Human Malaria Infection Research Articles

Role of human Pegivirus infections in whole Plasmodium falciparum sporozoite vaccination and controlled human malaria infection in African volunteers

BackgroundDiverse vaccination outcomes and protection levels among different populations pose a serious challenge to the development of an effective malaria vaccine. Co-infections are among many factors associated with immune dysfunction and sub-optimal vaccination outcomes. Chronic, asymptomatic viral infections can contribute to the modulation of vaccine efficacy through various mechanisms. Human Pegivirus-1 (HPgV-1) persists in immune cells thereby potentially modulating immune responses. We investigated whether Pegivirus infection influences vaccine-induced responses and protection in African volunteers undergoing whole P. falciparum sporozoites-based malaria vaccination and controlled human malaria infections (CHMI).MethodsHPgV-1 prevalence was quantified by RT-qPCR in plasma samples of 96 individuals before, post vaccination with PfSPZ Vaccine and after CHMI in cohorts from Tanzania and Equatorial Guinea. The impact of HPgV-1 infection was evaluated on (1) systemic cytokine and chemokine levels measured by Luminex, (2) PfCSP-specific antibody titers quantified by ELISA, (3) asexual blood-stage parasitemia pre-patent periods and parasite multiplication rates, (4) HPgV-1 RNA levels upon asexual blood-stage parasitemia induced by CHMI.ResultsThe prevalence of HPgV-1 was 29.2% (28/96) and sequence analysis of the 5′ UTR and E2 regions revealed the predominance of genotypes 1, 2 and 5. HPgV-1 infection was associated with elevated systemic levels of IL-2 and IL-17A. Comparable vaccine-induced anti-PfCSP antibody titers, asexual blood-stage multiplication rates and pre-patent periods were observed in HPgV-1 positive and negative individuals. However, a tendency for higher protection levels was detected in the HPgV-1 positive group (62.5%) compared to the negative one (51.6%) following CHMI. HPgV-1 viremia levels were not significantly altered after CHMI.ConclusionsHPgV-1 infection did not alter PfSPZ Vaccine elicited levels of PfCSP-specific antibody responses and parasite multiplication rates. Ongoing HPgV-1 infection appears to improve to some degree protection against CHMI in PfSPZ-vaccinated individuals. This is likely through modulation of immune system activation and systemic cytokines as higher levels of IL-2 and IL17A were observed in HPgV-1 infected individuals. CHMI is safe and well tolerated in HPgV-1 infected individuals. Identification of cell types and mechanisms of both silent and productive infection in individuals will help to unravel the biology of this widely present but largely under-researched virus.

T-follicular helper cells in malaria infection and roles in antibody induction.

Immunity to malaria is mediated by antibodies that block parasite replication to limit parasite burden and prevent disease. Cytophilic antibodies have been consistently shown to be associated with protection, and recent work has improved our understanding of the direct and Fc-mediated mechanisms of protective antibodies. Antibodies also have important roles in vaccine-mediated immunity. Antibody induction is driven by the specialized CD4+ T cells, T-follicular helper (Tfh) cells, which function within the germinal centre to drive B-cell activation and antibody induction. In humans, circulating Tfh cells can be identified in peripheral blood and are differentiated into subsets that appear to have pathogen/vaccination-specific roles in antibody induction. Tfh cell responses are essential for protective immunity from Plasmodium infection in murine models of malaria. Our understanding of the activation of Tfh cells during human malaria infection and the importance of different Tfh cell subsets in antibody development is still emerging. This review will discuss our current knowledge of Tfh cell activation and development in malaria, and the potential avenues and pitfalls of targeting Tfh cells to improve malaria vaccines.

A Molecular Signature for Il-10-Producing Th1 Cells in Protozoan Parasitic Diseases

Control of the intracellular parasites causing malaria and visceral leishmaniasis (VL) is dependent on IFNγ+Tbet+CD4+ T (Th1) cells generated by infected hosts. IL-10 produced by Th1 cells mitigates subsequent inflammation and related pathology. However, IL-10-producing Th1 (Tr1) cells can also promote parasite persistence, as well as impair immunity following re-infection or vaccination. Here, we identify molecular and phenotypic signatures that distinguish Th1 cells from Tr1 cells in experimental VL caused by Leishmania donovani infection of C57BL/6 mice and in Plasmodium falciparum-infected humans participating in controlled human malaria infection (CHMI) studies. We identify LAG3 as an immune checkpoint target in VL, describe heterogeneity of co-inhibitory receptor expression by Tr1 cells and discovered a role for the transcription factor Pbx1 in suppressing CD4+ T cell cytokine production. Together, this work provides new insights into Tr1 cell functions that offer opportunities for strategies to modulate their functions to improve anti-parasitic immunity.

1241. PfSPZ Vaccine Administered by Direct Venous Inoculation to Prevent Plasmodium falciparum Malaria is as Safe as Normal Saline – a Meta-analysis of 12 Randomized Controlled Clinical Trials

BackgroundSanaria’s PfSPZ Vaccine prevents Plasmodium falciparum (Pf) infection transmitted in the field and by controlled human malaria infection. Safety of PfSPZ Vaccine has been demonstrated in 12 randomized, double-blind, placebo-controlled trials (RCT) varying in regimen from 3 to 5 doses over 4 to 20 weeks and in size from 18 to 332 subjects in adults in the US and EU and 5-month to 65-year-olds in 5 countries in sub-Saharan Africa. This study was conducted to analyze solicited adverse event (AE) and laboratory data by random effects meta-analysis.MethodsPfSPZ Vaccine is composed of radiation-attenuated, aseptic, purified, cryopreserved Pf sporozoites (SPZ) administered by direct venous inoculation (DVI). Normal saline (NS) is always the placebo. Data from all completed RCTs were included as either age > 18 years (n=598) or age 5 months to 17 years (n=641). Any subject receiving at least one dose was included. A random-effects model was used to study vaccine safety and I2 to evaluate heterogeneity. Analysis was performed for any systemic solicited AE and for the most frequently observed AEs and laboratory abnormalities. Sensitivity analyses were performed by removal of trials with zero events to evaluate potential bias.ResultsWhen examined individually, only 1 trial had a significant difference between PfSPZ Vaccine and NS for any AE (myalgias in adults). In the adult meta-analysis, there was no difference in the random effects risk ratios (RR) for having any vaccine-related AEs (1.40, 95% confidence interval (CI) 0.88-2.28), or for fever (0.75, 0.24-2.35), headache (1.23, 0.74-2.02), fatigue (0.72, 0.19-2.54), or myalgia (1.09, 0.26-4.68). In the pediatric meta-analysis there was no difference between the RR for PfSPZ Vaccine and NS for any AE (0.84, 0.59-1.18) or for fever (1.09, 0.44-2.69). No significant differences in the most common grade 2 or higher laboratory abnormalities – declines in hemoglobin, neutrophil or platelet count – were detected. Sensitivity analysis did not change the results.ConclusionThere was no difference in risk for AEs or lab abnormalities between PfSPZ Vaccine and NS, indicating that PfSPZ Vaccine administered by DVI was extremely safe and well tolerated in 5-month- to 65-year-olds.DisclosuresLW Preston Church, MD, FIDSA, Sanaria Inc. (Employee)

Magnitude, Specificity, and Avidity of Sporozoite-Specific Antibodies Associate With Protection Status and Distinguish Among RTS,S/AS01 Dose Regimens

Abstract Background The malaria vaccine, RTS,S/AS01, demonstrated an enhanced efficacy (86.7%) in a delayed third fractional dose (0.1.7Fx) regimen in controlled human malaria infection trials compared with a standard full-dose (0.1.2) regimen (62.5%). To understand the humoral component of the RTS,S/AS01 vaccine-induced protection against sporozoite infection in these 2 regimens, we investigated the serum antibody dynamics of 0.1.2 and 0.1.7Fx groups vaccinees. Methods The specific binding responses (magnitude) and dissociation rates (avidity) of serum antibodies interaction with a recombinant Plasmodium falciparum circumsporozoite protein (CSP) and peptides corresponding to the central repeat region (NANP6), the C-terminal region (PF16), and the N-terminal junction (N-interface) of CSP, respectively, were measured using a Biolayer Interferometry assay. Results On the day of challenge, higher NANP6-specific antibody responses were associated with protection in the 0.1.2 group. In contrast, slower antibody dissociation rates for CSP and PF16 binding were observed in the protected 0.1.7Fx group. Protected vaccinees of both groups exhibited 2- to 3-fold higher N-interface peptide binding antibody responses. Conclusions Unlike the standard dose, the delayed-fractional third dose of RTS,S/AS01 induced higher avidity CSP and PF16 binding antibodies that were associated with protection against sporozoite infection.

Host and Environmental Factors that Influence Plasmodium knowlesi Malaria Infection in Humans: A Systematic Review

Introduction: Plasmodium knowlesi (P.knowlesi) is a zoonotic malaria parasite, transmitted between non-human primate hosts by the Anopheles (An.) mosquitos, and causing spill-over infections in humans where the parasite, vector, host, and human converge.
 Methods: The search was done electronically to explore for appropriate papers via PubMed, and Science Direct for articles published up to March 2020, containing the words “factors associated” or “environmental factors” or “individual factors” or “ecological factors” and “P.knowlesi” and “human” including synonyms and Medical Subject Heading (MeSH) terms. A total of 27 articles from PubMed Databases and 18 articles from Science Direct were selected to be assessed for eligibility. Out of it, a total of 13 articles were selected to be analysed.
 Results: Host factors such as sex and age, as well as occupation as individual factors, while environmental factors such as rainfall and geographic elevation have some association with P.knowlesi infection in humans. This zoonotic malaria poses unique challenges that will need to be addressed if all forms of malaria are to be eliminated based on the sustainable development goal (SDG).
 Conclusion: This article highlights the importance of disease ecologies such as climate and landscape and human-environment interactions such as the land use patterns, such as agriculture or infrastructure activities) to reduce the further increase of cases and mortality globally due to P.knowlesi infection. This review focuses mainly on the host and environmental factors that influence P.knowlesi Malaria Infection in Humans.

Structure of the Plasmodium-interspersed repeat proteins of the malaria parasite

The deadly symptoms of malaria occur as Plasmodium parasites replicate within blood cells. Members of several variant surface protein families are expressed on infected blood cell surfaces. Of these, the largest and most ubiquitous are the Plasmodium-interspersed repeat (PIR) proteins, with more than 1,000 variants in some genomes. Their functions are mysterious, but differential pir gene expression associates with acute or chronic infection in a mouse malaria model. The membership of the PIR superfamily, and whether the family includes Plasmodium falciparum variant surface proteins, such as RIFINs and STEVORs, is controversial. Here we reveal the structure of the extracellular domain of a PIR from Plasmodium chabaudi We use structure-guided sequence analysis and molecular modeling to show that this fold is found across PIR proteins from mouse- and human-infective malaria parasites. Moreover, we show that RIFINs and STEVORs are not PIRs. This study provides a structure-guided definition of the PIRs and a molecular framework to understand their evolution.

Controlled Human Malaria Infection Induces Long-Term Functional Changes in Monocytes.

Innate immune memory responses (also termed “trained immunity”) have been described in monocytes after BCG vaccination and after stimulation in vitro with microbial and endogenous ligands such as LPS, β-glucan, oxidized LDL, and monosodium urate crystals. However, whether clinical infections are also capable of inducing a trained immunity phenotype remained uncertain. We evaluated whether Plasmodium falciparum infection can induce innate immune memory by measuring monocyte-derived cytokine production from five volunteers undergoing Controlled Human Malaria Infection. Monocyte responses followed a biphasic pattern: during acute infection, monocytes produced lower amounts of inflammatory cytokines upon secondary stimulation, but 36 days after malaria infection they produced significantly more IL-6 and TNF-α in response to various stimuli. Furthermore, transcriptomic and epigenomic data analysis revealed a clear reprogramming of monocytes at both timepoints, with long-term changes of H3K4me3 at the promoter regions of inflammatory genes that remain present for several weeks after parasite clearance. These findings demonstrate an epigenetic basis of trained immunity induced by human malaria in vivo.

Dynamics and Outcomes of Plasmodium Infections in Grammomys surdaster (Grammomys dolichurus) Thicket Rats versus Inbred Mice

Investigations of malaria infection are often conducted by studying rodent Plasmodium species in inbred laboratory mice, but the efficacy of vaccines or adjunctive therapies observed in these models often does not translate to protection in humans. This raises concerns that mouse malaria models do not recapitulate important features of human malaria infections. African woodland thicket rats (Grammomys surdaster) are the natural host for the rodent malaria parasite Plasmodium berghei and the suspected natural host for Plasmodium vinckei vinckei. Previously, we reported that thicket rats are highly susceptible to diverse rodent parasite species, including P. berghei, Plasmodium yoelii, and Plasmodium chabaudi chabaudi, and are a more stringent model to assess the efficacy of whole-sporozoite vaccines than laboratory mice. Here, we compare the course of infection and virulence with additional rodent Plasmodium species, including various strains of P. berghei, P. yoelii, P. chabaudi, and P. vinckei, in thicket rats versus laboratory mice. We present evidence that rodent malaria parasite growth typically differs between the natural versus nonnatural host; G. surdaster limit infection by multiple rodent malaria strains, delaying and reducing peak parasitemia compared with laboratory mice. The course of malaria infection in thicket rats varied depending on parasite species and strain, resulting in self-cure, chronic parasitemia, or rapidly lethal infection, thus offering a variety of rodent malaria models to study different clinical outcomes in the natural host.

An open label study of the safety and efficacy of a single dose of weekly chloroquine and azithromycin administered for malaria prophylaxis in healthy adults challenged with 7G8 chloroquine-resistant Plasmodium falciparum in a controlled human malaria infection model

BackgroundMalaria remains the top infectious disease threat facing the U.S. military in many forward operating environments. Compliance with malaria chemoprophylaxis remains a critical component in preventing malaria in the deployed Service Member. Studies of previous military operations show that compliance is consistently higher with weekly versus daily dosing regimens. Current FDA approved weekly chemoprophylaxis options have contraindications that can limit prescribing. The combination of chloroquine (CQ) with azithromycin (AZ) has previously been shown to be an efficacious treatment option for malaria, has pharmacokinetics compatible with weekly dosing, and has shown synergy when combined in vitro.MethodsIn this open label study, 18 healthy volunteers, aged 18–50 years (inclusive), were randomly assigned to receive either 300 mg CQ or 300 mg CQ and 2 gm azithromycin (CQAZ) of directly observed therapy, weekly for 3 weeks prior to undergoing mosquito bite challenge with chloroquine-resistant Plasmodium falciparum. Volunteers that remained asymptomatic and had no evidence of parasitaemia continued to receive weekly post-exposure chemoprophylaxis for 3 weeks following malaria challenge. The primary endpoint was the number of volunteers that remained asymptomatic and had no evidence of parasitaemia 28 days after the malaria challenge.ResultsAll 6 (100%) volunteers randomized to the CQ control group became symptomatic with parasitaemia during the 28-day post-challenge period. Only 1/12 (8.3%) of volunteers in the CQAZ group developed symptoms and parasitaemia during the 28-day post-challenge period. However, after chemoprophylaxis was discontinued an additional 6 volunteers developed parasitaemia between days 28–41 after challenge, with 4 of 6 experiencing symptoms. 80% of subjects in the CQAZ group experienced treatment related gastrointestinal adverse events (including 13% that experienced severe nausea) compared to 38% in the CQ group. A comparison of the pharmacokinetics in the CQAZ group demonstrated higher azithromycin Cmax (p = 0.03) and AUC (p = 0.044) levels in those volunteers who never became parasitaemic compared to those who did.ConclusionGiven the high rate of side effects and poor efficacy when administered for 3 weeks before and after challenge, the combination of weekly chloroquine and azithromycin is a suboptimal regimen combination for weekly malaria chemoprophylaxis.Trial registration ClinicalTrials.gov NCT03278808

Multidose Priming and Delayed Boosting Improve Plasmodium falciparum Sporozoite Vaccine Efficacy Against Heterologous P. falciparum Controlled Human Malaria Infection.

A live-attenuated Plasmodium falciparum sporozoite (SPZ) vaccine (PfSPZ Vaccine) has shown up to 100% protection against controlled human malaria infection (CHMI) using homologous parasites (same P. falciparum strain as in the vaccine). Using a more stringent CHMI, with heterologous parasites (different P. falciparum strain), we assessed the impact of higher PfSPZ doses, a novel multi-dose prime regimen, and a delayed vaccine boost upon vaccine efficacy (VE). We immunized 4 groups that each contained 15 healthy, malaria-naive adults. Group 1 received 5 doses of 4.5 x 105 PfSPZ (Days 1, 3, 5, and 7; Week 16). Groups 2, 3, and 4 received 3 doses (Weeks 0, 8, and 16), with Group 2 receiving 9.0 × 105/doses; Group 3 receiving 18.0 × 105/doses; and Group 4 receiving 27.0 × 105 for dose 1 and 9.0 × 105 for doses 2 and 3. VE was assessed by heterologous CHMI after 12 or 24 weeks. Volunteers not protected at 12 weeks were boosted prior to repeat CHMI at 24 weeks. At 12-week CHMI, 6/15 (40%) participants in Group 1 (P = .04) and 3/15 (20%) participants in Group 2 remained aparasitemic, as compared to 0/8 controls. At 24-week CHMI, 3/13 (23%) participants in Group 3 and 3/14 (21%) participants in Group 4 remained aparasitemic, versus 0/8 controls (Groups 2-4, VE not significant). Postboost, 9/14 (64%) participants versus 0/8 controls remained aparasitemic (3/6 in Group 1, P = .025; 6/8 in Group 2, P = .002). Administering 4 stacked priming injections (multi-dose priming) resulted in 40% VE against heterologous CHMI, while dose escalation of PfSPZ using single-dose priming was not significantly protective. Boosting unprotected subjects improved VE at 24 weeks, to 64%. NCT02601716.

Proteome-wide analysis of a malaria vaccine study reveals personalized humoral immune profiles in Tanzanian adults.

Tanzanian adult male volunteers were immunized by direct venous inoculation with radiation-attenuated, aseptic, purified, cryopreserved Plasmodium falciparum (Pf) sporozoites (PfSPZ Vaccine) and protective efficacy assessed by homologous controlled human malaria infection (CHMI). Serum immunoglobulin G (IgG) responses were analyzed longitudinally using a Pf protein microarray covering 91% of the proteome, providing first insights into naturally acquired and PfSPZ Vaccine-induced whole parasite antibody profiles in malaria pre-exposed Africans. Immunoreactivity was identified against 2239 functionally diverse Pf proteins, showing a wide breadth of humoral response. Antibody-based immune 'fingerprints' in these individuals indicated a strong person-specific immune response at baseline, with little changes in the overall humoral immunoreactivity pattern measured after immunization. The moderate increase in immunogenicity following immunization and the extensive and variable breadth of humoral immune response observed in the volunteers at baseline suggest that pre-exposure reduces vaccine-induced antigen reactivity in unanticipated ways.

Safety, Pharmacokinetics, and Causal Prophylactic Efficacy of KAF156 in a Plasmodium falciparum Human Infection Study.

KAF156 is a novel antimalarial drug that is active against both liver- and blood-stage Plasmodium parasites, including drug-resistant strains. Here, we investigated the causal prophylactic efficacy of KAF156 in a controlled human malaria infection (CHMI) model. In part 1, healthy, malaria-naive participants received 800 mg KAF156 or placebo 3 hours before CHMI with P. falciparum-infected mosquitoes. In part 2, KAF156 was administered as single doses of 800, 300, 100, 50, or 20 mg 21 hours post-CHMI. All participants received atovaquone/proguanil treatment if blood-stage infection was detected or on day 29. For each cohort, 7-14 subjects were enrolled to KAF156 treatment and up to 4 subjects to placebo. KAF156 at all dose levels was safe and well tolerated. Two serious adverse events were reported-both resolved without sequelae and neither was considered related to KAF156. In part 1, all participants treated with KAF156 and none of those randomized to placebo were protected against malaria infection. In part 2, all participants treated with placebo or 20 mg KAF156 developed malaria infection. In contrast, 50 mg KAF156 protected 3 of 14 participants from infection, and doses of 800, 300, and 100 mg KAF156 protected all subjects against infection. An exposure-response analysis suggested that a 24-hour postdose concentration of KAF156 of 21.5 ng/mL (90% confidence interval, 17.66-25.32 ng/mL) would ensure a 95% chance of protection from malaria parasite infection. KAF156 was safe and well tolerated and demonstrated high levels of pre- and post-CHMI protective efficacy. NCT04072302.

Deliberately infecting healthy volunteers with malaria parasites: Perceptions and experiences of participants and other stakeholders in a Kenyan-based malaria infection study.

Controlled human malaria infection (CHMI) studies involve the deliberate infection of healthy volunteers with malaria parasites under controlled conditions to study immune responses and/or test drug or vaccine efficacy. An empirical ethics study was embedded in a CHMI study at a Kenyan research programme to explore stakeholders’ perceptions and experiences of deliberate infection and moral implications of these. Data for this qualitative study were collected through focus group discussions, in‐depth interviews and non‐participant observation. Sixty‐nine participants were involved, including CHMI study volunteers, community representatives and research staff. Data were managed using QSR Nvivo 10 and analysed using an inductive‐deductive approach, guided by ethics literature. CHMI volunteers had reasonable understanding of the study procedures. Decisions to join were influenced by study incentives, trust in the research institution, their assessment of associated burdens and motivation to support malaria vaccine development. However, deliberate malaria infection was a highly unusual research strategy for volunteers, community representatives and some study staff. Volunteers’ experiences of physical, emotional and social burdens or harms were often greater than anticipated initially, and fluctuated over time, related to specific procedures and events. Although unlikely to deter volunteers' participation in similar studies in furture, we argue that the dissonance between level of understanding of the burdens involved and actual experiences are morally relevant in relation to community engagement, informed consent processes, and ongoing support for volunteers and research staff. We further argue that ethics oversight of CHMI studies should take account of these issues in deciding whether consent, engagement and the balance of benefits and harms are reasonable in a given context.

IMRAS-A clinical trial of mosquito-bite immunization with live, radiation-attenuated P. falciparum sporozoites: Impact of immunization parameters on protective efficacy and generation of a repository of immunologic reagents.

BackgroundImmunization with radiation-attenuated sporozoites (RAS) by mosquito bite provides >90% sterile protection against Plasmodium falciparum (Pf) malaria in humans. RAS invade hepatocytes but do not replicate. CD8+ T cells recognizing parasite-derived peptides on the surface of infected hepatocytes are likely the primary protective mechanism. We conducted a randomized clinical trial of RAS immunization to assess safety, to achieve 50% vaccine efficacy (VE) against controlled human malaria infection (CHMI), and to generate reagents from protected and non-protected subjects for future identification of protective immune mechanisms and antigens.MethodsTwo cohorts (Cohort 1 and Cohort 2) of healthy, malaria-naïve, non-pregnant adults age 18–50 received five monthly immunizations with infected (true-immunized, n = 21) or non-infected (mock-immunized, n = 5) mosquito bites and underwent homologous CHMI at 3 weeks. Immunization parameters were selected for 50% protection based on prior clinical data. Leukapheresis was done to collect plasma and peripheral blood mononuclear cells.ResultsAdverse event rates were similar in true- and mock-immunized subjects. Two true- and two mock-immunized subjects developed large local reactions likely caused by mosquito salivary gland antigens. In Cohort 1, 11 subjects received 810–1235 infected bites; 6/11 (55%) were protected against CHMI vs. 0/3 mock-immunized and 0/6 infectivity controls (VE 55%). In Cohort 2, 10 subjects received 839–1131 infected bites with a higher first dose and a reduced fifth dose; 9/10 (90%) were protected vs. 0/2 mock-immunized and 0/6 controls (VE 90%). Three/3 (100%) protected subjects administered three booster immunizations were protected against repeat CHMI vs. 0/6 controls (VE 100%). Cohort 2 uniquely showed a significant rise in IFN-γ responses after the third and fifth immunizations and higher antibody responses to CSP.ConclusionsPfRAS were generally safe and well tolerated. Cohort 2 had a higher first dose, reduced final dose, higher antibody responses to CSP and significant rise of IFN-γ responses after the third and fifth immunizations. Whether any of these factors contributed to increased protection in Cohort 2 requires further investigation. A cryobank of sera and cells from protected and non-protected individuals was generated for future immunological studies and antigen discovery.Trial registrationClinicalTrials.gov NCT01994525.

Effect of immune regulatory pathways after immunization with GMZ2 malaria vaccine candidate in healthy lifelong malaria-exposed adults

BackgroundDespite appreciable immunogenicity in malaria-naive populations, many candidate malaria vaccines are considerably less immunogenic in malaria-exposed populations. This could reflect induction of immune regulatory mechanisms involving Human Leukocyte Antigen G (HLA-G), regulatory T (Treg), and regulatory B (Breg) cells. Here, we addressed the question whether there is correlation between these immune regulatory pathways and both plasmablast frequencies and vaccine-specific IgG concentrations. MethodsFifty Gabonese adults with lifelong exposure to Plasmodium spp were randomized to receive three doses of either 30 µg or 100 µg GMZ2-CAF01, or 100 µg GMZ2-alum, or control vaccine (rabies vaccine) at 4-week intervals. Only plasma and peripheral blood mononuclear cells isolated from blood samples collected before (D0) and 28 days after the third vaccination (D84) of 35 participants were used to measure sHLA-G levels and anti-GMZ2 IgG concentrations, and to quantify Treg, Breg and plasmablast cells. Vaccine efficacy was assessed using controlled human malaria infection (CHMI) by direct venous inoculation of Plasmodium falciparum sporozoites (PfSPZ Challenge). ResultsThe sHLA-G concentration increased from D0 to D84 in all GMZ2 vaccinated participants and in the control group, whereas Treg frequencies increased only in those receiving 30 µg or 100 µg GMZ2-CAF01. The sHLA-G level on D84 was associated with a decrease of the anti-GMZ2 IgG concentration, whereas Treg frequencies on D0 or on D84, and Breg frequency on D84 were associated with lower plasmablast frequencies. Importantly, having a D84:D0 ratio of sHLA-G above the median was associated with an increased risk of P. falciparum infection after sporozoites injection. ConclusionRegulatory immune responses are induced following immunization. Stronger sHLA-G and Treg immune responses may suppress vaccine induced immune responses, and the magnitude of the sHLA-G response increased the risk of Plasmodium falciparum infection after CHMI. These findings could have implications for the design and testing of malaria vaccine candidates in semi-immune individuals.

A delayed fractionated dose RTS,S AS01 vaccine regimen mediates protection via improved T follicular helper and B cell responses.

Malaria-071, a controlled human malaria infection trial, demonstrated that administration of three doses of RTS,S/AS01 malaria vaccine given at one-month intervals was inferior to a delayed fractional dose (DFD) schedule (62.5% vs 86.7% protection, respectively). To investigate the underlying immunologic mechanism, we analyzed the B and T peripheral follicular helper cell (pTfh) responses. Here, we show that protection in both study arms was associated with early induction of functional IL-21-secreting circumsporozoite (CSP)-specific pTfh cells, together with induction of CSP-specific memory B cell responses after the second dose that persisted after the third dose. Data integration of key immunologic measures identified a subset of non-protected individuals in the standard (STD) vaccine arm who lost prior protective B cell responses after receiving the third vaccine dose. We conclude that the DFD regimen favors persistence of functional B cells after the third dose.

The Ratiometric Transcript Signature MX2/GPR183 Is Consistently Associated With RTS,S-Mediated Protection Against Controlled Human Malaria Infection.

The RTS,S/AS01 vaccine provides partial protection against Plasmodium falciparum infection but determinants of protection and/or disease are unclear. Previously, anti-circumsporozoite protein (CSP) antibody titers and blood RNA signatures were associated with RTS,S/AS01 efficacy against controlled human malaria infection (CHMI). By analyzing host blood transcriptomes from five RTS,S vaccination CHMI studies, we demonstrate that the transcript ratio MX2/GPR183, measured 1 day after third immunization, discriminates protected from non-protected individuals. This ratiometric signature provides information that is complementary to anti-CSP titer levels for identifying RTS,S/AS01 immunized people who developed protective immunity and suggests a role for interferon and oxysterol signaling in the RTS,S mode of action.

Human Plasmodium vivax mosquito experimental transmission

Plasmodium vivax bench research greatly lags behind Plasmodium falciparum because of an inability to culture in vitro. A century ago, intentionally inducing a malaria infection was a strategy commonly used to cure late-stage syphilis. These controlled human malaria infections were used with expertise and persisted to the end of World War II. While controlled malaria liver-stage infection has been achieved for both P. vivax and P. falciparum, controlled human transmission to mosquitoes falls short for both species. In this issue of the JCI, Collins et al. present groundbreaking work that establishes a system to transmit P. vivax gametocytes from humans to mosquitoes. The authors injected a unique human isolate of P. vivax that reached high gametocyte density within weeks. This study provides a technical advance that will facilitate the study and eradication of the human parasite P. vivax.

Recombinase Polymerase Amplification and Lateral Flow Assay for Ultrasensitive Detection of Low-Density Plasmodium falciparum Infection from Controlled Human Malaria Infection Studies and Naturally Acquired Infections.

Microscopy and rapid diagnostic tests (RDTs) are the main diagnostic tools for malaria but fail to detect low-density parasitemias that are important for maintaining malaria transmission. To complement existing diagnostic methods, an isothermal reverse transcription-recombinase polymerase amplification and lateral flow assay (RT-RPA) was developed. We compared the performance with that of ultrasensitive reverse transcription-quantitative PCR (uRT-qPCR) using nucleic acid extracts from blood samples (n = 114) obtained after standardized controlled human malaria infection (CHMI) with Plasmodium falciparum sporozoites. As a preliminary investigation, we also sampled asymptomatic individuals (n = 28) in an area of malaria endemicity (Lambaréné, Gabon) to validate RT-RPA and assess its performance with unprocessed blood samples (dbRT-RPA). In 114 samples analyzed from CHMI trials, the positive percent agreement to uRT-qPCR was 90% (95% confidence interval [CI], 80 to 96). The negative percent agreement was 100% (95% CI, 92 to 100). The lower limit of detection was 64 parasites/ml. In Gabon, RT-RPA was 100% accurate with asymptomatic volunteers (n = 28), while simplified dbRT-RPA showed 89% accuracy. In a subgroup analysis, RT-RPA detected 9/10 RT-qPCR-positive samples, while loop-mediated isothermal amplification (LAMP) detected 2/10. RT-RPA is a reliable diagnostic test for asymptomatic low-density infections. It is particularly useful in settings where uRT-qPCR is difficult to implement.