Safety and infectivity of female cercariae in Schistosoma-naïve, healthy participants: a controlled human Schistosoma mansoni infection study

BACKGROUND

A controlled human infection model for schistosomiasis (CHI-S) can speed up vaccine development and provides insight into early immune responses following schistosome exposure. Recently, we established CHI-S model using single-sex male-only Schistosoma mansoni (Sm) cercariae in Schistosoma-naïve individuals. Given important differences in antigenic profile and human immune responses to schistosomes of different sex, we pioneered a single-sex female-only CHI-S model for future use in vaccine development.

METHODS

We exposed 13 healthy, Schistosoma-naïve adult participants to 10 (n = 3) or 20 (n = 10) female cercariae and followed for 20 weeks, receiving treatment with praziquantel (PZQ) 60 mg/kg at week 8 and 12 after exposure.

FINDINGS

The majority (11/13) participants reported rash and/or itch at the site of exposure, 5/13 had transient symptoms of acute schistosomiasis. Exposure to 20 cercariae led to detectable infection, defined as serum circulating anodic antigen levels >1.0 pg/mL, in 6/10 participants. Despite two rounds of PZQ treatment, 4/13 participants showed signs of persistent infection. Additional one- or three-day PZQ treatment (1 × 60 mg/kg and 3 × 60 mg/kg) or artemether did not result in cure, but over time three participants self-cured. Antibody, cellular, and cytokine responses peaked at week 4 post infection, with a mixed Th1, Th2, and regulatory profile. Cellular responses were (most) discriminative for symptoms.

INTERPRETATION

Female-only infections exhibit similar clinical and immunological profiles as male-only infections but are more resistant to PZQ treatment. This limits future use of this model and may have important implications for disease control programs.

FUNDING

European Union's Horizon 2020 (grant no. 81564).

Establishing a controlled hookworm human infection (CHHI) model for Africa: A report from the stakeholders meeting held in Lambaréné, Gabon, November 10-11, 2019

BACKGROUND

Hookworm is a major contributor to worldwide disease burden with over 230 million people infected. It has been identified as one of the Neglected Tropical Diseases that can be controlled and even eliminated through mass drug administration and other effective interventions. Mathematical models have shown that hookworm can only be eliminated via a vaccine. Controlled Hookworm Human Infection (CHHI) models can facilitate rapid development of vaccines and drugs.

METHODS

As a first step towards the establishment of CHHI in Africa, we held a stakeholders meeting in Lamberene, Gabon from 10 to 11 November 2019.

RESULTS

Discussions revolved around the roles of the different regulatory institutions concerned; the need to strengthen existing regulatory capacity and the role of legislation; creating Gabon-specific ethical guidelines to govern Controlled Human Infection (CHI) studies; development of a study protocol; consideration of cultural and social peculiarities; the need for regular joint review meetings between interested parties throughout the process of protocol implementation; and participant compensation. Moreover, operational considerations concerning the introduction of CHHI in Gabon include the use of the local strain of hookworm for the challenge infections, capacity building for the local production of challenge material, and the establishment of adequate quality assurance procedures.

CONCLUSION

The workshop addressed several of the anticipated hurdles to the successful implementation of CHHI in Gabon. It is our aim that this report will stimulate interest in the implementation of this model in the sub-Saharan African setting.

A Randomized Controlled Trial to Investigate Safety and Variability of Egg Excretion After Repeated Controlled Human Hookworm Infection

BACKGROUND

Controlled human hookworm infections could significantly contribute to the development of a hookworm vaccine. However, current models are hampered by low and unstable egg output, reducing generalizability and increasing sample sizes. This study aims to investigate the safety, tolerability, and egg output of repeated exposure to hookworm larvae.

METHODS

Twenty-four healthy volunteers were randomized, double-blindly, to 1, 2, or 3 doses of 50 Necator americanus L3 larvae at 2-week intervals. Volunteers were monitored weekly and were treated with albendazole at week 20.

RESULTS

There was no association between larval dose and number or severity of adverse events. Geometric mean egg loads stabilized at 697, 1668, and 1914 eggs per gram feces for the 1 × 50L3, 2 × 50L3, and 3 × 50L3 group, respectively. Bayesian statistical modeling showed that egg count variability relative to the mean was reduced with a second infectious dose; however, the third dose did not increase egg load or decrease variability. We therefore suggest 2 × 50L3 as an improved challenge dose. Model-based simulations indicates increased frequency of stool sampling optimizes the power of hypothetical vaccine trials.

CONCLUSIONS

Repeated infection with hookworm larvae increased egg counts to levels comparable to the field and reduced relative variability in egg output without aggravating adverse events.

CLINICAL TRIALS REGISTRATION

NCT03257072.

New Insights Into the Kinetics and Variability of Egg Excretion in Controlled Human Hookworm Infections

Four healthy volunteers were infected with 50 Necator americanus infective larvae (L3) in a controlled human hookworm infection trial and followed for 52 weeks. The kinetics of fecal egg counts in volunteers was assessed with Bayesian multilevel analysis, which revealed an increase between weeks 7 and 13, followed by an egg density plateau of about 1000 eggs/g of feces. Variation in egg counts was minimal between same-day measurements but varied considerably between days, particularly during the plateau phase. These analyses pave the way for the controlled human hookworm model to accelerate drug and vaccine efficacy studies.

CD39 and immune regulation in a chronic helminth infection: The puzzling case of Mansonella ozzardi

Background

Chronic helminth infections typically induce an immunoregulatory environment, with markedly reduced immune responses to both parasite-specific and unrelated bystander antigens. Here we tested whether these changes are also observed in human infections with Mansonella ozzardi, a neglected filarial nematode widely distributed across Latin America.

Methods

CD4⁺ T cell populations from microfilaremic (Fil+) and uninfected (Fil-) inhabitants in M. ozzardi-endemic riverine communities in Brazil were characterized by flow cytometry analysis. Plasma concentrations of a wide range of cytokines and chemokines were measured. We examined whether M. ozzardi infection is associated with suppressed in vitro lymphoproliferative and inflammatory cytokine responses upon stimulation with filarial antigen, unrelated antigens or mitogens.

Principal findings/Conclusions

Fil+ subjects had lower plasma levels of selected inflammatory cytokines, such as TNF-α, IL-8, and IL-6, than their Fil- counterparts. However, we found no evidence for attenuated T-cell responses to filarial antigens or co-endemic pathogens, such as malaria parasites and Toxoplasma gondii. CD4⁺ T cells expressing CD39, an ectonucleosidase involved in the generation of the anti-inflammatory molecule adenosine, were increased in frequency in Fil+ subjects, compared to uninfected controls. Significantly, such an expansion was directly proportional to microfilarial loads. Surprisingly, CD39 blocking with a neutralizing antibody suppressed antigen-driven lymphoproliferation in vitro, while decreasing inflammatory cytokine responses, in Fil+ and Fil- individuals. These findings suggest that circulating CD4⁺ CD39⁺ T cells comprise subsets with both regulatory and stimulatory roles that contribute to the immune homeostasis in chronic M. ozzardi infection.

Helminth infections downregulate immunity and reduce host’s inflammatory responses, but the filarial nematode Mansonella ozzardi, which is widely distributed across Latin America, appears to represent an exception to this rule. We found similar lymphoproliferative responses to filarial and unrelated antigens and comparable regulatory cytokine responses in subjects harboring M. ozzardi microfilariae, compared to local uninfected controls. The proportion of CD4⁺ T cell subtypes expressing CD39 was significantly increased in infected subjects and correlated positively with their microfilarial density. However, antibody blocking of CD39, an ectonucleosidase involved in the synthesis of the immunosuppresive molecule adenosine, paradoxically reduced, rather than promoted, antigen-driven lymphoproliferation in vitro. We suggest that CD39⁺ CD4⁺ T cells circulating in microfilaremics comprise both regulatory and stimulatory cell subsets that are concomitantly expanded. The balance between these cell subsets with opposing regulatory functions may be crucial to maintain immune homeostasis during chronic M. ozzardi infections.

Responses to malarial antigens are altered in helminth-infected children

Malaria and helminth infections often coincide in the same tropical regions. Studies of the consequences of helminth and malaria coinfection in humans have been few and are mainly epidemiological, with little information on cellular immune responses. In this study, we investigated the antimalarial immune responses of Ghanaian children living in a rural area with a high prevalence of both helminth infection and Plasmodium falciparum infection. Whole blood specimens were cultured with P. falciparum-infected red blood cells (iRBCs), and pro- and anti-inflammatory cytokines and immune regulatory molecules were measured. In response to iRBCs, levels of interleukin (IL)-10, but not tumor necrosis factor-alpha,were higher in samples from helminth-infected children than in those from uninfected children, as was expression of the regulatory molecules suppressor of cytokine signaling (SOCS)-3, Foxp3, and programmed death (PD)-1. Furthermore, a significant correlation was found between SOCS-3 gene expression and IL-10 production. These results indicate that the presence of helminth infection modulates the immune response to malarial parasites, making it more anti-inflammatory.

Responses to Toll-like receptor ligands in children living in areas where schistosome infections are endemic

To study the effect of repeated challenge of the innate immune system with pathogen-associated molecular patterns, cytokine responses to schistosomal lipids and bacterial lipopolysaccharide (LPS) were analyzed in schoolchildren living in an area in Gabon where schistosomiasis, a helminth infection that is chronic in nature, is endemic. A schistosomal phosphatidylserine (PS) fraction containing the Toll-like receptor (TLR)-2 ligand lyso-PS stimulated the production of interleukin (IL)-8, IL-10, IL-6, and tumor necrosis factor (TNF)-alpha in children without Schistosoma haematobium infection. However, in infected children, the responses to this stimulus were lower, in particular for production of IL-8 and TNF-alpha. Responses to the TLR4 ligand, LPS, followed a similar pattern. In contrast, schistosomal adult worm glycolipids that did not stimulate any of the TLRs tested induced IL-8 and IL-6 responses that were significantly higher in schistosome-infected children than in schistosome-uninfected children. These results indicate that relentless exposure to pathogens can lead to altered responses to TLR ligands.

A novel host-parasite lipid cross-talk. Schistosomal lyso-phosphatidylserine activates toll-like receptor 2 and affects immune polarization

Schistosome infections are characterized by prominent T cell hyporesponsiveness during the chronic stage of infection. We found that schistosome-specific phosphatidylserine (PS) activated TLR2 and affected dendritic cells such that mature dendritic cells gained the ability to induce the development of IL-10-producing regulatory T cells. Using mass spectrometry, schistosomal lysophosphatidylserine (lyso-PS) was identified as the TLR2-activating molecule. This activity appears to be a unique property of schistosomal lyso-PS, containing specific acyl chains, because neither a synthetic lyso-PS (16:0) nor PS isolated from the mammalian host activates TLR2. Taken together, these findings provide evidence for a novel host-parasite interaction that may be central to long term survival of the parasite and limited host pathology with implications beyond parasitology.