Unique T cell effector functions elicited by Plasmodium falciparum epitopes in malaria-exposed Africans tested by three T cell assays

Using two phases of the CD4 T cell response to blood-stage murine malaria to understand regulation of systemic immunity and placental pathology in Plasmodium falciparum infection

Plasmodium falciparum infection and malaria remain a risk for millions of children and pregnant women. Here, we seek to integrate knowledge of mouse and human T helper cell (Th) responses to blood-stage Plasmodium infection to understand their contribution to protection and pathology. Although there is no complete Th subset differentiation, the adaptive response occurs in two phases in non-lethal rodent Plasmodium infection, coordinated by Th cells. In short, cellular immune responses limit the peak of parasitemia during the first phase; in the second phase, humoral immunity from T cell-dependent germinal centers is critical for complete clearance of rapidly changing parasite. A strong IFN-γ response kills parasite, but an excess of TNF compared with regulatory cytokines (IL-10, TGF-β) can cause immunopathology. This common pathway for pathology is associated with anemia, cerebral malaria, and placental malaria. These two phases can be used to both understand how the host responds to rapidly growing parasite and how it attempts to control immunopathology and variation. This dual nature of T cell immunity to Plasmodium is discussed, with particular reference to the protective nature of the continuous generation of effector T cells, and the unique contribution of effector memory T cells.

Comprehensive Review of Human Plasmodium falciparum-Specific CD8+ T Cell Epitopes

Control of malaria is an important global health issue and there is still an urgent need for the development of an effective prophylactic vaccine. Multiple studies have provided strong evidence that Plasmodium falciparum-specific MHC class I-restricted CD8+ T cells are important for sterile protection against Plasmodium falciparum infection. Here, we present an interactive epitope map of all P. falciparum-specific CD8+ T cell epitopes published to date, based on a comprehensive data base (IEDB), and literature search. The majority of the described P. falciparum-specific CD8+ T cells were directed against the antigens CSP, TRAP, AMA1, and LSA1. Notably, most of the epitopes were discovered in vaccine trials conducted with malaria-naïve volunteers. Only few immunological studies of P. falciparum-specific CD8+ T cell epitopes detected in patients suffering from acute malaria or in people living in malaria endemic areas have been published. Further detailed immunological mappings of P. falciparum-specific epitopes of a broader range of P. falciparum proteins in different settings and with different disease status are needed to gain a more comprehensive understanding of the role of CD8+ T cell responses for protection, and to better guide vaccine design and to study their efficacy.

Characterization of T-cell immune responses in clinical trials of the candidate RTS,S malaria vaccine

The candidate malaria vaccine RTS,S has demonstrated 45.7% efficacy over 18 months against all clinical disease in a phase-III field study of African children. RTS,S targets the circumsporozoite protein (CSP), which is expressed on the Plasmodium sporozoite during the pre-erythrocyte stage of its life-cycle; the stage between mosquito bite and liver infection.

Early in the development of RTS,S, it was recognized that CSP-specific cell-mediated immunity (CMI) was required to complement CSP-specific antibody-mediated immunity. In reviewing RTS,S clinical studies, associations between protection and various types of CMI (CSP-specific CD4⁺ T cells and INF-γ ELISPOTs) have been identified, but not consistently. It is plausible that certain CD4⁺ T cells support antibody responses or co-operate with other immune-cell types to potentially elicit protection. However, the identities of vaccine correlates of protection, implicating either CSP-specific antibodies or T cells remain elusive, suggesting that RTS,S clinical trials may benefit from additional immunogenicity analyses that can be informed by the results of controlled human malaria infection studies.

Interferon-γ responses to Plasmodium falciparum vaccine candidate antigens decrease in the absence of malaria transmission

Background

Malaria elimination campaigns are planned or active in many countries. The effects of malaria elimination on immune responses such as antigen-specific IFN- γ responses are not well characterized.

Methods

IFN- γ responses to the P. falciparum antigens circumsporozoite protein, liver stage antigen-1, thrombospondin-related adhesive protein, apical membrane antigen-1, MB2, and merozoite surface protein-1 were tested by ELISA in 243 individuals in highland Kenya in April 2008, October 2008, and April 2009, after a one-year period of interrupted malaria transmission from April 2007 to March 2008.

Results

While one individual (0.4%) tested positive for P. falciparum by PCR inOctober 2008 and another two (0.9%) tested positive in April 2009, no clinical malaria cases were detected during weekly visits. Levels of IFN-γ to all antigens decreased significantly from April 2008 to April 2009 (all P < 0.001).

Discussion

Naturally acquired IFN- γ responses to P. falciparum antigensare short-lived in the absence of repeated P. falciparum infection. Even short periods of malaria interruption may significantly decrease IFN-γ responses to P. falciparum antigens.

Protective humoral immunity elicited by a needle-free malaria vaccine comprised of a chimeric Plasmodium falciparum circumsporozoite protein and a Toll-like receptor 5 agonist, flagellin

Immunization with Plasmodium sporozoites can elicit high levels of sterile immunity, and neutralizing antibodies from protected hosts are known to target the repeat region of the circumsporozoite (CS) protein on the parasite surface. CS-based subunit vaccines have been hampered by suboptimal immunogenicity and the requirement for strong adjuvants to elicit effective humoral immunity. Pathogen-associated molecular patterns (PAMPs) that signal through Toll-like receptors (TLRs) can function as potent adjuvants for innate and adaptive immunity. We examined the immunogenicity of recombinant proteins containing a TLR5 agonist, flagellin, and either full-length or selected epitopes of the Plasmodium falciparum CS protein. Mice immunized with either of the flagellin-modified CS constructs, administered intranasally (i.n.) or subcutaneously (s.c.), developed similar levels of malaria-specific IgG1 antibody and interleukin-5 (IL-5)-producing T cells. Importantly, immunization via the i.n. but not the s.c. route elicited sporozoite neutralizing antibodies capable of inhibiting >90% of sporozoite invasion in vitro and in vivo, as measured using a transgenic rodent parasite expressing P. falciparum CS repeats. These findings demonstrate that functional sporozoite neutralizing antibody can be elicited by i.n. immunization with a flagellin-modified P. falciparum CS protein and raise the potential of a scalable, safe, needle-free vaccine for the 40% of the world's population at risk of malaria.

Immunization with apical membrane antigen 1 confers sterile infection-blocking immunity against Plasmodium sporozoite challenge in a rodent model

Apical membrane antigen 1 (AMA-1) is a leading blood-stage malaria vaccine candidate. Consistent with a key role in erythrocytic invasion, AMA-1-specific antibodies have been implicated in AMA-1-induced protective immunity. AMA-1 is also expressed in sporozoites and in mature liver schizonts where it may be a target of protective cell-mediated immunity. Here, we demonstrate for the first time that immunization with AMA-1 can induce sterile infection-blocking immunity against Plasmodium sporozoite challenge in 80% of immunized mice. Significantly higher levels of gamma interferon (IFN-γ)/interleukin-2 (IL-2)/tumor necrosis factor (TNF) multifunctional T cells were noted in immunized mice than in control mice. We also report the first identification of minimal CD8(+) and CD4(+) T cell epitopes on Plasmodium yoelii AMA-1. These data establish AMA-1 as a target of both preerythrocytic- and erythrocytic-stage protective immune responses and validate vaccine approaches designed to induce both cellular and humoral immunity.

A statistical interaction between circumsporozoite protein-specific T cell and antibody responses and risk of clinical malaria episodes following vaccination with RTS,S/AS01E

The candidate malaria vaccine RTS,S/AS01_E provides significant but partial protection from clinical malaria. On in vitro circumsporozoite protein (CSP) peptide stimulation and intra-cellular cytokine staining of whole blood taken from 407 5–17 month-old children in a phase IIb trial of RTS,S/AS01_E, we identified significantly increased frequencies of two CSP-specific CD4+ T cells phenotypes among RTS,S/AS01_E vaccinees (IFNγ-IL2+TNF− and IFNγ-IL2+TNF+ CD4+ T cells), and increased frequency of IFNγ-IL2-TNF+ CD4+ T cells after natural exposure. All these T cells phenotypes were individually associated with reductions in the risk of clinical malaria, but IFNγ-IL2-TNF+ CD4+ T cells independently predicted reduced risk of clinical malaria on multi-variable analysis (HR = 0.29, 95% confidence intervals 0.15–0.54, p<0.0005). Furthermore, there was a strongly significant synergistic interaction between CSP-specific IFNγ-IL2-TNF+ CD4+ T cells and anti-CSP antibodies in determining protection against clinical malaria (p = 0.002). Vaccination strategies that combine potent cellular and antibody responses may enhance protection against malaria.

Single DermaVir immunization: dose-dependent expansion of precursor/memory T cells against all HIV antigens in HIV-1 infected individuals

Background

The GIHU004 study was designed to evaluate the safety and immunogenicity of three doses of DermaVir immunization in HIV-infected subjects on fully suppressive combination antiretroviral therapy (cART).

Methodology/Principal Findings

This first-in-human dose escalation study was conducted with three topical DermaVir doses targeted to epidermal Langerhans cells to express fifteen HIV antigens in draining lymph nodes: 0.1 mg DNA targeted to two, 0.4 mg and 0.8 mg DNA targeted to four lymph nodes. Particularly, in the medium dose cohort 0.1 mg DNA was targeted per draining lymph node via ∼8 million Langerhans cells located in 80 cm² epidermis area. The 28-days study with 48-week safety follow-up evaluated HIV-specific T cell responses against Gag p17, Gag p24 and Gag p15, Tat and Rev antigens. DermaVir-associated side effects were mild, transient and not dose-dependent. Boosting of HIV-specific effector CD4⁺ and CD8⁺ T cells expressing IFN-gamma and IL-2 was detected against several antigens in every subject of the medium dose cohort. The striking result was the dose-dependent expansion of HIV-specific precursor/memory T cells with high proliferation capacity. In low, medium and high dose cohorts this HIV-specific T cell population increased by 325-, 136,202 and 50,759 counts after 4 weeks, and by 3,899, 9,878 and 18,382 counts after one year, respectively, compared to baseline.

Conclusions/Significance

Single immunization with the DermaVir candidate therapeutic vaccine was safe and immunogenic in HIV-infected individuals. Based on the potent induction of Gag, Tat and Rev-specific memory T cells, especially in the medium dose cohort, we speculate that DermaVir boost T cell responses specific to all the 15 HIV antigens expressed from the single DNA. For durable immune reactivity repeated DermaVir immunization might be required since the frequency of DermaVir-boosted HIV-specific memory T cells decreased during the 48-week follow up.

Trial Registration

ClinicalTrial.gov NCT00712530.

Circumsporozoite-specific T cell responses in children vaccinated with RTS,S/AS01E and protection against P falciparum clinical malaria

BACKGROUND

RTS,S/AS01(E) is the lead candidate pre-erythrocytic malaria vaccine. In Phase IIb field trials the safety profile was acceptable and the efficacy was 53% (95%CI 31%-72%) for protecting children against clinical malaria caused by P. falciparum. We studied CS-specific T cell responses in order to identify correlates of protection.

METHODS AND FINDINGS

We used intracellular cytokine staining (for IL2, IFNγ, and TNFα), ex-vivo ELISPOTs (IFNγ and IL2) and IFNγ cultured ELISPOT assays to characterize the CS-specific cellular responses in 407 children (5-17 months of age) in a phase IIb randomized controlled trial of RTS,S/AS01(E) (NCT00380393). RTS,S/ AS01(E) vaccinees had higher frequencies of CS-specific CD4+ T cells producing IFNγ, TNFα or IL2 compared to control vaccinees. In a multivariable analysis TNFα(+) CD4(+) T cells were independently associated with a reduced risk for clinical malaria among RTS,S/AS01(E) vaccinees (HR = 0.64, 95%CI 0.49-0.86, p = 0.002). There was a non-significant tendency towards reduced risk among control vaccinees (HR = 0.80, 95%CI 0.62-1.03, p = 0.084), albeit with lower CS-specific T cell frequencies and higher rates of clinical malaria. When data from both RTS,S/AS01(E) vaccinees and control vaccinees were combined (with adjusting for vaccination group), the HR was 0.74 (95%CI 0.62-0.89, p = 0.001). After a Bonferroni correction for multiple comparisons (n-18), the finding was still significant at p = 0.018. There was no significant correlation between cultured or ex vivo ELISPOT data and protection from clinical malaria. The combination of TNFα(+) CD4(+) T cells and anti-CS antibody statistically accounted for the protective effect of vaccination in a Cox regression model.

CONCLUSIONS

RTS,S/AS01(E) induces CS-specific Th1 T cell responses in young children living in a malaria endemic area. The combination of anti-CS antibody concentrations titers and CS-specific TNFα(+) CD4(+) T cells could account for the level of protection conferred by RTS,S/AS01(E). The correlation between CS-specific TNFα(+) CD4(+) T cells and protection needs confirmation in other datasets.

Differential cellular recognition of antigens during acute Plasmodium falciparum and Plasmodium vivax malaria

BACKGROUND

Plasmodium falciparum and Plasmodium vivax are co-endemic in the Asia-Pacific region. Their capacity to induce and sustain diverse T-cell responses underpins protective immunity. We compared T-cell responses to the largely conserved merozoite surface protein-5 (PfMSP5) during acute and convalescent falciparum and vivax malaria.

METHODS

Lymphoproliferation and IFN--γ secretion to PfMSP5 and purified protein derivate were quantified in adults with falciparum (n=34), and vivax malaria (n=12) or asymptomatic residents (n=10) of Papua, Indonesia. Responses were reassessed 7-28 days following treatment.

RESULTS

The frequency of IFN-γ responders to PfMSP5 was similar in acute falciparum (63%) or vivax (67%) malaria. However, significantly more IFN-γ-secreting cells were detectable during vivax compared with falciparum infection. Purified protein derivative responses showed a similarly enhanced pattern. While rapidly lost in vivax patients, PfMSP5-specific responses in falciparum malaria remained to day 28. By contrast, frequency and magnitude of lymphoproliferation to PfMSP5 were similar for falciparum and vivax infections.

CONCLUSION

Cellular PfMSP5-specific responses are most frequent during either acute falciparum or vivax malaria, indicating functional T-cell responses to conserved antigens. Both effector and central memory T-cell functions are increased. Greater IFN-γ responses in acute P. vivax, suggest enhancement of pre-existing effector T-cells during acute vivax infection.

Immunogenicity and safety of an HIV-1 lipopeptide vaccine in healthy adults: a phase 2 placebo-controlled ANRS trial

BACKGROUND

French National Agency for Research on AIDS and Viral Hepatitis's HIV-LIPO-5 vaccine includes five HIV-1 peptides, containing multiple CD8 and CD4 T-cell epitopes and coupled to a palmitoyl tail. Whether HIV-LIPO-5 immunogenicity varies with the dose is unknown.

METHODS

HIV-negative volunteers were randomized to receive HIV-LIPO-5 vaccine at 50 microg/lipopeptide (N = 32), 150 microg/lipopeptide (N = 32), 500 microg/lipopeptide (N = 33) or placebo (N = 34) at weeks 0, 4, 12 and 24. HIV-1-specific CD8 (interferon-gamma ELISpot on peripheral blood mononuclear cells cultured for 12 days) and CD4 responses (peripheral blood mononuclear cell lymphoproliferation) were assessed at baseline, after each injection and at week 48.

RESULTS

Local reactions were dose-dependent but no differences in systemic reactions appeared between groups. Sustained (at least on two separate occasions) CD8 response rates to at least one given HIV-1 pool were obtained in 22 of 32 (69%), 21 of 33 (64%) and 21 of 34 (62%) individuals for LIPO-5 50, 150 and 500 groups, respectively (P < or = 0.0001 for all comparisons to the placebo). Cumulative CD4 response rates were obtained in 15 of 32 (47%), 18 of 33 (55%) and 15 of 34 (44%) individuals (P < 0.0001 for all comparisons to placebo). At week 48, CD8 responses persisted in 47 of 91 (52%) HIV-LIPO-5 recipients.

CONCLUSION

Doses of 50, 150 and 500 microg of French National Agency for Research on AIDS and Viral Hepatitis's HIV-LIPO-5 vaccine were able to elicit HIV-specific sustained CD8 and CD4 T-cell responses in healthy adults. Safety is good and all doses appear appropriate in further 'prime-boost' trials.

Why functional pre-erythrocytic and bloodstage malaria vaccines fail: a meta-analysis of fully protective immunizations and novel immunological model

BACKGROUND

Clinically protective malaria vaccines consistently fail to protect adults and children in endemic settings, and at best only partially protect infants.

METHODOLOGY/PRINCIPAL FINDINGS

We identify and evaluate 1916 immunization studies between 1965-February 2010, and exclude partially or nonprotective results to find 177 completely protective immunization experiments. Detailed reexamination reveals an unexpectedly mundane basis for selective vaccine failure: live malaria parasites in the skin inhibit vaccine function. We next show published molecular and cellular data support a testable, novel model where parasite-host interactions in the skin induce malaria-specific regulatory T cells, and subvert early antigen-specific immunity to parasite-specific immunotolerance. This ensures infection and tolerance to reinfection. Exposure to Plasmodium-infected mosquito bites therefore systematically triggers immunosuppression of endemic vaccine-elicited responses. The extensive vaccine trial data solidly substantiate this model experimentally.

CONCLUSIONS/SIGNIFICANCE

We conclude skinstage-initiated immunosuppression, unassociated with bloodstage parasites, systematically blocks vaccine function in the field. Our model exposes novel molecular and procedural strategies to significantly and quickly increase protective efficacy in both pipeline and currently ineffective malaria vaccines, and forces fundamental reassessment of central precepts determining vaccine development. This has major implications for accelerated local eliminations of malaria, and significantly increases potential for eradication.

The effect of placental malaria infection on cord blood and maternal immunoregulatory responses at birth

Placental malaria (PM), a frequent infection of pregnancy, provides an ideal opportunity to investigate the impact on immune development of exposure of the foetal immune system to foreign Ag. We investigated the effect of PM on the regulatory phenotype and function of cord blood cells from healthy Gambian newborns and peripheral blood cells from their mothers, and analyzed for effects on the balance between regulatory and effector responses. Using the gold standard for classifying PM we further distinguished between resolved infection and acute or chronic PM active at the time of delivery. We show that exposure to malarial Ag in utero results in the expansion of malaria-specific FOXP3(+) Treg and more generalized FOXP3(+) CD4(+) Treg in chronic and resolved PM, alongside increased Th1 pro-inflammatory responses (IFN-gamma, TNF-alpha, IFN-gamma:IL-10) in resolved PM infection only. These observations demonstrate a clear effect of exposure to malarial Ag in foetal life on the immune environment at birth, with a regulatory response dominating in the newborns with ongoing chronic PM, while those with resolved infection produce both regulatory and inflammatory responses. The findings might explain some of the adverse effects on the health of babies born to women with PM.

Human T cell recognition of the blood stage antigen Plasmodium hypoxanthine guanine xanthine phosphoribosyl transferase (HGXPRT) in acute malaria

Background

The Plasmodium purine salvage enzyme, hypoxanthine guanine xanthine phosphoribosyl transferase (HGXPRT) can protect mice against Plasmodium yoelii pRBC challenge in a T cell-dependent manner and has, therefore, been proposed as a novel vaccine candidate. It is not known whether natural exposure to Plasmodium falciparum stimulates HGXPRT T cell reactivity in humans.

Methods

PBMC and plasma collected from malaria-exposed Indonesians during infection and 7–28 days after anti-malarial therapy, were assessed for HGXPRT recognition using CFSE proliferation, IFNγ ELISPOT assay and ELISA.

Results

HGXPRT-specific T cell proliferation was found in 44% of patients during acute infection; in 80% of responders both CD4⁺ and CD8⁺ T cell subsets proliferated. Antigen-specific T cell proliferation was largely lost within 28 days of parasite clearance. HGXPRT-specific IFN-γ production was more frequent 28 days after treatment than during acute infection. HGXPRT-specific plasma IgG was undetectable even in individuals exposed to malaria for at least two years.

Conclusion

The prevalence of acute proliferative and convalescent IFNγ responses to HGXPRT demonstrates cellular immunogenicity in humans. Further studies to determine minimal HGXPRT epitopes, the specificity of responses for Plasmodia and associations with protection are required. Frequent and robust T cell proliferation, high sequence conservation among Plasmodium species and absent IgG responses distinguish HGXPRT from other malaria antigens.

The good, the bad and the ugly: how altered peptide ligands modulate immunity

BACKGROUND

The basis of T cell immune responses is the specific recognition of an immunogenic peptide epitope by a T cell receptor. Peptide alterations of such T cell epitopes with single or few amino acid variations can have drastic effects on the outcome of this recognition. These altered peptide ligands can act as modulators of immune responses as they are capable of downregulating or upregulating responses.

OBJECTIVE/METHODS

We review how altered peptide ligands can have 'good' 'bad' and 'ugly' outcomes in treating diseases.

RESULTS/CONCLUSION

Altered peptide ligands have been used as immunotherapeutics in autoimmune (and allergic) diseases, infectious diseases and cancer. In the next five years we anticipate seeing a number of altered peptide ligands in clinical trials, progressing from contradictory classifications of good, bad or ugly, to the exciting outcome of 'useful'.

T cell responses to mycobacterial catalase-peroxidase profile a pathogenic antigen in systemic sarcoidosis

Sarcoidosis is a systemic granulomatous disease associated with local epithelioid granulomas, CD4(+) T cells, and Th1 cytokines. The tissue Ags that drive this granulomatous inflammation are uncertain. In this study, we used IFN-gamma-ELISPOT assays and flow cytometry to assess lung and blood T cell responses to the candidate pathogenic Ag, Mycobacterium tuberculosis catalase-peroxidase (mKatG) in patients with sarcoidosis from two centers. Despite differences in patient phenotypic, genetic, and prognostic characteristics, we report that T cell responses to mKatG were remarkably similar in these cohorts, with higher frequencies of mKatG-reactive, IFN-gamma-expressing T cells in the blood of sarcoidosis patients compared with nontuberculosis sensitized healthy controls, and (in a subset) in greater numbers than T cells reactive to purified protein derivative. In sarcoidosis, mKatG-reactive CD4(+) Th1 cells preferentially accumulated in the lung, indicating a compartmentalized response. Patients with or without Löfgren syndrome had similar frequencies of mKatG specific IFN-gamma-expressing blood T cells. Circulating mKatG-reactive T cells were found in chronic active sarcoidosis but not in patients with inactive disease. Together, these results demonstrate that T cell responses to mKatG in sarcoidosis fit a profile expected for a pathogenic Ag, supporting an immunotherapeutic approach to this disease.

Ex vivo characterization of polyclonal memory CD8+ T-cell responses to PRAME-specific peptides in patients with acute lymphoblastic leukemia and acute and chronic myeloid leukemia

Preferentially expressed antigen of melanoma (PRAME) is aberrantly expressed in hematologic malignancies and may be a useful target for immunotherapy in leukemia. To determine whether PRAME is naturally immunogenic, we studied CD8(+) T-cell responses to 4 HLA-A*0201-restricted PRAME-derived epitopes (PRA100, PRA142, PRA300, PRA425) in HLA-A*0201-positive patients with acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), chronic myeloid leukemia (CML), and healthy donors. CD8(+) T cells recognizing PRAME peptides could be detected ex vivo in 4 of 10 ALL, 6 of 10 AML, 3 of 10 CML patients, and 3 of 10 donors by HLA-A2 tetramer analysis and flow cytometry for intracellular interferon-gamma. The frequency of PRAME-specific CD8(+) T cells was greater in patients with AML, CML, and ALL than healthy controls. All peptides were immunogenic in patients, while responses were only detected to PRA300 in donors. High PRAME expression in patient peripheral blood mononuclear cells was associated with responses to greater than or equal to 2 PRAME epitopes compared with low PRAME expression levels (4/7 vs 0/23, P = .001), suggesting a PRAME-driven T-cell response. PRAME-specific T cells were readily expanded in short-term cultures in donors and patients. These results provide evidence for spontaneous T cell reactivity against multiple epitopes of PRAME in ALL, AML, and CML. The potential for developing PRAME as a target for immunotherapy in leukemia deserves further exploration.

A comparison of IFNgamma detection methods used in tuberculosis vaccine trials

Interferon gamma (IFNgamma) is a critical component of the pro-inflammatory immune response that provides protection against Mycobacterium tuberculosis. In the absence of an immunological correlate of protection, antigen-specific production of IFNgamma is a commonly used marker of a protective immune response. To facilitate the evaluation of tuberculosis candidate vaccines three different IFNgamma detection methods were compared. The cultured whole blood ELISA, ex vivo IFNgamma ELISpot and whole blood ex vivo intracellular cytokine staining (ICS) assays were performed head-to-head during a Phase I clinical trial using the candidate vaccine MVA85A. Whilst all three assays detected significant increases in IFNgamma production immediately following vaccination, distinctions between the assays were apparent. Higher baseline IFNgamma responses were detected using the cultured whole blood ELISA, whereas the ex vivo ELISpot assay was the most sensitive in detecting long-term (52 weeks) post-vaccination responses. The whole blood ex vivo ICS assay provided novel information by dissecting the IFNgamma response into responding CD4, CD8 and gamma/delta T cell subsets. Future tuberculosis vaccine trials and immunology studies should ideally include a combination of ex vivo and cultured assays to ensure a thorough and multifaceted evaluation of the immune response is achieved.

Preclinical assessment of the receptor-binding domain of Plasmodium vivax Duffy-binding protein as a vaccine candidate in rhesus macaques

The receptor-binding domain of Plasmodium vivax Duffy-binding protein, region II (PvRII), is an attractive candidate for a vaccine against P. vivax malaria. Here, we have studied the safety and immunogenicity of recombinant PvRII in Macaca mulatta (rhesus monkeys). Recombinant PvRII with a C-terminal 6-histidine tag was expressed in E. coli, recovered from inclusion bodies, refolded into its functional conformation, purified to homogeneity and formulated with three adjuvants, namely, Alhydrogel, Montanide ISA 720 and the GSK proprietary Adjuvant System AS02A for use in immunogenicity studies. All the PvRII vaccine formulations tested were safe and highly immunogenic. The overall magnitude of the antibody response was significantly higher for both Montanide ISA 720 and AS02A formulations in comparison with Alhydrogel. Furthermore, there was a significant correlation between antibody recognition titers by ELISA and binding inhibition titers in in vitro binding assays. The PvRII vaccine formulations also induced IFN-gamma recall responses that were identified using ex vivo ELISPOT assays. These results provide support for further clinical development of a vaccine for P. vivax malaria based on recombinant PvRII.

Correlation of memory T cell responses against TRAP with protection from clinical malaria, and CD4 CD25 high T cells with susceptibility in Kenyans

Background

Immunity to malaria develops naturally in endemic regions, but the protective immune mechanisms are poorly understood. Many vaccination strategies aim to induce T cells against diverse pre-erythrocytic antigens, but correlates of protection in the field have been limited. The objective of this study was to investigate cell-mediated immune correlates of protection in natural malaria. Memory T cells reactive against thrombospondin-related adhesive protein (TRAP) and circumsporozoite (CS) protein, major vaccine candidate antigens, were measured, as were frequencies of CD4⁺ CD25^high T cells, which may suppress immunity, and CD56⁺ NK cells and γδ T cells, which may be effectors or may modulate immunity.

Methodology and Principal Findings

112 healthy volunteers living in rural Kenya were entered in the study. Memory T cells reactive against TRAP and CS were measured using a cultured IFNγ ELISPOT approach, whilst CD4⁺ CD25^high T cells, CD56⁺ NK cells, and γδ T cells were measured by flow cytometry. We found that T cell responses against TRAP were established early in life (<5 years) in contrast to CS, and cultured ELISPOT memory T cell responses did not correlate with ex-vivo IFNγ ELISPOT effector responses. Data was examined for associations with risk of clinical malaria for a period of 300 days. Multivariate logistic analysis incorporating age and CS response showed that cultured memory T cell responses against TRAP were associated with a significantly reduced incidence of malaria (p = 0.028). This was not seen for CS responses. Higher numbers of CD4⁺ CD25^high T cells, potentially regulatory T cells, were associated with a significantly increased risk of clinical malaria (p = 0.039).

Conclusions

These data demonstrate a role for central memory T cells in natural malarial immunity and support current vaccination strategies aimed at inducing durable protective T cell responses against the TRAP antigen. They also suggest that CD4⁺ CD25^high T cells may negatively affect naturally acquired malarial immunity.

Contribution of influenza immunity and virosomal-formulated synthetic peptide to cellular immune responses in a phase I subunit malaria vaccine trial

We have demonstrated recently in a phase Ia clinical trial that synthetic malaria peptides delivered by immuno-potentiating reconstituted influenza virosomes (IRIV) induced long-lived peptide-specific antibody responses in all volunteers. In the current ancillary study to this clinical trial we have investigated the cellular immune responses specific for IRIV and the surface bound synthetic malaria peptides tested. After vaccination, in 50% (8/16) of the volunteers at least one positive lymphoproliferative response specific for the 49mer peptide derived from the Plasmodium falciparum apical membrane antigen-1 (AMA-1) was observed with stimulation indices ranging from 2 to 4.5. All volunteers showed pre-existing IRIV specific cellular immunity assessed by ex vivo IFN-gamma ELISpot analysis and lymphoproliferation. The pre-existing influenza specific T cell responses did not interfere negatively with the induction of malaria peptide-specific humoral and cellular immune responses. Our results support the view that IRIV constitute a safe antigen delivery system for induction of peptide-specific immune responses in human populations.

Single-dose protection against Plasmodium berghei by a simian adenovirus vector using a human cytomegalovirus promoter containing intron A

Human adenovirus serotype 5 (AdH5) vector vaccines elicit strong immune responses to the encoded antigen and have been used in various disease models. We designed AdH5 vectors expressing antigen under the control of a human cytomegalovirus (HCMV) immediate-early promoter containing its intron A sequence. The transcriptional levels of antigen and immune responses to antigen for vectors with the HCMV promoter with the intron A sequence (LP) were greater than those for AdH5 vectors using the HCMV promoter sequence without intron A (SP). We compared an E1E3-deleted AdH5 adenoviral vector, which affords more space for insertion of foreign sequences, and showed it to be as immunogenic as an E1-deleted AdH5 vector. Neutralizing antibodies to AdH5 limit the efficacy of vaccines based on the AdH5 serotype, and simian adenoviral vectors offer an attractive option to overcome this problem. We constructed E1E3-deleted human and simian adenoviral vectors encoding the pre-erythrocytic-stage malarial antigen Plasmodium berghei circumsporozoite protein. We compared the immunogenicity and efficacy of AdC6, a recombinant simian adenovirus serotype 6 vector, in a murine malaria model to those of AdH5 and the poxviral vectors MVA and FP9. AdC6 induced sterile protection from a single dose in 90% of mice, in contrast to AdH5 (25%) and poxviral vectors MVA and FP9 (0%). Adenoviral vectors maintained potent CD8(+) T-cell responses for a longer period after immunization than did poxviral vectors and mainly induced an effector memory phenotype of cells. Significantly, AdC6 was able to maintain protection in the presence of preexisting immunity to AdH5.

The induction and persistence of T cell IFN-gamma responses after vaccination or natural exposure is suppressed by Plasmodium falciparum

Epidemiological observations suggest that T cell immunity may be suppressed in malaria-endemic areas. In vitro studies, animal models, and limited data in humans link immunosuppression with malaria, malnutrition, and other parasitic infections. However, there are no data to determine whether malaria-induced immunosuppression is significant in the long-term, or relative data comparing it with other factors in malaria-endemic areas, so as to measure the impact of malaria, other parasitic disease, nutritional status, age. and location on the acquisition and longevity of IFN-gamma responses in children in Kenya. We studied these factors in two cohorts of 1- to 6-year-old children in a malaria-endemic area. T cell responses were induced by vaccination in one cohort, and acquired as a result of natural exposure in a second cohort. Serial ELISPOT assays conducted over a 1-year period measured the induction and kinetics of IFN-gamma production in response to the malaria Ag thrombospondin-related adhesion protein. Induced responses in both cohorts and the longevity of response in the vaccinated cohort were fitted to potential explanatory variables. Parasitemia was prospectively associated with reduced IFN-gamma-producing T cells in both cohorts (by 15-25%), and both parasitemia and episodes of febrile malaria were associated with 19 and 31% greater attrition of T cell responses, respectively. Malaria may reduce the efficacy vaccinations such as bacillus Calmette-Guérin and investigational T cell-inducing vaccines, and may delay the acquisition of immunity following natural exposure to malaria and other pathogens.

Cellular immune responses induced with dose-sparing intradermal administration of HIV vaccine to HIV-uninfected volunteers in the ANRS VAC16 trial

OBJECTIVE

The objective was to compare the safety and cellular immunogenicity of intradermal versus intramuscular immunization with an HIV-lipopeptide candidate vaccine (LIPO-4) in healthy volunteers.

METHODOLOGY

A randomized, open-label trial with 24 weeks of follow-up was conducted in France at six HIV-vaccine trial sites. Sixty-eight healthy 21- to 55-year-old HIV-uninfected subjects were randomized to receive the LIPO-4 vaccine (four HIV lipopeptides linked to a T-helper-stimulating epitope of tetanus-toxin protein) at weeks 0, 4 and 12, either intradermally (0.1 ml, 100 microg of each peptide) or intramuscularly (0.5 ml, 500 microg of each peptide). Comparative safety of both routes was evaluated. CD8+ T-cell immune responses to HIV epitopes (ELISpot interferon-gamma assay) and tetanus toxin-specific CD4+ T-cell responses (lymphoproliferation) were assessed at baseline, two weeks after each injection, and at week 24.

RESULTS AND CONCLUSION

No severe, serious or life-threatening adverse events were observed. Local pain was significantly more frequent after intramuscular injection, but local inflammatory reactions were more frequent after intradermal immunization. At weeks 2, 6, 14 and 24, the respective cumulative percentages of induced CD8+ T-cell responses to at least one HIV peptide were 9, 33, 39 and 52 (intradermal group) or 14, 20, 26 and 37 (intramuscular group), and induced tetanus toxin-specific CD4+ T-cell responses were 6, 27, 33 and 39 (intradermal), or 9, 46, 54 and 63 (intramuscular). In conclusion, intradermal LIPO-4 immunization was well tolerated, required one-fifth of the intramuscular dose, and induced similar HIV-specific CD8+ T-cell responses. Moreover, the immunization route influenced which antigen-specific T-cells (CD4+ or CD8+) were induced.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00121121.

Immunogenicity of Anaplasma marginale type IV secretion system proteins in a protective outer membrane vaccine

Rickettsial pathogens in the genera Anaplasma and Ehrlichia cause acute infection in immunologically naive hosts and are major causes of tick-borne disease in animals and humans. Immunization with purified outer membranes induces protection against acute Anaplasma marginale infection and disease, and a proteomic and genomic approach recently identified 21 proteins within the outer membrane immunogen in addition to the well-characterized major surface proteins MSP1 to MSP5. Among the newly described proteins were the type IV secretion system (TFSS) proteins VirB9, VirB10, and conjugal transfer protein (CTP). In other gram-negative bacteria, TFSS proteins form channels, facilitate secretion of molecules, and are required for intracellular survival. However, TFSS proteins have not been explored as vaccine antigens. In this study we demonstrate that in Anaplasma marginale outer membrane-vaccinated cattle, VirB9, VirB10, and CTP are recognized by serum immunoglobulin G2 (IgG2) and stimulate memory T-lymphocyte proliferation and gamma interferon secretion. VirB9 induced the greatest proliferation in CD4+ T-cell lines, and VirB9-specific CD4+ T-cell clones responded to three A. marginale strains, confirming the VirB9-specific T-cell responses are directed against epitopes in the native protein. The three TFSS proteins are highly conserved with orthologous proteins in Anaplasma phagocytophilum, Ehrlichia chaffeensis, and Ehrlichia canis. Recognition of TFSS antigens by CD4+ T cells and by IgG2 from cattle immunized with the protective outer membrane fraction provides a rationale for including these proteins in development of vaccines against A. marginale and related pathogens.

A phase 2b randomised trial of the candidate malaria vaccines FP9 ME-TRAP and MVA ME-TRAP among children in Kenya

Objective:

The objective was to measure the efficacy of the vaccination regimen FFM ME-TRAP in preventing episodes of clinical malaria among children in a malaria endemic area. FFM ME-TRAP is sequential immunisation with two attenuated poxvirus vectors (FP9 and modified vaccinia virus Ankara), which both deliver the pre-erythrocytic malaria antigen construct multiple epitope–thrombospondin-related adhesion protein (ME-TRAP).

Design:

The trial was randomised and double-blinded.

Setting:

The setting was a rural, malaria-endemic area of coastal Kenya.

Participants:

We vaccinated 405 healthy 1- to 6-year-old children.

Interventions:

Participants were randomised to vaccination with either FFM ME-TRAP or control (rabies vaccine).

Outcome Measures:

Following antimalarial drug treatment children were seen weekly and whenever they were unwell during nine months of monitoring. The axillary temperature was measured, and blood films taken when febrile. The primary analysis was time to a parasitaemia of over 2,500 parasites/μl.

Results:

The regime was moderately immunogenic, but the magnitude of T cell responses was lower than in previous studies. In intention to treat (ITT) analysis, time to first episode was shorter in the FFM ME-TRAP group. The cumulative incidence of febrile malaria was 52/190 (27%) for FFM ME-TRAP and 40/197 (20%) among controls (hazard ratio = 1.52). This was not statistically significant (95% confidence interval [CI] 1.0–2.3; p = 0.14 by log-rank). A group of 346 children were vaccinated according to protocol (ATP). Among these children, the hazard ratio was 1.3 (95% CI 0.8–2.1; p = 0.55 by log-rank). When multiple malaria episodes were included in the analyses, the incidence rate ratios were 1.6 (95% CI 1.1–2.3); p = 0.017 for ITT, and 1.4 (95% CI 0.9–2.1); p = 0.16 for ATP. Haemoglobin and parasitaemia in cross-sectional surveys at 3 and 9 mo did not differ by treatment group. Among children vaccinated with FFM ME-TRAP, there was no correlation between immunogenicity and malaria incidence.

Conclusions:

No protection was induced against febrile malaria by this vaccine regimen. Future field studies will require vaccinations with stronger immunogenicity in children living in malarious areas.

Background: Malaria kills over a million people a year worldwide, and young children in sub-Saharan are particularly at risk. Cheap, safe, and effective vaccines are needed. One strategy involves a double-vaccination process. This approach (termed “prime-boost”) uses two different delivery methods to transmit the same antigen (part of a protein from the malaria parasite that can trigger an immune response). The two-step vaccination is designed to achieve a greater immune response than with just one vaccination. One research group, based in Oxford in the UK, is using an antigen called “ME-TRAP,” which is delivered using first a strain of modified fowlpox virus (called FP9), then a weakened vaccinia virus (called MVA). Previous studies done in adult UK volunteers have been promising, achieving an immune response and some protection against malaria when volunteers were deliberately infected. However, this approach has not been tested in the group most in need of a vaccine—young African children. Therefore a field trial was conducted among 405 healthy children aged 1–6 years, in a region of Kenya with year-round malaria transmission. Children were randomized to receive either the sequence of vaccines delivering ME-TRAP or to receive a rabies vaccine (as control, but which still gives the children some benefit for taking part in the trial). The children were followed up for nine months, and the primary aim of the trial was to compare the occurrence of clinical malaria (fever combined with malaria parasites in the blood) in the two groups.

What this trial shows: In the 387 children receiving vaccine and having at least one follow-up visit the vaccine did produce an immune response; however, this immune response did not seem to be protective, as the occurrence of malaria was slightly higher in the group receiving the candidate vaccine—although this difference was not statistically significant. Safety data were also collected; the number and severity of adverse events were similar between volunteers receiving the rabies vaccine and those receiving the candidate malaria vaccine, and any serious events were not judged to be linked to the vaccines by the trial's data safety monitoring board.

Strengths and limitations: The methods used in the trial were robust, using appropriate randomization procedures and blinding of participants and researchers. Outcome measures (clinical malaria, defined as fever together with parasites in the blood over 2,500/microliter) were clinically relevant. In order to detect cases of malaria in vaccinated children, health workers visited children weekly, and children with a temperature over 37.5 °C were tested for parasites in the blood. (In between the weekly visits, self-report and referral for assessment also allowed detection of cases.) This process of active detection of malaria cases (as opposed to obtaining data on clinical malaria only from self-report or referral) enables a smaller sample size to be used in the trial, but it is not clear whether this approach is more or less specific at picking up malaria cases than are passive methods. The researchers aimed to ensure that their case detection methods were specific; for children with normal temperature, but reported by their parents as feverish, parasite tests were done only if subsequent temperature readings were high.

Contribution to the evidence: Previous studies of ME-TRAP using the FP9 and MVA vectors have shown the candidate vaccine is safe and induces a strong immune response. The safety result was also supported by the findings of the current trial, conducted in young Kenyan children, but a 5-fold lower immune response was found compared to previous studies. The trial showed that this weak immune response was not effective at preventing clinical cases of malaria in this group of children, although it is not clear why the immune response was lower than expected.

Alternating vector immunizations encoding pre-erythrocytic malaria antigens enhance memory responses in a malaria endemic area

A heterologous prime-boost strategy has been developed to potently induce T cell responses to pre-erythrocytic malaria antigens. Efficacy in the field is likely to depend on both peak immunogenicity and the durability of responses. To improve both immunogenicity and durability of responses, 54 adult males from a malaria endemic area were immunized with different vaccination regimens, systematically varying antigenic insert and the number and sequence of component vaccinations. The component vaccinations were recombinant attenuated viruses, either fowlpox (FP) 9 or modified vaccinia virus Ankara (MVA). These were recombinant for either of two pre-erythrocytic malaria antigens (multiple epitope-thrombospondin-related adhesion protein, ME-TRAP, or circumsporozoite antigen (CS). ELISPOT assays were used to measure the effector and resting memory T cell responses. Sequence, antigen insert and number of vaccinations influenced immunogenicity, but the novel alternating vector immunizations generated the largest resting memory T cell populations. Effector responses were maintained at 84% of the peak response after 270 days. This durability of response is unprecedented. Classical prime-boost vaccination responses were at 5% of the peak after 270 days. Vaccines administered by heterologous prime-boost regimes are being developed for diverse pathogens and cancer. These data suggest these vaccines should also be administered by alternating vector regimens in clinical development.

Early gamma interferon and interleukin-2 responses to vaccination predict the late resting memory in malaria-naïve and malaria-exposed individuals

Two different cell populations respond to potent T-cell-inducing vaccinations. The induction and loss of effector cells can be seen using an ex vivo enzyme-linked immunospot (ELISPOT) assay, but the more durable resting memory response is demonstrable by a cultured ELISPOT assay. The relationship of the early effector response to durable resting memory is incompletely understood. Effector phenotype is usually identified by gamma interferon (IFN-gamma) production, but interleukin-2 (IL-2) has been specifically linked to the differentiation of memory cells. Here, IFN-gamma- and IL-2-secreting effector cells were identified by an ex vivo ELISPOT assay 1 week after vaccination and compared with the resting memory responses detected by a cultured ELISPOT assay 3 months later. The different kinetics and induction of IL-2 by different vaccines and natural exposure are described. Furthermore, both early IFN-gamma and IL-2 production independently predicted subsequent memory responses at 3 months in malaria-naïve volunteers, but only IFN-gamma predicted memory in malaria-exposed volunteers. However, dual ELISPOT assays were also performed on malaria-exposed volunteers to identify cells producing both cytokines simultaneously. This demonstrated that double-cytokine-producing cells were highly predictive of memory. This assay may be useful in predicting vaccinations most likely to generate stable, long-term memory responses.

Safety and immunogenicity of attenuated Salmonella enterica serovar Typhimurium delivering an HIV-1 Gag antigen via the Salmonella Type III secretion system

BACKGROUND

CKS257 (Salmonella typhimurium SL1344 DeltaphoP/phoQDelta aroA Deltaasd DeltastrA/strB pSB2131) is a live oral vaccine vector expressing HIV Gag.

METHODS

HIV Gag was expressed as a fusion protein of a Salmonella Type III secretion system protein SopE, from a balanced lethal asd-based plasmid. Eighteen healthy adults were given single escalating oral doses of 5 x 10(6) to 1 x 10(10)CFU of CKS257 and were monitored for clinical events, shedding and immune responses.

RESULTS

Adverse events were mild except at the highest dose. Volunteers shed the organism an average of 5.1 days (range 0-13 days). Eighty-three percent (15/18) of subjects had a mucosal immune response to Salmonella LPS and flagella by IgA ELISPOT assay. Seventy-two percent (13/18) of subjects seroconverted to Salmonella antigens. No volunteer had a response to recombinant Gag as measured by serology, IgA ELISPOT, or immediate ex vivo gamma-interferon ELISPOT response to Gag peptide pools. Two volunteers responded to Gag peptides by IL-2 ELISPOT, and 4 of 10 volunteers receiving >or=5 x 10(8)CFU had a response to HIV peptides in a cultured gamma-interferon ELISPOT assay.

CONCLUSIONS

Although immunogenicity of the HIV antigen needs augmentation, the attenuated Salmonella strain proved to be an excellent platform for vaccine development.

Pre-erythrocytic malaria vaccines: towards greater efficacy

The complex life cycle of the malaria parasite Plasmodium falciparum provides many options for vaccine design. Several new types of vaccine are now being evaluated in clinical trials. Recently, two vaccine candidates that target the pre-erythrocytic stages of the malaria life cycle - a protein particle vaccine with a powerful adjuvant and a prime-boost viral-vector vaccine - have entered Phase II clinical trials in the field and the first has shown partial efficacy in preventing malarial disease in African children. This Review focuses on the potential immunological basis for the encouraging partial protection induced by these vaccines, and it considers ways for developing more effective malaria vaccines.

Correlation of antiviral T-cell responses with suppression of viral rebound in chronic hepatitis B carriers: a proof-of-concept study

Despite recent advances in the chemotherapy of chronic hepatitis B (CHB), an effective viral suppression after cessation of therapy has not yet been achieved. To investigate whether hepatitis B virus (HBV)-specific T-cell responses are inducible and can contribute to the viral suppression after cessation of the therapy, we conducted a proof-of-concept study with a DNA vaccine comprising of most HBV genes plus genetically engineered interleukin-12 DNA (IL-12N222L) in 12 CHB carriers being treated with lamivudine (LAM). When the ex vivo and/or cultured IFN-gamma enzyme-linked immunospot (ELISPOT) assay was performed, the detectable HBV-specific IFN-gamma secreting T-cell responses were observed at the end of treatment and during a follow-up. These type 1T-cell responses, particularly CD4(+) memory T-cell responses could be maintained for at least 40 weeks after the therapy and correlated with virological responses, but not with alanine aminotransferase elevation. Moreover, DNA vaccination under LAM treatment appeared to be well-tolerated and showed 50% of virological response rate in CHB carriers. Thus, a combination therapy of the DNA vaccine with chemotherapy may be one of new immunotherapeutic methods for the cure of CHB.

Durable human memory T cells quantifiable by cultured enzyme-linked immunospot assays are induced by heterologous prime boost immunization and correlate with protection against malaria

Immunological memory is a required component of protective antimalarial responses raised by T cell-inducing vaccines. The magnitude of ex vivo IFN-gamma T cell responses is widely used to identify immunogenic vaccines although this response usually wanes and may disappear within weeks. However, protection in the field is likely to depend on durable central memory T cells that are not detected by this assay. To identify longer-lived memory T cells, PBMC from malaria-naive vaccinated volunteers who had received prime boost vaccinations with a combination of DNA and/or viral vectors encoding the multiepitope string-thrombospondin-related adhesion protein Ag were cultured in vitro with Ag for 10 days before the ELISPOT assay. Ex vivo T cell responses peaked at 7 days after the final immunization and declined substantially over 6 mo, but responses identified after T cell culture increased over the 6-mo period after the final immunization. Moreover, individual cultured ELISPOT responses at the day of challenge time point correlated significantly with degree of protection against malaria sporozoite challenge, whereas ex vivo responses did not, despite a correlation between the peak ex vivo response and magnitude of memory responses 6 mo later. This cultured assay identifies long-lasting protective T cell responses and therefore offers an attractive option for assessments of vaccine immunogenicity.

HLA-A2 supertype-restricted cell-mediated immunity by peripheral blood mononuclear cells derived from Malian children with severe or uncomplicated Plasmodium falciparum malaria and healthy controls

Understanding HLA-restricted adaptive host immunity to defined epitopes of malarial antigens may be required for the development of successful malaria vaccines. Fourteen epitopes of preerythrocytic malarial antigens known to mediate cytotoxic T-lymphocyte responses against target cells expressing HLA-A2-restricted epitopes were synthesized and pooled based on antigen: thrombospondin-related anonymous protein (TRAP), circumsporozoite protein (CSP), and export protein 1 (Exp-1) peptides. HLA-A2 supertype (*0201, *0202, *0205, *6802) peripheral blood mononuclear cells collected from 774 Malian children, aged 3 months to 14 years, with severe Plasmodium falciparum malaria matched to uncomplicated malaria or healthy controls were stimulated with the HLA-A2-restricted peptide pools. Significant gamma interferon production, determined by enzyme-linked immunospot assay to at least one of the three peptide pools, was observed in 24/58 (41%) of the severe malaria cases, 24/57 (42%) of the uncomplicated malaria cases, and 34/51 (67%) of the healthy controls. Significant lymphoproliferation to these peptides was observed in 12/44 (27%) of the severe malaria cases, 13/55 (24%) of the uncomplicated malaria cases, and 18/50 (36%) of the healthy controls. Responses to individual peptide pools were limited. These studies confirm the presence of adaptive cell-mediated immunity to preerythrocytic malaria antigens in volunteers from Mali and demonstrate that suballeles of the HLA-A2 supertype can effectively present antigenic epitopes. However, whether these immune responses to TRAP, CSP, and Exp-1 malarial proteins play a substantial role in protection remains a matter of controversy.

Does malaria suffer from lack of memory?

It is widely perceived that immunity to malaria is, to an extent, defective and that one component of this defective immune response is the inability to induce or maintain long-term memory responses. If true, this is likely to pose problems for development of an effective vaccine against malaria. In this article, we critically review and challenge this interpretation of the epidemiological and experimental evidence. While evasion and modulation of host immune responses clearly occurs and naturally acquired immunity is far from optimal, mechanisms to control blood-stage parasites are acquired and maintained by individuals living in endemic areas, allowing parasite density to be kept below the threshold for induction of acute disease. Furthermore, protective immunity to severe pathology is achieved relatively rapidly and is maintained in the absence of boosting by re-infection. Nevertheless, there are significant challenges to overcome. The need for multiple infections to acquire immunity means that young children remain at risk of infection for far too long. Persistent or frequent exposure to antigen seems to be required to maintain anti-parasite immunity (premunition). Lastly, pre-erythrocytic and sexual stages of the life cycle are poorly immunogenic, and there is little evidence of effective pre-erythrocytic or transmission-blocking immunity at the population level. While these problems might theoretically be due to defective immunological memory, we suggest alternative explanations. Moreover, we question the extent to which these problems are malaria-specific rather than generic (i.e. result from inherent limitations of the vertebrate immune system).

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.

Anaplasma marginale major surface protein 2 CD4+-T-cell epitopes are evenly distributed in conserved and hypervariable regions (HVR), whereas linear B-cell epitopes are predominantly located in the HVR

Organisms in the genus Anaplasma express an immunodominant major surface protein 2 (MSP2), composed of a central hypervariable region (HVR) flanked by highly conserved regions. Throughout Anaplasma marginale infection, recombination results in the sequential appearance of novel MSP2 variants and subsequent control of rickettsemia by the immune response, leading to persistent infection. To determine whether immune evasion and selection for variant organisms is associated with a predominant response against HVR epitopes, T-cell and linear B-cell epitopes were localized by measuring peripheral blood gamma interferon-secreting cells, proliferation, and antibody binding to 27 overlapping peptides spanning MSP2 in 16 cattle. Similar numbers of MSP2-specific CD4(+) T-cell epitopes eliciting responses of similar magnitude were found in conserved and hypervariable regions. T-cell epitope clusters recognized by the majority of animals were identified in the HVR (amino acids [aa] 171 to 229) and conserved regions (aa 101 to 170 and 272 to 361). In contrast, linear B-cell epitopes were concentrated in the HVR, residing within hydrophilic sequences. The pattern of recognition of epitope clusters by T cells and of HVR epitopes by B cells is consistent with the influence of protein structure on epitope recognition.

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.

A CD4(+) T-cell immune response to a conserved epitope in the circumsporozoite protein correlates with protection from natural Plasmodium falciparum infection and disease

Many human T-cell responses specific for epitopes in Plasmodium falciparum have been described, but none has yet been shown to be predictive of protection against natural malaria infection. Here we report a peptide-specific T-cell assay that is strongly associated with protection of humans in The Gambia, West Africa, from both malaria infection and disease. The assay detects interferon-gamma-secreting CD4(+) T cells specific for a conserved sequence from the circumsporozoite protein, which binds to many human leukocyte antigen (HLA)-DR types. The correlation was observed using a cultured, rather than an ex vivo, ELISPOT assay that measures central memory-'type T cells rather than activated effector T cells. These findings provide direct evidence for a protective role for CD4(+) T cells in humans, and a precise target for the design of improved vaccines against P. falciparum.

Cellular immunity induced by the recombinant Plasmodium falciparum malaria vaccine, RTS,S/AS02, in semi-immune adults in The Gambia

Vaccination of malaria-naive humans with recombinant RTS,S/AS02, which includes the C-terminus of the circumsporozoite protein (CS), has been shown to induce strong T cell responses to both the whole protein antigen and to peptides from CS. Here we show that strong T cell responses were also observed in a semi-immune population in The Gambia, West Africa. In a Phase I study, 20 adult male volunteers, lifelong residents in a malaria-endemic region, were given three doses of RTS,S/AS02 at 0, 1 and 6 months. Responses to RTS,S, hepatitis B surface antigen and peptides from CS were tested using lymphocyte proliferation, interferon (IFN)-gamma production in microcultures, and IFN-gamma ex vivo and cultured ELISPOT, before and after vaccination. Cytotoxic responses were tested only after vaccination and none were detected. Before vaccination, the majority of the volunteers (15/20) had detectable responses in at least one of the tests. After vaccination, responses increased in all assays except cytotoxicity. The increase was most marked for proliferation; all donors responded to RTS,S after the third dose and all except one donor responded to at least one peptide after the second or third dose. There was a lack of close association of peptide responses detected by the different assays, although in microcultures IFN-gamma responses were found only when proliferative responses were high, and responses by cultured ELISPOT and proliferation were found together more frequently after vaccination. We have therefore identified several peptide-specific T cell responses induced by RTS,S/AS02 which provides a mechanism to investigate potentially protective immune responses in the field.

Delivery of a heterologous antigen by a registeredSalmonellavaccine (STM1)

STM1 is an aro A(-) attenuated mutant of Salmonella enterica serovar Typhimurium, and is a well-characterised vaccine strain available to the livestock industry for the prevention of salmonellosis in chickens. This strain has potential for heterologous antigen delivery, and here we show that the strain can be used to deliver a model antigen, ovalbumin, to immune cells in vitro and in vivo. Two plasmid constructs expressing the ovalbumin gene were utilised, one of which uses a prokaryotic promoter and the other the CMV promoter (DNA vaccine). In vitro, STM1 carrying ovalbumin-encoding plasmids was able to invade dendritic cells and stimulate a CD8(+) cell line specific for the dominant ovalbumin epitope, SIINFEKL. In vivo, spleen cells were responsive to SIINFEKL after vaccination of mice with ovalbumin-encoding plasmids in STM1, and finally, humoral responses, including IgA, were induced after vaccination.

Association of interferon-gamma responses to pre-erythrocytic stage vaccine candidate antigens of Plasmodium falciparum in young Kenyan children with improved hemoglobin levels: XV. Asembo Bay Cohort Project

Previous studies in animal models have revealed an association between interferon-gamma (IFN-gamma), produced by CD8+ T cells and irradiated sporozoite-induced sterile immunity. To determine whether IFN-gamma can serve as a marker of pre-erythrocytic protective immunity in individuals naturally exposed to malaria, we characterized IFN-gamma and lymphocyte proliferative responses to previously defined CD8+ cytotoxic T lymphocyte (CTL) epitopes from six pre-erythrocytic stage antigens in 107 children six months to two years old from a community-based birth cohort in western Kenya. We found that IFN-gamma positive responders had higher hemoglobin (Hb) levels and significantly reduced prevalence of severe malarial anemia one month after the test compared with IFN-gamma non-responders, suggesting that IFN-gamma immune responses to these pre-erythrocytic antigens were associated with protection against malarial anemia. Children who responded by lymphocyte proliferation had a significantly longer time to first documented malaria parasitemia after birth; however, there was no correlation between the presence of lymphocyte proliferative response and higher Hb levels. We propose that IFN-gamma production could be used as a potential marker of protective immunity against malaria associated anemia in young children living in malaria holoendemic areas.

Comprehensive epitope analysis of human immunodeficiency virus type 1 (HIV-1)-specific T-cell responses directed against the entire expressed HIV-1 genome demonstrate broadly directed responses, but no correlation to viral load

Cellular immune responses play a critical role in the control of human immunodeficiency virus type 1 (HIV-1); however, the breadth of these responses at the single-epitope level has not been comprehensively assessed. We therefore screened peripheral blood mononuclear cells (PBMC) from 57 individuals at different stages of HIV-1 infection for virus-specific T-cell responses using a matrix of 504 overlapping peptides spanning all expressed HIV-1 proteins in a gamma interferon-enzyme-linked immunospot (Elispot) assay. HIV-1-specific T-cell responses were detectable in all study subjects, with a median of 14 individual epitopic regions targeted per person (range, 2 to 42), and all 14 HIV-1 protein subunits were recognized. HIV-1 p24-Gag and Nef contained the highest epitope density and were also the most frequently recognized HIV-1 proteins. The total magnitude of the HIV-1-specific response ranged from 280 to 25,860 spot-forming cells (SFC)/10(6) PBMC (median, 4,245) among all study participants. However, the number of epitopic regions targeted, the protein subunits recognized, and the total magnitude of HIV-1-specific responses varied significantly among the tested individuals, with the strongest and broadest responses detectable in individuals with untreated chronic HIV-1 infection. Neither the breadth nor the magnitude of the total HIV-1-specific CD8+-T-cell responses correlated with plasma viral load. We conclude that a peptide matrix-based Elispot assay allows for rapid, sensitive, specific, and efficient assessment of cellular immune responses directed against the entire expressed HIV-1 genome. These data also suggest that the impact of T-cell responses on control of viral replication cannot be explained by the mere quantification of the magnitude and breadth of the CD8+-T-cell response, even if a comprehensive pan-genome screening approach is applied.

Immunologic monitoring of cancer vaccine therapy: results of a workshop sponsored by the Society for Biological Therapy

The Society for Biological Therapy held a Workshop last fall devoted to immune monitoring for cancer immunotherapy trials. Participants included members of the academic and pharmaceutical communities as well as the National Cancer Institute and the Food and Drug Administration. Discussion focused on the relative merits and appropriate use of various immune monitoring tools. Six breakout groups dealt with assays of T-cell function, serologic and proliferation assays to assess B cell and T helper cell activity, and enzyme-linked immunospot assay, tetramer, cytokine flow cytometry, and reverse transcription polymerase chain reaction assays of T-cell immunity. General conclusions included: (1) future vaccine studies should be designed to determine whether T-cell dysfunction (tumor-specific and nonspecific) correlated with clinical outcome; (2) tetramer-based assays yield quantitative but not functional data (3) enzyme-linked immunospot assays have the lowest limit of detection (4) cytokine flow cytometry have a higher limit of detection than enzyme-linked immunospot assay, but offer the advantages of speed and the ability to identify subsets of reactive cells; (5) antibody tests are simple and accurate and should be incorporated to a greater extent in monitoring plans; (6) proliferation assays are imprecise and should not be emphasized in future studies; (7) the reverse transcription polymerase chain reaction assay is a promising research approach that is not ready for widespread application; and (8)there is a critical need to validate these assays as surrogates for vaccine potency and clinical effect. Current data and opinion support the use of a functional assay like the enzyme-linked immunospot assay or cytokine flow cytometry in combination with a quantitative assay like tetramers for immune monitoring. At present, assays appear to be most useful as measures of vaccine potency. Careful immune monitoring in association with larger scale clinical trials ultimately may enable the correlation of monitoring results with clinical benefit.