Growth-Inhibitory Antibodies Are Not Necessary for Protective Immunity to Malaria Infection

Abstract

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 Plasmodium yoelii rodent malaria model using the 19-kDa C-terminal fragment of merozoite surface protein 1 (MSP1 19 ). To examine the relationship between inhibitory antibodies in immunized mice and the immune state, as indicated by resistance to a blood-stage challenge, we used an allelic replacement strategy to generate a transgenic P. falciparum line that expresses MSP1 19 from P. yoelii. We show that MSP1 19 is functionally conserved across these two divergent Plasmodium species, and replacing PfMSP1 19 with PyMSP1 19 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 PfMSP1 19 , we developed an assay to measure the specific growth-inhibitory activity directed exclusively to the PyMSP1 19 protein in the sera from vaccinated animals. To validate this assay, sera from rabbits immunized with recombinant PyMSP1 19 were tested and showed specific inhibitory activity in a concentration-dependent manner. In mice that were immunized with recombinant PyMSP1 19 , the levels of PyMSP1 19 -specific inhibitory activity did not correlate with the total antibody levels measured by enzymelinked 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 prechallenge sera. These data indicated that growth-inhibitory activity measured in vitro was not a reliable predictor of immune status in vivo, and the reliance on this criterion to select vaccine candidates for human clinical trials may be misplaced. The transgenic lines further offer useful tools for comparing the efficacy of MSP1 19 -based vaccines that utilize different immunization regimens and antigen formulations.

Enhancing Blood-Stage Malaria Subunit Vaccine Immunogenicity in Rhesus Macaques by Combining Adenovirus, Poxvirus, and Protein-in-Adjuvant Vaccines

“…It is perhaps not surprising that single-modality vaccine platforms, aiming to induce a single type of immune response, have failed to impact on the blood-stage growth rates of P. falciparum in clinical trials. This is further supported by recent studies showing that growth-and invasioninhibitory Abs are not directly associated with, or do not completely account for, disease outcome in the field (29,30). …”

Malaria vaccine development based on merozoite surface proteins ofPlasmodium falciparum

“…While, the safety and immunogenicity of FMP1 was confirmed in Phase I trials, 16 a Phase II efficacy trial among young Kenyan children failed to elicit any protection. 6,8 Other MSP-1 vaccines that has been tested in clinical trials are MSP-1 42 C1 developed by MVDB, NIAID (NIH) which comprises of two variants of the MSP-1 42 (3D7 and FVO), 17 formulated with Alhydrogel with and without CPG7909. 18 Phase I studies of this vaccine generated moderate antibody responses, but minimal in vitro inhibitory activity.…”

“…Thus, it is likely that an immune response developed against MSP-1 42 may not recognize heterologous parasites in the field. As a result, it appears that MSP-1 19 which has a more conserved sequence than MSP-1 42 would be a better candidate antigen to test in a vaccine. In fact, a functional conservation of MSP-1 19 across distantly related Plasmodium species has also been reported, 33 which suggests that the 19 kDa region of MSP-1 has a crucial role not only in P. falciparum but in other Plasmodium species as well.…”

“…It is a 195 kDa, GPI anchored protein that coats the merozoite surface. 10 It undergoes proteolytic processing that result in the formation of several MSP-1 fragments, of which the 42 kDa (MSP-1 42 ) and 19 kDa fragment (MSP-1 19 ) have been most studied. 10 Secondary processing of the 42 kDa fragment into the 19 kDa and 33 kDa fragments is believed to be crucial for erythrocyte invasion.…”

“…Several MSP-1 based vaccines are at various phases of pre-clinical or clinical development of which FMP1 (falciparum malaria protein-1) 6,8,9 that comprises of MSP-1 42 (3D7) formulated with adjuvant, ASO2A, (GlaxoSmithKline, Belgium) is at the most advanced stage. FMP1 has induced significant growth inhibitory antibodies.…”