T-cell clonality in immune responses

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.

Activation of Th1 immunity is a common immune mechanism for the successful treatment of hepatitis B and C: tetramer assay and therapeutic implications

Both chronic hepatitis B and C virus (HBV and HCV) infections respond ineffectively to current antiviral therapies. Recent studies have suggested that treatment outcomes may depend on the development of type 1 T helper (Th1) and Th2 cell responses. Specifically, activation of Th1 immunity may play a major role in successfully treating hepatitis B and C. This model was revisited herein by evaluating immune responses in 36 HBV and 40 HCV patients with or without treatment, in an attempt to find a common immune mechanism for successful treatment. The immune responses in all examined cases were studied by peripheral blood mononuclear cell (PBMC) proliferation and cytokine responses to viral antigens, cytotoxic T lymphocyte (CTL) responses, enzyme-linked immunospot (ELISPOT) assay, and tetramer staining of virus-specific CD8+ T cells. The overall results revealed that all responders among both HBV- and HCV-infected cases displayed significantly higher PBMC proliferation to viral antigens with a predominant Th1 cytokine profile. Furthermore, the Th1-dominant responses were associated with significant enhancement of CTL activities and were correlated with ELISPOT data, while non-responders responded more weakly. During therapy, the numbers of tetramer-staining, virus-specific CD8+ T cells showed greater increases in responders than in non-responders (p = 0.001). The frequencies determined by the tetramer assay were approximately 200-fold higher than data estimated by limiting-dilution analysis. In conclusion, activation of Th1 immunity accompanied by enhancement of CTL activity during therapy is a common immune mechanism for successfully treating hepatitis B and C, and therefore may have important therapeutic implications.

Antigen-driven T-cell turnover

A mathematical model is developed to characterize the distribution of cell turnover rates within a population of T lymphocytes. Previous models of T-cell dynamics have assumed a constant uniform turnover rate; here we consider turnover in a cell pool subject to clonal proliferation in response to diverse and repeated antigenic stimulation. A basic framework is defined for T-cell proliferation in response to antigen, which explicitly describes the cell cycle during antigenic stimulation and subsequent cell division. The distribution of T-cell turnover rates is then calculated based on the history of random exposures to antigens. This distribution is found to be bimodal, with peaks in cell frequencies in the slow turnover (quiescent) and rapid turnover (activated) states. This distribution can be used to calculate the overall turnover for the cell pool, as well as individual contributions to turnover from quiescent and activated cells. The impact of heterogeneous turnover on the dynamics of CD4(+) T-cell infection by HIV is explored. We show that our model can resolve the paradox of high levels of viral replication occurring while only a small fraction of cells are infected.

Lack of proliferative capacity of human effector and memory T cells expressing killer cell lectinlike receptor G1 (KLRG1)

Adaptive immunity necessitates the proliferation of lymphocytes. In the mouse, we have previously shown that antigen-experienced T cells that have lost their proliferative potential express the killer cell lectinlike receptor G1 (KLRG1). By using a newly generated monoclonal antibody specific for human KLRG1, we now demonstrate that expression of KLRG1 also identifies T cells in humans that are capable of secreting cytokines but that fail to proliferate after stimulation. Furthermore, our data show that proliferative incapacity of CD8 T cells correlates better with KLRG1 expression than with absence of the CD28 marker. In peripheral blood lymphocytes (PBLs) from healthy adult donors, KLRG1 was expressed on 44% +/- 14% of CD8 and 18% +/- 10% of CD4 T cells. KLRG1 expression was restricted to antigen-experienced T cells. Here, KLRG1(+) cells were preferentially found in the CCR7(-) effector T-cell pool. Besides T cells, a significant portion (approximately 50%) of human natural killer (NK) cells expressed KLRG1. Interestingly, these KLRG1(+) NK cells were found exclusively in the CD56(dim) NK-cell subset. Thus, the expression of KLRG1 identifies a subset of NK cells and antigen-experienced T cells in humans that lack proliferative capacity.

Molecular and flow cytometric characterization of the CD4 and CD8 T-cell repertoire in patients with myelodysplastic syndrome

We studied 18 patients with myelodysplastic syndrome (MDS), measuring clonality and T-cell receptor Vbeta (TCRBV) expression of CD4 and CD8 T cells by polymerase chain reaction and by flow cytometric analysis of TCRBV families. The CD4 and CD8 T-cell repertoire in most MDS patients is characterized by an abnormal TCRBV-restricted expansion of T cells in CD4 and CD8 cells, and increased expression of the CD8 effector marker CD57 of multiple TCRBV in CD8 cells. Clonality analysis of CD4 and CD8 cells showed that seven of 10 patients analysed had a major clone in the CD8 cells but not in CD4 cells. Furthermore, in one patient we found that both the CD57- and CD57+ fraction contained the clone (which was absent from the TCRBV-negative fraction). These data suggest that, in MDS, multiple T-cell expansions can be found in both helper and cytotoxic T cells, and that, in the CD8 cells, T cells functionally differentiate in vivo from memory to effector T cells. Together, these data support the hypothesis of the involvement of T cells in the pathogenesis of MDS.

Differential kinetics of antigen-specific CD4+ and CD8+ T cell responses in the regression of retrovirus-induced sarcomas

Despite the accepted role for CD4+ T cells in immune control, little is known about the development of Ag-specific CD4+ T cell immunity upon primary infection. Here we use MHC class II tetramer technology to directly visualize the Ag-specific CD4+ T cell response upon infection of mice with Moloney murine sarcoma and leukemia virus complex (MoMSV). Significant numbers of Ag-specific CD4+ T cells are detected both in lymphoid organs and in retrovirus-induced lesions early during infection, and they express the 1B11-reactive activation-induced isoform of CD43 that was recently shown to define effector CD8+ T cell populations. Comparison of the kinetics of the MoMSV-specific CD4+ and CD8+ T cell responses reveals a pronounced shift toward CD8+ T cell immunity at the site of MoMSV infection during progression of the immune response. Consistent with an important early role of Ag-specific CD4+ T cell immunity during MoMSV infection, CD4+ T cells contribute to the generation of virus-specific CD8+ T cell immunity within the lymphoid organs and are required to promote an inflammatory environment within the virus-infected tissue.

Cytokine detection by ELISPOT: relevance for immunological studies in type 1 diabetes

Diabetes mellitus type 1 is a chronic disease in which the insulin-secreting ss-cells are selectively destroyed by an immune-mediated process. Autoantibodies directed against several islet antigens are useful parameters to estimate the risk to develop diabetes, but cell-mediated immunity involving T lymphocytes plays a major part in causing the specific destruction of ss-cells. T cells are characterized by their antigen-specificity, phenotype and cytokine-secreting profile. T cells that secrete cytokines of the T helper 1 (Th1) type have been shown to transfer diabetes in animal studies, in contrast to T helper 2 (Th2) cytokine-secreting T cells that are thought to be rather nondestructive. In the absence of phenotypic markers for Th1 and Th2 cells, several different approaches have been taken to examine T cell responses in detail. Methods involve T-cell proliferation assays, Enzyme-Linked-Immuno-Sorbent-Assay (ELISA) analysis of secreted cytokines and phenotype analysis applying flow cytometry. A more recent development is ELISPOT analysis, which enables the investigator to determine the qualitative and quantitative antigen-specific immune response on a single-cell level with regard to cytokine secretion. This article aims to give an introduction to the advantages and limitations inherent in the different techniques and their potential relevance for immunological studies in diabetes mellitus type 1.

Detection of alloreactive T cells by flow cytometry: a new test compared with limiting dilution assay

BACKGROUND

Frequencies of alloreactive T cells determined by limiting dilution assays (LDA) may not adequately reflect the donor-reactive immune status in transplant recipients. To reevaluate LDA frequencies, we developed a flow cytometry test for direct determination of alloreactive T-cell frequencies and compared these frequencies with classical LDA estimates of frequencies.

METHODS

For determination of frequencies by flow cytometry, peripheral blood lymphocytes (or lymphocytes taken from primary mixed lymphocyte culture) were stimulated with either Epstein-Barr virus-transformed lymphoblastoid cell lines or T cell-depleted spleen cells and stained for intracellular interferon (IFN)-gamma production and CD69. In lung transplant recipients, frequencies of IFN+ alloreactive T cells were compared with LDA frequencies, that is, cytotoxic T lymphocyte precursors and helper T lymphocyte precursors.

RESULTS

With flow cytometry, alloreactive T cells were detected after overnight allostimulation as IFN-gamma CD69bright cells (range, 0.1-0.58% and 0.1-0.66% of total CD4 and CD8 cells, respectively). Frequencies increased 25-fold or more when lymphocytes were prestimulated in primary mixed lymphocyte culture before testing. After lung transplantation, mean donor-specific IFN+ CD8 T-cell frequencies did not decrease as mean donor-specific LDA cytotoxic T lymphocyte precursor frequencies, whereas no difference was seen in pretransplantation samples or third-party-specific frequencies at both time points. Mean frequencies of IFN+ CD4 did not differ from helper T lymphocyte precursors at both time points, but frequencies did not correlate.

CONCLUSIONS

The flow cytometry test allows a direct measurement of alloreactive T-cell frequencies and demonstrates a discrepancy between donor-specific IFN+ CD8 T-cell frequencies and LDA CLTp after transplantation. This may be a result of the existence of "functional diverse" alloreactive T cells or of activation-induced cell death of donor-reactive T cells during long (LDA) culturing, which is avoided in the flow cytometry test.

Longitudinal analysis of the T-cell receptor (TCR)-VA and -VB repertoire in CD8+ T cells from individuals immunized with recombinant hepatitis B surface antigen

Recent studies have suggested that vaccination induces alterations in the T cell receptor (TCR) repertoire. We investigate the diversity of the TCR repertoire after immunization with a recombinant hepatitis B surface vaccine in seven healthy subjects in CD8+ T cells in peripheral blood lymphocytes. Cellular immune responses were monitored over time by sorting CD8 T cells followed by TCR-VA and -VB complementarity determining region 3 (CDR3) analysis. Frequency of individual VB families was determined by flow cytometry. TCR-VA/VB repertoires obtained from CD8+ T cells drawn after vaccination were compared to the TCR repertoire determined prior to vaccination. Monoclonal TCR transcripts could be detected exclusively in CD8+, but not in CD4+ T cells. Such monoclonal TCR transcripts were either stable in some individuals, or could only be detected at certain time points after vaccination. Sorting of monoclonal TCR-VB3+ T cells, which constituted up to 5% of the CD8+ T cell population from one individual, revealed that this T cell clone recognizes an epitope provided by the recombinant hepatitis B vaccine presented by MHC-class I on autologous antigen-presenting cells. Examination of the structural anatomy, defined by the TCR, and the frequency of T cells responding to the immunizing antigen may be helpful to provide surrogate markers to monitor cellular immune responses induced by protein antigens utilized for vaccination.

Oligoclonal CD4(+) T cell expansions in lung transplant recipients with obliterative bronchiolitis

Obliterative bronchiolitis (OB) is a dreaded and frequent complication of lung transplantation with a poorly understood immunopathogenesis. To further evaluate disease mechanisms, we used T cell antigen receptor (TCR) beta-chain variable region RNase protection assays, after polymerase chain reaction amplification of TCR cDNA, to quantitate circulating CD4(+) and CD8(+) repertoires of transplant recipients with OB or no evidence of rejection (NER). All six recipients with OB had markedly abnormal CD4 expansions (2.5 +/- 0.5 expansions/recipient) attributable to oligoclonal proliferations. Only two of six recipients with NER had a single, much lesser, CD4(+) abnormality each (p < 0.01). Moreover, one of these patients developed OB shortly thereafter, and the other NER abnormality may have predated transplantation. In contrast, CD8(+) expansions were common in both recipient populations. Findings of CD4(+) expansions had 100% sensitivity and 80% specificity for the presence or imminent development of OB. These data suggest proliferations of CD4(+) T cells are important in OB pathogenesis, and these are most likely part of a major histocompatibility complex Class II-dependent process of indirect alloantigen presentation. These CD4(+) clones are likely to have facultative helper functions for the multiple and diverse immune processes that have been implicated in OB. Furthermore, the close association of CD4(+) expansions with OB raises possibilities of development of novel diagnostic and therapeutic approaches.

Spontaneous regression of Kikuchi lymphadenopathy with oligoclonal T-cell populations favors a benign immune reaction over a T-cell lymphoma

To aid in the initial diagnosis of Kikuchi lymphadenitis and to assess whether the composition of the T cells might shed light on the pathogenesis, we used nested polymerase chain reaction tests followed by high-resolution gel electrophoresis to determine the pattern of T-cell antigen receptor rearrangement in 56 consecutive cases. Except for 1 unusual case with recurrent lymphadenopathy, none had a monoclonal beta or gamma rearrangement. Eight cases had a polyclonal pattern at both beta and gamma loci, 20 cases had a mixed polyclonal beta and oligoclonal gamma pattern, and 27 cases had an oligoclonal pattern at both loci. The high frequency of oligoclonality did not indicate an early-stage T-cell lymphoma in evolution, as confirmed by spontaneous resolution of the lymphadenopathy in all cases within 6 months. Rather, it is consistent with reports of oligoclonal T cells in a variety of immune reactions. We conclude that, in the vast majority of cases, absence of a monoclonal T-cell receptor rearrangement excludes the possibility of T-cell lymphoma, and the presence of an oligoclonal pattern implies a benign immune reaction.

Distinct bone marrow findings in T-cell granular lymphocytic leukemia revealed by paraffin section immunoperoxidase stains for CD8, TIA-1, and granzyme B

Unlike other leukemia types in which the bone marrow findings are diagnostic, the bone marrow pathology of T-cell granular lymphocytic leukemia (GLL) is subtle and ill-defined. In this study, bone marrow biopsy specimens from 36 patients with T-cell GLL and from 25 control patients with cytopenias and relative or absolute increases in blood large granular lymphocytes were studied by immunohistochemistry using antibodies to the cytolytic lymphocyte antigens CD8, CD56, CD57, TIA-1, and granzyme B. The goals were to clarify the bone marrow pathology of T-cell GLL and to refine the diagnostic criteria for T-cell GLL. Most bone marrow specimens from the T-cell GLL patients contained interstitially distributed clusters of at least 8 CD8(+) (83%) or TIA-1(+) (75%) lymphocytes or clusters of at least 6 granzyme B(+) (50%) lymphocytes. Interstitial clusters of CD8(+), TIA-1(+), or granzyme B(+) cells were present in 36%, 12%, and 0%, respectively, of the control bone marrows (all values significantly different, P <.001). An additional T-cell GLL disease-specific finding was the presence of linear arrays of intravascular CD8(+), TIA-1(+), or granzyme B(+) lymphocytes, found in 67% of cases of T-cell GLL and in none of the 25 control samples (P <.001). Staining for CD56 and CD57 was noncontributory. These findings clarify the bone marrow histopathology of T-cell GLL and provide an additional tool by which the discrete, abnormal lymphocyte population required for a diagnosis of T-cell GLL can be identified.

Allergic contact dermatitis: the cellular effectors

Contact hypersensitivity reactions are mediated by lymphocytic effector cells. Until recently it was believed that the most important of these were CD4+ T lymphocytes. However, there is growing evidence that in many instances the predominant effector cell may be a CD8+ T lymphocyte, with in some instances CD4+ cells performing a counter-regulatory function. Here we review the roles of CD4+ T helper (Th) cells and CD8+ T cytotoxic (Tc) cells, and their main functional subpopulations (respectively, Th1 and Th2 cells and Tc1 and Tc2 cells) in the elicitation of contact hypersensitivity reactions and consider the implications of effector cell selectivity for the biology of allergic contact dermatitis.

Oligoclonal T-cells in blood and target tissues of patients with anti-Hu syndrome

T-cell clones of unknown significance (TCUS), assessed by monoclonal or oligoclonal T-cell patterns in PCR-DGGE, were detected in blood of 7/9 patients with anti-Hu syndrome. Clonal patterns were also detected in 2/2 neoplastic lymph nodes, and in 2/2 inflamed dorsal root ganglia from three patients. Only some T-cell clones found in target tissues were also detected in blood or non-target tissues, and likely corresponded to TCUS. In one patient, an identical T-cell clone was found in both neoplastic lymph node tissue and dorsal root ganglia, but not in blood. Dorsal root-infiltrating lymphocytes were cytotoxic CD8(+) TIA-1(+) T-cells. They were often found in close contact to sensory neurons, most of which expressed MHC-1. Taken together, these data support a direct effector role of cytotoxic CD8(+) T-cells, the same clones being likely operative in sensory neuron damage and immune-mediated tumor growth control.

A stochastic model for CD8(+)T cell dynamics in human immunosenescence: implications for survival and longevity

We propose here a stochastic model for the CD 8(+)T lymphocyte dynamics on the long time-scale of the human lifespan. Our purpose has been to test the hypothesis, recently proposed on the basis of our experimental data (Fagnoni et al., 2000), that the depletion of virgin CD8(+)T lymphocytes can be considered a reliable biomarker related to the risk of death. This hypothesis is embedded in a more general theory of immunosenescence according to which the accumulation of antigen experienced (AE) T cells and the concomitant exhaustion of antigen non-experienced (ANE) T cells with age, mostly due to the chronic lifelong exposure to antigens, is a major characteristic of the remodeling of the human immune system with age. In our model we considered a deterministic balance of ANE and AE T cell concentrations plus a stochastic forcing, which describes the chronic antigenic stress fluctuations, assuming a mean genetically determined capability of individuals to respond to antigens. The major results of our model is the validation of the above-mentioned hypothesis, since the model is capable of fitting the experimental data concerning the changes of ANE T cell concentration over age, and at the same time to reproduce survival curves similar to the demographic ones. Furthermore, the stochastic process results in being responsible for the peculiar shape of the survival curves.

Divergence in the degree of clonal expansions in inflammatory T cell subpopulations mirrors HLA-associated risk alleles in genetically and clinically distinct subtypes of childhood arthritis

Clinically distinct forms of childhood arthritis are associated with different risk alleles of polymorphic loci within the MHC, which code for the antigen-presenting class I or class II molecules. We have compared the TCR diversity of synovial T cells from children with enthesitis-related (HLA-B27(+)) arthritis and oligoarticular arthritis (with class II MHC risk allele associations) in parallel with peripheral blood T cells from each child, using a high-resolution heteroduplex TCR analysis. We demonstrate that multiple clonal T cell expansions are present and persistent within the joint in both groups, but that there is disease-specific divergence in the dominant T cell subset containing these expansions. Thus, the largest clonotypes within the inflamed joints of children with class II-associated arthritis are within the CD4(+) synovial T cell population, while the dominant clones from children with enthesitis-related arthritis (associated with a class I allele) are within the CD8(+) synovial T cell population. These data provide powerful data to support the concept that recognition of MHC-peptide complexes by T cells plays a role in the pathogenesis of juvenile arthritis.

Kinetics of the T-cell receptor CD4 and CD8 V beta repertoire in HIV-1 vertically infected infants early treated with HAART

OBJECTIVES

To determine the kinetics and the relationship between the T-cell receptor V beta (TCRBV) complementary determining region 3 length, the CD4 T-cell count and HIV viral load changes in HIV-1 infected infants treated early with highly active antiretroviral therapy (HAART) during 1 year of follow-up.

DESIGN

Two HIV-1 vertically infected infants, two HIV-1 vertically exposed uninfected and two healthy controls were analysed by spectratyping. Evaluation of viral load, CD4 naive and memory cell counts and a proliferation test were also carried out.

METHODS

Twenty-six families and subfamilies of the TCR on CD4 and CD8 T cells were analyzed by spectratyping. Flow cytometric analysis on peripheral blood mononuclear cells for CD4CD45Ra, CD4CD45Ro, CD8CD38, proliferation tests and plasma viral load measurements were performed at baseline, 1, 6 and after 12 months of therapy.

RESULTS

HAART induced a marked reduction of viral load in both HIV-1 infected infants and an increase to normal CD4 T-cell count in the symptomatic infant. At baseline the TCRBV family distribution in the majority of CD8 and a few of the CD4 T cells was highly perturbed, with several TCRBV families showing a monoclonal/oligoclonal distribution. During HAART a normalization of the TCR repertoire in both CD8 and CD4 subsets occurred. TCR repertoire normalization was associated with a good virological and immunological response.

CONCLUSION

These results suggest that complete and early virus replication control as a result of early HAART leads to a marked reduction of T-cell oligoclonality and is an essential prerequisite to the development of a polyclonal immune response in HIV-1 infected infants.

Viral infections induce abundant numbers of senescent CD8 T cells

Viral infections are often accompanied by extensive proliferation of reactive CD8 T cells. After a defined number of divisions, normal somatic cells enter a nonreplicative stage termed senescence. In the present study we have identified the inhibitory killer cell lectin-like receptor G1 (KLRG1) as a unique marker for replicative senescence of murine CD8 T cells. KLRG1 expression was induced in a substantial portion (30-60%) of CD8 T cells in C57BL/6 mice infected with lymphocytic choriomeningitis virus (LCMV), vesicular stomatitis virus, or vaccinia virus. Similarly, KLRG1 was found on a large fraction of LCMV gp33 peptide-specific TCR-transgenic (tg) effector and memory cells activated in vivo using an adoptive transfer model. Transfer experiments with CFSE-labeled TCR-tg cells into LCMV-infected hosts further indicated that induction of KLRG1 expression required an extensive number of cell divisions. Most importantly, KLRG1(+) TCR-tg effector/memory cells could efficiently lyse target cells and secrete cytokines, but were severely impaired in their ability to proliferate after Ag stimulation. Thus, this study demonstrates that senescent CD8 T cells are induced in abundant numbers during viral infections in vivo.

Clonotypic structure of the human CD4+ memory T cell response to cytomegalovirus

High steady-state frequencies of CMV-specific CD4(+) memory T cells are maintained in CMV-exposed subjects, and these cells are thought to play a key role in the immunologic control of this permanent infection. However, the essential components of this response are poorly defined. Here, we report the use of a step-wise application of flow cytometric and molecular techniques to determine the number and size of the TCR Vbeta-defined clonotypes within freshly obtained CMV-specific CD4(+) memory T cell populations of four healthy, CMV-exposed human subjects. This analysis revealed a stable clonotypic hierarchy in which 1-3 dominant clonotypes are maintained in concert with more numerous subdominant and minor clonotypes. These dominant clonotypes accounted for 10-50% of the overall CMV response, and comprised from 0.3 to 4.0% of peripheral blood CD4(+) T cells. Two subjects displayed immunodominant responses to single epitopes within the CMV matrix phosphoprotein pp65; these single epitope responses were mediated by a single dominant clonotype in one subject, and by multiple subdominant and minor clonotypes in the other. Thus, the CMV-specific CD4(+) T cell memory repertoire in normal subjects is characterized by striking clonotypic dominance and the potential for epitope focusing, suggesting that primary responsibility for immunosurveillance against CMV reactivation rests with a handful of clones recognizing a limited array of CMV determinants. These data have important implications for the understanding of mechanisms by which a genetically stable chronic viral pathogen such as CMV is controlled, and offer possible insight into the failure of such control for a genetically flexible pathogen like HIV-1.

Differential regulation of antiviral T-cell immunity results in stable CD8+ but declining CD4+ T-cell memory

Emerging evidence indicates that CD8+ and CD4+ T-cell immunity is differentially regulated. Here we have delineated differences and commonalities among antiviral T-cell responses by enumeration and functional profiling of eight specific CD8+ and CD4+ T-cell populations during primary, memory and recall responses. A high degree of coordinate regulation among all specific T-cell populations stood out against an approximately 20-fold lower peak expansion and prolonged contraction phase of specific CD4+ T-cell populations. Surprisingly, although CD8+ T-cell memory was stably maintained for life, levels of specific CD4+ memory T cells gradually declined. However, this decay, which seemed to result from less efficient rescue from apoptosis, did not affect functionality of surviving virus-specific CD4+ T cells. Our results indicate that CD4+ T-cell memory might become limiting under physiological conditions and that conditions precipitating CD4+ T-cell loss might compromise protective immunity even in the presence of unimpaired CD8+ T-cell responses.

[Relationship between T-cells subsets and prognostic markers in HIV-1-infected children]

BACKGROUND

To investigate the relationship between peripheral blood T-cell subsets and both CD4+ T-cell percentage and viral load (VL) in HIV-1-infected children.

PATIENTS AND METHOD

We studied 50 HIV-1-infected children on antiretroviral therapy. T-cell subsets were determined by flow cytometry. The VL was quantified using standardized molecular methods.

RESULTS

Memory (CD45RO+), activated memory (CD45RO+HLA-DR+) and CD45RA-CD62L+ (memory cells expressing L-selectin) CD4+ and CD8+ T-cells correlated positively with the VL and negatively with the percentage of CD4+ T-cells. Inversely, naive CD4+ and CD8+ T-cells (CD45RA+CD62L+) correlated positively with the percentage of CD4+ T-cells and negatively with the VL. HLA-DR+, CD38+ or HLA-DR+CD38+CD4+ and CD8+ T-cells correlated also positively with the VL and negatively with the percentage of CD4+ T-cells (with the exception of CD4+CD38+ which did not show any association with the VL). CD8+CD28+ T-cells correlated positively with the percentage of CD4+ T-cells and negatively with the VL, whereas CD8+ CD57+ and CD8+CD28-CD57+ exhibited an opposite association.

CONCLUSIONS

Our data suggest a relationship between the different lymphocyte subsets (memory, naïve, activated and effector T-cells) and the most commonly used markers in clinical practice, namely the viral load and the CD4+ T-cell percentage. Some of these subsets may be useful to determine the virologic and immunologic status in HIV-1-infected children.

Evidence of T cell clonality in the infectious tolerance pathway: implications toward identification of regulatory T cells

We have shown that a rare population of regulatory CD4+ T cells plays a key role in the acquisition of infectious tolerance in rat sensitized recipients of cardiac allografts pretreated with nondepleting anti-CD4 mAb. This study was designed to analyze the TCR Vbeta expression patterns in this transplantation model. First, we used Vbeta-specific RT-PCR to show that there was no differential usage of TCR Vbeta genes by T cells mediating rejection or tolerance. Indeed, graft-infiltrating lymphocytes expressed most of the 22 known rat TCR Vbeta genes in both recipient groups, suggesting unrestricted TCR Vbeta repertoire in alloreactive T cells. Then, we applied CDR3 spectrotyping of TCR beta-chain to assess the clonality of T cells at different anatomic sites. CDR3 size restriction, indicative of the presence of T cell clones, was observed in graft-infiltrating lymphocytes but not in draining lymph nodes or spleen of tolerant hosts. Consisent with the clonal expansion, T cells in tolerated grafts exhibited the memory phenotype at a much higher percentage as compared with peripheral lymphoid organs. Moreover, in tolerated graft-infiltrating lymphocytes, the CD3 size restriction occurred in limited Vbeta gene families, with Vbeta8.1 and Vbeta18 most frequently detected. Hence, T cells at the graft site of tolerant recipients contain T cell clones expressing selective Vbeta genes. This phenotypic characteristics of the tolerogenic GILs may potentially be used as a novel marker to identify operational regulatory T cells in organ allograft recipients.

Oligoclonal T cell repertoire in cerebrospinal fluid of patients with inflammatory diseases of the nervous system

OBJECTIVE

To evaluate the T cell receptor beta chain variable region (TCRBV) gene usage ex vivo in CSF cells and peripheral blood mononuclear cells (PBMCs) collected from patients with autoimmune and inflammatory diseases of the nervous system.

METHODS

A novel sensitive seminestedpolymerase chain reaction coupled with heteroduplex analysis was developed.

RESULTS

Under these experimental conditions, the minimal number of cells required for the analysis of the whole T cell repertoire was established at 2.5x10(4)-sufficient to evaluate most of the samples collected during diagnostic lumbar punctures. In the 21 patients examined, restrictions in TCRBV gene family usage were not seen. However, using heteroduplex analysis, oligoclonal T cell expansions were found in the CSF of 13 patients and monoclonal expansions in five patients. The T cell abnormalities found did not correlate with intrathecal IgG production or with any clinical variable considered.

CONCLUSION

T cell clonal expansions, useful for further characterisation of pathogenetic T cells, can be found during the course of nervous system inflammations, but this abnormality is probably not disease specific.

The value of clonality in the diagnosis and follow-up of patients with cutaneous T-cell infiltrates

The diagnosis of early stages of cutaneous T-cell lymphoma (CTCL) is often difficult, especially for lesions that are at the borderline between reactive and neoplastic skin T-cell infiltrates. T-cell monoclonality in these lesions is considered by some to be an important prognostic factor of neoplastic evolution, whereas others claim that clonality can also be found in benign skin infiltrates and is therefore of limited diagnostic value. To address this controversy, the authors analyzed retrospectively eight patients with lymphocytic skin lesions who progressed to CTCL, and three patients with recurrent T-cell lymphocytic infiltrates who had not developed CTCL. From a total of 65 biopsies of eight progressing patients, 32 were diagnosed as histologically malignant and 33 were diagnosed as benign or borderline. The authors found clonality by either polymerase chain reaction or Southern blot analysis in 88% of malignant and in 79% of nonmalignant lesions. None of the 37 biopsies of non-progressing patients was clonal. These results indicate strongly that the presence of monoclonality in T-cell skin infiltrates is related closely to the risk of developing CTCL. The value of clonality as a marker of malignancy is supported by the absence of T-cell clonal populations in all infiltrates from patients who had not progressed to lymphoma.

The exon A (C77G) mutation is a common cause of abnormal CD45 splicing in humans

The leukocyte common (CD45) Ag is essential for normal T lymphocyte function and alternative splicing at the N terminus of the gene is associated with changes in T cell maturation and differentiation. Recently, a statistically significant association was reported in a large series of human thymus samples between phenotypically abnormal CD45 splicing and the presence of the CC chemokine receptor 5 deletion 32 (CCR5del32) allele, which confers resistance to HIV infection in homozygotes. We show here that abnormal splicing in these thymus samples is associated with the presence of the only established cause of CD45 abnormal splicing, a C77G transversion in exon A. In addition we have examined 227 DNA samples from peripheral blood of healthy donors and find no association between the exon A (C77G) and CCR5del32 mutations. Among 135 PBMC samples, tested by flow cytometric analysis, all those exhibiting abnormal splicing of CD45 also showed the exon A C77G transversion. We conclude that the exon A (C77G) mutation is a common cause of abnormal CD45 splicing and that further disease association studies of this mutation are warranted.

Identification of the T cell clones expanding within both CD8+CD28+ and CD8+CD28− T cell subsets in recipients of allogeneic hematopoietic cell grafts and its implication in post-transplant skewing of T cell receptor repertoire

We have previously reported that skewed repertoires of T cell receptor-beta chain variable region (TCRBV) and TCR-alpha chain variable region (TCRAV) are observed at an early period after allogeneic hematopoietic cell transplantation. Furthermore, we found that T lymphocytes using TCRBV24S1 were increased in 28% of the recipients of allogeneic grafts and an increase of TCRBV24S1 usage was shown to result from clonal expansions. Interestingly, the arginine residue was frequently present at the 3' terminal of BV24S1 segment and was followed by an acidic amino acid residue within the CDR3 region. These results suggest that these clonally expanded T cells are not randomly selected, but are expanded by stimulation with specific antigens. This study was undertaken to elucidate the mechanisms of the post-transplant skewing of TCR repertoires. Since the CD8(+)CD28(-)CD57(+) T cell subset has been reported to expand in the peripheral blood of patients receiving allogeneic hematopoietic cell grafts, we examined the TCRAV and TCRBV repertoires of the CD8(+)CD28(-) T cell and CD8(+)CD28(+) T cell subsets, and also determined the clonality of both T cell populations. In all three recipients examined, the CD8(+)CD28(-) T cell subset appeared to define the post-transplant TCR repertoire of circulating blood T cells. Moreover, the CDR3 length of TCRBV imposed constraints in both CD8(+)CD28(-) T cell and CD8(+)CD28(+) T cell subsets. The DNA sequences of the CDR3 region were determined, and the same clones were identified within both CD8(+)CD28(-) and CD8(+)CD28(+) T cell subsets in the same individuals. These results suggest that the clonally expanded CD8(+)CD28(-) T cells after allogeneic hematopoietic cell transplantation derive from the CD8(+)CD28(+) T cell subset, possibly by an antigen-driven mechanism, resulting in the skewed TCR repertoire.

Oligoclonal expansion of CD4(+)CD28(-) T lymphocytes in recipients of allogeneic hematopoietic cell grafts and identification of the same T cell clones within both CD4(+)CD28(+) and CD4(+)CD28(-) T cell subsets

Recipients of allogeneic bone marrow grafts have clonally expanded CD8(+)CD28(-) T lymphocytes during the early period after transplantation, which leads to skewing of T cell receptor (TCR) repertoires. Here, we have addressed the question of whether clonal expansion of CD28(-) T cells is also observed in CD4(+) T lymphocytes after human allogeneic hematopoietic cell transplantation. We found that the fraction of T cells lacking CD28 expression in the CD4(+) subset was increased after transplantation, and expanded CD4(+)CD28(-) T lymphocytes carrying certain TCRBV subfamilies showed limited TCR diversity. In order to further study the ontogeny of CD4(+)CD28(-) T cells, we analyzed the complementarity-determining region 3 (CDR3) of the TCR-beta chain of CD4(+)CD28(+) and CD4(+)CD28(-) cells. We identified the same T cell clones within both CD4(+)CD28(-) and CD4(+)CD28(+) T cell subsets. These results suggest that both subsets are phenotypic variants of the same T cell lineage.

Kinetics of TCR use in response to repeated epitope-specific immunization

Selection of T cell-directed immunization strategies is based extensively on discordant information derived from preclinical models. We characterized the kinetics of T cell selection in response to repeated antigenic challenge. By enumerating with epitope/HLA tetrameric complexes (tHLA) vaccine-elicited T cell precursor frequencies (Tc-pf) in melanoma patients exposed to the modified gp100 epitope gp100:209-217 (g209-2M) we observed in most patients that the Tc-pf increased with number of immunizations. One patient's kinetics were further characterized. Dissociation kinetics of g209-2M/tHLA suggested enrichment of T cell effector populations expressing TCR with progressively higher affinity. Furthermore, vaccine-elicited T cells maintained the ability to express IFN-gamma ex vivo and proliferate in vitro. Thus, repeated exposure to immunogenic peptides benefited immune competence. These results provide a rationale for immunization strategies.

Extensive clonal expansion of T lymphocytes causes contracted diversity of complementarity-determining region 3 and skewed T cell receptor repertoires after allogeneic hematopoietic cell transplantation

We previously described skewed repertoires of the T cell receptor-beta chain variable region (TCRBV) and the TCR-alpha chain variable region (TCRAV) soon after allogeneic hematopoietic cell transplantation. To determine the characteristics of skewed TCRBV after transplantation, we examined the clonality of T lymphocytes carrying skewed TCRBV subfamilies and determined the CDR3 sequences of expanded T cell clones. In all 11 recipients examined, TCR repertoires were skewed, with an increase of certain TCRBV subfamilies that differed among individuals. In nine of 11 patients, clonal/oligoclonal T cell expansion was observed, although the expanded T cells were not necessarily oligoclonal. The extent of expansion after transplantation appeared to predict clonality. The arginine (R)-X-X-glycine (G) sequence was identified in clonally expanded T cells from four of five recipients examined, and glutamic acid (E), aspartic acid (D) and alanine (A) were frequently inserted between R and G. These results suggest that T lymphocyte expansion may result from the response to antigens widely existing in humans, and that the extensive clonal expansion of a limited number of T cells leads to contracted CDR3 diversity and post-transplant skewed TCR repertoires.

The developing human immune system: T-cell receptor repertoire of children and young adults shows a wide discrepancy in the frequency of persistent oligoclonal T-cell expansions

While the T-cell receptor (TCR) repertoire of the newborn is highly diverse, a gradual alteration in diversity of the expressed TCR repertoire, in particular the oligoclonality of CD8+ T cells, occurs with increasing age. The timing of the initiation of this process is unknown. These changes are associated with an accumulation of T-cell expansions, thought to be in response to chronic antigen stimulation, frequently by persistent viruses such as Epstein-Barr virus (EBV) and cytomegalovirus (CMV). Using reverse transcription-polymerase chain reaction heteroduplex analysis we have characterized the TCR expression of CD4 and CD8 cells from healthy young children and adults in order to delineate the age range at which these oligoclonal populations appear. We demonstrate that considerable oligoclonality can occur, even in healthy young children, and also that these expanded clonotypes persist. These are shown by heteroduplex to be exclusively within the CD28- subpopulation. The presence of such oligoclonal expansions correlates closely with the percentage of CD8+ cells that have the CD28- phenotype. However, we also show that control of chronic infection with EBV or CMV may coexist with a highly diverse, polyclonal TCR repertoire well into adulthood. These studies suggest that many factors affect the overall regulation of clone size in response to chronic antigens during the development of the immune system.

The flow cytometric analysis of telomere length in antigen-specific CD8+ T cells during acute Epstein-Barr virus infection

Acute infectious mononucleosis (AIM) induced by Epstein-Barr virus (EBV) infection is characterized by extensive expansion of antigen-specific CD8+ T cells. One potential consequence of this considerable proliferative activity is telomere shortening, which predisposes the EBV-specific cells to replicative senescence. To investigate this, a method was developed that enables the simultaneous identification of EBV specificity of the CD8+ T cells, using major histocompatibility complex (MHC) class I/peptide complexes, together with telomere length, which is determined by fluorescence in situ hybridization. Despite the considerable expansion, CD8+ EBV-specific T cells in patients with AIM maintain their telomere length relative to CD8+ T cells in normal individuals and relative to CD4+ T cells within the patients themselves and this is associated with the induction of the enzyme telomerase. In 4 patients who were studied up to 12 months after resolution of AIM, telomere lengths of EBV-specific CD8+ T cells were unchanged in 3 but shortened in one individual, who was studied only 5 months after initial onset of infection. Substantial telomere shortening in EBV-specific CD8+ T cells was observed in 3 patients who were studied between 15 months and 14 years after recovery from AIM. Thus, although telomerase activation may preserve the replicative potential of EBV-specific cells in AIM and after initial stages of disease resolution, the capacity of these cells to up-regulate this enzyme after restimulation by the persisting virus may dictate the extent of telomere maintenance in the memory CD8+ T-cell pool over time.

Long-term immunological memory against viruses

This review addresses a novel facet of human T cell biology that constitutes a fundamental problem for long-term maintenance of immunological memory against viruses. The finite proliferative capacity of human T lymphocytes is sufficiently great to accommodate the waves of clonal expansion associated with primary and even secondary immune responses. However, long-term memory to viruses that establish latency and to repeatedly encountered viruses such as influenza may be severely impaired by "replicative senescence", a genetically programmed process affecting most somatic cell types of human origin. Consistent with this idea, memory CD8+ T cells with hallmarks of replicative senescence have been identified in vivo. Such cells may contribute to compromised viral immunity and response to vaccines, and furthermore, their very presence may negatively influence homeostatic mechanisms that control the size of the memory T cell pool in elderly persons.

Rapid whole blood analysis of virus-specific CD4 and CD8 T cell responses in persistent HIV infection

OBJECTIVES

Upon HIV infection, strong antiviral cytotoxic and helper T cell responses are generated. They are considered to be an important component in the control of HIV viral load. A simple and rapid whole blood assay was established to quantify and simultaneously characterize HIV-reactive CD4 and CD8 cells. The assay was applied to evaluate the effect of antiretroviral therapy on HIV-specific T cell responses.

METHODS

Whole blood of 33 HIV-infected individuals was specifically stimulated by HIV-1 Pr55gag, and activation-induced intracellular cytokine expression in CD4 and CD8 T cells was analysed by flow cytometry.

RESULTS

HIV-1-specific CD8 and CD4 T cells can be quantified simultaneously. As specific antigen, HIV-1 Pr55gag virus-like particles were superior to soluble protein, especially for the activation of CD8 T cells. In untreated individuals, a high frequency of HIV-specific T cells was observed. The frequency of CD8 T cells was consistently higher than the respective CD4 T cell response, thus demonstrating a dominance in CD8 T cell expansion in persistent HIV infection. Patients on antiretroviral therapy showed a significant reduction in HIV-specific CD4 and, even more strikingly, CD8 T cells.

CONCLUSION

The whole blood assay provides a rapid estimate of the total antiviral T cell resources, and is highly suited for a clinical setting. It may thus have widespread applications for the evaluation of vaccination strategies and immunotherapy. Because antiretroviral therapy significantly reduces both HIV-specific cytotoxic and helper T cell responses, future therapeutic strategies should aim at improving cellular antiviral immunity.

Clonal expansions in acute EBV infection are detectable in the CD8 and not the CD4 subset and persist with a variable CD45 phenotype

We have applied a sensitive global analysis of TCR heterogeneity to compare clonal dynamics of CD4(+) and CD8(+) T cells in acute infectious mononucleosis. Using this approach, we are able to identify a broad representation of the total virus-specific population without the bias of in vitro culture and then to track their phenotype and fate by their unique molecular footprint. We demonstrate a large number of Ag-driven clones using different TCRs in the acute phase, all CD8(+). The diverse large clones generated in the CD8 subset in response to this virus contrast with the complete lack of detectable clonal expansion in the CD4 compartment. Many of the same clones remain detectable in directly ex vivo CD8(+) T cells for at least a year after resolution of infectious mononucleosis, although the clone size is reduced. Thus, memory CD8 cells following EBV infection persist at relatively high circulating frequency and represent a subset of the large range of clonotypes comprising the acute effectors. Separation of samples into CD45RA (naive) and CD45RO (memory) fractions shows the accumulation of identical CDR3 region defined clonotypes in both CD45RO and CD45RA fractions and sequencing confirms that dominant long-lived monoclonal expansions can reside in the CD45RA pool.

Differential splicing of antigen-encoding RNA reduces endogenous epitope presentation that regulates the expansion and cytotoxicity of T cells

The activation of CTLs is dependent on the recognition of MHC-bound peptide present on the surface of APCs. We give evidence in this study that differential splicing of Ag-encoding RNA can decrease the antigenic dose in APCs and regulate the recall of human memory CTLs. Differential splicing of RNA that encoded an immunodominant HLA-B8-restricted CTL epitope of EBV reduced the functional presentation of this epitope, and consequently the in vitro expansion and activity of CTLs, as measured by MHC/peptide-tetramer staining and cytotoxicity assays. The reduced activity of the stimulated CTLs was not only due to lower numbers of Ag-specific CTLs but, surprisingly, was also characterized by decreased cytotoxicity of the CTLs to target cells presenting limiting amounts of the peptide epitope. As indicated by TCR repertoire analysis, the reduction in CTL activity was not caused by stimulation of distinct populations of TCR clonotypes. This study demonstrates how a common eukaryotic posttranscriptional mechanism of gene regulation can modulate the endogenous presentation of Ag and ultimately contribute to the fine tuning of immunological memory cells, which are important in the fight against pathogens and tumors and in autoimmunity.

Role of interferon-alpha and clonally expanded T cells in the immunotherapy of chronic myelogenous leukemia

Twenty five percent of patients in the chronic phase of chronic myelogenous leukemia (CML) are treated with interferon-alpha (IFN-alpha) to induce a cytogenic remission. In addition to its direct effects on leukemic cells, IFN-alpha has been shown to induce immunologic alterations, including upregulation of the expression of major histocompatibility (MHC) antigens in antigen-presenting cells (APCs), as well as augmentation of the activity of the lymphocytes against tumor cells. However, there has been little direct evidence supporting a causal interaction between cellular immunoreactivity and clinical responsiveness to IFN-alpha. We have shown that one approach to elucidate the immunological mechanisms by which IFN-alpha exerts its anti-CML activity is by analyzing therapy-induced modulation in T-cell receptor (TCR) Vbeta chain usage, using the reverse transcription-polymerase chain reaction (RT-PCR) followed by single-strand conformation (SSCP) analysis. This method is particularly attractive, since it provides an index of antigen-specific T cell expansion, but does not require the extraction and purification of the antigens involved in the T-cell response. T cell clones that express the Vbeta 10, 12, and 14 families predominate in the peripheral blood (PB) of CML patients. The enhanced expression of the Vbeta 9 and 20 families has been detected in IFN-alpha responsive patients but not patients who are poorly responsive to this agent. This suggests that expansion of T cells expressing these TCR Vbeta gene families may serve as a prognostic factors of the clinical responsiveness of CML patients to IFN-alpha. In addition, since T cell clones that express certain Vbeta families may react with a discrete set of antigenic peptides presented on the surface of malignant cells, a better understanding of the immunobiology of T cells in CML may allow for the design of increasing efficacious immune therapy for this disease.

Reduction of the HIV-1-infected T-cell reservoir by immune activation treatment is dose-dependent and restricted by the potency of antiretroviral drugs

BACKGROUND

Treatments combining T-cell activating agents and potent antiretroviral drugs have been proposed as a possible means of reducing the reservoir of long-lived HIV-1 infected quiescent CD4 T-cells.

OBJECTIVE

To analyse the effect of such therapies on HIV-1 dynamics and T-cell homeostasis.

DESIGN AND METHODS

A mathematical framework describing HIV-1 dynamics and T-cell homeostasis was developed. Three patients who were kept on a particularly potent course of highly active antiretroviral therapy (HAART) were treated with the anti-CD3 monoclonal antibody OKT3 and interleukin (IL)-2. Plasma HIV-RNA, and HIV-RNA and DNA in peripheral blood mononuclear cells and lymph node mononuclear cells were measured. These results and other published studies on the use of IL-2 alone were assessed using our mathematical framework.

RESULTS

We show that outcome of treatment is determined by the relative rates of depletion of the infected quiescent T-cell population by activation and of its replenishment through new infection. Which of these two processes dominates is critically dependent on both the potency of HAART and also the degree of T-cell activation induced. We demonstrate that high-level T-cell stimulation is likely to produce negative outcomes, both by failing to reduce viral reservoirs and by depleting the CD4 T-cell pool and disrupting CD4/CD8 T-cell homeostasis. In contrast, repeated low-level stimulation may both aid CD4 T-cell pool expansion and achieve a substantial reduction in the long-lived HIV-1 reservoir.

CONCLUSIONS

Our analysis suggests that although treatment that activates T-cells can reduce the long-lived HIV-1 reservoir, caution should be used as high-level stimulation may result in a negative outcome.

Initiation of antiretroviral therapy during primary HIV-1 infection induces rapid stabilization of the T-cell receptor β chain repertoire and reduces the level of T-cell oligoclonality

Major T-cell receptor β chain variable region (TCRBV) repertoire perturbations are temporally associated with the down-regulation of viremia during primary human immunodeficiency virus (HIV) infection and with oligoclonal expansion and clonal exhaustion of HIV-specific cytotoxic T lymphocytes (CTLs). To determine whether initiation of antiretroviral therapy (ART) or highly active antiretroviral therapy (HAART) during primary infection influences the dynamics of T-cell–mediated immune responses, the TCRBV repertoire was analyzed by semiquantitative polymerase chain reaction in serial blood samples obtained from 11 untreated and 11 ART-treated patients. Repertoire variations were evaluated longitudinally. Stabilization of the TCRBV repertoire was more consistently observed in treated as compared with untreated patients. Furthermore, the extent and the rapidity of stabilization were significantly different in treated versus untreated patients. TCRBV repertoire stabilization was positively correlated with the slope of HIV viremia in the treated group, suggesting an association between repertoire stabilization and virologic response to treatment. To test whether stabilization was associated with variations in the clonal complexity of T-cell populations, T-cell receptor (TCR) heteroduplex mobility shift assays (HMAs) were performed on sequential samples from 4 HAART-treated subjects. Densitometric analysis of HMA profiles showed a reduction in the number of TCR clonotypes in most TCRBV families and a significant decrease in the total number of clonotypes following 7 months of HAART. Furthermore, a biphasic decline in HIV-specific but not heterologous CTL clones was observed. This indicates that ART leads to a global reduction of CD8+T-cell oligoclonality and significantly modulates the mobilization of HIV-specific CTL during primary infection.

Is immune senescence reversible?

Many genes have been shown to be involved in the decline in immune function of the elderly. However, normal numbers of myeloid and lymphoid colonies can be grown from elderly bone marrow under optimal conditions and some thymic function is preserved well into adult life. It may also be possible to reverse partially declining thymic function by IL-7 treatment. Peripheral B and T cells show evidence of dysregulation with production of large clones, changes in subset distribution and altered signalling and cytokine production, particularly decreased IL-2 production in the mouse. The identification of these defects may lead to relatively simple procedures to improve vaccination for the elderly.

Protective DNA vaccination against organ-specific autoimmunity is highly specific and discriminates between single amino acid substitutions in the peptide autoantigen

DNA vaccines that encode encephalitogenic sequences in tandem can protect from subsequent experimental autoimmune encephalomyelitis induced with the corresponding peptide. The mechanism for this protection and, in particular, if it is specific for the amino acid sequence encoding the vaccine are not known. We show here that a single amino acid exchange in position 79 from serine (nonself) to threonine (self) in myelin basic protein peptide MBP68-85, which is a major encephalitogenic determinant for Lewis rats, dramatically alters the protection. Moreover, vaccines encoding the encephalitogenic sequence MBP68-85 do not protect against the second encephalitogenic sequence MBP89-101 in Lewis rats and vice versa. Thus, protective immunity conferred by DNA vaccination exquisitely discriminates between peptide target autoantigens. No bystander suppression was observed. The exact underlying mechanisms remain elusive because no simple correlation between impact on ex vivo responses and protection against disease were noted.

Biomarkers of immunosenescence within an evolutionary perspective: the challenge of heterogeneity and the role of antigenic load

Under an evolutionary perspective, antigens can be considered nothing else than chronic stressors that constituted the major selective pressure for immune system emergence and evolution. In this review, recent data are discussed under the hypothesis that human immunosenescence is the consequence of the continuous attrition caused by chronic antigenic overload/stress. The advantage of this theoretical approach is that a unifying hypothesis is proposed, which tries to fill in the current gap between the conceptualizations concerning the mechanisms which counteract aging and favor longevity in invertebrates and vertebrates. The hypothesis is that the immune system is, at a higher level of biological organization and complexity, the counterpart of the anti-stress response network identified in invertebrates as the major determinant of survival. We argue that some of the most important characteristics of immunosenescence, i.e. the accumulation and the clonal expansion of memory and effector T cells, the reduction/exhaustion of naive T cells, and the shrinkage of T cell repertoire, are compatible with this assumption. Thus, immunosenescence can be envisaged as a global reduction of the "immunological space." Concomitantly, immunosenescence results in the progressive generation of cellular mosaicism which is the consequence of the heterogeneous replicative histories and telomere shortening of T and B cell subsets, as well as hemopoietic stem cells. Most of the parameters affected by immunosenescence appear to be under genetic control, and future research on biomarkers should address this point. On the whole, immunosenescence can be taken as a proof that the beneficial effects of the immune system, devoted to the neutralization of dangerous/harmful agents early in life and in adulthood, turn to be detrimental late in life, in a period largely not foreseen by evolution. This perspective fits with basic assumptions of evolutionary theories of aging, such as antagonistic pleiotropy.

A clonal view of alphabeta T cell responses

Quantitative analyses of antigen (Ag)-specific alphabeta T cell populations have provided a large body of information on the natural course of T cell immune responses. New tools are now available to determine the clonal composition of Ag-specific pools in individual responders, an approach which offers direct insights into the generation of T cell immune responses and establishment of protective immunity. The present review discusses the parameters that determine the composition of Ag-specific T cell responses. Emphasis is placed on the role of the naive alphabeta T cell repertoire and on the dynamics of individual Ag-specific T cell clones during the successive phases of an immune response.