An Unstable Th Epitope of P. falciparum Fosters Central Memory T Cells and Anti-CS Antibody Responses

Abstract

Malaria is transmitted by Plasmodium-infected anopheles mosquitoes. Widespread resistance of mosquitoes to insecticides and resistance of parasites to drugs highlight the urgent need for malaria vaccines. The most advanced malaria vaccines target sporozoites, the infective form of the parasite. A major target of the antibody response to sporozoites are the repeat epitopes of the circumsporozoite (CS) protein, which span almost one half of the protein. Antibodies to these repeats can neutralize sporozoite infectivity. Generation of protective antibody responses to the CS protein (anti-CS Ab) requires help by CD4 T cells. A CD4 T cell epitope from the CS protein designated T* was previously identified by screening T cells from volunteers immunized with irradiated P. falciparum sporozoites. The T* sequence spans twenty amino acids that contains multiple T cell epitopes restricted by various HLA alleles. Subunit malaria vaccines including T* are highly immunogenic in rodents, non-human primates and humans. In this study we characterized a highly conserved HLA-DRβ1*04:01 (DR4) restricted T cell epitope (QNT-5) located at the C-terminus of T*. We found that a peptide containing QNT-5 was able to elicit long-term anti-CS Ab responses and prime CD4 T cells in HLA-DR4 transgenic mice despite forming relatively unstable MHC-peptide complexes highly susceptible to HLA-DM editing. We attempted to improve the immunogenicity of QNT-5 by replacing the P1 anchor position with an optimal tyrosine residue. The modified peptide QNT-Y formed stable MHC-peptide complexes highly resistant to HLA-DM editing. Contrary to expectations, a linear peptide containing QNT-Y elicited almost 10-fold lower long-term antibody and IFN-γ responses compared to the linear peptide containing the wild type QNT-5 sequence. Some possibilities regarding why QNT-5 is more effective than QNT-Y in inducing long-term T cell and anti-CS Ab when used as vaccine are discussed.

Progress in the Development of Subunit Vaccines against Malaria

“…QNT-Y peptide interacted strongly with major histocompatibility complex II (MHCII) receptors and formed stable MHC-peptide complexes. Disappointingly, IFN-γ and antibody responses induced by linear QNT-Y-containing peptide were significantly lower than those of wild-type QNT-5 peptide [49].…”

“…Parra-López et al designed a highly conserved HLA-DRβ1*04:01 (DR4) epitope (QNT-5 332-345 ) situated at the C-terminus of a CD4 T-cell epitope named T* (P. falciparum CS 326-345 ). HLA-DR4 transgenic mice were immunized with QNT-5 332-345 [49] to elicit long-term anti-CS antibody responses and prime CD4 + T-cells in mice. To improve QNT-5 332-345 immunogenicity, the P1 anchor position of the epitope was substituted with a tyrosine residue and named QNT-Y.…”

Association of HLA‐DM and HLA class II genes with antibody response induced by inactivated Japanese encephalitis vaccine

“…Evidence suggests that DM can bind to an empty HLA‐DR dimer and maintain functional activation 6 . In the presence of HLA‐DM, substituting L335 residues with tyrosine in the P1 side chain can improve the ability of HLA‐DR4 to bind and stabilise the MHC II peptide complex, which can improve the immunogenicity of the malaria vaccine 39 . Liu et al 40 measured the susceptibility of the corresponding HLA‐DR1‐A10L peptide complexes to be replaced by DM and demonstrated that the DM‐mediated dissociation half‐life is a strong and independent factor that governs peptide immunogenicity by favouring the presentation of peptides with greater kinetic stability in the presence of DM.…”

Distorted Immunodominance by Linker Sequences or other Epitopes from a Second Protein Antigen During Antigen-Processing

“…It is believed that an immunodominant epitope would emerge as dominant no matter where it might be placed within a carrier protein 12 . Factors such as kinetic stability, or DM resistance 51 are important contributors to immunodominance, although low affinity peptides have also been reported as dominant 52 . Thus, our new finding present a cautionary tale to vaccine designers 35 53 , warning against the assumption that placement of dominant determinants in any context would not affect their selection as dominant epitopes, or mixing several vaccines together might result in effective immunity against all the immunogens in the mix.…”