Amino acid sequences recognized by T cells: studies on a merozoite surface antigen from the FCQ-27/PNG isolate of Plasmodium falciparum.

Twenty-six overlapping peptides, spanning the entire FCQ-27/PNG sequence of the Plasmodium falciparum antigen known as merozoite surface antigen 2 were screened for their ability to induce the proliferation of peripheral blood lymphocytes (PBL) obtained from 12 donors living in Honiara, Solomon Islands where P. falciparum is endemic. A recombinant (r) form of MSA2, known as Ag 1609 was also screened in these assays and tetanus toxoid (TT) antigen was included as a control. The location of the predicted T cell determinants within MSA2 was examined using the algorithm, AMPHI and by scanning MSA2 for amino acid sequences showing the Rothbard motif. There were 13 predicted amphipathic helical sites and five examples of Rothbard sequences in the antigen. The location of these with regard to the peptides tested is shown. Nine of the 12 individuals responded to TT with high stimulation indices (greater than 4) being obtained in the majority of donors. Only three individuals responded to r-MSA2 with the stimulation indices (SI) in the range of 2.4-4.1. Peptides from both the constant and variable regions of MSA2 were recognized in the proliferative assays. However, the majority of the positive proliferative responses were to peptides which spanned the central variable region which included the two copies of the 32-amino-acid repeat occurring in the antigen. High SI comparable to those obtained to TT were seen in some individuals with some peptides. There was considerable variation between donors in number and nature of the peptides recognised and two donors did not respond to any of the antigens tested. The significance of these findings to vaccine development is discussed.