Protracted sterile protection with Plasmodium yoelii pre-erythrocytic genetically attenuated parasite malaria vaccines is independent of significant liver-stage persistence and is mediated by CD8(+) T cells
ABSTRACT Irradiation-attenuated sporozoite vaccinations confer sterile protection against malaria infection in animal models and humans. Persistent, nonreplicating parasite forms in the liver are presumably necessary for the maintenance of sterile immunity. A novel vaccine approach uses genetically attenuated parasites (GAPs) that undergo arrested development during liver infection. The fate of GAPs after immunization, their persistence in vaccinated animals, and the immune mechanisms that mediate protection are unknown. To examine the developmental defects of genetically attenuated liver stages in vivo, we created deletions of the UIS3 and UIS4 loci in the Plasmodium yoelii rodent malaria model (Pyuis3[-] and Pyuis4[-]). The low 50% infectious dose of P. yoelii in BALB/c mice provides the most sensitive infectivity model. We show that P. yoelii GAPs reach the liver, invade hepatocytes, and develop a parasitophorous vacuole but do not significantly persist 40 h after infection. A single dose of Pyuis4(-) sporozoites conferred complete protection, but full protection by Pyuis3(-) sporozoites required at least 2 immunizations. CD8(+) T cells were essential for protection, but CD4(+) T cells were not. Our results show that genetically distinct GAPs confer different degrees of protective efficacy and that live vaccine persistence in the liver is not necessary to sustain long-lasting protection. These findings have important implications for the development of a P. falciparum GAP malaria vaccine.
SourceAvailable from: Alexandra J Spencer[Show abstract] [Hide abstract]
ABSTRACT: The development of an efficacious vaccine against the Plasmodium parasite remains a top priority. Previous research has demonstrated the ability of a prime-boost virally vectored sub-unit vaccination regimen, delivering the liver-stage expressed malaria antigen TRAP, to produce high levels of antigen-specific T cells. The liver-stage of malaria is the main target of T cell-mediated immunity, yet a major challenge in assessing new T cell inducing vaccines has been the lack of a suitable pre-clinical assay. We have developed a flow-cytometry based in vitro T cell killing assay using a mouse hepatoma cell line, Hepa1-6, and Plasmodium berghei GFP expressing sporozoites. Using this assay, P. berghei TRAP-specific CD8+ T cell enriched splenocytes were shown to inhibit liver-stage parasites in an effector-to-target ratio dependent manner. Further development of this assay using human hepatocytes and P. falciparum would provide a new method to pre-clinically screen vaccine candidates and to elucidate mechanisms of protection in vitro.PLoS ONE 01/2015; 10(3-3):e0119880. DOI:10.1371/journal.pone.0119880 · 3.53 Impact Factor
[Show abstract] [Hide abstract]
ABSTRACT: Malaria parasites spend a critical phase of their life cycle inside hepatocytes, in an environment with complex and distinctive immunological features. Here I will discuss how the immunological features of the liver and the adaptations of malaria parasites interact, resulting in defective CD8+ T cell immunity. These processes are explored with a focus on the mechanism by which CD4+ T cells deliver help to CD8+ T cells, and specifically through their interaction with antigen-presenting cells (APCs), resulting in "licensing" of the APCs and enhanced capacity to optimally activate CD8+ T cells. Synthesis of the available evidence supports a model in which the parasite-mediated manipulation of programmed cell death in infected hepatocytes impairs the capacity of the liver's immune system to successfully license APCs and fully activate T cell immunity.Frontiers in Microbiology 11/2014; 5:617. DOI:10.3389/fmicb.2014.00617 · 3.94 Impact Factor
[Show abstract] [Hide abstract]
ABSTRACT: Plasmodium falciparum (P. falciparum) malaria remains a significant cause of mortality and morbidity throughout the world. Development of an effective vaccine would be a key intervention to reduce the considerable social and economic impact of malaria. We conducted a Phase Ia, non-randomized, clinical trial in 24 healthy, malaria-naïve adults of the chimpanzee adenovirus 63 (ChAd63) and modified vaccinia virus Ankara (MVA) replication-deficient viral vectored vaccines encoding the circumsporozoite protein (CS) of P. falciparum. ChAd63-MVA CS administered in a heterologous prime-boost regime was shown to be safe and immunogenic, inducing high-level T cell responses to CS. With a priming ChAd63 CS dose of 5×109 vp responses peaked at a mean of 1947 SFC/million PBMC (median 1524) measured by ELIspot 7 days after the MVA boost and showed a mixed CD4+/CD8+ phenotype. With a higher priming dose of ChAd63 CS dose 5×1010 vp T cell responses did not increase (mean 1659 SFC/million PBMC, median 1049). Serum IgG responses to CS were modest and peaked at day 14 post ChAd63 CS (median antibody concentration for all groups at day 14 of 1.3 µg/ml (range 0-11.9), but persisted throughout late follow-up (day 140 median antibody concentration groups 1B & 2B 0.9 µg/ml (range 0-4.7). ChAd63-MVA is a safe and highly immunogenic delivery platform for the CS antigen in humans which warrants efficacy testing. ClinicalTrials.gov NCT01450280.PLoS ONE 12/2014; 9(12):e115161. DOI:10.1371/journal.pone.0115161 · 3.53 Impact Factor