Impact of a Rapid Decline in Malaria Transmission on Antimalarial IgG Subclasses and Avidity

The impact of Plasmodium-driven immunoregulatory networks on immunity to malaria

Malaria, caused by infection with Plasmodium parasites, drives multiple regulatory responses across the immune landscape. These regulatory responses help to protect against inflammatory disease but may in some situations hamper the acquisition of adaptive immune responses that clear parasites. In addition, the regulatory responses that occur during Plasmodium infection may negatively affect malaria vaccine efficacy in the most at-risk populations. Here, we discuss the specific cellular mechanisms of immunoregulatory networks that develop during malaria, with a focus on knowledge gained from human studies and studies that involve the main malaria parasite to affect humans, Plasmodium falciparum. Leveraging this knowledge may lead to the development of new therapeutic approaches to increase protective immunity to malaria during infection or after vaccination.

Identification and serological responses to a novel Plasmodium vivax merozoite surface protein 1 (PvMSP-1) derived synthetic peptide: a putative biomarker for malaria exposure

Background

The integration of diagnostic methods holds promise for advancing the surveillance of malaria transmission in both endemic and non-endemic regions. Serological assays emerge as valuable tools to identify and delimit malaria transmission, serving as a complementary method to rapid diagnostic tests (RDT) and thick smear microscopy. Here, we evaluate the potential of antibodies directed against peptides encompassing the entire amino acid sequence of the PvMSP-1 Sal-I strain as viable serological biomarkers for P. vivax exposure.

Methods

We screened peptides encompassing the complete amino acid sequence of the Plasmodium vivax Merozoite Surface Protein 1 (PvMSP-1) Sal-I strain as potential biomarkers for P. vivax exposure. Here, immunodominant peptides specifically recognized by antibodies from individuals infected with P. vivax were identified using the SPOT-synthesis technique followed by immunoblotting. Two 15-mer peptides were selected based on their higher and specific reactivity in immunoblotting assays. Subsequently, peptides p70 and p314 were synthesized in soluble form using SPPS (Solid Phase Peptide Synthesis) and tested by ELISA (IgG, and subclasses).

Results

This study unveils the presence of IgG antibodies against the peptide p314 in most P. vivax-infected individuals from the Brazilian Amazon region. In silico B-cell epitope prediction further supports the utilization of p314 as a potential biomarker for evaluating malaria transmission, strengthened by its amino acid sequence being part of a conserved block of PvMSP-1. Indeed, compared to patients infected with P. falciparum and uninfected individuals never exposed to malaria, P. vivax-infected patients have a notably higher recognition of p314 by IgG1 and IgG3.

Serological evaluation of risk factors for exposure to malaria in a pre-elimination setting in Malaysian Borneo

Malaysia has reported no indigenous cases of P. falciparum and P. vivax for over 3 years. When transmission reaches such low levels, it is important to understand the individuals and locations where exposure risks are high, as they may be at greater risk in the case of a resurgence of transmission. Serology is a useful tool in low transmission settings, providing insight into exposure over longer durations than PCR or RDT. We ran blood samples from a 2015 population-based survey in northern Sabah, Malaysian Borneo on a multiplex bead assay. Using supervised machine learning methods, we characterised recent and historic exposure to Plasmodium falciparum and P. vivax and found recent exposure to P. falciparum to be very low, with exposure to both species increasing with age. We performed a risk-factor assessment on environmental, behavioural, demographic and household factors, and identified forest activity and longer travel times to healthcare as common risk-factors for exposure to P. falciparum and P. vivax. In addition, we used remote-sensing derived data and geostatistical models to assess environmental and spatial associations with exposure. We created predictive maps of exposure to recent P. falciparum in the study area and showed 3 clear foci of exposure. This study provides useful insight into the environmental, spatial and demographic risk factors for P. falciparum and P. vivax at a period of low transmission in Malaysian Borneo. The findings would be valuable in the case of resurgence of human malarias in the region.

Malaria-driven expansion of adaptive-like functional CD56-negative NK cells correlates with clinical immunity to malaria

Natural killer (NK) cells likely play an important role in immunity to malaria, but the effect of repeated malaria on NK cell responses remains unclear. Here, we comprehensively profiled the NK cell response in a cohort of 264 Ugandan children. Repeated malaria exposure was associated with expansion of an atypical, CD56neg population of NK cells that differed transcriptionally, epigenetically, and phenotypically from CD56dim NK cells, including decreased expression of PLZF and the Fc receptor γ-chain, increased histone methylation, and increased protein expression of LAG-3, KIR, and LILRB1. CD56neg NK cells were highly functional and displayed greater antibody-dependent cellular cytotoxicity than CD56dim NK cells. Higher frequencies of CD56neg NK cells were associated with protection against symptomatic malaria and high parasite densities. After marked reductions in malaria transmission, frequencies of these cells rapidly declined, suggesting that continuous exposure to Plasmodium falciparum is required to maintain this modified, adaptive-like NK cell subset.

Assessment of IgG3 as a serological exposure marker for Plasmodium vivax in areas with moderate-high malaria transmission intensity

A more sensitive surveillance tool is needed to identify Plasmodium vivax infections for treatment and to accelerate malaria elimination efforts. To address this challenge, our laboratory has developed an eight-antigen panel that detects total IgG as serological markers of P. vivax exposure within the prior 9 months. The value of these markers has been established for use in areas with low transmission. In moderate-high transmission areas, there is evidence that total IgG is more long-lived than in areas with low transmission, resulting in poorer performance of these markers in these settings. Antibodies that are shorter-lived may be better markers of recent infection for use in moderate-high transmission areas. Using a multiplex assay, the antibody temporal kinetics of total IgG, IgG1, IgG3, and IgM against 29 P. vivax antigens were measured over 36 weeks following asymptomatic P. vivax infection in Papua New Guinean children (n = 31), from an area with moderate-high transmission intensity. IgG3 declined faster to background than total IgG, IgG1, and IgM. Based on these kinetics, IgG3 performance was then assessed for classifying recent exposure in a cohort of Peruvian individuals (n = 590; age 3-85 years) from an area of moderate transmission intensity. Using antibody responses against individual antigens, the highest performance of IgG3 in classifying recent P. vivax infections in the prior 9 months was to one of the Pv-fam-a proteins assessed (PVX_125728) (AUC = 0.764). Surprisingly, total IgG was overall a better marker of recent P. vivax infection, with the highest individual classification performance to RBP2b_1986-2653 (PVX_094255) (AUC = 0.838). To understand the acquisition of IgG3 in this Peruvian cohort, relevant epidemiological factors were explored using a regression model. IgG3 levels were positively associated with increasing age, living in an area with (relatively) higher transmission intensity, and having three or more PCR-detected blood-stage P. vivax infections within the prior 13 months. Overall, we found that IgG3 did not have high accuracy for detecting recent exposure to P. vivax in the Peruvian cohort, with our data suggesting that this is due to the high levels of prior exposure required to acquire high IgG3 antibody levels.

Antibody dynamics in children with first or repeat Plasmodium falciparum infections

Immunoglobulin (Ig) production during and after infection with Plasmodium parasites is one of the greatest adaptive immune defenses the human host has against this parasite. Infection with P. falciparum has been shown to induce different B cell maturation responses dependent upon the age of the patient, number of previous exposures, and severity of the disease. Described here are dynamics of Ig responses to a panel of 32 P. falciparum antigens by patients followed for 42 days and classified individuals as showing characteristics of an apparent first P. falciparum infection (naïve) or a repeat exposure (non-naïve). Six parameters were modeled to characterize the dynamics of IgM, IgG1, IgG3, and IgA for these two exposure groups with differences assessed among Ig isotypes/subclasses and unique antigens. Naïve patients had significantly longer periods of time to reach peak Ig titer (range 4–7 days longer) and lower maximum Ig titers when compared with non-naïve patients. Modeled time to seronegativity was significantly higher in non-naïve patients for IgM and IgA, but not for the two IgG subclasses. IgG1 responses to Rh2030, HSP40, and PfAMA1 were at the highest levels for non-naïve participants and may be used to predict previous or nascent exposure by themselves. The analyses presented here demonstrate the differences in the development of the Ig response to P. falciparum if the infection represents a boosting response or a primary exposure. Consistency in Ig isotype/subclasses estimates and specific data for P. falciparum antigens can better guide interpretation of seroepidemiological data among symptomatic persons.

Plasmodium vivax malaria serological exposure markers: Assessing the degree and implications of cross-reactivity with P. knowlesi

Serological markers are a promising tool for surveillance and targeted interventions for Plasmodium vivax malaria. P. vivax is closely related to the zoonotic parasite P. knowlesi, which also infects humans. P. vivax and P. knowlesi are co-endemic across much of South East Asia, making it important to design serological markers that minimize cross-reactivity in this region. To determine the degree of IgG cross-reactivity against a panel of P. vivax serological markers, we assayed samples from human patients with P. knowlesi malaria. IgG antibody reactivity is high against P. vivax proteins with high sequence identity with their P. knowlesi ortholog. IgG reactivity peaks at 7 days post-P. knowlesi infection and is short-lived, with minimal responses 1 year post-infection. We designed a panel of eight P. vivax proteins with low levels of cross-reactivity with P. knowlesi. This panel can accurately classify recent P. vivax infections while reducing misclassification of recent P. knowlesi infections.

Malaria transmission-blocking conjugate vaccine in ALFQ adjuvant induces durable functional immune responses in rhesus macaques

Malaria transmission-blocking vaccines candidates based on Pfs25 and Pfs230 have advanced to clinical studies. Exoprotein A (EPA) conjugate of Pfs25 in Alhydrogel^® developed functional immunity in humans, with limited durability. Pfs230 conjugated to EPA (Pfs230D1-EPA) with liposomal adjuvant AS01 is currently in clinical trials in Mali. Studies with these conjugates revealed that non-human primates are better than mice to recapitulate the human immunogenicity and functional activity. Here, we evaluated the effect of ALFQ, a liposomal adjuvant consisting of TLR4 agonist and QS21, on the immunogenicity of Pfs25-EPA and Pfs230D1-EPA in Rhesus macaques. Both conjugates generated strong antibody responses and functional activity after two vaccinations though activity declined rapidly. A third vaccination of Pfs230D1-EPA induced functional activity lasting at least 9 months. Antibody avidity increased with each vaccination and correlated strongly with functional activity. IgG subclass analysis showed induction of Th1 and Th2 subclass antibody levels that correlated with activity.

B Cell Receptor Repertoire Analysis in Malaria-Naive and Malaria-Experienced Individuals Reveals Unique Characteristics of Atypical Memory B Cells

Malaria, caused by parasites of the Plasmodium genus, is responsible for significant morbidity and mortality globally. Chronic Plasmodium falciparum exposure affects the B cell compartment, leading to the accumulation of atypical memory B cells (atMBCs). IgM-positive (IgM+) and IgG+ atMBCs have not been compared in-depth in the context of malaria, nor is it known if atMBCs in malaria-experienced individuals are different from phenotypically similar B cells in individuals with no known history of Plasmodium exposure. To address these questions, we characterized the B cell receptor (BCR) repertoire of naive B cells (NBCs), IgM+ and IgG+ classical MBCs (cMBCs), and IgM+ and IgG+ atMBCs from 13 malaria-naive American adults and 7 malaria-experienced Ugandan adults. Our results demonstrate that P. falciparum exposure mainly drives changes in atMBCs. In comparison to malaria-naive adults, the BCR repertoire of Plasmodium-exposed adults showed increased levels of somatic hypermutation in the heavy chain V region in IgM+ and IgG+ atMBCs, shorter heavy chain complementarity-determining region 3 (HCDR3) in IgG+ atMBCs, and increased usage of IGHV3-73 in IgG+ cMBCs and both IgM+ and IgG+ atMBCs. Irrespective of Plasmodium exposure, IgM+ atMBCs closely resembled NBCs, while IgG+ atMBCs resembled IgG+ cMBCs. Physicochemical properties of the HCDR3 seemed to be intrinsic to cell type and independent of malaria experience. The resemblance between atMBCs from Plasmodium-exposed and naive adults suggests similar differentiation pathways regardless of chronic antigen exposure. Moreover, these data demonstrate that IgM+ and IgG+ atMBCs are distinct populations that should be considered separately in future analyses. IMPORTANCE Malaria, caused by Plasmodium parasites, still contributes to a high global burden of disease, mainly in children under 5 years of age. Chronic and recurrent Plasmodium infections affect the development of B cell memory against the parasite and promote the accumulation of atypical memory B cells (atMBCs), which have an unclear function in the immune response. Understanding where these cells originate from and whether they are beneficial in the immune response to Plasmodium will help inform vaccination development efforts. We found differences in B cell receptor (BCR) properties of atMBCs between malaria-naive and malaria-experienced adults that are suggestive of divergent selection processes, resulting in more somatic hypermutation and differential immunoglobulin heavy chain V (IGHV) gene usage. Despite these differences, atMBCs from malaria-naive and malaria-experienced adults also showed many similarities in BCR characteristics, such as physicochemical properties of the HCDR3 region, suggesting that atMBCs undergo similar differentiation pathways in response to different pathogens. Our study provides new insights into the effects of malaria experience on the B cell compartment and the relationships between atMBCs and other B cell populations.