ELISPOT assay for detection of peptide specific interferon-gamma secreting cells in rhesus macaques

Bioinformatic, Biochemical, and Immunological Mining of MHC Class I Restricted T Cell Epitopes for a Marburg Nucleoprotein Microparticle Vaccine

The Marburg virus (MARV), the virus responsible for Marburg hemorrhagic fever (MHF), is considered a top-priority pathogen for vaccine development. Recent outbreaks in Equatorial Africa have highlighted the urgency of MARV because of its high fatality rate and historical concerns about potential weaponization. Currently, there are no licensed vaccines for MARV. Existing vaccine candidates rely on attenuated recombinant vesicular stomatitis virus carrying MARV glycoprotein (VSVΔG) or the chimpanzee replication-defective adenovirus 3 vector ChAd3-MARV. Although these platforms provide significant protection in animal models, they face challenges because of their limited thermal stability and the need for cold storage during deployment in resource-poor areas. An alternative approach involves using adjuvanted poly (lactic-co-glycolic acid) (PLGA) microparticles loaded with synthetic peptides representing MHC class I-restricted T cell epitopes. This vaccine platform has demonstrated effectiveness in protecting against SARS-CoV-2 and EBoV disease in animal models and has the advantage of not requiring cold storage and remaining stable at room temperature for over six months. This report outlines the design, manufacturing, and in vivo immunogenicity testing of PLGA microparticle human vaccines designed to prevent Marburg hemorrhagic fever.

Validation of an IFN-gamma ELISpot assay to measure cellular immune responses against viral antigens in non-human primates

Adeno-Associated Virus (AAV)-based gene therapy vectors are in development for many inherited human disorders. In nonclinical studies, cellular immune responses mediated by cytotoxic T cells may target vector-transduced cells, which could impact safety and efficacy. Here, we describe the bioanalytical validation of an interferon-gamma (IFN-γ)-based Enzyme-Linked Immunospot (ELISpot) assay for measuring T cell responses against viral antigens in cynomolgus monkeys. Since ELISpots performed with antigen-derived peptides offer a universal assay format, method performance characteristics were validated using widely available peripheral blood mononuclear cells (PBMCs) responsive to cytomegalovirus peptides. The limit of detection and confirmatory cut point were established using statistical methods; precision, specificity, and linearity were confirmed. Monkey PBMCs from an AAV5 gene therapy study were then analyzed, using peptide pools spanning the vector capsid and transgene product. AAV5-specific T cell responses were detected only in 2 of 18 monkeys at Day 28, but not at Day 13 and 56 after vector administration, with no correlation to liver enzyme elevations or transgene expression levels. No transgene product-specific T cell responses occurred. In conclusion, while viral peptide-specific IFN-γ ELISpots can be successfully validated for monkey PBMCs, monitoring peripheral T cell responses in non-clinical AAV5 gene therapy studies was of limited value to interpret safety or efficacy.

How Can Elispot Add Information to Improve Knowledge on Tropical Diseases?

Elispot has been used as an important tool for detecting immune cells' products and functions and has facilitated the understanding of host-pathogen interaction. Despite the incredible diversity of possibilities, two main approaches have been developed: the immunopathogenesis and diagnosis/prognosis of infectious diseases as well as cancer research. Much has been described on the topics of allergy, autoimmune diseases, and HIV-Aids, however, Elispot can also be applied to other infectious diseases, mainly leishmaniasis, malaria, some viruses, helminths and mycosis usually classified as tropical diseases. The comprehension of the function, concentration and diversity of the immune response in the infectious disease is pointed out as crucial to the development of infection or disease in humans and animals. In this review we will describe the knowledge already obtained using Elispot as a method for accessing the profile of immune response as well as the recent advances in information about host-pathogen interaction in order to better understand the clinical outcome of a group of tropical and neglected diseases.

Analyzing cellular immunity to AAV in a canine model using ELISPOT assay

Adeno-associated viral (AAV) vector-mediated gene transfer represents a promising gene replacement strategy for treating various genetic diseases. One obstacle in using viral-derived vectors for in vivo gene delivery is the development of host immune responses to the vector. Recent studies have demonstrated cellular immune responses specific to capsid proteins of various AAV serotypes in animal models and in human trials for different diseases. We developed a canine-specific ELISPOT assay to detect such immunity in dogs received AAV treatment. Here, we describe in detail the use of a constructed panel of overlapping peptides spanning the entire VP1 sequence of AAV capsid protein to detect specific T-cell responses in peripheral blood in dogs following intramuscular injection of AAV. This high-throughput method allows the identification of T-cell epitopes without the need for large cell numbers and the need for major histocompatibility complex molecule-matched cell lines.

Sterile protection against Plasmodium knowlesi in rhesus monkeys from a malaria vaccine: comparison of heterologous prime boost strategies

Using newer vaccine platforms which have been effective against malaria in rodent models, we tested five immunization regimens against Plasmodium knowlesi in rhesus monkeys. All vaccines included the same four P. knowlesi antigens: the pre-erythrocytic antigens CSP, SSP2, and erythrocytic antigens AMA1, MSP1. We used four vaccine platforms for prime or boost vaccinations: plasmids (DNA), alphavirus replicons (VRP), attenuated adenovirus serotype 5 (Ad), or attenuated poxvirus (Pox). These four platforms combined to produce five different prime/boost vaccine regimens: Pox alone, VRP/Pox, VRP/Ad, Ad/Pox, and DNA/Pox. Five rhesus monkeys were immunized with each regimen, and five Control monkeys received a mock vaccination. The time to complete vaccinations was 420 days. All monkeys were challenged twice with 100 P. knowlesi sporozoites given IV. The first challenge was given 12 days after the last vaccination, and the monkeys receiving the DNA/Pox vaccine were the best protected, with 3/5 monkeys sterilely protected and 1/5 monkeys that self-cured its parasitemia. There was no protection in monkeys that received Pox malaria vaccine alone without previous priming. The second sporozoite challenge was given 4 months after the first. All 4 monkeys that were protected in the first challenge developed malaria in the second challenge. DNA, VRP and Ad5 vaccines all primed monkeys for strong immune responses after the Pox boost. We discuss the high level but short duration of protection in this experiment and the possible benefits of the long interval between prime and boost.

Methods for quantitating antigen-specific T cell responses using functional assays in rhesus macaques

Ex vivo enumeration of the absolute frequencies of antigen-specific T cells is key for evaluating the immunogenicity of T cell-based vaccines. Currently there are three methods that are widely used to quantify cellular immune responses: Enzyme-Linked Immuno Spot assay (ELISpot), Intracellular Cytokine Staining assay (ICS) and Tetramer assay. These three different assays offer different information. In this chapter, I discuss the two functional assays, ELISpot and ICS. The ELISpot and ICS assays use short term in vitro stimulation to assay the frequency and cytokine expression profiles of responding cells. The ELISpot assay scores spots of captured cytokine produced by individual cells whereas, ICS uses flow cytometry to profile individual cells for surface markers and the production of cytokines.

Protection of rhesus monkeys by a DNA prime/poxvirus boost malaria vaccine depends on optimal DNA priming and inclusion of blood stage antigens

Background

We have previously described a four antigen malaria vaccine consisting of DNA plasmids boosted by recombinant poxviruses which protects a high percentage of rhesus monkeys against Plasmodium knowlesi (Pk) malaria. This is a multi-stage vaccine that includes two pre-erythrocytic antigens, PkCSP and PkSSP2(TRAP), and two erythrocytic antigens, PkAMA-1 and PkMSP-1(42kD). The present study reports three further experiments where we investigate the effects of DNA dose, timing, and formulation. We also compare vaccines utilizing only the pre-erythrocytic antigens with the four antigen vaccine.

Methodology

In three experiments, rhesus monkeys were immunized with malaria vaccines using DNA plasmid injections followed by boosting with poxvirus vaccine. A variety of parameters were tested, including formulation of DNA on poly-lactic co-glycolide (PLG) particles, varying the number of DNA injections and the amount of DNA, varying the interval between the last DNA injection to the poxvirus boost from 7 to 21 weeks, and using vaccines with from one to four malaria antigens. Monkeys were challenged with Pk sporozoites given iv 2 to 4 weeks after the poxvirus injection, and parasitemia was measured by daily Giemsa stained blood films. Immune responses in venous blood samples taken after each vaccine injection were measured by ELIspot production of interferon-γ, and by ELISA.

Conclusions

1) the number of DNA injections, the formulation of the DNA plasmids, and the interval between the last DNA injection and the poxvirus injection are critical to vaccine efficacy. However, the total dose used for DNA priming is not as important; 2) the blood stage antigens PkAMA-1 and PkMSP-1 were able to protect against high parasitemias as part of a genetic vaccine where antigen folding is not well defined; 3) immunization with PkSSP2 DNA inhibited immune responses to PkCSP DNA even when vaccinations were given into separate legs; and 4) in a counter-intuitive result, higher interferon-γ ELIspot responses to the PkCSP antigen correlated with earlier appearance of parasites in the blood, despite the fact that PkCSP vaccines had a protective effect.

Development and evaluation of a real-time reverse transcription-PCR assay for quantification of gamma interferon mRNA to diagnose tuberculosis in multiple animal species

Tuberculosis of free-ranging and captive wildlife, including species implicated in the maintenance and transmission of Mycobacterium bovis, is a difficult disease to diagnose and control. Historically, diagnosis of tuberculosis has relied largely upon assays of cell-mediated immunity (CMI), such as tuberculin skin testing. This approach, however, is problematic or impractical for use with many wildlife species. Increasingly, in vitro diagnostic tests, including gamma interferon (IFN-gamma)-based assays, are replacing or complementing skin testing of cattle and humans. Analogous assays are unavailable for most wildlife because of a lack of species-specific immunological reagents. This report describes the development and validation of a whole-blood assay to quantify antigen-specific IFN-gamma mRNA expression by quantitative real-time reverse transcription-PCR. Oligonucleotide primers and probes were designed and tested for reactivity towards several susceptible species of interest with respect to tuberculosis infection. The assay was subsequently optimized to quantify the IFN-gamma mRNA expression in elk and red deer (Cervus elaphus) and was evaluated for its ability to detect mycobacterial antigen-specific responses of experimentally tuberculosis-infected animals. The assay was a simple, rapid, and sensitive measure of antigen-specific CMI. The IFN-gamma mRNA responses correlated well with IFN-gamma protein production and showed performance in determining an animal's infection status superior to that of either lymphocyte proliferation or IFN-gamma protein enzyme-linked immunosorbent assay methods. An additional advantage is the ease with which the assay can be modified to reliably quantify IFN-gamma expression by using consensus sequences of closely related species or of other species for which IFN-gamma sequence information is available.

Induction of multi-antigen multi-stage immune responses against Plasmodium falciparum in rhesus monkeys, in the absence of antigen interference, with heterologous DNA prime/poxvirus boost immunization

The present study has evaluated the immunogenicity of single or multiple Plasmodium falciparum (Pf) antigens administered in a DNA prime/poxvirus boost regimen with or without the poloxamer CRL1005 in rhesus monkeys. Animals were primed with PfCSP plasmid DNA or a mixture of PfCSP, PfSSP2/TRAP, PfLSA1, PfAMA1 and PfMSP1-42 (CSLAM) DNA vaccines in PBS or formulated with CRL1005, and subsequently boosted with ALVAC-Pf7, a canarypox virus expressing the CSLAM antigens. Cell-mediated immune responses were evaluated by IFN-γ ELIspot and intracellular cytokine staining, using recombinant proteins and overlapping synthetic peptides. Antigen-specific and parasite-specific antibody responses were evaluated by ELISA and IFAT, respectively. Immune responses to all components of the multi-antigen mixture were demonstrated following immunization with either DNA/PBS or DNA/CRL1005, and no antigen interference was observed in animals receiving CSLAM as compared to PfCSP alone. These data support the down-selection of the CSLAM antigen combination. CRL1005 formulation had no apparent effect on vaccine-induced T cell or antibody responses, either before or after viral boost. In high responder monkeys, CD4+IL-2+ responses were more predominant than CD8+ T cell responses. Furthermore, CD8+ IFN-γ responses were detected only in the presence of detectable CD4+ T cell responses. Overall, this study demonstrates the potential for multivalent Pf vaccines based on rational antigen selection and combination, and suggests that further formulation development to increase the immunogenicity of DNA encoded antigens is warranted.

Abeta42 gene vaccine prevents Abeta42 deposition in brain of double transgenic mice

Abeta42 peptide aggregation and deposition is an important component of the neuropathology of Alzheimer's disease (AD). Gene-gun mediated gene vaccination targeting Abeta42 is a potential method to prevent and treat AD. APPswe/PS1DeltaE9 transgenic (Tg) mice were immunized with an Abeta42 gene construct delivered by the gene gun. The vaccinated mice developed Th2 antibodies (IgG1) against Abeta42. The Abeta42 levels in brain were decreased by 41% and increased in plasma 43% in the vaccinated compared with control mice as assessed by ELISA analysis. Abeta42 plaque deposits in cerebral cortex and hippocampus were reduced by 51% and 52%, respectively, as shown by quantitative immunolabeling. Glial cell activation was also significantly attenuated in vaccinated compared with control mice. One rhesus monkey was vaccinated and developed anti-Abeta42 antibody. These new findings advance significantly our knowledge that gene-gun mediated Abeta42 gene immunization effectively induces a Th2 immune response and reduces the Abeta42 levels in brain in APPswe/PS1DeltaE9 mice. Abeta42 gene vaccination may be safe and efficient immunotherapy for AD.

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.

Defining blood processing parameters for optimal detection of cryopreserved antigen-specific responses for HIV vaccine trials

Interferon-gamma (IFN-gamma) ELISpot and intracellular cytokine staining (ICS) assays are routinely employed in clinical HIV vaccine trials to identify antigen-specific T cells in cryopreserved peripheral blood mononuclear cells (PBMC). Several parameters involved in blood collection, processing and shipping may influence immunological function of the resulting cells, including anticoagulant type, time from venipuncture to PBMC isolation/cryopreservation, method of PBMC isolation and procedure for sample shipping. We examined these parameters in single and multiple site studies, and found the length of time from venipuncture to cryopreservation is the most important parameter affecting performance of T cells in immunological assays. Comparing blood processed at 24 h after venipuncture with that processed within 8 h, we observed on average a modest reduction in PBMC viability ( approximately 8% decrease), a greater loss in cell recovery ( approximately 32%), and between 36-56% loss in IFN-gamma T cell frequencies by ELISpot assay. We also describe three cold shipping methods that maintain immunological function in appropriately cryopreserved PBMC. These data indicate that cryopreservation of PBMC should occur within 8 h of venipuncture for optimal performance. This narrow window for specimen processing has important implications in selecting and monitoring clinical sites with laboratory capacity to perform these procedures in future clinical trials.

Abeta42 gene vaccination reduces brain amyloid plaque burden in transgenic mice

OBJECTIVE

To demonstrate that in APPswe/PS1DeltaE9 transgenic mice, gene gun mediated Abeta42 gene vaccination elicits a high titer of anti-Abeta42 antibodies causal of a significant reduction of Abeta42 deposition in brain.

METHODS

Gene gun immunization is conducted with transgenic mice using the Abeta42 gene in a bacterial plasmid with the pSP72-E3L-Abeta42 construct. Enzyme-linked immunoabsorbent assays (ELISA) and Western blots are used to monitor anti-Abeta42 antibody levels in serum and Abeta42 levels in brain tissues. Enzyme-linked immunospot (ELISPOT) assays are used for detection of peripheral blood T cells to release gamma-interferon. Immunofluorescence detection of Abeta42 plaques and quantification of amyloid burden of brain tissue were measured and sections were analyzed with Image J (NIH) software.

RESULTS

Gene gun vaccination with the Abeta42 gene resulted in high titers of anti-Abeta42 antibody production of the Th2-type. Levels of Abeta42 in treated transgenic mouse brain were reduced by 60-77.5%. The Mann-Whitney U-test P=0.0286.

INTERPRETATION

We have developed a gene gun mediated Abeta42 gene vaccination method that is efficient to break host Abeta42 tolerance without using adjuvant and induces a Th2 immune response. Abeta42 gene vaccination significantly reduces the Abeta42 burden of the brain in treated APPswe/PS1DeltaE9 transgenic mice with no overlap between treated and control mice.

Anti-HBV immune responses in rhesus macaques elicited by electroporation mediated DNA vaccination

Electroporation has been shown to be an effective method to improve the efficiency of gene expression and the immunogenicity of DNA vaccines. In order to optimize the procedure and test for its efficacy in more clinically relevant large animal models, we examined the detailed immune responses in rhesus macaques after vaccination intramuscularly with electroporation using the plasmid encoding for HBV preS(2)-S antigen and an adjuvant plasmid encoding for hIL-2 and hIFN-gamma. Several important factors were examined, including the dose response relationships, the effect of various prime and boost regimens, and different combinations of electro-pulse parameters. The immune responses were closely followed for more than a year. The results showed that in rhesus macaques, electroporation can significantly enhance the immunogenicity of the DNA vaccines, resulting in greatly improved antibody responses as well as peptide-stimulated IFN-gamma producing T cell responses. In addition, we also reported the different antibody response behaviors resulted from different electro-pulse parameters. The detailed data would be useful to suggest possible optimization strategies for better DNA vaccine efficacy.

Quantification and characterization of myosin peptide-specific CD4+ T cells in autoimmune myocarditis

Characterization of autoantigen-specific CD4+ T cells at the single cell level is crucial for understanding the immunopathological mechanisms underlying autoimmune diseases. Cardiac myosin heavy chain (myhca) is the major autoantigen associated with autoimmune myocarditis both in humans and in experimental autoimmune myocarditis (EAM) in mice. In the current study, we evaluated two methods for the enumeration and phenotypic characterization of myhca-specific CD4+ T cells during the course of EAM. Both enzyme-linked immunospot (ELISPOT) and cytokine flow cytometry (CFC) assays were suitable for the detection and characterization of myhca-specific Th cells during acute myocardial inflammation and the late healing phase of the disease. Cytokine production of myhca-specific Th cells was restricted to interferon-gamma (IFNgamma). Only trace amounts of the Th2 cytokines IL-4 and IL-5 could be detected. Concomitant surface marker analysis in the CFC assay revealed the prototypical effector phenotype of myhca-specific Th1 cells during the acute phase of the disease. Taken together, the combination of both methods appears to be most appropriate for a comprehensive ex vivo single cell analysis of Th cells in heart-specific autoimmune disorders.

Scrub typhus vaccine candidate Kp r56 induces humoral and cellular immune responses in cynomolgus monkeys

A truncated recombinant 56-kDa outer membrane protein of the Karp strain of Orientia tsutsugamushi (Kp r56) was evaluated in cynomolgus monkeys (Macaca fascicularis) for immunogenicity and safety as a vaccine candidate for the prevention of scrub typhus. This recombinant antigen induced strong humoral and cellular immune responses in two monkeys and was found to be well tolerated. Antigen-specific immunoglobulin M (IgM) and IgG were produced to almost maximal levels within 1 week of a single immunization. Peripheral blood mononuclear cells from vaccinated animals showed an induction of antigen-specific proliferation and gamma interferon production. The Kp r56 was not as efficient as infection with live organisms in preventing reinfection but was able to reduce the inflammation produced at the site of challenge. This report describes the results of the first systematic study of the immunogenicity of a recombinant scrub typhus vaccine candidate in a nonhuman primate model.

DNA immunization against respiratory syncytial virus (RSV) in infant rhesus monkeys

A DNA vaccine was tested in infant Rhesus macaques to evaluate its safety, immunogenicity and protective efficacy. Monkeys were vaccinated and challenged with a clinical isolate of human RSV. Vaccinated animals developed humoral and cellular responses following inoculation with plasmid DNA encoding the fusion (F) and nucleoprotein (N), from closely related bovine RSV. Vaccinated monkeys had decreased RSV in their lungs post-infection, and there was a qualitative difference in histopathology observed between vaccinated and unvaccinated animals. The combined result of safety and immunogenicity in a neonatal primate model is encouraging, suggesting the feasibility of DNA vaccines against RSV in infants.

Protection of rhesus monkeys against infection with minimally pathogenic simian-human immunodeficiency virus: correlations with neutralizing antibodies and cytotoxic T cells

We studied the capacity of active immunization of rhesus monkeys with HIV-1 envelope protein (Env) to induce primary virus cross-reactive neutralizing antibodies to prevent infection following intravenous challenge with simian-human immunodeficiency virus (SHIV). Monkeys were immunized with the human immunodeficiency type 1 (HIV-1) strain R2 Env. Initially, the Env was expressed in vivo by an alphavirus replicon particle system, and then it was administered as soluble oligomeric gp140. Concurrently, groups of monkeys received expression vectors that encoded either simian immunodeficiency virus (SIV) gag/pol genes or no SIV genes in vivo to test the additional protective benefit of concurrent induction of virus-specific cell-mediated immune (CMI) responses. Groups of control monkeys received either the gag/pol regimen or sham immunizations. The antibodies induced by the Env immunization regimen neutralized diverse primary HIV-1 strains. Similarly, potent CMI responses were induced by the gag/pol regimen, as measured by gamma interferon enzyme-linked immunospot assays. Differences in the responses among groups of monkeys strongly suggested that there was interference between the Env and gag/pol immunization regimens. Complete protection of some of the monkeys against infection after intravenous challenge with the partially pathogenic SHIV(DH12R (Clone 7)) was associated independently with both neutralizing antibody and CMI responses. Protection was associated with SHIV(DH12 (Clone 7)) serum neutralizing antibody titers of > or =1:80 or with cellular immune responses corresponding to >2,000 spot forming cells per 10(6) peripheral blood mononuclear cells. Immunization was also associated with a reduction in the magnitude and duration of virus load. Induction of cross-reactive, primary HIV-1-neutralizing antibodies is feasible and, when potent, may result in complete protection against infection with a heterologous challenge virus strain.

Enumeration of antigen-specific CD8+ T lymphocytes by single-platform, HLA tetramer-based flow cytometry: a European multicenter evaluation

BACKGROUND

HLA class I peptide tetramers represent powerful diagnostic tools for detection and monitoring of antigen-specific CD8(+) T cells. The impetus for the current multicenter study is the critical need to standardize tetramer flow cytometry if it is to be implemented as a routine diagnostic assay. Hence, the European Working Group on Clinical Cell Analysis set out to develop and evaluate a single-platform tetramer-based method that used cytomegalovirus (CMV) as the antigenic model.

METHODS

Absolute numbers of CMV-specific CD8(+) T cells were obtained by combining the percentage of tetramer-binding cells with the absolute CD8(+) T-cell count. Six send-outs of stabilized blood from healthy individuals or CMV-carrying donors with CMV-specific CD8(+) T-cell counts of 3 to 10 cells/microl were distributed to 7 to 16 clinical sites. These sites were requested to enumerate CD8(+) T cells and, in the case of CMV-positive donors, CMV-specific subsets on three separate occasions using the standard method.

RESULTS

Between-site coefficients of variation of less than 10% (absolute CD8(+) T-cell counts) and approximately 30% (percentage and absolute numbers of CMV-specific CD8(+) T cells) were achieved. Within-site coefficients of variation were approximately 5% (absolute CD8(+) T-cell counts), approximately 9% (percentage CMV-specific CD8(+) T cells), and approximately 17% (absolute CMV-specific CD8(+) T-cell counts). The degree of variation tended to correlate inversely with the proportion of CMV-specific CD8(+) T-cell subsets.

CONCLUSIONS

The single-platform MHC tetramer-based method for antigen-specific CD8(+) T-cell counting has been evaluated by a European group of laboratories and can be considered a reproducible assay for routine enumeration of antigen-specific CD8(+) T cells.

Granulocyte contamination dramatically inhibits spot formation in AIDS virus-specific ELISpot assays: analysis and strategies to ameliorate

The interferon-gamma (IFNgamma) ELISpot assay has become the most critical tool for HIV vaccine evaluation. External factors affecting ELISpot results must be minimized for the data to be reliably used in vaccine research and development processes. In pre-clinical pigtail macaque studies analyzing HIV/SIV vaccine studies, we detected a strong correlation between levels of granulocytes contaminating PBMC preparations and reduction in the quality and quantity of spots in the IFNgamma ELISpot assay. Acute SHIV infection of macaques worsened granulocyte contamination of PBMC fractions and made the assay much less reliable in detecting SIV-specific T cell immunity compared to intracellular cytokine staining (ICS). This problem could be ameliorated by using an F(ab)(2) form of the MD-1 IFNgamma capture antibody, presumably reflecting that activation of granulocytes in the well by the Fc portion of the standard capture antibody disrupts spot formation. Improving the standard ELISpot assay by using an F(ab)(2) capture antibody should make it more reliable for use in critical vaccine development studies.

Gene vaccination to bias the immune response to amyloid-beta peptide as therapy for Alzheimer disease

BACKGROUND

The amyloid-beta (Abeta) peptide has a central role in the neurodegeneration of Alzheimer disease (AD). Immunization of AD transgenic mice with Abeta(1-42) (Abeta(42)) peptide reduces both the spatial memory impairments and AD-like neuropathologic changes in these mice. Therapeutic immunization with Abeta in patients with AD was shown to be effective in reducing Abeta deposition, but studies were discontinued owing to the development of an autoimmune, cell-mediated meningoencephalitis. We hypothesized that gene vaccination could be used to generate an immune response to Abeta(42) that produced antibody response but avoided an adverse cell-mediated immune effect.

OBJECTIVE

To develop an effective genetic immunization approach for treatment and prevention of AD without causing an autoimmune, cell-mediated meningoencephalitis.

METHODS

Mice were vaccinated with a plasmid that encodes Abeta(42), administered by gene gun. The immune response of the mice to Abeta(42) was monitored by measurement of (1) antibody levels by enzyme-linked immunosorbent assay (ELISA) and Western blot and (2) Abeta(42)-specific T-cell response as measured by interferon-gamma enzyme-linked immunospot (ELISPOT) assay.

RESULTS

Gene-gun delivery of the mouse Abeta(42) dimer gene induced significant humoral immune responses in BALB/c wild-type mice after 3 vaccinations in 10-day intervals. All 3 mice in the treated group showed significant humoral immune responses. The ELISPOT assay for interferon-gamma release with mouse Abeta(42) peptide and Abeta(9-18) showed no evident cytotoxic T-lymphocyte response. We further tested the responses of wild-type BALB/c mice to the monomer Abeta(42) gene vaccine. Western blot evaluation showed both human and mouse Abeta monomer gene vaccine elicited detectable humoral immune responses. We also introduced the human Abeta(42) monomer gene vaccine into AD double transgenic mice APPswe/PSEN1(A246E). Mice were vaccinated with plasmids that encode Abeta(1-42) and Abeta(1-16), or with plasmid without the Abeta gene. Treated mice showed significant humoral immune responses as demonstrated by ELISA and by Western blot. These mice also showed no significant cellular immune response as tested by ELISPOT. One of the treated mice was killed at 7 months of age for histological observations, and scattered amyloid plaques were noted in all layers of the cerebral cortex and in the hippocampus in both Abeta(42)- and control-vaccinated mice. No definite difference was discerned between the experimental and control animals.

CONCLUSIONS

Gene-gun-administered genetic immunization with the Abeta(42) gene in wild-type BALB/c and AD transgenic mice can effectively elicit humoral immune responses without a significant T-cell-mediated immune response to the Abeta peptide. This immunotherapeutic approach could provide an alternative active immunization method for therapy and prevention of AD.

Gamma interferon responses to Plasmodium falciparum liver-stage antigen 1 and thrombospondin-related adhesive protein and their relationship to age, transmission intensity, and protection against malaria

Gamma interferon (IFN-gamma) responses to the Plasmodium falciparum antigens liver-stage antigen 1 (LSA-1) and thrombospondin-related adhesive protein (TRAP) are thought to be important in protection against malaria. Optimal methods of testing and the effects of age and transmission intensity on these responses are unknown. IFN-gamma responses to LSA-1 and TRAP peptides were assessed by the enzyme-linked immunospot assay (ELISPOT) and enzyme-linked immunosorbent assay (ELISA) in children and adults from areas of stable and unstable malaria transmission in Kenya. Adults in the areas of stable and unstable transmission had similar frequencies and levels of IFN-gamma responses to LSA-1 and TRAP as determined by ELISPOT and ELISA. In contrast, IFN-gamma responses to the LSA-1 T3 peptide (assessed by ELISPOT) and to any LSA-1 peptide (assessed by ELISA) were less frequent in children in the area of unstable transmission than in children in the area of stable transmission. IFN-gamma responses to LSA-1 were more frequently detected by ELISA than by ELISPOT in the stable-transmission area. IFN-gamma responses detected by ELISA and ELISPOT did not correlate with each other. In children in the stable-transmission area, IFN-gamma responses to LSA-1 peptides assessed by ELISA, but not by ELISPOT, were associated with protection against clinical malaria and anemia. IFN-gamma responses to LSA-1 appear to require repeated P. falciparum exposure and/or increased age and, as measured by ELISA, are associated with protection against clinical malaria and anemia.

Cytokine flow cytometry: a multiparametric approach for assessing cellular immune responses to viral antigens

Considerable attention has been focused on CD8 and CD4 T cell responses as a major element of the cellular immune response to viral infections including human immunodeficiency virus (HIV) and hepatitis C virus (HCV). However, increasing evidence based on the recent introduction of more quantitative assays for measuring antigen-specific T cells has suggested that the role of these cells in the development of a protective immune response to a particular viral pathogen may be determined by a complex interplay of multiple virologic and cellular factors. Thus, measurements of only the frequencies of the T cell subsets participating in the response to viral pathogens may be an incomplete reflection of efficacy. In this review, we suggest that some measurable factors may influence the role of T cell immunity in conferring protection, including functional avidity, epitope breadth and specificity, proliferative capacity, cytokine repertoire, degree of anergy, and differentiation phenotype, as well as magnitude, of viral-specific CD4 and CD8 T cells. We suggest that automated cytokine flow cytometry (CFC) is an efficient approach to the measurement of the complex interplay of multiple immune parameters involved in immune protection. These ideas are discussed in the context of new developments in sample preparation and analysis automation.

Detection of antigen-specific T cell interferon γ expression by ELISPOT and cytokine flow cytometry assays in rhesus macaques

Both enzyme-linked immunospot (ELISPOT) and cytokine flow cytometry (CFC) methods have been developed for the detection of low-frequency, antigen-specific, cytokine-producing T cells following short-term in vitro stimulation. Peptide-based ELISPOT and CFC assays were compared for the quantitative detection of interferon gamma-positive (IFN-gamma+) antigen-specific T cells in rhesus macaques. Ten normal and nine simian immunodeficiency virus (SIV)-infected monkeys were tested for the detection of IFN-gamma+ memory T cells specific for p27(gag) peptides of SIV with both assays. The CFC assay detected more IFN-gamma+ cells than the ELISPOT assay and this assay was more informative in identifying the phenotype of responding cells. Cryopreserved cells were as functional as fresh cells in heparinized blood samples and compared to EDTA, heparin was the better anticoagulant for yielding IFN-gamma+ cells. Using overlapping peptide pools, 20-mer peptides were more efficient in stimulating CD4+ T cells than 15-mer peptides in the ELISPOT assay, but there was no significant difference between 20- and 15-mer peptides in detecting CD4 or CD8+, IFN-gamma+ T cells in the CFC assay.

Boosting of SIV-specific immune responses in rhesus macaques by repeated administration of Ad5hr-SIVenv/rev and Ad5hr-SIVgag recombinants

Previous non-human primate studies have shown replication competent adenovirus (Ad) HIVenv/rev and SIVenv/rev recombinants to be promising vaccine candidates. To broaden induced immunity in rhesus macaques, an Ad type 5 host range (Ad5hr) mutant vector with an inserted SIV gag gene was added to the vaccine regimen. Immunity to the encoded SIV Env, Rev, and Gag gene products was evaluated following two immunizations with the same recombinants. The vaccines were administered intranasally plus orally via stomach tube at weeks 0 and 12. The recombinants replicated well in the upper respiratory tract but poorly in the gut, suggesting enteric-coated capsules might improve oral delivery to the intestine. SIV-specific cellular immunity was induced in all 16 immunized macaques. Fourteen exhibited positive interferon-gamma (IFN-gamma) ELISPOT responses, and nine, including two lacking IFN-gamma responses, exhibited SIV-specific T-cell proliferative activity. IFN-gamma secreting peripheral blood mononuclear cells (PBMCs) in response to SIV Gag, Env, and Rev peptides were induced in 73, 53, and 27% of macaques, respectively, and were boosted two- to four-fold by the second immunization. A persistent response to Gag was evident at least 10 weeks thereafter. p11C tetramer staining confirmed elicitation of SIV Gag-specific CD8+ T-cells in Mamu-A*01 macaques. Proliferative responses were more frequent after the second immunization, and binding antibody titers to SIV gp120 were significantly boosted by the immunization regimen. We conclude that a second administration of recombinants based in the same Ad5hr vector can effectively boost immunity to inserted gene products, obviating development of several recombinants in different Ad serotypes for multiple immunizations.

Use of interleukin 15 to enhance interferon-gamma production by antigen-specific stimulated lymphocytes from rhesus macaques

The enzyme-linked immune spot (ELISPOT) assay is receiving increased attention as a means for quantifying antigen-specific CD8 T-cell responses in rhesus macaques. Further improving the sensitivity of this assay could aid in the evaluation of vaccine candidates and/or immune therapeutic candidates. Interleukin (IL)-15 has been demonstrated to stimulate expansion of human immunodeficiency virus (HIV)-specific cytotoxic T lymphocytes (CTL) and to regulate homeostatic proliferation of CD8+ memory cells. We evaluated the in vitro effect of IL-15 to increase the detection of interferon-gamma (IFN-gamma) production by antigen-specific stimulated lymphocytes from a group of rhesus macaques exposed to simian-human immunodeficiency virus (SHIV) and a second group infected with SIVmac251, before and after antiretroviral treatment (ART). Results from these studies demonstrate that the presence of IL-15 during stimulation in a peptide-based ELISPOT assay greatly enhanced IFN-gamma production in both SHIV and simian immunodeficiency virus (SIV)-infected macaques. IFN-gamma production was mainly mediated by CD8 lymphocytes. The optimal concentrations of IL-15 that give enhancement of IFN-gamma production to specific antigen, without a significant increase in the spontaneous IFN-gamma release, ranged from 0.5 to 2.5 ng/ml. The mean number of IFN-gamma spots was increased 3.1- to 3.6-fold in response to SIV gag or HIV env peptide pools, respectively, in peripheral blood mononuclear cells (PBMC) from SHIV-infected macaques. Similarly, in SIV-infected macaques, IL-15 increased the mean number of IFN-gamma spots 2.7-fold in response to both SIV gag and env peptide pools. In samples obtained after ART in the same macaques, the increase factor was 2.5 for SIV gag and 1.8 for the env peptide pools. Thus, the sensitivity of the ELISPOT assay can be enhanced by addition of IL-15. This modified assay will be useful for detection of low frequencies of IFN-gamma producing cells in rhesus macaques.

Dendritic cells enhance detection of antigen-specific cellular immune responses by lymphocytes from rhesus macaques immunized with an HIV envelope peptide cocktail vaccine

Detection and enumeration of functional antigen-specific T cells is important for understanding the breadth of cell-mediated immunity to infections and experimental vaccines. We tested the utility of dendritic cells (DC), the professional antigen presenting cells, in the enzyme-linked immunosorbent spot-forming cell assay (ELISPOT) for efficient monitoring of antigen-specific immunity in rhesus macaques vaccinated with an HIV envelope peptide-cocktail. Compared with direct antigen-specific stimulation of peripheral blood mononuclear cells, the DC-ELISPOT protocol involving co-culturing of macaque T cells with autologous DC pulsed with the various peptides from the vaccine cocktail yielded up to 18-fold higher numbers of interferon-gamma producing cells without increasing the background. Importantly, use of DC in the analyses revealed immune responses in vaccinated macaques that were otherwise undetectable. Similar data were obtained when recall responses to purified protein derivative were analyzed by the DC-ELISPOT method using blood samples from human volunteers. These data establish the importance of DC in improving detection sensitivity and eliminating false negative results, both essential for efficient monitoring of antigen-specific cellular immune responses.

Immunological and pathological evaluation of rhesus macaques infected with Leishmania major

Cutaneous leishmaniasis, a parasitic infection causing ulcerating skin lesions, is an important disease worldwide and urgently requires a vaccine. Animal models that closely mimic human disease are essential for designing preventive vaccines against Leishmania major. We have evaluated both biologic and immunologic parameters of cutaneous L. major infection in nonhuman primates. Naïve rhesus macaques or monkeys previously exposed to L. major were infected with varying doses of L. major metacyclic promastigotes, and lesion size was assessed over a 10-week period. Monkeys previously infected with L. major had much smaller lesions that resolved faster compared with those of naïve monkeys in response to the two higher doses of infection. Moreover, eight of nine naïve monkeys had parasites detected in their lesions during the course of the infection. In addition, the cellular infiltrate within the lesions was qualitatively and quantitatively different in naïve versus previously infected monkeys. Finally, an ELIspot assay determined that the magnitude and kinetics of responses differed between previously infected and naïve monkeys.

ELISpot and ELISA analysis of spontaneous, mitogen-induced and antigen-specific cytokine production in cynomolgus and rhesus macaques

Evaluation of cytokine production in macaques has been hampered by a lack of availability of optimized and standardized immunoassays such as ELISA and enzyme-linked immune spot assay (ELISpot); only a limited number of macaque cytokines have been assessed by ELISpot. Using monoclonal antibodies (mAb) to human cytokines that cross-react with cynomolgus and rhesus macaque interferon-gamma (IFN-gamma), interleukin (IL)-2, IL-4, IL-5, IL-6, IL-12, IL-13 and granulocyte monocyte colony-stimulating factor, we measured macaque cytokine production by ELISA and ELISpot. Quantitation of spontaneous as well as phytohemagglutinin (PHA)-induced cytokine production in peripheral blood mononuclear cells (PBMC) from rhesus and cynomolgus macaques and humans were compared. The proportional distribution of the different cytokines, in terms of PBMC synthesizing different cytokines as well as the levels of the different cytokines produced, were similar in all species. Spontaneous- and PHA-induced cytokine productions thus appear to be similarly regulated in macaques and man. ELISpot and ELISA assays for macaque IFN-gamma were further used to measure antigen-specific immune responses of PBMC from cynomolgus macaques exposed to, or vaccinated against, simian immunodeficiency virus (SIV). The establishment of reliable immunoassays for detection of macaque cytokines is of importance for future progress of research utilizing macaques as experimental animals.

Protection of rhesus macaques against lethal Plasmodium knowlesi malaria by a heterologous DNA priming and poxvirus boosting immunization regimen

We tested a cytokine-enhanced, multiantigen, DNA priming and poxvirus boosting vaccine regimen for prevention of malaria in the Plasmodium knowlesi-rhesus macaque model system. Animals were primed with a mixture of DNA plasmids encoding two preerythrocytic-stage proteins and two erythrocytic-stage proteins from P. knowlesi and combinations of the cytokines granulocyte-macrophage colony-stimulating factor, interleukin-4, and tumor necrosis factor alpha and were boosted with a mixture of four recombinant, attenuated vaccinia virus strains encoding the four P. knowlesi antigens. Two weeks after boosting, the geometric mean immunofluorescence titers in the immunized groups against sporozoites and infected erythrocytes ranged from 160 to 8,096 and from 1,810 to 5,120, respectively. The geometric mean anti-P. knowlesi circumsporozoite protein (PkCSP) titers ranged from 1,761 to 24,242. Peripheral blood mononuclear cells (PBMC) from the immunized monkeys produced gamma interferon (IFN-gamma) in response to incubation with pooled peptides from the PkCSP at frequencies of 10 to 571 spot-forming cells/10(6) PBMC. Following challenge with 100 infectious P. knowlesi sporozoites, 2 of 11 immunized monkeys were sterilely protected, and 7 of the 9 infected monkeys resolved their parasitemias spontaneously. In contrast, all four controls became infected and required treatment for overwhelming parasitemia. Early protection was strongly associated with IFN-gamma responses against a pool of peptides from the preerythrocytic-stage antigen, PkCSP. These findings demonstrate that a multistage, multiantigen, DNA priming and poxvirus boosting vaccine regimen can protect nonhuman primates from an otherwise lethal malaria sporozoite challenge.

Design and validation of an enzyme-linked immunospot assay for use in clinical trials of candidate HIV vaccines

The enzyme-linked immunosorbent (ELISPOT) assay, which enumerates peripheral blood mononuclear cells (PBMCs) releasing interferon gamma (IFN-gamma) on specific antigen stimulation, is becoming the assay of choice for evaluation of vaccine-induced cell-mediated immune responses in many clinical trials. A properly conducted trial requires the assays to be validated, especially should the trial lead to vaccine licensure. Here, the design and validation of an ELISPOT assay are described for use in clinical trials of candidate human immunodeficiency virus (HIV) vaccines, using a particular immunogen termed HIVA. This assay employs eight pools of 20 to 23 peptides each: seven pools are derived from the immunogen and one pool is derived from cytotoxic T cell epitopes of common human viruses serving as an internal positive control. The validation determined that first, the overall variation of a positive response of approximately 500 spot-forming units (SFU)/10(6) cells was 21%, while second, the average of 5 SFU/10(6) cells was detected for the seven HIVA-derived pools in HIV-uninfected individuals; third, a positive response to a peptide added to the assay pools was not occluded by the other pool peptides; fourth, the frequencies detected in fresh PBMCs were 2- to 3-fold higher compared with the same samples that had been cryopreserved; and finally, all seven HIV-derived pools induced IFN-gamma responses in PBMCs isolated from HIV-infected individuals. The limits of the validation of assays involving biological responses of living cells are discussed.

Vaccine-induced immune responses in rodents and nonhuman primates by use of a humanized human immunodeficiency virus type 1 pol gene

A synthetic gene consisting of the reverse transcriptase (RT) and integrase (IN) domains of human immunodeficiency virus type 1 (HIV-1) pol was constructed using codons most frequently used in humans. The humanized pol gave dramatically improved levels of Rev-independent, in vitro protein production in mammalian cells and elicited much stronger cellular immunity in rodents than did virus-derived gene. Specifically, BALB/c mice were immunized with plasmids and/or recombinant vaccinia virus constructs expressing the synthetic gene. High frequencies of Pol-specific T lymphocytes were detected in these animals by the gamma interferon enzyme-linked immunospot assay against pools of short overlapping peptides. Characterization of the stimulatory peptides from these pools indicates that the optimized gene constructs are able to effectively activate both CD4+ and CD8+ T cells. Immunization of rhesus macaques with DNA vaccines expressing the humanized pol coupled to a human tissue plasminogen activator leader sequence led to pronounced in vitro cytotoxic T-lymphocyte killing activities and enhanced levels of circulating Pol-specific T cells, comparable to those observed in HIV-1-infected human subjects. Thus, optimizing the immunogenic properties of HIV-1 Pol at the level of the gene sequence validates it as an antigen and provides an important step toward the construction of a potent pol-based HIV-1 vaccine component.

Enhanced activation of rhesus T cells by vectors encoding a triad of costimulatory molecules (B7-1, ICAM-1, LFA-3)

Since the rhesus is often used as a "gatekeeper" model for the evaluation of malaria and simian immunodeficiency virus (SIV)/HIV vaccines, the identification of strategies to enhance the activation of rhesus T cells would potentially aid in the generation of more potent vaccines directed against these infectious agents. Several molecules normally found on the surface of professional human APCs are capable of providing the second signals critical for T cell activation: B7-1 (CD80), ICAM-1 (CD54), and LFA-3 (CD58). With the exception of B7, T cell costimulatory molecules in the rhesus have not been identified. We have recently designed and characterized both recombinant vaccinia and recombinant avipox vectors containing the transgenes for a triad of human T cell costimulatory molecules (B7-1, ICAM-1, LFA-3; designated TRICOM). Here, we demonstrate the enhanced activation of rhesus T cells stimulated with rhesus APCs infected with TRICOM vectors in the presence of signal 1. Infection with TRICOM vectors led to significant improvement of APC capabilities in terms of reduction of the amount of signal 1 needed to activate naive T cells, and reduction in the amount of APCs required to activate T cells using a constant amount of signal 1. Antibody blocking studies demonstrated that each of the three costimulatory molecule transgenes contributed to the enhanced proliferation of T cells. TRICOM-enhanced T cell activation was shown to correspond to increases in type 1 cytokines and a reduced level of apoptosis. TRICOM-infected autologous B cells from rhesus immunized with either an SIV vaccine or a malaria vaccine stimulated significantly greater levels of IFN-gamma in response to specific peptide than stimulation with uninfected autologous B cells or B cells infected with wild-type vector. The ability to augment immune responses using poxvirus-based vaccines containing multiple costimulatory molecule transgenes can now be addressed in the rhesus macaque model.

Cross-priming as a predominant mechanism for inducing CD8(+) T cell responses in gene gun DNA immunization

DNA immunization induces CD8(+) CTL responses by bone marrow-derived APCs, which are directly transfected with a plasmid DNA and/or acquire Ags from DNA-transfected non-APCs. To investigate the relative contribution of DNA-transfected APCs vs non-APCs to the initiation of CD8(+) T cell responses, we used tissue-specific promoter-directed gene expression and adoptive transfer systems in gene gun DNA immunization. In this study, we demonstrated that non-APC-specific gene expressions induced significant CD8(+) CTL and IFN-gamma-producing cells and Ab responses, whereas APC-specific gene expressions led to moderate CTL and IFN-gamma-producers, but no Ab responses. Interestingly, mice immunized with a non-APC-specific plasmid induced more rapid, vigorous, and prolonged proliferation of adoptively transferred Ag-specific CD8(+) T cells than APC-specific plasmid-immunized mice. In addition, the in vivo proliferative responses elicited by a non-APC-specific plasmid administration were dependent on TAP, but were independent of CD4(+) T cell help. Collectively, our results suggest that cross-priming, in which Ags expressed in non-APCs are taken up, processed, and presented by APCs, plays an important role in the initiation, magnitude, and maintenance of CD8(+) T cell responses in gene gun DNA immunization.

Multistage multiantigen heterologous prime boost vaccine for Plasmodium knowlesi malaria provides partial protection in rhesus macaques

A nonhuman primate model for malaria vaccine development allowing reliable, stringent sporozoite challenge and evaluation of both cellular and antibody responses is needed. We therefore constructed a multicomponent, multistage DNA vaccine for the simian malaria species Plasmodium knowlesi including two preerythrocytic-stage antigens, the circumsporozoite protein (PkCSP) and sporozoite surface protein 2 (PkSSP2), and two blood stage antigens, apical merozoite antigen 1 (PkAMA1) and merozoite surface protein 1 (PkMSP1p42), as well as recombinant canarypox viruses encoding the four antigens (ALVAC-4). The DNA vaccine plasmids expressed the corresponding antigens in vitro and induced antiparasite antibodies in mice. Groups of four rhesus monkeys received three doses of a mixture of the four DNA vaccine plasmids and a plasmid encoding rhesus granulocyte-monocyte colony-stimulating factor, followed by boosting with a single dose of ALVAC-4. Three groups received the priming DNA doses by different routes, either by intramuscular needle injection, by intramuscular injection with a needleless injection device, the Biojector, or by a combination of intramuscular and intradermal routes by Biojector. Animals immunized by any route developed antibody responses against sporozoites and infected erythrocytes and against a recombinant PkCSP protein, as well as gamma interferon-secreting T-cell responses against peptides from PkCSP. Following challenge with 100 P. knowlesi sporozoites, 1 of 12 experimental monkeys was completely protected and the mean parasitemia in the remaining monkeys was significantly lower than that in 4 control monkeys. This model will be important in preclinical vaccine development.

Use of overlapping peptide mixtures as antigens for cytokine flow cytometry

Intracellular cytokine staining and flow cytometry can be used to measure T-cell responses to defined antigens. Although CD8+ T-cell responses to soluble proteins are inefficiently detected by this approach, peptides can be used as antigens. Using overlapping peptides spanning an entire protein sequence, CD8+ T-cell responses can be detected to multiple epitopes, regardless of HLA type. In this study, overlapping peptide mixes of various lengths were compared and 15 amino acid peptides with 11 amino acid overlaps were found to stimulate both CD4+ and CD8+ T-cell responses. Such peptide mixes stimulated CD4+ T-cell responses equivalent to those observed with whole recombinant protein, while simultaneously stimulating CD8+ T-cell responses much higher than those observed with whole protein. Although 8-12 amino acid peptides produced the highest level of CD8+ T-cell responses, 15 amino acid peptides were still very effective. Peptides that were 20 amino acids in length, however, did not stimulate strong CD8+ T-cell responses at the same peptide dose. The cytokine responses to individual epitopes added up approximately to the response to the entire mix, demonstrating that large mixes can detect responses in a quantitative fashion. Unlike whole protein antigens, peptide mixes were effective at stimulating responses in both cryopreserved PBMC and blood stored for 24 h at room temperature. Thus, overlapping 15 amino acid peptide mixes may facilitate the analysis of antigen-specific CD4+ and CD8+ T-cell responses by cytokine flow cytometry, using clinical specimens that include shipped blood or cryopreserved PBMC.