Mice deficient in perforin, CD4+ T cells, or CD28-mediated signaling maintain the typical immunodominance hierarchies of CD8+ T-cell responses to influenza virus

Discordant rearrangement of primary and anamnestic CD8+ T cell responses to influenza A viral epitopes upon exposure to bacterial superantigens: Implications for prophylactic vaccination, heterosubtypic immunity and superinfections

Infection with (SAg)-producing bacteria may precede or follow infection with or vaccination against influenza A viruses (IAVs). However, how SAgs alter the breadth of IAV-specific CD8⁺ T cell (T_CD8) responses is unknown. Moreover, whether recall responses mediating heterosubtypic immunity to IAVs are manipulated by SAgs remains unexplored. We employed wild-type (WT) and mutant bacterial SAgs, SAg-sufficient/deficient Staphylococcus aureus strains, and WT, mouse-adapted and reassortant IAV strains in multiple in vivo settings to address the above questions. Contrary to the popular view that SAgs delete or anergize T cells, systemic administration of staphylococcal enterotoxin B (SEB) or Mycoplasma arthritidis mitogen before intraperitoneal IAV immunization enlarged the clonal size of ‘select’ IAV-specific T_CD8 and reshuffled the hierarchical pattern of primary T_CD8 responses. This was mechanistically linked to the TCR Vβ makeup of the impacted clones rather than their immunodominance status. Importantly, SAg-expanded T_CD8 retained their IFN-γ production and cognate cytolytic capacities. The enhancing effect of SEB on immunodominant T_CD8 was also evident in primary responses to vaccination with heat-inactivated and live attenuated IAV strains administered intramuscularly and intranasally, respectively. Interestingly, in prime-boost immunization settings, the outcome of SEB administration depended strictly upon the time point at which this SAg was introduced. Accordingly, SEB injection before priming raised CD127^highKLRG1^low memory precursor frequencies and augmented the anamnestic responses of SEB-binding T_CD8. By comparison, introducing SEB before boosting diminished recall responses to IAV-derived epitopes drastically and indiscriminately. This was accompanied by lower Ki67 and higher Fas, LAG-3 and PD-1 levels consistent with a pro-apoptotic and/or exhausted phenotype. Therefore, SAgs can have contrasting impacts on anti-IAV immunity depending on the naïve/memory status and the TCR composition of exposed T_CD8. Finally, local administration of SEB or infection with SEB-producing S. aureus enhanced pulmonary T_CD8 responses to IAV. Our findings have clear implications for superinfections and prophylactic vaccination.

Exposure to bacterial superantigens (SAgs) is often a consequence of infection with common Gram-positive bacteria causing septic and toxic shock or food poisoning. How SAgs affect the magnitude, breadth and quality of infection/vaccine-elicited CD8⁺ T cell (T_CD8) responses to respiratory viral pathogens, including influenza A viruses (IAVs), is far from clear. Also importantly, superinfections with IAVs and SAg-producing bacteria are serious clinical occurrences during seasonal and pandemic flu and require urgent attention. We demonstrate that two structurally distinct SAgs, including staphylococcal enterotoxin B (SEB), unexpectedly enhance primary T_CD8 responses to ‘select’ IAV-derived epitopes depending on the TCR makeup of the responding clones. Intriguingly, the timing of exposure to SEB dictates the outcome of prime-boost immunization. Seeing a SAg before priming raises memory precursor frequencies and augments anamnestic T_CD8 responses. Conversely, a SAg encounter before boosting renders T_CD8 prone to death or exhaustion and impedes recall responses, thus likely compromising heterosubtypic immunity to IAVs. Finally, local exposure to SEB increases the pulmonary response of immunodominant IAV-specific T_CD8. These findings shed new light on how bacterial infections and SAgs influence the effectiveness of anti-IAV T_CD8 responses, and have, as such, wide-ranging implications for preventative vaccination and infection control.

Influenza A Virus Infection Induces Viral and Cellular Defective Ribosomal Products Encoded by Alternative Reading Frames

The importance of antiviral CD8⁺ T cell recognition of alternative reading frame (ARF)-derived peptides is uncertain. In this study, we describe an epitope (NS1-ARF2_1-8) present in a predicted 14-residue peptide encoded by the +1 register of NS1 mRNA in the influenza A virus (IAV). NS1-ARF2_1-8 elicits a robust, highly functional CD8⁺ T cell response in IAV-infected BALB/c mice. NS1-ARF2_1-8 is presented from unspliced NS mRNA, likely from downstream initiation on a Met residue that comprises the P1 position of NS1-ARF2_1-8 Derived from a 14-residue peptide with no apparent biological function and negligible impacts on IAV infection, infectivity, and pathogenicity, NS1-ARF2_1-8 provides a clear demonstration of how immunosurveillance exploits natural errors in protein translation to provide antiviral immunity. We further show that IAV infection enhances a model cellular ARF translation, which potentially has important implications for virus-induced autoimmunity.

Pulmonary Dendritic Cell Subsets Shape the Respiratory Syncytial Virus-Specific CD8+ T Cell Immunodominance Hierarchy in Neonates

Young infants are generally more susceptible to viral infections and experience more severe disease than do adults. CD8⁺ T cells are important for viral clearance, and although often ineffective in neonates they can be protective when adequately stimulated. Using a murine CB6F1/J hybrid model of respiratory syncytial virus (RSV) infection, we previously demonstrated that the CD8⁺ T cell immunodominance hierarchy to two RSV-derived epitopes, K^dM2_82-90 and D^bM_187-195, was determined by the age at infection. To determine whether age-dependent RSV-specific CD8⁺ T cell responses could be modified through enhanced innate signaling, we used TLR4 or TLR9 agonist treatment at the time of infection, which remarkably changed the neonatal codominant response to an adult-like K^dM2_82-90 CD8⁺ T cell immunodominant response. This shift was associated with an increase in the number of conventional dendritic cells, CD11b⁺ and CD103⁺ dendritic cells, in the lung-draining lymph node, as well as increased expression of the costimulatory molecule CD86. The magnitude of the K^dM2_82-90 CD8⁺ T cell response in TLR agonist-treated neonates could be blocked with Abs against CD80 and CD86. These studies demonstrate the age-dependent function of conventional dendritic cells, their role in determining immunodominance hierarchy, and epitope-specific CD8⁺ T cell requirements for costimulation, all of which influence the immune response magnitude. The unique impact of TLR agonists on neonatal T cell responses is important to consider for RSV vaccines designed for young infants.

Reduced interferon-α production by dendritic cells in type 1 diabetes does not impair immunity to influenza virus

The increased risk and persistence of infections in diabetic condition is probably associated with defects in the cellular immune responses. We have previously shown a decrease in the production of interferon (IFN)-α by dendritic cells (DCs) in diabetic subjects. The basal level of IFN-α in splenic plasmacytoid DCs (pDCs) is also lower in non-obese diabetic (NOD) mice compared to prediabetic mice. The objective of this study was to analyse the ability of diabetic mice to mobilize innate and CD8(+) T cell-mediated immune response to influenza A virus (IAV) with the live influenza A/Puerto Rico/8/1934 H1N1 (PR8) strain or with its immunodominant CD8(+) T cell epitopes. We found that following immunization with IAV, the level of IFN-α in diabetic mice was increased to the level in prediabetic mice. Immunization of NOD mice with the immunodominant IAV PR8 peptide induced clonal expansion of IFN-γ-producing CD8(+) T cells similar to the response observed in prediabetic mice. Thus, diabetic and prediabetic NOD mice have a similar capacity for IFN-α and IFN-γ production by pDCs and CD8(+) T cells, respectively. Therefore, the DC-related immune defect in diabetic NOD mice does not impair their capacity to develop an effective immune response to IAV. Our results suggest that reduced IFN-α production by diabetic human and mouse DCs is not an impediment to an effective immunity to IAV in type 1 diabetic subjects vaccinated with live attenuated influenza vaccine.

Quantitative and qualitative deficits in neonatal lung-migratory dendritic cells impact the generation of the CD8+ T cell response

CD103+ and CD11b+ populations of CD11c+MHCIIhi murine dendritic cells (DCs) have been shown to carry antigens from the lung through the afferent lymphatics to mediastinal lymph nodes (MLN). We compared the responses of these two DC populations in neonatal and adult mice following intranasal infection with respiratory syncytial virus. The response in neonates was dominated by functionally-limited CD103+ DCs, while CD11b+ DCs were diminished in both number and function compared to adults. Infecting mice at intervals through the first three weeks of life revealed an evolution in DC phenotype and function during early life. Using TCR transgenic T cells with two different specificities to measure the ability of CD103+ DC to induce epitope-specific CD8+ T cell responses, we found that neonatal CD103+ DCs stimulate proliferation in a pattern distinct from adult CD103+ DCs. Blocking CD28-mediated costimulatory signals during adult infection demonstrated that signals from this costimulatory pathway influence the hierarchy of the CD8+ T cell response to RSV, suggesting that limited costimulation provided by neonatal CD103+ DCs is one mechanism whereby neonates generate a distinct CD8+ T cell response from that of adults.

Immunodomination during peripheral vaccinia virus infection

Immunodominance is a fundamental property of CD8(+) T cell responses to viruses and vaccines. It had been observed that route of administration alters immunodominance after vaccinia virus (VACV) infection, but only a few epitopes were examined and no mechanism was provided. We re-visited this issue, examining a panel of 15 VACV epitopes and four routes, namely intradermal (i.d.), subcutaneous (s.c.), intraperitoneal (i.p.) and intravenous (i.v.) injection. We found that immunodominance is sharpened following peripheral routes of infection (i.d. and s.c.) compared with those that allow systemic virus dissemination (i.p. and i.v.). This increased immunodominance was demonstrated with native epitopes of VACV and with herpes simplex virus glycoprotein B when expressed from VACV. Responses to some subdominant epitopes were altered by as much as fourfold. Tracking of virus, examination of priming sites, and experiments restricting virus spread showed that priming of CD8(+) T cells in the spleen was necessary, but not sufficient to broaden responses. Further, we directly demonstrated that immunodomination occurs more readily when priming is mainly in lymph nodes. Finally, we were able to reduce immunodominance after i.d., but not i.p. infection, using a VACV expressing the costimulators CD80 (B7-1) and CD86 (B7-2), which is notable because VACV-based vaccines incorporating these molecules are in clinical trials. Taken together, our data indicate that resources for CD8(+) T cell priming are limiting in local draining lymph nodes, leading to greater immunodomination. Further, we provide evidence that costimulation can be a limiting factor that contributes to immunodomination. These results shed light on a possible mechanism of immunodomination and highlight the need to consider multiple epitopes across the spectrum of immunogenicities in studies aimed at understanding CD8(+) T cell immunity to viruses.

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.

From crucial to negligible: functional CD8⁺ T-cell responses and their dependence on CD4⁺ T-cell help

CD8(+) T cells play an important role in controlling pathogenic infections and are therefore key players in the immune response. It has been shown that among other factors CD4(+) T cells can shape the magnitude as well as the quality of primary and/or secondary CD8(+) T-cell responses. However, due to the complexity and the differences among diverse immunization or infection models, the overall requirement, the time points, as well as the specific mechanism(s) of CD4(+) T-cell help may differ substantially. Here, we summarize current knowledge about the differential requirement of CD4(+) T-cell help in promoting primary CD8(+) T-cell responses as well as establishing functional memory CD8(+) T cells in various experimental settings.

Antibody breadth and protective efficacy are increased by vaccination with computationally optimized hemagglutinin but not with polyvalent hemagglutinin-based H5N1 virus-like particle vaccines

One of the challenges for developing an H5N1 influenza vaccine is the diversity of antigenically distinct isolates within this subtype. Previously, our group described a novel hemagglutinin (HA) derived from a methodology termed computationally optimized broadly reactive antigen (COBRA). This COBRA HA, when used as an immunogen, elicits a broad antibody response against H5N1 isolates from different clades. In this report, the immune responses elicited by the COBRA HA virus-like particle (VLP) vaccine were compared to responses elicited by a mixture of VLPs expressing representative HA molecules from clade 2.1, 2.2, and 2.3 primary H5N1 isolates (polyvalent). The COBRA HA VLP vaccine elicited higher-titer antibodies to a panel of H5N1 HA proteins than did the other VLPs. Both COBRA and polyvalent vaccines protected vaccinated mice and ferrets from experimental infection with highly lethal H5N1 influenza viruses, but COBRA-vaccinated animals had decreased viral replication, less inflammation in the lungs of mice, and reduced virus recovery in ferret nasal washes. Both vaccines had similar cellular responses postchallenge, indicating that higher-titer serum antibodies likely restrict the duration of viral replication. Furthermore, passively transferred immune serum from the COBRA HA VLP-vaccinated mice protected recipient animals more efficiently than immune serum from polyvalent-vaccinated mice. This is the first report comparing these two vaccine strategies. The single COBRA HA antigen elicited a broader antibody response and reduced morbidity and viral titers more effectively than a polyvalent mixture of primary H5N1 HA antigens.

Multiple layers of CD80/86-dependent costimulatory activity regulate primary, memory, and secondary lymphocytic choriomeningitis virus-specific T cell immunity

The lymphocytic choriomeningitis virus (LCMV) system constitutes one of the most widely used models for the study of infectious disease and the regulation of virus-specific T cell immunity. However, with respect to the activity of costimulatory and associated regulatory pathways, LCMV-specific T cell responses have long been regarded as relatively independent and thus distinct from the regulation of T cell immunity directed against many other viral pathogens. Here, we have reevaluated the contribution of CD28-CD80/86 costimulation in the LCMV system by use of CD80/86-deficient mice, and our results demonstrate that a disruption of CD28-CD80/86 signaling compromises the magnitude, phenotype, and/or functionality of LCMV-specific CD8(+) and/or CD4(+) T cell populations in all stages of the T cell response. Notably, a profound inhibition of secondary T cell immunity in LCMV-immune CD80/86-deficient mice emerged as a composite of both defective memory T cell development and a specific requirement for CD80 but not CD86 in the recall response, while a related experimental scenario of CD28-dependent yet CD80/86-independent secondary CD8(+) T cell immunity suggests the existence of a CD28 ligand other than CD80/86. Furthermore, we provide evidence that regulatory T cells (T(REG)s), the homeostasis of which is altered in CD80/86(-/-) mice, contribute to restrained LCMV-specific CD8(+) T cell responses in the presence of CD80/86. Our observations can therefore provide a more coherent perspective on CD28-CD80/86 costimulation in antiviral T cell immunity that positions the LCMV system within a shared context of multiple defects that virus-specific T cells acquire in the absence of CD28-CD80/86 costimulation.

Adjuvants that stimulate TLR3 or NLPR3 pathways enhance the efficiency of influenza virus-like particle vaccines in aged mice

There is intense interest in the design and use of vaccine strategies against influenza to enhance protective immune responses in the elderly. To address the need for improved influenza vaccines for the aged, two inflammatory adjuvants, Imject(®) alum (a stimulator of the Nod-like receptor, Nalp3) and poly I:C (a toll-like receptor type 3 ligand), were used during vaccination with novel influenza virus-like particles (VLP). Adult (4 month old) or aged (24 month old) mice were vaccinated with VLPs alone or in combination with adjuvant. VLP-vaccinated adult mice were protected from a lethal influenza virus challenge without the use of either adjuvant. In contrast, only aged mice that were vaccinated with VLPs plus adjuvant survived challenge, whereas ∼33% of the mice vaccinated with VLP only survived challenge. Mice vaccinated with adjuvant only did not survive challenge despite similar levels of activation of CD11b(+)/CD11c(+) dendritic cells in the lungs. The protection was not associated with HAI titers or HA specific CD8(+) T cells, since both adjuvants boosted the VLP-induced serum HAI titers and CD8(+) responses in adult mice, but not aged mice. Influenza VLPs used in combination with two different inflammatory adjuvants during vaccination allow for the immune system to overcome the deficiency in the aged immune system to influenza virus infection.

Gamma interferon and perforin control the strength, but not the hierarchy, of immunodominance of an antiviral CD8+ T cell response

The two major antiviral effector mechanisms of CD8(+) T cells are thought to be perforin (Prf)-mediated cell lysis and gamma interferon (IFN-γ)-mediated induction of an antiviral state. By affecting the expression of proteins involved in antigen presentation, IFN-γ is also thought to shape the magnitude and specificity of the CD8(+) T cell response. Here we studied the roles of Prf and IFN-γ in shaping the effector and memory CD8(+) T cell responses to vaccinia virus (VACV). IFN-γ deficiency resulted in increased numbers of anti-VACV effector and memory CD8(+) T cells, which were partly dependent on increased virus loads. On the other hand, Prf-deficient mice showed an increase in the number of VACV-specific CD8(+) T cells only in the memory phase. Treatment of the mice with the antiviral drug cidofovir reduced the numbers of effector and memory cells closer to wild-type levels in IFN-γ-deficient mice and reduced the numbers of memory CD8(+) T cells to wild-type levels in Prf-deficient mice. These data suggest that virus loads are the main reason for the increased strength of the CD8 response in IFN-γ- and Prf-deficient mice. Neither Prf deficiency nor IFN-γ deficiency had an effect on the immunodominance hierarchy of five K(b)-restricted CD8(+) T cell determinants either during acute infection or after recovery. Thus, our work shows that CD8(+) T cell immunodominance during VACV infection is not affected by the effects of IFN-γ on the antigen presentation machinery.

From optical bench to cageside: intravital microscopy on the long road to rational vaccine design

No antiviral vaccine is perfect. For some important pathogens, there are no effective vaccines. Many current vaccines are based on the working principles of Jenner and Pasteur, that is, empiric administration of attenuated or inactivated forms of the pathogen. Tapping the full potential of vaccination requires a thorough understanding of the mechanism of immune activation by pathogens and their individual components. Though the rate of discovery continues to accelerate, the complexity of the immune system is daunting, particularly when integrated into the overall physiology of the host. Here, we review the application of multiphoton microscopy to examine host-pathogen interactions, focusing on our recent efforts to understand mouse CD8(+) T-cell responses to viruses at the level of cellular interactions in lymph nodes draining the infection site. We also discuss our recent efforts to understand the influence of the sympathetic nervous system on antiviral immunity, with the ultimate goal of appreciating the traditional elements of immunity as just one facet of the total organismal response to infection and immunization.

T Cell receptor clonotype influences epitope hierarchy in the CD8+ T cell response to respiratory syncytial virus infection

CD8+ T cell responses are important for recognizing and resolving viral infections. To better understand the selection and hierarchy of virus-specific T cell responses, we compared the T cell receptor (TCR) clonotype in parent and hybrid strains of respiratory syncytial virus-infected mice. K(d)M2(82-90) (SYIGSINNI) in BALB/c and D(b)M(187-195) (NAITNAKII) in C57Bl/6 are both dominant epitopes in parent strains but assume a distinct hierarchy, with K(d)M2(82-90) dominant to D(b)M(187-195) in hybrid CB6F1/J mice. The dominant K(d)M2(82-90) response is relatively public and is restricted primarily to the highly prevalent Vβ13.2 in BALB/c and hybrid mice, whereas D(b)M(187-195) responses in C57BL/6 mice are relatively private and involve multiple Vβ subtypes, some of which are lost in hybrids. A significant frequency of TCR CDR3 sequences in the D(b)M(187-195) response have a distinct "(D/E)WG" motif formed by a limited number of recombination strategies. Modeling of the dominant epitope suggested a flat, featureless structure, but D(b)M(187-195) showed a distinctive structure formed by Lys(7). The data suggest that common recombination events in prevalent Vβ genes may provide a numerical advantage in the T cell response and that distinct epitope structures may impose more limited options for successful TCR selection. Defining how epitope structure is interpreted to inform T cell function will improve the design of future gene-based vaccines.

Factors influencing immunodominance hierarchies in TCD8+ -mediated antiviral responses

CD8(+) T-lymphocytes (T(CD8+)) perform a critical role in immunity against tumors and virus infections. A central feature of T(CD8+) immune responses is immunodominance: the observation that T(CD8+) responses consist of a limited collection of specificities with a structured hierarchy. These immunodominance hierarchies result from a complex combination of factors. Major roles are played by peptide binding affinity, T-cell repertoire, and antigen processing and presentation. While the bulk of our information comes from mouse model systems, an increasing number of human studies suggest that immunodominance will be even more complicated. This review outlines current knowledge of T(CD8+ )immunodominance to viral antigens and discusses the relevance and importance of a thorough understanding for the rational design of vaccines that elicit effective T(CD8+) responses.

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.

Altered CD8(+) T cell immunodominance after vaccinia virus infection and the naive repertoire in inbred and F(1) mice

Previous studies of CD8(+) T cell immunodominance after primary virus infection of F(1) mice compared with their inbred parents have generally concluded that no dramatic changes occur. In this study, we revisit this issue using vaccinia virus (VACV), which has a large genome, a recently defined immunodominance hierarchy in mice, and is a candidate vector for vaccines. We found that immunogenicity of VACV peptides defined using inbred mice was highly variable in F(1) progeny: some peptides were equally immunogenic in F(1) and inbred, whereas others elicited responses that were reduced by >90% in F(1) mice. Furthermore, the dominance of a peptide in the relevant inbred parent did not predict whether it would be poorly immunogenic in F(1) mice. This result held using F(1) hybrids of MHC-congenic mice, suggesting that MHC differences alone were responsible. It was also extended to foreign epitopes expressed by an rVACV vaccine. F(1) mice were less able to mount responses to the poorly immunogenic peptides when used as a sole immunogen, ruling out immunodomination. In addition, conserved TCR Vbeta usage between inbred and F(1) mice did not always correlate with strong responses in F(1) mice. However, direct estimation of naive precursor numbers showed that these were reduced in F(1) compared with inbred mice for specificities that were poorly immunogenic in the hybrids. These data have implications for our understanding of the extent to which MHC diversity alters the range of epitopes that are immunogenic in outbred populations.

Impaired immune responses in the lungs of aged mice following influenza infection

Background

Each year, influenza virus infection causes severe morbidity and mortality, particularly in the most susceptible groups including children, the elderly (>65 years-old) and people with chronic respiratory diseases. Among the several factors that contribute to the increased susceptibility in elderly populations are the higher prevalence of chronic diseases (e.g. diabetes) and the senescence of the immune system.

Methods

In this study, aged and adult mice were infected with sublethal doses of influenza virus (A/Puerto Rico/8/1934). Differences in weight loss, morbidity, virus titer and the kinetics of lung infiltration with cells of the innate and adaptive immune responses were analyzed. Additionally, the main cytokines and chemokines produced by these cells were also assayed.

Results

Compared to adult mice, aged mice had higher morbidity, lost weight more rapidly, and recovered more slowly from infection. There was a delay in the accumulation of granulocytic cells and conventional dendritic cells (cDCs), but not macrophages in the lungs of aged mice compared to adult animals. The delayed infiltration kinetics of APCs in aged animals correlated with alteration in their activation (CD40 expression), which also correlated with a delayed detection of cytokines and chemokines in lung homogenates. This was associated with retarded lung infiltration by natural killer (NK), CD4⁺ and CD8⁺ T-cells. Furthermore, the percentage of activated (CD69+) influenza-specific and IL-2 producer CD8+ T-cells was higher in adult mice compared to aged ones. Additionally, activation (CD69+) of adult B-cells was earlier and correlated with a quicker development of neutralizing antibodies in adult animals.

Conclusion

Overall, alterations in APC priming and activation lead to delayed production of cytokines and chemokines in the lungs that ultimately affected the infiltration of immune cells following influenza infection. This resulted in delayed activation of the adaptive immune response and subsequent delay in clearance of virus and prolonged illness in aged animals. Since the elderly are the fastest growing segment of the population in developed countries, a better understanding of the changes that occur in the immune system during the aging process is a priority for the development of new vaccines and adjuvants to improve the immune responses in this population.

A trivalent virus-like particle vaccine elicits protective immune responses against seasonal influenza strains in mice and ferrets

There is need for improved human influenza vaccines, particularly for older adults who are at greatest risk for severe disease, as well as to address the continuous antigenic drift within circulating human subtypes of influenza virus. We have engineered an influenza virus-like particle (VLP) as a new generation vaccine candidate purified from the supernatants of Sf9 insect cells following infection by recombinant baculoviruses to express three influenza virus proteins, hemagglutinin (HA), neuraminidase (NA), and matrix 1 (M1). In this study, a seasonal trivalent VLP vaccine (TVV) formulation, composed of influenza A H1N1 and H3N2 and influenza B VLPs, was evaluated in mice and ferrets for the ability to elicit antigen-specific immune responses. Animals vaccinated with the TVV formulation had hemagglutination-inhibition (HAI) antibody titers against all three homologous influenza virus strains, as well as HAI antibodies against a panel of heterologous influenza viruses. HAI titers elicited by the TVV were statistically similar to HAI titers elicited in animals vaccinated with the corresponding monovalent VLP. Mice vaccinated with the TVV had higher level of influenza specific CD8+ T cell responses than a commercial trivalent inactivated vaccine (TIV). Ferrets vaccinated with the highest dose of the VLP vaccine and then challenged with the homologous H3N2 virus had the lowest titers of replicating virus in nasal washes and showed no signs of disease. Overall, a trivalent VLP vaccine elicits a broad array of immunity and can protect against influenza virus challenge.

Murine norovirus infection has no significant effect on adaptive immunity to vaccinia virus or influenza A virus

Murine norovirus (MNV) is endemic in many research mouse colonies. Although MNV infections are typically asymptomatic in immunocompetent mice, the effects of MNV infection on subsequent experimental viral infections are poorly documented. Here, we infected C57BL/6 mice with MNV and then with either vaccinia virus or influenza A virus. MNV infection had no effect on CD8(+) T-cell or antibody responses to secondary viruses or to secondary virus-induced morbidity or mortality. While our findings suggest that MNV has little influence on host immunity in immunocompetent mice, we would urge caution regarding the potential effects of MNV on immune responses to viruses and other pathogens, which must be determined on a system-by-system basis.

The aryl hydrocarbon receptor is a modulator of anti-viral immunity

Although immune modulation by AhR ligands has been studied for many years, the impact of AhR activation on host defenses against viral infection has not, until recently, garnered much attention. The development of novel reagents and model systems, new information regarding anti-viral immunity, and a growing appreciation for the global health threat posed by viruses have invigorated interest in understanding how environmental signals affect susceptibility to and pathological consequences of viral infection. Using influenza A virus as a model of respiratory viral infection, recent studies show that AhR activation cues signaling events in both leukocytes and non-immune cells. Functional alterations include suppressed lymphocyte responses and increased inflammation in the infected lung. AhR-mediated events within and extrinsic to hematopoietic cells has been investigated using bone marrow chimeras, which show that AhR alters different elements of the immune response by affecting different tissue targets. In particular, suppressed CD8(+) T cell responses are due to deregulated events within leukocytes themselves, whereas increased neutrophil recruitment to and IFN-gamma levels in the lung result from AhR-regulated events extrinsic to bone marrow-derived cells. This latter discovery suggests that epithelial and endothelial cells are overlooked targets of AhR-mediated changes in immune function. Further support that AhR influences host cell responses to viral infection are provided by several studies demonstrating that AhR interacts directly with viral proteins and affects viral latency. While AhR clearly modulates host responses to viral infection, we still have much to understand about the complex interactions between immune cells, viruses, and the host environment.

Terminal deoxynucleotidyl transferase establishes and broadens antiviral CD8+ T cell immunodominance hierarchies

The action of TdT on mouse TCR genes accounts for approximately 90% of T cell repertoire diversity. We report that in TdT-/- mice, total T(CD8+) responses to influenza and vaccinia viruses are reduced by approximately 30% relative to wild-type mice. We find that T(CD8+) responses to three subdominant influenza virus determinants are reduced to background values in TdT-/- mice while responses to three immunodominant determinants undergo a major reshuffling. A similar reshuffling occurs in T(CD8+) responses to immunodominant vaccinia virus determinants, and is clearly based on broad differences in TCR family usage and CDR3 length between wild-type and TdT-/- mice. These findings demonstrate that TdT plays a critical role in the magnitude and breadth of anti-viral T(CD8+) responses toward individual determinants and suggests that germline TCR repertoire bias toward the most dominant determinants is a major factor in establishing immunodominance hierarchies.

Epitope-specific TCRbeta repertoire diversity imparts no functional advantage on the CD8+ T cell response to cognate viral peptides

TCR repertoire diversity has been convincingly shown to facilitate responsiveness of CD8+ T cell populations to mutant virus peptides, thereby safeguarding against viral escape. However, the impact of repertoire diversity on the functionality of the CD8+ T cell response to cognate peptide-MHC class I complex (pMHC) recognition remains unclear. Here, we have compared TCRbeta chain repertoires of three influenza A epitope-specific CD8+ T cell responses in C57BL/6 (B6) mice: D(b)NP(366-374), D(b)PA(224-233), and a recently described epitope derived from the +1 reading frame of the influenza viral polymerase B subunit (residues 62-70) (D(b)PB1-F2(62)). Corresponding to the relative antigenicity of the respective pMHCs, and irrespective of the location of prominent residues, the D(b)PA(224)- and D(b)PB1-F2(62)-specific repertoires were similarly diverse, whereas the D(b)NP(366) population was substantially narrower. Importantly, parallel analysis of response magnitude, cytotoxicity, TCR avidity, and cytokine production for the three epitope-specific responses revealed no obvious functional advantage conferred by increased T cell repertoire diversity. Thus, whereas a diverse repertoire may be important for recognition of epitope variants, its effect on the response to cognate pMHC recognition appears minimal.

Pathogenic epitopes, heterologous immunity and vaccine design

Vaccine strategies are focused on developing protective responses to immunogenic peptide epitopes of pathogens that are normally recognized by T and B cells. However, some epitopes stimulate crossreactive T-cell responses between pathogens and can prime a host to damaging pathology on infection with the crossreactive pathogen. The removal of potentially pathogenic epitopes from vaccines might enhance prophylaxis and reduce the risk of side effects of vaccine-associated disease.

Substantial research has been directed towards the development of a new generation of vaccines that are based on the inclusion of immunogenic epitopes in recombinant vectors. Here we examine the evidence that under certain conditions immunogenic epitopes can do more harm than good and might therefore be considered pathogenic. We suggest that the specific removal of such pathogenic epitopes from vaccines might increase their prophylactic potential, while minimizing the risk of side-effects from vaccine use.

Cross-competition of CD8+ T cells shapes the immunodominance hierarchy during boost vaccination

CD8⁺ T cell responses directed against multiple pathogen-derived epitopes are characterized by defined immunodominance hierarchy patterns. A possible explanation for this phenomenon is that CD8⁺ T cells of different specificities compete for access to epitopes on antigen-presenting cells, and that the outcome of this so-called cross-competition reflects the number of induced T cells. In our study using a vaccinia virus infection model, we found that T cell cross-competition is highly relevant during boost vaccination, thereby shaping the immunodominance hierarchy in the recall. We demonstrate that competition was of no importance during priming and was unaffected by the applied route of immunization. It strongly depended on the timing of viral antigen expression in infected APCs, and it was characterized by poor proliferation of T cells recognizing epitopes derived from late viral proteins. To our knowledge, this is the first demonstration of the functional importance of T cell cross-competition during a viral infection. Our findings provide a basis for novel strategies for how boost vaccination to defined antigens can be selectively improved. They give important new insights into the design of more efficient poxviral vectors for immunotherapy.

The Context of Epitope Presentation Can Influence Functional Quality of Recalled Influenza A Virus-Specific Memory CD8+ T Cells

Lipopeptide constructs offer a novel strategy for eliciting effective cellular and humoral immunity by directly targeting the vaccine Ag to dendritic cells. Importantly, it is not known how closely immunity generated after lipopeptide vaccination mimics that generated after natural infection. We have used a novel lipopeptide vaccine strategy to analyze both the quantity and quality of CD8(+) T cell immunity to an influenza A virus epitope derived from the acidic polymerase protein (PA(224)) in B6 mice. Vaccination with the PA(224) lipopeptide resulted in accelerated viral clearance after subsequent influenza virus infection. The lipopeptide was also effective at recalling secondary D(b)PA(224) responses in the lung. Lipopeptide recalled D(b)PA(224)-specific CTL produced lower levels of IFN-gamma and TNF-alpha, but produced similar levels of IL-2 when compared with D(b)PA(224)-specific CTL recalled after virus infection. Furthermore, lipopeptide- and virus-recalled CTL demonstrated similar TCR avidity. Interestingly, lipopeptide administration resulted in expansion of D(b)PA(224)-specific CTL using a normally subdominant TCRBV gene segment. Overall, these results demonstrate that protective CTL responses elicited by lipopeptide vaccines can be correlated with TCR avidity, IL-2 production, and broad TCR repertoire diversity. Furthermore, factors that impact the quality of immunity are discussed. These factors are important considerations when evaluating the efficacy of novel vaccine strategies that target dendritic cells for eliciting cellular immunity.

Disregulated influenza A virus-specific CD8+ T cell homeostasis in the absence of IFN-gamma signaling

Recent studies indicate that IFN-gamma may influence both the expansion and the trafficking of virus-specific CD8+ CTL, though the effects are not necessarily consistent for different models of viral and bacterial disease. Influenza A virus infection of mice deficient for IFN-gamma (IFN-gamma(-/-)) or deficient for the IFN-gamma receptor 1 (IFNGR1(-/-)) was, when compared with the wild-type (WT) B6 controls, associated with increased Ag-specific CD8+ T cell counts in the spleen and mediastinal lymph nodes. At the same time, fewer of these CTL effectors were found in the bronchoalveolar lavage population recovered from the IFN-gamma(-/-) mice. Comparable effects were observed for WT mice treated with a neutralizing IFN-gamma-specific mAb. Transfer of WT memory Thy1.1(+) CD8+ populations into Thy1.2+ B6 IFN-gamma(-/-) or IFNGR1(-/-) mice followed by intranasal virus challenge demonstrated both that IFN-gamma produced by the host was important for the regulation of Ag-specific CTL numbers and that IFN-gamma was likely to act directly on the T cells themselves. In addition, the prevalence of CTLs undergoing apoptosis in spleen was lower when measured directly ex vivo for IFN-gamma(-/-) vs WT B6 mice. The present analysis is the first comprehensive demonstration that IFN-gamma signaling can differentially regulate both Ag-specific CTL homeostasis in secondary lymphoid organs and trafficking to a site of virus-induced pathology.

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.

Direct CD28 costimulation is required for CD8+ T cell-mediated resistance to an acute viral disease in a natural host

Previous studies have suggested that, differing from model Ags, viruses that replicate extensively in the host still induce normal CD8+ T cell responses in the absence of CD28 costimulation. Because these studies were performed with viruses that do not normally cause acute disease, an important remaining question is whether CD28 costimulation is required for CD8+ T cell-mediated resistance to widely replicating but pathogenic viruses. To address this question, we studied the role of CD28 costimulation in CD8+ T cell-mediated resistance to mousepox, a disease of the mouse caused by the natural mouse pathogen, the ectromelia virus (ECTV). C57BL/6 (B6) mice are naturally resistant to mousepox, partly due to a fast and strong CD8+ T cell response. We found that B6 mice deficient in CD28 (CD28 knockout (KO)) are highly susceptible to lethal mousepox during the early stages of ECTV infection but can be protected by immunization with the antigenically related vaccinia virus (VACV) or by adoptive transfer of CD28 KO anti-VACV memory CD8+ cells. Of interest, a thorough comparison of the CD8+ T cell responses to ECTV and VACV suggests that the main reason for the susceptibility of CD28 KO mice to mousepox is a reduced response at the early stages of infection. Thus, while in the absence of CD28 costimulation the end point strength of the T cell responses to nonpathogenic viruses may appear normal, CD28 costimulation increases the speed of the T cell response and is essential for resistance to a life-threatening acute viral disease.

Confronting complexity: real-world immunodominance in antiviral CD8+ T cell responses

Antiviral CD8(+) T cells respond to only a minute fraction of the potential peptide determinants encoded by viral genomes. Immunogenic determinants can be ordered into highly reproducible hierarchies based on the magnitude of cognate CD8(+) T cell responses. Until recently, this phenomenon, termed immunodominance, was largely defined and characterized in model systems utilizing a few strains of inbred mice infected with a handful of viruses with limited coding capacity. Here, I review work that has extended immunodominance studies to viruses of greater complexity and to the real world of human antiviral immunity.

Perforin and Fas cytolytic pathways coordinately shape the selection and diversity of CD8+-T-cell escape variants of influenza virus

Antigenic variation is a viral strategy exploited to promote survival in the face of the host immune response and represents a major challenge for efficient vaccine development. Influenza viruses are pathogens with high transmissibility and mutation rates, enabling viral escape from immunity induced by prior infection or vaccination. Intense selection from neutralizing antibody drives antigenic changes in the surface glycoproteins, resulting in emergence of new strains able to reinfect hosts immune to previously circulating viruses. CD8+ cytotoxic T cells (CTLs) also provide protective immunity from influenza virus infection and may contribute to the antigenic evolution of influenza viruses. Utilizing mice transgenic for an influenza virus NP366-374 peptide-specific T-cell receptor, we demonstrated that the respiratory tract is a suitable site for generation of escape variants of influenza virus selected by CTL in vivo. In this report the contributions of the perforin and Fas pathways utilized by influenza virus-specific CTLs in viral clearance and selection of CTL escape variants have been evaluated. While transgenic CTLs deficient in either perforin- or Fas-mediated pathways are efficient in initial pulmonary viral control, variant virus emergence was observed in all the mice studied, although the spectrum of viral CTL escape variants selected varied profoundly. Thus, a less-restricted repertoire of escape variants was observed in mice with an intact perforin cytotoxic pathway compared with a limited variant diversity in perforin pathway-deficient mice, although maximal variant diversity was observed in mice having both Fas and perforin pathways intact. We conclude that selection of viral CTL escape variants reflects coordinate action between the tightly controlled perforin/granzyme pathway and the more promiscuous Fas/FasL pathway.

Regulatory T cells suppress CD8+ T cell responses induced by direct priming and cross-priming and moderate immunodominance disparities

Little is known regarding the participation of CD4+ CD25+ regulatory T cells (Treg) in TCD8+ responses. In this study, we show that Treg depletion via treatment with anti-CD25 mAb (PC61) significantly enhances TCD8+ responses to influenza A virus, vaccinia virus, and SV40-transformed cells induced by either direct priming or cross-priming. PC61 did not enhance TCD8+ responses in CD4-deficient mice, providing the initial demonstration that PC61 acts on a subset of TCD4+, and not on other cells that express either CD25 or a fortuitously cross-reactive Ag. We further show that Treg selectively suppress responses to the most immunodominant TCD8+ determinants in the three systems examined. Therefore, Treg influence TCD8 immunodominance hierarchies by moderating disparities in responses to different determinants.

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

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

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

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

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

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

Vaccination with an Acidic Polymerase Epitope of Influenza Virus Elicits a Potent Antiviral T Cell Response but Delayed Clearance of an Influenza Virus Challenge

The mechanisms underlying epitope selection and the potential impact of immunodominance hierarchies on peptide-based vaccines are not well understood. Recently, we have shown that two immunodominant MHC class I-restricted epitopes, NP(366-374)/D(b) (nucleoprotein (NP)) and PA(224-233)/D(b) (acidic polymerase (PA)), which drive the CD8(+) T cell response to influenza virus infection in C57BL/6 mice, are differentially expressed on infected cells. Whereas NP appears to be strongly expressed on all infected cells, PA appears to be strongly expressed on dendritic cells but only weakly expressed on nondendritic cells. Thus, the immune response to influenza virus may involve T cells specific for epitopes, such as PA, that are poorly expressed at the site of infection. To examine the consequences of differential Ag presentation on peptide vaccination, we compared the kinetics of the T cell response and influenza virus clearance in mice vaccinated with the NP or PA peptide. Vaccination with either the NP or PA peptide resulted in accelerated and enhanced Ag-specific T cell responses at the site of infection following influenza virus challenge. These T cells were fully functional in terms of their ability to produce IFN-gamma and TNF-alpha and to mediate cytolytic activity. Despite this enhancement of the Ag-specific T cell response, PA vaccination had a detrimental effect on the clearance of influenza virus compared with unvaccinated or NP-vaccinated mice. These data suggest that differential Ag presentation impacts the efficacy of T cell responses to specific epitopes and that this needs to be considered for the development of peptide-based vaccination strategies.

Role of T cell costimulation in anti-viral immunity

Members of both the CD28 and TNFR families can have costimulatory roles in T cell activation. Gene targeted mice as well as in vivo blocking experiments have established distinct roles for CD28/B7; ICOS/ICOSL; CD27/CD70; 4-1BB/4-1BBL and OX40/OX40L during viral infection. Many issues remain to be addressed, including the timing and location of the interactions, the possibility of partial redundancy between related family members and the molecular basis for the specific phenotypes observed in the different gene targeted mice.

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

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

Reversal in the Immunodominance Hierarchy in Secondary CD8+ T Cell Responses to Influenza A Virus: Roles for Cross-Presentation and Lysis-Independent Immunodomination

Immunodominance is a central feature of CD8+ T cell (TCD8+) responses to pathogens, transplants, and tumors. Determinants occupy a stable position in an immunodominance hierarchy (alpha-, beta-, etc.) defined by the frequencies of responding TCD8+. In this paper, we study the mechanistic basis for place-swapping between alpha- (acid polymerase (PA)(224-233)) and beta-determinants (nuclear protein 366-374) in primary vs secondary anti-influenza A virus (IAV) responses in mice. This phenomena was recently correlated with the inability of IAV-infected nondendritic cells (DCs) to generate PA(224-233), and it was proposed that secondary TCD8+ are principally activated by IAV-infected epithelial cells, while primary TCD8+ are activated by IAV-infected DCs. In this study, we show that the inability of non-DCs to generate PA(224-232) is relative rather than absolute, and that the preferential use of cross-priming in secondary anti-IAV responses can also account for the revised hierarchy. We further show that immunodomination of PA(224-233)-specific TCD8+ by nucleoprotein 366-374-specific TCD8+ plays a critical role in the phenomena, and that this is unlikely to be mediated by TCD8+ lysis of APCs or other cells.

Deletion of naive CD8 T cells requires persistent antigen and is not programmed by an initial signal from the tolerogenic APC

Activation of naive CD8 T cells in vivo requires the recognition of cognate peptide-MHC complexes on APCs. Depending upon the activation status of the APC, such recognition will promote either a vigorous immune response or T cell tolerance and deletion. Recent studies suggest that the initial signals provided by APCs are sufficient to program the proliferation of naive CD8 T cells and their differentiation into effector cells. In this study, we sought to determine whether an initial encounter with tolerogenic APCs was sufficient to program deletion of naive CD8 T cells. Surprisingly, we find that regardless of whether naive CD8 T cells were stimulated by activated or quiescent APCs, transfer of the activated T cells into an Ag-free host was sufficient to ensure survival. Thus, although the extent of clonal expansion and development of effector function is determined by the activation status of the stimulatory APC, peripheral clonal deletion requires persistent Ag and is not determined by the initial stimulatory event.

Deficient anti-listerial immunity in the absence of perforin can be restored by increasing memory CD8+ T cell numbers

Compared with wild-type (WT) mice, Listeria monocytogenes (LM)-vaccinated perforin-deficient (PKO) mice have elevated levels of CD8(+) T cell memory, but exhibit reduced levels of protection against virulent LM. In this study, Ag-specific CD8(+) T cells from LM-vaccinated WT and PKO mice were used in adoptive transfer assays to determine the contribution of perforin-dependent cytolysis in protective immunity to LM. Perforin deficiency resulted in an approximately 5-fold reduction in the per-cell protective capacity of Ag-specific memory CD8(+) T cells that was not caused by differences in memory cell quality as measured by CD62L/CD27 expression, TCR repertoire use, functional avidity, differences in expansion of Ag-specific cells upon infection, or maintenance of memory levels over time. However, perforin-deficient CD8(+) T cells exhibited reduced in vivo cytotoxic function compared to WT CD8(+) T cells. Consistent with the existence of perforin-independent effector pathways, double-vaccinated PKO mice were as resistant to challenge with LM as single-vaccinated WT mice. Thus, increasing the number of memory CD8(+) T cells can overcome diminished per-cell protective immunity in the absence of perforin.

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

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

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

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

Epitope-based vaccines: an update on epitope identification, vaccine design and delivery

The basic premise of the epitope-based approach to vaccine development is that, in certain cases, the responses induced by the natural immunogen are not optimal, and can be improved upon by isolation or optimization of specific components of the response. For example, immunodominance is a key factor limiting the type and breadth of adaptive immunity. Recent advances in understanding the mechanisms of immunodominance thus represent an opportunity to further develop the epitope-based approach.