CYTOTOXIC T-CELL REACTIVITY AND HLA-B35 BINDING OF THE VARIANT PLASMODIUM-FALCIPARUM CIRCUMSPOROZOITE PROTEIN CD8(+) CTL EPITOPE IN NATURALLY EXPOSED KENYAN ADULTS

In this study, we have investigated the extent of natural polymorphism in the CD8(+) cytotoxic T lymphocyte (CTL) determinant (amino acids 3 68-390) of circumsporozoite (CS) protein of Plasmodium falciparum fiel d isolates from a holoendemic region of Kenya, and determined how this variation affects the CTL reactivities in clinically immune adults an d binding specificities to human histocompatibility leukocyte antigen (HLA)-B35. Among the eight variant sequences that were found in this r egion, four were new and not seen in parasites from other geographical regions. When synthetic peptides corresponding to the eight variants were used to test the presence of CTL response in different donors, a different spectrum of CTL reactivity to these variants was noticed. Wh ile CTL from some donors recognized the P1 sequence (the most prevalen t type of sequence) but not P8 (another major variant), other donors s howed a reverse pattern of reactivity. Although none of the donors was able to recognize all the variants, CTL responses to ail the eight va riant sequences were found in this population. An octamer peptide with P1 sequence KPKDELDY in this polymorphic determinant was known to bin d HLA-B35. When we tested the effect of natural variation in this octa mer sequence on HLA-B35 binding, it became evident that SP13 with D -- > N substitution retained its binding specificity to HLA-B35. On the o ther hand,the SP12 octamer sequence which had two substitutions did no t bind HLA-B35. The most interesting finding was the observation that a D --> G substitution at position 374 rescued the binding ability of SP14, which otherwise could not bind to this HLA molecule due to E --> Q amino acid substitution at position 372. To our knowledge, this is the first demonstration showing that a natural polymorphism can rescue the binding specificity to an HLA-class I molecule that was lost due to another natural amino acid substitution. Altogether, these results demonstrate that natural polymorphism in the CS protein affects both t he CTL reactivity and the ability to bind to HLA-B35.