HIV-1-specific CD4+ T lymphocyte turnover and activation increase upon viral rebound

Elucidating Specificity Opens a Window to the Complexity of Both the Innate and Adaptive Immune Systems

Science is a tedious and painstaking business. Many discoveries are considered incremental, individually not necessarily earth shattering, but collectively providing the critical broad framework on which pivotal insights can emerge. Transformational discoveries spring from this knowledge legacy of others and spur a fervent discovery process, often driven by technological developments. The seminal discovery of major histocompatibility class restriction I (MHCI) and its role in antiviral infections by Doherty and Zinkernagel in 1974 was one such discovery—the key that unlocked the treasure chest to the rich tapestry of the diversity of the immune system. An army of researchers have teased apart the different elements of the immune response, which now brings us to a deeper understanding of immune memory and protective immunity. In this process, it has uncovered a multitude of cell types that bridge the innate and adaptive arms of the immune system—blurring the line between these two branches—and ultimately fortifying the development of long-term immune protection.

Maternal-Fetal rejection reactions are unconstrained in preeclamptic women

The risk factors for preeclampsia, extremes of maternal age, changing paternity, concomitant maternal autoimmunity, and/or birth intervals greater than 5 years, suggest an underlying immunopathology. We used peripheral blood and lymphocytes from the UteroPlacental Interface (UPI) of 3rd trimester healthy pregnant women in multicolor flow cytometry-and in vitro suppression assays. The major end-point was the characterization of activation markers, and potential effector functions of different CD4-and CD8 subsets as well as T regulatory cells (Treg). We observed a significant shift of peripheral CD4 -and CD8- T cells from naïve to memory phenotype in preeclamptic women compared to healthy pregnant women consistent with long-standing immune activation. While the proportions of the highly suppressive Cytokine and Activated Treg were increased in preeclampsia, Treg tolerance toward fetal antigens was dysfunctional. Thus, our observations indicate a long-standing inflammatory derangement driving immune activation in preeclampsia; in how far the Treg dysfunction is caused by/causes this immune activation in preeclampsia will be the object of future studies.

Cytomegalovirus Infection Leads to Development of High Frequencies of Cytotoxic Virus-Specific CD4+ T Cells Targeted to Vascular Endothelium

Cytomegalovirus (CMV) infection elicits a very strong and sustained intravascular T cell immune response which may contribute towards development of accelerated immune senescence and vascular disease in older people. Virus-specific CD8+ T cell responses have been investigated extensively through the use of HLA-peptide tetramers but much less is known regarding CMV-specific CD4+ T cells. We used a range of HLA class II-peptide tetramers to investigate the phenotypic and transcriptional profile of CMV-specific CD4+ T cells within healthy donors. We show that such cells comprise an average of 0.45% of the CD4+ T cell pool and can reach up to 24% in some individuals (range 0.01–24%). CMV-specific CD4+ T cells display a highly differentiated effector memory phenotype and express a range of cytokines, dominated by dual TNF-α and IFN-γ expression, although substantial populations which express IL-4 were seen in some donors. Microarray analysis and phenotypic expression revealed a profile of unique features. These include the expression of CX3CR1, which would direct cells towards fractalkine on activated endothelium, and the β2-adrenergic receptor, which could permit rapid response to stress. CMV-specific CD4+ T cells display an intense cytotoxic profile with high level expression of granzyme B and perforin, a pattern which increases further during aging. In addition CMV-specific CD4+ T cells demonstrate strong cytotoxic activity against antigen-loaded target cells when isolated directly ex vivo. PD-1 expression is present on 47% of cells but both the intensity and distribution of the inhibitory receptor is reduced in older people. These findings reveal the marked accumulation and unique phenotype of CMV-specific CD4+ T cells and indicate how such T cells may contribute to the vascular complications associated with CMV in older people.

Cytomegalovirus (CMV) is a member of the herpesvirus family and most humans carry chronic CMV infection. This drives the development of large expansions of CD8+ CMV-specific T cells, which increase further during ageing. CMV infection is associated with vascular disease and increased risk of mortality in older people, which may be related to damage from this CMV-specific immune response. Here we used a set of novel reagents called HLA class II tetramers to make a detailed study of CMV-specific CD4+ T cells. We show that CMV-specific CD4+ T cells are found at remarkably high frequencies within blood, representing up to a quarter of all such white cells. In addition they demonstrate a range of unique features. Firstly they carry a chemokine receptor that directs the cells to activated endothelial cells within blood vessels. Secondly, they express epinephrine receptors which would allow them to respond rapidly to stress. Finally, these CD4+ T cells are unique as they are strongly cytotoxic and equipped with the ability to directly kill virally-infected cells. HLA class II tetramers therefore reveal a profile of unique features which provide insight into how CMV-specific CD4+ T cells may be involved in vascular immunopathology.

MHC II tetramers visualize human CD4+ T cell responses to Epstein-Barr virus infection and demonstrate atypical kinetics of the nuclear antigen EBNA1 response

Characterization of the human EBV-specific CD4⁺ T cell response using MHC II tetramers reveals the latent EBV antigen response is more frequent than the lytic response with a delayed EBNA1 response that coincides with diminished cross-presentation.

Virus-specific CD4⁺ T cells are key orchestrators of host responses to viral infection yet, compared with their CD8⁺ T cell counterparts, remain poorly characterized at the single cell level. Here we use nine MHC II–epitope peptide tetramers to visualize human CD4⁺ T cell responses to Epstein–Barr virus (EBV), the causative agent of infectious mononucleosis (IM), a disease associated with large virus-specific CD8⁺ T cell responses. We find that, while not approaching virus-specific CD8⁺ T cell expansions in magnitude, activated CD4⁺ T cells specific for epitopes in the latent antigen EBNA2 and four lytic cycle antigens are detected at high frequencies in acute IM blood. They then fall rapidly to values typical of life-long virus carriage where most tetramer-positive cells display conventional memory markers but some, unexpectedly, revert to a naive-like phenotype. In contrast CD4⁺ T cell responses to EBNA1 epitopes are greatly delayed in IM patients, in line with the well-known but hitherto unexplained delay in EBNA1 IgG antibody responses. We present evidence from an in vitro system that may explain these unusual kinetics. Unlike other EBNAs and lytic cycle proteins, EBNA1 is not naturally released from EBV-infected cells as a source of antigen for CD4⁺ T cell priming.

HIV controllers maintain a population of highly efficient Th1 effector cells in contrast to patients treated in the long term

HIV controllers are rare individuals who spontaneously control HIV replication in the absence of antiretroviral therapy. To identify parameters of the CD4 response that may contribute to viral control rather than merely reflect a persistently low viremia, we compared the T helper profiles in two groups of patients with more than 10 years of viral suppression: HIV controllers from the Agence Nationale de Recherche sur le SIDA et les Hépatites Virales (ANRS) CO18 cohort (n = 26) and efficiently treated patients (n = 16). Cells specific for immunodominant Gag and cytomegalovirus (CMV) peptides were evaluated for the production of 10 cytokines and cytotoxicity markers and were also directly quantified ex vivo by major histocompatibility complex (MHC) class II tetramer staining. HIV controller CD4(+) T cells were characterized by a higher frequency of gamma interferon (IFN-γ) production, perforin(+)/CD107a(+) expression, and polyfunctionality in response to Gag peptides. While interleukin 4 (IL-4), IL-17, and IL-21 production did not differ between groups, the cells of treated patients produced more IL-10 in response to Gag and CMV peptides, pointing to persistent negative immunoregulation after long-term antiretroviral therapy. Gag293 tetramer-positive cells were detected at a high frequency (0.12%) and correlated positively with IFN-γ-producing CD4(+) T cells in the controller group (R = 0.73; P = 0.003). Tetramer-positive cells were fewer in the highly active antiretroviral therapy (HAART) group (0.04%) and did not correlate with IFN-γ production, supporting the notion of a persistent immune dysfunction in HIV-specific CD4(+) T cells of treated patients. In conclusion, HIV controllers maintained a population of highly efficient Th1 effectors directed against Gag in spite of a persistently low antigenemia, while patients treated in the long term showed a loss of CD4 effector functions.

Safety, immunogenicity and efficacy of pneumococcal conjugate vaccine in HIV-infected individuals

Streptococcus pneumoniae is the leading bacterial opportunistic infection in HIV-infected individuals. Anti-retroviral treatment (ART) of HIV-infected individuals reduces their risk of invasive pneumococcal disease (IPD), however, it remains 20- to 40-fold greater compared with age-matched general population. This review summarizes the available published data on the immunogenicity, safety and efficacy of pneumococcal polysaccharide-protein conjugate vaccines (PCV) in HIV-infected children and adults.   Several studies have demonstrated that PCV are safe in the HIV-infected persons. Although PCV are immunogenic in HIV-infected infants, the antibodies produced are functionally impaired, there is possibly a lack or loss of anamnestic responses and immunity declines in later life However, quantitative and qualitative antibody responses to PCV in HIV-infected infants are enhanced when vaccination occurs whilst on ART, as well as if vaccination occurs when the CD4+ cell percentage is ≥ 25% and if the nadir CD4+ is > 15%. Although the efficacy of PCV was lower, the vaccine preventable burden of hospitalization for IPD and clinical pneumonia were 18-fold and 9-fold greater, respectively, in HIV-infected children compared with -uninfected children. In HIV-infected adults, PCV vaccination induces more durable and functional antibody responses in individuals on ART at the time of vaccination than in ART-naive adults, independently of baseline CD4+ cell count, although there does not appear to be much benefit from a second-dose of PCV. PCV has also been shown to reduce the risk of recurrent IPD by 74% in HIV-infected adults not on ART, albeit, also with subsequent decline in immunity and protection.

Interruptions of antiretroviral therapy in human immunodeficiency virus infection: are they detrimental to neurocognitive functioning?

Because interruptions of antiretroviral treatment may entail clinical risks for human immunodeficiency virus (HIV)-infected individuals, we investigated their impact on neurocognitive functioning. Cross-sectional study was carried out, comparing HIV-infected persons who had interrupted antiretroviral therapy in the past (interruption group, IG) with persons who had never discontinued therapy (noninterruption group, NIG). Interruption was defined as the discontinuation of highly active antiretroviral therapy (HAART) for more than 15 days after previous treatment of at least 15 days. All the participants were on therapy. Demographic, clinical, and neurocognitive variables were assessed. The primary end point was the percentage of people with neurocognitive impairment. The score in different neurocognitive domains was a secondary end point. A total of 83 subjects participated in the study (IG: n = 27; NIG: n = 56). Demographic and clinical characteristics were balanced between the groups, except for years since HIV diagnosis (IG, 13.8; NIG, 10.2 [P = .003]). The percentage of people with neurocognitive impairment was significantly higher in the IG group (IG, 59.25%; NIG, 33.92% [P = 0.02]). As for scores in neurocognitive domains, individuals in the IG showed worse neurocognitive functioning, and significant differences in attention/working memory and information processing speed were found. The adjusted analysis supported the unadjusted analysis. In this study, a higher prevalence of neurocognitive impairment was detected in HIV-infected persons who had interrupted antiretroviral therapy in the past. Additionally, neurocognitive functioning was observed to be more impaired in the same individuals. Further studies should examine the potential negative effects of antiretroviral therapy interruptions on neurocognitive functioning.

Regulation of virus-specific CD4+ T cell function by multiple costimulatory receptors during chronic HIV infection

Elevated expression of inhibitory receptors on virus-specific T cells has been implicated as a mechanism by which viruses evade host immune surveillance. Blockade of these pathways during chronic infection leads to increased T cell function and improved immune control of viral replication. To explore the association between costimulatory receptors and HIV replication, we examined the expression of programmed death 1 (PD-1), CTLA-4, T cell Ig domain and mucin domain 3 (TIM-3), and CD28 on HIV-specific CD4(+) T cells from HIV-infected subjects. Greater than 30% of HIV-specific CD4(+) T cells from untreated subjects coexpressed PD-1, CTLA-4, and TIM-3, whereas <2% of CMV- or varicella-zoster virus-specific CD4(+) T cells expressed all three receptors. Coexpression of all three inhibitory receptors on HIV-specific CD4(+) T cells was more strongly correlated with viral load compared with the expression of each receptor individually. Suppression of HIV replication with antiretroviral therapy was associated with decreased expression of all three inhibitory receptors on HIV-specific CD4(+) T cells. Surprisingly, a high percentage of HIV-specific CD4(+) T cells that expressed inhibitory receptors also coexpressed CD28. In vitro blockade of PD-1 binding concurrent with stimulation through CD28 synergistically increased HIV-specific CD4(+) T cell proliferation to a greater extent than did either alone. These findings indicate that HIV-specific CD4(+) T cell responses during chronic infection are regulated by complex patterns of coexpressed inhibitory receptors and that the synergistic effect of inhibitory receptor blockade and stimulation of costimulatory receptors could be used for therapeutic augmentation of HIV-specific CD4(+) T cell function.

HIV-1 infection impairs the bronchoalveolar T-cell response to mycobacteria

RATIONALE

The risk of developing active tuberculosis in persons with latent Mycobacterium tuberculosis infection is substantially increased shortly after HIV-1 seroconversion. Immune responses in the lung are important to restrict the growth of M. tuberculosis to prevent the development of disease.

OBJECTIVES

To investigate innate and adaptive immune responses to M. tuberculosis in bronchoalveolar lavage from HIV-1-infected persons without active tuberculosis.

METHODS

Peripheral blood was drawn and bronchoalveolar lavage (BAL) performed on healthy, HIV-1-uninfected (n = 21) and HIV-1-infected (n = 15) adults. Growth of M. tuberculosis was assessed in monocytes and alveolar macrophages. Cytokine expression by mycobacteria-specific CD4 and CD8 T cells was measured by intracellular cytokine staining or IFN-gamma ELISpot.

MEASUREMENTS AND MAIN RESULTS

Mycobacterial growth in monocytes or alveolar macrophages from HIV-1-infected and -uninfected persons did not differ. Total CD4 T-cell frequencies in BAL were lower in HIV-1-infected than in HIV-1-uninfected persons (P < 0.001). Mycobacteria (bacillus Calmette-Guérin)-specific CD4 T-cell responses in BAL were severely impaired: Frequencies of cells expressing IFN-gamma or tumor necrosis factor (TNF)-alpha, as well as polyfunctional cells, expressing IFN-gamma, TNF-alpha, and IL-2 together, were lower in HIV-1-infected persons than in uninfected controls (P < 0.01 for all).

CONCLUSIONS

In addition to a total CD4 T-cell deficit, the function of mycobacteria-specific CD4 T cells is significantly impaired in the lung of HIV-1-infected persons, which may account for the HIV-1-associated elevated risk for developing tuberculosis.

Human immunodeficiency virus type 1 Nef induces programmed death 1 expression through a p38 mitogen-activated protein kinase-dependent mechanism

Chronic viral infection is characterized by the functional impairment of virus-specific T-cell responses. Recent evidence has suggested that the inhibitory receptor programmed death 1 (PD-1) is specifically upregulated on antigen-specific T cells during various chronic viral infections. Indeed, it has been reported that human immunodeficiency virus (HIV)-specific T cells express elevated levels of PD-1 and that this expression correlates with the viral load and inversely with CD4(+) T-cell counts. More importantly, antibody blockade of the PD-1/PD-L1 pathway was sufficient to both increase and stimulate virus-specific T-cell proliferation and cytokine production. However, the mechanisms that mediate HIV-induced PD-1 upregulation are not known. Here, we provide evidence that the HIV type 1 (HIV-1) accessory protein Nef can transcriptionally induce the expression of PD-1 during infection in vitro. Nef-induced PD-1 upregulation requires its proline-rich motif and the activation of the downstream kinase p38. Further, inhibition of Nef activity by p38 MAPK inhibitor effectively blocked PD-1 upregulation, suggesting that p38 MAPK activation is an important initiating event in Nef-mediated PD-1 expression in HIV-1-infected cells. These data demonstrate an important signaling event of Nef in HIV-1 pathogenesis.

Human immunodeficiency virus (HIV)-specific T helper responses fail to predict CD4+ T cell decline following short-course treatment at primary HIV-1 infection

Early anti-retroviral treatment (ART) in primary human immunodeficiency virus (HIV) infection (PHI) may have unique, restorative immunological and virological benefits which could enhance clinical outcomes. However, the sustainability of these HIV-specific immune responses and their impact on clinical outcome remains unclear. We present a 3-year longitudinal clinical and immunological follow-up of a single-arm, prospective study assessing the long-term impact of a short-course of ART (SCART) during PHI. Twenty-eight subjects with defined PHI received 3 months of SCART at HIV-1 seroconversion. HIV-specific interferon-gamma+ CD4+ T cell responses, CD4 cell counts and plasma viral loads were assessed prospectively. Clinical outcome was defined as the time taken from PHI to a fall in CD4 cell counts <350 cells/mul on two or more occasions. Of 28 patients, 25 (89%) had detectable HIV-specific CD4+ helper responses at baseline. Five of 11 (45%) patients had preserved HIV-specific CD4+ responses 3 years after stopping SCART. Neither the presence nor magnitude of HIV-1-specific T helper responses either at baseline or 3 years following SCART cessation predicted clinical outcome. Rebound viraemia associated with stopping SCART did not diminish HIV-1-specific CD4+ responses. Long-term (>3 years) preservation of virus-specific CD4+ cells occurred in 45% of patients receiving SCART in PHI. There was no correlation between either the presence or magnitude of these responses and clinical outcome.

Class II major histocompatibility complex tetramer staining: progress, problems, and prospects

The use of major histocompatibility complex (MHC) tetramers in the detection and analysis of antigen-specific T cells has become more widespread since its introduction 11 years ago. Early challenges in the application of tetramer staining to CD4+ T cells centred around difficulties in the expression of various class II MHC allelic variants and the detection of low-frequency T cells in mixed populations. As many of the technical obstacles to class II MHC tetramer staining have been overcome, the focus has returned to uncertainties concerning how oligomer valency and T-cell receptor/MHC affinity affect tetramer binding. Such issues have become more important with an increase in the number of studies relying on direct ex vivo analysis of antigen-specific CD4+ T cells. In this review we discuss which problems in class II MHC tetramer staining have been solved to date, and which matters remain to be considered.

Ex vivo analysis of human T lymphotropic virus type 1-specific CD4+ cells by use of a major histocompatibility complex class II tetramer composed of a neurological disease-susceptibility allele and its immunodominant peptide

HLA-DRB1*0101 is associated with susceptibility to human T lymphotropic virus type 1 (HTLV-1)-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Here, we used a synthetic tetramer of DRB1*0101 and its epitope peptide to analyze HTLV-1-specific CD4(+) T cells ex vivo. The frequency of tetramer(+)CD4(+) T cells was significantly greater in patients with HAM/TSP than in healthy HTLV-1 carriers (HCs) at a given proviral load and correlated with HTLV-1 tax messenger RNA expression in HCs but not in patients with HAM/TSP. These cells displayed an early to intermediate effector memory phenotype and were preferentially infected by HTLV-1. T cell receptor gene analyses of 2 unrelated DRB1*0101-positive patients with HAM/TSP showed similar Vbeta repertoires and amino acid motifs in complementarity-determining region 3. Our data suggest that efficient clonal expansion of virus-specific CD4(+) T cells in patients with HAM/TSP does not simply reflect higher viral burden but rather reflects a rapid turnover caused by preferential infection and/or in vivo stimulation by major histocompatibility complex-peptide complexes.

Human CD4+ T cell epitopes from vaccinia virus induced by vaccination or infection

Despite the importance of vaccinia virus in basic and applied immunology, our knowledge of the human immune response directed against this virus is very limited. CD4⁺ T cell responses are an important component of immunity induced by current vaccinia-based vaccines, and likely will be required for new subunit vaccine approaches, but to date vaccinia-specific CD4⁺ T cell responses have been poorly characterized, and CD4⁺ T cell epitopes have been reported only recently. Classical approaches used to identify T cell epitopes are not practical for large genomes like vaccinia. We developed and validated a highly efficient computational approach that combines prediction of class II MHC-peptide binding activity with prediction of antigen processing and presentation. Using this approach and screening only 36 peptides, we identified 25 epitopes recognized by T cells from vaccinia-immune individuals. Although the predictions were made for HLA-DR1, eight of the peptides were recognized by donors of multiple haplotypes. T cell responses were observed in samples of peripheral blood obtained many years after primary vaccination, and were amplified after booster immunization. Peptides recognized by multiple donors are highly conserved across the poxvirus family, including variola, the causative agent of smallpox, and may be useful in development of a new generation of smallpox vaccines and in the analysis of the immune response elicited to vaccinia virus. Moreover, the epitope identification approach developed here should find application to other large-genome pathogens.

Although the routine use of vaccinia virus for vaccination against smallpox was stopped after eradication of this disease, there is a possibility for an accidental or intentional release of this virus. In response to this challenge, vaccination of at least emergency personnel has been suggested. However, adverse reactions induced by the smallpox vaccine have had a negative impact in the success of this program. For these reasons development of new smallpox vaccines is a public health priority. Identification of strong helper T cell epitopes is central to these efforts. However, identification of T cell epitopes in large genomes like vaccinia is difficult using current screening methods. In this work, we develop a new computational approach for prediction of T cell epitopes, validate it using epitopes already identified by classical methods, and apply it to the prediction of vaccinia epitopes. Twenty-five of 36 peptides containing predicted sequences were recognized by T cells from individuals exposed to vaccinia virus. These peptides are highly conserved across the orthopox virus family and may be useful in development of a new generation of smallpox vaccines and in the analysis of the immune response against vaccinia virus.

Tracking virus-specific CD4+ T cells during and after acute hepatitis C virus infection

BACKGROUND

CD4+ T cell help is critical in maintaining antiviral immune responses and such help has been shown to be sustained in acute resolving hepatitis C. In contrast, in evolving chronic hepatitis C CD4+ T cell helper responses appear to be absent or short-lived, using functional assays.

METHODOLOGY/PRINCIPAL FINDINGS

Here we used a novel HLA-DR1 tetramer containing a highly targeted CD4+ T cell epitope from the hepatitis C virus non-structural protein 4 to track number and phenotype of hepatitis C virus specific CD4+ T cells in a cohort of seven HLA-DR1 positive patients with acute hepatitis C in comparison to patients with chronic or resolved hepatitis C. We observed peptide-specific T cells in all seven patients with acute hepatitis C regardless of outcome at frequencies up to 0.65% of CD4+ T cells. Among patients who transiently controlled virus replication we observed loss of function, and/or physical deletion of tetramer+ CD4+ T cells before viral recrudescence. In some patients with chronic hepatitis C very low numbers of tetramer+ cells were detectable in peripheral blood, compared to robust responses detected in spontaneous resolvers. Importantly we did not observe escape mutations in this key CD4+ T cell epitope in patients with evolving chronic hepatitis C.

CONCLUSIONS/SIGNIFICANCE

During acute hepatitis C a CD4+ T cell response against this epitope is readily induced in most, if not all, HLA-DR1+ patients. This antiviral T cell population becomes functionally impaired or is deleted early in the course of disease in those where viremia persists.

Predictors of disease progression in HIV infection: a review

During the extended clinically latent period associated with Human Immunodeficiency Virus (HIV) infection the virus itself is far from latent. This phase of infection generally comes to an end with the development of symptomatic illness. Understanding the factors affecting disease progression can aid treatment commencement and therapeutic monitoring decisions. An example of this is the clear utility of CD4+ T-cell count and HIV-RNA for disease stage and progression assessment. Elements of the immune response such as the diversity of HIV-specific cytotoxic lymphocyte responses and cell-surface CD38 expression correlate significantly with the control of viral replication. However, the relationship between soluble markers of immune activation and disease progression remains inconclusive. In patients on treatment, sustained virological rebound to >10,000 copies/mL is associated with poor clinical outcome. However, the same is not true of transient elevations of HIV RNA (blips). Another virological factor, drug resistance, is becoming a growing problem around the globe and monitoring must play a part in the surveillance and control of the epidemic worldwide. The links between chemokine receptor tropism and rate of disease progression remain uncertain and the clinical utility of monitoring viral strain is yet to be determined. The large number of confounding factors has made investigation of the roles of race and viral subtype difficult, and further research is needed to elucidate their significance. Host factors such as age, HLA and CYP polymorphisms and psychosocial factors remain important, though often unalterable, predictors of disease progression. Although gender and mode of transmission have a lesser role in disease progression, they may impact other markers such as viral load. Finally, readily measurable markers of disease such as total lymphocyte count, haemoglobin, body mass index and delayed type hypersensitivity may come into favour as ART becomes increasingly available in resource-limited parts of the world. The influence of these, and other factors, on the clinical progression of HIV infection are reviewed in detail, both preceding and following treatment initiation.

The role of cytokines which signal through the common gamma chain cytokine receptor in the reversal of HIV specific CD4(+) and CD8(+) T cell anergy

Background

HIV specific T cells are putatively anergic in vivo. IL-2, a member of a class of cytokines that binds to receptors containing the common gamma chain (γc) has been shown to reverse anergy. We examined the role of γc cytokines in reversing HIV specific T cell anergy.

Methods

PBMC from untreated HIV-infected individuals were briefly exposed to a panel of γc cytokines, and frequencies of gag specific T cells were enumerated by intracellular IFN-γ flow cytometry.

Results

Of the γc cytokines, brief exposure to IL-2, IL-15, or combined IL-15/IL-7 significantly enhanced (range 2–7 fold) the CD4⁺ and CD8⁺ T cell IFN-γ responses to HIV gag, with IL-15 giving the greatest enhancement. The effects of cytokines were not due to enhanced proliferation of pre-existing antigen specific cells, but were due to a combination of enhanced cytokine production from antigen specific T cells plus activation of non-epitope specific T cells.

Conclusions

These observations support the notion that a significant number of HIV specific T cells are circulating in an anergic state. IL-2, IL-7 and particularly IL-15 as an immune modulator to reverse HIV-1 specific T cell anergy should be investigated, with the caveat that non-specific activation of T cells may also be induced.

Identification of key peptide-specific CD4+ T cell responses to human cytomegalovirus: implications for tracking antiviral populations

Human cytomegalovirus (CMV) infection is normally controlled effectively by the immune response, including CD4(+) T cells. Large numbers of these cells are present in healthy seropositive individuals but their loss in immunosuppression leads to reactivation and disease. Tracking such responses in vivo is hampered by poor definition of their peptide targets. In this study, we defined the key targets of the peptide-specific CD4(+) T cell responses to the CMV pp65 protein using functional assays and a peptide library. Despite a good deal of interindividual variation in the numbers of peptides recognized, responses to CMV pp65 were strikingly targeted at three key epitopes. A response to one or more of these three key peptides was seen in all individuals tested (P < 0.0001) and this finding was tested and reproduced in a second independent population. The most common response identified was that to a DR53 restricted epitope, aa281-295. HLA-DR1 restricted CMV pp65-specific populations, although reproducibly detected, were of low frequency ex vivo. However, it was possible to detect and phenotype these cells using an enrichment protocol and this revealed them to have 'effector memory' status although, in contrast to CD8(+) T cell responses, these were CD45RA(-). These data suggest that CD4(+) T cell responses to CMV can be identified reliably using a pool of just three peptides. This simple approach will provide a robust and reliable as well as economic method for tracking peptide specific populations in health and disease.

CD4+ T cell targeting of human immunodeficiency virus type 1 (HIV-1) peptide sequences present in vivo during chronic, progressive HIV-1 disease

We previously detected HIV-1 Gag-specific CD4+ T cells recognizing reference strain viral epitopes in subjects with progressive, chronic infection. To test whether these CD4+ T cells persist in vivo by failing to recognize autologous HIV-1 epitopes, we compared autologous plasma HIV-1 p24 nucleotide sequences with targeted HXB.2 strain Gag p24 CD4+ T cell epitopes in nine chronically infected, untreated subjects. In five responding subjects, 10 of 26 HXB.2 strain p24 peptides targeted by CD4+ T cells exactly matched autologous plasma viral sequences. Four subjects with plasma viral loads >100,000 copies/mL had no measurable p24-specific CD4+ T cell responses despite carrying HIV-1 strains that matched HXB.2 sequences at predicted epitopes. These results show that HIV-1-specific CD4+ T cells can persist in chronic HIV-1 infection despite recognition of epitopes present in vivo. However, with high-level in vivo HIV-1 replication, CD4+ T cells targeting autologous HIV-1 may be non-responsive or absent.

Infection of CD127+ (interleukin-7 receptor+) CD4+ cells and overexpression of CTLA-4 are linked to loss of antigen-specific CD4 T cells during primary human immunodeficiency virus type 1 infection

We recently found that human immunodeficiency virus (HIV)-specific CD4+ T cells express coreceptor CCR5 and activation antigen CD38 during early primary HIV-1 infection (PHI) but then rapidly disappear from the circulation. This cell loss may be due to susceptibility to infection with HIV-1 but could also be due to inappropriate apoptosis, an expansion of T regulatory cells, trafficking out of the circulation, or dysfunction. We purified CD38+++CD4+ T cells from peripheral blood mononuclear cells, measured their level of HIV-1 DNA by PCR, and found that about 10% of this population was infected. However, a small subset of HIV-specific CD4+) T cells also expressed CD127, a marker of long-term memory cells. Purified CD127+CD4+ lymphocytes contained fivefold more copies of HIV-1 DNA per cell than did CD127-negative CD4+ cells, suggesting preferential infection of long-term memory cells. We observed no apoptosis of antigen-specific CD4+ T cells in vitro and only a small increase in CD45RO+CD25+CD127dimCD4+ T regulatory cells during PHI. However, 40% of CCR5+CD38+++ CD4+ T cells expressed gut-homing integrins, suggesting trafficking through gut-associated lymphoid tissue (GALT). Furthermore, 80% of HIV-specific CD4+ T cells expressed high levels of the negative regulator CTLA-4 in response to antigen stimulation in vitro, which was probably contributing to their inability to produce interleukin-2 and proliferate. Taken together, the loss of HIV-specific CD4+ T cells is associated with a combination of an infection of CCR5+ CD127+ memory CD4+ T cells, possibly in GALT, and a high expression of the inhibitory receptor CTLA-4.

Preferential infection shortens the life span of human immunodeficiency virus-specific CD4+ T cells in vivo

CD4(+) T-cell help is essential for effective immune responses to viruses. In human immunodeficiency virus (HIV) infection, CD4(+) T cells specific for HIV are infected by the virus at higher frequencies than other memory CD4(+) T cells. Here, we demonstrate that HIV-specific CD4(+) T cells are barely detectable in most infected individuals and that the corresponding CD4(+) T cells exhibit an immature phenotype compared to both cytomegalovirus (CMV)-specific CD4(+) T cells and other memory CD4(+) T cells. However, in two individuals, we observed a rare and diametrically opposed pattern in which HIV-specific CD4(+) T-cell populations of large magnitude exhibited a terminally differentiated immunophenotype; these cells were not preferentially infected in vivo. Clonotypic analysis revealed that the HIV-specific CD4(+) T cells from these individuals were cross-reactive with CMV. Thus, preferential infection can be circumvented in the presence of cross-reactive CD4(+) T cells driven to maturity by coinfecting viral antigens, and this physical proximity rather than activation status per se is an important determinant of preferential infection based on antigen specificity. These data demonstrate that preferential infection reduces the life span of HIV-specific CD4(+) T cells in vivo and thereby compromises the generation of effective immune responses to the virus itself; further, this central feature in the pathophysiology of HIV infection can be influenced by the cross-reactivity of responding CD4(+) T cells.

Expansion and contraction of HIV-specific CD4 T cells with short bursts of viremia, but physical loss of the majority of these cells with sustained viral replication

Chronic infection with the HIV results in poor HIV-specific CD4 T cell proliferation, but more recent analyses using intracellular cytokine staining demonstrated that IFN-gamma-producing, HIV-specific CD4 T cells can be detected for years in HIV-infected subjects. Because it is not known whether the majority of HIV-specific T cells are lost or become dysfunctional, we examined the kinetics of the T cell response over an extended period of time using a panel of 10 HLA-DR tetramers loaded with HIV p24 peptides. Tetramer+ CD4 T cells were present at a relatively high frequency during acute infection, but the size of these populations substantially contracted following suppression of viral replication. Short-term cessation of antiretroviral therapy resulted in a burst of viral replication and concomitant expansion of tetramer+ CD4 T cells, and these populations again contracted following reinitiation of therapy. The kinetics with which these cell populations contracted were characteristic of effector T cells, a conclusion that was supported by their phenotypic (CCR7-CD45RA-) and functional properties (IFN-gamma+). Continued high-level viremia resulted in the physical loss of the majority of tetramer+ CD4 T cells, and the decline of HIV p24-specific CD4 T cells occurred more rapidly and was more substantial than the reduction of total CD4 T cell numbers. We conclude that the population of HIV p24-specific CD4 T cells is initially responsive to changes in the levels of viral Ags, but that the majority of these cells are lost in a setting of chronic viremia.