus for re-activation disease during immunodeficient states such as AIDS.

Cutting edge: central memory T cells do not show accelerated proliferation or tissue infiltration in response to localized herpes simplex virus-1 infection

Memory T cells mount an enhanced response to secondary infections. Such an enhancement has been attributed in part to the ability of memory cells to more rapidly respond to cognate stimulation. In this study we have examined the rapidity with which murine CD8(+) memory T cells respond to a localized infection with HSV. Although central memory T cells (TcM), but not the effector memory T cells, mounted a strong recall response to secondary infection, the kinetics of TcM proliferation, the magnitude of their expansion, and their infiltration into infected nonlymphoid tissues were not advanced compared with that observed for naive T cells. These findings imply that it is the lack of accelerated proliferation kinetics and the subsequent delayed dissemination into the periphery that limits the ability of TcM to rapidly control localized virus replication.

Influenza virus receptor specificity and cell tropism in mouse and human airway epithelial cells

Recent human infections caused by the highly pathogenic avian influenza virus H5N1 strains emphasize an urgent need for assessment of factors that allow viral transmission, replication, and intra-airway spread. Important determinants for virus infection are epithelial cell receptors identified as glycans terminated by an alpha2,3-linked sialic acid (SA) that preferentially bind avian strains and glycans terminated by an alpha2,6-linked SA that bind human strains. The mouse is often used as a model for study of influenza viruses, including recent avian strains; however, the selectivity for infection of specific respiratory cell populations is not well described, and any relationship between receptors in the mouse and human lungs is incompletely understood. Here, using in vitro human and mouse airway epithelial cell models and in vivo mouse infection, we found that the alpha2,3-linked SA receptor was expressed in ciliated airway and type II alveolar epithelial cells and was targeted for cell-specific infection in both species. The alpha2,6-linked SA receptor was not expressed in the mouse, a factor that may contribute to the inability of some human strains to efficiently infect the mouse lung. In human airway epithelial cells, alpha2,6-linked SA was expressed and functional in both ciliated and goblet cells, providing expanded cellular tropism. Differences in receptor and cell-specific expression in these species suggest that differentiated human airway epithelial cell cultures may be superior for evaluation of some human strains, while the mouse can provide a model for studying avian strains that preferentially bind only the alpha2,3-linked SA receptor.

Addition of a Prominent Epitope Affects Influenza A Virus-Specific CD8+T Cell Immunodominance Hierarchies When Antigen Is Limiting

A reverse genetics strategy was used to insert the OVA peptide (amino acid sequence SIINFEKL; OVA(257-264)) into the neuraminidase stalk of both the A/PR8 (H1N1) and A/HKx31 (H3N2) influenza A viruses. Initial characterization determined that K(b)OVA257 is presented on targets infected with PR8-OVA and HK-OVA without significantly altering D(b) nucleoprotein (NP)366 presentation. There were similar levels of K(b)OVA257- and D(b)NP366-specific CTL expansion following both primary and secondary intranasal challenge. Interestingly, while variable, the presence of the immunodominant K(b)OVA257-specific response resulted in diminished D(b) acidic polymerase224- and K(b) basic polymerase subunit 1(703)-, but not D(b)NP366-specific responses and didn't alter endogenous influenza A virus-specific immunodominance hierarchies. However, challenging PR8-OVA-primed mice with HK-OVA via the i.p. route, and thereby limiting Ag dose, led to a reduction in the magnitude of all the influenza A virus-specific responses measured. A similar reduction in CTL response to native epitopes was also seen following primary respiratory HK-OVA infection of mice that received substantial numbers of K(b)OVA257-specific TCR transgenic T cells. Thus, during the course of infection, the generation of individual virus-specific CTL responses is independently regulated. However, in cases in which Ag is limiting, or high precursor frequency, the presence of immunodominant CTL responses can impact on the magnitude of other specific populations. Therefore, depending on both the size of the T cell precursor pool and the mode of Ag presentation, the addition of a major epitope can diminish the size of endogenous, influenza-specific CD8+ T cell responses, although never to the point that these are totally compromised.

Establishment and recall of CD8 + T‐cell memory in a model of localized transient infection

The influenza A virus model of localized, transient respiratory infection provides a well-defined experimental system for dissecting the induction and maintenance of CD8+ T-cell memory. This review focuses on quantitative and qualitative aspects of the prominent D(b)NP366- and D(b)PA224-specific CD8+ T-cell responses in virus-infected B6 mice. The different virus-specific effector and memory sets are compared by phenotypic [CD62L, interleukin-7 receptor-alpha (IL-7Ralpha), and IL-15Rbeta expression] and functional [interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), and IL-2 production] analyses. Most clonotypes [defined by T-cell receptor (TCR) CDR3beta sequence] generated during the acute phase of infection survive into memory, with those expressing the more consensus 'canonical' TCRs being the major contributors to the recall response. The extent of clonal expansion and the size of memory CD8+ T-cell populations has been characterized for mice challenged with either wildtype or mutant viruses, where broadly equivalent D(b)NP366 and D(b)PA224 expression was achieved by disabling the peptides in their native configuration, then expressing them in the viral neuraminidase protein. Combining the clonotypic and antigen dose analyses led to a somewhat mechanistic conclusion that the magnitude of any virus-specific CD8+ T-cell response will be a direct function of antigen dose and the size of the naïve or memory CD8+ T-cell precursor pool.

Influenza and the challenge for immunology

The continued westward dissemination of H5N1 influenza A viruses in avian populations and the nearly 50% mortality rate of humans infected with H5N1 are a source of great international concern. A mutant H5N1 virus with the capability to spread rapidly between humans could cause a global catastrophe. Governments have reacted by developing national response plans, stockpiling antiviral drugs and speeding up the development and approval of vaccines. Here we summarize what is known about the interaction between influenza A viruses and the mammalian host response, specifically emphasizing issues that might be of interest to the broader immunology community.

A virus-specific CD8+ T cell immunodominance hierarchy determined by antigen dose and precursor frequencies

Immunodominance hierarchies are a substantial, but poorly understood, characteristic of CD8(+) T cell-mediated immunity. Factors influencing the differential responses to the influenza A virus nucleoprotein (NP(366-374)) and acid polymerase (PA(224-233)) peptides presented by H2D(b) have been analyzed by disabling (N5--> Q substitution) these peptides in their native configuration, then expressing them in the viral neuraminidase protein. This strategy of shifting epitopes within the same viral context resulted in an apparent equalization of D(b)NP(366) [epitope consisting of viral nucleoprotein (NP) amino acid residues 366-374 complexed with the H2D(b) MHC class I glycoprotein] and D(b)PA(224) (H2D(b)+PA(224-233)) epitope abundance after direct infection in vitro and induced reproducible changes in the magnitude of the D(b)NP(366)- and D(b)PA(224)-specific T cell subsets generated after infection of mice. Comparison of D(b)NP(366)- and D(b) PA(224)-specific CD8(+) T cell responses induced from the native configuration and from the viral neuraminidase stalk demonstrated that the size of both primary and secondary responses is influenced by relative epitope levels and that, at least after secondary challenge, the magnitude of responses is also determined by CD8(+) T cell precursor frequency. Thus, this immunodominance hierarchy is a direct function of antigen dose and T cell numbers.

Evaluation of bystander recruitment and cytotoxic functions of the IFN-gamma producing alloreactive CD8+ T cells in mice

Vigorous CTL response against alloantigens, which is the main effector mechanism in acute allograft rejection, has been well described. Studies to monitor these responses in a quantitative manner has recently taken a new turn following the introduction of new quantitative flow cytometric methods such as CFSE cell proliferation and intracellular cytokine staining as alternatives to the conventional LDA assays. Although this technique has frequently been used in allogeneic systems in recent years, potential recruitment of non-antigen-specific bystander CD8+ T cells in the antigen-specific population has not been studied in detail. In addition, the degree of association between the cytotoxicity function and the IFN-gamma expression of CD8+ T cells has not been elucidated. In this study, we have investigated the bystander recruitment in a mouse allogeneic setting using ex vivo mixed lymphocyte culture (MLC) expanded allogeneic CD8+ T cells. By using a fluorescent labeling technique, primary unsensitized CD8+ cells were monitored for their potential to be stimulated to produce IFN-gamma when present in close proximity to activated cells during a 6-h incubation period. In addition, by using two different approaches, direct flow cytometric sorting of IFN-gamma producing cells as well as a direct comparison of IFN-gamma expression and cytolysis in LDA wells, we were able to determine the cytotoxic capacity of IFN-gamma producing CD8+ T cells. Our results demonstrated that antigen-non-specific CD8+ T cells are not recruited to produce IFN-gamma in vitro by alloantigen activated T cells. In addition, our results showed only a moderate correlation between the two functions of the alloreactive CD8+ T cells, and might also suggest the existence of non-cytotoxic subpopulations.

Genetically programmed B lymphocytes are highly efficient in inducing anti-virus protective immunity mediated by central memory CD8 T cells

The hallmarks of specific T cell immunity include proliferative expansion, acquisition of effector function and memory T cell formation. Here, we used priming with B lymphocytes transgenic for the dominant epitope (NP366-374) of the influenza virus nucleoprotein, to study the characteristics of the CD8 T cell memory response in C57Bl/6 mice and elucidate which subset of CD8 T cells memory mediates protection from disease. We found that (i) the size of the memory CTL response is independent of the priming dose and is similar to that induced by the live virus, (ii) priming with a low dose (3 x 10(2)cells/inoculum) of transgenic B lymphocytes confers a protective memory CTL response, and (iii) protection from disease is mediated by central memory (T(CM)) CD8 T cells.

Reducing the Stimulation of CD8+T Cells during Infection with Intracellular Bacteria Promotes Differentiation Primarily into a Central (CD62LhighCD44high) Subset

During infection with lymphocytic choriomeningitis virus, CD8(+) T cells differentiate rapidly into effectors (CD62L(low)CD44(high)) that differentiate further into the central memory phenotype (CD62L(high)CD44(high)) gradually. To evaluate whether this CD8(+) T cell differentiation program operates in all infection models, we evaluated CD8(+) T cell differentiation during infection of mice with recombinant intracellular bacteria, Listeria monocytogenes (LM) and Mycobacterium bovis (BCG), expressing OVA. We report that CD8(+) T cells primed during infection with the attenuated pathogen BCG-OVA differentiated primarily into the central subset that correlated to reduced attrition of the primed cells subsequently. CD8(+) T cells induced by LM-OVA also differentiated into central phenotype cells first, but the cells rapidly converted into effectors in contrast to BCG-OVA. Memory CD8(+) T cells induced by both LM-OVA as well as BCG-OVA were functional in that they produced cytokines and proliferated extensively in response to antigenic stimulation after adoptive transfer. During LM-OVA infection, if CD8(+) T cells were guided to compete for access to APCs, then they received reduced stimulation that was associated with increased differentiation into the central subset and reduced attrition subsequently. Similar effect was observed when CD8(+) T cells encountered APCs selectively during the waning phase of LM-OVA infection. Taken together, our results indicate that the potency of the pathogen can influence the differentiation and fate of CD8(+) T cells enormously, and the extent of attrition of primed CD8(+) T cells correlates inversely to the early differentiation of CD8(+) T cells primarily into the central CD8(+) T cell subset.

Frequency, Specificity, and Sites of Expansion of CD8+T Cells during Primary Pulmonary Influenza Virus Infection

We have used intracellular cytokine staining and MHC class I tetramer binding in conjunction with granzyme B protease expression and in vivo BrdU uptake to characterize the primary murine CD8(+) T cell response to pulmonary influenza virus infection. We have observed that the majority (>90%) of the CD8(+) T cell response to the A/Japan/305/57 virus in the lung at the peak of the response (days 9-11) is directed to four epitopes (three dominant and one subdominant). Using induction of granzyme B as a surrogate to identify specific activated CD8(+) T cells, we found that an unexpectedly large fraction ( approximately 70%) of lung-infiltrating CD8(+) T cells expressed granzyme B on day 6 of infection when estimates by MHC tetramer/intracellular cytokine staining yielded substantially lower frequencies ( approximately 30%). In addition, by using intranasal administration of BrdU during infection, we obtained evidence for proliferative expansion of activated CD8(+) T cells in the infected lung early (days 5-7) in the primary response. These results suggest that the frequency and number of specific CTL present in the lung early in infection may be underestimated by standard detection methods, and primary CD8(+) T cell expansion may occur in both secondary lymphoid organs and the infected lung.

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.

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CD8 T cell responses to viral infections in sequence

Our current understanding of virus-specific T cell responses has been shaped by model systems with mice, where naive animals are infected with a single viral pathogen. Paradigms derived from such models, however, may not always be applicable to a natural setting, where a host is exposed to numerous pathogens over its lifetime. Accumulating data in animal models and with some human diseases indicate that a host's prior history of infections can impact the specificity of future CD8 T cell responses, even to unrelated viruses. This can have both beneficial and detrimental consequences for the host, including altered clearance of virus, distinct forms of immunopathology, and substantial changes in the pool of memory T cells. Here we will describe the characteristics of CD8 T cells and the dynamics of their response to heterologous viral infections in sequence.

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.

Fewer CTL, not enhanced NK cells, are sufficient for viral clearance from the lungs of immunocompromised mice

Activation of the aryl hydrocarbon receptor (AhR) causes numerous defects in anti-viral immunity, including suppressed CTL generation and impaired host resistance. However, despite a reduced CTL response, mice that survive infection clear the virus. Therefore, we examined the contribution of NK cells and pro-inflammatory cytokines to viral clearance in influenza virus-infected mice exposed to TCDD, the most potent AhR agonist. Infection caused transient increases in pulmonary TNFalpha, IL-1, and IFNalpha/beta levels, but neither the kinetics nor magnitude of this response was affected by AhR activation. No IL-18 was detected at any time point examined. Exposure to TCDD enhanced NK cell numbers in the lung but did not affect their IFNgamma production. Furthermore, depletion of NK cells did not alter anti-viral cytolytic activity. In contrast, removal of CD8+ T cells ablated virus-specific cytolytic activity. These results demonstrate that the pulmonary CTL response to influenza virus is robust and few CTL are necessary for viral clearance.

A Switch in Costimulation from CD28 to 4-1BB during Primary versus Secondary CD8 T Cell Response to Influenza In Vivo

4-1BBL(-/-) mice exhibit normal primary CD8 T cell responses to influenza virus, but show decreased CD8 T cell numbers late in the primary response as well as decreased secondary responses. In contrast, CD28(-/-) mice are defective in initial CD8 T cell expansion. Using agonistic anti-4-1BB Ab to replace the CD28 or 4-1BB signal, we examined the timing of the required signals for CD28 vs 4-1BB costimulation. A single dose of agonistic anti-4-1BB Ab added only during priming restores the secondary CD8 T cell response in CD28(-/-) mice. Once the T cell numbers in the primary response reach a minimum threshold, a full secondary response is achieved even in the absence of CD28. In contrast, anti-4-1BB added during priming fails to correct the defective secondary response in 4-1BBL(-/-) mice, whereas addition of anti-4-1BB during challenge fully restores this response. Thus, there is a switch in costimulatory requirement from CD28 to 4-1BB during primary vs recall responses. Adoptive transfer studies show that T cells primed in 4-1BBL(-/-) or wild-type mice are equally capable of re-expansion when rechallenged in wild-type mice. These studies rule out a model in which signals delivered through 4-1BB during priming program the T cells to give a full recall response and suggest that 4-1BB-4-1BBL interactions take place at later stages in the immune response. The results indicate that anti-4-1BB or 4-1BBL therapy will be most effective during the boost phase of a prime-boost vaccination strategy.

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.

Differential antigen presentation regulates the changing patterns of CD8+ T cell immunodominance in primary and secondary influenza virus infections

The specificity of CD8+ T cell responses can vary dramatically between primary and secondary infections. For example, NP366-374/Db- and PA224-233/Db-specific CD8+ T cells respond in approximately equal numbers to a primary influenza virus infection in C57BL/6 mice, whereas NP366-374/Db-specific CD8+ T cells dominate the secondary response. To investigate the mechanisms underlying this changing pattern of immunodominance, we analyzed the role of antigen presentation in regulating the specificity of the T cell response. The data show that both dendritic and nondendritic cells are able to present the NP366-374/Db epitope, whereas only dendritic cells effectively present the PA224-233/Db epitope after influenza virus infection, both in vitro and in vivo. This difference in epitope expression favored the activation and expansion of NP366-374/Db-specific CD8+ memory T cells during secondary infection. The data also show that the immune response to influenza virus infection may involve T cells specific for epitopes, such as PA224-233/Db, that are poorly expressed at the site of infection. In this regard, vaccination with the PA224-233 peptide actually had a detrimental effect on the clearance of a subsequent influenza virus infection. Thus, differential antigen presentation impacts both the specificity of the T cell response and the efficacy of peptide-based vaccination strategies.

Analysis of clonotype distribution and persistence for an influenza virus-specific CD8+ T cell response

The spectrum of TCR V beta usage is compared for primary and recall CD8(+)D(b)PA(224)(+) T cell responses in mice with influenza pneumonia. Single-cell RT-PCR established that the same clonotypes were present in the lymphoid tissue and in the virus-infected lung. Longitudinal analysis indicated that the memory TCR repertoire reflects the primary response, with no decrease in diversity prior to (or after) secondary challenge. The re-engagement of memory T cells looked to be stochastic in this localized, transient infection. Analysis of clonotypes from the blood, spleen, regional lymph nodes, bone marrow, lung, and liver over a 200 day interval showed no evidence of selective localization or loss. The long-term distribution of memory T cells seemed to be essentially random.

Primary and secondary immunocompetence in mixed allogeneic chimeras

Targeted disruption of T cell costimulatory pathways, particularly CD28 and CD40, has allowed for the development of minimally myeloablative strategies for the induction of mixed allogeneic chimerism and donor-specific tolerance across full MHC barriers. In this study we analyze in depth the ability of mixed allogeneic chimeras in two strain combinations to mount effective host-restricted and donor-restricted antiviral CD4 and CD8 responses, as well as the impact of development of mixed chimerism on the maintenance of pre-existing memory populations. While antiviral CD8 responses in mixed chimeras following acute viral infection with lymphocytic choriomeningitis virus Armstrong or vaccinia virus are largely host-restricted, donor-restricted CD8 responses as well as host- and donor-restricted CD4 responses are also readily detected, and virus is promptly cleared. We further demonstrate that selection of donor-restricted T cells in mixed chimeras is principally mediated by bone marrow-derived cells in the thymus. Conversely, we find that mixed chimeras exhibit a deficit in their ability to deal with a chronic lymphocytic choriomeningitis virus clone 13 infection. Encouragingly, pre-existing memory populations are largely unaffected by the development of high level mixed chimerism and maintain the ability to control viral rechallenge. Our results suggest that while pre-existing T cell memory and primary immunocompetence to acute infection are preserved in mixed allogeneic chimeras, MHC class I and/or class II tissue matching may be required to fully preserve immunocompetence in dealing with chronic viral infections.

Production of TARC and MDC by naive T cells in asthmatic patients

The helper (Th)2 cell-attracting chemokines thymus and activation-regulated chemokine (TARC) and macrophage-derived chemokine (MDC) are ligands for the chemokine receptor CCR4. A number of cellular sources of TARC and MDC have been identified, including not only macrophages, dendritic cells, and natural killer cells, but also bronchial epithelial cells. Recent studies report that TARC and MDC may serve as pivotal chemokines for the development of Th2-dominated experimental allergen-induced asthma. This study was designed to assess TARC and MDC production by CD4+ T cells, including naive T cells and memory/effector T cells, purified from peripheral blood mononuclear cells in patients with asthma. Asthmatic subjects included in this study had mild asthmatic symptoms, positive skin test responses to house dust mite allergen, and elevated level of Dermatophagoides farinae immunoglobulin E in the sera. CD4+ T cells--CD45RA+ CD4+ T cells--as naive T cells and CD45RO+ CD4+ T cells--as memory/effector T cells--were purified by negative selection from peripheral blood mononuclear cells obtained from asthmatic patients (n = 6) and healthy controls (n = 6). These cells and established Th1/Th2 cell lines were then cultured in the presence of both anti-CD3 and -CD28 antibodies. After 48 hr of incubation, concentrations of TARC, MDC, interleukin (IL)-4, IL-5, and interferon-gamma in the supernatants were measured by enzyme-linked immunoadsorbent assay. Reverse transcriptase-polymerase chain reaction was performed to analyze mRNA expression of TARC and MDC. Our results clearly showed that TARC and MDC were produced by activated CD45RA+ CD4+ T cells rather than by activated CD45RO+ CD4+ T cells, and the levels of these chemokines in the asthmatic patients were higher than those in the healthy controls. Furthermore, these chemokines production by Th2 cell lines were greater than those by Th1 cell lines, but the level were smaller than those by naive T cells. Our studies suggest that TARC and MDC are produced by naive T cells rather than by memory/effector T cells, including Th2 cells, in asthmatic patients, and these chemokines were produced at modest levels in any T-cell populations from healthy controls. Taken together, naive T cells in asthma have a peculiar function to produce TRAC and MDC, which contribute to local migration of Th2 cells into lung and lymphoid tissues, along with a function as precursor for memory/effector T cell. This novel function of naive T cells may be implicated in the development of asthma.

Enhanced type 1 immunity after secondary viral challenge in mice primed as neonates

The goal of infant immunization against viral infection is to develop protective long term memory responses. Priming neonatal mice with a low dose of Cas-Br-E murine leukemia virus (Cas) results in adult-like, type 1 protective responses. However, other studies suggest that Ag priming of neonates leads to an increase in type 2 secondary responses even when primary responses were type 1. We assessed whether type 1 CD8+ T cell-mediated responses developed in murine neonates are maintained after secondary challenge with Cas in adulthood. Despite the induction of significant anti-viral CD8+-mediated cytotoxic T lymphocyte and IFN-gamma responses, initial neonatal priming led to a lower frequency of virus-specific T cells compared with adult priming. Adult frequencies were reached in mice primed as neonates only after secondary challenge in adulthood. A nonspecific and transient CD4+-mediated IL-4 response was present in all groups after secondary challenge with Cas or medium, indicating that this rise in type 2 cytokine production was not unique to mice that had been primed as neonates. Rather, type 1 anti-viral memory CD8+ T cell responses developed in neonatal mice are stable, protective, and enhanced after secondary challenge.

Bacterial superantigen exposure after resolution of influenza virus infection perturbs the virus-specific memory CD8(+)-T-cell repertoire

Heterologous viral infections have been shown to impact the preexisting memory CD8(+)-T-cell repertoire. Bacterial superantigens are products of common human pathogenic bacteria, including staphylococci and streptococci, that are potent T-cell-stimulatory molecules. In this report, we show that exposure to staphylococcal enterotoxin B, a bacterial superantigen, causes a selective functional deletion of cross-reactive influenza virus-specific CD8(+) memory T cells. This perturbation of the memory repertoire can have a significant impact on viral clearance after secondary challenge.

The pas de deux of viruses and CD8 T cells

The vertebrate immune system has evolved to deal with parasitic life forms, so it is hardly surprising that experiments with pathogens have proved illuminating for immunology. Those of us who have worked for years with infectious processes are acutely conscious that we do little more than probe the vast 'experiment of nature.' There is no place for doctrinaire rigidity in this extraordinarily complex area of biology. Though we have obviously tried to do rational experiments, much of the novelty that has been brought in to immunology from the analysis of the virus-specific host response has, in a very real sense, been the product of what we now call 'discovery science'. The following relates some of the research that I was involved in personally and attempts to put it in the context of both the history of the field and the events of the time. Virus infections, particularly HIV, pose enormous challenges for the future. It is generally helpful to know a little of what went before.

Dynamics of memory T cell proliferation under conditions of heterologous immunity and bystander stimulation

By examining adoptively transferred CSFE-labeled lymphocytic choriomeningitis virus (LCMV)-immune donor T cells in Thy-1 congenic hosts inoculated with viruses or with the cytokine inducer poly(I:C), strikingly different responses of bona fide memory T cells were found in response to different stimuli. Poly(I:C) (cytokine) stimulation caused a limited synchronized division of memory CD8 T cells specific to each of five LCMV epitopes, with no increase and sometimes a loss in number, and no change in their epitope hierarchy. Homologous LCMV infection caused more than seven divisions of T cells specific for each epitope, with dramatic increases in number and minor changes in hierarchy. Infections with the heterologous viruses Pichinde and vaccinia (VV) caused more than seven divisions and increases in number of T cells specific to some putatively cross-reactive but not other epitopes and resulted in substantial changes in the hierarchy of the LCMV-specific T cells. Hence, there can be memory T cell division without proliferation (i.e., increase in cell number) in the absence of Ag and division with proliferation in the presence of Ag from homologous or heterologous viruses. Heterologous protective immunity between viruses is not necessarily reciprocal, given that LCMV protects against VV but VV does not protect against LCMV. VV elicited proliferation of LCMV-induced CD8 and CD4 T cells, whereas LCMV did not elicit proliferation of VV-induced T cells. Thus, depending on the pathogen and the sequence of infection, a heterologous agent may selectively stimulate the memory pool in patterns consistent with heterologous immunity.

No one is naive: the significance of heterologous T-cell immunity

Memory T cells that are specific for one virus can become activated during infection with an unrelated heterologous virus, and might have roles in protective immunity and immunopathology. The course of each infection is influenced by the T-cell memory pool that has been laid down by a host's history of previous infections, and with each successive infection, T-cell memory to previously encountered agents is modified. Here, we discuss evidence from studies in mice and humans that shows the importance of this phenomenon in determining the outcome of infection.

IL-21 limits NK cell responses and promotes antigen-specific T cell activation: a mediator of the transition from innate to adaptive immunity

IFNalpha/beta, IL-12, and IL-15 regulate NK cell activation and expansion, but signals triggering resolution of the NK response upon induction of adaptive immunity remain to be defined. We now report that IL-21, a product of activated T cells, may serve this function. Mice lacking IL-21R (IL-21R(-/-)) had normal NK cell development but no detectable responses to IL-21. IL-21 enhanced cytotoxic activity and IFNgamma production by activated murine NK cells but did not support their viability, thus limiting their duration of activation. Furthermore, IL-21 blocked IL-15-induced expansion of resting NK cells, thus preventing the initiation of further innate responses. In contrast, IL-21 enhanced the proliferation, IFNgamma production, and cytotoxic function of CD8(+) effector T cells in an allogeneic MLR. These observations suggest that IL-21 promotes the transition between innate and adaptive immunity.

Epitope-specific evolution of human CD8(+) T cell responses from primary to persistent phases of Epstein-Barr virus infection

Primary virus infection often elicits a large CD8(+) T cell response which subsequently contracts to a smaller memory T cell pool; the relationship between these two virus-specific populations is not well understood. Here we follow the human CD8(+) T cell response to Epstein-Barr virus (EBV) from its primary phase in infectious mononucleosis (IM) through to the persistent carrier state. Using HLA-A2.1 or B8 tetramers specific for four lytic cycle and three latent cycle epitopes, we find marked differences in the epitope-specific composition of the T cell populations between the two phases of infection. The primary response is dominated by lytic epitope specificities which are severely culled (and in one case extinguished) with resolution of the acute infection; in contrast latent epitope specificities are less abundant, if present at all, in acute IM but often then increase their percentage representation in the CD8 pool. Even comparing epitopes of the same type, the relative size of responses seen in primary infection does not necessarily correlate with that seen in the longer term. We also follow the evolution of phenotypic change in these populations and show that, from a uniform CD45RA(-)RO(+)CCR7(-) phenotype in IM, lytic epitope responses show greater reversion to a CD45RA(+)RO(-) phenotype whereas latent epitope responses remain CD45RA(-)RO(+) with a greater tendency to acquire CCR7. Interestingly these phenotypic distinctions reflect the source of the epitope as lytic or latent, and not the extent to which the response has been amplified in vivo.