IMMUNIZATION WITH SPF66 AND SUBSEQUENT INFECTION WITH HOMOLOGOUS AND HETEROLOGOUS PLASMODIUM-FALCIPARUM PARASITES

In an area of intense transmission, a malaria vaccine could reduce inf ection due to the parasite types represented in the vaccine, but have no detectable effect on the overall frequency of infection if it did n ot protect against infection with heterologous parasites. These studie s were performed to determine whether immunization with SPf66 decrease d infection with homologous parasites containing the 11 amino acid pep tide from merozoite surface protein-1 (MSP-1) in SPf66, or increased i nfection due to heterologous parasites containing heterologous (altern ative) MSP-1 sequences. Based on this 11 amino acid peptide (YSLFQKEKM VL), three forward primers (S,Q,V) were designed to amplify the MSP-1 sequence present in SPf66, and 3 additional forward primers (G,H,I) to amplify the alternative MSP-1 sequence (YGLFHKEKMIL). This strategy w as validated by polymerase chain reaction (PCR) amplification and dide oxy sequencing with 14 cloned laboratory isolates, which demonstrated that each primer amplified one MSP-1 sequence or the other, but not bo th. The technique was then used to examine filter paper blots from an SPf66 vaccine study of 69 subjects in Saradidi, Kenya. In that study, the prevalence of infection with YSLFQKEKMVL, or YGLFHKEKMIL type para sites was unaffected by immunization with SPf66 (based on PCR amplific ation with the S,Q,V,G, H and I primers, respectively). These results suggest that immunization with SPf66 does not produce a selective effe ct in vivo. They demonstrate a molecular method to test for selection in vivo as an indirect measure of vaccine efficacy.