High frequencies of virus-specific CD8+ T-cell precursors

Lipid order and charge protect killer T cells from accidental death

Killer T cells (cytotoxic T lymphocytes, CTLs) maintain immune homoeostasis by eliminating virus-infected and cancerous cells. CTLs achieve this by forming an immunological synapse with their targets and secreting a pore-forming protein (perforin) and pro-apoptotic serine proteases (granzymes) into the synaptic cleft. Although the CTL and the target cell are both exposed to perforin within the synapse, only the target cell membrane is disrupted, while the CTL is invariably spared. How CTLs escape unscathed remains a mystery. Here, we report that CTLs achieve this via two protective properties of their plasma membrane within the synapse: high lipid order repels perforin and, in addition, exposed phosphatidylserine sequesters and inactivates perforin. The resulting resistance of CTLs to perforin explains their ability to kill target cells in rapid succession and to survive these encounters. Furthermore, these mechanisms imply an unsuspected role for plasma membrane organization in protecting cells from immune attack.

Development of an animal model of progressive vaccinia in nu/nu mice and the use of bioluminescence imaging for assessment of the efficacy of monoclonal antibodies against vaccinial B5 and L1 proteins

Bioluminescence imaging (BLI) was used to follow dissemination of recombinant vaccinia virus (VACV) expressing luciferase (IHD-J-Luc) in BALB/c nu/nu mice treated post-challenge with monoclonal antibodies (MAbs) against L1 and B5 VACV proteins in a model of Progressive Vaccinia (PV). Areas Under the flux Curve (AUC) were calculated for viral loads in multiple organs in individual mice. Following scarification with 10⁵ pfu, IHD-J-Luc VACV undergoes fast replication at the injection site and disseminates rapidly to the inguinal lymph nodes followed by spleen, liver, and axillary lymph nodes within 2-3 days and before primary lesions are visible at the site of scarification. Extension of survival in nude mice treated with a combination of anti-B5 and anti-L1 MAbs 24 h post challenge correlated with a significant reduction in viral load at the site of scarification and delayed systemic dissemination. Nude mice reconstituted with 10⁴ T cells prior to challenge with IHD-J-Luc, and treated with MAbs post-challenge, survived infection, cleared the virus from all organs and scarification site, and developed anti-VACV IgG and VACV-specific polyfunctional CD8⁺ T cells that co-expressed the degranulation marker CD107a, and IFNγ and TNFα cytokines. All T cell reconstituted mice survived intranasal re-challenge with IHD-J-Luc (10⁴ pfu) two months after the primary infection. Thus, using BLI to monitor VACV replication in a PV model, we showed that anti-VACV MAbs administered post challenge extended survival of nude mice and protected T cell reconstituted nude mice from lethality by reducing replication at the site of scarification and systemic dissemination of VACV.

A new mechanism shapes the naïve CD8+ T cell repertoire: the selection for full diversity

During thymic T cell differentiation, TCR repertoires are shaped by negative, positive and agonist selection. In the thymus and in the periphery, repertoires are also shaped by strong inter-clonal and intra-clonal competition to survive death by neglect. Understanding the impact of these events on the T cell repertoire requires direct evaluation of TCR expression in peripheral naïve T cells. Several studies have evaluated TCR diversity, with contradictory results. Some of these studies had intrinsic technical limitations since they used material obtained from T cell pools, preventing the direct evaluation of clonal sizes. Indeed with these approaches, identical TCRs may correspond to different cells expressing the same receptor, or to several amplicons from the same T cell. We here overcame this limitation by evaluating TCRB expression in individual naïve CD8⁺ T cells. Of the 2269 Tcrb sequences we obtained from 13 mice, 99% were unique. Mathematical analysis of the data showed that the average number of naïve peripheral CD8⁺ T cells expressing the same TCRB is 1.1 cell. Since TCRA co-expression studies could only increase repertoire diversity, these results reveal that the number of naïve T cells with unique TCRs approaches the number of naïve cells. Since thymocytes undergo multiple rounds of divisions after TCRB rearrangement and 3-5% of thymocytes survive thymic selection events the number of cells expressing the same TCRB was expected to be much higher. Thus, these results suggest a new repertoire selection mechanism, which strongly selects for full TCRB diversity.

CD4 T Helper Cells Instruct Lymphopenia-Induced Memory-Like CD8 T Cells for Control of Acute LCMV Infection

Lymphopenic conditions lead to expansion of memory-like T cells (T_ML), which develop from naïve T cells by spontaneous proliferation. T_ML cells are often increased in the elderly population, AIDS patients, and patients recovering from radio- or chemotherapy. At present, it is unclear whether T_ML cells can efficiently respond to foreign antigen and participate in antiviral immunity. To address this question, we analyzed the immune response during acute low-dose infection with lymphocytic choriomeningitis virus-WE in T cell lymphopenic CD4Cre/R-diphtheria toxin alpha (DTA) mice in which most peripheral T cells show a T_ML phenotype. On day 8 after infection, the total number of effector T cells and polyfunctional IFN-γ and TNF-α producing CD8 T cells were three- to fivefold reduced in CD4Cre/R-DTA mice as compared to controls. Viral clearance and the humoral immune response were severely impaired in CD4Cre/R-DTA mice although CTLs efficiently killed transferred target cells in vivo. Transfer of naïve CD4 T cells but not anti-PD-L1 blockade restored the expansion of antigen-specific polyfunctional CD8 T cells and resulted in lower viral titers. This finding indicates that under lymphopenic conditions endogenous CD4 T_ML cell lack the capacity to promote expansion of CTLs. However, CD8 T_ML cells retain sufficient functional plasticity to participate in antiviral immunity in the presence of appropriate help by fully functional CD4 T cells. This capacity might be exploited to develop treatments for improvement of CD8 T cell functions under various clinical settings of lymphopenia.

Protein Translation Activity: A New Measure of Host Immune Cell Activation

We describe the in vivo ribopuromycylation (RPM) method, which uses a puromycin-specific Ab to fluorescently label ribosome-bound puromycylated nascent chains, enabling measurement of translational activity via immunohistochemistry or flow cytometry. Tissue staining provides a unique view of virus-induced activation of adaptive, innate, and stromal immune cells. RPM flow precisely quantitates virus-induced activation of lymphocytes and innate immune cells, and it provides a unique measure of immune cell deactivation and quiescence. Using RPM we find that high endothelial cells in draining lymph nodes rapidly increase translation in the first day of vaccinia virus infection. We also find a population of constitutively activated splenic T cells in naive mice and further that most bone marrow T cells activate 3 d after vaccinia virus infection. Bone marrow T cell activation is nonspecific, IL-12-dependent, and induces innate memory T cell phenotypic markers. Thus, RPM measures translational activity to uniquely identify cell populations that participate in the immune response to pathogens, other foreign substances, and autoantigens.

Modeling the dynamics of neonatal CD8+ T-cell responses

Neonates are particularly susceptible to a number of infections, and the neonatal CD8+ T cell response demonstrates differences in both the phenotype and magnitude of responses to infection compared with adults. However, the underlying basis for these differences is unclear. We have used a mathematical modeling approach to analyze the dynamics of neonatal and adult CD8+ T cell responses following in vitro stimulation and in vivo infection, which allows us to dissect key cell-intrinsic differences in expansion, differentiation and memory formation. We found that neonatal cells started dividing 8 hrs earlier and proliferated at a faster rate (0.077 day⁻¹ vs 0.105 day⁻¹) than adult cells in vitro. In addition, neonatal cells also differentiated more rapidly, as measured by the loss in CD62L and Ly6C expression. We extended our mathematical modeling to analysis of neonatal and adult CD8+ T cells responding in vivo and demonstrated that neonatal cells divide more slowly than adult cells after day 4 post-infection. However, neonatal cells differentiate more rapidly, up-regulating more KLRG-1 per division than adult cells (20% vs. 5%). The dynamics of memory formation were also found to be different, with neonatal effector cells showing increased death (1.0 day⁻¹ vs. 2.45 day⁻¹). Comparison of the division of human cord blood and adult naïve cells stimulated in vitro showed more division in cord blood derived cells, consistent with the observations in mice. This work highlights differences of the cell-intrinsic division and differentiation program in neonatal CD8+ T cells.

Type 1 interferon licenses naïve CD8 T cells to mediate anti-viral cytotoxicity

Naïve T cells, unlike memory T cells, exhibit very limited effector function in response to cognate antigen, but exposure to type 1 interferon (IFN) prior to cognate antigen allows for rapid manifestation of effector functions. A full assessment of the functions of these IFN-sensitized otherwise naïve T cells has not been made, nor has their capacity to be effector cells in vivo. We describe here that IFN-sensitized naïve T cells in the absence of cognate antigen adopt a partial activated phenotype distinguished by the upregulation of the surface activation marker CD69, effector-associated transcription factors Eomes and IRF4, and cytotoxicity effector molecule granzyme B. IFN-sensitized naive T cells lysed target cells in vivo and responded to low concentrations and affinities of cognate ligands. We suggest that this rapid and sensitive effector function of IFN-conditioned naïve CD8 T cells may play a role in pathogen control and help ward off superinfections.

Understanding Experimental LCMV Infection of Mice: The Role of Mathematical Models

Virus infections represent complex biological systems governed by multiple-level regulatory processes of virus replication and host immune responses. Understanding of the infection means an ability to predict the systems behaviour under various conditions. Such predictions can only rely upon quantitative mathematical models. The model formulations should be tightly linked to a fundamental step called “coordinatization” (Hermann Weyl), that is, the definition of observables, parameters, and structures that enable the link with a biological phenotype. In this review, we analyse the mathematical modelling approaches to LCMV infection in mice that resulted in quantification of some fundamental parameters of the CTL-mediated virus control including the rates of T cell turnover, infected target cell elimination, and precursor frequencies. We show how the modelling approaches can be implemented to address diverse aspects of immune system functioning under normal conditions and in response to LCMV and, importantly, make quantitative predictions of the outcomes of immune system perturbations. This may highlight the notion that data-driven applications of meaningful mathematical models in infection biology remain a challenge.

Postchallenge administration of brincidofovir protects healthy and immune-deficient mice reconstituted with limited numbers of T cells from lethal challenge with IHD-J-Luc vaccinia virus

Protection from lethality by postchallenge administration of brincidofovir (BCV, CMX001) was studied in normal and immune-deficient (nude, nu/nu) BALB/c mice infected with vaccinia virus (VACV). Whole-body bioluminescence imaging was used to record total fluxes in the nasal cavity, lungs, spleen, and liver and to enumerate pox lesions on tails of mice infected via the intranasal route with 10(5) PFU of recombinant IHD-J-Luc VACV expressing luciferase. Areas under the flux curve (AUCs) were calculated for individual mice to assess viral loads. A three-dose regimen of 20 mg/kg BCV administered every 48 h starting either on day 1 or day 2 postchallenge protected 100% of mice. Initiating BCV treatment earlier was more efficient in reducing viral loads and in providing protection from pox lesion development. All BCV-treated mice that survived challenge were also protected from rechallenge with IHD-J-Luc or WRvFire VACV without additional treatment. In immune-deficient mice, BCV protected animals from lethality and reduced viral loads while animals were on the drug. Viral recrudescence occurred within 4 to 9 days, and mice succumbed ∼10 to 20 days after treatment termination. Nude mice reconstituted with 10(5) T cells prior to challenge with 10(4) PFU of IHD-J-Luc and treated with BCV postchallenge survived the infection, cleared the virus from all organs, and survived rechallenge with 10(5) PFU of IHD-J-Luc VACV without additional BCV treatment. Together, these data suggest that BCV protects immunocompetent and partially T cell-reconstituted immune-deficient mice from lethality, reduces viral dissemination in organs, prevents pox lesion development, and permits generation of VACV-specific memory.

IMPORTANCE

Mass vaccination is the primary element of the public health response to a smallpox outbreak. In addition to vaccination, however, antiviral drugs are required for individuals with uncertain exposure status to smallpox or for whom vaccination is contraindicated. Whole-body bioluminescence imaging was used to study the effect of brincidofovir (BCV) in normal and immune-deficient (nu/nu) mice infected with vaccinia virus, a model of smallpox. Postchallenge administration of 20 mg/kg BCV rescued normal and immune-deficient mice partially reconstituted with T cells from lethality and significantly reduced viral loads in organs. All BCV-treated mice that survived infection were protected from rechallenge without additional treatment. In immune-deficient mice, BCV extended survival. The data show that BCV controls viral replication at the site of challenge and reduces viral dissemination to internal organs, thus providing a shield for the developing adaptive immunity that clears the host of virus and builds virus-specific immunological memory.

CD28-, CD45RA(null/dim) and natural killer-like CD8+ T cells are increased in peripheral blood of women with low-grade cervical lesions

Background

In response to antigen naive CD8+, T cells differentiate into effector cells, which express Natural killer (NK) receptors, lose CD28 expression, and die by apoptosis. However, in smaller quantities, the cells are retained for subsequent exposure to the same antigen. Knowledge is limited regarding whether the percentages of CD28-, Effector memory (EMRA^null/dim), and the CD16+/CD56 + CD8+ T cells of women with low-grade cervical lesions are altered at a systemic level.

Methods

We enrolled in this study women controls and women with Human papilloma virus infection (HPV-I) without associated cellular neoplastic changes and with Cervical Intraepithelial Neoplastic-I (CIN-I). Flow cytometry (FC) was performed for measurement of CD28-, memory subset, and NK-like CD8 + T cells, and IL-17, IFN-gamma, Tumor necrosis factor (TNF)-alpha, Interleukin (IL)-10, IL-6, IL-4, and IL-2. Finally, we genotyped the HPV.

Results

The CIN-I group increased the CD8 + CD28− and CD16+/56+ T cell percentage compared with that of HPV-I and controls (p <0.01), and CD8 + CCR7-CD45RA^null/dim (EMRA^null/dim) T cells were also increased in the CIN-I group compared with the controls (p <0.01). These two study groups were HPV- genotyped; 49% were HPV18+, and we did not observe differences in cytokine levels among all groups.

Conclusions

Increased levels of CD28-, EMRA^null/dim, and CD16+/CD56 + CD8+ T cells of peripheral blood in women with CIN-I may be associated with persistent HPV infection and could exert an influence on progression to cervical cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s12935-014-0097-5) contains supplementary material, which is available to authorized users.

The number of responding CD4 T cells and the dose of antigen conjointly determine the TH1/TH2 phenotype by modulating B7/CD28 interactions

Our previous in vivo studies show that both the amount of Ag and the number of available naive CD4 T cells affect the Th1/Th2 phenotype of the effector CD4 T cells generated. We examined how the number of OVA-specific CD4 TCR transgenic T cells affects the Th1/Th2 phenotype of anti-SRBC CD4 T cells generated in vivo upon immunization with different amounts of OVA-SRBC. Our observations show that a greater number of Ag-dependent CD4 T cell interactions are required to generate Th2 than Th1 cells. We established an in vitro system that recapitulates our main in vivo findings to more readily analyze the underlying mechanism. The in vitro generation of Th2 cells depends, as in vivo, upon both the number of responding CD4 T cells and the amount of Ag. We demonstrate, using agonostic/antagonistic Abs to various costimulatory molecules or their receptors, that the greater number of CD4 T cell interactions, required to generate Th2 over Th1 cells, does not involve CD40, OX40, or ICOS costimulation, but does involve B7/CD28 interactions. A comparison of the level of expression of B7 molecules by APC and CD4 T cells, under different conditions resulting in the substantial generation of Th1 and Th2 cells, leads us to propose that the critical CD28/B7 interactions, required to generate Th2 cells, may directly occur between CD4 T cells engaged with the same B cell acting as an APC.

Virtual memory CD8 T cells display unique functional properties

Previous studies revealed the existence of foreign antigen-specific memory phenotype CD8 T cells in unimmunized mice. Considerable evidence suggests this population, termed "virtual memory" (VM) CD8 T cells, arise via physiological homeostatic mechanisms. However, the antigen-specific function of VM cells is poorly characterized, and hence their potential contribution to immune responses against pathogens is unclear. Here we show that naturally occurring, polyclonal VM cells have unique functional properties, distinct from either naïve or antigen-primed memory CD8 T cells. In striking contrast to conventional memory cells, VM cells showed poor T cell receptor-induced IFN-γ synthesis and preferentially differentiated into central memory phenotype cells after priming. Importantly, VM cells showed efficient control of Listeria monocytogenes infection, indicating memory-like capacity to eliminate certain pathogens. These data suggest naturally arising VM cells display unique functional traits, allowing them to form a bridge between the innate and adaptive phase of a response to pathogens.

Effects of postchallenge administration of ST-246 on dissemination of IHD-J-Luc vaccinia virus in normal mice and in immune-deficient mice reconstituted with T cells

Whole-body bioimaging was used to study dissemination of vaccinia virus (VACV) in normal and in immune deficient (nu(-)/nu(-)) mice protected from lethality by postchallenge administration of ST-246. Total fluxes were recorded in the liver, spleen, lungs, and nasal cavities of live mice after intranasal infection with a recombinant IHD-J-Luc VACV expressing luciferase. Areas under the flux curve were calculated for individual mice to assess viral loads. Treatment for 2 to 5 days of normal BALB/c mice with ST-246 at 100 mg/kg starting 24 h postchallenge conferred 100% protection and reduced viral loads in four organs compared to control mice. Mice also survived after 5 days of treatment with ST-246 at 30 mg/kg, and yet the viral loads and poxes were higher in these mice compared to 100-mg/kg treatment group. Nude mice were not protected by ST-246 alone or by 10 million adoptively transferred T cells. In contrast, nude mice that received T cells and 7-day treatment with ST-246 survived infection and exhibited reduced viral loads compared to nonreconstituted and ST-246-treated mice after ST-246 was stopped. Similar protection of nude mice was achieved using adoptively transferred 1.0 and 0.1 million, but not 0.01 million, purified T cells or CD4(+) or CD8(+) T cells in conjunction with ST-246 treatment. These data suggest that ST-246 protects immunocompetent mice from lethality and reduces viral dissemination in internal organs and poxvirus lesions. Furthermore, immune-deficient animals with partial T cell reconstitution can control virus replication after a course of ST-246 and survive lethal vaccinia virus challenge.

NK cells controlling virus-specific T cells: Rheostats for acute vs. persistent infections

Viral infections characteristically induce a cytokine-driven activated natural killer (NK) cell response that precedes an antigen-driven T cell response. These NK cells can restrain some but not all viral infections by attacking virus-infected cells and can thereby provide time for an effective T cell response to mobilize. Recent studies have revealed an additional immunoregulatory role for the NK cells, where they inhibit the size and functionality of the T cell response, regardless of whether the viruses are themselves sensitive to NK cells. This subsequent change in T cell dynamics can alter patterns of immunopathology and persistence and implicates NK cells as rheostat-like regulators of persistent infections.

Increasing viral dose causes a reversal in CD8+ T cell immunodominance during primary influenza infection due to differences in antigen presentation, T cell avidity, and precursor numbers

T cell responses are characterized by the phenomenon of immunodominance (ID), whereby peptide-specific T cells are elicited in a reproducible hierarchy of dominant and subdominant responses. However, the mechanisms that give rise to ID are not well understood. We investigated the effect of viral dose on primary CD8(+) T cell (T(CD8+)) ID by injecting mice i.p. with various doses of influenza A virus and assessing the primary T(CD8+) response to five dominant and subdominant peptides. Increasing viral dose enhanced the overall strength of the T(CD8+) response, and it altered the ID hierarchy: specifically, NP(366-374) T(CD8+) were dominant at low viral doses but were supplanted by PA(224-233) T(CD8+) at high doses. To understand the basis for this reversal, we mathematically modeled these T(CD8+) responses and used Bayesian statistics to obtain estimates for Ag presentation, T(CD8+) precursor numbers, and avidity. Interestingly, at low viral doses, Ag presentation most critically shaped ID hierarchy, enabling T(CD8+) specific to the more abundantly presented NP(366-374) to dominate. By comparison, at high viral doses, T(CD8+) avidity and precursor numbers appeared to be the major influences on ID hierarchy, resulting in PA(224-233) T(CD8+) usurping NP(366-374) cells as the result of higher avidity and precursor numbers. These results demonstrate that the nature of primary T(CD8+) responses to influenza A virus is highly influenced by Ag dose, which, in turn, determines the relative importance of Ag presentation, T(CD8+) avidity, and precursor numbers in shaping the ID hierarchy. These findings provide valuable insights for future T(CD8+)-based vaccination strategies.

At low precursor frequencies, the T-cell response to chronic self-antigen results in anergy without deletion

The behavior of self-reactive T cells in the peripheral immune system has often been studied by following the fate of adoptively transferred antigen-specific T cells in antigen expressing mice. In most cases, after a period of expansion, such cells undergo a slow clonal deletion, accompanied by the onset of anergy and/or suppression in the remaining cells. Here, we demonstrate that at initial frequencies approaching those found in normal repertoires, it is possible to completely avoid deletion and still maintain peripheral tolerance. At starting numbers of <1000 T cells, stimulation by chronic self-antigens resulted in a period of robust clonal expansion, followed by a steady plateau phase extending beyond 4 months. Despite their stable persistence, the self-reactive T cells did not convert to a Foxp3⁺ fate. However, they displayed a considerable block in their ability to make IL-2, consistent with the onset of anergy - in a precursor frequency or deletion independent fashion.

The activation, by antigen, of naïve TCR transgenic CD4 T cells cultured at physiological, rather than artificially high, frequencies more accurately reflects the in vivo activation of normal numbers of naïve CD4(+) T cells

The majority of in vitro studies investigating the activation of naïve TCR transgenic T cells routinely employ an artificially high frequency of such cells. To assess whether employing high frequencies of TCR transgenic cells in vitro accurately reflects the in vivo activation of a normal number of T cells, we cultured between 300 and 3×10(6) Rag2(-/-) DO11.10 T cells per well under otherwise identical conditions. We find that those T cells cultured at low frequencies proliferate more and are more potently activated, as assessed by the expression of CD44 and CD62L, each giving rise to a much larger number of cytokine producing cells, comparable to the number generated in vivo when a normal number of CD4(+) T cells are activated. The effect of T cell frequency on the level of their activation was not due to differences in MHCII or CD80/86 expression by B cells, the major APC population present, nor to increased death of B cells in high frequency cultures. Taken together, our observations illustrate the necessity of culturing naïve TCR transgenic CD4(+) T cells at a physiological frequency if one is to more accurately recapitulate the in vivo activation of naïve CD4(+) T cells.

Population dynamics of naive and memory CD8 T cell responses after antigen stimulations in vivo

The extent to which the progeny of one primary memory CD8 T cell differs from the progeny of one naive CD8 T cell of the same specificity remains an unresolved question. To explore cell-autonomous functional differences between naive and memory CD8 T cells that are not influenced by differences in the priming environment, an experimental model has been developed in which physiological numbers of both populations of cells were cotransferred into naive hosts before Ag stimulation. Interestingly, naive CD8 T cells undergo greater expansion in numbers than do primary memory CD8 T cells after various infections or immunizations. The intrinsic ability of one naive CD8 T cell to give rise to more effector CD8 T cells than one memory CD8 T cell is independent of the number and quality of primary memory CD8 T cells present in vivo. The sustained proliferation of newly activated naive CD8 T cells contributed to their greater magnitude of expansion. Additionally, longitudinal analyses of primary and secondary CD8 T cell responses revealed that on a per-cell basis naive CD8 T cells generate higher numbers of long-lived memory cells than do primary memory CD8 T cells. This enhanced "memory generation potential" of responding naive CD8 T cells occurred despite the delayed contraction of secondary CD8 T cell responses. Taken together, the data in this study revealed previously unappreciated differences between naive and memory CD8 T cells and will help further define the functional potential for both cell types.

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.

Modulating numbers and phenotype of CD8+ T cells in secondary immune responses

Prime-boost regimens are frequently used to increase the number of memory CD8(+) T cells and thus the protective capacity of experimental vaccinations; however, it is currently unknown how the frequency and phenotype of primary (1 degrees ) memory CD8(+) T cells impact the quantity and phenotype of secondary (2 degrees ) memory CD8(+) T-cell populations. Here, we show that 2 degrees infections of mice that received different 1 degrees infections and/or immunizations generated similar numbers of 2 degrees effector and memory CD8(+) T cells. Remarkably, this result was independent of the numbers and phenotype of 1 degrees memory CD8(+) T cells present at the time of rechallenge. However, after adoptive transfer of low numbers of 1 degrees memory CD8(+) T cells, a linear correlation between 1 degrees memory CD8(+) T-cell input and 2 degrees memory CD8(+) T-cell numbers was observed. These data suggest that, above a very low threshold, boosting of 1 degrees memory CD8(+) T-cell populations elicits 2 degrees immune responses of similar magnitude. Therefore, our study has important implications for the design of prime-boost regimens that aim to generate protective CD8(+) T-cell-mediated immunity.

Analysis of A47, an immunoprevalent protein of vaccinia virus, leads to a reevaluation of the total antiviral CD8+ T cell response

Vaccinia virus (VACV) is the prototypic orthopoxvirus and was the live vaccine used to eradicate smallpox. In addition, VACV is a possible vector for recombinant vaccines. Despite these reasons for study, the roles of many VACV genes are unknown, and some fundamental aspects, such as the total size of immune responses, remain poorly characterized. VACV gene A47L is of interest because it is highly transcribed, has no sequence similarity to any nonpoxvirus gene, and contains a larger-than-expected number of CD8(+) T cell epitopes. Here it is shown that A47L is not required for growth in vitro and does not contribute to virulence in mice. However, we confirmed that this one protein primes CD8(+) T cells to three different epitopes in C57BL/6 mice. In the process, one of these epitopes was redefined and shown to be the most dominant in A47 and one of the more highly ranked in VACV as a whole. The relatively high immunogenicity of this epitope led to a reevaluation of the total CD8(+) T cell response to VACV. By the use of two methods, the true size of the response was found to be around double previous estimates and at its peak is on the order of 60% of all CD8(+) T cells. We speculate that more CD8(+) T cell epitopes remain to be mapped for VACV and that underestimation of responses is unlikely to be unique to VACV, so there would be merit in revisiting this issue for other viruses.

Tumor necrosis factor receptor/tumor necrosis factor family members in antiviral CD8 T-cell immunity

CD8 memory T cells can play a critical role in protection against repeated exposure to infectious agents such as viruses, yet can also contribute to the immunopathology associated with these pathogens. Understanding the mechanisms that control effective memory responses has important ramifications for vaccine design and in the management of adverse immune reactions. Recent studies have implicated several members of the tumor necrosis factor receptor (TNFR) family as key stimulatory and inhibitory molecules involved in the regulation of CD8 T cells. In this review, we discuss their control of the generation, persistence, and reactivation of CD8 T cells during virus infection.

Heterologous immunity between viruses

Immune memory responses to previously encountered pathogens can sometimes alter the immune response to and the course of infection of an unrelated pathogen by a process known as heterologous immunity. This response can lead to enhanced or diminished protective immunity and altered immunopathology. Here, we discuss the nature of T-cell cross-reactivity and describe matrices of epitopes from different viruses eliciting cross-reactive CD8(+) T-cell responses. We examine the parameters of heterologous immunity mediated by these cross-reactive T cells during viral infections in mice and humans. We show that heterologous immunity can disrupt T-cell memory pools, alter the complexity of the T-cell repertoire, change patterns of T-cell immunodominance, lead to the selection of viral epitope-escape variants, alter the pathogenesis of viral infections, and, by virtue of the private specificity of T-cell repertoires within individuals, contribute to dramatic variations in viral disease. We propose that heterologous immunity is an important factor in resistance to and variations of human viral infections and that issues of heterologous immunity should be considered in the design of vaccines.