A panel of MHC class I restricted viral peptides for use as a quality control for vaccine trial ELISPOT assays

Enhanced rates and magnitude of immune responses detected against an HIV vaccine: effect of using an optimized process for isolating PBMC

Quantitative analysis of cell-mediated immune responses induced by candidate HIV vaccines requires robust procedures for collecting and processing human peripheral mononuclear blood cells (PBMCs). We evaluated several parameters in order to optimize a sample handling process that would be suitable for a multicenter clinical trial. Among the findings, systematic increases in the magnitude of IFN-gamma ELISpot responses were observed when the time from blood collection to PBMC freezing was reduced to <12 h. By implementing these improvements within an ongoing clinical trial, the estimated immunologic response rates to an adenovirus- based HIV vaccine increased by more than 20 percentage points to approximately 80% of the vaccine recipients against any of the vaccine antigens and the average levels of T cell response improved more than 3-fold. These studies establish the importance of optimal conditions for PBMC collection and handling to the success of a clinical development program.

A dual color ELISPOT method for the simultaneous detection of IL-2 and IFN-gamma HIV-specific immune responses

The single color IFN-gamma ELISPOT assay has become a standard for assessing HIV-specific immune responses in HIV-infected subjects. However, recent data suggests that single cytokine detection for immune monitoring of HIV-infected individuals may not be sufficient to fully describe virus-specific immune responses. Here, we have designed and validated a dual color ELISPOT assay capable of detecting both IL-2 and IFN-gamma secreting cells simultaneously in response to HIV antigens. We found that a cell input number of 200,000 cells/well provided a good balance between limited availability of cells due to blood volume restrictions and ability to detect all cytokine secretion patterns. The simultaneous detection of IL-2 and IFN-gamma resulted in a decreased magnitude of IFN-gamma but not IL-2 responses. Measures of intra- and inter-assay variability for the dual color ELISPOT assay were comparable to that seen for single cytokine ELISPOT assay with coefficients of variation below 20% for IL-2, IFN-gamma and dual secretion. Although CD8+ T cells mediated most HIV-specific responses in infected subjects, CD4+ T cells mediated responses to HIV were also detected. Features of this assay such as high throughput, cell number requirement and cytokine choice should make this assay a valuable tool for screening for HIV-specific immune responses in several clinically relevant settings.

Identification of Novel HLA-A*0201-restricted epitopes in recent-onset type 1 diabetic subjects and antibody-positive relatives

Cytotoxic T-lymphocytes (CTLs) are considered to be essential for beta-cell destruction in type 1 diabetes. However, few islet-associated peptides have been demonstrated to activate autoreactive CTLs from type 1 diabetic subjects. In an effort to identify novel epitopes, we used matrix-assisted algorithms to predict peptides of glial fibrillary acidic protein (GFAP), prepro-islet amyloid polypeptide (ppIAPP), and islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) that likely bind to HLA-A*0201 with a strong affinity and contain a COOH-terminal proteasomal cleavage site. Seven peptides stabilized HLA-A*0201 expression in binding assays and were used to stimulate peripheral blood mononuclear cells and were evaluated for granzyme B secretion. We found that 5 of 13 type 1 diabetic subjects and 4 of 6 antibody-positive relatives exhibited greater numbers of granzyme B-secreting cells in response to at least one putative epitope compared with healthy control subjects. The most prevalent responses in antibody-positive and type 1 diabetic subjects were to ppIAPP(9-17). Other peptides recognized by type 1 diabetic or antibody-positive subjects included GFAP(143-151), IGRP(152-160), and GFAP(214-222). These data implicate peptides of ppIAPP, GFAP, and IGRP as CTL epitopes for a heterogenous CD8(+) T-cell response in type 1 subjects and antibody-positive relatives.

Substantial improvements in performance indicators achieved in a peripheral blood mononuclear cell cryopreservation quality assurance program using single donor samples

Storage of high-quality cryopreserved peripheral blood mononuclear cells (PBMC) is often a requirement for multicenter clinical trials and requires a reproducibly high standard of practice. A quality assurance program (QAP) was established to assess an Australia-wide network of laboratories in the provision of high-quality PBMC (determined by yield, viability, and function), using blood taken from single donors (human immunodeficiency virus [HIV] positive and HIV negative) and shipped to each site for preparation and cryopreservation of PBMC. The aim of the QAP was to provide laboratory accreditation for participation in clinical trials and cohort studies which require preparation and cryopreservation of PBMC and to assist all laboratories to prepare PBMC with a viability of >80% and yield of >50% following thawing. Many laboratories failed to reach this standard on the initial QAP round. Interventions to improve performance included telephone interviews with the staff at each laboratory, two annual wet workshops, and direct access to a senior scientist to discuss performance following each QAP round. Performance improved substantially in the majority of sites that initially failed the QAP (P = 0.002 and P = 0.001 for viability and yield, respectively). In a minority of laboratories, there was no improvement (n = 2), while a high standard was retained at the laboratories that commenced with adequate performance (n = 3). These findings demonstrate that simple interventions and monitoring of PBMC preparation and cryopreservation from multiple laboratories can significantly improve performance and contribute to maintenance of a network of laboratories accredited for quality PBMC fractionation and cryopreservation.

Distinct memory CD4+ T-cell subsets mediate immune recognition of Epstein Barr virus nuclear antigen 1 in healthy virus carriers

CD4+ T cells, specific for transforming latent infection with the Epstein Barr virus (EBV), consistently recognize the nuclear antigen 1 of EBV (EBNA1). EBNA1-specific effector CD4+ T cells are primarily T-helper 1 (TH1) polarized. Here we show that most healthy EBV carriers have such IFN-secreting EBNA1-specific CD4+ T cells at a frequency of 0.03% of circulating CD4+ T cells. In addition, healthy carriers have a large pool of CD4+ T cells that proliferated in response to EBNA1 and consisted of distinct memory-cell subsets. Despite continuous antigen presence due to persistent EBV infection, half of the proliferating EBNA1-specific CD4+ T cells belonged to the central-memory compartment (TCM). The remaining EBNA1-specific CD4+ T cells displayed an effector-memory phenotype (TEM), of which a minority rapidly secreted IFN upon stimulation with EBNA1. Based on chemokine receptor analysis, all EBNA1-specific TCM CD4+ T cells were TH1 committed. Our results suggest that protective immune control of chronic infections, like EBV, includes a substantial reservoir of TCM CD4+ TH1 precursors, which continuously fuels TH1-polarized effector cells.

Flow cytometry controls, instrument setup, and the determination of positivity

A frequent goal of flow cytometric analysis is to classify cells as positive or negative for a given marker, or to determine the precise ratio of positive to negative cells. This requires good and reproducible instrument setup, and careful use of controls for analyzing and interpreting the data. The type of controls to include in various kinds of flow cytometry experiments is a matter of some debate and discussion. In this tutorial, we classify controls in various categories, describe the options within each category, and discuss the merits of each option.

Rapid assessment of the antigenic integrity of tetrameric HLA complexes by human monoclonal HLA antibodies

The ability of tetrameric major histocompatibility complex (MHC) class I-peptide complexes (tetramers) to detect antigen-specific T lymphocyte responses has yielded significant information about the generation of in vivo immunity in numerous antigenic systems. Here we present a novel method for rapid validation of tetrameric HLA molecules based on the presence of allodeterminants. Human monoclonal antibodies (mAbs) recognizing polymorphic determinants on HLA class I were immobilized on polystyrene microparticles and used to probe the structural integrity of tetrameric HLA class I molecules by flow cytometry. A total of 22 tetramers, based on HLA-A1, A2, A3, A24, B7 and B8 were reactive with their counterpart mAbs, thus confirming their antigenic integrity. A positive outcome of this mAb test ensures that tetrameric HLA class I can be used with greater confidence in subsequent functional assays.

Statistical positivity criteria for the analysis of ELISpot assay data in HIV-1 vaccine trials

The enzyme-linked immunospot (ELISpot) assay is one of the leading technologies used to measure cellular responses in many settings including HIV vaccine clinical trials. HIV-1-specific effector T lymphocyte responses are considered necessary in the control of infection. Accurate measurement and summary of cellular immune responses are critical to HIV research. ELISpot assay readout is often dichotomized into positive/negative responses according to some pre-specified criteria. Given the increase in the number of replicates used for each stimulation with current assay configurations in the HIV Vaccine Trials Network (HVTN) laboratories, a new approach is now possible. We propose an objective criteria based on a hypothesis-driven method that controls the false positive rate while maintaining sensitivity to each of the antigen-specific responses under study. The new approach is compared to other commonly employed criteria using real and simulated data.

Characterization of the IFN-gamma T-cell responses to immediate early antigens in humans with genital herpes

Background

The IFN-γ ELISPOT assay has been used to examine the T-cell repertoire for many disease states in humans but, as yet, not genital herpes. Using overlapping synthetic peptide libraries, an IFN-γ ELISPOT assay was established that could measure CD4 and CD8 T-cell responses to HSV-2 antigens in patients with genital herpes.

Results

In unexpanded T-cells isolated from peripheral blood, CD4 responses were readily measured against four immediate early antigens (ICP0, ICP4, ICP22 and ICP27), VP22 and gD. The CD4 responses were characterized by a low number of positive cells which produced large ELISPOTs. CD4 responses had a broad specificity and within individual patients several of the test antigens were recognized. In contrast, CD8 responses were found only in approximately 50% of patients and were typically specific to a single antigen. When disease status and immune responses were compared, an enhanced CD4 response to ICP4 in patients with a low recurrence rate was found. The ICP4 response was striking in three HSV-1 single positive genital herpes patients.

Conclusion

The survey of T-cell responses is an important step to understand the host cellular immune response in individuals with genital herpes. The assay described here has the capability of measuring CD4 and CD8 T-cell responses that may be used to correlate disease status with specific immune responses. In an evaluation of 18 subjects a trend of positive responses to an immediate early protein, ICP4, was found in individuals that had a low rate of disease recurrence.

Direct exosome stimulation of peripheral humanT cells detected by ELISPOT

Exosomes from APC are nano-vesicles that can induce antigen-specific T cell responses and are presently explored as therapeutic tools in different clinical settings. Investigations of the capacity of exosomes to stimulate T cells in vitro have mostly been performed on T cell hybridomas, clones or lines. Whether exosomes can stimulate T cells directly or need the presence of dendritic cells (DC) is debated. We could detect exosome-induced antigen-specific CD8(+) T cell responses in peripheral blood from humans. Exosomes from monocyte-derived DC (MDDC) were loaded with a mix of 23 immunogenic peptides from EBV, CMV and influenza virus, and added to autologous peripheral CD8(+) T cells. IFN-gamma-producing cells were detected by enzyme-linked immunospot assay (ELISPOT). MDDC-exosomes induced IFN-gamma production in CD8(+) T cells without addition of DC. The response was exosome dose dependent, and dependent on exosomal MHC class I. Furthermore, we detected an enhanced T cell stimulatory capacity by exosomes from lipopolysaccharide-matured MDDC compared to exosomes from immature MDDC. Exosomes could also induce TNF-alpha production. These results show, for the first time, that exosomes can directly stimulate human peripheral CD8(+) T cells in an antigen-specific manner and that ELISPOT is a suitable method for detecting exosome-induced peripheral T cell responses. This system may provide a useful tool when developing exosomes as therapeutic agents.

Induction of multifunctional human immunodeficiency virus type 1 (HIV-1)-specific T cells capable of proliferation in healthy subjects by using a prime-boost regimen of DNA- and modified vaccinia virus Ankara-vectored vaccines expressing HIV-1 Gag coupled to CD8+ T-cell epitopes

A double-blind randomized phase I trial was conducted in human immunodeficiency virus type 1 (HIV-1)-negative subjects receiving vaccines vectored by plasmid DNA and modified vaccinia virus Ankara (MVA) expressing HIV-1 p24/p17 gag linked to a string of CD8(+) T-cell epitopes. The trial had two groups. One group received either two doses of MVA.HIVA (2x MVA.HIVA) (n=8) or two doses of placebo (2x placebo) (n=4). The second group received 2x pTHr.HIVA followed by one dose of MVA.HIVA (n=8) or 3x placebo (n=4). In the pTHr.HIVA-MVA.HIVA group, HIV-1-specific T-cell responses peaked 1 week after MVA.HIVA vaccination in both ex vivo gamma interferon (IFN-gamma) ELISPOT (group mean, 210 spot-forming cells/10(6) cells) and proliferation (group mean stimulation index, 37), with assays detecting positive responses in four out of eight and five out of eight subjects, respectively. No HIV-1-specific T-cell responses were detected in either assay in the 2x MVA.HIVA group or subjects receiving placebo. Using a highly sensitive and reproducible cultured IFN-gamma ELISPOT assay, positive responses mainly mediated by CD4(+) T cells were detected in eight out of eight vaccinees in the pTHr.HIVA-MVA.HIVA group and four out of eight vaccinees in the 2x MVA.HIVA group. Importantly, no false-positive responses were detected in the eight subjects receiving placebo. Of the 12 responders, 11 developed responses to previously identified immunodominant CD4(+) T-cell epitopes, with 6 volunteers having responses to more than one epitope. Five out of 12 responders also developed CD8(+) T-cell responses to the epitope string. Induced T cells produced a variety of anti-viral cytokines, including tumor necrosis factor alpha and macrophage inflammatory protein 1 beta. These data demonstrate that prime-boost vaccination with recombinant DNA and MVA vectors can induce multifunctional HIV-1-specific T cells in the majority of vaccinees.

Definition of an HPV18/45 cross-reactive human T-cell epitope after DNA immunisation of HLA-A2/KB transgenic mice

Although human papillomavirus (HPV) types 16 and 18 are the most common types associated with cervical cancer worldwide, other related HPV types such as HPV 35, 45 and 58 have significant prevalence in geographically distinct populations. For development of global prophylactic and therapeutic vaccine strategies, it is important to study immune responses against these viruses and to define the degree of cross-reactivity between related HPV types. To investigate the potential for T cell cross-reactivity after vaccination, HLA-A2/Kb transgenic mice were immunised with DNA plasmid constructs containing HPV18 and 45 E6 and E7. Splenocytes from immunised mice were tested in direct ELIspot assays against overlapping pools of HPV 18 peptides. Immunisation with either HPV18 or HPV45 E6 DNA produced dominant T cell responses against an epitope (KCIDFYSRI) that was shared between HPV18 and HPV45. This peptide was shown to bind to HLA-A*0201 but not Db or Kb molecules on the cell surface. Furthermore this peptide was shown to be immunogenic in vitro to human T cells from 2 out of 3 HLA-A2+ healthy donors. Collectively, these results demonstrate that HPV 18 and 45 E6 DNA vaccines are immunogenic in mice and demonstrate that cross-reactive T cell responses against closely related HPV types can be induced in vivo. The use of the HLA-A2/Kb transgenic mice allowed definition of an HLA-A*0201 binding peptide epitope that would have been rejected on the basis of predicted major histocompatibility complex binding affinity.

Defective IL-2 production by HIV-1-specific CD4 and CD8 T cells in an adolescent/young adult cohort

Here we investigate the effect of viremia and the influence of HAART on the frequency and quality of HIVspecfic T cells in an adolescent/young adult cohort. Measurements of viral loads and the magnitude and quality of antiviral cellular immune responses were performed on 14 HAART-naive and 8 treated HIV-1-infected adolescents. Cross-sectional correlations between viral load and cellular immune responses were determined and data were analyzed by viral load (<4000, 4000-40,000, and >40,000 copies/ml plasma) and patient treatment status. All 22 patients showed a broad IFN-gamma ELISPOT response that was proportional to viral load (r = 0.53, p = 0.02), recognizing an average of five to eight peptide pools throughout Gag, Pol, Env, Tat, Rev, and Nef. Intracellular cytokine staining was performed with pools of overlapping peptides corresponding to HIV Gag to distinguish CD8 response from CD4 response. Among untreated patients with increased viral load there was a constant IFN-gamma CD8 response but a declining IFN-gamma CD4 response. HIV-specific IL-2 production was consistently low in CD8 cells but inversely related to viral load in CD4 cells (r = -0.52, p = 0.02). In this crosssectional analysis, time on HAART was associated with an increased frequency of antiviral IFN-gamma- and IL-2-coproducing CD4 cells (r = 0.98, p <0.001), but not of antiviral CD8 cells. Our results suggest that T cells coproducing IL-2 and IFN-gamma are a better marker for immunological competence than T cells producing IFN-gamma alone. They also suggest that HAART may be associated with an improved capacity for IL-2 production by antiviral CD4 T cells in a time-dependent manner. Longitudinal studies are clearly necessary to assess the impact of HAART on these parameters.

Successful transduction of liver in hemophilia by AAV-Factor IX and limitations imposed by the host immune response

We have previously shown that a single portal vein infusion of a recombinant adeno-associated viral vector (rAAV) expressing canine Factor IX (F.IX) resulted in long-term expression of therapeutic levels of F.IX in dogs with severe hemophilia B. We carried out a phase 1/2 dose-escalation clinical study to extend this approach to humans with severe hemophilia B. rAAV-2 vector expressing human F.IX was infused through the hepatic artery into seven subjects. The data show that: (i) vector infusion at doses up to 2 x 10(12) vg/kg was not associated with acute or long-lasting toxicity; (ii) therapeutic levels of F.IX were achieved at the highest dose tested; (iii) duration of expression at therapeutic levels was limited to a period of approximately 8 weeks; (iv) a gradual decline in F.IX was accompanied by a transient asymptomatic elevation of liver transaminases that resolved without treatment. Further studies suggested that destruction of transduced hepatocytes by cell-mediated immunity targeting antigens of the AAV capsid caused both the decline in F.IX and the transient transaminitis. We conclude that rAAV-2 vectors can transduce human hepatocytes in vivo to result in therapeutically relevant levels of F.IX, but that future studies in humans may require immunomodulation to achieve long-term expression.

HIV-Specific Cytotoxic Cell Frequencies Measured Directly Ex Vivo by the Lysispot Assay Can Be Higher or Lower Than the Frequencies of IFN-γ-Secreting Cells: Anti-HIV Cytotoxicity Is Not Generally Impaired Relative to Other Chronic Virus Responses

CD8(+) T cells in HIV-infected patients are believed to contribute to the containment of the virus and the delay of disease progression. However, the frequencies of HIV-specific CD8(+) T cells, as measured by IFN-gamma secretion and tetramer binding, often do not correlate with a delay in disease progression during chronic infection. Using the Lysispot and ELISPOT assays, we measured the frequencies of cytotoxic and IFN-gamma-secreting T cells responding to overlapping peptides from Gag, Nef, Env, and Pol consensus HIV-1 clade B sequences. PBMC from the majority of HIV-infected subjects have significant frequencies of HIV-specific cells that killed targets within 5 h directly ex vivo. The relative frequencies of IFN-gamma-secreting and cytotoxic cells varied markedly between different HIV peptide pools within the same patient, and some T cells lysed targets without secreting IFN-gamma. These results indicate that measurement of IFN-gamma production alone may be insufficient to evaluate the breadth of the HIV-specific T cell response. Also, neither the CTL to IFN-gamma ratios nor the ex vivo CTL frequencies specific for different HIV proteins were consistently lower than responses specific for two other chronic viral infections, human CMV and EBV, within the same subjects. Thus ex vivo cytotoxic T cell frequencies do not provide evidence for a model of "preterminal differentiation" of HIV-specific CD8(+) T cells during chronic HIV infection. Analysis of the frequency of directly cytotoxic HIV-specific T cells may be of considerable value in the assessment of disease progression and the potential efficacy of HIV vaccines.

Rapid qualitative and quantitative analysis of T-cell responses in HIV-1-infected individuals receiving successful HAART and HIV-1 sero-negative controls: concomitant assessment of perforin, IFN-gamma and IL-4 secretion

The enzyme-linked immunospot (ELIspot) assay is a highly sensitive and valuable tool for determining the frequency of cytokine-secreting T cells. It is essential to determine both frequencies and functional capabilities of antigen-specific T cells, including cytokine secretion, degranulation, and cytotoxicity in order to obtain a fuller picture of the immune status of an individual. We describe here for the first time a perforin-release ELIspot assay which, when used in combination with IFN-gamma and IL-4 ELIspots, permits rapid assessment of these functional parameters for antigen-specific T cells. Whole antigen or peptides from HIV-1, recall and other viral antigens were used for in vitro stimulation. Anti-HIV-1 responses in treated chronically infected individuals were weak, both in terms of perforin and IFN-gamma production. Tetanus toxoid stimulation was associated with moderate perforin release and a predominantly type-2 IL-4 producing response, whilst herpes simplex virus antigen stimulation resulted in perforin release but only a weak type-1 IFN-gamma response. Anti-cytomegalovirus responses generated high levels of perforin in conjunction with IFN-gamma. Cytokines IL-2 and IL-12/IL-15 induced perforin release coupled with an IFN-gamma type-1 response. Perforin release strongly correlated with IFN-gamma production to individual influenza, Epstein-Barr virus or cytomegalovirus MHC class I restricted peptides, in an HIV-1 sero-negative cohort, indicating a cytolytic type-1 CD8+ T-cell response. Evaluation of immunogenicity and putative efficacy of candidate vaccines using IFN-gamma will not be as informative alone as when combined with perforin and IL-4 evaluations, which allow assessment of specific cytotoxic potential without extensive cell culture.

Cell-mediated immune responses to influenza vaccination in healthy volunteers and allogeneic stem cell transplant recipients

Influenza is one of the most common respiratory diseases in humans. The response to vaccination is frequently poor in immunosuppressed individuals. The aim of the present study was to develop an enzyme-linked immunospot (ELISPOT) assay for measuring of the specific T-cell response to influenza vaccination. In all, 18 healthy subjects and six stem cell transplantation (SCT) patients tested before and 4 weeks after influenza vaccination were included in the present study. Peripheral blood lymphocytes were stimulated with four influenza peptides; three based on sequences from the hemagglutinin and one from the M1 protein. The ELISPOT assay and the measurement of intracellular IFN-gamma production were used to determine the cell-mediated responses after stimulation with the peptides. Influenza vaccination elicited strong cell-mediated immune responses in the healthy controls to all four peptides with 3.2-6.9-fold increases in the number of IFN-gamma producing spots/10(6) cells. By intracellular staining, it was suggested that CD4+ cells mediated the responses to the hemagglutinin peptides. In contrast, there was no increase in the number of IFN-gamma producing cells response after vaccination in the six SCT patients. In conclusion, our results suggest that the ELISPOT assay might be used as a complement to serology for monitoring of future influenza vaccine studies in SCT patients.

Dendritic cells expand Epstein Barr virus specific CD8+ T cell responses more efficiently than EBV transformed B cells

Adoptive transfer of Epstein Barr virus (EBV) specific cytotoxic T lymphocytes (CTLs) has been successfully applied in the treatment of EBV associated post-transplant lymphoproliferative disease (PTLD). In most studies EBV transformed B cells (LCLs) have been used for the induction of EBV specific T cell lines. Application of this approach to other EBV associated tumors is difficult, because LCLs focus T cell expansion toward immunodominant EBV antigens that are not expressed in EBV associated Hodgkin's lymphoma and nasopharyngeal carcinoma. Therefore, we compared dendritic cells (DCs) with LCLs for CD8+ T cell stimulation against dominant and subdominant EBV antigens. DCs expanded tenfold more EBNA3A and LMP2 specific CD8+ T cells than LCL and also stimulated EBV specific CTL from PTLD patients. Both, DCs and LCLs stimulations led to the expansion of high affinity T cells, capable to target EBV transformed B cells. While LCLs and DCs expressed MHC class I and II products at similar levels, DCs showed a higher expression of costimulatory and adhesion molecules. This resulted in more efficient T cell conjugate formation with DCs than with LCLs. We propose the use of DCs for stimulation of EBV specific T cells in active or passive immunotherapy of EBV associated malignancies.

HLA class I-restricted responses to vaccinia recognize a broad array of proteins mainly involved in virulence and viral gene regulation

We have analyzed by ex vivo ELISPOT the anti-vaccinia cytotoxic T lymphocyte responses of peripheral blood mononuclear cells from humans vaccinated with Dryvax vaccine. More than 6,000 peptides from 258 putative vaccinia ORFs predicted to bind the common molecules of the HLA A1, A2, A3, A24, B7, and B44 supertypes were screened with peripheral blood mononuclear cells of 31 vaccinees. A total of 48 epitopes derived from 35 different vaccinia antigens were identified, some of which (B8R, D1R, D5R, C10L, C19L, C7L, F12, and O1L) were recognized by multiple donors and contain multiple epitopes recognized in the context of different HLA types. The antigens recognized tend to be >100 residues in length and are expressed predominantly in the early phases of infection, although some late antigens were also recognized. Viral genome regulation and virulence factor were recognized most frequently, whereas few structural proteins were immunogenic. Finally, most epitopes were highly conserved among vaccinia virus Western Reserve, variola major and modified vaccinia Ankara, supporting their potential use in vaccine and diagnostic applications.

IL-2 absorption affects IFN-γ and IL-5, but not IL-4 producing memory T cells in double color cytokine ELISPOT assays

Cytokine assays are gaining increasing importance for human immune monitoring because they reliably detect antigen-specific T cells in primary PBMC, even at low clonal sizes. Double color ELISPOT assays permit the simultaneous visualization of cells producing two different cytokines. Permitting the simultaneous assessment of type 1 and 2 immunity and due to the limited numbers of PBMC available from human study subjects, double color assays should be particularly attractive for clinical trials. Since the performance of double color assays has not yet been validated, we set out to compare them to single color measurements. Testing the recall antigen-induced cytokine response of PBMC, we found that double color assays regularly provided lower numbers of IFN-gamma and IL-5 spots than single color measurements when IL-2 detection was part of the double color assay. We showed that the inhibitory effect resulted from IL-2 absorption and could be overcome by either antibody free preactivation cultures or by inclusion of anti-CD28 antibody. In contrast, the simultaneous detection of IL-2 did not affect the numbers of IL-4 spots. Therefore, unlike IL-2/IL-4 and IFN-gamma/IL-5 assays, IL-2/IFN-gamma, and IL-2/IL-5 assays require compensation for the IL-2 capture to provide accurate numbers for the frequencies of cytokine producing memory T cells.

Single-round HIV type 1 neutralization measured by ELISPOT technique in primary human cells

To accurately quantify HIV-1 neutralizing antibody titers in primary human cells, we developed a single round, focus-forming unit (FFU) reduction assay in human peripheral blood mononuclear cells (PBMC). Infected PBMC were enumerated by a reverse ELISPOT technique in which they were incubated under agarose in the presence of a protease inhibitor in anti-p24 antibody-coated microtiter plates. Viral p24, secreted in the immediate vicinity of infected cells and captured by immobilized antibodies, was subsequently stained using gold-labeled anti-p24-antibody and a precipitating silver substrate. The resulting spots were counted visually, without the aid of a microscope, and percent neutralization titers were determined using curve-fitting software. Results of this ELISPOT neutralization assay (ENA) for 15 HIV-positive human specimens were compared with results from a standard PBMC neutralization assay (standard assay) that measured neutralization as a function of p24 concentration by enzyme immunoassay (EIA). The ENA measures FFU reduction of both syncytium-inducing (SI) and non-syncytium-inducing (NSI) primary isolates. Completed assay plates may be retained as a physical record of results or saved as an image using a flat-bed computer scanner.

Recovery of CD8+ T-Cell Function During Systemic Chemotherapy in Advanced Ovarian Cancer

Immunologic approaches are emerging as new treatment options in several types of cancer. However, whereas the ability of patients to develop potent CD8+ T-cell responses is crucial for efficient antitumor responses, immunocompetence and T-cell function are not tested routinely in patients entering immunotherapy. The objective of our study was to monitor T-cell function in advanced cancer and during chemotherapy. CD8+ T-cell function of 21 patients with advanced ovarian cancer (stages III-IV) was assessed by cytokine flow cytometry following stimulation of 42 PBMC samples with a panel of synthetic viral peptides in vitro, consisting of pan-Caucasian epitopes. CD8+ T-cell responses were significantly lower in patients with high levels (>200 units/mL) of Ca125 (marker of tumor load and progression) than in those with low Ca125 levels (P = 0.0013). In longitudinal studies of nine patients, chemotherapy was associated with decreasing Ca125 levels in seven cases and also with improvement or maintenance of CD8+ T-cell function in seven cases. After the full course of chemotherapy, five of nine patients in remission displayed potent CD8+ T-cell responses, whereas four of nine patients in progression displayed low or decreasing T-cell responses, pointing toward a correlation between T-cell function and clinical response. Our results show for the first time that CD8+ T-cell function is not permanently suppressed in advanced cancer and successful chemotherapy is associated with improved antigen-specific T-cell reactivity. We suggest that functional assays determining T-cell immunocompetence can be valuable tools for optimizing cancer immunotherapy for improved clinical success.

Comprehensive analyses of CD8+ T cell responses during longitudinal study of acute human hepatitis C

We comprehensively studied the cellular immune response during acute human hepatitis C virus (HCV) infection by monthly prospective sampling of persons at high risk of infection. In 19 of 23 subjects, interferon-gamma-secreting T cells specific for 1 or more peptides spanning the entire HCV polyprotein were detected 1 to 3 months after infection. The median time to development of interferon gamma responses to HCV peptides was 33 days (range, 29-50 days), and these responses peaked between 180 and 360 days. Nineteen subjects had sufficient follow-up to determine outcome, with 15 (79%) developing persistent viremia and 4 (21%) clearing viremia spontaneously. Of those with progression to chronic infection and detectable T cell responses, all lost recognition of one or more antigens recognized during acute infection, and the median reduction in the magnitude of responses was 85%. Most significantly, despite ongoing viremia, those who had persistent infection did not develop new epitope specificities after the first 6 months of infection. In conclusion, in most individuals, the CD8+ T cell responses generated early in HCV infection decline in peripheral blood and are not replaced with new responses. Supplementary material for this article can be found on the HEPATOLOGY website (http://www.interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

Detection of human perforin by ELISpot and ELISA: Ex vivo identification of virus-specific cells

The perforin (PFN) protein is essential for the elimination of target cells by cytotoxic T lymphocytes (CTL) and natural killer (NK) cells. The study of cells releasing PFN has been hampered by a lack of sensitive methods. We therefore produced PFN-reactive monoclonal antibodies (mAb) and developed capture enzyme-linked immunosorbent (ELISA) and enzyme-linked immunospot (ELISpot) assays. Three mAbs were generated and shown to react with unique determinants of PFN. All mAbs recognized intracellular PFN in human peripheral blood mononuclear cell (PBMC) as assessed by flow cytometry and immunohistochemistry. Functional PFN capture ELISA and ELISpot assays were developed utilizing two of the mAbs for capture and the third mAb for detection. When examining PFN release by the YT lymphoma cell line, the ELISpot displayed a greater detection sensitivity than the ELISA. Assessment of PFN release by a CTL clone using ELISpot gave results consistent with a parallel (51)Cr-release cytotoxicity assay. Moreover, PFN release by PBMC could be quantified by ELISpot and ELISA after ex vivo stimulation with defined CTL epitopes from common viruses. These novel immunoassays will be valuable for further investigations of the mechanisms underlying granule-mediated apoptosis. In addition, the capture immunoassays could provide tools for studying CTL responses in infectious and tumor diseases as well as for vaccine development.

A vaccinia-based elispot assay for detection of CD8+ T cells from HIV-1 infected children

HIV-specific CD8+ T lymphocytes participate in the control of viral replication in infected patients. These responses are of low intensity in young infants and are decreased by antiretroviral therapy. In the present study, we report on a recombinant Vaccinia virus (rVV)-based Elispot assay for the detection of HIV-specific CD8+ T cells immediately after isolation of peripheral blood mononuclear cells (PBMC). The rVV-based assay was highly sensitive; 48 out of 50 children had a positive response against the rVV encoding HIV Env-Gag-Pol antigen. Interferon-gamma was produced by CD8+ T cells, and CD14+/15+ cells were the main cell subset presenting antigens expressed by rVV. We observed that the cell input per well had a critical influence on the sensitivity of the assay. Results from the ex vivo Elispot assay correlated poorly with those of the 51Cr release assay performed after expansion of PBMC in vitro; thus, both assays gave information on different subsets and/or functions of the HIV-specific T cell response.

Standardization of cytokine flow cytometry assays

BACKGROUND

Cytokine flow cytometry (CFC) or intracellular cytokine staining (ICS) can quantitate antigen-specific T cell responses in settings such as experimental vaccination. Standardization of ICS among laboratories performing vaccine studies would provide a common platform by which to compare the immunogenicity of different vaccine candidates across multiple international organizations conducting clinical trials. As such, a study was carried out among several laboratories involved in HIV clinical trials, to define the inter-lab precision of ICS using various sample types, and using a common protocol for each experiment (see additional files online).

RESULTS

Three sample types (activated, fixed, and frozen whole blood; fresh whole blood; and cryopreserved PBMC) were shipped to various sites, where ICS assays using cytomegalovirus (CMV) pp65 peptide mix or control antigens were performed in parallel in 96-well plates. For one experiment, antigens and antibody cocktails were lyophilised into 96-well plates to simplify and standardize the assay setup. Results ((CD4+)cytokine+ cells and (CD8+)cytokine+ cells) were determined by each site. Raw data were also sent to a central site for batch analysis with a dynamic gating template. Mean inter-laboratory coefficient of variation (C.V.) ranged from 17-44% depending upon the sample type and analysis method. Cryopreserved peripheral blood mononuclear cells (PBMC) yielded lower inter-lab C.V.'s than whole blood. Centralized analysis (using a dynamic gating template) reduced the inter-lab C.V. by 5-20%, depending upon the experiment. The inter-lab C.V. was lowest (18-24%) for samples with a mean of > 0.5% IFNgamma + T cells, and highest (57-82%) for samples with a mean of < 0.1% IFNgamma + cells.

CONCLUSION

ICS assays can be performed by multiple laboratories using a common protocol with good inter-laboratory precision, which improves as the frequency of responding cells increases. Cryopreserved PBMC may yield slightly more consistent results than shipped whole blood. Analysis, particularly gating, is a significant source of variability, and can be reduced by centralized analysis and/or use of a standardized dynamic gating template. Use of pre-aliquoted lyophilized reagents for stimulation and staining can provide further standardization to these assays.

Reduced frequency of FOXP3+ CD4+CD25+ regulatory T cells in patients with chronic graft-versus-host disease

Chronic graft-versus-host disease (cGVHD) is a major complication of allogeneic hematopoietic stem cell transplantation but the immune mechanisms leading to the diverse clinical manifestations of cGVHD remain unknown. In this study, we examined regulatory T cells (Tregs) in 57 transplant recipients (30 with cGVHD and 27 without active cGVHD) and 26 healthy donors. Phenotypic studies demonstrated decreased frequency of CD4+CD25+ T cells in patients with cGVHD compared with patients without cGVHD (P < .001) and healthy individuals (P < .001). Gene expression of Treg transcription factor FOXP3 was reduced in cGVHD patients compared with patients without cGVHD (P = .009) or healthy donors (P = .01). T-cell receptor excision circle (TREC) assays for the evaluation of thymus activity revealed fewer TRECs in both transplant groups compared with healthy donors (P < .001 and P = .02, respectively) although no difference was observed between patients with or without cGVHD (P = .13). When tested in functional assays, Tregs from both patient cohorts and healthy individuals mediated equivalent levels of suppression. Collectively, these studies indicate that patients with active cGVHD have reduced frequencies of Tregs but the function of these cells remains normal. These findings support the development of new strategies to increase the number of Tregs following allogeneic hematopoietic stem cell transplantation to prevent or correct cGVHD.

Cellular immune selection with hepatitis C virus persistence in humans

Hepatitis C virus (HCV) infection frequently persists despite substantial virus-specific cellular immune responses. To determine if immunologically driven sequence variation occurs with HCV persistence, we coordinately analyzed sequence evolution and CD8+ T cell responses to epitopes covering the entire HCV polyprotein in subjects who were followed prospectively from before infection to beyond the first year. There were no substitutions in T cell epitopes for a year after infection in a subject who cleared viremia. In contrast, in subjects with persistent viremia and detectable T cell responses, we observed substitutions in 69% of T cell epitopes, and every subject had a substitution in at least one epitope. In addition, amino acid substitutions occurred 13-fold more often within than outside T cell epitopes (P < 0.001, range 5-38). T lymphocyte recognition of 8 of 10 mutant peptides was markedly reduced compared with the initial sequence, indicating viral escape. Of 16 nonenvelope substitutions that occurred outside of known T cell epitopes, 8 represented conversion to consensus (P = 0.015). These findings reveal two distinct mechanisms of sequence evolution involved in HCV persistence: viral escape from CD8+ T cell responses and optimization of replicative capacity.

Immune responses toPlasmodium vivax pre-erythrocytic stage antigens in naturally exposed Duffy-negative humans: a potential model for identification of liver-stage antigens

Duffy antigen is the receptor used by Plasmodium vivax to invade erythrocytes. Consequently, individuals lacking Duffy antigen [Fy(-)] do not develop blood-stage infections. We hypothesized that naturally exposed Fy(-) humans may develop immune responses mainly to pre-erythrocytic stages and could be used to study acquired immunity to P. vivax and to identify liver-stage antigens. We report here that antibody and IFN-gamma responses to known sporozoite antigens were significantly induced by natural exposure in Fy(-) humans, whereas responses to blood-stage antigens were significantly induced in Fy(+) humans. IFN-gamma responses to sporozoite antigens were lower in Fy(+) than in Fy(-) humans, indicating that in Fy(+) humans blood-stage infections may have suppressed T cell responses to pre-erythrocytic stages. We evaluated the immune responses to 18 novel P. vivax homologs of P. falciparum sporozoite proteins identified from the P. vivax genome sequence. Eight proteins recalled IFN-gamma responses in P. vivax-exposed but not in unexposed individuals. Of these, 3 antigens elicited IFN-gamma responses in Fy(-) but not in Fy(+) individuals. These results suggest that differential immune responses observed in naturally exposed Fy(-) and Fy(+) individuals can be exploited to identify P. vivax stage-specific antigens.

Human immunodeficiency virus (HIV)-specific gamma interferon secretion directed against all expressed HIV genes: relationship to rate of CD4 decline

Immune responses to human immunodeficiency virus (HIV) are detected at all stages of infection and are believed to be responsible for controlling viremia. This study seeks to determine whether gamma interferon (IFN-gamma)-secreting HIV-specific T-cell responses influence disease progression as defined by the rate of CD4 decline. The study population consisted of 31 subjects naive to antiretroviral therapy. All were monitored clinically for a median of 24 months after the time they were tested for HIV-specific responses. The rate of CD4+-T-cell loss was calculated for all participants from monthly CD4 counts. Within this population, 17 subjects were classified as typical progressors, 6 subjects were classified as fast progressors, and 8 subjects were classified as slow progressors. Peripheral blood mononuclear cells were screened for HIV-specific IFN-gamma responses to all expressed HIV genes. Among the detected immune responses, 48% of the recognized peptides were encoded by Gag and 19% were encoded by Nef gene products. Neither the breadth nor the magnitude of HIV-specific responses correlated with the viral load or rate of CD4 decline. The breadth and magnitude of HIV-specific responses did not differ significantly among typical, fast, and slow progressors. These results support the conclusion that although diverse HIV-specific IFN-gamma-secreting responses are mounted during the asymptomatic phase, these responses do not seem to modulate disease progression rates.

HIV-1-specific IFN-gamma/IL-2-secreting CD8 T cells support CD4-independent proliferation of HIV-1-specific CD8 T cells

Functional and phenotypic characterization of virus-specific CD8 T cells against cytomegalovirus, Epstein-Barr virus, influenza (flu), and HIV-1 were performed on the basis of the ability of CD8 T cells to secrete IFN-gamma and IL-2, to proliferate, and to express CD45RA and CCR7. Two functional distinct populations of CD8 T cells were identified: (i) dual IFN-gamma/IL-2-secreting cells and (ii) single IFN-gamma-secreting cells. Virus-specific IFN-gamma/IL-2-secreting CD8 T cells were CD45RA-CCR7-, whereas single IFN-gamma CD8 T cells were either CD45RA-CCR7- or CD45RA+CCR7-. The proportion of virus-specific IFN-gamma/IL-2-secreting CD8 T cells correlated with that of proliferating CD8 T cells, and the loss of HIV-1-specific IL-2-secreting CD8 T cells was associated with that of HIV-1-specific CD8 T cell proliferation. Substantial proliferation of virus-specific CD8 T cells (including HIV-1-specific CD8 T cells) was also observed in CD4 T cell-depleted populations or after stimulation with MHC class I tetramer-peptide complexes. IL-2 was the factor responsible for the CD4-independent CD8 T cell proliferation. These results indicate that IFN-gamma/IL-2-secreting CD8 T cells may promote antigen-specific proliferation of CD8 T cells even in the absence of helper CD4 T cells.

Phenotypic and functional analysis of EBV-specific memory CD8 cells in SLE

T cell dysfunction has been described in systemic lupus erythematosus (SLE). However, the specific phenotype and function of antigen-specific CD8 cells is less clear. Here we determined phenotype and function of Epstein-Barr virus (EBV)-specific CD8 cells at the single-cell level in SLE. HLA-A2-restricted EBV-BMLF-1-specific CD8 cells were enumerated by flow cytometry using tetramers in SLE and healthy control subjects. Antigen-specific CD8 cells were analyzed for expression of differentiation, activation, proliferation, and anti-apoptotic markers. EBV-specific, other virus-specific (specific against a viral peptide pool consisting of cytomegalovirus, EBV and influenza virus peptides), and mitogen-induced CD8 cell function was assessed by INF-gamma ELISPOT assay. Frequencies of EBV-specific CD8 cells tended to be greater in SLE subjects than in healthy control subjects (p=0.07). While over 10% of EBV-specific CD8 cells were capable of producing IFN-gamma in four out of five healthy control subjects, such proportions of EBV-specific CD8 cells capable of IFN-gamma production were observed in only one out of six SLE subjects (p=0.04). In contrast, viral peptide pool-specific and mitogen-induced IFN-gamma-producing T cell function was intact in SLE subjects. Phenotypic analysis revealed EBV-specific CD8 cells to be in an early to intermediate differentiation and resting memory state in both groups. While EBV-specific CD8 cells are similar in phenotype, their frequency tends to be increased, and function appears to be decreased in SLE. Therefore, an impaired EBV-specific CD8 immune response may exist in SLE, potentially contributing to disease pathogenesis.

Depletion of Regulatory T Cells in HIV Infection Is Associated with Immune Activation

Immune activation during chronic HIV infection is a strong clinical predictor of death and may mediate CD4(+) T cell depletion. Regulatory T cells (Tregs) are CD4(+)CD25(bright)CD62L(high) cells that actively down-regulate immune responses. We asked whether loss of Tregs during HIV infection mediates immune activation in a cross-sectional study of 81 HIV-positive Ugandan volunteers. We found that Treg number is strongly correlated with both CD4(+) and CD8(+) T cell activation. In multivariate modeling, this relationship between Treg depletion and CD4(+) T cell activation was stronger than any other clinical factor examined, including viral load and absolute CD4 count. Tregs appear to decline at different rates compared with other CD4(+) T cells, resulting in an increased regulator to helper ratio in many patients with advanced disease. We hypothesize that this skewing may contribute to T cell effector dysfunction. Our findings suggest Tregs are a major contributor to the immune activation observed during chronic HIV infection.

CD8+ T cells from most HIV-1-infected patients, even when challenged with mature dendritic cells, lack functional recall memory to HIV gag but not other viruses

Chronically HIV-1-infected patients fail to contain their viremia despite high frequencies of HIV-1-specific, IFN-gamma-producing CD8(+) T cells. However, these cells are known to exhibit both phenotypic and functional defects. We tested if mature dendritic cells (DC) could correct defective HIV-1 gag-specific T cell responses and if responses to other viral antigens were comparably affected. The circulating gag-specific CD8(+) T cells in fresh blood reliably produced IFN-gamma but lacked IL-2 and high perforin levels and failed to expand significantly during culture with mature DC presenting HIV-1 gag peptides. In contrast, CD8(+) T cells from long-term nonprogressors contained gag-specific IFN-gamma and IL-2 double producers, and the numbers of IFN-gamma producers expanded approximately 15-fold during culture with DC. DC from chronically infected patients could expand IFN-gamma- and IL-2-producing cells specific for influenza, cytomegalovirus and Epstein Barr virus, and the expansions were comparable to those in healthy donors. When the proliferative capacity of CD8(+) T cells from progressor patients was assessed by CFSE dilution, proliferation to other viral antigens was more vigorous than to HIV-1 gag. Therefore, monocyte-derived DC from HIV patients present viral antigens effectively, but there is a selective inability to expand CD8(+) IFN-gamma-producing and IFN-gamma and IL-2 double-producing T cells when challenged with HIV-1 gag.

Diverse CD8+ T-Cell Responses to Renal Cell Carcinoma Antigens in Patients Treated with an Autologous Granulocyte-Macrophage Colony-Stimulating Factor Gene-Transduced Renal Tumor Cell Vaccine

A phase I clinical trial with granulocyte-macrophage colony-stimulating factor tumor cell vaccines in patients with metastatic renal cell carcinoma (RCC) showed immune cell infiltration at vaccine sites and delayed-type hypersensitivity (DTH) responses to autologous tumor cells indicative of T-cell immunity. To further characterize RCC T-cell responses and identify relevant RCC-associated antigens, we did a detailed analysis of CD8+ T-cell responses in two vaccinated RCC patients who generated the greatest magnitude of DTH response and also displayed a strong clinical response to vaccination (&gt;90% reduction in metastatic tumor volume). Three separate CD8+ T-cell lines (and subsequent derived clones) derived from patient 24 recognized distinct RCC-associated antigens. One recognized a shared HLA-A*0201-restricted antigen expressed by both renal cancer cells and normal kidney cells. This recognition pattern correlated with a positive DTH test to normal kidney cells despite no evidence of impairment of renal function by the patient's remaining kidney after vaccination. A second line recognized a shared HLA-C7-restricted antigen that was IFN-γ inducible. A third line recognized a unique HLA-A*0101-restricted RCC antigen derived from a mutated KIAA1440 gene specific to the tumor. In addition, two independent CTL lines and three clones were also generated from patient 26 and they recognized autologous tumor cells restricted through HLA-A*0205, HLA-A/B/C, and HLA-B/C. These results show that paracrine granulocyte-macrophage colony-stimulating factor tumor vaccines may generate a diverse repertoire of tumor-reactive CD8+ T-cell responses and emphasize the importance of polyvalency in the design of cancer immunotherapies.

Characterization of Host Immunity to cytomegalovirus pp150 (UL32)

The basic phosphoprotein 150 (pp150), the product of UL32 (unique long domain 32) gene of human cytomegalovirus (CMV), is an abundant component of the viral tegument and a target of human leukocyte antigen (HLA)-restricted cytotoxic T cells (CTLs) after infection. Identification of minimal cytotoxic epitopes (MCEs) from this CMV protein is of importance for peptide-based vaccines and immunotherapeutic approaches. Several pp150-specific CTL clones were derived from peripheral blood mononuclear cells of healthy CMV-positive donors with autologous fibroblasts infected either with CMV AD169 or with a recombinant vaccinia virus expressing full-length pp150 protein. HLA A*0301- and HLA A*6801-restricted CD8+ pp150 T-cell clones derived from different donors were found to efficiently kill autologous CMV-infected fibroblasts. Fine mapping of each MCE first used a T-cell epitope prediction algorithm. Overlapping peptides within the recognized regions were screened. The analysis identified pp150(792-802) and pp150(945-955) as MCEs for the HLA A*6801 and the HLA A*0301 pp150 clones, respectively. In vitro stimulation by recombinant modified vaccinia Ankara virus expressing full-length pp150 elicited high frequencies of CMV-CTL and interferon gamma production specific for the MCE identified in all subjects. The consistent presence of pp150 T cells in CMV-exposed individuals supports a role for this antigen in shaping the antiviral CTL response and indicates that pp150 could be a pivotal constituent of prophylactic and therapeutic CMV vaccines.

Effect of Serum and Antioxidants on the Immunogenicity of Protein Kinase C-Activated Chronic Lymphocytic Leukemia Cells

Since the intrinsically poor immunogenicity of chronic lymphocytic leukemia (CLL) cells might be a key factor in allowing them to avoid immune control mechanisms, the development of methods to enhance CLL cell immunogenicity might lead to improved disease control. The ability of CLL cells to stimulate T cells was increased significantly by the protein kinase C (PKC) agonist phorbol myristic acetate (PMA). However, under serum-free conditions, PMA-activated CLL cells died within 48 hours. Antioxidants, such as 2-mercaptoethanol (2-ME), or fetal calf serum could prevent the death of these cells but caused them to enter distinct states of differentiation. In the presence of 2-ME, PMA-activated CLL cells extended dendritic-like protrusions and exhibited increased T-cell stimulatory capacity. In the presence of serum, PMA-activated CLL cells developed fewer dendrites, made less IL-10 and more IL-12 p40 mRNA transcripts, and showed an increased capacity to induce IFN-gamma production by T cells. The effects of serum on the promotion of type 1 immune responses by phorbol ester-activated CLL cells were dominant and correlated with activation of the NF-kappaB signaling pathway. Other PKC agonists, such as Bryostatin-1 and a synthetic Bryostatin analog (Picolog), had similar effects on CLL cells. The observation that CLL cells can acquire features of dendritic cells that promote type 1 immunity may find clinical application in immunotherapeutic strategies for this disease.

Results of an ELISPOT proficiency panel conducted in 11 laboratories participating in international human immunodeficiency virus type 1 vaccine trials

We used an external quality assurance (EQA) panel to assess laboratory competency and comparability when performing ELISPOT assays in support of human immunodeficiency virus type 1 (HIV-1) vaccine trials. Cell recovery, viability, and frequency of interferon-gamma (IFN-gamma)-secreting cells after antigen stimulation were obtained from 11 laboratories on a coded panel of 11 peripheral blood mononuclear cell samples. The median recovery and viability before plating for all samples were 35% and 86%, respectively, with notable interlaboratory and intrasample variability. Empirical as well as statistical analysis methods were used to define positive ELISPOT responses. Remarkable concordance between laboratories was obtained in defining a qualitative assessment of responder/nonresponder status to antigens, but the frequency of responding cells varied among the laboratories. This study highlights the need for better standardization of protocols and reagents to obtain reliable and reproducible data that may support immunogenicity studies, vaccine regulatory submissions, and licensure.

Epitope specificity and longevity of a vaccine-induced human T cell response against HPV18

Persistent human papillomavirus (HPV) type 16 and 18 infection can lead to pre-malignant and malignant diseases of the lower genital tract. Several lines of evidence suggest that T cell responses can control HPV infection. However, relative to other human viruses, strong effector memory T cell responses against HPV have been difficult to detect. We used an in vitro stimulation step prior to enzyme-linked immunospot assays to identify IFN-γ-secreting T cells specific for HPV16 and 18 E6/E7 peptides. This allowed the detection of HPV-specific CD4⁺ T cells that were not evident in direct ex vivo assays. T cell responses against HPV16 or 18 peptides were detected in healthy volunteers (7/9) and patients with lower genital tract neoplasia (10/20). Importantly, this assay allowed tracking of vaccine-induced T cell responses in nine patients, following inoculation with a live recombinant vaccinia virus (HPV16 and 18 E6/E7, TA-HPV). Novel vaccine-induced T cell responses were demonstrated in five patients, but no clinical responses (lesion regressions) were seen. For one vaccinated patient, the T cell response was mapped to a single dominant HPV18 E7 epitope and this response was sustained for >3 years. Our data suggest that systemic memory T cells against HPV16 and 18, induced naturally or by TA-HPV vaccination, are relatively rare. Nevertheless, the assay system developed allowed estimation of magnitude, epitope specificity, and longevity of vaccine-induced CD4⁺ T cell responses. This will be useful for vaccine design and measurement of immunological endpoints in clinical trials.

Identification of a naturally processed cytotoxic CD8 T-cell epitope of coxsackievirus B4, presented by HLA-A2.1 and located in the PEVKEK region of the P2C nonstructural protein

The adaptive immune system generates CD8 cytotoxic T lymphocytes (CTLs) as a major component of the protective response against viruses. Knowledge regarding the nature of the peptide sequences presented by HLA class I molecules and recognized by CTLs is thus important for understanding host-pathogen interactions. In this study, we focused on identification of a CTL epitope generated from coxsackievirus B4 (CVB4), a member of the enterovirus group responsible for several inflammatory diseases in humans and often implicated in the triggering and/or acceleration of the autoimmune disease type 1 diabetes. We identified a 9-mer peptide epitope that can be generated from the P2C nonstructural protein of CVB4 (P2C(1137-1145)) and from whole virus by antigen-presenting cells and presented by HLA-A2.1. This epitope is recognized by effector memory (gamma interferon [IFN-gamma]-producing) CD8 T cells in the peripheral blood at a frequency of responders that suggests that it is a major focus of the anti-CVB4 response. Short-term CD8 T-cell lines generated against P2C(1137-1145) are cytotoxic against peptide-loaded target cells. Of particular interest, the epitope lies within a region of viral homology with the diabetes-related autoantigen, glutamic acid decarboxylase-65 (GAD(65)). However, P2C(1137-1145)-specific cytotoxic T lymphocyte (CTL) lines were not activated to produce IFN-gamma by the GAD(65) peptide homologue and did not show cytotoxic activity in the presence of appropriately labeled targets. These results describe the first CD8 T-cell epitope of CVB4 that will prove useful in the study of CVB4-associated disease.

DNA vaccines against human immunodeficiency virus type 1

Development of a vaccine against human immunodeficiency virus type 1 (HIV-1) is the main hope for controlling the acquired immunodeficiency syndrome pandemic. An ideal HIV vaccine should induce neutralizing antibodies, CD4+ helper T cells, and CD8+ cytotoxic T cells. While the induction of broadly neutralizing antibodies remains a highly challenging goal, there are a number of technologies capable of inducing potent cell-mediated responses in animal models, which are now starting to be tested in humans. Naked DNA immunization is one of them. This review focuses on the stimulation of HIV-specific T cells and discusses in the context of the current 'state-of-art' of DNA vaccines, the areas where this technology might assist either alone or as a part of more complex vaccine formulations in the HIV vaccine development.

Optimization of an elispot assay to detect cytomegalovirus-specific CD8+ T lymphocytes

Various arguments suggest that CD8+ T lymphocytes play a major role in the control of cytomegalovirus (CMV) infection. The detection of CMV-specific CD8+ T cells may therefore provide additional information about CMV virus detection to predict the risk of development of CMV disease, especially in immunodepressed transplant recipients. We compared and tested various experimental conditions to optimize an enzyme-linked immunospot assay (Elispot) assay for the detection of CMV-specific CD8+ T lymphocytes. The indirect Elispot assay with one six-day in vitro sensitization step was found to be the most sensitive method to detect CMV-specific CD8+ T cells compared to direct Elispot with unfractionated peripheral blood mononuclear cells or purified CD8+ T cells. We showed that low doses of interleukin-2 during the in vitro culture enhanced the sensitivity of this test, and tetramer staining was performed to verify the high efficiency of this in vitro stimulation step. We directly loaded the specific CMV peptide during the Elispot assay and demonstrated that the use of T2 cells did not improve its sensitivity. Elispot for the detection of interferon-gamma appears to be more sensitive and reliable than measurement of tumor necrosis factor alpha or granzyme B. This technique was successfully applied to detect CMV-specific CD8+ T cells in human leukocyte antigen A2 (HLA-A2) and HLA-B7 healthy patients and in one lymphopenic post-transplant patient with positive CMV serology. This highly sensitive test may be a useful tool to assess T-cell immunity directed against CMV in immunodepressed patients.

Memory T-cell-mediated immune responses specific to an alternative core protein in hepatitis C virus infection

In vitro studies have described the synthesis of an alternative reading frame form of the hepatitis C virus (HCV) core protein that was named F protein or ARFP (alternative reading frame protein) and includes a domain coded by the +1 open reading frame of the RNA core coding region. The expression of this protein in HCV-infected patients remains controversial. We have analyzed peripheral blood from 47 chronically or previously HCV-infected patients for the presence of T lymphocytes and antibodies specific to the ARFP. Anti-ARFP antibodies were detected in 41.6% of the patients infected with various HCV genotypes. Using a specific ARFP 99-amino-acid polypeptide as well as four ARFP predicted class I-restricted 9-mer peptides, we show that 20% of the patients display specific lymphocytes capable of producing gamma interferon, interleukin-10, or both cytokines. Patients harboring three different viral genotypes (1a, 1b, and 3) carried T lymphocytes reactive to genotype 1b-derived peptides. In longitudinal analysis of patients receiving therapy, both core and ARFP-specific T-cell- and B-cell-mediated responses were documented. The magnitude and kinetics of the HCV antigen-specific responses differed and were not linked with viremia or therapy outcome. These observations provide strong and new arguments in favor of the synthesis, during natural HCV infection, of an ARFP derived from the core sequence. Moreover, the present data provide the first demonstration of the presence of T-cell-mediated immune responses directed to this novel HCV antigen.

Evaluating the cytotoxicity of innate immune effector cells using the GrB ELISPOT assay

Background

This study assessed the Granzyme B (GrB) ELISPOT as a viable alternative to the ⁵¹Cr-release assay for measuring cytotoxic activity of innate immune effector cells. We strategically selected the GrB ELISPOT assay because GrB is a hallmark effector molecule of cell-mediated destruction of target cells.

Methods

We optimized the GrB ELISPOT assay using the human-derived TALL-104 cytotoxic cell line as effectors against K562 target cells. Titration studies were performed to assess whether the ELISPOT assay could accurately enumerate the number of GrB-secreting effector cells. TALL-104 were treated with various secretion inhibitors and utilized in the GrB ELISPOT to determine if GrB measured in the ELISPOT was due to degranulation of effector cells. Additionally, CD107a expression on effector cells after effector-target interaction was utilized to further confirm the mechanism of GrB release by TALL-104 and lymphokine-activated killer (LAK) cells. Direct comparisons between the GrB ELISPOT, the IFN-γ ELISPOT and the standard ⁵¹Cr-release assays were made using human LAK cells.

Results

Titration studies demonstrated a strong correlation between the number of TALL-104 and LAK effector cells and the number of GrB spots per well. GrB secretion was detectable within 10 min of effector-target contact with optimal secretion observed at 3–4 h; in contrast, optimal IFN-γ secretion was not observed until 24 h. The protein secretion inhibitor, brefeldin A, did not inhibit the release of GrB but did abrogate IFN-γ production by TALL-104 cells. GrB secretion was abrogated by BAPTA-AM (1,2-bis-(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid tetra(acetoxymethyl) ester), which sequesters intracellular Ca²⁺, thereby preventing degranulation. The number of effector cells expressing the degranulation associated glycoprotein CD107a increased after interaction with target cells and correlated with the stimulated release of GrB measured in the ELISPOT assay.

Conclusions

Because of its high sensitivity and ability to estimate cytotoxic effector cell frequency, the GrB ELISPOT assay is a viable alternative to the ⁵¹Cr-release assay to measure MHC non-restricted cytotoxic activity of innate immune cells. Compared to the IFN-γ ELISPOT assay, the GrB ELISPOT may be a more direct measure of cytotoxic cell activity. Because GrB is one of the primary effector molecules in natural killer (NK) cell-mediated killing, detection and enumeration of GrB secreting effector cells can provide valuable insight with regards to innate immunological responses.

Minor histocompatibility antigen DBY elicits a coordinated B and T cell response after allogeneic stem cell transplantation

We examined the immune response to DBY, a model H-Y minor histocompatibility antigen (mHA) in a male patient with chronic graft-versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplant from a human histocompatibility leukocyte antigen (HLA)-identical female sibling. Patient peripheral blood mononuclear cells were screened for reactivity against a panel of 93 peptides representing the entire amino acid sequence of DBY. This epitope screen revealed a high frequency CD4⁺ T cell response to a single DBY peptide that persisted from 8 to 21 mo after transplant. A CD4⁺ T cell clone displaying the same reactivity was established from posttransplant patient cells and used to characterize the T cell epitope as a 19-mer peptide starting at position 30 in the DBY sequence and restricted by HLA-DRB1*1501. Remarkably, the corresponding X homologue peptide was also recognized by donor T cells. Moreover, the T cell clone responded equally to mature HLA-DRB1*1501 male and female dendritic cells, indicating that both DBY and DBX peptides were endogenously processed. After transplant, the patient also developed antibodies that were specific for recombinant DBY protein and did not react with DBX. This antibody response was mapped to two DBY peptides beginning at positions 118 and 536. Corresponding DBX peptides were not recognized. These studies provide the first demonstration of a coordinated B and T cell immune response to an H-Y antigen after allogeneic transplant. The specificity for recipient male cells was mediated by the B cell response and not by donor T cells. This dual DBX/DBY antigen is the first mHA to be identified in the context of chronic GVHD.

Evaluation of Elispot assays: influence of method and operator on variability of results

In this study, a comprehensive comparative analysis of different evaluation methods of Elispot plates was performed. Three investigators using three different evaluation approaches read 50 randomly selected wells at three different time points. The methods were: (1) manual evaluation using a stereomicroscope, (2) automated evaluation using an image analysis reader system with reading parameters established by each investigator, and (3) automated evaluation using a reader system with preset reading parameters using assay-specific controls. We demonstrate that manual evaluation had the highest variability both within the same method and when comparing all methods, followed by automated evaluation with investigator-dependent parameters. The variability was low only when all investigators used the same parameters established using assay-specific controls. This variability was independent of operator or spot number per well. Based on this study, recommendations for standardization and validation procedures of Elispot assay performance and evaluation procedures are presented.

Immune reconstitution following autologous transfers of CD3/CD28 stimulated CD4+ T cells to HIV-infected persons

We have previously shown that adoptive transfer of in vitro CD3/CD28 activated autologous CD4(+) T cells results in increased CD4 counts and CD4/CD8 ratios in HIV+ subjects. In this report, analysis of variable beta (Vbeta) chain T cell receptor (TCR) repertoire showed that CD3/CD28 stimulation was able to increase polyclonality within skewed spectra types in vitro. In vivo, two of eight subjects showed increase in TCR diversity and importantly, in no subject did a highly skewed in vivo repertoire emerge. Measurement of proliferative response to alloantigen showed increases following infusions. Response to pharmacological stimulus and lectin via Interferon-gamma ELISpot assay showed increases in a subset of subjects following infusions. However, interferon-gamma response to HIV antigens and peptides declined concurrent with stable or diminishing latent infectious viral load in CD4(+) T cells. These data provide further evidence that adoptive transfer of activated autologous CD4(+) T cells can augment the immune system.

Impact of intrapeptide epitope location on CD8 T cell recognition: implications for design of overlapping peptide panels

BACKGROUND

Antigen-specific CD8 T cells following infection or immunization are typically assessed by measuring interferon-gamma production after stimulation with overlapping peptides spanning the region of interest. The effect of epitope location within such peptides is not known but may influence recognition.

OBJECTIVE

To examine if peptides containing the appropriate C-terminal anchor amino acid residue would provide more sensitive detection of T cell responses. The impact was examined of epitope location within overlapping peptides on recognition of epitope-specific CD8 T cell responses.

METHODS

C-terminal amino acid residues were analyzed in well-defined optimal epitopes for HIV, Epstein-Barr virus, cytomegalovirus and influenza and in peptide-binding motifs. Recognition of known epitopes within longer synthesized peptides by peripheral blood mononuclear cells or CD8 T cell lines was tested using interferon-gamma Elispot at various peptide concentrations.

RESULTS

Only 9 of 20 amino acids served as the C-terminal anchor position in 96% of described optimal epitopes and in 95% of peptide-binding motifs. A CD8 T cell response to an epitope within a longer peptide is best detected when the epitope is situated at the C-terminal end of the longer peptide, both when using peptides designed to include the optimal epitope at every possible position and when comparing responses towards optimal epitopes and corresponding overlapping peptides in a larger group of subjects.

CONCLUSION

When using overlapping peptides to screen for CD8 T cell responses, more sensitive detection will be achieved using known C-terminal anchor amino acid residues at the C-terminus.

Hierarchical targeting of subtype C human immunodeficiency virus type 1 proteins by CD8+ T cells: correlation with viral load

An understanding of the relationship between the breadth and magnitude of T-cell epitope responses and viral loads is important for the design of effective vaccines. For this study, we screened a cohort of 46 subtype C human immunodeficiency virus type 1 (HIV-1)-infected individuals for T-cell responses against a panel of peptides corresponding to the complete subtype C genome. We used a gamma interferon ELISPOT assay to explore the hypothesis that patterns of T-cell responses across the expressed HIV-1 genome correlate with viral control. The estimated median time from seroconversion to response for the cohort was 13 months, and the order of cumulative T-cell responses against HIV proteins was as follows: Nef > Gag > Pol > Env > Vif > Rev > Vpr > Tat > Vpu. Nef was the most intensely targeted protein, with 97.5% of the epitopes being clustered within 119 amino acids, constituting almost one-third of the responses across the expressed genome. The second most targeted region was p24, comprising 17% of the responses. There was no correlation between viral load and the breadth of responses, but there was a weak positive correlation (r = 0.297; P = 0.034) between viral load and the total magnitude of responses, implying that the magnitude of T-cell recognition did not contribute to viral control. When hierarchical patterns of recognition were correlated with the viral load, preferential targeting of Gag was significantly (r = 0.445; P = 0.0025) associated with viral control. These data suggest that preferential targeting of Gag epitopes, rather than the breadth or magnitude of the response across the genome, may be an important marker of immune efficacy. These data have significance for the design of vaccines and for interpretation of vaccine-induced responses.

Antiviral immunity and T-regulatory cell function are retained after selective alloreactive T-cell depletion in both the HLA-identical and HLA-mismatched settings

Nonselective T-cell depletion reduces the incidence of severe graft-versus-host disease after allogeneic hematopoietic stem cell transplantation, but the cost is delayed and disordered antigen-specific immune reconstitution and increased infection. We use a method of selective depletion of alloreactive T cells expressing the activation marker CD69 after coculture with stimulator cells in a modified or standard mixed lymphocyte reaction. The technique has been shown to reduce alloreactivity while retaining third-party responses in vitro and, in a mismatched murine model, led to donor T-cell engraftment with a virtual absence of graft-versus-host disease and increased survival. We show in a human HLA-mismatched and unrelated HLA-identical setting that this technique retains >80% of specific cellular antiviral activity by cytomegalovirus-tetramer analysis and cytomegalovirus/Epstein-Barr virus peptide-stimulated interferon-gamma ELISpot assay. Furthermore, CD4(+) CD25(+) T-regulatory cells are not removed by this method of selective allodepletion and retain their function in suppressing allogeneic proliferative responses. Preservation of antiviral cytotoxic T lymphocytes in selectively allodepleted stem cell grafts would lead to improved antiviral immunity after transplantation. The retention of immunosuppressive CD4(+) CD25(+) T-regulatory cells could lead to more ordered immune reconstitution and further suppress alloreactive responses after transplantation.