Journal of Infectious Diseases Advance Access published April 8, 2014 1
Dynamics of the antibody response to Plasmodium falciparum infection in African children
Michael T White1,*, Jamie T Griffin1, Onome Akpogheneta2,3, David J Conway2,3, Kwadwo A Koram4, Eleanor M Riley5,†, Azra C Ghani1,† MRC Centre for Outbreak Analysis & Modelling, Department of Infectious Disease Epidemiology, Imperial College
London Medical Research Council Laboratories, Fajara, The Gambia
3
Department of Pathogen Molecular Biology, London School of Hygiene & Tropical Medicine, Keppel Street, London
4
Noguchi Memorial Institute for Medical Research, University of Ghana, Legon, Ghana
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Department of Immunology and Infection, London School of Hygiene & Tropical Medicine, Keppel Street, London
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†
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These authors contributed equally to this study.
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Corresponding author: Dr Michael T. White, MRC Centre for Outbreak Analysis and Modelling, Department of Infectious Disease Epidemiology, Imperial College London, London, W2 1PG, UK, e‐mail: [email protected], phone: 0044 20 7594 3946
© The Author 2014. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved. For Permissions, please e‐mail: [email protected].
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Abstract Background
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Acquired immune responses to malaria have widely been perceived to be short‐lived, with previously immune individuals suffering a loss of immunity when they move from malaria‐endemic areas. However long‐lived P. falciparum specific antibody responses lasting for an individual’s lifetime are frequently observed. Methods and Findings
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cohort studies of African children to estimate the half‐lives of circulating Immunoglobulin G (IgG) antibodies and IgG
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antibody secreting cells (ASC). Antibody responses in African children can be described by a model including both short‐lived ASCs (half‐life in the range 2‐10 days) responsible for boosting antibody titres following infection, and long‐lived ASCs (half‐life in the range 3‐9 years) responsible for maintaining sustained humoral responses.
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Comparison of antibody responses in the younger Ghanaian cohort and the older Gambian cohort suggests that young children are less able to generate the long‐lived ASCs necessary to maintain the circulating antibodies that
Conclusions
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may provide protection against reinfection.
The rapid decay of antibodies following exposure to malaria and the maintenance of sustained antibody responses
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can be explained in terms of populations of short‐lived and long‐lived ASCs.
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We fit mathematical models of the dynamics of antibody titres to Plasmodium falciparum antigens from longitudinal
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Introduction In malaria‐endemic areas, young children bear the major burden of disease whereas older children and adults acquire substantial protection from severe malaria and death following exposure, but rarely, if ever, acquire sterile
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immunity[1]. It is widely perceived that acquired immune responses to malaria are short‐lived[1, 2], and that previously immune individuals suffer a loss of immunity when they move away from malaria‐endemic areas.
However, the true picture is more complex[3, 4]. Infants and young children are particularly vulnerable to malaria after physiological adaptations and maternally‐acquired antibodies have waned and before the development of
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acquisition of immunity to malaria requires multiple infections. Although substantial protection from severe malaria
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and death is acquired after a small number of infections[6], episodes of febrile disease may continue for many years. While frequent reinfection is required to maintain high concentrations of anti‐malarial antibodies, and antibody responses can appear transient, especially in young children[2, 7‐9], there is evidence that other components of the
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immune response to malaria are long‐lived[3]. Circulating memory B cells (MBC) specific for P. falciparum antigens can be detected at least eight years after the most recent infection[10, 11] and may persist for the lifetime of the individual[12]. Furthermore, estimates of the half‐life of antibody seropositivity indicate that individuals may remain
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seropositive for life for antibodies to conserved or relatively conserved antigens, even in areas of low ongoing malaria transmission[13].
The cellular and molecular determinants of the duration of anti‐malarial antibody responses have been investigated in mouse models[14, 15] but remain poorly studied in humans. MBCs specific to Plasmodium antigens are detectable
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among human peripheral blood mononuclear cells (PBMC) and studies have reported their expansion and contraction[16], and their longevity[10, 11, 17]. However, there are no comparable studies of longevity of ASCs in
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humans exposed to malaria. Such studies would be difficult to undertake, as ASCs are located primarily in bone marrow and lymphoid organs and are detectable in blood only in the short window between differentiation and
migration to the bone marrow[15]. However, mouse models reveal a strong correlation between numbers of ASCs in tissues and serum antibody concentrations, suggesting that antibody titres are a good surrogate for ASC numbers[14].
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effective adaptive immunity[5]. In contrast to other childhood infections, such as measles, rubella and varicella,
4 To obtain quantitative estimates of the immune parameters determining the antibody response to malaria, we analysed data from two longitudinal cohort studies of antibody responses to P. falciparum infection in African children. In the first study, 151 Ghanaian children were followed from birth for over 2 years to investigate the
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association between maternal antibody and protection from malaria[18]. Serum samples were analysed by enzyme‐ linked immunosorbent assays (ELISA) for the presence of antibodies to the antigens apical membrane antigen 1
(AMA‐1), merozoite surface protein 1 (MSP‐1), merozoite surface protein 2 (MSP‐2), and circumsporozoite protein
(CSP). Infants in malaria‐endemic areas are born with maternally‐acquired IgG antibodies but no ASCs of their own,
IgG titres upon exposure to P. falciparum antigens, after which antibody titres decay again. In the second study, 124 Gambian children were followed for 3 months during the dry season (when there was little or no ongoing malaria
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transmission) to investigate antibody dynamics following exposure to P. falciparum during the previous wet season[8, 9]. Serum samples were analysed by ELISA for antibodies to the antigens AMA‐1, MSP‐1, MSP‐2, and
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erythrocyte binding antigen 175 (EBA‐175). The Gambian children ranged in age from 1 to 6 years, too old for maternal antibodies to be present in appreciable quantities[5]. Figure 1 shows the population‐level pattern of boosting and decay of antibodies in both cohorts of children, as well as sample antibody trajectories from individual
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children. Longitudinal measurements of parasitaemia were available for most children. However, there was poor agreement between detection of parasites and boosting of antibody titres, with P. falciparum parasites frequently detected without a boost in titres, titres boosted without detectable infection, and titres to one antigen boosted without an
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accompanying boost to titres of other antigens. This could be due to poor sensitivity or specificity of the parasite detection methods, polymorphism of parasite antigens, persistent antigen presentation by follicular dendritic cells[19], bystander activation[20] or clonal imprinting (original antigenic sin[7]). We therefore analysed the
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dynamics of antibodies to each antigen separately. When antibody titres were boosted above some threshold
between consecutive samples (Figure S1), we assumed antigen exposure to have occurred (which may or may not
coincide with detection of parasites).
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hence antibody titres decay in the first months of life. Throughout follow‐up, children acquire ASC and boost their
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Results Three nested models of the immunological processes that might underlie antibody dynamics (Figure 2) were fitted to the Ghanaian and Gambian data. The simplest, Model 1, assumes that antibodies are generated in a single, short
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pulse following P. falciparum infection and thereafter decay exponentially. Model 2 assumes that ASCs are
generated in response to infection and decay exponentially. Throughout their life the population of ASCs generates antibodies which also decay exponentially. Finally Model 3 is similar to Model 2 but assumes that infection induces two populations of ASCs, one of which is short‐lived and the other long‐lived[21]; these two ASC populations are
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datasets are shown in Table 1. Sample model fits for four children are presented in Figure 3.
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Under the simplest model (Model 1), we estimate antibody half‐lives in the range 9‐40 days in Ghanaian children and 60‐103 days in Gambian children. Under Model 2, the observed antibody response in the Ghanaian children was estimated to be due to ASCs with half‐life in the range of 2‐5 days, generating antibodies with half‐life in the range of
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17‐30 days. The antibody response in the Gambian children was estimated to be due to ASCs with half‐life in the range of 95‐270 days, generating antibodies with half‐life in the range of 11‐12 days. Models 1 and 2 capture the
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short‐lived phase of the antibody response in the Ghanaian cohort, but identify a phase of intermediate duration on the Gambian cohort.
In contrast, Model 3, in which two populations of ASCs are incorporated (thus more accurately representing the underlying immunological processes), produced consistent estimates for the half‐lives of both antibodies and ASCs
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between the Ghanaian and Gambian children. The half‐life of antibodies was estimated to be 14‐21 days in Ghanaian children and 4‐11 days in Gambian children. These half‐lives of the antibody response are similar to an estimate of 9.8 days previously reported in Kenyan children[2]. The half‐life of maternally acquired antibodies from the
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Ghanaian cohort was estimated to be 13‐45 days. These figures are consistent with estimates of the half‐life of passively acquired IgG in adults, which varies from 11‐70 days depending on initial serum concentration and IgG subclass[22]. As different malaria antigens have been found to induce different IgG subclasses (e.g. with IgG1
predominating for AMA‐1 and IgG3 predominating for MSP‐2) and the extent of this varies with age and duration of exposure[23], an analysis of the proportions and kinetics of each IgG subclass for each antigen may begin to explain the differences in estimated antibody half‐life shown in Table 1. Differences in estimated half‐lives between antigens
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both assumed to generate antibodies but to decay at different rates. The parameters providing the best fit to both
6 may be partially due to differences in the proportion of different IgG subclasses since different subclasses are catabolised at different rates [24]. The shorter half‐life of a child’s own IgG antibodies compared to maternally acquired IgG antibodies may be explained by antibodies being used up to clear parasites in the case of active
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infection. Under Model 3, the half‐life of short‐lived ASCs was estimated to be 2‐3 days in Ghanaian children and 4‐10 days in Gambian children, whilst the half‐life of long‐lived ASCs was estimated to be 5‐9 years in Ghanaian infants and 3‐5 years in Gambian children. These estimates suggest a sustained, long‐lived immune response in the order of years
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responses (Model 1 and Kinyanjui[2]). Furthermore, 81‐95% of ASCs are estimated to be short‐lived in Ghanaian children compared to 68‐83% in Gambian children. This difference in the production of long‐lived ASCs may be in
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part attributable to differences in age as has previously been observed for the Gambian cohort[8]. Whilst there were insufficient data to formally fit an age effect, the results are consistent with the capacity to mount a sustained
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response (captured here as a higher proportion of long‐lived ASCs) improving with age. Moreover, in the youngest infants (less than 100 days old) from the Ghanaian cohort, antibody titres were rarely boosted, even though the infants were regularly exposed to Plasmodium parasites. This suggests that young infants may be unable to mount
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their own B cell response, perhaps because high avidity maternal antibodies sequester the antigen and reduce its availability to prime the child’s naïve B cells.
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Discussion
The models described here are necessary simplifications of the complex processes underlying the generation of
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antibody responses. Antigen exposure is assumed to induce rapid proliferation and differentiation of naïve B cells or MBCs into ASCs and sudden bursts of IgG secretion from newly minted ASCs leads to a sharp increase in antibody titre (boosting). However blood‐stage malaria infections can persist for weeks or months (up to at least 40 weeks for a single parasite clone in the Ghanaian cohort[25]), continuously exposing emerging naïve B cells and MBCs to
antigen and generating less discrete waves of ASCs and antibodies. A model incorporating data on the duration of infection may provide additional insights into the maintenance of long‐lived antibody responses. Furthermore, the simple models investigated here do not distinguish between primary infection and re‐infection[26]. It has been
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rather than days, revealing the limitations of models that do not capture the observed biphasic decay in humoral
7 observed in mice that functional MBCs generated in a primary malaria infection give rise to a faster ASC and antibody response upon re‐infection[14]. Alternative model formulations could consider the possibility of faster and stronger secondary antibody responses and a greater likelihood of MBCs differentiating into long‐lived ASCs
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compared to naïve B cells. However, our data were insufficient to distinguish between these models of increased complexity. More complex models could be tested against data from mouse studies, where frequent sampling of both the humoral and cellular components of immunity is possible [14, 15].
The data and model presented here can explain both the transient antibody response observed in very young
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infection. It also suggests that there may be a lower age limit for the efficient generation of long‐lived antibody secreting cells.This information may be useful in interpreting the recent RTS,S/AS01 malaria vaccine trials in which a
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bi‐phasic exponential decay of anti‐CSP antibody titres was observed (with titres waning rapidly in the first few weeks after vaccination followed by a slower decay during extended follow up over several years [27]) and in which
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vaccine efficacy was higher in those receiving the vaccine at 5‐17 months of age group than in those vaccinated at 6‐ 10 weeks of age[28, 29]. Comparable longitudinal studies of other naturally acquired infections are needed to determine whether the kinetics of humoral immune responses to malaria are typical of other infections or are in
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some way aberrant. More extensive sampling of narrowly‐defined age cohorts throughout child development will also be important, as a substantial age effect on antibody longevity has previously been seen among children between 3 and 8 years old in The Gambia [Alpogheneta et al. 2008]. Also, the findings of Model 3 incorporating both short and long‐lived components of the anti‐malarial antibody response need to be validated against data from
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other longitudinal studies. Importantly, these studies will need both a period of frequent sampling to detect the short‐lived antibody decay and extended follow up to capture long term trends. Finally, the indication that malaria infections can give rise to very long‐lived ASCs, which continue to secrete low levels of antibodies for very long
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periods of time in the absence of re‐infection, provides a plausible explanation for the maintenance of protective
immunity after decades of negligible malaria exposure in migrants [3] or long periods of effective malaria control in previously endemic areas [30] and has implications for the use of serology for mapping temporal changes in malaria transmission [31].
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children and the development of sustained humoral immunity following repeated exposure to P. falciparum
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Materials and Methods Model 1
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IgG antibody is generated in boosts following exposure to P. falciparum antigens and decays with half‐life d a days. When IgG antibody is generated at rate the antibody dynamics can be described by
dA rA dt
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log(2) log(2) is acquired at is the rate of decay of antibody. If maternal antibody Am decaying at rate m dm da
birth and antibody is generated in boosts of size i following antigen exposure at time i then equation (1) can be
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solved to give
A(t ) Am e mt H (t , i ) i e r (t i )
(2)
where H is a step function defined as
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i
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0 t i H (t , i ) 1 t i
(3)
At the start of follow up the Gambian children do not have maternal antibody, but instead have some initial titre of naturally‐acquired antibody A0 .
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Model 2
Model 2 assumes rapid proliferation of ASCs following exposure to P. falciparum antigens. These plasma cells persist
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with half‐life d b days secreting IgG antibody throughout their life. When ASCs are generated at a rate , the antibody
kinetics can be described by
dB cB dt dA gB rA dt
(4)
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where r
(1)
9 where c
log(2) is the rate of decay of cells, and g is the rate at which antibody is secreted by plasma cells. If db
maternal antibody is acquired at birth and plasma cells are generated in boosts of size i at times i then equation (4)
A(t ) Am e mt H (t , i ) i
i r c
e
c ( t i )
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can be solved to give
e r (t i )
(5)
The constant g has been absorbed into the boost size i by normalization. At the start of follow up the Gambian
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some amount of ASCs B0 .
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Model 3
Model 2 is extended so that following antigen exposure, short‐lived ASCs with half‐life d s days (of the order of
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days/weeks) and long‐lived ASCs with half‐life dl (of the order of months/years) are generated. Short‐lived ASCs are responsible for the rapid rise in antibody titres following initial antigen exposure. Long‐lived ASCs form a vital part of
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immunological memory and are responsible for maintaining antibody titres after the infection has been cleared. If plasma cells are generated at rate , a proportion of which are short‐lived and 1 long‐lived, then the antibody
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dynamics can be described by
where cs
dBs cs Bs dt dBl (1 ) cl Bl dt dA gBs gBl rA dt
(6)
log(2) log(2) is the rate of decay of long‐lived is the rate of decay of short‐lived plasma cells, and cl dl ds
plasma cells. If maternal antibody is acquired at birth and plasma cells are generated in boosts of size i at
times i then equation (6) can be solved to give
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children do not have maternal antibody, but instead have some initial titre of naturally‐acquired antibody A0 , and
10
1 cl ( t i ) r ( t i ) e A(t ) Am e mt H (t , i ) i e cs ( t i ) e r (t i ) e r cl i r cs
(7)
Setting 1 , Model 3 simplifies to Model 2. At the start of follow up the Gambian children do not have maternal
Bs 0 , and long‐lived ASCs Bl 0 . Model fitting
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The models were fitted to individual‐level longitudinal data on antibody titres from the Ghanaian and Gambian cohorts. A mixed‐effects framework was utilised where the mean and standard deviation of each parameter is
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estimated. For example, for Model 1 we estimate the mean antibody half‐life of all children in the cohort da, and the standard deviation of the antibody half‐life within the cohort Σa. Parameters were estimated using Bayesian Markov Chain Monte Carlo methods (see Supplementary Information for more details). Posterior median parameter
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estimates are presented in Table 1. We do not present formal statistical model comparisons due to the challenges of comparing mixed‐effects models (see Supplementary Information for further discussion).
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antibody, but instead have some initial titre of naturally‐acquired antibody A0 , and some amount of short‐lived ASCs
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References 1. Langhorne J, Ndungu FM, Sponaas AM, Marsh K. Immunity to malaria: more questions than answers. Nat Immunol
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2008; 9:725‐32. 2. Kinyanjui SM, Conway DJ, Lanar DE, Marsh K. IgG antibody responses to Plasmodium falciparum merozoite antigens in Kenyan children have a short half‐life. Malar J 2007; 6.
3. Struik SS, Riley EM. Does malaria suffer from lack of memory? Immunol Rev 2004; 201:268‐90.
us
malaria infection. Immunology Immunopathogenesis Malaria 2005; 297:71‐102.
infection? Parasite Immunol 2001; 23:51‐9.
an
5. Riley EM, Wagner GE, Akanmori BD, Koram KA. Do maternally acquired antibodies protect infants from malaria
M
6. Gupta S, Snow RW, Donnelly CA, Marsh K, Newbold C. Immunity to non‐cerebral severe malaria is acquired after one or two infections. Nat Med 1999; 5:340‐3.
pt ed
7. Taylor RR, Egan A, McGuinness D, et al. Selective recognition of malaria antigens by human serum antibodies is not genetically determined but demonstrates some features of clonal imprinting. Int Immunol 1996; 8:905‐15. 8. Akpogheneta OJ, Duah NO, Tetteh KKA, et al. Duration of naturally acquired antibody responses to blood‐stage Plasmodium falciparum is age dependent and antigen specific. Infect Immun 2008; 76:1748‐55.
ce
9. Akpogheneta OJ, Dunyo S, Pinder M, Conway DJ. Boosting antibody responses to Plasmodium falciparum merozoite antigens in children with highly seasonal exposure to infection. Parasite Immunol 2010; 32:296‐304.
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10. Wipasa J, Suphavilai C, Okell LC, et al. Long‐Lived Antibody and B Cell Memory Responses to the Human Malaria Parasites, Plasmodium falciparum and Plasmodium vivax. PloS Path 2010; 6.
11. Ndungu FM, Olotu A, Mwacharo J, et al. Memory B cells are a more reliable archive for historical antimalarial responses than plasma antibodies in no‐longer exposed children. PNAS USA 2012; 109:8247‐52.
12. Migot F, Chougnet C, Henzel D, et al. Anti‐malaria antibody‐producing B cell frequencies in adults after a Plasmodium falciparum outbreak in Madagascar. Clin Exp Immunol 1995; 102:529‐34.
Downloaded from http://jid.oxfordjournals.org/ at Ondokuz Mayis University on May 19, 2014
4. Achtman AH, Bull PC, Stephens R, Langhorne J. Longevity of the immune response and memory to blood‐stage
12 13. Drakeley CJ, Corran PH, Coleman PG, et al. Estimating medium‐ and long‐term trends in malaria transmission by using serological markers of malaria exposure. PNAS USA 2005; 102:5108‐13.
after a Single Plasmodium chabaudi Infection in Mice. PloS Path 2009; 5.
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14. Ndungu FM, Cadman ET, Coulcher J, et al. Functional Memory B Cells and Long‐Lived Plasma Cells Are Generated
15. Nduati EW, Ng DHL, Ndungu FM, Gardner P, Urban BC, Langhorne J. Distinct kinetics of memory B‐cell and
plasma‐cell responses in peripheral blood following a blood‐stage Plasmodium chabaudi infection in mice. PloS One
16. Weiss GE, Traore B, Kayentao K, et al. The Plasmodium falciparum‐Specific Human Memory B Cell Compartment
an
Expands Gradually with Repeated Malaria Infections. PloS Path 2010; 6.
17. Dorfman JR, Bejon P, Ndungu FM, et al. B cell memory to 3 Plasmodium falciparum blood‐stage antigens in a malaria‐endemic area. J Infect Dis 2005; 191:1623‐30.
M
18. Riley EM, Wagner GE, Ofori MF, et al. Lack of association between maternal antibody and protection of African infants from malaria infection. Infect Immun 2000; 68:5856‐63.
pt ed
19. Tew JG, Wu JH, Qin DH, Helm S, Burton GF, Szakal AK. Follicular dendritic cells and presentation of antigen and costimulatory signals to B cells. Immunol Rev 1997; 156:39‐52. 20. Bernasconi NL, Traggiai E, Lanzavecchia A. Maintenance of serological memory by polyclonal activation of human memory B cells. Science 2002; 298:2199‐202.
ce
21. Slifka MK, Antia R, Whitmire JK, Ahmed R. Humoral immunity due to long‐lived plasma cells. Immunity 1998; 8:363‐72.
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22. Waldmann TA. Disorders of immunoglobulin metabolism. N Eng J Med 1969; 281:1170‐7.
23. Tongren JE, Drakeley CJ, McDonald SLR, et al. Target antigen, age, and duration of antigen exposure independently regulate immunoglobulin G subclass switching in malaria. Infect Immun 2006; 74:257‐64.
24. Spiegelberg HL. Biological activities of immunoglobulins of different classes and subclasses. Advances Immunol 1974; 19:259‐94.
Downloaded from http://jid.oxfordjournals.org/ at Ondokuz Mayis University on May 19, 2014
us
2010; 5:e15007.
13 25. Franks S, Koram KA, Wagner GE, et al. Frequent and persistent, asymptomatic Plasmodium falciparum infections in African infants, characterized by multilocus genotyping. J Infect Dis 2001; 183:796‐804. 26. Moncunill G, Mayor A, Jimenez A, et al. Cytokine and Antibody Responses to Plasmodium falciparum in Naive
cr ipt
Individuals during a First Malaria Episode: Effect of Age and Malaria Exposure. PloS One 2013; 8.
27. Olotu A, Fegan G, Wambua J, et al. Four‐Year Efficacy of RTS,S/AS01E and Its Interaction with Malaria Exposure. N Eng J Med 2013; 368:1111‐20.
African Children. N Eng J Med 2011; 365:1863‐75.
Eng J Med 2012; 367:2284‐95.
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29. Mian‐McCarthy S, Agnandji ST, Lell B, et al. A Phase 3 Trial of RTS,S/AS01 Malaria Vaccine in African Infants. N
M
30. Deloron P, Chougnet C. Is immunity to malaria really short‐lived. Parasitology Today 1992; 8:375‐8. 31. Corran P, Coleman P, Riley E, Drakeley C. Serology: a robust indicator of malaria transmission intensity? Trends
Ac
ce
pt ed
Parasitol 2007; 23:575‐82.
Downloaded from http://jid.oxfordjournals.org/ at Ondokuz Mayis University on May 19, 2014
us
28. Agnandji ST, Lell B, Soulanoudjingar SS, et al. First Results of Phase 3 Trial of RTS,S/AS01 Malaria Vaccine in
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Funding sources MTW was supported by a grant from The Bill & Melinda Gates Foundation. JTG was supported by a fellowship from the UK Medical Research Council. ACG acknowledges support from the UK Medical Research Council and The Bill &
Conflict of Interests
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The authors declare they have no conflict of interest.
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Figure legends
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Figure 1 Figure title: Population‐level antibody dynamics
Population‐level dynamics of antibody titres to AMA‐1 (blue), MSP‐1 (pink), MSP‐2 (yellow), CSP (green) and EBA‐175
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(purple) in cohorts of 151 Ghanaian children and 124 Gambian children. The solid coloured lines show polynomial fits of the median antibody titre in each cohort. The shaded regions capture 50% and 95% of the variation in the observed data. Antibody trajectories from two randomly‐selected children from each cohort are shown in dashed lines. Ghanaian children were followed from birth for approximately 800 days. Maternal blood samples were
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collected by venepuncture, and children’s samples were collected at birth, 2, 4, and 6 weeks after birth, and then every 4 weeks, by heel prick. Blood samples were tested for parasites by microscopy and parasite DNA by
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polymerase chain reaction (PCR). The Gambian children were followed for approximately 3 months after the end of the rainy season. Blood samples were collected every 2 weeks and tested for malaria parasites by microscopy. In the Ghanaian children there is some evidence for a bi‐phasic decay, with antibody titres dropping rapidly immediately after boosting and then decaying at a slower rate over a period of months to years. For the Gambian children, a slow
decay in antibody titres is observed, punctuated by occasional boosts followed by rapid decay.
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Melinda Gates Foundation. Sample and data collection in Ghana was funded by the Wellcome Trust (040328).
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Figure 2 Figure title: Schematic representation of the fitted models.
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The top row represents how each model captures the underlying immunological processes, the middle row depicts the change in antibody titres over time (red – maternally‐acquired antibodies; blue – antibodies generated by short‐ lived ASCs; green – antibodies generated by long‐lived ASCs) and the bottom row shows the mathematical
description. In Model 1, antibodies (A) are generated in sharp boosts following infection (at rate α) and then decay
exponentially (at rate r). In Model 2, ASCs (B) are generated (at rate β) in response to antigen exposure and decay at
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following antigen exposure, a proportion ρ being short‐lived ASCs (Bs) and decaying at rate cs, and a proportion 1‐ρ
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being long‐lived ASCs (Bl) and decaying at rate cl. Antibodies (A) are produced by all ASCs at rate g and decay at rate
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r. In all three models IgG antibody can additionally be acquired maternally.
Figure 3
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Figure title: Sample model fits for individual‐level antibody dynamics. Model predicted antibody dynamics for two Ghanaian children under Model 1 (a‐b), Model 2 (c‐d), and Model 3 (e‐f). Model predicted antibody dynamics for two Gambian children under Model 1 (g‐h), Model 2 (i‐j), and Model 3 (k‐l). Dots represent data points, and the continuous lines represent model fits. The presence (red) or absence
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(black) of P. falciparum parasites detected by microscopy is indicated at the top of each plot. There was poor agreement between the time of detection of parasites and boosting of antibody titres. Furthermore, in a given
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individual the times of boosting of antibodies to different antibodies often differ. Note that the sustained antibody response in Ghanaian child 2 following infection is poorly captured by Models 1 and 2 (b,d), but is captured by Model 3 (f).
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rate c. Antibodies (A) are produced by ASCs at rate g and decay at rate r. In Model 3, ASCs are generated (at rate β)
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Table 1
AMA-1
MSP-1
MSP-2
CSP
EBA-175
maternal antibody ½‐life
dm
45 (37 ‐ 56)
30 (25 ‐ 38)
21 (18 ‐ 24)
13 (7 ‐ 24)
–
IgG antibody ½‐life
da
25 (18 ‐ 37)
40 (31 ‐ 56)
33 (27 ‐ 43)
9 (7 ‐ 13)
–
Model 2
maternal antibody ½‐life
dm
44 (37 ‐ 56)
32 (26 ‐ 43)
IgG antibody ½‐life
da
30 (25 ‐ 37)
29 (24 ‐ 36)
ASC ½‐life
db
2 (1 ‐ 8)
5 (3 ‐ 8)
Model 3
maternal antibody ½‐life
dm
46 (38 ‐ 57)
33 (27 ‐ 45)
IgG antibody ½‐life
da
17 (13 ‐ 21)
short‐lived ASC ½‐life
ds
long‐lived ASC ½‐life
dl
16 (10 ‐ 30)
–
28 (22 ‐ 34)
17 (13 ‐ 23)
–
5 (4 ‐ 9)
3 (2 ‐ 5)
–
27 (22 ‐ 36)
24 (13 ‐ 52)
–
19 (16 ‐ 23)
21 (16 ‐ 26)
14 (11 ‐ 20)
–
2.5 (2.1 ‐ 3.1)
2.4 (1.9 ‐ 3.1)
2.3 (1.9 ‐ 2.8)
3.0 (2.4 ‐ 4.0)
–
2956
1901
3444
2881
–
(1829 ‐ 4513)
(1115 ‐ 3228)
(2107 ‐ 4702)
(1530 ‐ 4639)
0.95 (0.89 ‐ 0.98)
0.84 (0.74 ‐ 0.92)
0.95 (0.89 ‐ 0.98)
0.81 (0.64 ‐ 0.97)
–
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22 (19 ‐ 26)
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proportion short‐lived
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Gambian cohort (1 – 6 years)
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Model 1
da
60 (39 ‐ 100)
72 (49 ‐ 113)
60 (32 ‐ 112)
–
103 (65 ‐ 176)
Model 2
IgG antibody ½‐life
da
12 (9 ‐ 16)
11 (8 ‐ 16)
11 (8 ‐ 14)
–
12 (9 ‐ 16)
ASC ½‐life
db
214 (123 ‐ 442)
108 (66 ‐ 224)
95 (48 ‐ 232)
–
270 (147 ‐ 567)
Model 3
IgG antibody ½‐life
da
7 (6 ‐ 9)
7 (6 ‐ 9)
4 (3 ‐ 5)
–
11 (6 ‐ 18)
short‐lived ASC ½‐life
ds
4 (3 ‐ 8)
10 (5 ‐ 17)
4 (3 ‐ 5)
–
10 (5 ‐ 18)
long‐lived ASC ½‐life
dl
1050
859
1020
–
1607
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IgG antibody ½‐life
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Model 1
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Ghanaian cohort (0 – 2 years)
17 proportion short‐lived
(612 ‐ 1742)
(473 ‐ 1565)
(547 ‐ 1939)
0.83 (0.74 ‐ 0.89)
0.70 (0.61 ‐ 0.82)
0.75 (0.67 ‐ 0.82)
(1030 ‐ 2549) –
0.68 (0.59 ‐ 0.77)
Table title: Estimated parameters for antibody dynamics in each cohort.
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Posterior median parameter estimates and 95% credible intervals for all three Models fitted to both the Ghanaian
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long‐lived ASCs (dl) and the proportion of the response (1‐ρ) that is long‐lived are correlated – a sustained antibody
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response may be due to a small number of long‐lived cells or a larger number of cells with shorter half‐life. The estimated half‐lives for the long‐lived ASCs are greater than the duration of longitudinal follow up in both cohorts, and hence are partially informed by prior information on the duration of antibody response. Estimates of the
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variation between individuals of each parameter are presented in Table S2.
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and Gambian datasets. All units, apart from proportions, are in days. In Model 3, the estimates of the half‐lives of
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20
Dynamics of the antibody response to Plasmodium falciparum infection in African children
Michael T White1,*, Jamie T Griffin1, Onome Akpogheneta2,3, David J Conway2,3, Kwadwo A Koram4, Eleanor M Riley5,†, Azra C Ghani1,† MRC Centre for Outbreak Analysis & Modelling, Department of Infectious Disease Epidemiology, Imperial College
London Medical Research Council Laboratories, Fajara, The Gambia
3
Department of Pathogen Molecular Biology, London School of Hygiene & Tropical Medicine, Keppel Street, London
4
Noguchi Memorial Institute for Medical Research, University of Ghana, Legon, Ghana
5
Department of Immunology and Infection, London School of Hygiene & Tropical Medicine, Keppel Street, London
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2
*
†
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These authors contributed equally to this study.
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Corresponding author: Dr Michael T. White, MRC Centre for Outbreak Analysis and Modelling, Department of Infectious Disease Epidemiology, Imperial College London, London, W2 1PG, UK, e‐mail: [email protected], phone: 0044 20 7594 3946
© The Author 2014. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved. For Permissions, please e‐mail: [email protected].
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1
2
Abstract Background
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Acquired immune responses to malaria have widely been perceived to be short‐lived, with previously immune individuals suffering a loss of immunity when they move from malaria‐endemic areas. However long‐lived P. falciparum specific antibody responses lasting for an individual’s lifetime are frequently observed. Methods and Findings
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cohort studies of African children to estimate the half‐lives of circulating Immunoglobulin G (IgG) antibodies and IgG
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antibody secreting cells (ASC). Antibody responses in African children can be described by a model including both short‐lived ASCs (half‐life in the range 2‐10 days) responsible for boosting antibody titres following infection, and long‐lived ASCs (half‐life in the range 3‐9 years) responsible for maintaining sustained humoral responses.
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Comparison of antibody responses in the younger Ghanaian cohort and the older Gambian cohort suggests that young children are less able to generate the long‐lived ASCs necessary to maintain the circulating antibodies that
Conclusions
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may provide protection against reinfection.
The rapid decay of antibodies following exposure to malaria and the maintenance of sustained antibody responses
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can be explained in terms of populations of short‐lived and long‐lived ASCs.
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We fit mathematical models of the dynamics of antibody titres to Plasmodium falciparum antigens from longitudinal
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Introduction In malaria‐endemic areas, young children bear the major burden of disease whereas older children and adults acquire substantial protection from severe malaria and death following exposure, but rarely, if ever, acquire sterile
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immunity[1]. It is widely perceived that acquired immune responses to malaria are short‐lived[1, 2], and that previously immune individuals suffer a loss of immunity when they move away from malaria‐endemic areas.
However, the true picture is more complex[3, 4]. Infants and young children are particularly vulnerable to malaria after physiological adaptations and maternally‐acquired antibodies have waned and before the development of
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acquisition of immunity to malaria requires multiple infections. Although substantial protection from severe malaria
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and death is acquired after a small number of infections[6], episodes of febrile disease may continue for many years. While frequent reinfection is required to maintain high concentrations of anti‐malarial antibodies, and antibody responses can appear transient, especially in young children[2, 7‐9], there is evidence that other components of the
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immune response to malaria are long‐lived[3]. Circulating memory B cells (MBC) specific for P. falciparum antigens can be detected at least eight years after the most recent infection[10, 11] and may persist for the lifetime of the individual[12]. Furthermore, estimates of the half‐life of antibody seropositivity indicate that individuals may remain
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seropositive for life for antibodies to conserved or relatively conserved antigens, even in areas of low ongoing malaria transmission[13].
The cellular and molecular determinants of the duration of anti‐malarial antibody responses have been investigated in mouse models[14, 15] but remain poorly studied in humans. MBCs specific to Plasmodium antigens are detectable
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among human peripheral blood mononuclear cells (PBMC) and studies have reported their expansion and contraction[16], and their longevity[10, 11, 17]. However, there are no comparable studies of longevity of ASCs in
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humans exposed to malaria. Such studies would be difficult to undertake, as ASCs are located primarily in bone marrow and lymphoid organs and are detectable in blood only in the short window between differentiation and
migration to the bone marrow[15]. However, mouse models reveal a strong correlation between numbers of ASCs in tissues and serum antibody concentrations, suggesting that antibody titres are a good surrogate for ASC numbers[14].
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effective adaptive immunity[5]. In contrast to other childhood infections, such as measles, rubella and varicella,
4 To obtain quantitative estimates of the immune parameters determining the antibody response to malaria, we analysed data from two longitudinal cohort studies of antibody responses to P. falciparum infection in African children. In the first study, 151 Ghanaian children were followed from birth for over 2 years to investigate the
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association between maternal antibody and protection from malaria[18]. Serum samples were analysed by enzyme‐ linked immunosorbent assays (ELISA) for the presence of antibodies to the antigens apical membrane antigen 1
(AMA‐1), merozoite surface protein 1 (MSP‐1), merozoite surface protein 2 (MSP‐2), and circumsporozoite protein
(CSP). Infants in malaria‐endemic areas are born with maternally‐acquired IgG antibodies but no ASCs of their own,
IgG titres upon exposure to P. falciparum antigens, after which antibody titres decay again. In the second study, 124 Gambian children were followed for 3 months during the dry season (when there was little or no ongoing malaria
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transmission) to investigate antibody dynamics following exposure to P. falciparum during the previous wet season[8, 9]. Serum samples were analysed by ELISA for antibodies to the antigens AMA‐1, MSP‐1, MSP‐2, and
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erythrocyte binding antigen 175 (EBA‐175). The Gambian children ranged in age from 1 to 6 years, too old for maternal antibodies to be present in appreciable quantities[5]. Figure 1 shows the population‐level pattern of boosting and decay of antibodies in both cohorts of children, as well as sample antibody trajectories from individual
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children. Longitudinal measurements of parasitaemia were available for most children. However, there was poor agreement between detection of parasites and boosting of antibody titres, with P. falciparum parasites frequently detected without a boost in titres, titres boosted without detectable infection, and titres to one antigen boosted without an
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accompanying boost to titres of other antigens. This could be due to poor sensitivity or specificity of the parasite detection methods, polymorphism of parasite antigens, persistent antigen presentation by follicular dendritic cells[19], bystander activation[20] or clonal imprinting (original antigenic sin[7]). We therefore analysed the
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dynamics of antibodies to each antigen separately. When antibody titres were boosted above some threshold
between consecutive samples (Figure S1), we assumed antigen exposure to have occurred (which may or may not
coincide with detection of parasites).
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hence antibody titres decay in the first months of life. Throughout follow‐up, children acquire ASC and boost their
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Results Three nested models of the immunological processes that might underlie antibody dynamics (Figure 2) were fitted to the Ghanaian and Gambian data. The simplest, Model 1, assumes that antibodies are generated in a single, short
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pulse following P. falciparum infection and thereafter decay exponentially. Model 2 assumes that ASCs are
generated in response to infection and decay exponentially. Throughout their life the population of ASCs generates antibodies which also decay exponentially. Finally Model 3 is similar to Model 2 but assumes that infection induces two populations of ASCs, one of which is short‐lived and the other long‐lived[21]; these two ASC populations are
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datasets are shown in Table 1. Sample model fits for four children are presented in Figure 3.
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Under the simplest model (Model 1), we estimate antibody half‐lives in the range 9‐40 days in Ghanaian children and 60‐103 days in Gambian children. Under Model 2, the observed antibody response in the Ghanaian children was estimated to be due to ASCs with half‐life in the range of 2‐5 days, generating antibodies with half‐life in the range of
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17‐30 days. The antibody response in the Gambian children was estimated to be due to ASCs with half‐life in the range of 95‐270 days, generating antibodies with half‐life in the range of 11‐12 days. Models 1 and 2 capture the
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short‐lived phase of the antibody response in the Ghanaian cohort, but identify a phase of intermediate duration on the Gambian cohort.
In contrast, Model 3, in which two populations of ASCs are incorporated (thus more accurately representing the underlying immunological processes), produced consistent estimates for the half‐lives of both antibodies and ASCs
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between the Ghanaian and Gambian children. The half‐life of antibodies was estimated to be 14‐21 days in Ghanaian children and 4‐11 days in Gambian children. These half‐lives of the antibody response are similar to an estimate of 9.8 days previously reported in Kenyan children[2]. The half‐life of maternally acquired antibodies from the
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Ghanaian cohort was estimated to be 13‐45 days. These figures are consistent with estimates of the half‐life of passively acquired IgG in adults, which varies from 11‐70 days depending on initial serum concentration and IgG subclass[22]. As different malaria antigens have been found to induce different IgG subclasses (e.g. with IgG1
predominating for AMA‐1 and IgG3 predominating for MSP‐2) and the extent of this varies with age and duration of exposure[23], an analysis of the proportions and kinetics of each IgG subclass for each antigen may begin to explain the differences in estimated antibody half‐life shown in Table 1. Differences in estimated half‐lives between antigens
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both assumed to generate antibodies but to decay at different rates. The parameters providing the best fit to both
6 may be partially due to differences in the proportion of different IgG subclasses since different subclasses are catabolised at different rates [24]. The shorter half‐life of a child’s own IgG antibodies compared to maternally acquired IgG antibodies may be explained by antibodies being used up to clear parasites in the case of active
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infection. Under Model 3, the half‐life of short‐lived ASCs was estimated to be 2‐3 days in Ghanaian children and 4‐10 days in Gambian children, whilst the half‐life of long‐lived ASCs was estimated to be 5‐9 years in Ghanaian infants and 3‐5 years in Gambian children. These estimates suggest a sustained, long‐lived immune response in the order of years
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responses (Model 1 and Kinyanjui[2]). Furthermore, 81‐95% of ASCs are estimated to be short‐lived in Ghanaian children compared to 68‐83% in Gambian children. This difference in the production of long‐lived ASCs may be in
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part attributable to differences in age as has previously been observed for the Gambian cohort[8]. Whilst there were insufficient data to formally fit an age effect, the results are consistent with the capacity to mount a sustained
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response (captured here as a higher proportion of long‐lived ASCs) improving with age. Moreover, in the youngest infants (less than 100 days old) from the Ghanaian cohort, antibody titres were rarely boosted, even though the infants were regularly exposed to Plasmodium parasites. This suggests that young infants may be unable to mount
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their own B cell response, perhaps because high avidity maternal antibodies sequester the antigen and reduce its availability to prime the child’s naïve B cells.
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Discussion
The models described here are necessary simplifications of the complex processes underlying the generation of
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antibody responses. Antigen exposure is assumed to induce rapid proliferation and differentiation of naïve B cells or MBCs into ASCs and sudden bursts of IgG secretion from newly minted ASCs leads to a sharp increase in antibody titre (boosting). However blood‐stage malaria infections can persist for weeks or months (up to at least 40 weeks for a single parasite clone in the Ghanaian cohort[25]), continuously exposing emerging naïve B cells and MBCs to
antigen and generating less discrete waves of ASCs and antibodies. A model incorporating data on the duration of infection may provide additional insights into the maintenance of long‐lived antibody responses. Furthermore, the simple models investigated here do not distinguish between primary infection and re‐infection[26]. It has been
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rather than days, revealing the limitations of models that do not capture the observed biphasic decay in humoral
7 observed in mice that functional MBCs generated in a primary malaria infection give rise to a faster ASC and antibody response upon re‐infection[14]. Alternative model formulations could consider the possibility of faster and stronger secondary antibody responses and a greater likelihood of MBCs differentiating into long‐lived ASCs
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compared to naïve B cells. However, our data were insufficient to distinguish between these models of increased complexity. More complex models could be tested against data from mouse studies, where frequent sampling of both the humoral and cellular components of immunity is possible [14, 15].
The data and model presented here can explain both the transient antibody response observed in very young
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infection. It also suggests that there may be a lower age limit for the efficient generation of long‐lived antibody secreting cells.This information may be useful in interpreting the recent RTS,S/AS01 malaria vaccine trials in which a
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bi‐phasic exponential decay of anti‐CSP antibody titres was observed (with titres waning rapidly in the first few weeks after vaccination followed by a slower decay during extended follow up over several years [27]) and in which
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vaccine efficacy was higher in those receiving the vaccine at 5‐17 months of age group than in those vaccinated at 6‐ 10 weeks of age[28, 29]. Comparable longitudinal studies of other naturally acquired infections are needed to determine whether the kinetics of humoral immune responses to malaria are typical of other infections or are in
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some way aberrant. More extensive sampling of narrowly‐defined age cohorts throughout child development will also be important, as a substantial age effect on antibody longevity has previously been seen among children between 3 and 8 years old in The Gambia [Alpogheneta et al. 2008]. Also, the findings of Model 3 incorporating both short and long‐lived components of the anti‐malarial antibody response need to be validated against data from
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other longitudinal studies. Importantly, these studies will need both a period of frequent sampling to detect the short‐lived antibody decay and extended follow up to capture long term trends. Finally, the indication that malaria infections can give rise to very long‐lived ASCs, which continue to secrete low levels of antibodies for very long
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periods of time in the absence of re‐infection, provides a plausible explanation for the maintenance of protective
immunity after decades of negligible malaria exposure in migrants [3] or long periods of effective malaria control in previously endemic areas [30] and has implications for the use of serology for mapping temporal changes in malaria transmission [31].
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children and the development of sustained humoral immunity following repeated exposure to P. falciparum
8
Materials and Methods Model 1
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IgG antibody is generated in boosts following exposure to P. falciparum antigens and decays with half‐life d a days. When IgG antibody is generated at rate the antibody dynamics can be described by
dA rA dt
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log(2) log(2) is acquired at is the rate of decay of antibody. If maternal antibody Am decaying at rate m dm da
birth and antibody is generated in boosts of size i following antigen exposure at time i then equation (1) can be
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solved to give
A(t ) Am e mt H (t , i ) i e r (t i )
(2)
where H is a step function defined as
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0 t i H (t , i ) 1 t i
(3)
At the start of follow up the Gambian children do not have maternal antibody, but instead have some initial titre of naturally‐acquired antibody A0 .
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Model 2
Model 2 assumes rapid proliferation of ASCs following exposure to P. falciparum antigens. These plasma cells persist
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with half‐life d b days secreting IgG antibody throughout their life. When ASCs are generated at a rate , the antibody
kinetics can be described by
dB cB dt dA gB rA dt
(4)
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where r
(1)
9 where c
log(2) is the rate of decay of cells, and g is the rate at which antibody is secreted by plasma cells. If db
maternal antibody is acquired at birth and plasma cells are generated in boosts of size i at times i then equation (4)
A(t ) Am e mt H (t , i ) i
i r c
e
c ( t i )
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can be solved to give
e r (t i )
(5)
The constant g has been absorbed into the boost size i by normalization. At the start of follow up the Gambian
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some amount of ASCs B0 .
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Model 3
Model 2 is extended so that following antigen exposure, short‐lived ASCs with half‐life d s days (of the order of
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days/weeks) and long‐lived ASCs with half‐life dl (of the order of months/years) are generated. Short‐lived ASCs are responsible for the rapid rise in antibody titres following initial antigen exposure. Long‐lived ASCs form a vital part of
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immunological memory and are responsible for maintaining antibody titres after the infection has been cleared. If plasma cells are generated at rate , a proportion of which are short‐lived and 1 long‐lived, then the antibody
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dynamics can be described by
where cs
dBs cs Bs dt dBl (1 ) cl Bl dt dA gBs gBl rA dt
(6)
log(2) log(2) is the rate of decay of long‐lived is the rate of decay of short‐lived plasma cells, and cl dl ds
plasma cells. If maternal antibody is acquired at birth and plasma cells are generated in boosts of size i at
times i then equation (6) can be solved to give
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children do not have maternal antibody, but instead have some initial titre of naturally‐acquired antibody A0 , and
10
1 cl ( t i ) r ( t i ) e A(t ) Am e mt H (t , i ) i e cs ( t i ) e r (t i ) e r cl i r cs
(7)
Setting 1 , Model 3 simplifies to Model 2. At the start of follow up the Gambian children do not have maternal
Bs 0 , and long‐lived ASCs Bl 0 . Model fitting
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The models were fitted to individual‐level longitudinal data on antibody titres from the Ghanaian and Gambian cohorts. A mixed‐effects framework was utilised where the mean and standard deviation of each parameter is
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estimated. For example, for Model 1 we estimate the mean antibody half‐life of all children in the cohort da, and the standard deviation of the antibody half‐life within the cohort Σa. Parameters were estimated using Bayesian Markov Chain Monte Carlo methods (see Supplementary Information for more details). Posterior median parameter
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estimates are presented in Table 1. We do not present formal statistical model comparisons due to the challenges of comparing mixed‐effects models (see Supplementary Information for further discussion).
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antibody, but instead have some initial titre of naturally‐acquired antibody A0 , and some amount of short‐lived ASCs
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References 1. Langhorne J, Ndungu FM, Sponaas AM, Marsh K. Immunity to malaria: more questions than answers. Nat Immunol
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2008; 9:725‐32. 2. Kinyanjui SM, Conway DJ, Lanar DE, Marsh K. IgG antibody responses to Plasmodium falciparum merozoite antigens in Kenyan children have a short half‐life. Malar J 2007; 6.
3. Struik SS, Riley EM. Does malaria suffer from lack of memory? Immunol Rev 2004; 201:268‐90.
us
malaria infection. Immunology Immunopathogenesis Malaria 2005; 297:71‐102.
infection? Parasite Immunol 2001; 23:51‐9.
an
5. Riley EM, Wagner GE, Akanmori BD, Koram KA. Do maternally acquired antibodies protect infants from malaria
M
6. Gupta S, Snow RW, Donnelly CA, Marsh K, Newbold C. Immunity to non‐cerebral severe malaria is acquired after one or two infections. Nat Med 1999; 5:340‐3.
pt ed
7. Taylor RR, Egan A, McGuinness D, et al. Selective recognition of malaria antigens by human serum antibodies is not genetically determined but demonstrates some features of clonal imprinting. Int Immunol 1996; 8:905‐15. 8. Akpogheneta OJ, Duah NO, Tetteh KKA, et al. Duration of naturally acquired antibody responses to blood‐stage Plasmodium falciparum is age dependent and antigen specific. Infect Immun 2008; 76:1748‐55.
ce
9. Akpogheneta OJ, Dunyo S, Pinder M, Conway DJ. Boosting antibody responses to Plasmodium falciparum merozoite antigens in children with highly seasonal exposure to infection. Parasite Immunol 2010; 32:296‐304.
Ac
10. Wipasa J, Suphavilai C, Okell LC, et al. Long‐Lived Antibody and B Cell Memory Responses to the Human Malaria Parasites, Plasmodium falciparum and Plasmodium vivax. PloS Path 2010; 6.
11. Ndungu FM, Olotu A, Mwacharo J, et al. Memory B cells are a more reliable archive for historical antimalarial responses than plasma antibodies in no‐longer exposed children. PNAS USA 2012; 109:8247‐52.
12. Migot F, Chougnet C, Henzel D, et al. Anti‐malaria antibody‐producing B cell frequencies in adults after a Plasmodium falciparum outbreak in Madagascar. Clin Exp Immunol 1995; 102:529‐34.
Downloaded from http://jid.oxfordjournals.org/ at Ondokuz Mayis University on May 19, 2014
4. Achtman AH, Bull PC, Stephens R, Langhorne J. Longevity of the immune response and memory to blood‐stage
12 13. Drakeley CJ, Corran PH, Coleman PG, et al. Estimating medium‐ and long‐term trends in malaria transmission by using serological markers of malaria exposure. PNAS USA 2005; 102:5108‐13.
after a Single Plasmodium chabaudi Infection in Mice. PloS Path 2009; 5.
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14. Ndungu FM, Cadman ET, Coulcher J, et al. Functional Memory B Cells and Long‐Lived Plasma Cells Are Generated
15. Nduati EW, Ng DHL, Ndungu FM, Gardner P, Urban BC, Langhorne J. Distinct kinetics of memory B‐cell and
plasma‐cell responses in peripheral blood following a blood‐stage Plasmodium chabaudi infection in mice. PloS One
16. Weiss GE, Traore B, Kayentao K, et al. The Plasmodium falciparum‐Specific Human Memory B Cell Compartment
an
Expands Gradually with Repeated Malaria Infections. PloS Path 2010; 6.
17. Dorfman JR, Bejon P, Ndungu FM, et al. B cell memory to 3 Plasmodium falciparum blood‐stage antigens in a malaria‐endemic area. J Infect Dis 2005; 191:1623‐30.
M
18. Riley EM, Wagner GE, Ofori MF, et al. Lack of association between maternal antibody and protection of African infants from malaria infection. Infect Immun 2000; 68:5856‐63.
pt ed
19. Tew JG, Wu JH, Qin DH, Helm S, Burton GF, Szakal AK. Follicular dendritic cells and presentation of antigen and costimulatory signals to B cells. Immunol Rev 1997; 156:39‐52. 20. Bernasconi NL, Traggiai E, Lanzavecchia A. Maintenance of serological memory by polyclonal activation of human memory B cells. Science 2002; 298:2199‐202.
ce
21. Slifka MK, Antia R, Whitmire JK, Ahmed R. Humoral immunity due to long‐lived plasma cells. Immunity 1998; 8:363‐72.
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22. Waldmann TA. Disorders of immunoglobulin metabolism. N Eng J Med 1969; 281:1170‐7.
23. Tongren JE, Drakeley CJ, McDonald SLR, et al. Target antigen, age, and duration of antigen exposure independently regulate immunoglobulin G subclass switching in malaria. Infect Immun 2006; 74:257‐64.
24. Spiegelberg HL. Biological activities of immunoglobulins of different classes and subclasses. Advances Immunol 1974; 19:259‐94.
Downloaded from http://jid.oxfordjournals.org/ at Ondokuz Mayis University on May 19, 2014
us
2010; 5:e15007.
13 25. Franks S, Koram KA, Wagner GE, et al. Frequent and persistent, asymptomatic Plasmodium falciparum infections in African infants, characterized by multilocus genotyping. J Infect Dis 2001; 183:796‐804. 26. Moncunill G, Mayor A, Jimenez A, et al. Cytokine and Antibody Responses to Plasmodium falciparum in Naive
cr ipt
Individuals during a First Malaria Episode: Effect of Age and Malaria Exposure. PloS One 2013; 8.
27. Olotu A, Fegan G, Wambua J, et al. Four‐Year Efficacy of RTS,S/AS01E and Its Interaction with Malaria Exposure. N Eng J Med 2013; 368:1111‐20.
African Children. N Eng J Med 2011; 365:1863‐75.
Eng J Med 2012; 367:2284‐95.
an
29. Mian‐McCarthy S, Agnandji ST, Lell B, et al. A Phase 3 Trial of RTS,S/AS01 Malaria Vaccine in African Infants. N
M
30. Deloron P, Chougnet C. Is immunity to malaria really short‐lived. Parasitology Today 1992; 8:375‐8. 31. Corran P, Coleman P, Riley E, Drakeley C. Serology: a robust indicator of malaria transmission intensity? Trends
Ac
ce
pt ed
Parasitol 2007; 23:575‐82.
Downloaded from http://jid.oxfordjournals.org/ at Ondokuz Mayis University on May 19, 2014
us
28. Agnandji ST, Lell B, Soulanoudjingar SS, et al. First Results of Phase 3 Trial of RTS,S/AS01 Malaria Vaccine in
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Funding sources MTW was supported by a grant from The Bill & Melinda Gates Foundation. JTG was supported by a fellowship from the UK Medical Research Council. ACG acknowledges support from the UK Medical Research Council and The Bill &
Conflict of Interests
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The authors declare they have no conflict of interest.
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Figure legends
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Figure 1 Figure title: Population‐level antibody dynamics
Population‐level dynamics of antibody titres to AMA‐1 (blue), MSP‐1 (pink), MSP‐2 (yellow), CSP (green) and EBA‐175
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(purple) in cohorts of 151 Ghanaian children and 124 Gambian children. The solid coloured lines show polynomial fits of the median antibody titre in each cohort. The shaded regions capture 50% and 95% of the variation in the observed data. Antibody trajectories from two randomly‐selected children from each cohort are shown in dashed lines. Ghanaian children were followed from birth for approximately 800 days. Maternal blood samples were
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collected by venepuncture, and children’s samples were collected at birth, 2, 4, and 6 weeks after birth, and then every 4 weeks, by heel prick. Blood samples were tested for parasites by microscopy and parasite DNA by
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polymerase chain reaction (PCR). The Gambian children were followed for approximately 3 months after the end of the rainy season. Blood samples were collected every 2 weeks and tested for malaria parasites by microscopy. In the Ghanaian children there is some evidence for a bi‐phasic decay, with antibody titres dropping rapidly immediately after boosting and then decaying at a slower rate over a period of months to years. For the Gambian children, a slow
decay in antibody titres is observed, punctuated by occasional boosts followed by rapid decay.
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Melinda Gates Foundation. Sample and data collection in Ghana was funded by the Wellcome Trust (040328).
15
Figure 2 Figure title: Schematic representation of the fitted models.
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The top row represents how each model captures the underlying immunological processes, the middle row depicts the change in antibody titres over time (red – maternally‐acquired antibodies; blue – antibodies generated by short‐ lived ASCs; green – antibodies generated by long‐lived ASCs) and the bottom row shows the mathematical
description. In Model 1, antibodies (A) are generated in sharp boosts following infection (at rate α) and then decay
exponentially (at rate r). In Model 2, ASCs (B) are generated (at rate β) in response to antigen exposure and decay at
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following antigen exposure, a proportion ρ being short‐lived ASCs (Bs) and decaying at rate cs, and a proportion 1‐ρ
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being long‐lived ASCs (Bl) and decaying at rate cl. Antibodies (A) are produced by all ASCs at rate g and decay at rate
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r. In all three models IgG antibody can additionally be acquired maternally.
Figure 3
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Figure title: Sample model fits for individual‐level antibody dynamics. Model predicted antibody dynamics for two Ghanaian children under Model 1 (a‐b), Model 2 (c‐d), and Model 3 (e‐f). Model predicted antibody dynamics for two Gambian children under Model 1 (g‐h), Model 2 (i‐j), and Model 3 (k‐l). Dots represent data points, and the continuous lines represent model fits. The presence (red) or absence
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(black) of P. falciparum parasites detected by microscopy is indicated at the top of each plot. There was poor agreement between the time of detection of parasites and boosting of antibody titres. Furthermore, in a given
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individual the times of boosting of antibodies to different antibodies often differ. Note that the sustained antibody response in Ghanaian child 2 following infection is poorly captured by Models 1 and 2 (b,d), but is captured by Model 3 (f).
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rate c. Antibodies (A) are produced by ASCs at rate g and decay at rate r. In Model 3, ASCs are generated (at rate β)
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Table 1
AMA-1
MSP-1
MSP-2
CSP
EBA-175
maternal antibody ½‐life
dm
45 (37 ‐ 56)
30 (25 ‐ 38)
21 (18 ‐ 24)
13 (7 ‐ 24)
–
IgG antibody ½‐life
da
25 (18 ‐ 37)
40 (31 ‐ 56)
33 (27 ‐ 43)
9 (7 ‐ 13)
–
Model 2
maternal antibody ½‐life
dm
44 (37 ‐ 56)
32 (26 ‐ 43)
IgG antibody ½‐life
da
30 (25 ‐ 37)
29 (24 ‐ 36)
ASC ½‐life
db
2 (1 ‐ 8)
5 (3 ‐ 8)
Model 3
maternal antibody ½‐life
dm
46 (38 ‐ 57)
33 (27 ‐ 45)
IgG antibody ½‐life
da
17 (13 ‐ 21)
short‐lived ASC ½‐life
ds
long‐lived ASC ½‐life
dl
16 (10 ‐ 30)
–
28 (22 ‐ 34)
17 (13 ‐ 23)
–
5 (4 ‐ 9)
3 (2 ‐ 5)
–
27 (22 ‐ 36)
24 (13 ‐ 52)
–
19 (16 ‐ 23)
21 (16 ‐ 26)
14 (11 ‐ 20)
–
2.5 (2.1 ‐ 3.1)
2.4 (1.9 ‐ 3.1)
2.3 (1.9 ‐ 2.8)
3.0 (2.4 ‐ 4.0)
–
2956
1901
3444
2881
–
(1829 ‐ 4513)
(1115 ‐ 3228)
(2107 ‐ 4702)
(1530 ‐ 4639)
0.95 (0.89 ‐ 0.98)
0.84 (0.74 ‐ 0.92)
0.95 (0.89 ‐ 0.98)
0.81 (0.64 ‐ 0.97)
–
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an
22 (19 ‐ 26)
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proportion short‐lived
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Gambian cohort (1 – 6 years)
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Model 1
da
60 (39 ‐ 100)
72 (49 ‐ 113)
60 (32 ‐ 112)
–
103 (65 ‐ 176)
Model 2
IgG antibody ½‐life
da
12 (9 ‐ 16)
11 (8 ‐ 16)
11 (8 ‐ 14)
–
12 (9 ‐ 16)
ASC ½‐life
db
214 (123 ‐ 442)
108 (66 ‐ 224)
95 (48 ‐ 232)
–
270 (147 ‐ 567)
Model 3
IgG antibody ½‐life
da
7 (6 ‐ 9)
7 (6 ‐ 9)
4 (3 ‐ 5)
–
11 (6 ‐ 18)
short‐lived ASC ½‐life
ds
4 (3 ‐ 8)
10 (5 ‐ 17)
4 (3 ‐ 5)
–
10 (5 ‐ 18)
long‐lived ASC ½‐life
dl
1050
859
1020
–
1607
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IgG antibody ½‐life
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Model 1
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Ghanaian cohort (0 – 2 years)
17 proportion short‐lived
(612 ‐ 1742)
(473 ‐ 1565)
(547 ‐ 1939)
0.83 (0.74 ‐ 0.89)
0.70 (0.61 ‐ 0.82)
0.75 (0.67 ‐ 0.82)
(1030 ‐ 2549) –
0.68 (0.59 ‐ 0.77)
Table title: Estimated parameters for antibody dynamics in each cohort.
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Posterior median parameter estimates and 95% credible intervals for all three Models fitted to both the Ghanaian
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long‐lived ASCs (dl) and the proportion of the response (1‐ρ) that is long‐lived are correlated – a sustained antibody
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response may be due to a small number of long‐lived cells or a larger number of cells with shorter half‐life. The estimated half‐lives for the long‐lived ASCs are greater than the duration of longitudinal follow up in both cohorts, and hence are partially informed by prior information on the duration of antibody response. Estimates of the
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variation between individuals of each parameter are presented in Table S2.
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and Gambian datasets. All units, apart from proportions, are in days. In Model 3, the estimates of the half‐lives of
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