Growth-inhibitory antibodies are not necessary for protective immunity to malaria infection

The absence of a validated surrogate marker for the immune state has complicated the design of a subunit vaccine against asexual stages of Plasmodium falciparum. In particular, it is not known whether the capacity to induce antibodies that inhibit parasite growth in vitro is an important criterion for selection of P. falciparum proteins to be assessed in human vaccine trials. We examined this issue in the P. yoelii rodent malaria model using the 19 kDa C-terminal fragment of merozoite surface protein 1 (MSP119). To examine the relationship between inhibitory antibodies in immunised mice and the immune state, as indicated by resistance to a blood stage challenge, we have used an allelic replacement strategy to generate a transgenic P. falciparum line that expresses MSP119 from P. yoelii. We show that MSP119 is functionally conserved across these two divergent Plasmodium species, and replacing PfMSP119 with PyMSP119 has no detectable effect on parasite growth in vitro. By comparing growth rates of this transgenic line with a matched transgenic line that expresses the endogenous PfMSP119, we have developed an assay to measure the specific growth inhibitory activity directed exclusively to the PyMSP119 protein in the sera from vaccinated animals. To validate this assay, sera from rabbits immunised with recombinant PyMSP119 were tested and showed specific inhibitory activity in a concentration-dependent manner. In mice that were immunised with recombinant PyMSP119, the levels of PyMSP119-specific inhibitory activity did not correlate with the total antibody levels measured by enzyme-linked immunosorbent assay. Furthermore, they did not correlate with resistance to subsequent blood stage infection, and some mice with complete protection showed no detectable inhibitory activity in their pre-challenge sera. These data....