B-CELL AND T-CELL RESPONSES IN CONGENIC MICE TO REPEAT SEQUENCES OF THE MALARIA ANTIGEN-PF332 - EFFECTS OF THE NUMBER OF REPEATS

The Plasmodium falicparum antigen Pf332 comprises degenerated 11-amino -acid repeats with regularly spaced pairs of glutamic acid. Epitopes f ormed by such repeats are recognized by polyclonal and monoclonal anti bodies that interfere with the life cycle of the blood stages of the m alaria parasite. In order to study the immunogenicity of one such Pf33 2 repeat sequence (SVTEEIAEEDK), fusion proteins containing ZZ (two Ig G binding domains of staphylococcal protein A) and dimers, trimers or tetramers of the malarial sequence were injected into mice. To analyse possible major histocompatibility complex class II restrictions of th e immune response, mice of different H-2 haplotypes were used. A signi ficant antibody response was elicited by administration of all the thr ee fusion proteins in mice expressing the I-A(k) allele (B10.BR, B10.A (2R) and B10.A(4R)) whereas B10 and C57BL/6 (H-2(b)) mice were low res ponders. In comparison, B10.D2 (H-2(d)) mice were low responders to fu sion proteins with 2 or 3 repeats but responded well to the protein co ntaining 4 repeats. Lymph node cells from B10.BR (H-2(k)) mice, primed in vivo with ZZ-fusion proteins containing either 2 or 4 repeats, pro liferated in vitro in response to repeat sequences fused to ZZ or to a n unrelated fusion partner, as well as to a synthetic peptide containi ng less than two repeats. In contrast, a response of lymph node cells from B10.D2 (H-2(d)) mice was only obtained when a fusion protein cont aining 4 repeats was used both for in vivo priming and in vitro restim ulation. Antibodies from all responding mice reacted with the native p arasite protein in immunofluorescence. However, when measured by bindi ng to a panel of synthetic peptides, the fine specificities of the ant ibodies were found to depend on the number of Pf332 repeats used for i mmunization. Thus, the number of repeat sequences influences both B- a nd T-cell responses. Since immunodominant repeat regions are a common feature for several malarial antigens involved in immune protection, t he present findings have implications for the design of proper immunog ens to be incorporated in malaria vaccines.