In vivo proliferation of naïve and memory influenza-specific CD8(+) T cells

Protective function and durability of mouse lymph node-resident memory CD8+ T cells

Protective lung tissue-resident memory CD8⁺T cells (Trm) form after influenza A virus (IAV) infection. We show that IAV infection of mice generates CD69⁺CD103⁺and other memory CD8⁺T cell populations in lung-draining mediastinal lymph nodes (mLNs) from circulating naive or memory CD8⁺T cells. Repeated antigen exposure, mimicking seasonal IAV infections, generates quaternary memory (4M) CD8⁺T cells that protect mLN from viral infection better than 1M CD8⁺T cells. Better protection by 4M CD8⁺T cells associates with enhanced granzyme A/B expression and stable maintenance of mLN CD69⁺CD103⁺4M CD8⁺T cells, vs the steady decline of CD69⁺CD103⁺1M CD8⁺T cells, paralleling the durability of protective CD69⁺CD103⁺4M vs 1M in the lung after IAV infection. Coordinated upregulation in canonical Trm-associated genes occurs in circulating 4M vs 1M populations without the enrichment of canonical downregulated Trm genes. Thus, repeated antigen exposure arms circulating memory CD8⁺T cells with enhanced capacity to form long-lived populations of Trm that enhance control of viral infections of the mLN.

Fucosyltransferase Induction during Influenza Virus Infection Is Required for the Generation of Functional Memory CD4+ T Cells

T cells mediating influenza viral control are instructed in lymphoid and nonlymphoid tissues to differentiate into memory T cells that confer protective immunity. The mechanisms by which influenza virus-specific memory CD4⁺ T cells arise have been attributed to changes in transcription factors, cytokines and cytokine receptors, and metabolic programming. The molecules involved in these biosynthetic pathways, including proteins and lipids, are modified to varying degrees of glycosylation, fucosylation, sialation, and sulfation, which can alter their function. It is currently unknown how the glycome enzymatic machinery regulates CD4⁺ T cell effector and memory differentiation. In a murine model of influenza virus infection, we found that fucosyltransferase enzymatic activity was induced in effector and memory CD4⁺ T cells. Using CD4⁺ T cells deficient in the Fut4/7 enzymes that are expressed only in hematopoietic cells, we found decreased frequencies of effector cells with reduced expression of T-bet and NKG2A/C/E in the lungs during primary infection. Furthermore, Fut4/7^-/- effector CD4⁺ T cells had reduced survival with no difference in proliferation or capacity for effector function. Although Fut4/7^-/- CD4⁺ T cells seeded the memory pool after primary infection, they failed to form tissue-resident cells, were dysfunctional, and were unable to re-expand after secondary infection. Our findings highlight an important regulatory axis mediated by cell-intrinsic fucosyltransferase activity in CD4⁺ T cell effectors that ensure the development of functional memory CD4⁺ T cells.

Adjuvanting influenza hemagglutinin vaccine with a human pulmonary surfactant-mimicking synthetic compound SF-10 induces local and systemic cell-mediated immunity in mice

We reported previously that intranasal instillation of a synthetic human pulmonary surfactant with a carboxy vinyl polymer as a viscosity improver, named SF-10, shows potent adjuvanticity for humoral immunity in mice and cynomolgus monkeys. SF-10 effectively induces influenza hemagglutinin vaccine (HAv)-specific IgA in nasal and lung washes and IgG in sera with their neutralizing activities. Since CD8⁺ T cell-mediated protection is an important requirement for adaptive immunity, we investigated in this study the effects of SF-10 with antigen on local and systemic cell-mediated immunity. Nasal instillation of ovalbumin, a model antigen, combined with SF-10 efficiently delivered antigen to mucosal dendritic and epithelial cells and promoted cross-presentation in antigen presenting cells, yielding a high percentage of ovalbumin-specific cytotoxic T lymphocytes in the nasal mucosa, compared with ovalbumin alone. Nasal immunization of HAv-SF-10 also induced HAv-specific cytotoxic T lymphocytes and upregulated granzyme B expression in splenic CD8⁺ T cells with their high cytotoxicity against target cells pulsed with HA peptide. Furthermore, nasal vaccination of HAv-SF-10 significantly induced higher cytotoxic T lymphocytes-mediated cytotoxicity in the lungs and cervical lymph nodes in the early phase of influenza virus infection compared with HAv alone. Protective immunity induced by HAv-SF-10 against lethal influenza virus infection was partially and predominantly suppressed after depletion of CD8⁺ and CD4⁺ T cells (induced by intraperitoneal injection of the corresponding antibodies), respectively, suggesting that CD4⁺ T cells predominantly and CD8⁺ T cells partially contribute to the protective immunity in the advanced stage of influenza virus infection. These results suggest that SF-10 promotes effective antigen delivery to antigen presenting cells, activates CD8⁺ T cells via cross-presentation, and induces cell-mediated immune responses against antigen.

Optimal Use of Vaccines for Control of Influenza A Virus in Swine

Influenza A virus in swine (IAV-S) is one of the most important infectious disease agents of swine in North America. In addition to the economic burden of IAV-S to the swine industry, the zoonotic potential of IAV-S sometimes leads to serious public health concerns. Adjuvanted, inactivated vaccines have been licensed in the United States for over 20 years, and there is also widespread usage of autogenous/custom IAV-S vaccines. Vaccination induces neutralizing antibodies and protection against infection with very similar strains. However, IAV-S strains are so diverse and prone to mutation that these vaccines often have disappointing efficacy in the field. This scientific review was developed to help veterinarians and others to identify the best available IAV-S vaccine for a particular infected herd. We describe key principles of IAV-S structure and replication, protective immunity, currently available vaccines, and vaccine technologies that show promise for the future. We discuss strategies to optimize the use of available IAV-S vaccines, based on information gathered from modern diagnostics and surveillance programs. Improvements in IAV-S immunization strategies, in both the short term and long term, will benefit swine health and productivity and potentially reduce risks to public health.

CD28 expression is required after T cell priming for helper T cell responses and protective immunity to infection

The co-stimulatory molecule CD28 is essential for activation of helper T cells. Despite this critical role, it is not known whether CD28 has functions in maintaining T cell responses following activation. To determine the role for CD28 after T cell priming, we generated a strain of mice where CD28 is removed from CD4⁺ T cells after priming. We show that continued CD28 expression is important for effector CD4⁺ T cells following infection; maintained CD28 is required for the expansion of T helper type 1 cells, and for the differentiation and maintenance of T follicular helper cells during viral infection. Persistent CD28 is also required for clearance of the bacterium Citrobacter rodentium from the gastrointestinal tract. Together, this study demonstrates that CD28 persistence is required for helper T cell polarization in response to infection, describing a novel function for CD28 that is distinct from its role in T cell priming.

DOI:http://dx.doi.org/10.7554/eLife.03180.001

Invasion by a bacterium or virus typically activates a mammalian host's immune system to eliminate the pathogen. The cells of the so-called ‘innate immune system’ are the body's first line of defense against infection, and these cells patrol the organs and tissues in an effort to locate and eliminate pathogens quickly. The innate immune response is rapid and non-specific, but often cannot completely clear an infection. When necessary, innate immune cells will escalate the immune response by activating the second branch of the immune system, called the ‘adaptive immune system’. This specifically targets and eradicates an invading pathogen.

T cells are essential components of the adaptive immune system, and these cells can be readily distinguished from other types of cell by proteins called T cell receptors (or TCRs) found on their surface. There are also different types of T cell, each with a specific function. T helper cells, for example, help other adaptive immune cells to mature and activate, which involves these immune cells proliferating and developing into more specialized cells.

For a T cell to activate, two events must occur at the same time. First, the TCR must recognize and bind to a fragment of the pathogen that is presented to it by an innate immune cell. And second, ‘co-stimulatory molecules’ present on the surfaces of both the T cell and the same innate immune cell must interact. Using these two signals to activate a T cell helps to ensure the adaptive immune response is not ‘unleashed‘ unnecessarily.

Co-stimulatory molecules have become popular targets for therapies aimed at treating autoimmune disorders—where the immune system attacks and destroys the body's own tissues. One of the most well studied co-stimulatory molecules expressed by T cells is called CD28; however, it remained unknown whether CD28 is involved in any processes after T cell activation.

Now, Linterman et al. reveal that the CD28 co-stimulatory molecule plays a number of roles in addition to T cell activation. For example, a newly developed mouse model showed that CD28 must remain on the surface of T helper cells after they have been activated for these cells to effectively specialize. Linterman et al. also discovered that CD28 helps different T helper cell subtypes to develop.

Linterman et al. demonstrate that CD28 is critical throughout a host's response to infection, and suggest that if CD28 is lost on activated T cells (which happens during aging, HIV infection and autoimmune diseases) the responses of T helper cells become limited. Furthermore, these findings reveal that treatments that target the CD28 co-stimulatory molecule will also affect on-going immune responses.

DOI:http://dx.doi.org/10.7554/eLife.03180.002

Universal immunity to influenza must outwit immune evasion

Although an influenza vaccine has been available for 70 years, influenza virus still causes seasonal epidemics and worldwide pandemics. Currently available vaccines elicit strain-specific antibody (Ab) responses to the surface haemagglutinin (HA) and neuraminidase (NA) proteins, but these can be ineffective against serologically-distinct viral variants and novel subtypes. Thus, there is a great need for cross-protective or "universal" influenza vaccines to overcome the necessity for annual immunization against seasonal influenza and to provide immunity to reduce the severity of infection with pandemic or outbreak viruses. It is well established that natural influenza infection can provide cross-reactive immunity that can reduce the impact of infection with distinct influenza type A strains and subtypes, including H1N1, H3N2, H2N2, H5N1, and H7N9. The key to generating universal influenza immunity through vaccination is to target functionally-conserved regions of the virus, which include epitopes on the internal proteins for cross-reactive T cell immunity or on the HA stem for broadly reactive Ab responses. In the wake of the 2009 H1N1 pandemic, broadly neutralizing antibodies (bnAbs) have been characterized and isolated from convalescent and vaccinated individuals, inspiring development of new vaccination techniques to elicit such responses. Induction of influenza-specific T cell responses through vaccination has also been recently examined in clinical trials. Strong evidence is available from human and animal models of influenza to show that established influenza-specific T cell memory can reduce viral shedding and symptom severity. However, the published evidence also shows that CD8(+) T cells can efficiently select immune escape mutants early after influenza virus infection. Here, we discuss universal immunity to influenza viruses mediated by both cross-reactive T cells and Abs, the mechanisms of immune evasion in influenza, and propose how to counteract commonly occurring immune-escape variants.

Immunological assessment of influenza vaccines and immune correlates of protection

Influenza vaccines remain the primary public health tool in reducing the ever-present burden of influenza and its complications. In seeking more immunogenic, more effective and more broadly cross-protective influenza vaccines, the landscape of influenza vaccines is rapidly expanding, both in near-term advances and next-generation vaccine design. Although the first influenza vaccines were licensed over 60 years ago, the hemagglutination-inhibition antibody titer is currently the only universally accepted immune correlate of protection against influenza. However, hemagglutination-inhibition titers appear to be less effective at predicting protection in populations at high risk for severe influenza disease; older adults, young children and those with certain medical conditions. The lack of knowledge and validated methods to measure alternate immune markers of protection against influenza remain a substantial barrier to the development of more immunogenic, broadly cross-reactive and effective influenza vaccines. Here, the authors review the knowledge of immune effectors of protection against influenza and discuss assessment methods for a broader range of immunological parameters that could be considered in the evaluation of traditional or new-generation influenza vaccines.

Role of CD8(+) T-cell immunity in influenza infection: potential use in future vaccine development

Continued circulation of the highly pathogenic avian H5N1 influenza A virus has many people worried that an influenza pandemic is imminent. Compounding this is the realization that H5N1 vaccines based on current influenza vaccine technology (designed to generate protective antibody responses) may be suboptimal at providing protection. As a consequence, there is recent interest in vaccine strategies that elicit cellular immunity, particularly the cytotoxic T lymphocyte response, in an effort to provide protection against a potential pandemic. A major issue is the lack of information about the precise role that these 'hitmen' of the immune system have in protecting against both pandemic and seasonal influenza. We need to know more about how the induction and maintenance of cytotoxic T lymphocytes after influenza infection can impact protection from further infection. The challenge is then to use this information in the design of vaccines that will protect against pandemic influenza and will help optimize CD8(+) killer T-cell responses in other infections.

Identification of novel avian influenza virus derived CD8+ T-cell epitopes

Avian influenza virus (AIV) infection is a continuing threat to both humans and poultry. Influenza virus specific CD8+ T cells are associated with protection against homologous and heterologous influenza strains. In contrast to what has been described for humans and mice, knowledge on epitope-specific CD8+ T cells in chickens is limited. Therefore, we set out to identify AIV-specific CD8+ T-cell epitopes. Epitope predictions based on anchor residues resulted in 33 candidate epitopes. MHC I inbred chickens were infected with a low pathogenic AIV strain and sacrificed at 5, 7, 10 and 14 days post infection (dpi). Lymphocytes isolated from lung, spleen and blood were stimulated ex vivo with AIV-specific pooled or individual peptides and the production of IFNγ was determined by ELIspot. This resulted in the identification of 12 MHC B12-restricted, 3 B4-restricted and 1 B19-restricted AIV- specific CD8+ T-cell epitopes. In conclusion, we have identified novel AIV-derived CD8+ T-cell epitopes for several inbred chicken strains. This knowledge can be used to study the role of CD8+ T cells against AIV infection in a natural host for influenza, and may be important for vaccine development.

Lung CD103+ dendritic cells efficiently transport influenza virus to the lymph node and load viral antigen onto MHC class I for presentation to CD8 T cells

The uptake, transport, and presentation of Ags by lung dendritic cells (DCs) are central to the initiation of CD8 T cell responses against respiratory viruses. Although several studies have demonstrated a critical role of CD11b(low/neg)CD103(+) DCs for the initiation of cytotoxic T cell responses against the influenza virus, the underlying mechanisms for its potent ability to prime CD8 T cells remain poorly understood. Using a novel approach of fluorescent lipophilic dye-labeled influenza virus, we demonstrate that CD11b(low/neg)CD103(+) DCs are the dominant lung DC population transporting influenza virus to the posterior mediastinal lymph node as early as 20 h postinfection. By contrast, CD11b(high)CD103(neg) DCs, although more efficient for taking up the virus within the lung, migrate poorly to the lymph node and remain in the lung to produce proinflammatory cytokines instead. CD11b(low/neg)CD103(+) DCs efficiently load viral peptide onto MHC class I complexes and therefore uniquely possess the capacity to potently induce proliferation of naive CD8 T cells. In addition, the peptide transporters TAP1 and TAP2 are constitutively expressed at higher levels in CD11b(low/neg)CD103(+) DCs, providing, to our knowledge, the first evidence of a distinct regulation of the Ag-processing pathway in these cells. Collectively, these results show that CD11b(low/neg)CD103(+) DCs are functionally specialized for the transport of Ag from the lung to the lymph node and also for efficient processing and presentation of viral Ags to CD8 T cells.

Modeling of influenza-specific CD8+ T cells during the primary response indicates that the spleen is a major source of effectors

The biological parameters that determine the distribution of virus-specific CD8(+) T cells during influenza infection are not all directly measurable by experimental techniques but can be inferred through mathematical modeling. Mechanistic and semimechanistic ordinary differential equations were developed to describe the expansion, trafficking, and disappearance of activated virus-specific CD8(+) T cells in lymph nodes, spleens, and lungs of mice during primary influenza A infection. An intensive sampling of virus-specific CD8(+) T cells from these three compartments was used to inform the models. Rigorous statistical fitting of the models to the experimental data allowed estimation of important biological parameters. Although the draining lymph node is the first tissue in which Ag-specific CD8(+) T cells are detected, it was found that the spleen contributes the greatest number of effector CD8(+) T cells to the lung, with rates of expansion and migration that exceeded those of the draining lymph node. In addition, models that were based on the number and kinetics of professional APCs fit the data better than those based on viral load, suggesting that the immune response is limited by Ag presentation rather than the amount of virus. Modeling also suggests that loss of effector T cells from the lung is significant and time dependent, increasing toward the end of the acute response. Together, these efforts provide a better understanding of the primary CD8(+) T cell response to influenza infection, changing the view that the spleen plays a minor role in the primary immune response.

T cell costimulatory molecules in anti-viral immunity: Potential role in immunotherapeutic vaccines

T lymphocyte activation is required to eliminate or control intracellular viruses. The activation of T cells requires both an antigen specific signal, involving the recognition of a peptide/major histocompatibility protein complex by the T cell receptor, as well as additional costimulatory signals. In chronic viral diseases, T cell responses, although present, are unable to eliminate the infection. By providing antigens and costimulatory molecules together, investigators may be able to increase and broaden the immune response, resulting in better immunological control or even elimination of the infection. Recent progress in understanding the function of costimulatory molecules suggests that different costimulatory molecules are involved in initial immune responses than are involved in recall responses. These new developments have important implications for therapeutic vaccine design. In this review the authors discuss the function of T cell costimulatory molecules in immune system activation and their potential for enhancing the efficacy of therapeutic vaccines.

Environmental and antigen receptor-derived signals support sustained surveillance of the lungs by pathogen-specific cytotoxic T lymphocytes

Viral infections often gain access to the body of their host by exploiting areas of natural vulnerability, such as the semipermeable surfaces of mucosal tissues which are adapted for adsorption of nutrients and other diffusible molecules. Once the microbes have crossed the epithelial barrier, they can disperse to other tissues where eradication may not be possible. The best opportunity for successful immune intervention is immediately after infection while the pathogen is confined to a localized area of the body. Cytotoxic T lymphocytes (CTL) which reside at the site where the infection begins can make an important contribution to immunity by reducing early dissemination of the infection. Because the lungs provide easy access points for many pathogens to enter the body, they require protection from many complementary mechanisms, including pathogen-specific cytotoxic T cells. In this study we show that an enduring response to pathogen-derived peptide antigens facilitates sustained surveillance of the lungs by pathogen-specific CTL during the recovery from influenza virus infection. Our studies show that these processed peptide antigens reinforce expression of two homing receptors (CD69 and CD103) which help recently activated virus-specific CTL colonize the lungs during a mild inflammatory response. We suggest that this requirement for prolonged antigen presentation to reinforce local CTL responses in the lungs explains why protective cellular immunity quickly declines following influenza virus infection and other viral infections that enter the body via mucosal tissues.

Qualitatively different memory CD8+ T cells are generated after lymphocytic choriomeningitis virus and influenza virus infections

Viral infections often induce robust T cell responses that are long-lived and protective. However, it is unclear to what degree systemic versus mucosal infection influences the generation of effector and memory T cells. In this study, we characterized memory CD8(+) T cells generated after respiratory influenza virus infection and compared the phenotypic and functional qualities of these cells with memory T cells generated after systemic infection with lymphocytic choriomeningitis virus (LCMV). Using a recombinant influenza virus expressing the LCMV gp(33-41) epitope and TCR transgenic CD8(+) T cells with a fixed TCR, we compared responses to the same Ag delivered by mucosal or systemic viral infection. Memory cells generated postinfection with either virus showed only a few phenotypic differences. Yet, influenza memory T cells produced lower amounts of effector cytokines upon restimulation and displayed reduced proliferation compared with LCMV-induced memory cells. Strikingly, we observed reduced expansion of spleen- and, in particular, lung-derived influenza memory cells after recall in vivo, which correlated with reduced early protection from secondary infection. These findings suggest that qualitatively different memory CD8(+) T cells are generated after respiratory or systemic virus infections.

Primary CTL response magnitude in mice is determined by the extent of naive T cell recruitment and subsequent clonal expansion

CD8+ T cell responses to viral infection are characterized by the emergence of dominant and subdominant CTL populations. The immunodominance hierarchies of these populations are highly reproducible for any given spectrum of virus-induced peptide-MHCI complexes and are likely determined by multiple factors. Recent studies demonstrate a direct correlation between naive epitope-specific CD8+ T cell precursor (CTLp) frequency and the magnitude of the response after antigen challenge. Thus, the number of available precursors in the naive pool has emerged as a key predictor of immunodominance. In contrast to this, we report here no consistent relationship between CTLp frequency and the subsequent magnitude of the immune response for 4 influenza virus-derived epitopes following intranasal infection of mice with influenza A virus. Rather, the characteristic, antigen-driven T cell immunodominance hierarchy was determined by the extent of recruitment from the available pool of epitope-specific precursors and the duration of their continued expansion over the course of the infection. These findings suggest possibilities for enhancing protective immune memory by maximizing both the size and diversity of typically subdominant T cell responses through rational vaccine design.

Antigen specificity determines the pro- or antitumoral nature of CD8+ T cells

Although CD8+ T cells are usually considered antitumoral, several recent studies report that the cells can also promote tumor progression. Using the melanoma cell line B16 as a murine model of pulmonary metastasis, we examined whether the pro- versus antitumoral effects of CD8+ T cells relate to their Ag specificity. Results of the study indicate that although CD8+ T cells specific for tumor Ags promote tumor rejection, CD8+ T cells specific for unrelated Ags promote tumor progression. We found the effect to be partly attributable to CD8+ T cells dampening effective antitumor NK cell responses. Notably, activation of CD8+ T cell responses by an unrelated stimulus, in this case infection with influenza virus, increased the number of pulmonary tumor nodules. These data provide a rationale for previously unexplained data identifying contrasting roles for CD8+ T cells in tumor progression.

Innate immune control and regulation of influenza virus infections

Adaptive immune responses are critical for the control and clearance of influenza A virus (IAV) infection. However, in recent years, it has become increasingly apparent that innate immune cells, including natural killer cells, alveolar macrophages (aMphi), and dendritic cells (DC) are essential following IAV infection in the direct control of viral replication or in the induction and regulation of virus-specific adaptive immune responses. This review will discuss the role of these innate immune cells following IAV infection, with a particular focus on DC and their ability to induce and regulate the adaptive IAV-specific immune response.

Transience of MHC Class I-restricted antigen presentation after influenza A virus infection

Antigen expressed as MHC Class I glycoprotein (pMHCI) complexes on dendritic cells is the primary driver of CD8(+) T cell clonal expansion and differentiation. As we seek to define the molecular differences between acutely stimulated cytotoxic T lymphocyte (CTL) effectors and long-lived memory T cells, it is essential that we understand the duration of in vivo pMHCI persistence. Although infectious influenza A virus is readily cleared by mammalian hosts, that does not necessarily mean that all influenza antigen is totally eliminated. An exhaustive series of carefully controlled adoptive transfer experiments using 3 different carboxy fluorescein diacetate succinimidyl ester-labeled T cell receptor-transgenic CTL populations and a spectrum of genetically engineered and wild-type influenza A viruses provided no evidence for pMHCI persistence over the 30-60-d interval after virus challenge. Molecular profiles identified in antigen-specific T cells at this time may thus be considered to reflect established immunologic memory and not recent CTL activation from a persistent pMHCI pool.

Kinetics of major histocompatibility class I antigen presentation in acute infection

Ag presentation within the regional lymph node is crucial for the initiation of CD8+ T cell responses following viral infection. The magnitude and quality of the CD8+ T cell response are regulated by the interplay between the size of the APC population and duration of Ag presentation. To understand how these parameters are finely regulated during an immune response, we have investigated the dynamics of Ag presentation in influenza A virus and HSV-1 infection. In both infections, APC production was calculated to occur over the first few days of infection, after which there was slow exponential decay over a period of up to 2 wk. This production rate is most likely determined by the Ag availability and recruitment and/or maturation rate of dendritic cells. APC production was found to closely parallel lymph node cell recruitment in both infections. This was greatest in the first 6 h of infection for HSV and over the second and third day for influenza. In HSV infection, the peak production also coincides with peak viral levels. By contrast, in influenza infection, APC production ceased between the third and fourth day despite the presence of high levels of virus until 5 days after infection. These analyses demonstrate that two quite different self-limiting infections generate the APC necessary to drive T cell responses early in infection at different rates. Understanding how such contrasting kinetics of Ag presentation impacts on the growth and size of developing protective T cell populations has important implications for the design of vaccines and immunotherapies.

Influenza neuraminidase as a vaccine antigen

The neuraminidase protein of influenza viruses is a surface glycoprotein that shows enzymatic activity to remove sialic acid, the viral receptor, from both viral and host proteins. The removal of sialic acid from viral proteins plays a key role in the release of the virus from the cell by preventing the aggregation of the virus by the hemagglutinin protein binding to other viral proteins. Antibodies to the neuraminidase protein can be protective alone in animal challenge studies, but the neuraminidase antibodies appear to provide protection in a different manner than antibodies to the hemagglutinin protein. Neutralizing antibodies to the hemagglutinin protein can directly block virus entry, but protective antibodies to the neuraminidase protein are thought to primarily aggregate virus on the cell surface, effectively reducing the amount of virus released from infected cells. The neuraminidase protein can be divided into nine distinct antigenic subtypes, where there is little cross-protection of antibodies between subtypes. All nine subtypes of neuraminidase protein are commonly found in avian influenza viruses, but only selected subtypes are routinely found in mammalian influenza viruses; for example, only the N1 and N2 subtypes are commonly found in both humans and swine. Even within a subtype, the neuraminidase protein can have a high level of antigenic drift, and vaccination has to specifically be targeted to the circulating strain to give optimal protection. The levels of neuraminidase antibody also appear to be critical for protection, and there is concern that human influenza vaccines do not include enough neuraminidase protein to induce a strong protective antibody response. The neuraminidase protein has also become an important target for antiviral drugs that target sialic acid binding which blocks neuraminidase enzyme activity. Two different antiviral drugs are available and are widely used for the treatment of seasonal influenza in humans, but antiviral resistance appears to be a growing concern for this class of antivirals.

Toward a broadly protective influenza vaccine

The current inactivated influenza virus vaccines induce antibodies that protect against closely related virus strains. They do not, however, protect against antibody-escape variants of seasonal influenza A viruses or new pandemic influenza A viruses emerging from non-human reservoirs. Might boosting influenza A virus-specific CD8+ T cell memory diminish the danger posed by these variant viruses? Pre-existing CD8+ T cell-mediated immunity directed at peptides from conserved internal proteins of the influenza A virus does not prevent infection, but it can promote early virus clearance and decrease morbidity in mice. In this issue of the JCI, Lee et al. show that people who have not been exposed to avian influenza A (H5N1) viruses have cross-reactive CD8+ T cell memory to a wide range of H5N1 peptides (see the related article beginning on page 3478). These peptides could be used to add a CD8+ T cell component to current antibody-focused vaccine strategies with a view to reducing the impact of infection with novel influenza A viruses.

Activation of invariant NKT cells enhances the innate immune response and improves the disease course in influenza A virus infection

Invariant NKT (iNKT) cells have an indubitable role in antiviral immunity, although the mechanisms by which these cells exert their functions are not fully elucidated. With the emerging importance of high-pathogenicity influenza A virus infections in humans, we questioned whether iNKT cells contribute to immune defence against influenza A virus and whether activation of these cells influences outcome. We show that activation of iNKT cells with alpha-galactosylceramide (alpha-GC) during influenza virus infection transiently enhanced early innate immune response without affecting T cell immunity, and reduced early viral titres in lungs of C57BL/6 mice. This is accompanied by a better disease course with improved weight loss profile. Temporal changes in iNKT cells in the liver, blood and lungs suggest activation and migration of iNKT cells from the liver to the lungs in mice that were administered alpha-GC. Improvement in viral titres appears dependent on activation of iNKT cells via the intraperitoneal route since intranasal administration of alpha-GC did not have the same effect. We conclude that activation of iNKT cells enhances early innate immune response in the lungs and contribute to antiviral immunity and improved disease course in influenza A virus infection.

Monitoring of vaccine-specific gamma interferon induction in genital mucosa of mice by real-time reverse transcription-PCR

Monitoring of T-cell responses in genital mucosa has remained a major challenge because of the absence of lymphoid aggregates and the low abundance of T cells. Here we have adapted to genital tissue a sensitive real-time reverse transcription-PCR (TaqMan) method to measure induction of gamma interferon (IFN-gamma) mRNA transcription after 3 h of antigen-specific activation of CD8 T cells. For this purpose, we vaccinated C57BL/6 mice subcutaneously with human papillomavirus type 16 L1 virus-like particles and monitored the induction of CD8 T cells specific to the L1(165-173) H-2D(b)-restricted epitope. Comparison of the responses induced in peripheral blood mononuclear cells and lymph nodes (LN) by L1-specific IFN-gamma enzyme-linked immunospot assay and TaqMan determination of the relative increase in L1-specific IFN-gamma mRNA induction normalized to the content of CD8b mRNA showed a significant correlation, despite the difference in the readouts. Most of the cervicovaginal tissues could be analyzed by the TaqMan method if normalization to glyceraldehyde-3-phosphate dehydrogenase mRNA was used and a significant L1-specific IFN-gamma induction was found in one-third of the immunized mice. This local response did not correlate with the immune responses measured in the periphery, with the exception of the sacral LN, an LN draining the genital mucosa, where a significant correlation was found. Our data show that the TaqMan method is sensitive enough to detect antigen-specific CD8 T-cell responses in the genital mucosa of individual mice, and this may contribute to elaborate effective vaccines against genital pathogens.

Cross-clade protective immune responses to influenza viruses with H5N1 HA and NA elicited by an influenza virus-like particle

Background

Vaccination is a cost-effective counter-measure to the threat of seasonal or pandemic outbreaks of influenza. To address the need for improved influenza vaccines and alternatives to egg-based manufacturing, we have engineered an influenza virus-like particle (VLP) as a new generation of non-egg or non-mammalian cell culture-based candidate vaccine.

Methodology/Principal Findings

We generated from a baculovirus expression system using insect cells, a non-infectious recombinant VLP vaccine from both influenza A H5N1 clade 1 and clade 2 isolates with pandemic potential. VLPs were administered to mice in either a one-dose or two-dose regimen and the immune responses were compared to those induced by recombinant hemagglutinin (rHA). Both humoral and cellular responses were analyzed. Mice vaccinated with VLPs were protected against challenge with lethal reassortant viruses expressing the H5N1 HA and NA, regardless if the H5N1 clade was homologous or heterologous to the vaccine. However, rHA-vaccinated mice showed considerable weight loss and death following challenge with the heterovariant clade virus. Protection against death induced by VLPs was independent of the pre-challenge HAI titer or cell-mediated responses to HA or M1 since vaccinated mice, with low to undetectable cross-clade HAI antibodies or cellular responses to influenza antigens, were still protected from a lethal viral challenge. However, an apparent association rate of antibody binding to HA correlated with protection and was enhanced using VLPs, particularly when delivered intranasally, compared to rHA vaccines.

Conclusion/Significance

This is the first report describing the use of an H5N1 VLP vaccine created from a clade 2 isolate. The results show that a non-replicating virus-like particle is effective at eliciting a broadened, cross-clade protective immune response to proteins from emerging H5N1 influenza isolates giving rise to a potential pandemic influenza vaccine candidate for humans that can be stockpiled for use in the event of an outbreak of H5N1 influenza.

Memory CD8+ T cells require CD28 costimulation

CD8(+) T cells are a critical component of the adaptive immune response against infections and tumors. A current paradigm in immunology is that naive CD8(+) T cells require CD28 costimulation, whereas memory CD8(+) T cells do not. We show here, however, that during viral infections of mice, costimulation is required in vivo for the reactivation of memory CD8(+) T cells. In the absence of CD28 costimulation, secondary CD8(+) T cell responses are greatly reduced and this impairs viral clearance. The failure of CD8(+) T cells to expand in the absence of CD28 costimulation is CD4(+) T cell help independent and is accompanied by a failure to down-regulate Bcl-2 and by cell cycle arrest. This requirement for CD28 costimulation was shown in both influenza A and HSV infections. Thus, contrary to current dogma, memory CD8(+) T cells require CD28 costimulation to generate maximal secondary responses against pathogens. Importantly, this CD28 requirement was shown in the context of real infections were multiple other cytokines and costimulators may be up-regulated. Our findings have important implications for pathogens, such as HIV and measles virus, and tumors that evade the immune response by failing to provide CD28 costimulation. These findings also raise questions about the efficacy of CD8(+) T cell-based vaccines against such pathogens and tumors.

Discriminating between different pathways of memory CD8+ T cell differentiation

Despite the rapid accumulation of quantitative data on the dynamics of CD8(+) T cell responses following acute viral or bacterial infections of mice, the pathways of differentiation of naive CD8(+) T cells into memory during an immune response remain controversial. Currently, three models have been proposed. In the "stem cell-associated differentiation" model, following activation, naive T cells differentiate into stem cell-like memory cells, which then convert into terminally differentiated short-lived effector cells. In the "linear differentiation" model, following activation, naive T cells first differentiate into effectors, and after Ag clearance, effectors convert into memory cells. Finally, in the "progressive differentiation" model, naive T cells differentiate into memory or effector cells depending on the amount of specific stimulation received, with weaker stimulation resulting in formation of memory cells. This study investigates whether the mathematical models formulated from these hypotheses are consistent with the data on the dynamics of the CD8(+) T cell response to lymphocytic choriomeningitis virus during acute infection of mice. Findings indicate that two models, the stem cell-associated differentiation model and the progressive differentiation model, in which differentiation of cells is strongly linked to the number of cell divisions, fail to describe the data at biologically reasonable parameter values. This work suggests additional experimental tests that may allow for further discrimination between different models of CD8(+) T cell differentiation in acute infections.

Efficacy of intranasal administration of a truncated NS1 modified live influenza virus vaccine in swine

In the U.S., despite available swine influenza virus (SIV) vaccines, multiple influenza subtypes as well as antigenic and genetic variants within subtypes continue to circulate in the swine population. One of the challenges to control and eliminate SIV is that the currently used inactivated influenza virus vaccines do not provide adequate cross-protection against multiple antigenic variants of SIV in the field. We previously generated a recombinant H3N2 swine influenza virus (SIV) based on the influenza A/SW/TX/4199-2/98 virus (TX98) containing an NS1 gene expressing a truncated NS1 protein of 126 amino acids, TX98-NS1Delta126 virus. This recombinant strain was demonstrated to be highly attenuated in swine and showed potential for use as a modified live-virus vaccine (MLV) after intratracheal application in pigs. However, this route of inoculation is not practical for vaccination in the field. In the present study, we first compared intramuscular and intranasal routes of application of the MLV, and found that the intranasal route was superior in priming the local (mucosal) immune response. Pigs were then vaccinated via the intranasal route and challenged with wild type homologous TX98 H3N2 virus, with a genetic and antigenic variant H3N2 SIV (influenza A/SW/CO/23619/99 virus, CO99) and a heterosubtypic H1N1 SIV (influenza A/SW/IA/00239/2004 virus, IA04). The intranasally vaccinated pigs were completely protected against homologous challenge. In addition, MLV vaccination provided nearly complete protection against the antigenic H3N2 variant CO99 virus. When challenged with the H1N1 IA04 virus, MLV vaccinated animals displayed reduced fever and virus titers despite minimal reduction in lung lesions. In vaccinated pigs, there was no serologic cross-reactivity by HI assays with the heterologous or heterosubtypic viruses. However, there appeared to be substantial cross-reactivity in antibodies at the mucosal level with the CO99 virus in MLV vaccinated pigs.

Avian influenza: should China be alarmed?

Avian influenza has emerged as one of the primary public health concern of the 21st century. Influenza strain H5N1 is capable of incidentally infecting humans and other mammals. Since their reemergence in 2003, highly pathogenic avian influenza A (H5N1) viruses have been transmitted from poultry to humans (by direct or indirect contact with infected birds) in several provinces of Mainland China, which has resulted in 22 cases of human infection and has created repercussions for the Chinese economy. People have been concerned whether a new pandemic will occur in the future. The eradication of pathogenic avian influenza viruses appears to be the most effective way to prevent an influenza pandemic. This paper will examine the features of H5N1, including incidence, infection, immunity, clinical management, prevention and control, and therapy in Mainland China.

Heterogeneity of effector phenotype for acute phase and memory influenza A virus-specific CTL

Ag-specific, CD8+ CTLs clear influenza A viruses from the lung via granzyme (Gzm) and perforin-dependent mechanisms. Ex vivo analysis of perforin-Gzm mRNA profiles demonstrated substantial heterogeneity in patterns of effector mRNA transcription of CD8+ D(b)NP(366)- or D(b)PA(224)-specific CTL. The only difference between the two epitope-specific sets was apparent very early after infection with similar molecular profiles seen in peak primary and secondary responses and in long-term memory. Surprisingly, memory T cells also expressed a diverse pattern of effector mRNA profile with an emphasis on GzmB and, surprisingly, GzmK. This analysis thus defines how naive, effector, and memory T cells differ in cytotoxic potential and provides novel insight into the molecular signatures of effector molecules observed at various stages after infection.

Failure of protection and enhanced pneumonia with a US H1N2 swine influenza virus in pigs vaccinated with an inactivated classical swine H1N1 vaccine

Two US swine influenza virus (SIV) isolates, A/Swine/Iowa/15/1930 H1N1 (IA30) and A/Swine/Minnesota/00194/2003 H1N2 (MN03), were evaluated in an in vivo vaccination and challenge model. Inactivated vaccines were prepared from each isolate and used to immunize conventional pigs, followed by challenge with homologous or heterologous virus. Both inactivated vaccines provided complete protection against homologous challenge. However, the IA30 vaccine failed to protect against the heterologous MN03 challenge. Three of the nine pigs in this group had substantially greater percentages of lung lesions, suggesting the vaccine potentiated the pneumonia. In contrast, priming with live IA30 virus provided protection from nasal shedding and virus replication in the lung in MN03 challenged pigs. These data indicate that divergent viruses that did not cross-react serologically did not provide complete cross-protection when used in inactivated vaccines against heterologous challenge and may have enhanced disease. In addition, live virus infection conferred protection against heterologous challenge.

Collagen distribution and expression of collagen-binding alpha1beta1 (VLA-1) and alpha2beta1 (VLA-2) integrins on CD4 and CD8 T cells during influenza infection

Most viral infections occur in extralymphoid tissues, yet the mechanisms that regulate lymphocytes in these environments are poorly understood. One feature common to many extralymphoid environments is an abundance of extracellular matrix. We have studied the expression of two members of the beta(1) integrin family of collagen-binding receptors, alpha(1)beta(1) and alpha(2)beta(1) (CD49a, VLA-1 and CD49b, VLA-2, respectively), on CD4 and CD8 T cells during the response to influenza infection in the lung. Flow cytometry showed that whereas T cells infiltrating the lung and airways can express both CD49a and CD49b, CD49a expression was most strongly associated with the CD8+ subset. Conversely, though fewer CD4+ T cells expressed CD49a, most CD4+ cells in the lung tissue or airways expressed CD49b. This reciprocal pattern suggested that CD4 and CD8 T cells might localize differently within the lung tissue and this was supported by immunofluorescent analysis. CD8+ cells tended to localize in close proximity to the collagen IV-rich basement membranes of either the airways or blood vessels, whereas CD4+ cells tended to localize in the collagen I-rich interstitial spaces, with few in the airways. These observations suggest that CD4 T cell interaction with the tissue microenvironment is distinct from CD8 T cells and support the concept that CD4+ T cells in peripheral tissues are regulated differently than the CD8 subset.

Location rather than CD62L phenotype is critical in the early establishment of influenza-specific CD8+ T cell memory

The rapid recall of influenza virus-specific CD8(+) T cell effector function is protective, although our understanding of T cell memory remains incomplete. Recent debate has focused particularly on the CD62L lymph node homing receptor. The present analysis shows that although functional memory can be established from both CD62L(hi) and CD62L(lo) CD8(+) T cell subsets soon after initial encounter between naïve precursors and antigen, the optimal precursors are CD8(+)CD44(hi)CD25(lo) immune lymphocytes isolated from draining lymph nodes on day 3.5 after influenza virus infection. Analysis of primed T cells at different times after challenge indicates that the capacity to transfer memory is diminished at the peak of the primary cytotoxic T lymphocyte response, challenging speculations that the transition to memory first requires full differentiation to effector status. It seems that location rather than CD62Lhi/lo phenotype may be the more profitable focus for further dissection of the early establishment of T cell memory.

Influenza neuraminidase antibodies provide partial protection for chickens against high pathogenic avian influenza infection

Protection of chickens against avian influenza (AI) is mostly attributed to production of antibodies against the viral glycoprotein hemagglutinin, whereas less is known about the protective role of antibodies to the other surface glycoprotein neuraminidase (NA). Therefore, vaccines encoding NA antigen (e.g., DNA and alphavirus-based virus like replicon particles (VRP)) or baculovirus-expressed recombinant NA (rN2) were tested for their ability to protect against highly pathogenic AI (HPAI) in chickens. Vaccination with A/Pheasant/Maryland/4457/93 (Ph/MD) rN2 protein produced significantly higher levels of NA-inhibition (NI) activity and 88% protection from HPAI H5N2 challenge than vaccination with Ph/MD N2 DNA (25% protection). Vaccination with Ph/MD N2 VRP a minimum of two times also produced high levels of NI activity and protection against HPAI challenge (63% protection). Vaccination with VRP encoding an N2 gene that was genetically distant from the challenge virus N2 failed to protect chickens. Vaccines producing higher levels of NI activity conferred partial protection, but failed to affect viral shedding. Consideration of the homology between vaccine and challenge virus isolate NA genes may provide improved immunity if high levels of NI activity are obtained.

Virus-specific CD8+ T cells in the liver: armed and ready to kill

Influenza A virus infection of C57BL/6 mice is a well-characterized model for studying CD8+ T cell-mediated immunity. Analysis of primary and secondary responses showed that the liver is highly enriched for CD8+ T cells specific for the immunodominant H2D(b)NP(366-374) (D(b)NP(366)) epitope. Functional analysis established that these liver-derived virus-specific CD8+ T cells are fully competent cytotoxic effectors and IFN-gamma secretors. In addition, flow cytometric analysis of early apoptotic cells showed that these influenza-specific CD8+ T cells from liver are as viable as those in the spleen, bronchoalveolar lavage, mediastinal lymph nodes, or lung. Moreover, cytokine profiles of the influenza-specific CD8+ T cells recovered from different sites were consistent with the bronchoalveolar lavage, rather than liver population, being the most susceptible to activation-induced cell death. Importantly, adoptively transferred influenza virus-specific CD8+ T cells from the liver survived and were readily recalled after virus challenge. Together, these results show clearly that the liver is not a "graveyard" for influenza virus-specific CD8+ T cells.

IL-18, but not IL-12, is required for optimal cytokine production by influenza virus-specific CD8+ T cells

The potent innate cytokines IL-12 and IL-18 are considered to be important antigen-independent mediators of IFN-gamma production by NK cells and T lymphocytes. The present analysis addresses the physiological role of IL-12 and IL-18 in the generation of virus-specific CD8+ T cells. Both wt C57BL/6J (B6) mice and mice with disrupted IL-12p40 (IL-12p40(-/-)) or IL-18 (IL-18(-/-)) genes were infected with an influenza A virus and the characteristics of the resultant epitope-specific CD8+ T cell responses were compared. While IL-12 appeared to have no notable effect on either virus growth or on CD8+ T cell response profiles, the absence of IL-18 was associated with delayed virus clearance from the lung and, despite normal numbers, a significantly reduced production of IFN-gamma, TNF-alpha, and IL-2 by epitope-specific CD8+ T cells. While this cytokine phenotype was broadly maintained in IL-12p40/IL-18 double-knockout mice, no evidence was seen for any additive effect. Together, our results suggest that IL-18, but not IL-12, induces optimal, antigen-specific production of key cytokines by CD8+ T cells for the efficient clearance of influenza virus from the lungs of infected mice.

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.

Vaccination of pigs against swine influenza viruses by using an NS1-truncated modified live-virus vaccine

Swine influenza viruses (SIV) naturally infect pigs and can be transmitted to humans. In the pig, genetic reassortment to create novel influenza subtypes by mixing avian, human, and swine influenza viruses is possible. An SIV vaccine inducing cross-protective immunity between different subtypes and strains circulating in pigs is highly desirable. Previously, we have shown that an H3N2 SIV (A/swine/Texas/4199-2/98 [TX98]) containing a deleted NS1 gene expressing a truncated NS1 protein of 126 amino acids, NS1black triangle126, was attenuated in swine. In this study, 4-week-old pigs were vaccinated with the TX98 NS1black triangle126 modified live virus (MLV). Ten days after boosting, pigs were challenged with wild-type homologous H3N2 or heterosubtypic H1N1 SIV and sacrificed 5 days later. The MLV was highly attenuated and completely protected against challenge with the homologous virus. Vaccinated pigs challenged with the heterosubtypic H1N1 virus demonstrated macroscopic lung lesions similar to those of the unvaccinated H1N1 control pigs. Remarkably, vaccinated pigs challenged with the H1N1 SIV had significantly lower microscopic lung lesions and less virus shedding from the respiratory tract than did unvaccinated, H1N1-challenged pigs. All vaccinated pigs developed significant levels of hemagglutination inhibition and enzyme-linked immunosorbent assay titers in serum and mucosal immunoglobulin A antibodies against H3N2 SIV antigens. Vaccinated pigs were seronegative for NS1, indicating the potential use of the TX98 NS1black triangle126 MLV as a vaccine to differentiate infected from vaccinated animals.

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.

Kupffer cell-dependent hepatitis occurs during influenza infection

Respiratory infections, including influenza in humans, are often accompanied by a hepatitis that is usually mild and self-limiting. The mechanism of this kind of liver damage is not well understood. In the present study, we show that influenza-associated hepatitis occurs due to the formation of inflammatory foci that include apoptotic hepatocytes, antigen-specific CD8(+) T cells, and Kupffer cells. Serum aminotransaminase levels were elevated, and both the histological and serum enzyme markers of hepatitis were increased in secondary influenza infection, consistent with a primary role for antigen-specific T cells in the pathogenesis. No virus could be detected in the liver, making this a pure example of "collateral damage" of the liver. Notably, removal of the Kupffer cells prevented the hepatitis. Such hepatic collateral damage may be a general consequence of expanding CD8(+) T-cell populations during many extrahepatic viral infections, yielding important implications for liver pathobiology.

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.

Kupffer cell-dependent hepatitis occurs during influenza infection

Respiratory infections, including influenza in humans, are often accompanied by a hepatitis that is usually mild and self-limiting. The mechanism of this kind of liver damage is not well understood. In the present study, we show that influenza-associated hepatitis occurs due to the formation of inflammatory foci that include apoptotic hepatocytes, antigen-specific CD8(+) T cells, and Kupffer cells. Serum aminotransaminase levels were elevated, and both the histological and serum enzyme markers of hepatitis were increased in secondary influenza infection, consistent with a primary role for antigen-specific T cells in the pathogenesis. No virus could be detected in the liver, making this a pure example of "collateral damage" of the liver. Notably, removal of the Kupffer cells prevented the hepatitis. Such hepatic collateral damage may be a general consequence of expanding CD8(+) T-cell populations during many extrahepatic viral infections, yielding important implications for liver pathobiology.

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.

Measuring Ag-specific immune responses: understanding immunopathogenesis and improving diagnostics in infectious disease, autoimmunity and cancer

Characterization of antigen-specific immune responses at the single-cell level has been made possible by recent advancements in reagent and technology development, combined with increasing knowledge of molecular mechanisms. Fluorescently labelled MHC-peptide multimers and antigens identify directly specific T and B cells, respectively, whereas dynamic assays exploit mediator production or secretion, or the changes in surface expression of other proteins, to identify specific lymphocytes--some techniques enabling the recovery of viable cells. Meanwhile, multiparameter flow cytometry has emerged as the most versatile platform for integrating most of these methods. As the complexity of experimental data increases, so does the level of technical sophistication required for analysis and interpretation, both in terms of basic research and modern medicine, with new applications for infectious diseases, autoimmunity and cancer.

CD8+ lymphocytes do not mediate protection against acute superinfection 20 days after vaccination with a live attenuated simian immunodeficiency virus

In order to test the hypothesis that CD8+ cytotoxic T lymphocytes mediate protection against acute superinfection, we depleted >99% of CD8+ lymphocytes in live attenuated simian immunodeficiency virus macC8 (SIVmacC8) vaccinees from the onset of vaccination, maintained that depletion for 20 days, and then challenged with pathogenic, wild-type SIVmacJ5. Vaccinees received 5 mg per kg of humanized anti-CD8 monoclonal antibody (MAb) 1 h before inoculation, followed by the same dose again on days 3, 7, 10, 13, and 17. On day 13, peripheral CD8+ T lymphocytes were >99% depleted in three out of four anti-CD8 MAb-treated vaccinees. At this time attenuated SIVmacC8 viral RNA loads in anti-CD8 MAb-treated vaccinees were significantly higher than control vaccinees treated contemporaneously with nonspecific human immunoglobulin. Lymphoid tissue CD8+ T lymphocyte depletion was >99% in three out of four anti-CD8 MAb-treated vaccinees on the day of wild-type SIVmacJ5 challenge. All four control vaccinees and three out of four anti-CD8 MAb-treated vaccinees were protected against detectable superinfection with wild-type SIVmacJ5. Although superinfection with wild-type SIVmacJ5 was detected at postmortem in a single anti-CD8 MAb-treated vaccinee, this did not correlate with the degree of preceding CD8+ T lymphocyte depletion. Clearance of attenuated SIVmacC8 viremia coincided with recovery of normal CD8+ T lymphocyte counts between days 48 and 76. These results support the view that cytotoxic T lymphocytes are important for host-mediated control of SIV primary viremia but do not indicate a central role in protection against acute superinfection conferred by inoculation with live attenuated SIV.

Vaccine route, dose and type of delivery vector determine patterns of primary CD8+ T cell responses

The dynamics of primary CD8+ T cell responses following administration of modified virus Ankara (MVA)- and DNA-vectored vaccines was investigated in a mouse model. To overcome the low frequency of naive antigen-specific precursors and follow the early expansion events, naive CFSE-labelled T cell receptor-transgenic F5 lymphocytes were transferred into syngeneic non-transgenic recipients prior to vaccination. Using the i.d., i.v. and i.m. routes and increasing recombinant MVA (rMVA) vaccine doses, the primary response was analysed on a divisional basis at local and distant lymphoid organs at various times after vaccination. The results indicated that F5 cell divisions were initiated in the local draining lymph nodes and cells only after five to six divisions appeared at more distant sites. The rMVA dose affected frequencies of cells entering division and at the peak response. When priming induced by rMVA and plasmid DNA was compared, dramatic differences in the cycling patterns were observed with plasmid DNA inducing a response slower and more sustained over the first 2 wk than rMVA. Both rMVA and DNA induced comparable IFN-gamma production, which increased with cell divisions. Taken together, the vaccine type, dose and route have a strong influence on the spatial and temporal patterns of initial T cell responses.

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

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