Non-competitive resource exploitation within mosquito shapes within-host malaria infectivity and virulence

Malaria is a fatal human parasitic disease transmitted by a mosquito vector. Although the evolution of within-host malaria virulence has been the focus of many theoretical and empirical studies, the vector’s contribution to this process is not well understood. Here, we explore how within-vector resource exploitation would impact the evolution of within-host Plasmodium virulence. By combining within-vector dynamics and malaria epidemiology, we develop a mathematical model, which predicts that non-competitive parasitic resource exploitation within-vector restricts within-host parasite virulence. To validate our model, we experimentally manipulate mosquito lipid trafficking and gauge within-vector parasite development and within-host infectivity and virulence. We find that mosquito-derived lipids determine within-host parasite virulence by shaping development (quantity) and metabolic activity (quality) of transmissible sporozoites. Our findings uncover the potential impact of within-vector environment and vector control strategies on the evolution of malaria virulence.

The evolution of within-host malaria virulence has been studied, but the vector’s contribution isn’t well understood. Here, Costa et al. show that non-competitive parasitic resource exploitation within-vector, in particular lipid trafficking, restricts within-host infectivity and virulence of the parasite.

Schistosoma haematobium infection affects Plasmodium falciparum-specific IgG responses associated with protection against malaria

We have previously shown that antibody responses directed to Plasmodium falciparum merozoite surface protein (MSP)-1, MSP-2 and glutamate-rich protein (GLURP) are associated with anti-malarial protection in residents of the Niakhar area of Senegal. In the same area, urinary schistosomiasis is frequent and we therefore assessed the possible influence of Schistosoma haematobium infection on these protective anti-malarial IgG responses. After adjustment for confounders, we found that the levels of IgG1 directed to MSP1 and GLURP were significantly lower in helminth carriers. The higher circulating levels of interleukin (IL)-10 present in the plasma of co-infected individuals were associated with decreased anti-plasmodial IgG responses, particularly of those directed to MSP-2. Our data thus reveal a modulation of P. falciparum-specific immune responses in the presence of a trematode helminth infection, potentially increasing infected individuals' risk of plasmodial infection or disease.

Role of heat-labile serum factor or host complement in the inhibition of Plasmodium falciparum sporogonic stages in Anopheles stephensi by gametocyte carriers' serological factors

This study investigated the significance of serum complement on transmission-reducing activity (TRA) of field sera from 24 infected Plasmodium falciparum gametocyte carriers (from Cameroon) against cultured NF54 P. falciparum. Laboratory-reared Anopheles stephensi were given infectious blood meals prepared either with sera from naïve Dutch donor (AB type) or pair-matched field serum samples, both with and without active complement. TRA of serum factors and host complement on mosquito infection rate and oocyst intensity were divided into the various components involved in the early stages of sporogony. The majority (>80%) of sera tested showed positive antibody titres to Pfs230, the relevant complement-dependent target of transmission-reducing mechanisms. Regardless of the presence of active complement, bloodmeals with field sera exhibited significantly lower infection rates and oocyst intensity than the control group. Serological reactivity in Capture-ELISA against Pfs230 was significantly correlated with the reduction of parasite infectivity. Contrary to our expectation, the presence of active complement in the mosquito bloodmeal did not increase parasite losses and therefore the magnitude of transmission reduction by individual immune sera. Our findings on P. falciparum are consistent with previous studies on animal hosts of Plasmodium, indicating that early P. falciparum sporogonic stages may be insensitive to the antibody-dependent pathways of complement in human serum.

Parasite infectivity and immunity to Plasmodium falciparum gametocytes in Gambian children

Immunity to the sexual stages of Plasmodium falciparum can be induced during natural infections. Characterization of this immunity may facilitate the design of a transmission-blocking vaccine (TBV). This study aimed to assess the prevalence and serological correlates of functional transmission-blocking immunity in Gambian children (aged 1-4 years old) who were P. falciparum gametocyte carriers. Serological assays showed 100% response to fixed, whole parasites but only 42% to live gametes. Responses to the antigens Pfs230 and Pfs48/45 were 54.1% and 37.3%, respectively, in an IgG1 ELISA. 14/55 sera were capable of reducing the infectivity of laboratory isolate NF54 in a standard membrane-feeding assay (SMFA). This activity was strongly correlated with IgG1 responses to Pfs48/45 (r = 0.49, P < 0.001) and to a serological reaction with epitopes of the same molecule (r = 0.38, P = 0.003). A weaker correlation was observed with IgG1 to Pfs230 (r = 0.29, P = 0.03). In direct membrane feeding assays (DMFA) with autologous isolates, sera from 4/29 children showed transmission-blocking activity. There was no correlation with serological assays and the DMFA or between the SMFA and DMFA. This may be caused by variation in sexual stage antigens and/or alternative modes of transmission-blocking immunity, both of which have implications for vaccine implementation.

Stage-specific effects of host plasma factors on the early sporogony of autologous Plasmodium falciparum isolates within Anopheles gambiae

Summary Quantitatively assessing the impact of naturally occurring transmission-blocking (TB) immunity on malaria parasite sporogonic development may provide a useful interpretation of the underlying mechanisms. Here, we compare the effects of plasma derived from 23 naturally infected gametocyte carriers (OWN) with plasma from donors without previous malaria exposure (AB) on the early sporogonic development of Plasmodium falciparum in Anopheles gambiae. Reduced parasite development efficiency was associated with mosquitoes taking a blood meal mixed with the gametocyte carriers' own plasma, whereas replacing autologous plasma with non-immune resulted in the highest level of parasite survival. Seven days after an infective blood meal, 39.1% of the gametocyte carriers' plasma tested showed TB activity as only a few macrogametocytes ingested along with immune plasma ended up as ookinetes but subsequent development was blocked in the presence of immune plasma. In other experiments (60.9%), the effective number of parasites declined dramatically from one developmental stage to the next, and resulted in an infection rate that was two-fold lower in OWN than in AB infection group. These findings are in agreement with those in other reports and go further by quantitatively examining at which transition stages TB immunity exerts its action. The transitions from macrogametocytes to gamete/zygote and from gamete/zygote to ookinete were identified as main targets. However, the net contribution of host plasma factors to these interstage parasite reductions was low (5-20%), suggesting that irrespective of the host plasma factors, mosquito factors might also lower the survival level of parasites during the early sporogonic development.

Plasmodium falciparum: membrane feeding assays and competition ELISAs for the measurement of transmission reduction in sera from Cameroon

The effect of natural malaria transmission-blocking factors in the blood of Plasmodium falciparum gametocyte carriers was assessed in two types of functional bioassays. In the direct membrane feeding assay (DMFA), a comparison is made between the infectivity of gametocytes from a naturally infected gametocyte carrier in the presence of autologous plasma and the infectivity in the presence of replacement plasma from nonimmune donors. In the standard membrane feeder assay (SMFA), cultured NF54 gametocytes are used to measure the capacity of endemic sera to block transmission. In the DMFA, 18 out of 48 sera (37.5%) from Cameroonian gametocyte carriers reduced transmission significantly, while in the SMFA 22 out of 48 sera (45.8%) produced transmission reduction. There was a positive correlation between both assays (r + 0.41, P < 0.05). Antibodies against epitopes of transmission-blocking target antigens Pfs48/45 and Pfs230 were measured in competition ELISAs and compared with the results of DMFA and SMFA. Serological reactivity in competition ELISAs against three epitopes of Pfs48/45 was significantly higher in the group of transmission-reducing sera in both the DMFA and the SMFA, especially for epitope III. No significant difference was found for Pfs230 antibodies (epitope I). Sensitivity of the serological assays was approximately 60%, with a specificity of around 70%. Serological tests cannot replace the functional bioassay in field situations as yet, but can contribute in the selection of sera for SMFA evaluation.

Transmission-blocking effects of sera from malaria-exposed individuals on Plasmodium falciparum isolates from gametocyte carriers

Sera from donors exposed to malaria were tested for their ability to block the transmission of isolates from Cameroonian Plasmodium falciparum gametocyte carriers. Sera were selected from amongst Cameroonian and Gambian donors who had positive antibody reactivity against the surface of activated gametes and against epitopes of Pfs 48/45 (a potential transmission-blocking vaccine candidate antigen). Aliquots of washed blood from gametocyte carriers were resuspended in test and control sera and fed to An. gambiae mosquitoes via a membrane feeder. Comparisons of the prevalence and intensity of infections is dissected mosquitoes showed variations in the ability of sera to block the transmission of the different isolates. Sera were identified that had little or no blocking effect on the transmission of isolates unless the isolate was poorly infectious. Some sera completely blocked the transmission of some isolates whilst having little or no effect on others. The observed variation in transmission-modulating activity may have implications for the development of a transmission-blocking vaccine.

Mechanisms that reduce transmission of Plasmodium falciparum malaria in semiimmune and nonimmune persons

Transmission of Plasmodium falciparum can be reduced by immune factors present in the mosquito blood meal. Specific antibodies and white blood cells (WBCs) can interact with the sexual stages of the parasite inside the mosquito midgut. The relative contribution of serum factors and WBCs on transmission reduction in gametocyte carriers from an endemic area in Cameroon and in travelers with a first malaria experience was studied. Blood from these gametocyte carriers was fed to mosquitoes through membrane feeders after serum replacement, WBC depletion, or both. In most imported malaria cases, serum factors, WBCs, or both showed a significant effect on transmission reduction, while infectiousness of gametocyte carriers from Cameroon was reduced by humoral plasma factors only. In addition, the infectivity of gametocytes from semiimmune carriers was significantly lower compared with that of nonimmune carriers, and infectivity was independent of gametocyte density and the presence of WBCs or plasma factors (or both) in the blood meal.

Toxoplasma retinitis/encephalitis 9 months after allogeneic bone marrow transplantation

In a 34-year-old patient toxoplasma retinitis/encephalitis developed 9 months after bone marrow transplantation. The BMT was complicated with a serious GVHD. Although she initially responded well to antibiotic therapy she died 2 years after BMT due to progressive infection.

Acidosis in severe childhood malaria

Data were prospectively collected on 306 Kenyan children, including blood gases in 258 (75%). Severe malaria caused a predominantly high-anion-gap metabolic acidosis in at least 43% of children. Children with coma and respiratory distress (CM + RD) had greater evidence of renal dysfunction, lower mean pH and higher mean plasma osmolality than those with respiratory distress (RD) or coma (CM) as isolated findings (mean urea 10.7 vs. 6.0 vs. 4.3 mmol/l; mean creatinine 97 vs. 74 vs. 58 mumol/l; mean osmolality 301 vs. 288 vs. 283 mosmol/l; and mean pH 7.16 vs. 7.29 vs. 7.39, respectively, p < 0.001 for each comparison of CM + RD vs. RD or CM). In addition, children with CM + RD had a higher mean blood lactate (6.7 vs. 3.3 mmol/l, p < 0.001), a lower mean haemoglobin (5.5 vs. 7.0 g/dl, p = 0.002) and a lower mean age (26.4 vs. 41.9 months, p < 0.001) than children with CM and accounted for 15/24 (63%) of all deaths. These and previous data implicate hypovolaemia and renal impairment in the pathogenesis of metabolic acidosis in severe childhood malaria. In children who are acidotic, anaemia is strongly associated with lactic acidaemia and may therefore contribute to its pathogenesis. These data also imply that coma in acidotic children (CM + RD) and those with an isolated encephalopathy (CM) may result from quite different pathophysiological mechanisms.

Depletion of CD4+ or CD8+ T-cells prevents Plasmodium berghei induced cerebral malaria in end-stage disease

The role of T-cells in development of experimental cerebral malaria was analysed in C57B1/6J and C57B1/10 mice infected with Plasmodium berghei K173 or Plasmodium berghei ANKA by treatment with anti-CD4 or anti-CD8 mAbs. Mice were protected against cerebral malaria (CM) when anti-CD4 or anti-CD8 mAbs were injected before or during infection. Even in mice in end-stage disease, i.e. with a body temperature below 35.5 degrees C, treatment with anti-CD4 or anti-CD8 antibodies or the combination protected against CM, whereas chloroquine treatment was completely ineffective in inhibiting further development of the cerebral syndrome.

Estimation of the infectious reservoir of Plasmodium falciparum in natural vector populations based on oocyst size

A method for determining the infectious reservoir of malaria (K) and vector survival rate (P) by measuring oocyst size and discriminating between the most recent and other infections is described. In the laboratory the mean diameter of 3 d oocysts in Anopheles gambiae, kept at 26 degrees C, was 11.5 microns and the mean diameter at day 5 was 24.5 microns. Oocyst sizes in wild caught mosquitoes from southern Tanzania, that had fed on the occupants of bed nets with holes in the sides, were more variable. 2060 A. gambiae s.l. and 1982 A. funestus were examined for oocysts 3 d after feeding; 796 and 654 oocysts from the 153 and 170 infected females, respectively, were measured. Because of misclassification errors, the use of a simple cut-off model, in which all oocysts less than 17.5 microns in diameter were considered to have arisen from the most recent feed, was thought to overestimate K and underestimate P. A statistical model which allows for overlap in the oocyst size distributions is described. Estimates of the infectious reservoir derived from this model were 2.8% for A. gambiae s.l. and 4.2% for A. funestus, and the estimated survival rates per gonotrophic cycle were 65.5% and 52.9%, respectively. The utility of measuring oocyst size in naturally infected mosquitoes is discussed.

Association between anti-Pfs48/45 reactivity and P. falciparum transmission-blocking activity in sera from Cameroon

Pfs48/45, a sexual stage parasite protein doublet of P. falciparum, is a target of antibodies which inhibit the development of the parasite in the mosquito. Twenty-eight (54%) out of 52 sera of gametocyte carriers from Cameroon reduced infectivity in the mosquito membrane feeding bioassay to less than 20% of the controls. These 52 sera were analysed by competition ELISAs for the presence of antibodies capable of competing the binding of six monoclonal antibodies (MoAbs) directed against five different epitopes on Pfs48/45. The percentage of these 52 Cameroon sera that competed with one of the MoAbs ranged from 13% (epitope I) to 33% (epitope IIc). Comparison of activity in the transmission-blocking assay (> or = 80%) and in the Pfs48/45 competition ELISA show a relative specificity of 100% (24 of 24) and a relative sensitivity of 75% (21 of 28). Non-blocking sera showed no competition with any of the MoAbs. These MoAbs were further used to study the diversity of epitopes among isolates of P. falciparum using a two-site ELISA. MoAbs against epitope I, III and V reacted with four different isolates whereas epitope II could be subdivided into three epitopes. None of the isolates reacted with MoAb 3G12 (epitope IV). Using these four different isolates, the competition ELISA titre varies from 1/20 to 1/80 and no significant differences are found between the isolates except for epitope II where only three out of 11 positives for epitope IIa were also positive for epitope IIc.

Cytokine profiles in bronchoalveolar lavage fluid and blood in HIV-seronegative patients with Pneumocystis carinii pneumonia

Concentrations and ex vivo production of interleukin-1beta (IL-1beta), tumour necrosis factor-alpha (TNF), interleukin-6 (IL-6), interleukin-1 receptor antagonist (IL-1RA) and TNF soluble receptors were followed in bronchoalveolar lavage (BAL) fluid and blood from 10 HIV-seronegative patients with Pneumocystis carinii pneumonia (PCP) and compared with values found in healthy volunteers. During the acute phase of PCP, TNF but not IL-6 or IL-1beta was detectable in BAL fluid. At that time, plasma concentrations of the proinflammatory cytokines were low, whereas plasma concentrations of the anti-inflammatory cytokines were high. The ex vivo production capacity of proinflammatory cytokines was suppressed in the acute phase, in the blood as well as at the site of infection. During convalescence the production capacity of the blood cells normalized. The IL-1RA production capacity of the alveolar cells was also suppressed in the acute phase, but preserved in blood cells.

Measurement by membrane feeding of reduction in Plasmodium falciparum transmission induced by endemic sera

The standard laboratory test for reduction in malaria transmission is based on the measurement of oocyst numbers in mosquitoes fed on blood meals containing test and control sera. Interpretation of the results, however, is often hampered by the large variation in numbers of infected mosquitoes and oocysts. The objective of this study was to compare 3 measures for the assessment of transmission reduction (so-called R values) and to define the experimental criteria that allow interpretation of the results. To determine variability in R values of control sera, a replicate experiment was performed with 10 non-endemic sera of Dutch blood donors. Furthermore, 2 measures for calculation of transmission reduction were compared in a triplicate experiment using Plasmodium falciparum, Anopheles gambiae and malaria endemic sera. Calculations using the geometric mean of Williams are currently used to identify blocking and non-blocking sera. However, calculations using log-transformed data could distinguish more gradual levels of transmission reduction activity by endemic sera--i.e. blocking, reducing and non-blocking activity. Grading of transmission reduction activity is important for epidemiological studies on transmission immunity and for validation of future transmission-blocking vaccines.

Transmission blockade of Plasmodium falciparum malaria by anti-Pfs230-specific antibodies is isotype dependent

By use of the parental hybridoma cell line 63F2A2 that produces specific antibodies of immunoglobulin isotype G1 (IgG1; 63F2A2.1) against Pfs230, we attempted to enrich for the synthesis of the downstream switch variant IgG2b and IgG2a monoclonal antibodies (MAbs) of the hybridoma cell line (63F2A2.2b and 63F2A2.2a, respectively). The parental IgG1 did not reduce the Plasmodium falciparum transmission in a bioassay irrespective of the presence of complement. MAbs 63F2A2.2b and 63F2A2.2a were effective in reducing the infectivity of P. falciparum parasites to Anopheles gambiae mosquitoes in membrane-feeding experiments. A transmission reduction of 91% was accomplished by the 63F2A2.2b switch variant, and a reduction of greater than 99% was accomplished by the 63F2A2.2a switch variant, but only in the presence of active human complement. Subsequently, the transmission-reducing effect of MAb 63F2A2.2b or 63F2A2.2a was confirmed in vitro by the rapid lysis of newly formed macrogametes or zygotes in the presence of active complement. MAb 63F2A2.1 did not lyse the newly formed macrogametes or zygotes irrespective of the presence of complement.

A comparison of transmission-blocking activity with reactivity in a Plasmodium falciparum 48/45-kD molecule-specific competition enzyme-linked immunosorbent assay

Monoclonal antibodies (MAbs) 32F1 and 32F3 react with two independent epitopes of a protein doublet with molecular weights of 48 and 45 kilodaltons (kD) expressed on the surface of Plasmodium falciparum (Pfs48/45) macrogametes and zygotes; only 32F3 blocks transmission. These MAbs were used to develop a Pfs48/45-specific competition enzyme-linked immunosorbent assay (ELISA) using 32F1 to capture antigen and labeled 32F3 for quantification and analysis of the contribution of antibodies in human serum to transmission-blocking activity. A comparison analysis was used to determine agreement of competition ELISA titers and transmission-blocking activity as observed in the bioassay in three groups of serum samples: 37 from European travelers with previous exposure to malaria, 56 from gametocyte carriers, and 66 from schoolchildren from a malaria-endemic area in Cameroon. The index of agreement between outcomes of the ELISA and transmission-blocking assay in gametocyte carriers and in travelers was specifically defined as fair-to-moderate; in schoolchildren the agreement was not significant. The combined analysis of all sera showed a significant and fair-to-moderate agreement between the results of the competition ELISA and the transmission-blocking assay, with a relative specificity of 94% (of 105 cases negative in the transmission-blocking assay, 99 were also negative in the competition ELISA) and a relative sensitivity of 44% (of 54 cases positive in the transmission-blocking assay, 24 were also positive in the competition ELISA). This study shows that a positive C48/45-ELISA is indicative for transmission-blocking activity in the mosquito assay, while a negative result does not exclude transmission-blocking activity.

Patterns of proinflammatory cytokines and inhibitors during typhoid fever

Cytokines and inhibitors in plasma were measured in 44 patients with typhoid fever. Ex vivo production of the cytokines was analyzed in a whole blood culture system with and without lipopolysaccharide (LPS). Acute phase circulating concentrations of cytokines (+/- SD) were as follows: interleukin (IL)-1 beta, < 140 pg/mL; tumor necrosis factor-alpha (TNF alpha), 130 +/- 50 pg/mL; IL-6, 96 +/- 131 pg/mL; and IL-8, 278 +/- 293 pg/mL. Circulating inhibitors were elevated in the acute phase: IL-1 receptor antagonist (IL-1RA) was 2304 +/- 1427 pg/mL and soluble TNF receptors 55 and 75 were 4973 +/- 2644 pg/mL and 22,865 +/- 15,143 pg/mL, respectively. LPS-stimulated production of cytokines was lower during the acute phase than during convalescence (mean values: IL-1 beta, 2547 vs. 6576 pg/mL; TNF alpha, 2609 vs. 6338 pg/mL; IL-6, 2416 vs. 7713 pg/mL). LPS-stimulated production of IL-1RA was higher in the acute than during the convalescent phase (5608 vs. 3977 pg/mL). Inhibited production of cytokines during the acute phase may be due to a switch from a proinflammatory to an antiinflammatory mode.

Transmission blocking immunity as observed in a feeder system and serological reactivity to Pfs 48/45 and Pfs230 in field sera

Monoclonal antibodies (mAbs) and human sera from gametocyte carriers were applied in the bio-assay to test for their transmission-blocking capacity. Competition ELISA's have been developed for the detection of natural transmission blocking antibodies. Approximately 55% of the sera blocking in the bio-assay gave positive results in these competition ELISA's.

Comparison of serological tests and bio-assay for malaria transmission blocking capacity in field sera

Competition ELISAs have been developed for natural transmission blocking antibodies. Approximately 50% of the sera blocking in the conventional mosquito feeding experiments, gave positive results in these competition ELISAs. Attempts to adapt competition ELISAs to a field application have been partly successful.