Viral persistence alters CD8 T-cell immunodominance and tissue distribution and results in distinct stages of functional impairment

Cutting edge: emergence of CD127high functionally competent memory T cells is compromised by high viral loads and inadequate T cell help

In this report we have inspected whether difficulties in controlling viral infections negatively impacts the generation of CD127(high) memory T cells. Using both MHC class I and II tetramers we reveal that CD127(low) T cells are not necessarily rapidly deleted but can persist in a pseudoeffector state in which they display the hallmarks of activated effector cells but are functionally inferior. CD127(high) cells can, however, emerge if the infection is contained. We also show that in the absence of CD4 T cell help significant populations of CD127(high) CD8 T cells fail to emerge. Analyses of cytokine-producing activities by both mouse and human CD8 T cells further document that the extended maintenance of T cells in a CD127(low) state has functional consequences which manifest as an impairment of IL-2 production.

Oscillating CD8(+) T cell effector functions after antigen recognition in the liver

When hepatitis B virus (HBV)-specific CD8(+) cytotoxic T lymphocytes (CTLs) are adoptively transferred into HBV transgenic mice, they enter the liver, recognize antigen, secrete interferon gamma (IFNgamma), inhibit viral replication, and kill their target cells, causing hepatitis. In the current study, we examined the impact of antigen recognition on the evolution of the activation phenotype, antiviral effector functions, expansion and contraction kinetics, and compartmentalization of the transferred CTLs. The results reveal that noncytolytic and cytolytic effector functions and expansion-contraction kinetics of the CTLs are regulated asynchronously and in an oscillatory manner as a consequence of antigen recognition in the liver and in association with PD-1 upregulation. We suggest that such oscillations maximize viral clearance and minimize tissue injury during HBV infection and that poor coordination of these events could lead to viral persistence and chronic liver disease.

Non-malignant clonal expansions of CD8+ memory T cells in aged individuals

CD8(+) T cells provide a major line of defense against intracellular pathogens. Upon encounter with antigen, CD8(+) T cells go through three distinct phases involving proliferation, contraction, and differentiation to become eventually long-lived CD8(+) memory T cells. CD8(+) memory T cells provide long-term protection against infection by intracellular pathogens. CD8(+) memory T-cell proliferation and survival are regulated by many factors, including cytokines, and CD8(+) memory T cells are stably maintained over a period of months to years. In aged humans and mice, however, there are significant alterations to the CD8(+) memory T-cell compartment with frequent development of monoclonal expansions of CD8(+) memory T cells in healthy individuals. Interestingly, CD8(+) clonal expansions are not malignant and do not progress to lymphomas, suggesting that these cells must still be under certain constraints. In this review, we discuss our current understanding of factors that contribute to and regulate these CD8(+) clonal expansions as well as the impact of CD8(+) clonal expansions on immune function of the aged. In addition, we discuss similarities and differences between CD8(+) clonal expansions observed in humans and mice, and we postulate that CD8(+) clonal expansions represent a spectrum of biological outcomes ranging from antigen-driven to antigen-independent phenomena.

Evolution of diverse antiviral CD8+ T cell populations after murine cytomegalovirus infection

Cytomegalovirus (CMV) is a major human pathogen normally controlled by cellular immune responses. The infection can be modeled in the mouse using murine CMV (MCMV). During the latent phase of infection, two different patterns of CD8(+) T cell responses have been observed: some specificities show increasing frequencies over time ("memory inflation"), while others, which are present acutely, are barely detectable at later time points. This distinction is independent of initial immunodominance. We analyzed the extent to which such responses differ functionally and tracked both their population distribution and their evolution over time. We observed two clear patterns of memory development that diverged early after infection. Acutely, CD8(+) T cells directed against all epitopes showed similar activation, phenotype and distribution. Thereafter, one set of responses ("inflationary") increased in frequency over time, was found in high numbers in non-lymphoid organs and was associated with an activated (CD28(low) CD27(low)CD122(low)) phenotype. In contrast, CD8(+) T cells responses specific for other MCMV epitopes ("non-inflationary") showed a slow reversion to a classical "central" memory phenotype without enrichment in non-lymphoid organs. A simple model to describe the equilibrium state in MCMV is presented, which may point to previously unexplored antiviral populations present after human CMV infection.

Late Priming and Variability of Epitope-Specific CD8+T Cell Responses during a Persistent Virus Infection

Control of persistently infecting viruses requires that antiviral CD8(+) T cells sustain their numbers and effector function. In this study, we monitored epitope-specific CD8(+) T cells during acute and persistent phases of infection by polyoma virus, a mouse pathogen that is capable of potent oncogenicity. We identified several novel polyoma-specific CD8(+) T cell epitopes in C57BL/6 mice, a mouse strain highly resistant to polyoma virus-induced tumors. Each of these epitopes is derived from the viral T proteins, nonstructural proteins produced by both productively and nonproductively (and potentially transformed) infected cells. In contrast to CD8(+) T cell responses described in other microbial infection mouse models, we found substantial variability between epitope-specific CD8(+) T cell responses in their kinetics of expansion and contraction during acute infection, maintenance during persistent infection, as well as their expression of cytokine receptors and cytokine profiles. This epitope-dependent variability also extended to differences in maturation of functional avidity from acute to persistent infection, despite a narrowing in TCR repertoire across all three specificities. Using a novel minimal myeloablation-bone marrow chimera approach, we visualized priming of epitope-specific CD8(+) T cells during persistent virus infection. Interestingly, epitope-specific CD8(+) T cells differed in CD62L-selectin expression profiles when primed in acute or persistent phases of infection, indicating that the context of priming affects CD8(+) T cell heterogeneity. In summary, persistent polyoma virus infection both quantitatively and qualitatively shapes the antiviral CD8(+) T cell response.

Human immunodeficiency virus (HIV)-specific gamma interferon secretion directed against all expressed HIV genes: relationship to rate of CD4 decline

Immune responses to human immunodeficiency virus (HIV) are detected at all stages of infection and are believed to be responsible for controlling viremia. This study seeks to determine whether gamma interferon (IFN-gamma)-secreting HIV-specific T-cell responses influence disease progression as defined by the rate of CD4 decline. The study population consisted of 31 subjects naive to antiretroviral therapy. All were monitored clinically for a median of 24 months after the time they were tested for HIV-specific responses. The rate of CD4+-T-cell loss was calculated for all participants from monthly CD4 counts. Within this population, 17 subjects were classified as typical progressors, 6 subjects were classified as fast progressors, and 8 subjects were classified as slow progressors. Peripheral blood mononuclear cells were screened for HIV-specific IFN-gamma responses to all expressed HIV genes. Among the detected immune responses, 48% of the recognized peptides were encoded by Gag and 19% were encoded by Nef gene products. Neither the breadth nor the magnitude of HIV-specific responses correlated with the viral load or rate of CD4 decline. The breadth and magnitude of HIV-specific responses did not differ significantly among typical, fast, and slow progressors. These results support the conclusion that although diverse HIV-specific IFN-gamma-secreting responses are mounted during the asymptomatic phase, these responses do not seem to modulate disease progression rates.

Dendritic cells as therapeutic vaccines against cancer

Mouse studies have shown that the immune system can reject tumours, and the identification of tumour antigens that can be recognized by human T cells has facilitated the development of immunotherapy protocols. Vaccines against cancer aim to induce tumour-specific effector T cells that can reduce the tumour mass, as well as tumour-specific memory T cells that can control tumour relapse. Owing to their capacity to regulate T-cell immunity, dendritic cells are increasingly used as adjuvants for vaccination, and the immunogenicity of antigens delivered by dendritic cells has now been shown in patients with cancer. A better understanding of how dendritic cells regulate immune responses will allow us to better exploit these cells to induce effective antitumour immunity.

HIV-1-specific IFN-gamma/IL-2-secreting CD8 T cells support CD4-independent proliferation of HIV-1-specific CD8 T cells

Functional and phenotypic characterization of virus-specific CD8 T cells against cytomegalovirus, Epstein-Barr virus, influenza (flu), and HIV-1 were performed on the basis of the ability of CD8 T cells to secrete IFN-gamma and IL-2, to proliferate, and to express CD45RA and CCR7. Two functional distinct populations of CD8 T cells were identified: (i) dual IFN-gamma/IL-2-secreting cells and (ii) single IFN-gamma-secreting cells. Virus-specific IFN-gamma/IL-2-secreting CD8 T cells were CD45RA-CCR7-, whereas single IFN-gamma CD8 T cells were either CD45RA-CCR7- or CD45RA+CCR7-. The proportion of virus-specific IFN-gamma/IL-2-secreting CD8 T cells correlated with that of proliferating CD8 T cells, and the loss of HIV-1-specific IL-2-secreting CD8 T cells was associated with that of HIV-1-specific CD8 T cell proliferation. Substantial proliferation of virus-specific CD8 T cells (including HIV-1-specific CD8 T cells) was also observed in CD4 T cell-depleted populations or after stimulation with MHC class I tetramer-peptide complexes. IL-2 was the factor responsible for the CD4-independent CD8 T cell proliferation. These results indicate that IFN-gamma/IL-2-secreting CD8 T cells may promote antigen-specific proliferation of CD8 T cells even in the absence of helper CD4 T cells.

CD8 memory T cells: cross-reactivity and heterologous immunity

Virus-specific memory T cell populations demonstrate plasticity in antigen recognition and in their ability to accommodate new memory T cell populations. The degeneracy of T cell antigen recognition and the flexibility of diverse antigen-specific repertoires allow the host to respond to a multitude of pathogens while accommodating these numerous large memory pools in a finite immune system. These cross-reactive memory T cells can be employed in immune responses and mediate protective immunity, but they can also induce life-threatening immunopathology or impede transplantation tolerance and graft survival. Here we discuss examples of altered viral pathogenesis occurring as a consequence of heterologous T cell immunity and propose models for the maintenance of a dynamic pool of memory cells.

Generation and maintenance of immunological memory

The key feature of the adaptive immune response is its specificity and the ability to generate and maintain memory. Preexisting antibodies in the circulation and at the mucosa provide the first line of defense against re-infection by extracellular as well as intracellular pathogens. Memory T cells are an important second line of defense against intracellular pathogens, and in particular against microbes that can cause chronic or latent infection. In this article we will review our current understanding of the generation and maintenance of B cell and T cell memory.

Inverse correlation between IL-7 receptor expression and CD8 T cell exhaustion during persistent antigen stimulation

Persistence is a hallmark of infection by viruses such as HIV, hepatitis B virus, hepatitis C virus and LCMV. In the case of LCMV, persistence may often be associated with exhaustion of CD8(+) T cells. We demonstrate here that persistent antigen suppressed IL-7Ralpha expression and this correlated with T cell exhaustion and reduced expression of the anti-apoptotic molecule B cell leukemia/lymphoma 2 (Bcl-2). In contrast, exposure to short-lived antigen only temporarily suppressed IL-7Ralpha expression, failed to induce T cell exhaustion, and primed T cells. Persistent antigen also suppressed IL-7Ralpha expression on primed T cells and this correlated with exhaustion of a previously stable primed T cell population. These findings suggest that antigen longevity regulates T cell fate.

Adjuvant IL-7 or IL-15 overcomes immunodominance and improves survival of the CD8+ memory cell pool

Current models of T cell memory implicate a critical role for IL-7 in the effector-to-memory transition, raising the possibility that IL-7 therapy might enhance vaccine responses. IL-7 has not been studied, to our knowledge, before now for adjuvant activity. We administered recombinant human IL-7 (rhIL-7) to mice during immunization against the male antigen HY and compared these results with those obtained from mice immunized with rhIL-2 and rhIL-15. Administration of rhIL-7 or rhIL-15, but not rhIL-2, increased effector cells directed against these dominant antigens and dramatically enhanced CD8(+) effectors to subdominant antigens. The mechanisms by which the cytokines augmented effector pool generation were multifactorial and included rhIL-7-mediated costimulation and rhIL-15-mediated augmentation of the proliferative burst. The contraction phase of the antigen-specific response was exaggerated in cytokine-treated mice; however, CD8(+) memory pools in rhIL-7- or rhIL-15-treated groups demonstrated superior long-term survival resulting in quantitative advantages that remained long after the cytokines were discontinued, as demonstrated by improved survival after challenge with an HY-expressing tumor undertaken several weeks after cytokine cessation. These results confirm the adjuvant activity of rhIL-15 and demonstrate that rhIL-7 also serves as a potent vaccine adjuvant that broadens immunity by augmenting responses to subdominant antigens and improving the survival of the CD8(+) T cell memory pool.

Rapid Functional Exhaustion and Deletion of CTL following Immunization with Recombinant Adenovirus

Replication-deficient adenoviruses (recombinant adenovirus (rec-AdV)) expressing different transgenes are widely used vectors for gene therapy and vaccination. In this study, we describe the tolerization of transgene-specific CTL following administration of beta-galactosidase (beta gal)-recombinant adenovirus (Ad-LacZ). Using MHC class I tetramers to track beta gal-specific CTL, we found that a significant expansion of beta gal-specific CTL was restricted to a very narrow dose range. Functional analysis revealed that adenovirus-induced beta gal-specific CTL produced only very low amounts of effector cytokines and were unable to exhibit cytolytic activity in a 51Cr release assay. Furthermore, Ad-LacZ vaccination failed to efficiently clear established beta gal-positive tumors. The impaired function of Ad-LacZ-induced CTL correlated with the presence of persisting beta gal Ag in the liver. A further increase in the peripheral Ag load by injection of Ad-LacZ into SM-LacZ transgenic mice which express beta gal as self-Ag exclusively in peripheral nonlymphoid organs, resulted in the physical deletion of beta gal-specific CTL. Our results indicate first that CTL deletion in the course of adenoviral vaccination is preceded by their functional impairment and second, that the outcome of rec-AdV vaccination depends critically on the Ag load in peripheral tissues.

Signature for long-term vaccine-mediated control of a Simian and human immunodeficiency virus 89.6P challenge: stable low-breadth and low-frequency T-cell response capable of coproducing gamma interferon and interleukin-2

In 2001, we reported 20 weeks of control of challenge with the virulent 89.6P chimera of simian and human immunodeficiency viruses (SHIV-89.6P) by a Gag-Pol-Env vaccine consisting of DNA priming and modified vaccinia virus Ankara boosting. Here we report that 22 out of 23 of these animals successfully controlled their viremia until their time of euthanasia at 200 weeks postchallenge. At euthanasia, all animals had low to undetectable viral loads and normal CD4 counts. During the long period of viral control, gamma interferon (IFN-gamma)-producing antiviral T cells were present at unexpectedly low breadths and frequencies. Most animals recognized two CD8 and one CD4 epitope and had frequencies of IFN-gamma-responding T cells from 0.01 to 0.3% of total CD8 or CD4 T cells. T-cell responses were remarkably stable over time and, unlike responses in most immunodeficiency virus infections, maintained good functional characteristics, as evidenced by coproduction of IFN-gamma and interleukin-2. Overall, high titers of binding and neutralizing antibody persisted throughout the postchallenge period. Encouragingly, long-term control was effective in macaques of diverse histocompatibility types.

Epitope down-modulation as a mechanism for the coexistence of competing T-cells

Efficient immune responses against pathogens are frequently characterized by the simultaneous targeting of multiple epitopes. However, it remains unclear how the targeting of multiple epitopes is maintained in the face of competition for antigenic stimulation. Here, we investigate this question by using mathematical models of the population dynamics of a viral pathogen, antigen presentation sites and T-cells. We first show that direct competition for access to antigen presenting sites and indirect competition through killing of the pathogen select for dominance of the T-cell response with the highest affinity for its epitope. We then incorporate in our model that epitopes can become down-modulated following interaction with epitope specific T-cells. We demonstrate that epitope down-modulation leads to differentiation of epitope presentation on antigen presenting sites. This differentiation promotes the coexistence of multiple epitope specific responses. Hence, we propose that the functional relevance of epitope down-modulation may be to enable the persistence of a broad immune response despite competition for antigenic stimulation.

Reduced hepatitis B virus (HBV)-specific CD4+ T-cell responses in human immunodeficiency virus type 1-HBV-coinfected individuals receiving HBV-active antiretroviral therapy

Functional hepatitis B virus (HBV)-specific T cells are significantly diminished in individuals chronically infected with HBV compared to individuals with self-limiting HBV infection or those on anti-HBV therapy. In individuals infected with human immunodeficiency virus type 1 (HIV-1), coinfection with HBV is associated with an increased risk of worsening liver function following antiviral therapy and of more rapid HBV disease progression. Total HBV-specific T-cell responses in subjects with diverse genetic backgrounds were characterized by using a library of 15-mer peptides overlapping by 11 amino acids and spanning all HBV proteins. The magnitude and breadth of CD4(+) and CD8(+) T-cell responses to HBV in peripheral blood were examined by flow cytometry to detect gamma interferon production following stimulation with HBV peptide pools. Chronic HBV carriers (n = 34) were studied, including individuals never treated for HBV infection (n = 7), HBV-infected individuals receiving anti-HBV therapy (n = 13), and HIV-1-HBV-coinfected individuals receiving anti-HBV therapy (n = 14). CD4(+) and CD8(+) HBV-specific T-cell responses were more frequently detected and the CD8(+) T-cell responses were of greater magnitude and breadth in subjects on anti-HBV treatment than in untreated chronic HBV carriers. There was a significant inverse correlation between detection of a HBV-specific T-cell response and HBV viral load. HBV-specific CD4(+) and CD8(+) T-cell responses were significantly (fivefold) reduced compared with HIV-specific responses. Although, the frequency and breadth of HBV-specific CD8(+) T-cell responses were comparable in the monoinfected and HIV-1-HBV-coinfected groups, HBV-specific CD4(+) T-cell responses were significantly reduced in HIV-1-HBV-coinfected individuals. Therefore, HIV-1 infection has a significant and specific effect on HBV-specific T-cell immunity.

Divergent in vitro and in vivo correlates of HIV-specific T-cell responses during onset of HIV viraemia

BACKGROUND

Cellular immune responses to HIV-1 have been examined mainly in peripheral blood mononuclear cells (PBMC). During onset of HIV replication and antigenaemia after discontinuation of highly active antiretroviral therapy (HAART), PBMC may theoretically contain HIV-specific T cells that are qualitatively and quantitatively different from specific T cells dominating in the tissues. PBMC responses throughout HIV immunotherapy trials may therefore be skewed during recurrent viraemia.

OBJECTIVE

To compare cellular HIV-specific in vitro responses in PBMC during onset of HIV viraemia with corresponding in vivo responses, represented by classical delayed-type hypersensitivity tests (DTH).

METHODS

HIV patients (n = 38), pre-immunized with four HIV-1 p24-like consensus peptides (Vacc-4x) during HAART, were subjected to a 14-week treatment interruption with recurrent HIV viraemia. Proliferative T-cell responses to Vacc-4x p24 peptides, HIV p24 protein, and cytomegalovirus (CMV) proteins were measured in PBMC. Corresponding Vacc-4x peptide DTH were expressed as skin infiltrate areas after 48 h.

RESULTS

After 14 weeks without HAART, HIV-1 RNA increased to 72,500 copies/ml (median). The Vacc-4x p24 peptide- and HIV-1 p24 protein-induced T-cell proliferation concurrently decreased by 81 and 93% in PBMC during viraemia (medians, P < or = 0.03), whereas proliferative responses to CMV antigens were stable. In contrast, the Vacc-4x DTH areas, rather tended to increase by 36% (P = 0.08) and contained infiltrates dominated by proliferating T cells and macrophages.

CONCLUSIONS

Divergent in vitro and in vivo HIV-specific cellular immune responses were found during recurrent HIV viraemia. The clinical relevance of both surrogate markers for HIV-related immune responses should be compared in future studies.

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.

Characterization of functional and phenotypic changes in anti-Gag vaccine-induced T cell responses and their role in protection after HIV-1 infection

Worldwide HIV-1 vaccine efforts are guided by the principle that HIV-specific T cell responses may provide protection from infection or delay overt disease. However, no clear correlates of T cell-mediated immune protection have been identified. Here, we examine in a HLA-B27(+) HIV seronegative vaccinee persistent HIV-specific vaccine-induced anti-Gag CD4(+) and CD8(+) T cell responses. Although these responses exhibited those characteristics (multifunctionality, appropriate memory phenotype, and targeting of epitopes associated with long-term nonprogression) predicted to correlate with protection from infection, the subject became HIV infected. After HIV infection, the vaccine-induced CD8(+) T cells expanded, but both CD4(+) and CD8(+) T cell responses acquired the functional and phenotypic patterns characteristic of chronic HIV infection. The virus quickly escaped the vaccine-induced T cell response, and the subject progressed more rapidly than expected for someone expressing the HLA-B27 allele. These data suggest that control of HIV by vaccine-elicited HIV-specific T cell responses may be difficult, even when the T cell response has those characteristics predicted to provide optimal protection.

T cells undergo rapid ON/OFF but not ON/OFF/ON cycling of cytokine production in response to antigen

Inflammatory cytokines such as IFN-gamma and TNF produced by Ag-stimulated CD4(+) and CD8(+) T cells are important in defense against microbial infection. However, production of these cytokines must be tightly regulated to prevent immunopathology. Previous studies, conducted with BALB/c mice, have suggested that 1) CD8(+) T cells maintain IFN-gamma production but transiently produce TNF in the continued presence of Ag and 2) lymphocytic choriomeningitis virus-specific and in vitro-propagated effector CD8(+) T cells could rapidly cycle IFN-gamma production ON/OFF/ON in response to Ag exposure, removal, and re-exposure. In contrast with CD8(+) T cells, our results show that Listeria monocytogenes-specific CD4(+) T cells from C57BL/6 mice rapidly initiate (ON cycling) and maintain production of both IFN-gamma and TNF in the continued presence of Ag. Upon Ag removal, production of both cytokines rapidly ceases (OFF cycling). However, if the initial stimulation was maximal, Ag-specific CD4(+) T cells were unable to reinitiate cytokine production after a second Ag exposure. Furthermore, L. monocytogenes-specific CD8(+) T cells in the same mice and lymphocytic choriomeningitis virus-specific CD8(+) T cells in BALB/c mice also underwent ON/OFF cycling, but if the initial Ag stimulus was maximal, they could not produce IFN-gamma after Ag re-exposure. As the initial Ag dose was reduced, the number of cells producing cytokine in response to the second Ag exposure exhibited a corresponding increase. However, T cells that were marked for IFN-gamma secretion during the first stimulation did not contribute cytokine production during the second stimulation. Thus, T cells are not able to undergo rapid ON/OFF/ON cytokine cycling in vitro in response to Ag.

Breast Milk-Derived Antigen-Specific CD8+ T Cells: An Extralymphoid Effector Memory Cell Population in Humans

Although mouse studies have demonstrated the presence of an effector memory population in nonlymphoid tissues, the phenotype of human CD8(+) T cells present in such compartments has not been characterized. Because of the relatively large number of CD8(+) T cells present in breast milk, we were able to characterize the phenotype of this cell population in HIV-infected and uninfected lactating women. CMV, influenza virus, EBV, and HIV-specific CD8(+) T cells as measured by the IFN-gamma ELISPOT and MHC class I tetramer staining were all present at greater frequencies in breast milk as compared with blood. Furthermore, a greater percentage of the breast milk CD8(+) T cells expressed the intestinal homing receptor, CD103, and the mucosal homing receptor CCR9. Breast milk T cells were predominantly CD45RO(+)HLADR(+) and expressed low levels of CD45RA, CD62L, and CCR7 consistent with an effector memory population. Conversely, T cells derived from blood were mainly characterized as central memory cells (CCR7(+)CD62L(+)). These results demonstrate a population of extralymphoid CD8(+) T cells with an effector memory phenotype in humans, which could contribute to enhanced local virologic control and the relative lack of HIV transmission via this route.

The functional avidity of virus-specific CD8+ T?cells is down-modulated in Borna disease virus-induced immunopathology of the central nervous system

Borna disease virus (BDV) infection of the central nervous system (CNS) leads to severe neurological symptoms in susceptible MRL mice. The disease is mainly mediated by CD8+ T cells specific for the immunodominant epitope TELEISSI in the BDV nucleoprotein. In this study, TELEISSI/MHC class I tetramers were used to directly visualize antigen-specific CD8+ T cells. We found that on average approximately 30% of the ex vivo analyzed CD8+ T cells in the CNS of diseased mice were specific for TELEISSI. Unexpectedly, the frequency of tetramer-reactive brain-derived CD8+ T cells doubled following overnight culture in the absence of antigen. The majority of CD8+ T cells showed enhanced tetramer binding without up-regulation of T cell receptor surface expression. The frequency of IFN-gamma-secreting CD8+ T cells after antigen-specific stimulation was higher in overnight cultures than in freshly isolated BDV-specific brain lymphocytes, and enhanced tetramer binding correlated with elevated sensitivity to lower levels of peptide antigen in cytotoxicity assays. These results indicate that the functional avidity of virus-specific CD8+ T cells was down-modulated in vivo. Thus, quantification of tissue-infiltrating CD8+ T cells by the tetramer technique must be interpreted with caution as it may underestimate the real frequency of antigen-specific CD8+ T cells.

Differential regulation of virus-specific T-cell effector functions following activation by peptide or innate cytokines

Robust CD8+ T-cell activation is vital for the recovery from many viral infections and is orchestrated via the integration of signals delivered through surface molecules, including the T-cell antigen receptors (TcRs) and cytokine receptors. Little is known about how virus-specific T cells interpret sequential or combined stimulation through these receptors, which must undoubtedly occur in vivo during antiviral immune responses. When measured in real time, peptide antigen and the cytokines, interleukin 12 (IL-12) and IL-18, independently regulate the on/off kinetics of protective (interferon γ, tumor necrosis factor α) and immunomodulatory (IL-2, CD40L) cytokine production by activated T cells and memory T cells. The remarkable differences in effector functions elicited by innate or adaptive signals (IL-12/ IL-18 or peptide, respectively) illustrate the complex and stringent regulation of cytokine expression by CD8+ T cells. Together, these results indicate how antiviral T cells incorporate multiple signals from their local microenvironment and tailor their cytokine responses accordingly.

Shortening the infectious period does not alter expansion of CD8 T cells but diminishes their capacity to differentiate into memory cells

Following a primary immune response, a portion of effector T cells gives rise to long-lived memory cells. Although primary expansion and differentiation of effector CD8 T cells is dictated by a brief exposure to Ag, it is unclear whether full memory differentiation is also programmed within the same short window. By carefully modulating the kinetics of Listeria monocytogenes infection, we analyzed the requirements for the programming of effector and memory T cell development in vivo. We find that although limiting the infectious period to the first 24-48 h does not impact the size of the primary CD8 response, the ensuing memory population is significantly diminished. This effect is particularly pronounced in the development of tissue-homing memory cells and is inversely proportional to the initial infectious dose. In contrast to CD8 responses, the differentiation of primary CD4 responses was highly dependent on the continued presence of the infection. Shortening the duration of the infection greatly reduced the development of CD4 effector responses in the spleen and prevented their trafficking to peripheral sites of infection. We propose that the stimulus received by CD8 T cells during the early stages of infection largely contribute to the differentiation of CD8 effector cells, whereas continued or distinct signals received at later stages influence their ability to differentiate into memory cells.

Apoptosis and loss of virus-specific CD8+ T-cell memory

CD8⁺ T-cell memory to viruses is stable in the absence but volatile in the presence of other infections. Apoptotic events that occur early in acute infections delete pre-existing memory T cells, leaving the host with reduced memory (except for cross-reactive responses) to previously encountered viruses. Apoptotic events also silence the acute immune response, leaving the host with a residual population of memory T cells. Persistent infections can induce apoptotic deletions of memory T cells that are specific to the persisting virus and to previously encountered pathogens.

Natural history of a recurrent feline coronavirus infection and the role of cellular immunity in survival and disease

We describe the natural history, viral dynamics, and immunobiology of feline infectious peritonitis (FIP), a highly lethal coronavirus infection. A severe recurrent infection developed, typified by viral persistence and acute lymphopenia, with waves of enhanced viral replication coinciding with fever, weight loss, and depletion of CD4+ and CD8+ T cells. Our combined observations suggest a model for FIP pathogenesis in which virus-induced T-cell depletion and the antiviral T-cell response are opposing forces and in which the efficacy of early T-cell responses critically determines the outcome of the infection. Rising amounts of viral RNA in the blood, consistently seen in animals with end-stage FIP, indicate that progression to fatal disease is the direct consequence of a loss of immune control, resulting in unchecked viral replication. The pathogenic phenomena described here likely bear relevance to other severe coronavirus infections, in particular severe acute respiratory syndrome, for which multiphasic disease progression and acute T-cell lymphopenia have also been reported. Experimental FIP presents a relevant, safe, and well-defined model to study coronavirus-mediated immunosuppression and should provide an attractive and convenient system for in vivo testing of anticoronaviral drugs.

The role of bystander T cells in CNS pathology and pathogen clearance

It is generally accepted that both self- and pathogen-specific T lymphocytes have the potential to mediate immunopathogenesis and contribute to a variety of human ailments. Despite this unfortunate tendency to induce tissue injury, these cells are guided by interactions with peptide-loaded major histocompatibility complexes (MHC) and adhere appropriately to a vital evolutionary constraint imposed by the host: specificity. More recently, a series of studies have demonstrated that bystander T cells of an irrelevant specificity can bypass peptide/MHC restriction and become active participants in immunopathology. This review critically evaluates the role of bystander T cells in immunopathogenesis and pathogen clearance in the periphery as well as the central nervous system and attempts to establish the likelihood of their participation in human disease.

The quantal theory of how the immune system discriminates between "self and non-self"

In the past 50 years, immunologists have accumulated an amazing amount of information as to how the immune system functions. However, one of the most fundamental aspects of immunity, how the immune system discriminates between self vs. non-self, still remains an enigma. Any attempt to explain this most intriguing and fundamental characteristic must account for this decision at the level of the whole immune system, but as well, at the level of the individual cells making up the immune system. Moreover, it must provide for a molecular explanation as to how and why the cells behave as they do. The "Quantal Theory", proposed herein, is based upon the "Clonal Selection Theory", first proposed by Sir McFarland Burnet in 1955, in which he explained the remarkable specificity as well as diversity of recognition of everything foreign in the environment. The "Quantal Theory" is built upon Burnet's premise that after antigen selection of cell clones, a proliferative expansion of the selected cells ensues. Furthermore, it is derived from experiments which indicate that the proliferation of antigen-selected cell clones is determined by a quantal, "all-or-none", decision promulgated by a critical number of cellular receptors triggered by the T Cell Growth Factor (TCGF), interleukin 2 (IL2). An extraordinary number of experiments reported especially in the past 20 years, and detailed herein, indicate that the T cell Antigen Receptor (TCR) behaves similarly, and also that there are several critical numbers of triggered TCRs that determine different fates of the T cells. Moreover, the fates of the cells appear ultimately to be determined by the TCR triggering of the IL2 and IL2 receptor (IL2R) genes, which are also expressed in a very quantal fashion. The "Quantal Theory" states that the fundamental decisions of the T cell immune system are dependent upon the cells receiving a critical number of triggered TCRs and IL2Rs and that the cells respond in an all-or-none fashion. The "Quantal Theory" accounts fully for the development of T cells in the thymus, and such fundamental cellular fates as both "positive" and "negative" selection, as well as the decision to differentiate into a "Regulatory T cell" (T-Reg). In the periphery, the "Quantal Theory" accounts for the decision to proliferate or not in response to the presence of an antigen, either non-self or self, or to differentiate into a T-Reg. Since the immune system discriminates between self and non-self antigens by the accumulated number of triggered TCRs and IL2Rs, therapeutic manipulation of the determinants of these quantal decisions should permit new approaches to either enhance or dampen antigen-specific immune responses.

Pathogen-host standoff: immunity to polyomavirus infection and neoplasia

Polyomaviruses establish persistent infection in a variety of hosts, including humans, where they pose an oncogenic threat under conditions of depressed immune function. Control of persistent infection by these DNA tumor viruses requires continuous immunosurveillance by functionally competent antiviral CD8+ T cells. Repetitive antigen encounter by these T cells, however, often leads to their deletion or inactivation. Elucidation of the in vivo mechanisms that sustain antigen-specific CD8+ T-cell effector activity in the face of persistent antigen is essential for devising immunotherapeutic strategies against viral oncogenesis.

Cytolytic CD8+ T cells recognizing CFP10 are recruited to the lung after Mycobacterium tuberculosis infection

Optimum immunity against Mycobacterium tuberculosis requires both CD4⁺ and CD8⁺ T cells. In contrast with CD4⁺ T cells, few antigens are known that elicit CD8⁺ T cells during infection. CD8⁺ T cells specific for culture filtrate protein-10 (CFP10) are found in purified protein derivative positive donors, suggesting that CFP10 primes CD8⁺ T cells in vivo. Using T cells from M. tuberculosis–infected mice, we identified CFP10 epitopes recognized by CD8⁺ T cells and CD4⁺ T cells. CFP10-specific T cells were detected as early as week 3 after infection and at their peak accounted for up to 30% of CD8⁺ T cells in the lung. IFNγ-producing CD8⁺ and CD4⁺ T cells recognizing CFP10 epitopes were preferentially recruited to the lungs of M. tuberculosis–infected mice. In vivo cytolytic activity of CD8⁺ T cells specific for CFP10 and TB10.3/10.4 proteins was detected in the spleen, pulmonary lymph nodes, and lungs of infected mice. The cytolytic activity persisted long term and could be detected 260 d after infection. This paper highlights the cytolytic function of antigen-specific CD8⁺ T cells elicited by M. tuberculosis infection and demonstrates that large numbers of CFP10-specific cytolytic CD8⁺ T cells are recruited to the lung after M. tuberculosis infection.

Reduction of retrovirus-induced immunosuppression by in vivo modulation of T cells during acute infection

Chronic infection with Friend retrovirus is associated with suppressed antitumor immune responses. In the present study we investigated whether modulation of T-cell responses during acute infection would restore antitumor immunity in persistently infected mice. T-cell modulation was done by treatments with DTA-1 anti- glucocorticoid-induced tumor necrosis factor receptor monoclonal antibodies. The DTA-1 monoclonal antibody is nondepleting and delivers costimulatory signals that both enhance the activation of effector T cells and inhibit suppression by regulatory T cells. DTA-1 therapy produced faster Th1 immune responses, significant reductions in both acute virus loads and pathology and, most importantly, long-term improvement of CD8(+) T-cell-mediated antitumor responses.

The role of programming in memory T-cell development

Recent studies suggest that memory T-cell differentiation continues for weeks or months following antigen clearance, although commitment to the memory lineage occurs during the effector stage of development. Several variables associated with priming, such as the duration of antigenic stimulation, degree of co-stimulation, cytokine environment, and CD4(+) T-cell help, may program epigenetic qualitative differences into the ensuing effector and memory populations. Defining what memory qualities best protect the organism from re-infection, as well as how commitment to the memory lineage is specified following T-cell activation remains an important goal.

Antigen-independent memory CD8 T cells do not develop during chronic viral infection

Memory T cells can persist for extended periods in the absence of antigen, and long-term T cell immunity is often seen after acute infections. Paradoxically, there have been observations suggesting that T cell memory may be antigen-dependent during chronic infections. To elucidate the underlying mechanisms we have compared memory CD8 T cell differentiation during an acute versus chronic infection by using the mouse model of infection with lymphocytic choriomeningitis virus. We found that during a chronic infection virus-specific CD8 T cells failed to acquire the cardinal memory T cell property of long-term antigen-independent persistence. These chronically stimulated CD8 T cells were unable to undergo homeostatic proliferation, responded poorly to IL-7 and IL-15, and expressed reduced levels of the IL-7 and IL-15 receptors, thus providing a possible mechanism for the inability of these cells to persist long term in the absence of antigen. In striking contrast, virus-specific memory CD8 T cells that developed after an acute lymphocytic choriomeningitis virus infection could persist without antigen, were capable of self-renewal because of homeostatic proliferation, responded efficiently to IL-7 and IL-15, and expressed high levels of receptors for these two cytokines. Thus, memory CD8 T cells generated after acute infections are likely to have a competitive advantage over CD8 T cells that develop during chronic infections. These findings raise concerns about using vaccines that may persist and also suggest that there may be limitations and challenges in designing effective immunological interventions for the treatment of chronic infections and tumors.

Preapoptotic phenotype of viral epitope-specific CD8 T cells precludes memory development and is an intrinsic property of the epitope

Virus-specific CD8 T cells after clearance of infection reduce their number in lymphoid organs by apoptotic death and by migration into peripheral tissues. During and after infection, many lymphocytic choriomeningitis virus (LCMV)-specific CD8 T cells in lymphoid but not peripheral tissues are in a preapoptotic state, as detected by the early apoptosis marker annexin V. In this report, we investigated the significance of this preapoptotic state and how it may be influenced by viral epitope specificity. Stimulation with anti-CD3 or IL-2 in vitro postponed DNA fragmentation in annexin V+ cells, but adoptive transfer studies in vivo showed that this preapoptotic phenotype precluded the development of functional memory. CD8 T cells specific to LCMV epitopes NP396 and gp33 differed in their preapoptotic state, with NP396-specific T cells binding more annexin V than gp33-specific T cells. These epitope- and tissue-dependent differences were seen in primary, memory, and secondary responses and in mice receiving different displays of Ag by infection with LCMV strains of different tropisms or by infection with vaccinia virus recombinants expressing LCMV proteins. Thus, the epitope-dependent differences in apoptosis were independent of virus tropisms, duration of Ag exposure, and competition within APCs, and were an intrinsic property of the epitope. The tissue-dependent and epitope-dependent preapoptotic state correlated with reduced expression of IL-7Ralpha.

Limited durability of viral control following treated acute HIV infection

BACKGROUND

Early treatment of acute HIV infection with highly active antiretroviral therapy, followed by supervised treatment interruption (STI), has been associated with at least transient control of viremia. However, the durability of such control remains unclear. Here we present longitudinal follow-up of a single-arm, open-label study assessing the impact of STI in the setting of acute HIV-1 infection.

METHODS AND FINDINGS

Fourteen patients were treated during acute HIV-1 infection and subsequently subjected to an STI protocol that required retreatment if viral load exceeded 50,000 RNA copies/ml plasma or remained above 5,000 copies/ml for more than three consecutive weeks. Eleven of 14 (79%) patients were able to achieve viral loads of less than 5,000 RNA copies/ml for at least 90 d following one, two, or three interruptions of treatment. However, a gradual increase in viremia and decline in CD4+ T cell counts was observed in most individuals. By an intention-to-treat analysis, eight (57%), six (43%), and three (21%) of 14 patients achieved a maximal period of control of 180, 360, and 720 d, respectively, despite augmentation of HIV-specific CD4+ and CD8+ T cell responses. The magnitude of HIV-1-specific cellular immune responses before treatment interruption did not predict duration of viremia control. The small sample size and lack of concurrent untreated controls preclude assessment of possible clinical benefit despite failure to control viremia by study criteria.

CONCLUSIONS

These data indicate that despite initial control of viremia, durable viral control to less than 5,000 RNA copies/ml plasma in patients following treated acute HIV-1 infection occurs infrequently. Determination of whether early treatment leads to overall clinical benefit will require a larger and randomized clinical trial. These data may be relevant to current efforts to develop an HIV-1 vaccine designed to retard disease progression rather than prevent infection since they indicate that durable maintenance of low-level viremia may be difficult to achieve.

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.

Kinetics of virus-specific CD8+ T cells and the control of human immunodeficiency virus infection

Several primate models indicate that cytotoxic T lymphocyte-inducing vaccines may be unable to prevent human immunodeficiency virus infection but may have a long-term benefit in controlling viral replication and delaying disease progression. Here we show that analysis of the kinetics of antigen-specific CD8+ T-cell expansion suggests a delay in activation following infection that allows unimpeded early viral replication. Viral kinetics do not differ between controls and vaccinees during this delay phase. An increase in virus-specific CD8+ T-cell numbers around day 10 postinfection coincides with a slowing in viral replication in vaccinees and reduces peak viral loads by around 1 log. However, this response is too little too late to prevent establishment of persistent infection.

Immunoproteasomes Down-Regulate Presentation of a Subdominant T Cell Epitope from Lymphocytic Choriomeningitis Virus

The cytotoxic T cell response to pathogens is usually directed against a few immunodominant epitopes, while other potential epitopes are either subdominant or not used at all. In C57BL/6 mice, the acute cytotoxic T cell response against lymphocytic choriomeningitis virus is directed against immunodominant epitopes derived from the glycoprotein (gp33-41) and the nucleoprotein (NP396-404), while the gp276-286 epitope remains subdominant. Despite extensive investigations, the reason for this hierarchy between epitopes is not clear. In this study, we show that the treatment of cells with IFN-gamma enhanced the presentation of gp33-41, whereas presentation of the gp276-286 epitope from the same glycoprotein was markedly reduced. Because proteasomes are crucially involved in epitope generation and because IFN-gamma treatment in vitro and lymphocytic choriomeningitis virus infection in vivo lead to a gradual replacement of constitutive proteasomes by immunoproteasomes, we investigated the role of proteasome composition on epitope hierarchy. Overexpression of the active site subunits of immunoproteasomes LMP2, LMP7, and MECL-1 as well as overexpression of LMP2 alone suppressed the presentation of the gp276-286 epitope. The ability to generate gp276-286-specific CTLs was enhanced in LMP2- and LMP7-deficient mice, and macrophages from these mice showed an elevated presentation of this epitope. In vitro digests demonstrated that fragmentation by immunoproteasomes, but not constitutive proteasomes led to a preferential destruction of the gp276 epitope. Taken together, we show that LMP2 and LMP7 can at least in part determine subdominance and shape the epitope hierarchy of CTL responses in vivo.

Visualizing the viral burden: phenotypic and functional alterations of T cells and APCs during persistent infection

Persistent viral infections continue to present major public health problems. Failure to achieve virus control confronts the immune system with a chronic viral burden that may involve immune cells themselves and directly compromise the functionality of effector lymphocytes and APCs. In this study we use the lymphocytic choriomeningitis virus system for persistent viral infection of its natural murine host and use analytical techniques for direct ex vivo visualization of virus-infected immune cells. We report that virtually all cells of the immune system can be infected, but the distribution of the viral burden is differentially allocated to lymphocyte and APC subsets of defined phenotypes. Importantly, the profile of immune cell infection found in the blood is broadly representative for the pattern of cellular infection in most organs and is independent of the presence of Abs or complement. By direct comparison of virus-infected and uninfected cell subsets, we demonstrate that lymphocytic choriomeningitis virus-infected T cells show preferential activation, skewed cytokine profiles, and increased apoptosis. In contrast, increased activation of APCs is generalized and independent of the presence of viral Ag. Our data indicate that specific patterns of immune cell infection are associated with distinct forms of immunostimulatory and immunosuppressive alterations that may provide insights into autoimmune processes associated with infectious disease and offer clues for therapeutic interventions aimed at restoration of complete immunity.

High resolution analysis of cellular immune responses in resolved and persistent hepatitis C virus infection

BACKGROUND & AIMS

Cellular immune responses are thought to play a key role in the resolution of primary HCV infection. Although it has been consistently shown that CD4+ T-cell responses are maintained in those with spontaneous resolution but lost in those with persistent infection, the role of CD8+ T-cell responses remains controversial. Previous studies have largely focused on limited HLA alleles and predefined CD8+ T-cell epitopes, and, thus, comprehensive studies remain to be performed.

METHODS

To understand the composition of the immune response associated with spontaneous resolution, we comprehensively mapped CD8+ T-cell responses in 20 HLA-diverse persons with resolved HCV infection, using HCV peptides spanning the entire genome. We analyzed the magnitude, breadth, function, and phenotype using ELISpot, class-I tetramers, intracellular cytokine staining, and cytolytic assays. We studied in parallel HCV-specific responses and viral sequence variation in persistent infection.

RESULTS

Responses in individuals with resolved infection were strong and broad with robust proliferation in response to antigen. Responses in those persistently infected were rarely detected ex vivo and, when present, were narrowly directed and weak. However, they also proliferated in vitro. Dominant target epitopes differed among individuals in both cohorts, despite frequently shared HLA-alleles.

CONCLUSIONS

These data indicate that persisting, strong CD8+ T-cell responses are observed in the majority of persons with resolved HCV infection and provide support for strategies to boost CD8+ T-cell responses for the prevention or treatment of HCV infection but also highlight the diversity of responses that may need to be elicited to provide protection.

Prospects for an AIDS vaccine: three big questions, no easy answers

The unremitting devastation created by the AIDS pandemic will probably only be controlled when a vaccine is developed that is safe, effective, affordable, and simple enough to permit implementation in developing countries where the impact of AIDS is most severe. Although formidable practical, political, economic, social, and ethical challenges face the AIDS vaccine development effort, the most fundamental challenges now reside at the level of the basic biology of HIV-1 infection and pathogenesis. Of these biological considerations, three questions loom especially large: can we design immunogens that will elicit neutralising antibodies that are reactive against a wide variety of primary HIV isolates; will vaccine-elicited cytotoxic T cells be fundamentally better at controlling HIV-1 replication and ameliorating disease progression than those responses that arise during natural HIV infection; and to what extent will the tremendous global genetic diversity of HIV-1 compromise the breadth of vaccine-elicited protective immunity and the overall effectiveness of an AIDS vaccine? Although these are three exceptionally challenging questions, they are now being approached with clear hypotheses whose testing is being facilitated by an ever-improving array of technologies for vaccine design and immunological characterisation. The extent to which the field of AIDS vaccine research can now come together to answer these questions in the best coordinated, most efficient manner will probably be an important determinant of how and when an effective AIDS vaccine will be developed.

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.

Increased expression of the natural killer cell inhibitory receptor CD85j/ILT2 on antigen-specific effector CD8 T cells and its impact on CD8 T-cell function

We investigated whether inhibitory natural killer cell receptor (iNKR) expression contributes to impaired antigen-specific cytotoxicity and interferon-gamma (IFN-gamma) production by CD8 T cells during chronic infection. iNKR immunoglobulin-like transcript-2 (ILT2/CD85j) is expressed on 40-55% of cytomegalovirus (CMV)-, Epstein-Barr virus (EBV)- and human immunodeficiency virus (HIV)-specific CD8 T cells in both healthy and HIV-infected donors. Other iNKRs (CD158a, b1, e1/e2, k, CD94/NKG2A) are expressed on only a small minority of CD8 T cells and are not preferentially expressed on tetramer-staining virus-specific cells. In normal donors, ILT2 is expressed largely on perforin(+) CD27(-) effector cells. However, in HIV-infected donors, only a third of ILT2(+) cells are also perforin(+). In both normal and HIV-infected donors, ILT2(+) cells are prone to spontaneous apoptosis. Therefore, ILT2 is normally expressed during effector cytotoxic T-lymphocyte (CTL) differentiation, but can also be expressed when effector maturation is incomplete, as in HIV infection. The effect of ILT2 on CD8 cell function was assessed by preincubating effector cells with ILT2 antibody. While blocking ILT2 engagement has no appreciable effect on cytotoxicity, it increases antiviral IFN-gamma production by approximately threefold in both normal and HIV-infected donors. Thus, ILT2 expression, increased on antiviral CD8 cells in chronic infection, may interfere with protective CD8 T-cell function by suppressing IFN-gamma production.

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.

CD8+ T cell contraction is controlled by early inflammation

Pathogen-specific CD8(+) T cells expand in number after infection and then their numbers invariably contract by 90-95%, leaving a stable memory cell pool. The chief features of this response are programmed early after infection; however, the factors regulating contraction are mostly undefined. Here we show that antibiotic treatment before Listeria monocytogenes infection induced numbers of protective memory CD8(+) T cells similar to those in control infected mice, by a pathway without contraction. The absence of contraction correlated with decreased early inflammation and interferon-gamma production and an increased fraction of CD8(+) T cells expressing the interleukin 7 receptor at the peak of the response. Thus, contraction is controlled by early inflammation but is not essential for the generation of protective memory CD8(+) T cells after infection.

Superinfecting mycobacteria home to established tuberculous granulomas

A central paradox of tuberculosis immunity is that reinfection and bacterial persistence occur despite vigorous host immune responses concentrated in granulomas, which are organized structures that form in response to infection. Prevailing models attribute reinfection and persistence to bacterial avoidance of host immunity via establishment of infection outside primary granulomas. Alternatively, persistence is attributed to a gradual bacterial adaptation to evolving host immune responses. We show here that superinfecting Mycobacterium marinum traffic rapidly into preexisting granulomas, including their caseous (necrotic) centers, through specific mycobacterium-directed and host cell-mediated processes, yet adapt quickly to persist long term therein. These findings demonstrate a failure of established granulomas, concentrated foci of activated macrophages and antigen-specific immune effector cells, to eradicate newly deposited mycobacteria not previously exposed to host responses.

The level of virus-specific T-cell and macrophage recruitment in porcine reproductive and respiratory syndrome virus infection in pigs is independent of virus load

Porcine reproductive and respiratory syndrome virus (PRRSV) is the most important infectious disease agent of pigs worldwide, causing reproductive failure in pregnant sows and respiratory problems in nursing and growing pigs. PRRSV infection is characterized by a prolonged viremia of 30 or more days and an extended persistent infection of lymphoid tissues. To better understand the immunological basis for prolonged acute and persistent PRRSV infection, we have examined the cell-mediated immune (CMI) response throughout the course of infection and compared the results to the local distribution and abundance of PRRSV in infected tissues. PRRSV-specific T cells, enumerated by gamma interferon enzyme-linked immunospot assay, did not appear until 2 weeks after PRRSV inoculation, and their abundance exhibited substantial variation over time and among animals. In all cases the T-cell response was transient. High levels of viral RNA were present in lymphoid tissues of all animals in the acute phase of infection. Viral loads were decreased 1,000-fold or more in persistent infections, with the primary sites of persistence being tonsil, sternal lymph node, and inguinal lymph node. The abundance of virus-specific T cells in either acutely or persistently infected animals was highly variable and showed no correlation to the level of virus in lymphoid tissues. No significant difference in antigen-specific T-cell abundance was observed in secondary lymphoid tissues in either acute or persistent infection except for tonsil, in which the number of responding cells was extremely low. CD4(+)- and CD8(+)-T-cell frequencies did not change after PRRSV infection, though a decrease in gammadelta T cells was observed. Macrophages, the permissive cell type for PRRSV, were present in various levels in all tissue preparations and were not in proportion to local virus load. These findings indicate that a weak CMI response contributes to prolonged PRRSV infection and suggests that PRRSV suppresses T-cell recognition of infected macrophages. Thus, the slow but eventual resolution of PRRSV infection may be dependent on limiting permissive macrophages and on innate immune factors.