A Pfs48/45-based vaccine to block Plasmodium falciparum transmission: phase 1, open-label, clinical trial

BACKGROUND

The stalling global progress in malaria control highlights the need for novel tools for malaria elimination, including transmission-blocking vaccines. Transmission-blocking vaccines aim to induce human antibodies that block parasite development in the mosquito and mosquitoes becoming infectious. The Pfs48/45 protein is a leading Plasmodium falciparum transmission-blocking vaccine candidate. The R0.6C fusion protein, consisting of Pfs48/45 domain 3 (6C) and the N-terminal region of P. falciparum glutamate-rich protein (R0), has previously been produced in Lactococcus lactis and elicited functional antibodies in rodents. Here, we assess the safety and transmission-reducing efficacy of R0.6C adsorbed to aluminium hydroxide with and without Matrix-M™ adjuvant in humans.

METHODS

In this first-in-human, open-label clinical trial, malaria-naïve adults, aged 18-55 years, were recruited at the Radboudumc in Nijmegen, the Netherlands. Participants received four intramuscular vaccinations on days 0, 28, 56 and 168 with either 30 µg or 100 µg of R0.6C and were randomised for the allocation of one of the two different adjuvant combinations: aluminium hydroxide alone, or aluminium hydroxide combined with Matrix-M1™ adjuvant. Adverse events were recorded from inclusion until 84 days after the fourth vaccination. Anti-R0.6C and anti-6C IgG titres were measured by enzyme-linked immunosorbent assay. Transmission-reducing activity of participants' serum and purified vaccine-specific immunoglobulin G was assessed by standard membrane feeding assays using laboratory-reared Anopheles stephensi mosquitoes and cultured P. falciparum gametocytes.

RESULTS

Thirty-one participants completed four vaccinations and were included in the analysis. Administration of all doses was safe and well-tolerated, with one related grade 3 adverse event (transient fever) and no serious adverse events occurring. Anti-R0.6C and anti-6C IgG titres were similar between the 30 and 100 µg R0.6C arms, but higher in Matrix-M1™ arms. Neat participant sera did not induce significant transmission-reducing activity in mosquito feeding experiments, but concentrated vaccine-specific IgGs purified from sera collected two weeks after the fourth vaccination achieved up to 99% transmission-reducing activity.

CONCLUSIONS

R0.6C/aluminium hydroxide with or without Matrix-M1™ is safe, immunogenic and induces functional Pfs48/45-specific transmission-blocking antibodies, albeit at insufficient serum concentrations to result in transmission reduction by neat serum. Future work should focus on identifying alternative vaccine formulations or regimens that enhance functional antibody responses.

TRIAL REGISTRATION

The trial is registered with ClinicalTrials.gov under identifier NCT04862416.

Baseline TGF-β correlates with protection after immunization with Plasmodium falciparum sporozoites in the Controlled Human Malaria Infection model

BACKGROUND

Here we assessed a possible relationship between baseline TGF-β concentrations and acquisition of sterile immunity after Plasmodium falciparum sporozoite immunization.

METHODS

TGF-β concentrations were determined in samples of 65 malaria-naive volunteers in 4 studies either prior to and after challenge infection, or prior to and after first immunizing infection under chemoprophylaxis with P. falciparum sporozoites.

RESULTS

High baseline TGF-β concentrations were associated with rapid acquisition of sterile protection (p=0.028).

CONCLUSION

Baseline TGF-β concentrations predict the efficiency of acquisition of sterile immunity following sporozoite immunisation and may represent a steady-state regulatory mechanism to keep in check immune systems with a low threshold for activation.

[Development of vaccines against malaria; progress with contributions from the Netherlands]

Five years ago, the very first malaria vaccine, RTS,S/AS01, received a positive evaluation by the European Medicines Agency. Although this vaccine does not achieve the WHO's target of 75% protection, it does set the standard for all new vaccine candidates. In this article, we describe the progress made in the development of several second-generation malaria vaccines, an area where Dutch research has made major contributions. These include vaccines with live, attenuated Plasmodium falciparumsporozoites and transmission-blocking vaccines. Thanks in part to Dutch contributions, the development of vaccines against malaria has recently made significant progress on the way to the finish line: a vaccine that provides protection to the most vulnerable population - young children in Africa.

Ocular syphilis acquired through oral sex in two HIV-infected patients

Two cases of ocular syphilis are described in HIV-infected individuals after unprotected oral sex. The primary syphilitic lesion remained unnoticed and lues was therefore only diagnosed after visual symptoms developed.