Selection Dynamics Of Circumsporozoite Protein (Csp) Vaccine Target In Ghana: The Contribution Of Human Leukocyte Antigen (Hla) Variation

Abstract

Implementation of RTS,S/AS01 vaccine for malaria is underway in three (3) African countries, Ghana, Kenya, and Malawi. This vaccine, which targets the Plasmodium falciparum circumsporozoite protein (CSP) provides partial protection for infants and children against clinical and severe malaria infections. Reasons for this reduced efficacy or immunogenicity are poorly understood, but CSP variation has been implicated. Human leukocyte antigen (HLA) has also been observed to influence RTS,S-mediated protection. This study aims to define the variants of CSP and determine its distribution between Begoro and Cape Coast in Ghana over three years. Further, the influence of HLA genotype in terms of parasite frequency and RTS,S/AS01 response was assessed. About 50μl of peripheral blood was collected from participants in Begoro and Cape Coast in 2014, 2015, and 2016, dried blood spot (DBS) prepared and DNA was extracted. The C-terminal of Plasmodium falciparum CSP and the human leukocyte antigen (HLA) class II gene in humans were deep sequenced. The translated amino acid haplotypes of the CSP were aligned and compared to the reference 3D7 vaccine strain. The HLA class II haplotypes were grouped into superfamily and their association with the CSP variants was ascertained. The CSP haplotypes are evenly distributed between Begoro and Cape Coast. There were 31 Th2R haplotypes in Begoro and 30 Th2R haplotypes in Cape Coast; 15 Th3R haplotypes in Begoro and 13 in Cape Coast. About 83.9% of Th2R and 96.5% of Th3R haplotypes in Begoro are shared with Cape Coast. The amino acid changes with reference to the 3D7 vaccine strain at the Th2R epitope range from 1 to 6 and 1 to 4 at theTh3R epitope. There is a 53% and 60% reduction in the 3D7 Th2R and Th3R haplotypes, respectively, from 2014 to 2016, but 3D7 is still common in Ghana, Kenya, and Malawi. The 3D7 haplotype does not correlate with HLA-DRB1, but there is with HLA-DQA1 and HLA-DPB1. Begoro and Cape Coast are two different ecological zones in Ghana but the parasite population is homogenous. The Th2R epitope of CSP is polymorphic than the Th3R epitope and this higher polymorphism is driving a higher non-synonymous amino acid substitution at the Th2R epitope than the Th3R epitope which may have vaccine implication. A decline in frequency of 3D7 parasite population may also affect the performance in the vaccine in Begoro and Cape Coast. Initial correlations indicate that HLA-DPB1 (01:01/17:01) correlates with the 3D7 vaccine strain, but HLA-DPB1 (01:01/17:01) and other variants of HLA-DQA1 also correlates with other Th2R haplotypes and may compete with the vaccine haplotype for antigen presentation to CD+4 T cells. This may have implications for the efficacy of the RTS,S/AS01 vaccine in Ghana.