Pattern of antibody responses to Plasmodium falciparum antigens in individuals differentially exposed to Anopheles bites

Prevalence of culicine salivary antibodies in non-human primates living in national parks in Thailand

Macaques are widely distributed in Thailand with remarkable adaptation to living close to humans in residential, religious sites, markets and tourist areas. They play an essential role in the persistence of pathogens in the environment. As reservoir hosts, they are exposed to hematophagous vectors that secrete saliva, a cocktail of bioactive molecules including antigenic components stimulating host antibody production. Subsequent to the detection of mosquito-borne pathogens in macaques living in national parks, we aimed to determine the seroprevalence of antibodies to crude salivary gland extracts (SGEs) from culicine mosquitoes (Aedes aegypti [Linnaeus, 1762], Ae. albopictus [Skuse, 1895] and Culex quinquefasciatus [Say, 1823]) and compare individual titres between macaque species/national parks (33, Macaca arctoides [I. Geoffroy Saint-Hilaire, 1831] [Primates: Cercopithecidae] from Kaeng Krachan, 23 M. leonina leonina [Blyth, 1863] [Primates: Cercopithecidae] from Khao Yai and four M. fascicularis [Raffles, 1821] [Primates: Cercopithecidae] from Mu Ko Ranong). The anti-mosquito SGE antibodies found in 60 macaques from three national parks indicate varying levels of host-vector exposure. Macaque antibody titres were high against culicine mosquitoes. However, the significant difference among national parks (or macaque species) was only observed against Cx. quinquefasciatus. Correlation analysis of titres between Aedes SGE and arboviruses revealed a significantly more intense immune response against Ae. albopictus in DENV3-positive M. arctoides. Current findings support the concept of salivary biomarkers using accessible SGE, available from mosquito colonies of interest. However, we observed cross-reactivity between Aedes species because of crude SGE containing species-shared proteins. Nevertheless, a potential risk of pathogen transmission is emphasised between national park visitors and macaques via mosquitoes as bridge vectors. This information contributes to preventive measures against mosquito bites, including those implemented in tourist areas.

Biomarker of Anopheles exposure in Ghanaian children with haemoglobin S and C

Heterozygous carriers of haemoglobin S and C (HbAS and HbAC) have a reduced risk of severe malaria but are not protected from Plasmodium falciparum infection, suggesting that the protection involves acquired immunity. During a blood meal, female Anopheles mosquitoes inject saliva that can elicit a host antibody response, which can serve as a proxy for exposure to Plasmodium infection. Previous studies have shown that the peptide gSG6-P1 of An. gambiae saliva is antigenic and highly Anopheles specific. Here, we used plasma samples from 201 Ghanaian children with wild-type haemoglobin (HbAA), HbAS, and HbAC to evaluate antibody levels against gSG6-P1 as a serological biomarker of Anopheles exposure and, therefore of P. falciparum infection risk. Malaria antigen (PfCSP, GLURP, Pfs230, and HB3VAR06)-specific IgG levels, demographic data, and data regarding P. falciparum infection and malaria control practices were also analysed. Children with active P. falciparum infection had higher antibody levels against all antigens, and those with HbAS and HbAC had significantly higher antibody levels against Pfs230. Pfs230-specific IgG correlated negatively with gSG6-P1-specific IgG in children with HbAC. Our results highlight the importance of studying the role of haemoglobinopathies in malaria transmission to improve control interventions.

Mosquito bites and stage-specific antibody responses against Plasmodium falciparum in southern Ghana

BACKGROUND

The human host elicits specific immune responses after exposure to various life stages of the malaria parasite as well as components of mosquito saliva injected into the host during a mosquito bite. This study describes differences in IgG responses against antigens derived from the sporozoite (PfCSP), asexual stage parasite (PfEBA175) and the gametocyte (Pfs230), in addition to an Anopheles gambiae salivary gland antigen (gSG6-P1), in two communities in Ghana with similar blood stage malaria parasite prevalence.

METHODS

This study used archived plasma samples collected from an earlier cross-sectional study that enrolled volunteers aged from 6 months to 70 years from Simiw, peri-urban community (N = 347) and Obom, rural community (N = 291). An archived thick and thin blood smear for microscopy was used for the estimation of Plasmodium parasite density and species and DNA extraction from blood spots and P. falciparum confirmation was performed using PCR. This study used the stored plasma samples to determine IgG antibody levels to P. falciparum and Anopheles salivary antigens using indirect ELISA.

RESULTS

Individuals from Simiw had significantly higher levels of IgG against mosquito gSG6-P1 [median (95%CI)] [2.590 (2.452-2.783) ng/mL] compared to those from Obom [2.119 (1.957-2.345) ng/mL], p < 0.0001. Both IgG responses against Pfs230proC (p = 0.0006), and PfCSP (p = 0.002) were significantly lower in volunteers from Simiw compared to the participants from Obom. The seroprevalence of PfEBA-175.5R (p = 0.8613), gSG6-P1 (p = 0.0704), PfCSP (p = 0.7798) IgG were all similar in Obom and Simiw. However, Pfs230 seroprevalence was significantly higher at Obom compared to Simiw (p = 0.0006). Spearman correlation analysis showed no significant association between IgG responses against gSG6-P1, PfCSP, Pfs230proC and PfEBA-175.5R and parasite density at both Obom and Simiw (p > 0.05).

CONCLUSION

In conclusion, the study showed that participants from Simiw had higher concentrations of circulating gSG6-P1 IgG antibodies but lower concentrations of P. falciparum antibodies, PfCSP IgG and Pfs230proC IgG compared to participants from Obom.

Malaria: influence of Anopheles mosquito saliva on Plasmodium infection

Malaria is caused by Plasmodium protozoa that are transmitted by anopheline mosquitoes. Plasmodium sporozoites are released with saliva when an infected female mosquito takes a blood meal on a vertebrate host. Sporozoites deposited into the skin must enter a blood vessel to start their journey towards the liver. After migration out of the mosquito, sporozoites are associated with, or in proximity to, many components of vector saliva in the skin. Recent work has elucidated how Anopheles saliva, and components of saliva, can influence host-pathogen interactions during the early stage of Plasmodium infection in the skin. Here, we discuss how components of Anopheles saliva can modulate local host responses and affect Plasmodium infectivity. We hypothesize that therapeutic strategies targeting mosquito salivary proteins can play a role in controlling malaria and other vector-borne diseases.

Characterization of Anopheles gambiae D7 salivary proteins as markers of human-mosquito bite contact

BACKGROUND

Malaria is transmitted when infected Anopheles mosquitoes take a blood meal. During this process, the mosquitoes inject a cocktail of bioactive proteins that elicit antibody responses in humans and could be used as biomarkers of exposure to mosquito bites. This study evaluated the utility of IgG responses to members of the Anopheles gambiae D7 protein family as serological markers of human-vector contact.

METHODS

The D7L2, D7r1, D7r2, D7r3, D7r4 and SG6 salivary proteins from An. gambiae were expressed as recombinant antigens in Escherichia coli. Antibody responses to the salivary proteins were compared in Europeans with no prior exposure to malaria and lifelong residents of Junju in Kenya and Kitgum in Uganda where the intensity of malaria transmission is moderate and high, respectively. In addition, to evaluate the feasibility of using anti-D7 IgG responses as a tool to evaluate the impact of vector control interventions, we compared responses between individuals using insecticide-treated bednets to those who did not in Junju, Kenya where bednet data were available.

RESULTS

We show that both the long and short forms of the D7 salivary gland antigens elicit a strong antibody response in humans. IgG responses against the D7 antigens reflected the transmission intensities of the three study areas, with the highest to lowest responses observed in Kitgum (northern Uganda), Junju (Kenya) and malaria-naïve Europeans, respectively. Specifically, the long form D7L2 induced an IgG antibody response that increased with age and that was lower in individuals who slept under a bednet, indicating its potential as a serological tool for estimating human-vector contact and monitoring the effectiveness of vector control interventions.

CONCLUSIONS

This study reveals that D7L2 salivary antigen has great potential as a biomarker of exposure to mosquito bites and as a tool for assessing the efficacy of vector control strategies such as bednet use.

Anopheles salivary antigens as serological biomarkers of vector exposure and malaria transmission: A systematic review with multilevel modelling

Background

Entomological surveillance for malaria is inherently resource-intensive and produces crude population-level measures of vector exposure which are insensitive in low-transmission settings. Antibodies against Anopheles salivary proteins measured at the individual level may serve as proxy biomarkers for vector exposure and malaria transmission, but their relationship is yet to be quantified.

Methods

A systematic review of studies measuring antibodies against Anopheles salivary antigens (PROSPERO: CRD42020185449). Multilevel modelling (to account for multiple study-specific observations [level 1], nested within study [level 2], and study nested within country [level 3]) estimated associations between seroprevalence with Anopheles human biting rate (HBR) and malaria transmission measures.

Results

From 3981 studies identified in literature searches, 42 studies across 16 countries were included contributing 393 study-specific observations of anti-Anopheles salivary antibodies determined in 42,764 samples. A positive association between HBR (log transformed) and seroprevalence was found; overall a twofold (100% relative) increase in HBR was associated with a 23% increase in odds of seropositivity (OR: 1.23, 95% CI: 1.10-1.37; p<0.001). The association between HBR and Anopheles salivary antibodies was strongest with concordant, rather than discordant, Anopheles species. Seroprevalence was also significantly positively associated with established epidemiological measures of malaria transmission: entomological inoculation rate, Plasmodium spp. prevalence, and malarial endemicity class.

Conclusions

Anopheles salivary antibody biomarkers can serve as a proxy measure for HBR and malaria transmission, and could monitor malaria receptivity of a population to sustain malaria transmission. Validation of Anopheles species-specific biomarkers is important given the global heterogeneity in the distribution of Anopheles species. Salivary biomarkers have the potential to transform surveillance by replacing impractical, inaccurate entomological investigations, especially in areas progressing towards malaria elimination.

Funding

Australian National Health and Medical Research Council, Wellcome Trust.

First evaluation of antibody responses to Culex quinquefasciatus salivary antigens as a serological biomarker of human exposure to Culex bites: A pilot study in Côte d'Ivoire

Background

Culex mosquitoes are vectors for a variety of pathogens of public health concern. New indicators of exposure to Culex bites are needed to evaluate the risk of transmission of associated pathogens and to assess the efficacy of vector control strategies. An alternative to entomological indices is the serological measure of antibodies specific to mosquito salivary antigens. This study investigated whether the human IgG response to both the salivary gland extract and the 30 kDa salivary protein of Culex quinquefasciatus may represent a proxy of human exposure to Culex bites.

Methodology/Principal findings

A multidisciplinary survey was conducted with children aged 1 to 14 years living in neighborhoods with varying exposure to Culex quinquefasciatus in the city of Bouaké, Côte d’Ivoire. Children living in sites with high exposure to Cx quinquefasciatus had a significantly higher IgG response to both salivary antigens compared with children living in the control site where only very few Culex were recorded. Moreover, children from any Culex-high exposed sites had significantly higher IgG responses only to the salivary gland extract compared with children from the control village, whereas no difference was noted in the anti-30 kDa IgG response. No significant differences were noted in the specific IgG responses between age and gender. Sites and the use of a bed net were associated with the level of IgG response to the salivary gland extract and to the 30 kDa antigen, respectively.

Conclusions/Significance

These findings suggest that the IgG response to Culex salivary gland extracts is suitable as proxy of exposure; however, the specificity to the Culex genus needs further investigation. The lower antigenicity of the 30 kDa recombinant protein represents a limitation to its use. The high specificity of this protein to the Culex genus makes it an attractive candidate and other specific antibody responses might be more relevant as a biomarker of exposure. These epidemiological observations may form a starting point for additional work on developing serological biomarkers of Culex exposure.

The evaluation of exposure to mosquitoes is a key parameter in assessing the risk of transmission of associated pathogens, including zoonoses. Entomological methods represent the gold standard but have several limitations, and efforts are being made to develop new indicators to accurately assess human–Culex contact. This study showed the IgG response to Culex quinquefasciatus salivary gland extract is suitable proxy of exposure to Culex bites. The lower antigenicity of the 30 kDa recombinant protein represents a limitation to its use. The high specificity of this protein to the Culex genus makes it an attractive candidate and other isotypic antibody responses specific to this salivary antigen might be more relevant as a biomarker of exposure.

Influence of Host-Related Factors and Exposure to Mosquito Bites on the Dynamics of Antibody Response to Plasmodium falciparum Antigens

Humoral immunity to Plasmodium falciparum is acquired after repeated infections, and can lead to clinical protection. This study aimed to evaluate how human-, parasite-, and environment-related determinants can modulate the dynamics of IgG responses to Plasmodium falciparum after an infection. Individuals (n = 68, average age = 8.2 years) with uncomplicated malaria were treated with ACT and followed up for 42 days. IgG responses to P. falciparum merozoite antigens (PfMSP1, PfMSP3, PfAMA1, PfGLURP-R0), to whole schizont extract (PfSchz), and to Anopheles gSG6-P1 and Aedes Nterm–34 kDa salivary peptides were measured. Regression analyses were used to identify factors that influence the dynamics of IgG response to P. falciparum antigen between D0 and D42, including demographic and biological factors and the level of exposure to mosquito bites. The dynamics of IgG response to P. falciparum differed according to the antigen. According to multivariate analysis, IgG responses to PfSchz and to PfGLURP-R0 appear to be affected by exposure to Aedes saliva and are associated with age, parasite density, and anti-Plasmodium pre-existing immune response at study inclusion. The present work shows that human exposure to Aedes saliva may contribute, in addition to other factors, to the regulation of anti-Plasmodium immune responses during a natural infection.