T cell immunity to herpes simplex viruses in seronegative subjects: silent infection or acquired immunity?

During the course of investigating T cell responses to HSV among volunteers entering trials of investigational genital herpes vaccines, 6 of the 24 immunocompetent subjects with no prior history of oral/labial or genital herpes possessed HSV-specific T cell immunity but, by multiple determinants of even the most sensitive serological assays, remained seronegative to HSV-1 and -2. Of these six immune seronegative (IS; HSV-seronegative with HSV-specific T cell responses) subjects, two had transient HSV-specific T cell responses, while four had CD4(+) and CD8(+) T cell responses directed at HSV that persisted for up to 4 years. CD4(+) T cell clones were isolated that recognized and had high binding affinities to epitopes in HSV-2 tegument proteins. All six IS subjects had potential sexual exposure to an HSV-2-infected sexual partner. Oral and genital mucosal secretions were sampled and tested for the presence of infectious HSV and HSV DNA. No evidence of HSV was detected in >1500 samples obtained from these IS subjects. The identification of persistent T cell responses to HSV in seronegative subjects is a novel finding in the herpesvirus field and suggests either undetected infection or acquired immunity in the absence of infection. Understanding the basis of these acquired immune responses may be critical in developing effective vaccines for genital herpes.

The gp49B1 inhibitory receptor regulates the IFN-gamma responses of T cells and NK cells

The magnitude and diversity of Ag-specific T cell effector activity have been proposed to be controlled by an integration of positive signals transduced by the TCR and negative signals originating from inhibitory cell surface molecules. Although the lectin family of NK cell-associated inhibitory receptors has been reported to regulate the function of murine CTLs, gp49B1, the Ig superfamily member is not known to be expressed on T cells. Moreover, the consequences of the lack of an endogenously expressed NK cell-associated inhibitory receptor on T cell functions are not known. We report that gp49B1 is expressed by nearly all activated CD8 and CD4 T cells in addition to NK cells during an immune response to viral, bacterial, or tumor challenge. Kinetics of gp49B1 expression parallel functional capability and subside in the memory phase. Following vaccinia viral infection, IFN-gamma production by both subsets of T cells and NK cells is enhanced in gp49B1-deficient mice compared with gp49B1(+/+) mice. The stimulation threshold for IFN-gamma production is also lower in gp49B1-deficient T cells. In contrast, no significant differences were observed in the cytotoxic responses. We conclude that gp49B1 is a unique inhibitory receptor that is induced in multiple lineages of innate and adaptive immune cells during an infection and controls their IFN-gamma, but not cytotoxic responses.

Direct visualization of cross-reactive effector and memory allo-specific CD8 T cells generated in response to viral infections

CD8 T cell cross-reactivity between heterologous viruses has been shown to provide protective immunity, induce immunopathology, influence the immunodominance of epitope-specific T cell responses, and shape the overall memory population. Virus infections also induce cross-reactive allo-specific CTL responses. In this study, we quantified the allo-specific CD8 T cells elicited by infection of C57BL/6 (B6) mice with lymphocytic choriomeningitis virus (LCMV). Cross-reactive LCMV-specific CD8 T cells were directly visualized using LCMV peptide-charged MHC tetramers to costain T cells that were stimulated to produce intracellular IFN-gamma in response to allogeneic target cells. The cross-reactivity between T cells specific for LCMV and allogeneic Ags was broad-based, in that it involved multiple LCMV-derived peptides, but there were distinctive patterns of reactivity against allogeneic cells with different haplotypes. Experiments indicated that this cross-reactivity was not due to the expression of two TCR per cell, and that the patterns of allo-reactivity changed during sequential infection with heterologous viruses. The allo-specific CD8 T cells generated by LCMV infection were maintained at relatively high frequencies in the memory pool, indicating that memory allo-specific CD8 T cell populations can arise as a consequence of viral infections. Mice previously infected with LCMV and harboring allo-specific memory T cells were refractory to the induction of tolerance to allogeneic skin grafts.

Direct phenotypic analysis of human MHC class I antigen presentation: visualization, quantitation, and in situ detection of human viral epitopes using peptide-specific, MHC-restricted human recombinant antibodies

The advent in recent years of the application of tetrameric arrays of class I peptide-MHC complexes now enables us to detect and study rare populations of Ag-specific CD8(+) T cells. However, available methods cannot visualize or determine the number and distribution of these TCR ligands on individual cells nor detect APCs in tissues. In this study, we describe for the first time studies of human class I peptide-MHC ligand presentation. These studies were facilitated by applying novel tools in the form of peptide-specific, HLA-A2-restricted human recombinant Abs directed toward a viral epitope derived from human T cell lymphotropic virus type I. Using a large human Ab phage display library, we isolated a large panel of recombinant Fab Abs that are specific for a particular peptide-MHC class I complex in a peptide-dependent, MHC-restricted manner. We used these Abs to visualize the specific complex on APCs and virus-infected cells by flow cytometry, to quantify the number of, and visualize in situ, a particular complex on the surface of APCs bearing complexes formed by naturally occurring active intracellular processing of the cognate viral Ag. These findings demonstrate our ability to transform the unique fine specificity, but low intrinsic affinity of TCRs into high affinity soluble Ab molecules endowed with a TCR-like specificity toward human viral epitopes. These molecules may prove to be crucial useful tools for studying MHC class I Ag presentation in health and disease as well as for therapeutic purposes in cancer, infectious diseases, and autoimmune disorders.

CpG-A oligonucleotides induce a monocyte-derived dendritic cell-like phenotype that preferentially activates CD8 T cells

Human B cells and plasmacytoid dendritic cells recognize CpG motifs within microbial DNA via Toll-like receptor 9. Two functionally distinct types of CpG motif containing oligonucleotides (CpG ODN) have been described, CpG-A and CpG-B. In contrast to CpG-B, CpG-A induces high amounts of type I IFN (IFN-alpha and IFN-beta) in plasmacytoid dendritic cells. In the present study, we examined the effects of CpG-A on human primary monocytes. In PBMC stimulated with CpG-A and GM-CSF, monocytes showed excellent survival, increased in size and granularity, and within 3 days developed a dendritic cell-like phenotype that was characterized by down-regulation of CD14, partial up-regulation of CCR7, and an increased surface expression of costimulatory and Ag-presenting molecules. This effect could be inhibited by a combination of blocking Abs to type I IFN, and no such CpG-A-induced changes were observed in purified monocytes. Although IL-12 production by this dendritic cell-like phenotype required additional stimulation with CD40 ligand, this cell type spontaneously up-regulated IL-15 expression. Consistent with the known effect of IL-15 on effector and memory CD8 T cells, the frequency of CCR7(-)/CD45RA(-) CD8 T cells was selectively increased in allogeneic T cell assays. Furthermore, this dendritic cell type was more potent to support both the generation and the IFN-gamma production of autologous influenza matrix peptide-specific memory CD8 T cells as compared with dendritic cells generated in the presence of GM-CSF and IL-4. In conclusion, monocytes exposed to the cytokine milieu provided by CpG-A rapidly develop a dendritic cell-like phenotype that is well equipped to support CD8 T cell responses.

Infections that induce autoimmune diabetes in BBDR rats modulate CD4+CD25+ T cell populations

Viruses are believed to contribute to the pathogenesis of autoimmune type 1A diabetes in humans. This pathogenic process can be modeled in the BBDR rat, which develops pancreatic insulitis and type 1A-like diabetes after infection with Kilham's rat virus (RV). The mechanism is unknown, but does not involve infection of the pancreatic islets. We first documented that RV infection of BBDR rats induces diabetes, whereas infection with its close homologue H-1 does not. Both viruses induced similar humoral and cellular immune responses in the host, but only RV also caused a decrease in splenic CD4(+)CD25(+) T cells in both BBDR rats and normal WF rats. Surprisingly, RV infection increased CD4(+)CD25(+) T cells in pancreatic lymph nodes of BBDR but not WF rats. This increase appeared to be due to the accumulation of nonproliferating CD4(+)CD25(+) T cells. The results imply that the reduction in splenic CD4(+)CD25(+) cells observed in RV-infected animals is virus specific, whereas the increase in pancreatic lymph node CD4(+)CD25(+) cells is both virus and rat strain specific. The data suggest that RV but not H-1 infection alters T cell regulation in BBDR rats and permits the expression of autoimmune diabetes. More generally, the results suggest a mechanism that could link an underlying genetic predisposition to environmental perturbation and transform a "regulated predisposition" into autoimmune diabetes, namely, failure to maintain regulatory CD4(+)CD25(+) T cell function.

nef gene is required for robust productive infection by simian immunodeficiency virus of T-cell-rich paracortex in lymph nodes

The pathogenesis of AIDS virus infection in a nonhuman primate AIDS model was studied by comparing plasma viral loads, CD4(+) T-cell subpopulations in peripheral blood mononuclear cells, and simian immunodeficiency virus (SIV) infection in lymph nodes for rhesus macaques infected with a pathogenic molecularly cloned SIVmac239 strain and those infected with its nef deletion mutant (Deltanef). In agreement with many reports, whereas SIVmac239 infection induced AIDS and depletion of memory CD4(+) T cells in 2 to 3 years postinfection (p.i.), Deltanef infection did not induce any manifestation associated with AIDS up to 6.5 years p.i. To explore the difference in SIV infection in lymphoid tissues, we biopsied lymph nodes at 2, 8, 72, and 82 weeks p.i. and analyzed them by pathological techniques. Maximal numbers of SIV-infected cells (SIV Gag(+), Env(+), and RNA(+)) were detected at 2 weeks p.i. in both the SIVmac239-infected animals and the Deltanef-infected animals. In the SIVmac239-infected animals, most of the infected cells were localized in the T-cell-rich paracortex, whereas in the Deltanef-infected animals, most were localized in B-cell-rich follicles and in the border region between the paracortex and the follicles. Analyses by double staining of CD68(+) macrophages and SIV Gag(+) cells and by double staining of CD3(+) T cells and SIV Env(+) cells revealed that SIV-infected cells were identified as CD4(+) T cells in either the SIVmac239 or the Deltanef infection. Whereas the many functions of Nef protein were reported from in vitro studies, our finding of SIVmac239 replication in the T-cell-rich paracortex in the lymph nodes supports the reported roles of Nef protein in T-cell activation and enhancement of viral infectivity. Furthermore, the abundance of SIVmac239 infection and the paucity of Deltanef infection in the T-cell-rich paracortex accounted for the differences in viral replication and pathogenicity between SIVmac239 and the Deltanef mutant. Thus, our in vivo study indicated that the nef gene enhances SIV replication by robust productive infection in memory CD4(+) T cells in the T-cell-rich region in lymphoid tissues.

Cutting edge: impairment of dendritic cells and adaptive immunity by Ebola and Lassa viruses

Acute infection of humans with Ebola and Lassa viruses, two principal etiologic agents of hemorrhagic fevers, often results in a paradoxical pattern of immune responses: early infection, characterized by an outpouring of inflammatory mediators such as TNF-alpha, IL-1 beta, and IL-6, vs late stage infections, which are associated with poor immune responses. The mechanisms underlying these diverse outcomes are poorly understood. In particular, the role played by cells of the innate immune system, such as dendritic cells (DC), is not known. In this study, we show that Ebola and Lassa viruses infect human monocyte-derived DC and impair their function. Monocyte-derived DC exposed to either virus fail to secrete proinflammatory cytokines, do not up-regulate costimulatory molecules, and are poor stimulators of T cells. These data represent the first evidence for a mechanism by which Ebola and Lassa viruses target DC to impair adaptive immunity.

Detection of suppressor T lymphocytes and estimation of their frequency in limiting dilution assays by generalized linear regression modeling

The estimate of the frequency of suppressor T lymphocytes in unfractionated cell populations remains challenging, mainly because these regulatory cells do not display specific immunophenotypic markers. In this paper, we describe a novel theoretical approach for quantifying the frequency of suppressor cells. This method is based on limiting dilution data modeling, and allows the simultaneous estimation of the frequencies of both proliferating and suppressor cells. We used previously published biological data, characterizing the inhibiting activity of suppressor T cell clones. Starting from these data, we propose a mathematical model describing the interaction between suppressor and proliferating T cells, and applied to a Poisson process. Limiting dilution data corresponding to this non-single-hit, suppressor two-target Poisson model were artificially generated, then modeled according to a generalized linear regression procedure. Deviation from the single-hit Poisson model was revealed by a statistical slope test, and a stepwise analysis of the regression appeared to be an efficient method that strongly argued in favor of the presence of suppressor cells. By using the frequency of proliferating T cells calculated in the first step of the regression, we demonstrated the possibility to provide a reasonable estimate of the frequency of suppressor T cells. Based on these findings, a practical decision-making procedure is given to perform standard analyses of limiting dilution data.

Human CD8 T cells of the peripheral blood contain a low CD8 expressing cytotoxic/effector subpopulation

Heterogeneity of lymphocyte populations demonstrates the diversity of cellular immune responses and provide a better understanding of the immune system. CD3+ CD8+ T cells exhibit a low CD8 expressing (CD8low) population in flow cytometric analysis of peripheral blood T cells. In healthy donors, this population consists of 0.2-7.0% of all CD8 T cells. The majority of the CD8low T cell population showed an elevated expression of CD25, CD45RA, and CD95L, and low levels of CD28, CD62L and CD45RO. Circulating CD8low T cells resemble cytotoxic effector cells because they express cytolytic mediators and are able to execute cytotoxicity. A restricted T cell receptor profile with increased Vbeta9, Vbeta14 and Vbeta23 expression was observed and the CD8low T cell population contain Epstein-Barr virus-specific T cells. Therefore, the CD8low population represent a subset of activated CD8 effector T cells, resulting most probably from a continuous and/or balanced immune response to intracellular pathogens.

Immediate early effector functions of virus-specific CD8+CCR7+ memory cells in humans defined by HLA and CC chemokine ligand 19 tetramers

Memory T cells exhibit a high degree of heterogeneity in terms of their phenotype and functional characteristics. It has been proposed that the CCR7 chemokine receptor divides memory T cell populations into central memory T cells and effector memory T cells with distinct functions in secondary immune responses. We were interested whether this hypothesis holds true in experiments performed on Ag-specific CD8(+) T cells. To identify CCR7(+) cells, we engineered a fluorescent ligand for CCR7; results with the new CC chemokine ligand 19 chemotetramer were verified by staining with a CCR7 mAb. Staining with the CC chemokine ligand 19 chemotetramer reveals two subsets within CCR7(+) cells: a CCR7(int) population containing memory cells and a CCR7(high) population containing naive T cells. Phenotypic analysis of MHC class I/peptide tetramer-positive cells revealed that HLA-A2-restricted CMV-specific CD8 T cells exhibit the lowest percentage of CCR7(+) cells (0.5-5%), while HLA-A2-restricted flu- and HLA-B8-restricted EBV-specific CD8 T cells showed the highest (45-70%). Intracellular staining of unstimulated cells revealed that both CCR7(int)- and CCR7(-)-specific CD8 T cells exhibit a detectable level of perforin. Both CCR7(int) and CCR7(-) Ag-specific CD8(+) T cells produced IFN-gamma and TNF-alpha following short-term peptide stimulation. Therefore, our finding that CCR7(+)CD8(+) T cells are able to exert immediate effector functions requires a substantial revision to the central and effector memory hypothesis.

Primary and secondary immunocompetence in mixed allogeneic chimeras

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

Memory inflation: continuous accumulation of antiviral CD8+ T cells over time

CD8+ T lymphocytes play an important role in the control of intracellular pathogens during both acute and persistent infections. This is particularly true in the case of persistent herpesviruses such as human CMV, which are typified by large virus-specific CD8+ T cell populations during viral latency. To understand the origin of these populations and the factors shaping them over time, we investigated the CD8+ T cell response after murine CMV (MCMV) infection. The kinetics of the acute response were characterized by rapid expansion of activated T cells, followed by a contraction phase. Thereafter, we observed a striking pattern, where MCMV-specific memory CD8+ T cells steadily accumulated over time, with 20% of all CD8+ T cells at 1 year specific for one MCMV epitope. Accumulation of MCMV-specific CD8+ T lymphocytes was seen in all organs tested and was associated with continuous activation of specific CD8+ T lymphocytes, primarily within lymph nodes. The pattern of accumulation was observed in only two of five epitopes tested, and was accompanied by a gradual restriction in usage of the variable region of the TCR beta-chain over time. This novel pattern of a virus-specific CD8+ T cell response suggests that continuous or repetitive exposure to Ag can slowly mold memory T cell populations over time. This may be relevant for understanding the evolution of the large human CMV-specific CD8+ T cell populations seen in humans.

Cutting edge: immediate RANTES secretion by resting memory CD8 T cells following antigenic stimulation

The efficiency of CD8 memory response relies partially on the modification of cellular functional capacities. To identify effector functions that can be modified following priming, we have compared the chemokines produced by naive and memory CD8 T cells. Our results show that in contrast to naive cells, resting memory CD8 T cells contain high levels of RANTES mRNA. As a result, they have the capacity to rapidly secrete RANTES upon ex vivo antigenic stimulation. In contrast to that of IFN-gamma, RANTES secretion is mainly due to the translation of the pre-existing mRNA.

Preferential escape of subdominant CD8+ T cells during negative selection results in an altered antiviral T cell hierarchy

Negative selection is designed to purge the immune system of high-avidity, self-reactive T cells and thereby protect the host from overt autoimmunity. In this in vivo viral infection model, we show that there is a previously unappreciated dichotomy involved in negative selection in which high-avidity CD8(+) T cells specific for a dominant epitope are eliminated, whereas T cells specific for a subdominant epitope on the same protein preferentially escape deletion. Although this resulted in significant skewing of immunodominance and a substantial depletion of the most promiscuous T cells, thymic and/or peripheral deletion of high-avidity CD8(+) T cells was not accompanied by any major change in the TCR V beta gene family usage or an absolute deletion of a single preferred complementarity-determining region 3 length polymorphism. This suggests that negative selection allows high-avidity CD8(+) T cells specific for subdominant or cryptic epitopes to persist while effectively deleting high-avidity T cells specific for dominant epitopes. By allowing the escape of subdominant T cells, this process still preserves a relatively broad peripheral TCR repertoire that can actively participate in antiviral and/or autoreactive immune responses.

Transfer of TCR genes into mature T cells is accompanied by the maintenance of parental T cell avidity

The adoptive transfer of tumor-specific T cells expanded in vitro can be of significant therapeutic value in select cancer patients. This strategy is limited though, as it is often difficult, if not impossible, to obtain T cells of clinical value. The transfer of TCR genes to mature T cells to generate tumor-reactive T cells provides a potential mechanism to overcome these limitations. To evaluate the feasibility of such an approach and the quality of the resulting T cells, we generated replication-deficient retroviral vectors using the well-characterized OT-1 TCR genes. After transducing murine T cells, we were able to expand large numbers of Ag-specific T cells that were functionally active against tumor cells expressing the relevant Ag. Furthermore, we found that T cells expressing retrovirally encoded TCR had avidity that was similar to that of the parental clone. This maintenance of avidity was despite variable expression of the retrovirally encoded TCR and the presence of potentially competing endogenous TCRs. These results suggest that the inherent qualities of the TCR, as dictated by the coding sequence, are the most critical parameters in the generation of high-avidity T cells.

Nonspecific recruitment of memory CD8+ T cells to the lung airways during respiratory virus infections

Previous studies have shown that heterologous viral infections have a significant impact on pre-existing memory T cell populations in secondary lymphoid organs through a combination of cross-reactive and bystander effects. However, the impact of heterologous viral infections on effector/memory T cells in peripheral sites is not well understood. In this study, we have analyzed the impact of a heterologous influenza virus infection on Sendai virus-specific CD8(+) effector/memory cells present in the lung airways. The data show a transient increase in the numbers of Sendai virus nucleoprotein 324-332/K(b)-specific CD8(+) memory T cells in the airways of the influenza-infected mice peaking around day 4 postinfection. Intratracheal transfer studies and 5-bromo-2'-deoxyuridine incorporation demonstrate that this increase is due to the recruitment of resting memory cells into the airways. In addition, the data show that these immigrating memory cells are phenotypically distinct from the resident memory T cells of the lung airways. A similar influx of nonproliferating Sendai virus nucleoprotein 324-332/K(b)-specific CD8(+) memory T cells is also induced by a secondary (homologous) infection with Sendai virus. Together, these data suggest that inflammation can accelerate memory T cell migration to nonlymphoid tissues and is a part of the normal recall response during respiratory infections.

Regulation of T cell migration during viral infection: role of adhesion molecules and chemokines

T cell mediated immunity and in particular CD8+ T cells are pivotal for the control of most viral infections. T cells exclusively exert their antiviral effect through close cellular interaction with relevant virus-infected target cells in vivo. It is therefore imperative that efficient mechanisms exist, which will rapidly direct newly generated effector T cells to sites of viral replication. In the present report we have reviewed our present knowledge concerning the molecular interactions, which are important in targeting of effector CD8+ T cells to sites of viral infection.

Phenotypic analysis of peripheral blood gammadelta T lymphocytes and their targeting by human immunodeficiency virus type 1 in vivo

There is increasing evidence that a wider range of lymphoid cell types other than CD4(+) T helper lymphocytes are infected with HIV-1 in vivo, including CD8 lymphocytes, natural killer cells, and reticulodendritic cells. Each potentially contributes to the reservoir of infected cells that resist antiviral treatment and to the impairment of immune responses in AIDS. By quantitative PCR for HIV proviral sequences we have now obtained evidence for substantial infection of gammadelta lymphocytes, contributing 3-45% of the proviral load in peripheral blood. A large proportion of gammadelta lymphocytes constitutively expressed the chemokine receptors CCR5 and CXCR4, with evidence for marked up-regulation of CD8 in samples from HIV-infected individuals, corresponding to an activated phenotype. That gammadelta lymphocytes might be susceptible to HIV infection was investigated using in vitro infectivity assays of recombinant HIV-expressing green fluorescent protein, followed by flow cytometry. gammadelta, CD4, and CD8 lymphocytes were each productively infected, with gammadelta lymphocytes showing the greatest susceptibility. For each cell type, blocking assays with an anti-CD4 monoclonal antibody indicated that entry was CD4-dependent.

Alterations in T-cell receptor Vbeta repertoire of CD4 and CD8 T lymphocytes in human immunodeficiency virus-infected children

Perturbations in the T-cell receptor (TCR) Vbeta repertoire were assessed in the CD4 and CD8 T lymphocytes of human immunodeficiency virus (HIV)-infected children who were receiving therapy during the chronic phase of infection by flow cytometry (FC) and PCR analysis. By FC, representation of 21 TCR Vbeta subfamilies was assessed for an increased or decreased percentage in CD4 and CD8 T cells, and by PCR, 22 TCR Vbeta subfamilies of CD4 and CD8 T cells were analyzed by CDR3 spectratyping for perturbations and reduction in the number of peaks, loss of Gaussian distribution, or clonal dominance. The majority of the TCR Vbeta subfamilies were examined by both methods and assessed for deviation from the norm by comparison with cord blood samples. The CD8-T-lymphocyte population exhibited more perturbations than the CD4 subset, and clonal dominance was present exclusively in CD8 T cells. Of the 55 total CD8-TCR Vbeta families classified with clonal dominance by CDR3 spectratyping, only 18 of these exhibited increased expression by FC. Patients with high numbers of CD8-TCR Vbeta families with decreased percentages had reduced percentages of total CD4 T cells. Increases in the number of CD4-TCR Vbeta families with increased percentages showed a positive correlation with skewing. Overall, changes from normal were often discordant between the two methods. This study suggests that the assessment of HIV-induced alterations in TCR Vbeta families at cellular and molecular levels yields different information and that our understanding of the immune response to HIV is still evolving.

IL-4-producing CD8+ T cells with a CD62L++(bright) phenotype accumulate in a subgroup of older adults and are associated with the maintenance of intact humoral immunity in old age

An increased production of proinflammatory cytokines occurs in a high percentage of elderly persons and is associated with an impaired humoral immune response. However, high IL-4 production has also been observed in old age. We now demonstrate an IL-4-producing subpopulation of CD8+ T cells in a subgroup of healthy older adults. This T cell subset is substantial in size and has a characteristic phenotype expressing CD45RO, CD28, CD62L, and CD25. IL-4-producing CD8+ T cells produce large amounts of IL-2 but not IFN-gamma or perforin, and these cells do not have a regulatory suppressive effect on other T cells. In vivo IL-4-producing CD8+ T cells can be stably detected over a year. When put into culture they also have a stable cytokine production pattern but fail to produce perforin even in the presence of IL-12. This special T cell type does not occur in persons under the age of 40, but is present in 36% of the persons >60 years of age. In this age group, IL-4-producing CD8+ T cells are more frequent in persons who are still capable of raising a humoral immune response following immunization than in others who fail to produce protective Abs after vaccination. Our results suggest that CD8+ T cells with a CD62L++(bright) phenotype accumulate in a subgroup of older adults. Due to their phenotype that enables them to migrate into lymphoid tissues and to their capacity to produce IL-4, these cells may counterbalance the overproduction of proinflammatory cytokines in old age.

The effects of experimental infection with bovine diarrhoea virus (BVDV) on lymphocyte subpopulations in the peripheral blood of pigs

The aim of the study was to analyse the dynamics of selected lymphocytes subpopulations in peripheral blood of pigs infected with BVDV, using flow cytometric method. The examinations were performed on eighteen healthy, pestivirus-free pigs divided into 3 groups. Pigs in the group 1 were intranasally infected with two virulent reference strains of BVDV: NADL2 and NADL8. Pigs of group 2 were included into experimentally infected ones. Group 3 consisted of control pigs free from infection. The percentage of CD2+ lymphocytes was gradually decreasing during the experiment. Finally a decrease by 20% was noted in group 2 and by 9% in group 1. A slight decrease of CD4+ cells and more significant decrease of CD8+ subpopulation were observed. The CD4:CD8 ratio changed from approximately 3:5 to 1:1 in pigs of group 2, and from 1:2 to 1:3 in pigs of group 1. There were only small fluctuations regarding TcR(gamma)delta+ cells. The level of these lymphocytes increased during first hours post infection and then decreased to the initial value. Obtained results may indicate that infection with species-unspecific pestiviruses causes similar, but less significant, changes within lymphocyte subpopulations than infection with species-specific pestiviruses.

High avidity CD8+ T cells are the initial population elicited following viral infection of the respiratory tract

Following intranasal administration, the model paramyxovirus simian virus 5 (SV5) establishes an infection in the respiratory tract of mice, which is subsequently cleared by CD8+ T cells. In this study, we sought to understand the maturation of the antiviral immune response over time by assessing the functional avidity of the responding T cells and the expansion of immunodominant populations. Surprisingly, we determined that the initial response to Ag at day 3 (d3) in the mediastinal lymph node was exclusively high avidity. However, by d5 postinfection, low avidity cells were approximately 50% of the responding T cell population. Following secondary exposure to SV5, high avidity CD8+ T cells again are the exclusive cell type present at early times postinfection (d2). Similarly, high avidity cells were preferentially elicited at d3 following infection with the unrelated vaccinia virus. We also made the observation that the immunodominance profile has not been established at d3 postinfection with SV5. However, by d5 a clear immunodominance pattern arises and is permanently maintained. These data indicate that high avidity cells are the predominant population responding at early times postinfection following respiratory infection with SV5 or vaccinia virus. However, as the response progresses, low avidity cells are activated/expanded to a greater extent compared with high avidity cells.

Apoptosis in lymph nodes and changes in lymphocyte subpopulations in peripheral blood of pigs infected with porcine rubulavirus

In a first experiment, five pigs were inoculated intranasally with porcine rubulavirus (PoRV) at 5 days of age and killed 7 days post-infection (pi). In a second experiment, four pigs were infected with the same virus at 17 days of age and killed at 9 or 15 days pi. Control piglets in each experiment received uninfected cell culture supernate. All PoRV-infected pigs developed respiratory and nervous signs, and histological lesions of non-suppurative encephalitis and interstitial pneumonia. All control pigs remained clinically normal and did not have histological lesions. Significantly increased numbers of apoptotic cells were detected by terminal deoxynucleotidyl transferase biotin-dUTP nick end labelling (TUNEL) in tonsil and lymph nodes of the pigs infected at 7 days of age and killed at 7 days pi. Significantly increased percentages of CD2(+) and CD8(+) T lymphocytes were also found in peripheral blood of these animals at this time, while the percentages of CD4(+) and MHC class II lymphocytes were significantly reduced. Significantly increased numbers of apoptotic cells were detected in lymphoid tissues of the pigs infected at 17 days of age and killed at 9 days pi. The percentages of CD2(+), CD8(+) and MHC class II lymphocytes in peripheral blood were also significantly increased at this time; the percentage of MHC class II lymphocytes remained elevated at 15 days pi. These results indicate that induction of apoptosis is an important mechanism in the pathogenesis of PoRV infection in young pigs, and that this virus induces changes in lymphocyte subpopulations in peripheral blood.

Induction of telomerase activity and maintenance of telomere length in virus-specific effector and memory CD8+ T cells

Acute viral infections induce extensive proliferation and differentiation of virus-specific CD8+ T cells. One mechanism reported to regulate the proliferative capacity of activated lymphocytes is mediated by the effect of telomerase in maintaining the length of telomeres in proliferating cells. We examined the regulation of telomerase activity and telomere length in naive CD8+ T cells and in virus-specific CD8+ T cells isolated from mice infected with lymphocytic choriomeningitis virus. These studies reveal that, compared with naive CD8+ T cells, which express little or no telomerase activity, Ag-specific effector and long-lived memory CD8+ T cells express high levels of telomerase activity. Despite the extensive clonal expansion that occurs during acute lymphocytic choriomeningitis virus infection, telomere length is maintained in both effector and memory CD8+ T cells. These results suggest that induction of telomerase activity in Ag-specific effector and memory CD8+ T cells is important for the extensive clonal expansion of both primary and secondary effector cells and for the maintenance and longevity of the memory CD8+ T cell population.