- Open Access
Phenotypic characterization of Plasmodium berghei responsive CD8+ T cells after immunization with live sporozoites under chloroquine cover
© Brando et al.; licensee BioMed Central Ltd. 2014
- Received: 12 September 2013
- Accepted: 2 February 2014
- Published: 12 March 2014
An effective malaria vaccine remains elusive. The most effective experimental vaccines confer only limited and short-lived protection despite production of protective antibodies. However, immunization with irradiated sporozoites, or with live sporozoites under chloroquine cover, has resulted in long-term protection apparently due to the generation of protective CD8+ T cells. The nature and function of these protective CD8+ T cells has not been elucidated. In the current study, the phenotype of CD8+ T cells generated after immunization of C57BL/6 mice with live Plasmodium berghei sporozoites under chloroquine cover was investigated.
Female C57BL/6 mice, C57BL/6 mice B2 macroglobulin −/− [KO], or invariant chain−/− [Ic KO] [6–8 weeks old] were immunized with P. berghei sporozoites and treated daily with 800 μg/mouse of chloroquine for nine days. This procedure of immunization is referred to as “infection/cure”. Mice were challenged by inoculating intravenously 1,000 infectious sporozoites. Appearance of parasitaemia was monitored by Giemsa-stained blood smears.
By use of MHC I and MHC II deficient animals, results indicate that CD8+ T cells are necessary for full protection and that production of protective antibodies is either CD4+ T helper cells dependent and/or lymphokines produced by CD4 cells contribute to the protection directly or by helping CD8+ T cells. Further, the phenotype of infection/cure P. berghei responsive CD8+ T cells was determined to be KLRG1high CD27low CD44high and CD62Llow.
The KLRG1high CD27low CD44high and CD62Llow phenotype of CD8+ T cells is associated with protection and should be investigated further as a candidate correlate of protection.
- Protective Antibody
- Plasmodium Berghei
- Irradiate Sporozoite
- Residual Protective Effect
While reduction in malaria cases has been reported in many countries, malaria remains among the world’s most prevalent and fatal infectious disease. In 2011, it was estimated there were 216 million malaria episodes and 655,000 malaria deaths . With the majority of the deaths occurring in children less than five years of age, and with almost half of the world population at risk, an effective vaccine against malaria is urgently needed .
Natural exposures to malaria infections do not immediately induce immunity leaving infants and young children in endemic areas susceptible to multiple episodes of the disease. Eventually, partial immunity is acquired in older children and adults, affording them protection against clinical symptoms and/or severe disease. However, protection is not sterile  and the immune responses to Plasmodium parasites are short-lived . This is attributed to short half-life of protective antibodies  and to a cellular response [5–7] too weak to grant protection [8–11]. Sterile protection has however been achieved experimentally in both animal models of malaria and in malaria-naive humans immunized with whole live sporozoites .
Intravenous administration of irradiated sporozoites renders long-lasting protection [13–17] by a mechanism mediated largely by CD8+ T cells [18–23]. Although there is data in favour of other mechanisms  and different mice strains show different susceptibility to malaria [25, 26], CD8+ T cell remain the main player in this model of protection. Evidence of protection conferred by immunization with sporozoites under chloroquine (CQ) cover was initially demonstrated in mice and rats using Plasmodium berghei[27, 28]. More recently, infection under CQ cover has also been shown to induce long-lasting protection in malaria-naïve human [29, 30] and murine models [31, 32]. In these models, CD8+ T cells appear to play an important role in protection [24, 29–32]. Taken together, these data suggest that while natural exposure elicits weakly protective humoral and cellular immunity, strategies such as inoculation of irradiated sporozoites or viable sporozoites under CQ cover induce CD8+ T cells which, alone or in combination with antibodies, provide long-lasting protection. A better understanding of the mechanics of these protective cells is fundamental to vaccine development efforts.
In this study, P. berghei murine model was used to characterize the phenotype of CD8+ T cells generated under CQ cover and to associate these phenotypes with protection from a lethal challenge.
Female C57BL/6 mice, C57BL/6 mice B2 macroglobulin −/− (KO), or invariant chain−/− (Ic KO) (6–8 weeks old) were purchased from the Jackson Laboratory (Bar Harbor, ME). These animals were housed at the Walter Reed Army Institute of Research (WRAIR) animal facility and handled according to institutional guidelines. All procedures were reviewed and approved by the WRAIR Animal Care and Use Committee (IACUC) and were performed in a facility accredited by the Association for Assessment and Accreditation of Laboratory Animal Care International. Euthanasia methods were compliant with the approved IACUC protocol, with frequent supervision by the institute’s veterinary team. After the animals were challenged, strict humane endpoints criteria were adhered to by frequently monitoring the health of the animals in order to avoid or terminate unrelieved pain and/or distress. Animals positive for parasitaemia for two consecutive days were euthanized before showing distress and/or pain by inhalation of CO2 from a pressurized tank in a chamber. This was followed by cervical dislocation prior to disposal of the dead mice.
Sporozoites (Spz) from P. berghei ANKA strain were extracted from salivary glands of Anopheles stephensi mosquitoes 16–21 days post blood meal as previously described . Mosquitoes were maintained in the WRAIR Entomology Branch Insectary facilities.
Reagents, antibodies and chemicals
Cell media and CFSE were purchased from Life Technologies (Carlsbad, CA). Purified antibodies or fluorochrome labeled antibodies were purchased from eBioscience (San Diego, CA). Brefeldine A, Giemsa stain and CQ were purchased from Sigma (St. Louis, MO).
Immunization and treatment
Experimental animals were inoculated two or three times intravenously with 10,000 or 20,000 Spz (in 100 ul PBS), ten days apart and treated daily with 800 μg/mouse (100 ul in PBS) of CQ, administered intraperitoneally (i.p.), starting from the day of the first inoculation until nine days post last inoculation. This procedure of immunization is referred to as “infection/cure”. For the CQ control group, the same regimen of CQ as that of the experimental group was administered with the exception that these animals were not inoculated with Spz. The absolute control group received PBS only.
Challenge and assessment of protection
For challenge experiments, mice were inoculated intravenously with 1000 infectious Spz. Challenge was performed at two weeks post suspension of CQ treatment. Appearance of parasitaemia was monitored by Giemsa-stained blood smears. Animals free of parasitaemia for two weeks were considered protected.
Mice were euthanized by CO2 inhalation as described above. Livers and spleens were removed, and livers were perfused with 10 ml PBS. Single cell suspensions of lymphocytes were made from both organs (liver infiltrating lymphocytes and splenocytes). Cells were re-suspended in PBS and used for analysis or transfer. Peripheral blood for blood lymphocyte analysis was collected from the tail vein in a heparinized vial.
Serum and splenocyte transfer
Recipient mice in transfer experiments received either 500 μL/mouse of undiluted serum intraperitoneally, 20 × 10^6 splenocytes re-suspended in 500 μL of PBS by injection into the tail vein, or a combination of both.
Flow cytometry analysis
Analysis was performed with an LSR II cytometer (Beckon Dickinson San Diego, CA) and data were analysed with Flow-Jo software (BD). The panel of fluorochrome conjugated antibodies for flow cytometry included: CD3- PrcP, CD4-Pacific Blue, CD8-V500 or Pacific Orange, CD45RB or CD44- Alexa fluoro −700, CD27-APC, CD127 –PE, KLRG-1 FITC. When the intracellular cytokine IFNγ was tested, the panel used was: CD8-PO, KLRG-1-FITC, CD62-PrCPcy5.5, CD44 Alexa-700, CD27-APC, IFN- γ-PE. In all assays UV-viability dye was included. Flow cytometry antibodies were purchased from Life Technologies (Carlsbad, CA).
Abbreviation for fluorochrome
Allophycocyanin (APC), Pyridine-chlorophyll proteins-Cy-5 (PrCP-Cy5), Phycoerythrin (PE), Fluorescein (FITC). Antibodies are indicated by the marker recognizedfluorochrome, e.g. anti CD3 Fluorescein-conjugated was CD3-FITC.
Three cycles of infection/cure results in protection
Immunization regimen with Spz used for infection/cure of mice (10 animals/group)
Protection requires both CD8+ T cells and antibodies
The phenotype of infection/cure P. berghei responsive CD8+ T cells is KLRG1high CD27low CD44high and CD62Llow
CD8+ T cells containing KLRG1 high , CD27 low phenotype
Mean CD8+ KLRG1high, CD27lowcells (%)
95% CI of mean
Number of lymphocytes/ml × 10^3
Absolute number of CD + KLRG1high, CD27lowT cells × 10^3/ml of blood
Identifying T cell phenotype associated with protection
Mean CD8+ KLRG1 high , CD27 low T cell (%) expressed in circulating peripheral blood of control, protected and non-protected animals
Mean CD8+ KLRG1high, CD27lowcells (%)
95% CI of mean
Little progress has been made in elucidating the requirement for immune protection against malaria infection. Protection has not been achieved even with high titers of antibodies against known liver antigen , blood stage antigen [34, 35] or antigen specific CD8+ T cells produced with adenovirus immunization . In addition, human immunization with irradiated sporozoites has shown limited success unless sporozoites are injected intravenously . Conversely, immunization with irradiated sporozoites or live sporozoites under CQ cover induces protection [29, 31]. In the current study, the infection/cure C57BL/6 mouse model was used to elucidate the immune protection conferred during malaria infection by characterizing the nature of CD8+ T cells. Recent studies showed that both CD4+ T cells and CD8+ T cells are necessary to mediate immunity to liver stage malaria parasites [31, 32]. This was demonstrated by immunizing BALB/c mice with Plasmodium yoelii iRBC under CQ cover and then treating with depleting antibodies against CD4+ T cells and CD8+ T cells. In this study, the infection/cure CD8+ T cells deficient (MHC I KO) mice resulted in only partial protection from lethal challenge, thus implicating CD8+ T cells in protection. Passive transfer with serum or cell alone also failed to confer 100% protection, thus indicating the cooperation of antigen specific cells and antibodies in granting protection. MHC II animals were also only partially protected. The most likely explanation is that the protective antibodies generated during infection/cure need CD4+ T cell help, which is lost in CD4+ T cell deficient animals. However, the role of CD4+ T cell in this model cannot be excluded is also to support the generation of protective CD8+ T cells or that they are directly involved in parasite killing. It will be interesting to perform transfer experiments to test the protective effect of serum from infection/cure MHC II animals enriched with CD4+ T cells from infection/cure wildtype animals. This will be important to discriminate between the roles of CD4+ T cells as pure ‘helper’ for antibodies production or more direct involved in anti-plasmodium activity. From this study and that by Belnoue et al., data strongly suggest that both a humoral response and CD8+ T cells are required for immunity to liver stage P. berghei parasites [31, 32].
In an effort to further elucidate immune protection, CD8+ T cells in peripheral blood and splenocytes were characterized. The distinct populations of CD8+ T cells with phenotype KLRG1high, CD27low, CD44high, CD62Llow were shown to be associated with protection. This population is absent in the intrahepatic lymphocytes. In the classical model of protection by immunization with irradiated sporozoites, the putative protective CD8+ T cells are found mostly in the liver [18, 19] and produce IFNγ. In a recent study, Nganou-Makandop et al. reported comparable levels of hepatic CD8+ T cells with a CD44high, CD62Llow phenotype that produce IFNγ in response to PMA/Iono. In this study, mice were either immunized with radiation-attenuated sporozoites or infected with P. berghei under CQ cover. In the current study, the CD8+ KLRG1high, CD27low, CD44high, CD62Llow phenotype that was associated with protection was not present in the liver of immunized animals. A possible explanation of the apparent discrepancy is that more than one population of CD8+ T cells contributes to protection. One can speculate that the splenic CD8+ KLRG1high, CD27low, CD44high, CD62Llow cells and the intrahepatic CD8+ CD44high, CD62Llow cells are the same antigen specific population whose phenotype and organ-homing is modulated by antigen exposure. In explorative experiments, it was observed that splenic and peripheral blood CD8+ KLRG1high, CD27low, CD44high, CD62Llow T cells respond to in vitro stimulation with anti CD3 and in vivo sporozoites infection by producing IFNg (see Additional file 1: Figure S1 and Figure S2). This is consistent with CD8+ KLRG1high, CD27low, CD44high, CD62Llow T cells participating to protection by producing IFNg. However, these cells do not account for all the IFNg produced in response to in vitro stimulation with anti CD3 (see Additional file 1: Figure S1 and Figure S2), or in vivo stimulation with sporozoites see Additional file 1: Figure S1 and Figure S2). It is, therefore, conceivable that other CD8+ T cells with low expression of KLRG1 are generated during this infection/cure regimen. Such cells respond to sporozoite infection with IFNg production and display an elevated ability to respond to T cell receptor triggered by releasing IFNg. The role of this CD8+ T cell phenotype in protection remains to be determined. KLRG1 has been shown to down regulate T cell receptor signaling [38, 39], thus suggesting that the acquisition of the KLRG1high phenotype is important for effector memory population after antigen encounter and execution of effector function.
KLRG1high, CD27low, CD44high, CD62Llow are the circulating CD8+ T cells generated during infection which are associated with protection and thus represent a correlate of protection. Passive transfer of purified/enriched KLRG1high, CD27low, CD44high, CD62Llow CD8+ T cells will be critical in demonstrating a direct correlation of this phenotype in protection.
Material has been reviewed by the Walter Reed Army Institute of Research. There is no objection to its presentation and/or publication. The opinions or assertions contained herein are the private views of the author, and are not to be construed as official, or as reflecting true views of the Department of the Army or the Department of Defense. Research was conducted in compliance with the Animal Welfare Act and other federal statutes and regulations relating to animals and experiments involving animals and adheres to principles stated in the Guide for the Care and Use of Laboratory Animals, NRC Publication, 1996 edition.
- WHO: World Malaria Report 2011. 2011, Geneva: World Health Organization,http://www.who.int/malaria/world_malaria_report_2011/en/,Google Scholar
- The malERA Consultative Group on Vaccines: A research agenda for malaria eradication: vaccines. PLoS Med. 2011, 8: e1000398-PubMed CentralView ArticleGoogle Scholar
- Marsh K, Kinyanjui S: Immune effector mechanisms in malaria. Parasite Immunol. 2006, 28: 51-60. 10.1111/j.1365-3024.2006.00808.x.View ArticlePubMedGoogle Scholar
- Achtman AH, Bull PC, Stephens R, Langhorne J: Longevity of the immune response and memory to blood-stage malaria infection. Curr Top Microbiol Immunol. 2005, 297: 71-102.PubMedGoogle Scholar
- Migot F, Chougnet C, Raharimalala L, Astagneau P, Lepers JP, Deloron P: Human immune responses to the Plasmodium falciparum ring-infected erythrocyte surface antigen [Pf155/RESA] after a decrease in malaria transmission in Madagascar. Am J Trop Med Hyg. 1993, 48: 432-439.PubMedGoogle Scholar
- Wipasa J, Okell L, Sakkhachornphop S, Suphavilai C, Chawansuntati K, Liewsaree W, Hafalla JC, Riley EM: Short-lived IFN-gamma effector responses, but long-lived IL-10 memory responses, to malaria in an area of low malaria endemicity. PLoS Pathog. 2011, 7: e1001281-10.1371/journal.ppat.1001281.PubMed CentralView ArticlePubMedGoogle Scholar
- Zevering Y, Khamboonruang C, Rungruengthanakit K, Tungviboonchai L, Ruengpipattanapan J, Bathurst I, Barr P, Good MF: Life-spans of human T-cell responses to determinants from the circumsporozoite proteins of Plasmodium falciparum and Plasmodium vivax. Proc Natl Acad Sci U S A. 1994, 91: 6118-6122. 10.1073/pnas.91.13.6118.PubMed CentralView ArticlePubMedGoogle Scholar
- Bejon P, Mwacharo J, Kai O, Todryk S, Keating S, Lowe B, Lang T, Mwangi TW, Gilbert SC, Peshu N, Marsh K, Hill AV: The induction and persistence of T cell IFN-gamma responses after vaccination or natural exposure is suppressed by Plasmodium falciparum. J Immunol. 2007, 179: 4193-4201.PubMed CentralView ArticlePubMedGoogle Scholar
- Dent AE, Chelimo K, Sumba PO, Spring MD, Crabb BS, Moormann AM, Tisch DJ, Kazura JW: Temporal stability of naturally acquired immunity to merozoite surface protein-1 in Kenyan adults. Malar J. 2009, 8: 162-10.1186/1475-2875-8-162.PubMed CentralView ArticlePubMedGoogle Scholar
- Flanagan KL, Mwangi T, Plebanski M, Odhiambo K, Ross A, Sheu E, Kortok M, Lowe B, Marsh K, Hill AV: Ex-vivo interferon-gamma immune response to thrombospondin-related adhesive protein in coastal Kenyans: longevity and risk of Plasmodium falciparum infection. Am J Trop Med Hyg. 2003, 68: 421-430.PubMedGoogle Scholar
- Hviid L, Theander TG, Jakobsen PH, Bu-Zeid YA, Abdulhadi NH, Saeed BO, Jepsen S, Bayoumi RA, Bendtzen K, Jensen JB: Cell-mediated immune responses to soluble Plasmodium falciparum antigens in residents from an area of unstable malaria transmission in the Sudan. APMIS. 1990, 98: 594-604. 10.1111/j.1699-0463.1990.tb04976.x.View ArticlePubMedGoogle Scholar
- Crompton PD, Pierce SK, Miller LH: Advances and challenges in malaria vaccine development. J Clin Invest. 2010, 120: 4168-4178. 10.1172/JCI44423.PubMed CentralView ArticlePubMedGoogle Scholar
- Guilbride DL, Gawlinski P, Guilbride PD: Why functional pre-erythrocytic and bloodstage malaria vaccines fail: a meta-analysis of fully protective immunizations and novel immunological model. PLoS One. 2010, 5: e10685-10.1371/journal.pone.0010685.PubMed CentralView ArticlePubMedGoogle Scholar
- Nussenzweig RS, Vanderberg J, Most H, Orton C: Protective immunity produced by the injection of x-irradiated sporozoites of plasmodium berghei. Nature. 1967, 216: 160-162. 10.1038/216160a0.View ArticlePubMedGoogle Scholar
- Mueller AK, Labaied M, Kappe SH, Matuschewski K: Genetically modified Plasmodium parasites as a protective experimental malaria vaccine. Nature. 2005, 433: 164-167. 10.1038/nature03188.View ArticlePubMedGoogle Scholar
- Gwadz RW, Cochrane AH, Nussenzweig V, Nussenzweig RS: Preliminary studies on vaccination of rhesus monkeys with irradiated sporozoites of Plasmodium knowlesi and characterization of surface antigens of these parasites. Bull World Health Organ. 1979, 57 (Suppl 1): 165-173.PubMed CentralPubMedGoogle Scholar
- Hoffman SL, Goh LM, Luke TC, Schneider I, Le TP, Doolan DL, Sacci J, de la Vega P, Dowler M, Paul C, Gordon DM, Stoute JA, Church LW, Sedegah M, Heppner DG, Ballou WR, Richie TL: Protection of humans against malaria by immunization with radiation-attenuated Plasmodium falciparum sporozoites. J Infect Dis. 2002, 185: 1155-1164. 10.1086/339409.View ArticlePubMedGoogle Scholar
- Krzych U, Schwenk J: The dissection of CD8 T cells during liver-stage infection. Curr Top Microbiol Immunol. 2005, 297: 1-24.PubMedGoogle Scholar
- Berenzon D, Schwenk RJ, Letellier L, Guebre-Xabier M, Williams J, Krzych U: Protracted protection to Plasmodium berghei malaria is linked to functionally and phenotypically heterogeneous liver memory CD8+ T cells. J Immunol. 2003, 171: 2024-2034.View ArticlePubMedGoogle Scholar
- Weiss WR, Sedegah M, Beaudoin RL, Miller LH, Good MF: CD8+ T cells [cytotoxic/suppressors] are required for protection in mice immunized with malaria sporozoites. Proc Natl Acad Sci U S A. 1988, 85: 573-576. 10.1073/pnas.85.2.573.PubMed CentralView ArticlePubMedGoogle Scholar
- Nganou-Makamdop K, van Gemert GJ, Arens T, Hermsen CC, Sauerwein RW: Long term protection after immunization with P. berghei sporozoites correlates with sustained IFNγ responses of hepatic CD8+ memory T cells. PLoS One. 2012, 7: e36508-10.1371/journal.pone.0036508.PubMed CentralView ArticlePubMedGoogle Scholar
- Guebre-Xabier M, Schwenk R, Krzych U: Memory phenotype CD8[+] T cells persist in livers of mice protected against malaria by immunization with attenuated Plasmodium berghei sporozoites. Eur J Immunol. 1999, 29: 3978-3986. 10.1002/(SICI)1521-4141(199912)29:12<3978::AID-IMMU3978>3.0.CO;2-0.View ArticlePubMedGoogle Scholar
- Jobe O, Lumsden J, Mueller AK, Williams J, Silva-Rivera H, Kappe SH, Schwenk RJ, Matuschewski K, Krzych U: Genetically attenuated Plasmodium berghei liver stages induce sterile protracted protection that is mediated by major histocompatibility complex Class I-dependent interferon-gamma-producing CD8+ T cells. J Infect Dis. 2007, 196: 599-607. 10.1086/519743.PubMed CentralView ArticlePubMedGoogle Scholar
- Schofield L, Villaquiran J, Ferreira A, Schellekens H, Nussenzweig R, Nussenzweig V: Gamma interferon, CD8+ T cells and antibodies required for immunity to malaria sporozoites. Nature. 1987, 330: 664-666. 10.1038/330664a0.View ArticlePubMedGoogle Scholar
- Sayles PC, Wassom DL: Immunoregulation in murine malaria, Susceptibility of inbred mice to infection with Plasmodium yoelii depends on the dynamic interplay of host and parasite genes. J Immunol. 1988, 141: 241-248.PubMedGoogle Scholar
- Chen G, Feng H, Liu J, Qi ZM, Wu Y, Guo SY, Li DM, Wang JC, Cao YM: Characterization of immune responses to single or mixed infections with P. yoelii 17XL and P. chabaudi AS in different strains of mice. Parasitol Int. 2010, 59: 400-406. 10.1016/j.parint.2010.05.005.View ArticlePubMedGoogle Scholar
- Beaudoin RL, Strome CP, Mitchell F, Tubergen TA: Plasmodium berghei: immunization of mice against the ANKA strain using the unaltered sporozoite as an antigen. Exp Parasitol. 1977, 42: 1-5. 10.1016/0014-4894(77)90054-6.View ArticlePubMedGoogle Scholar
- Meuwissen JH, Golenser J, Verhave JP: Development of effective antisporozoite immunity by natural bites of Plasmodium-berghei-infected mosquitoes in rats under prophylactic treatment with various drug regimens. Isr J Med Sci. 1978, 14: 601-605.PubMedGoogle Scholar
- Roestenberg M, McCall M, Hopman J, Wiersma J, Luty AJ, van Gemert GJ, van de Vegte-Bolmer M, van Schaijk B, Teelen K, Arens T, Spaarman L, de Mast Q, Roeffen W, Snounou G, Rénia L, van der Ven A, Hermsen CC, Sauerwein R: Protection against a malaria challenge by sporozoite inoculation. N Engl J Med. 2009, 361: 468-477. 10.1056/NEJMoa0805832.View ArticlePubMedGoogle Scholar
- Roestenberg M, Teirlinck AC, McCall MB, Teelen K, Makamdop KN, Wiersma J, Arens T, Beckers P, van Gemert G, van de Vegte-Bolmer M, van der Ven AJ, Luty AJ, Hermsen CC, Sauerwein RW: Long-term protection against malaria after experimental sporozoite inoculation: an open-label follow-up study. Lancet. 2011, 377: 1770-1776. 10.1016/S0140-6736(11)60360-7.View ArticlePubMedGoogle Scholar
- Belnoue E, Costa FT, Frankenberg T, Vigario AM, Voza T, Leroy N, Rodrigues MM, Landau I, Snounou G, Rénia L: Protective T cell immunity against malaria liver stage after vaccination with live sporozoites under chloroquine treatment. J Immunol. 2004, 172: 2487-2495.View ArticlePubMedGoogle Scholar
- Belnoue E, Voza T, Costa FTM, Gruner AC, Mauduit M, Rosa DS, Depinay N, Kayibanda M, Vigário AM, Mazier D, Snounou G, Sinnis P, Rénia L: Vaccination with live Plasmodium yoelii blood stage parasites under chloroquine cover induces cross-stage immunity against malaria liver stage. J Immunol. 2008, 181: 8552-8558.View ArticlePubMedGoogle Scholar
- Cummings JF, Spring MD, Schwenk RJ, Ockenhouse CF, Kester KE, Polhemus ME, Walsh DS, Yoon IK, Prosperi C, Juompan LY, Lanar DE, Krzych U, Hall BT, Ware LA, Stewart VA, Williams J, Dowler M, Nielsen RK, Hillier CJ, Giersing BK, Dubovsky F, Malkin E, Tucker K, Dubois MC, Cohen JD, Ballou WR, Heppner DG: Recombinant Liver Stage Antigen-1 [LSA-1] formulated with AS01 or AS02 is safe, elicits high titer antibody and induces IFN-gamma/IL-2 CD4+ T cells but does not protect against experimental Plasmodium falciparum infection. Vaccine. 2010, 28: 5135-5144. 10.1016/j.vaccine.2009.08.046.View ArticlePubMedGoogle Scholar
- Polhemus ME, Magill AJ, Cummings JF, Kester KE, Ockenhouse CF, Lanar DE, Dutta S, Barbosa A, Soisson L, Diggs CL, Robinson SA, Haynes JD, Stewart VA, Ware LA, Brando C, Krzych U, Bowden RA, Cohen JD, Dubois MC, Ofori-Anyinam O, De-Kock E, Ballou WR, Heppner DG: Phase I dose escalation safety and immunogenicity trial of Plasmodium falciparum apical membrane protein [AMA-1] FMP2.1, adjuvanted with AS02A, in malaria-naive adults at the Walter Reed Army Institute of Research. Vaccine. 2007, 25: 4203-4212. 10.1016/j.vaccine.2007.03.012.View ArticlePubMedGoogle Scholar
- Sheehy SH, Duncan CJ, Elias SC, Biswas S, Collins KA, O'Hara GA, Halstead FD, Ewer KJ, Mahungu T, Spencer AJ, Miura K, Poulton ID, Dicks MD, Edwards NJ, Berrie E, Moyle S, Colloca S, Cortese R, Gantlett K, Long CA, Lawrie AM, Gilbert SC, Doherty T, Nicosia A, Hill AV, Draper SJ: Phase Ia clinical evaluation of the safety and immunogenicity of the Plasmodium falciparum blood-stage antigen AMA1 in ChAd63 and MVA vaccine vectors. PLoS ONE. 2012, 7: e31208-10.1371/journal.pone.0031208.PubMed CentralView ArticlePubMedGoogle Scholar
- Forbes EK, Biswas S, Collins KA, Gilbert SC, Hill AV, Draper SJ: Combining liver- and blood-stage malaria viral-vectored vaccines: investigating mechanisms of CD8+ T cell interference. J Immunol. 2011, 187: 3738-3750. 10.4049/jimmunol.1003783.PubMed CentralView ArticlePubMedGoogle Scholar
- Epstein JE, Tewari K, Lyke KE, Sim BK, Billingsley PF, Laurens MB, Gunasekera A, Chakravarty S, James ER, Sedegah M, Richman A, Velmurugan S, Reyes S, Li M, Tucker K, Ahumada A, Ruben AJ, Li T, Stafford R, Eappen AG, Tamminga C, Bennett JW, Ockenhouse CF, Murphy JR, Komisar J, Thomas N, Loyevsky M, Birkett A, Plowe CV, Loucq C, Edelman R, Richie TL, Seder RA, Hoffman SL: Live attenuated malaria vaccine designed to protect through hepatic CD8+ T cell immunity. Science. 2011, 334: 475-480. 10.1126/science.1211548.View ArticlePubMedGoogle Scholar
- Wilson DC, Matthews S, Yap GS: IL-12 signaling drives CD8+ T cell IFN-gamma production and differentiation of KLRG1+ effector subpopulations during Toxoplasma gondii Infection. J Immunol. 2008, 180: 5935-5945.View ArticlePubMedGoogle Scholar
- Beyersdorf NB, Ding X, Karp K, Hanke T: Expression of inhibitory ‘killer cell lectin-like receptor G1’ identifies unique subpopulations of effector and memory CD8 T cells. Eur J Immunol. 2001, 31: 3443-3452. 10.1002/1521-4141(200112)31:12<3443::AID-IMMU3443>3.0.CO;2-J.View ArticlePubMedGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.