A malaria sporozoite surface antigen distinct from the circumsporozoite protein

Monoclonal antibody NYS4 recognizes a single 140 kDa antigen on the surface of Plasmodium yoelii sporozoites, an antigen which is distinct from the extensively characterized circumsporozoite (CS) protein. To more thoroughly characterize this additional surface component, a genomic expression library was screened with NYS4 and an immunoreactive clone (M4) was obtained which expressed part of the antigen gene. The deduced amino acid sequence of the M4 peptide included two unique repetitive sequences of amino acids and a conserved sequence motif which is found in several proteins including the CS protein (region II). The cloned DNA hybridized to a PCR (polymerase chain reaction) amplified sporozoite mRNA demonstrating the sporozoite-stage expression of this gene. A synthetic peptide of one of the repeats, (Asn-Pro-Asn-Glu-Pro-Ser), was recognized by NYS4 and mice immunized with (Asn-Pro-Asn-Glu-Pro-Ser)3 conjugated to KLH (keyhole limpet haemocyanin) produced high levels of antibodies that reacted with the surface of sporozoites and specifically to the 140 kDa antigen. Thus, at least two different proteins are on the surface of the P. yoelii sporozoite indicating that the immunoreactive exterior of the infective stage of malaria parasites is more antigenically complex than previously thought.