dc.description.abstract | Phase Ib and lIb paediatric clinical trials in children between the ages of 12 and 48
months were conducted in Kombewa division, western Kenya between 2003 and 2006, where
the safety and efficacy of Falciparum Merozoite Protein-1 (FMP 1), a merozoite surface protein-
142(MSP-142) malaria vaccine were tested. MSP-142 comprises MSP-119, which elicits protective
humoral immune responses according to preclinical studies, and MSP-133 that contains T cell
epitopes believed to be essential for supporting the induction of immunity. Limited but
significant polymorphisms occur within the conserved C-terminal block of the msp-l gene,
where four key single nucleotide polymorphisms (SNPs) (E/Q, T/K, SIN and RIG) are present
and allow for the definition of the dominant MSP-119 haplotypes in mixed clone infections.
FMP1 was developed from the 307 Plasmodium strain which has the ETSR MSP-119 haplotype
and it is not known whether this vaccine would generate an effective immune response against
parasite with various MSP-l19 haplotypes in natural infections and whether this would be
significant with regard to overall vaccine efficacy.
The work described here constitutes the molecular analysis of MSP-119 haplotypes of
infecting parasites in vaccinated and non-vaccinated children in a phase Ib and a phase Ilb trial.
• The phase 1b trial had three FMP 1 vaccine dose cohorts (10 ug, 25 ug and 50 ug) and a rabies
vaccine control arm, while the Phase lIb had the full vaccine dose (50 ug) arm and a rabies
vaccine control arm. In the phase I trial genotyping of DNA 'samples was done at enrolment and
one month post-vaccination by PCR-sequencing and by Real Time quantitative PCR (RT-qPCR)
so as to validate the latter assay. For sequencing analysis, a 400 bp amplicon covering all of
block 17 of the msp-l gene which encodes MSP-119 was amplified from each sample and then
sequenced before determining the MSP-l19 haplotype. Haplotypes resolved by RT-qPCR
involved first determining the dominant allele at each MSP-119 SNP using the threshold cycle
(Ct) values of each allele before linking together these alleles. Analysis of MSP-119 haplotypes
for the Phase lIb study was conducted at enrolment, one month post-vaccination and for children
with clinical malaria using RT-qPCR. Haplotype prevalence was then compared between the
vaccine and control groups.
Eight different MSP-119 haplotypes were identified from the sequence data (EKNG,
EKSG, EKSR, ETNG, ETSR, QKNG, QKSG and QKSR) and by RT-qPCR (EKNG, EKSR,
ETNG, ETSR, QKNG, QKSR, QTNG and QTSR). Haplotype prevalence at enrolment
determined through direct sequencing of PCR products revealed a predominance of EKNG
(49%) and QKNG (40%) while ETSR, the vaccine haplotype, represented only 3% of these
infections with other lesser prevalent haplotypes (EKSR, EKSG, ETNG and QKSG) together
constituting the remaining 8%. The RT-qPCR results for predicted MSP-119 haplotypes were
highly comparable to sequencing: 73% agreement at Day 0 and 91% agreement at Day 90.
Six different MSP-119 haplotypes (EKNG, EKSR, ETNG, ETSR, QKNG and QKSR etc)
were predicted from the RT-qPCR allele data in the phase lIb trial. At enrolment, EKNG and
QKNG were still the major MSP-119 haplotypes as predicted from RT-qPCR (EKNG 65%,
QKNG 16%). Other haplotypes were at lower levels with EKSR at 13%, the vaccine haplotype
(ETSR) at 4% and QKSR and ETNG together making up the remaining 2%. While the predicted
haplotype frequencies following vaccination did not differ between the vaccine and control
groups among asymptomatic children at one month post-vaccination, significantly fewer T
alleles were observed in the vaccine group (49%) at one month post-vaccination (95% CI 38% -
60%) compared to the control group (64%, 95% CI 53% - 73%, P = 0.04). Comparison of the
complete haplotypes in 381 children (176 vaccinees and 205 controls) who had one or more
malaria episodes post-vaccination indicated no significant difference in haplotype distribution
between the vaccine and control groups when considered altogether or as separate episodes.
However, among children who fell sick during the first four months following vaccination there
was a significant lower prevalence of the E allele among vaccines: 54% (95%CI 42% - 65%)
compared to 69% (95%CI 58% - 79%, P = 0.04) in the controls. In addition, a trend towards
significance (p = 0.08) was noted in the time to first clinical episode with the vaccine haplotype
(ETSR) among vaccinees suggesting a vaccine-induced delay.
The results from the phase Ib showed that the RT-qPCR assay developed was reliable for
haplotyping. Furthermore, the observations from the phase lIb trial implied that FMP 1 induced a
subtle homologous effect on parasites with the ETSR haplotype. This was the expectation based
on previous investigations that examined the specificity of antibodies generated by FMP 1. The
results also support previous observations that suggest that effective immune responses induced
by FMP 1 seem to be diminished in malaria exposed populations. Given the apparent low level
homologous impact and the absence of any detectable effect on heterologous haplotype strains,
consideration must be given toward boosting the efficacy ofFMPI through the testing and
possible combination with antigens from other strains or other vaccine candidate antigen
altogether. Future studies should address these concerns and also unequivocally substantiate
the vaccine effects noted in this study. | en |
dc.description.department | a
Department of Psychiatry, University of Nairobi, ; bDepartment of Mental Health, School of Medicine,
Moi University, Eldoret, Kenya | |