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Successful Induction of CD8 T Cell-Dependent Protection Against Malaria by Sequential Immunization with DNA and Recombinant Poxvirus of Neonatal Mice Born to Immune Mothers ¹

Martha Sedegah^*, Maria Belmonte^*, Judith E. Epstein^*, Claire-Anne Siegrist, Walter R. Weiss^*, Trevor R. Jones^*, Minh Lu^*, Daniel J. Carucci^* and Stephen L. Hoffman^2,*

^* Malaria Program, Naval Medical Research Center, Silver Spring, MD 20910; and World Health Organization Collaborating Centre for Neonatal Vaccinology, Department of Pathology and Pediatrics, University of Geneva, Geneva, Switzerland

	Abstract

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In some parts of Africa, 50% of deaths attributed to malaria occur in infants less than 8 mo. Thus, immunization against malaria may have to begin in the neonatal period, when neonates have maternally acquired Abs against malaria parasite proteins. Many malaria vaccines in development rely upon CD8 cells as immune effectors. Some studies indicate that neonates do not mount optimal CD8 cell responses. We report that BALB/c mice first immunized as neonates (7 days) with a Plasmodium yoelii circumsporozoite protein (PyCSP) DNA vaccine mixed with a plasmid expressing murine GM-CSF (DG) and boosted at 28 days with poxvirus expressing PyCSP were protected (93%) as well as mice immunized entirely as adults (70%). Protection was dependent on CD8 cells, and mice had excellent anti-PyCSP IFN- and cytotoxic T lymphocyte responses. Mice born of mothers previously exposed to P. yoelii parasites or immunized with the vaccine were protected and had excellent T cell responses. These data support assessment of this immunization strategy in neonates/young infants in areas in which malaria exacts its greatest toll.

	Introduction

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Plasmodium falciparum is the most virulent species of the four malaria parasite species that infect humans, being responsible for 500 million new infections and 1–3 million deaths annually (1). In many parts of the world, more than half the mortality of malaria occurs in infants. Thus, to be optimally effective, a malaria vaccine will have to be effective in infants, and immunization may have to be initiated in the neonatal period.

P. falciparum DNA vaccines have been shown to be immunogenic in humans (2, 3), and strategies in which DNA vaccines are boosted with recombinant poxvirus expressing the same proteins have been shown to be protective in mice (4, 5, 6) and nonhuman primates (7). Vaccines based on this strategy have now entered human clinical trials (8).

Most infants who will be immunized in malaria-endemic areas will be born to mothers with previous malaria experience and immunity. The efficacy of malaria vaccines in neonates and infants in malaria-endemic areas could be adversely affected by maternally acquired Abs against the vaccine Ags (9, 10, 11, 12) and the immaturity of infants’ immune systems (13).

Mice first immunized as neonates have been useful in studying the effect of maternal Abs and immune immaturity on vaccine responses to human diseases such as tetanus, respiratory syncytial virus, and measles (10). We therefore used the Plasmodium yoelii murine malaria model to study the efficacy of immunizing (priming) 7-day-old neonatal mice with a DNA plasmid expressing the P. yoelii circumsporozoite protein (PyCSP) ³ and boosting them at the age of 28 days with a recombinant poxvirus expressing the PyCSP. This specific regimen was chosen for study in neonatal mice, because our extensive previous studies (4, 6, 14) have demonstrated it to be much more effective than any other regimen we have assessed in eliciting the CD8 T cell responses, we and others consider it to be critical for protective immunity against the liver stages of Plasmodium sp. parasites (15, 16). Also this has been the most protective regimen we have assessed in the P. yoelii system (4, 5, 6, 14). Our studies demonstrated that priming of 7-day-old pups from either nonimmune or immune mothers with the PyCSP prime-boost regimen and boosting them with recombinant poxvirus at 28 days of age resulted in excellent CD8 T cell responses and protection.

	Materials and Methods

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Mice

Eight-week-old BALB/cByJ mice purchased from The Jackson Laboratory (Bar Harbor, ME) were bred to produce pups. Breeding cages were checked daily and new births were recorded.

Parasites and infections

P. yoelii 17XNL nonlethal strain, clone 1.1 sporozoites were obtained by dissection of Anopheles stephensi salivary, as described (17). For challenge of immunized mice, 50 sporozoites were injected into the tail vein 2 wk postimmunization. Giemsa-stained blood films were prepared and evaluated on days 5–14 postchallenge. Up to 50 oil-immersion fields (x1000) were examined before calling a slide negative. Protection was defined as the complete absence of blood-stage parasitemia at all time points. Before mating, some adult females were infected by injecting 10,000 sporozoites. All had positive blood films on day 7, and negative blood films on day 21.

Immunogens and immunizations

The plasmid DNA constructs encoding the P. yoelii circumsporozoite protein (pPyCSP), designated as D; the murine GM-CSF plasmid, designated as G; the recombinant attenuated vaccinia virus (COPAK) construct encoding the PyCSP designated in this work as V; control empty plasmid; and control virus have been described (4, 6, 18). The doses of D (100 µg), G (30 µg), and V (2 x 10⁷ PFUs) were always the same. In all cases, immunizations were administered at 3-wk intervals. D and DG (a mixture of D and G) were injected i.m. in two 25-µl injections, one in each tibialis anterior muscle using a 0.3-ml insulin syringe and 29–1/2-gauge needle. V was delivered i.p. in 0.1 ml PBS using a 1-ml insulin syringe with a 26–1/2-gauge needle. Mice either received DG as the first dose and V as the second dose (DGV), or D as the first and second doses (DD).

Groups

Before mating, adult female mice (mothers) had either been infected with P. yoelii parasites and been shown to have cleared the parasites without intervention (mother:recovered); primed with DG and boosted with V 3 wk later (mother:DGV); primed with D and boosted with D 3 wk later (mother:DD); or had no intervention (mother:naive). These mothers were mated with naive males to produce pups between 3 and 9 wk after either documented recovery from infection or last immunization. The gestation period was 3 wk. Neonates in this study refer to 7-day-old pups (19). Seven- to eleven-week-old adult naive BALB/c mice were also immunized and included in some studies.

Ab assays

P. yoelii sporozoite and erythrocytic stage-specific Abs in sera were assayed by immunofluorescent staining of air-dried P. yoelii sporozoites or infected erythrocytes (4).

ELISPOT assay for detecting IFN--secreting CD8 cells

P. yoelii CTL MHC-restricted epitope-specific IFN--producing CD8 cells were determined, as described (4). Briefly, freshly isolated spleen cells from mice that had received their second immunization 14 days earlier were used. Measurements were made after 36 h of incubation of spleen cells with irradiated P815 cells pulsed with 1 µM of the P. yoelii CTL synthetic peptide SYVPSAEQI. Spleen cells were also incubated with unpulsed irradiated P815 target cells to serve as controls. The number of Ag-specific spots (number of spots with peptide-pulsed targets minus the number of spots with unpulsed targets) corresponding to IFN--producing cells in wells was enumerated using the Zeiss (Oberkochen, Germany) ELISPOT reader. The results were expressed as the number of IFN--secreting cells per 10⁶ spleen cells.

Ex vivo CTL assays

Ex vivo CTL assays (no period of in vitro incubation) were conducted, as previously described (4), using E:T ratios of 150:1, 75:1, and 38:1.

In vivo depletion of CD8 cells

Neonates were primed with DG and boosted with V 3 wk later, and depleted of CD8 cells in vivo before challenge with infective sporozoites 2 wk after boost. Mice received single daily i.p. injections of 0.5 mg of mAb 2.43 for 3 consecutive days (6) and then were challenged with sporozoites. Control mice received control rat Ig. PBMCs were prepared from three mice in each group, and microfluorometric analysis (FACS) was done to confirm the effectiveness of the in vivo depletions 3 days after the last Ab injection.

Statistical analysis

For protective efficacy, proportions of infected vs noninfected animals in control and experimental groups were compared using the ² or Fisher exact test (two tailed) as appropriate. To compare relative immunogenicity of the vaccine in the different pups studied, mean values were compared using independent samples t test. SPSS for windows, version 8.0, was used for all statistical analysis.

	Results

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Kinetics of maternal Abs transferred to pups

To evaluate the effect of different levels of maternal Ab on immunization, we followed the kinetics of maternally transferred Abs in neonates. Sera were prepared from mice from two litters for weeks 1 and 2 bleeds. Weeks 3, 5, 8, and 10 bleeds came from mice in a single litter. An example of the kinetics of anti-sporozoite and anti-erythrocytic stage Abs in neonates born to mothers previously infected with P. yoelii (mother:recovered) is shown in Table I. Ab titers were highest in sera from 2-wk-old mice. Table II outlines the anti-sporozoite Ab levels in pups born from mothers previously immunized by priming with a mixture of pPyCSP (D) plus GM-CSF plasmid (G) (day 0) and boosting with recombinant attenuated vaccinia virus expressing PyCSP (V) 3 wks later (mother:DGV). Titers were highest at 2 wk after birth.

Anti-parasite Ab response. Table III shows representative data on the Ab titers to sporozoites by the indirect fluorescent Ab test in mice 2 wk after boost in the different groups of neonates studied simultaneously. Geometric mean Ab titers after immunization were significantly higher (mean difference significant at the p < 0.05 level) in the immunized pups born to naive mothers (mother:naive) (27,024), compared with the levels measured in the pups born to mother:recovered (6,089), mother:DGV (1,560), or mothers who were primed with pPyCSP (D) (day 0) and boosted with pPyCSP (D) 3 wk later (mother:DD) (7,241). The geometric mean Ab titers of pups from mother:DGV were significantly lower than those in pups from mother:recovered and mother:DD (p < 0.05). The geometric mean Ab titers of pups from mother:recovered and mother:DD mothers did not differ significantly (Table III).

View larger version (53K): [in this window] [in a new window]   FIGURE 1. CD8 T cell responses in DGV-immunized mice. A, IFN--producing cells in ELISPOT. Mice born of mothers never exposed to P. yoelii or immunized with a P. yoelii vaccine (mother:naive), or from mothers previously exposed (mother:recovered) or immunized (mother:DGV or mother:DD) (see Materials and Methods) were immunized on day 7 with D or DG and boosted with V on day 28. Immunized adult mice (Adults) were the positive controls, and unimmunized mice were the negative controls (Control). Spleen cells from individual mice, five to eight immunized mice per group, were obtained 2 wk after the recombinant poxvirus boost (V). The number of Ag-specific IFN--producing cells was determined and expressed as the number of spot-forming cells per million spleen cells. (The error bars indicate the SD of the mean counts.) B, Ex vivo CTL assay. Spleen cells from same mice used for the ELISPOT assay were used directly as effectors in a chromium release ex vivo CTL assay (see Materials and Methods). Data are presented as net percentage of specific lysis at 150:1 E:T ratio. For both the IFN- ELISPOT and ex vivo CTL assays, the mean spot-forming cells per million and the mean net specific lysis for each of the six groups were compared in a one-way ANOVA (with Scheffe post hoc test). The only significant differences seen were between the control group and each of the other five immunized groups, p values ranging between 0.003 and <0.0009.

We next assessed whether maternally transferred Abs provided protection. To achieve this, pups born to mother:recovered were challenged with 50 infectious P. yoelii sporozoites i.v. 2 wk after they were born, at a time when they were still being nursed, and had the highest level of maternally acquired Abs. In one litter, 6 of 6 pups born to mother:recovered were completely protected against sporozoite challenge (Table IV). However, upon rechallenge at 26 days after first challenge, i.e., 40 days after birth, all but 1 mouse became infected, although 4 of 5 had a slight prolongation of prepatent period. In the two challenges, 10 of 10 age-matched control mice, born to naive mothers, became infected with blood-stage parasites 5 days after sporozoite injection. The experiment was repeated, and in a second litter, only 1 of 7 pups born to mother:recovered was completely protected against the sporozoite challenge (Table IV), and the remaining 6 had delayed patency. No mice from three different litters from mother:DGV were protected against challenge with sporozoites 2 or 3 wk after birth (data not shown).

Neonates from different litters from mother:naive and adult mice were immunized at the same time. Experiments were repeated using different litters, and representative data showing the results of protection after challenge are outlined in Table V, experiment A. Protection of neonates from mother:naive and adults by DGV immunization regimen was similar (neonates, 13 of 14 mice protected (93%), and adults, 7 of 10 mice protected (70%); comparison of neonates vs adults p = 0.272, Fisher’s exact test, two-tailed). CD8 depletion eliminated protection in 10 of 11 neonatal mice that received the DGV (Table V, experiment B). Immunization of mother:naive neonates with two doses of pPyCSP (DD) gave no protection (data not shown). Two doses of pPyCSP (DD) given at a 3-wk interval give little to no protection of adult BALB/c mice (6).

	Discussion

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For the first 2–4 mo after birth, infants born of immune mothers in P. falciparum-endemic areas rarely develop severe or fatal malaria. A major protective factor is thought to be maternally acquired Abs that provide enough protection to limit morbidity and mortality. However, in areas with extremely intense transmission of P. falciparum, such as northern Ghana (20) and western Kenya (21), a significant proportion of serious disease and P. falciparum-associated mortality occurs between 4 and 8 mo of age. This means that to be optimally effective in preventing serious morbidity and mortality, a P. falciparum vaccine will have to be first administered to neonates or young infants, and the immunization series will have to be completed by 4–6 mo of age. For logistic reasons, this means that the vaccine will have to be administered as part of the expanded program for immunization.

There are two major concerns regarding administration of a vaccine to individuals this young. The first is that the same maternally acquired Abs that protect against infection will interact with the vaccine proteins and render them nonimmunogenic, thereby limiting the immunogenicity and protective efficacy of the vaccine (9, 10, 11, 12). The second is that particularly for a vaccine designed to induce protective T cell responses, neonates and infants will not be able to mount such protective T cell responses after immunization because of immaturity of their immune systems (13).

To begin to model the situation in the field, we conducted studies in three groups of mice: 1) neonatal mice born of mothers that were naive to Plasmodium sp. infection or vaccines; 2) neonates born of mothers that had previously been infected and recovered from a rodent malaria parasite, P. yoelii; or 3) neonates born of mothers immunized with the same P. yoelii vaccine that their offspring later received.

The studies reported in this work demonstrated that BALB/c mice first immunized at 7 days of age (neonatal) with a PyCSP DNA vaccine and boosted 3 wk later with a recombinant poxvirus expressing the PyCSP mounted protective immune responses comparable to mice first immunized with DNA vaccines at 6 wk of age (adult) and boosted 3 wk later. Furthermore, mice born of mothers previously exposed to P. yoelii or immunized with the PyCSP DNA prime-poxvirus boost approach were protected by the prime-boost immunization regimen as well as mice born of malaria and vaccination naive mothers.

Protection of adult mice after immunization with the PyCSP DNA prime-poxvirus boost vaccine is dependent on CD8 cells and IFN- (6). As compared with adult mice, the mice first immunized as neonates had similar levels of protective immunity (Tables V, experiment A), IFN activity (Fig. 1a), and ex vivo CTL activity (Fig. 1b). Furthermore, protection against sporozoite challenge was dependent on CD8 T cells (Table V, experiment B).

Interestingly, Ab responses in the mice born of immune mothers were significantly lower than Ab responses in mice born of naive mothers (Table III). Furthermore, Ab responses in mice born of naive mothers and first immunized at 7 days of age were significantly lower than Ab responses found in adult mice. In the experiment depicted in Table V, experiment A, the geometric mean titer in immunized neonates, 24,963 was at least 4 times lower than it was in immunized adults, 108,044 (p = 0.013). These data suggest that maternally acquired Abs led to Ag neutralization and reduction of Ab responses, but not T cell responses. They also suggest that the capacity of mice first immunized as neonates to produce Ab responses, but not T cell responses, is reduced as compared with adult mice. We have no data to explain why the mice born of immune mothers made good T cell responses, but not Ab responses, as compared with the mice born of naive mothers. One explanation is that the Ab responses require intact Ag, whereas the T cell responses, which are elicited by processed peptides bound to MHC molecules, were unaffected by Ag/Ab interactions.

In malaria-endemic areas, it is widely believed that infants acquire immunity passively from their mothers and are protected from malaria during the early months of life. We showed that maternally acquired Ab apparently protected pups challenged at 2 wk after birth, but the protection was short-lived. There was no detectable protection at 40 days after birth. In the Plasmodium berghei white rat model system, mothers transferred immunity to their newborns (22). In an area of intense and perennial malaria transmission, infections in older infants lasted longer than those contracted during the first 2 mo of life (23). Other studies have reported no correlation between the age of onset of malaria and the level of cord serum total IgG, IgM, and Abs to P. falciparum Ags (24).

We have simulated in mice some of the factors that will have to be addressed in administering a malaria vaccine to infants in a malaria-endemic area. Priming with a DNA plasmid expressing the PyCSP in the neonatal period (7 days of age), and boosting with a recombinant vaccine virus expressing the PyCSP 3 wk later elicited excellent protective immunity, regardless of whether the newborn had passively acquired immunity from the mother. These findings are encouraging and support the execution of similar studies in human infants.

	Acknowledgments

 
We thank HM1 Robert Arcilla, HM2 Thomas Smalls, and Steve Matheny for excellent technical assistance and for providing P. yoelii sporozoites.

	Footnotes

 
¹ This work was supported by the Naval Medical Research Center Malaria Vaccine Research Work Unit 6000 RAD1 F A0309. The assertions in this work are the private ones of the authors and are not to be construed as official or as reflecting the views of the U.S. Navy or the Naval service at large. The experiments reported in this study were conducted according to the principles set forth in the "Guide for the Care and Use of Laboratory Animals," Institute of Laboratory Animal Research, National Research Council, National Academy Press (1996).

² Address correspondence and reprint requests to Dr. Stephen L. Hoffman, Sanaria, 308 Argosy Drive, Gaithersburg, MD 20878. E-mail address: slhoffman{at}sanaria.com

³ Abbreviation used in this paper: PyCSP, P. yoelii circumsporozoite protein.

Received for publication April 11, 2003. Accepted for publication July 8, 2003.

	References

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1. Breman, J. G.. 2001. The ears of the hippopotamus: manifestations, determinants, and estimates of the malaria burden. Am. J. Trop. Med. Hyg. 64:1.
2. Wang, R., D. L. Doolan, T. P. Le, R. C. Hedstrom, K. M. Coonan, Y. Charoenvit, T. R. Jones, P. Hobart, M. Margalith, J. Ng, et al 1998. Induction of antigen-specific cytotoxic T lymphocytes in humans by a malaria DNA vaccine. Science 282:476.[Abstract/Free Full Text]
3. Wang, R., J. Epstein, F. M. Baraceros, E. J. Gorak, Y. Charoenvit, D. J. Carucci, R. C. Hedstrom, N. Rahardjo, T. Gay, P. Hobart, et al 2001. Induction of CD4⁺ T cell-dependent CD8⁺ type 1 responses in humans by a malaria DNA vaccine. Proc. Natl. Acad. Sci. USA 98:10817.[Abstract/Free Full Text]
4. Sedegah, M., T. R. Jones, M. Kaur, R. C. Hedstrom, P. Hobart, J. A. Tine, S. L. Hoffman. 1998. Boosting with recombinant vaccinia increases immunogenicity and protective efficacy of malaria DNA vaccine. Proc. Natl. Acad. Sci. USA 95:7648.[Abstract/Free Full Text]
5. Schneider, J., S. C. Gilbert, T. J. Blanchard, T. Hanke, K. J. Robson, C. M. Hannan, M. Becker, R. Sinden, G. L. Smith, A. V. Hill. 1998. Enhanced immunogenicity for CD8⁺ T cell induction and complete protective efficacy of malaria DNA vaccination by boosting with modified vaccinia virus Ankara. Nat. Med. 4:397.[Medline]
6. Sedegah, M., W. Weiss, J. B. Sacci, Jr., Y. Charoenvit, R. Hedstrom, K. Gowda, V. F. Majam, J. Tine, S. Kumar, P. Hobart, S. L. Hoffman. 2000. Improving protective immunity induced by DNA-based immunization: priming with antigen and GM-CSF encoding plasmid DNA and boosting with antigen expressing recombinant poxvirus. J. Immunol. 164:5905.[Abstract/Free Full Text]
7. Rogers, W. O., W. R. Weiss, A. Kumar, J. C. Aguiar, J. A. Tine, R. Gwadz, J. G. Harre, K. Gowda, D. Rathore, S. Kumar, S. L. Hoffman. 2002. Protection of rhesus macaques against lethal Plasmodium knowlesi malaria by a heterologous DNA priming and poxvirus boosting immunization regimen. Infect. Immun. 70:4329.[Abstract/Free Full Text]
8. Moorthy, V. S., S. McConkey, M. Roberts, P. Gothard, N. Arulanantham, P. Degano, J. Schneider, C. Hannan, M. Roy, S. C. Gilbert, et alSafety of DNA and modifiedvaccinia virus Ankara vaccines against liver-stage P. falciparum malaria in non-immune volunteers. Vaccine 21:2004.
9. Siegrist, C. A., C. Barrios, X. Martinez, C. Brandt, M. Berney, M. Cordova, J. Kovarik, P. H. Lambert. 1998. Influence of maternal antibodies on vaccine responses: inhibition of antibody but not T cell responses allows successful early prime-boost strategies in mice. Eur. J. Immunol. 28:4138.[Medline]
10. Siegrist, C. A.. 2000. Vaccination in the neonatal period and early infancy. Int. Rev. Immunol. 19:195.[Medline]
11. Barrios, C., P. Brawand, M. Berney, C. Brandt, P. H. Lambert, C. A. Siegrist. 1996. Neonatal and early life immune responses to various forms of vaccine antigens qualitatively differ from adult responses: predominance of a Th2-biased pattern which persists after adult boosting. Eur. J. Immunol. 26:1489.[Medline]
12. Harte, P. G., J. H. Playfair. 1983. Failure of malaria vaccination in mice born to immune mothers. II. Induction of specific suppressor cells by maternal IgG. Clin. Exp. Immunol. 51:157.[Medline]
13. Gans, H. A., A. M. Arvin, J. Galinus, L. Logan, R. DeHovitz, Y. Maldonado. 1998. Deficiency of the humoral immune response to measles vaccine in infants immunized at age 6 months. J. Am. Med. Assoc. 280:527.[Abstract/Free Full Text]
14. Sedegah, M., G. T. Brice, W. O. Rogers, D. L. Doolan, Y. Charoenvit, T. R. Jones, V. F. Majam, A. Belmonte, M. Lu, M. Belmonte, et al 2002. Persistence of protective immunity to malaria induced by DNA priming and poxvirus boosting: characterization of effector and memory CD8⁺-T-cell populations. Infect. Immun. 70:3493.[Abstract/Free Full Text]
15. Hoffman, S. L., D. Isenbarger, G. W. Long, M. Sedegah, A. Szarfman, L. Waters, M. R. Hollingdale, P. H. van der Miede, D. S. Finbloom, W. R. Ballou. 1989. Sporozoite vaccine induces genetically restricted T cell elimination of malaria from hepatocytes. Science 244:1078.[Abstract/Free Full Text]
16. Doolan, D. L., S. L. Hoffman. 2000. The complexity of protective immunity against liver-stage malaria. J. Immunol. 165:1453.[Abstract/Free Full Text]
17. Belmonte, M., T. R. Jones, M. Lu, R. Arcilla, T. Smalls, A. Belmonte, J. Rosenbloom, D. J. Carucci, M. Sedegah. 2003. The infectivity of Plasmodium yoelii in different strains of mice. J. Parasitol. 89:602.[Medline]
18. Weiss, W. R., K. J. Ishii, R. C. Hedstrom, M. Sedegah, M. Ichino, K. Barnhart, D. M. Klinman, S. L. Hoffman. 1998. A plasmid encoding murine granulocyte-macrophage colony-stimulating factor increases protection conferred by a malaria DNA vaccine. J. Immunol. 161:2325.[Abstract/Free Full Text]
19. Dadaglio, G., C. M. Sun, R. Lo-Man, C. A. Siegrist, C. Leclerc. 2002. Efficient in vivo priming of specific cytotoxic T cell responses by neonatal dendritic cells. J. Immunol. 168:2219.[Abstract/Free Full Text]
20. Baird, J. K., O. Seth Agyei, G. C. Utz, K. Koram, M. J. Barcus, F. N. Binka, T. R. Jones, S. L. Hoffman, F. N. Nkrumah. 2000. Seasonal malaria attack rates in infants and small children in northern Ghana. Am. J. Trop. Med. Hyg. 66:280.
21. Hoffman, S. L., C. N. Oster, C. V. Plowe, G. R. Woollett, J. C. Beier, J. D. Chulay, R. A. Wirtz, M. R. Hollingdale, M. Mugambi. 1987. Naturally acquired antibodies to sporozoites do not prevent malaria: vaccine development implications. Science 237:639.[Abstract/Free Full Text]
22. Desowitz, R. S.. 1999. Plasmodium berghei in the white rat: severe malaria of pregnancy does not occur in the progeny of mothers infected during gestation. Ann. Trop. Med. Parasitol. 93:415.[Medline]
23. Kitua, A. Y., T. Smith, P. L. Alonso, H. Masanja, H. Urassa, C. Menendez, J. Kimario, M. Tanner. 1996. Plasmodium falciparum malaria in the first year of life in an area of intense and perennial transmission. Trop. Med. Int. Health 1:475.[Medline]
24. Achidi, E. A., L. S. Salimonu, H. Perlmann, P. Perlmann, K. Berzins, A. I. Williams. 1996. Lack of association between levels of transplacentally acquired Plasmodium falciparum-specific antibodies and age of onset of clinical malaria in infants in a malaria endemic area of Nigeria. Acta Trop. 61:315.[Medline]

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