Malaria represents a significant global health threat. In 2018, there were an estimated 228 million malaria cases and 405,000 deaths worldwide, most of which were caused by the Plasmodium falciparum parasite. With no licensed malaria vaccine currently available, and the leading clinical candidate (RTS,S/AS01, trade name Mosquirix™) achieving just 30% to 40% efficacy, more effective protection is required. mRNA vaccines are a promising approach, especially following recent successes in tackling COVID-19. To determine whether these might protect against malaria, Mallory et al. designed an mRNA vaccine based on an immunodominant Plasmodium falciparum coat protein (circumsporozoite protein, PfCSP, expressed at the invasive stage), which they used to perform a series of in vitro and in vivo studies. Their findings, published in npj Vaccines, show this approach to provide sterilizing immunity in mice and mark an important step toward developing a similar malaria vaccine for human use.
PfCSP can be expressed in mammalian cells
The PfCSP mRNA construct used by Mallory et al., produced by TriLink, was initially transfected into Chinese Hamster Ovary (CHO) cells to determine the magnitude and duration of expression – both critical factors in the induced immune response. Western blot analysis of samples harvested at 8, 24, and 48 hours following transfection showed cellular protein levels to peak between 8 and 24 hours, reducing dramatically by 48 hours. However, encapsulating the PfCSP mRNA in lipid nanoparticles (LNPs) prior to transfection meant the CHO cells could express the protein for at least 48 hours in vitro.
PfCSP mRNA-LNP elicits an antigen-specific response
Having shown LNPs to increase the duration of protein expression, Mallory et al. next investigated the effects on the cellular immune response when LNP-encapsulated PfCSP mRNA was used to immunize mice. Three different LNP formulations (LNP1, LNP2, and LNP3) and two mRNA doses (10 µg and 30 µg) were administered in two immunizations separated by a 3-week interval; the splenocytes were then isolated and stimulated with a PfCSP overlapping 15-mer peptide pool prior to cytokine expression analysis. Notably, the LNP1 10 µg dose group demonstrated IL-2 levels consistent with a potent and specific cell-mediated response.
Boost immunizations have an essential role
Both dose number and immunization schedule have been reported to influence immunity against malaria. For this reason, Mallory et al. compared a single 10 µg dose regimen with multiple 10 µg doses of PfCSP mRNA-LNP1 administered across different intervals, measuring PfCSP-specific antibody levels using an ELISA and monitoring the cellular immune response as previously described. Key findings included the observation that administering a third 10 μg dose of PfCSP mRNA-LNP1 promoted a Th1-skewed response known to correlate with positive outcomes in parasitic infections.
PfCSP mRNA-LNP1 induces protection in mice
To better understand the potential for PfCSP mRNA-LNP1 to be used as a protective vaccine, Mallory et al. next immunized mice three times with 10 μg mRNA at a 3-week interval before challenging them intravenously with an infectious Plasmodium berghei PfCSP transgenic parasite. Although additional studies are ongoing, PfCSP mRNA-LNP1 was shown to provide sterile protection (a unique immune status that prevents parasitic infection in the host) in a proportion of cases, laying an important foundation for future efforts aimed at combatting the global threat of malaria.
Article Reference: Mallory KL, Taylor JA, Zou X et al., Messenger RNA expressing PfCSP induces functional, protective immune responses against malaria in mice, npj Vaccines. 2021 doi: https://doi.org/10.1038/s41541-021-00345-0