Treatment-reinfection Study Research Articles

Antibodies to PfsEGXP, an Early Gametocyte-Enriched Phosphoprotein, Predict Decreased Plasmodium falciparum Gametocyte Density in Humans.

Antigametocyte-specific immune responses may regulate Plasmodium falciparum gametocyte density, providing the rationale for pursuing transmission-blocking vaccines (TBVs) that target gametocytes in the human host. To identify novel antigametocyte TBV antigens, we interrogated the gametocyte proteome with our whole proteome differential screening method using plasma from a treatment-reinfection study conducted in western Kenya. At the start of the high-transmission season, 144 males (12-35 years) were enrolled and treated with quinine and doxycycline, peripheral venous blood samples were obtained, volunteers were observed, and weekly blood films were obtained for 18 weeks to quantify gametocytemia. Using plasma pooled from individuals with low versus high gametocyte carriage, we differentially screened a P falciparum gametocyte stage complementary deoxyribonucleic acid expression library. We identified 8 parasite genes uniquely recognized by gametocyte-resistant but not by gametocyte-susceptible individuals. Antibodies to one of these antigens, PfsEGXP, predicted lower gametocytemia measured over the 18-week transmission season (P = .021). When analyzed dichotomously, anti-PfsEGXP responders had 31% lower gametocyte density over 18 weeks of follow-up, compared with nonresponders (P = .04). PfsEGXP is one of the first reported gametocyte-specific target of antibodies that predict decreased gametocyte density in humans and supports our novel TBV antigen discovery platform.

Antibodies to the Plasmodium falciparum Proteins MSPDBL1 and MSPDBL2 Opsonize Merozoites, Inhibit Parasite Growth, and Predict Protection From Clinical Malaria.

Increasing evidence suggests that antibodies against merozoite surface proteins (MSPs) play an important role in clinical immunity to malaria. Two unusual members of the MSP-3 family, merozoite surface protein duffy binding-like (MSPDBL)1 and MSPDBL2, have been shown to be extrinsically associated to MSP-1 on the parasite surface. In addition to a secreted polymorphic antigen associated with merozoite (SPAM) domain characteristic of MSP-3 family members, they also contain Duffy binding-like (DBL) domain and were found to bind to erythrocytes, suggesting that they play a role in parasite invasion. Antibody responses to these proteins were investigated in a treatment-reinfection study conducted in an endemic area of Papua New Guinea to determine their contribution to naturally acquired immunity. Antibodies to the SPAM domains of MSPDBL1 and MSPDBL2 as well as the DBL domain of MSPDBL1 were found to be associated with protection from Plasmodium falciparum clinical episodes. Moreover, affinity-purified anti-MSPDBL1 and MSPDBL2 were found to inhibit in vitro parasite growth and had strong merozoite opsonizing capacity, suggesting that protection targeting these antigens results from ≥2 distinct effector mechanisms. Together these results indicate that MSPDBL1 and MSPDBL2 are important targets of naturally acquired immunity and might constitute potential vaccine candidates.

Association of antibodies to Plasmodium falciparum reticulocyte binding protein homolog 5 with protection from clinical malaria.

Emerging evidence suggests that antibodies against merozoite proteins involved in Plasmodium falciparum invasion into the red blood cell (RBC) play an important role in clinical immunity to malaria. The protein family of parasite antigens known as P. falciparum reticulocyte binding protein-like homolog (PfRh) is required for RBC invasion. PfRh5 is the only member within the PfRh family that cannot be genetically deleted, suggesting it plays an essential role in parasite survival. This antigen forms a complex with the cysteine-rich P. falciparum Rh5 interacting protein (PfRipr), on the merozoite surface during RBC invasion. The PfRh5 ectodomain sequence and a C-terminal fragment of PfRipr were cloned and expressed in Escherichia coli and baculovirus-infected cells, respectively. Immunization of rabbits with these recombinant proteins induced antibodies able to inhibit growth of various P. falciparum strains. Antibody responses to these proteins were investigated in a treatment–re-infection study conducted in an endemic area of Papua New Guinea (PNG) to determine their contribution to naturally acquired immunity. Antibody titers to PfRh5 but not PfRipr showed strong association with protection against P. falciparum clinical episodes. When associations with time-to-first infection were analyzed, high antibody levels against PfRh5 were also found to be associated with protection from high-density infections but not from re-infection. Together these results indicate that PfRh5 is an important target of protective immunity and constitutes a promising vaccine candidate.

Genetic and Household Determinants of Predisposition to Human Hookworm Infection in a Brazilian Community

Predisposition to heavy or light human hookworm infection is consistently reported in treatment-reinfection studies. A significant role for host genetics in determining hookworm infection intensity has also been shown, but the relationship between host genetics and predisposition has not been investigated. A treatment-reinfection study was conducted among 1302 individuals in Brazil. Bivariate variance components analysis was used to estimate heritability for pretreatment and reinfection intensity and to estimate the contribution of genetic and household correlations between phenotypes to the overall phenotypic correlation (ie, predisposition). Heritability for hookworm egg count was 17% before treatment and 25% after reinfection. Predisposition to heavy or light hookworm infection was observed, with a phenotypic correlation of 0.34 between pretreatment and reinfection intensity. This correlation was reduced to 0.23 after including household and environmental covariates. Genetic and household correlations were 0.41 and 1, respectively, and explained 88% of the adjusted phenotypic correlation. Predisposition to human hookworm infection in this area results from a combination of host genetics and consistent differences in exposure, with the latter explained by household and environmental factors. Unmeasured individual-specific differences in exposure did not contribute to predisposition.

THE RISK OF MALARIAL INFECTIONS AND DISEASE IN PAPUA NEW GUINEAN CHILDREN

In a treatment re-infection study of 206 Papua New Guinean school children, we examined risk of reinfection and symptomatic malaria caused by different Plasmodium species. Although children acquired a similar number of polymerase chain reaction-detectable Plasmodium falciparum and P. vivax infections in six months of active follow-up (P. falciparum = 5.00, P. vivax = 5.28), they were 21 times more likely to develop symptomatic P. falciparum malaria (1.17/year) than P. vivax malaria (0.06/year). Children greater than nine years of age had a reduced risk of acquiring P. vivax infections of low-to-moderate (>150/microL) density (adjusted hazard rate [AHR] = 0.65 and 0.42), whereas similar reductions in risk with age of P. falciparum infection was only seen for parasitemias > 5,000/microL (AHR = 0.49) and symptomatic episodes (AHR = 0.51). Infection and symptomatic episodes with P. malariae and P. ovale were rare. By nine years of age, children have thus acquired almost complete clinical immunity to P. vivax characterized by a very tight control of parasite density, whereas the acquisition of immunity to symptomatic P. falciparum malaria remained incomplete. These observations suggest that different mechanisms of immunity may be important for protection from these malaria species.

PRO-INFLAMMATORY CYTOKINES AND C-REACTIVE PROTEIN ARE ASSOCIATED WITH UNDERNUTRITION IN THE CONTEXT OF SCHISTOSOMA JAPONICUM INFECTION

Schistosomiasis is associated with undernutrition, but the mechanisms involved remain unknown. We analyzed baseline and follow-up data from a longitudinal treatment-reinfection study in N = 477 Schistosoma japonicum-infected subjects 7-20 years of age from Leyte, the Philippines. After baseline treatment with praziquantel, follow-up visits were scheduled every 3 months for 18 months; stool, venous blood, and anthropometric measurements were collected at each visit. Cytokine production by peripheral blood mononuclear cells (PBMCs) stimulated with specific S. japonicum antigens was measured once 4 weeks after treatment. After adjustment for confounders, S. japonicum intensity was associated with decreased serum albumin and Z-scores (all P < 0.05) and with increased serum C-reactive protein (CRP) and interleukin (IL)-6. CRP was associated with decreased albumin and Z-scores (all P < 0.01). Production of IL-1b and tumor necrosis factor (TNF)-alpha in response to worm antigen was associated with decreased albumin (both P < 0.005) and height-for-age Z-score (TNF-alpha only, P = 0.05). S. japonicum-associated undernutrition may, in part, result directly from inflammation.

T-Helper-2 Cytokine Responses to Sj97 Predict Resistance to Reinfection withSchistosoma japonicum

Although schistosomiasis is effectively treated with Praziquantel, rapid reinfection with rebound morbidity precludes effective control based on chemotherapy alone and justifies current efforts to develop vaccines for these parasites. Using a longitudinal treatment-reinfection study design with 616 participants 7 to 30 years of age, we evaluated the relationship between cytokine responses to Schistosoma japonicum soluble adult worm extract (SWAP), Sj97, Sj22.6, and Sj67, measured 4 weeks after treatment with Praziquantel, and resistance to reinfection in a population from Leyte, The Philippines, where S. japonicum is endemic. S. japonicum transmission was high: 54.8% and 91.1% were reinfected within 6 and 18 months, respectively. A Th2 bias in the following cytokine ratios, interleukin-4 (IL-4)/IL-12, IL-5/IL-12, IL-13/IL-12, IL-4/gamma-IFN (IFN-gamma), IL-5/IFN-gamma, and IL-13/IFN-gamma, in response to SWAP predicted a 1.4- to 2.9-month longer time to reinfection (P < 0.05) and a 27 to 55% lower intensity of reinfection (P < 0.05). Similarly, a Th2 bias in response to Sj97 predicted a 1.6- to 2.2-month longer time to reinfection (P < 0.05) and a 30 to 41% lower intensity of reinfection (P < 0.05). Only a high IL-5/IL-10 ratio in response to Sj22.6 predicted a 3.0-month-longer time to reinfection (P = 0.03). Cytokine responses to Sj67 were not associated with protection. In a large population-based treatment-reinfection study we found that Th2 responses to SWAP and Sj97 consistently predicted resistance to reinfection. These findings underscore Th2-type immune responses as central in human resistance to S. japonicum and support Sj97 as a leading vaccine candidate for this parasite.

Immune responses in human necatoriasis: association between interleukin-5 responses and resistance to reinfection.

Cytokine and proliferative responses to Necator americanus infection were measured in a treatment-reinfection study of infected subjects from an area of Papua New Guinea where N. americanus is highly endemic. Before treatment, most subjects produced detectable interleukin (IL)-4 (97%), IL-5 (86%), and interferon (IFN)- gamma (64%) in response to adult N. americanus antigen. Pretreatment IFN- gamma responses were negatively associated with hookworm burden, decreasing by 18 pg/mL for each increase of 1000 eggs/gram (epg) (n=75; P<.01). Mean IFN- gamma responses increased significantly after anthelmintic treatment, from 166 to 322 pg/mL (n=42; P<.01). The intensity of reinfection was significantly negatively correlated with pretreatment IL-5 responses, decreasing by 551 epg for each 100 pg/mL increase in production of IL-5 (n=51; P<.01). These data indicate that there is a mixed cytokine response in necatoriasis, with worm burden-associated suppression of IFN- gamma responses to adult N. americanus antigen. Resistance to reinfection is associated with the parasite-specific IL-5 response.

Antibodies to the Plasmodium falciparum antigens circumsporozoite protein, thrombospondin-related adhesive protein, and liver-stage antigen 1 vary by ages of subjects and by season in a highland area of Kenya.

Immunoglobulin G (IgG) antibodies to three vaccine candidate preerythrocytic Plasmodium falciparum antigens were evaluated in children and adults in an epidemic-prone highland area of Kenya during rainy (high-transmission) and dry (low-transmission) seasons. The frequencies and median levels of IgG antibodies to circumsporozoite protein (CSP) and thrombospondin-related adhesive protein (TRAP) were compared to the frequencies and median levels of IgG antibodies to liver-stage antigen 1 (LSA-1) reported previously. The frequencies and median levels of IgG antibodies to CSP and TRAP were similar in children and adults in the rainy season, but they were lower in children than in adults in the dry season. The frequencies and median levels of antibodies to LSA-1 were lower in children than in adults in both the rainy and dry seasons. Antibodies to CSP and LSA-1 were primarily members of the IgG1 and IgG3 subclasses, while antibodies to TRAP were primarily members of the IgG3 and IgG4 subclasses. In a treatment-reinfection study following dry season testing, antibodies to TRAP were associated with a trend toward protection from infection in children (P = 0.051) but not in adults. Antibodies to LSA-1 and CSP did not correlate with protection in children or adults. In this highland area of Kenya with unstable transmission, IgG antibodies to preerythrocytic P. falciparum antigens vary in subjects by age and season, and the protective effects of these antibodies against infection may be different in adults and children.

Factors influencing resistance to reinfection with Plasmodium falciparum.

A treatment-reinfection study design was used to investigate the relationships between host immunologic and/or genetic factors and resistance to reinfection with Plasmodium falciparum. Sixty-one children in Gabon were enrolled in a cross-sectional study to measure the prevalence of each human plasmodial species. All were given amodiaquine for radical cure of parasites, and 40 were subsequently followed-up for 30 weeks. Successive blood smears were examined to measure the delay of reappearance in blood of asexual stages of P. falciparum parasites. Presence of infection during the cross-sectional survey was associated with male sex, non-deficient glucose-6-phosphate dehydrogenase activity, plasma interleukin-10 level, and anti-LSA-Rep antibody concentration. Resistance to reinfection was related to the presence of anti-LSA-J antibodies, and the absence of anti-LSA-Rep antibodies. Moreover, P. malariae-infected subjects were usually co-infected with P. falciparum, and were also more rapidly reinfected with P. falciparum after treatment, compared with those without P. malariae infection.