ABSTRACTThe C-terminal 19-kDa domain of merozoite surface protein 1 (MSP119) is the target of protective antibodies but alone is poorly immunogenic. Previously, using thePlasmodium yoeliimurine model, we fusedP. yoeliiMSP119(PyMSP119) with full-lengthP. yoeliimerozoite surface protein 8 (MSP8). Upon immunization, the MSP8-restricted T cell response provided help for the production of high and sustained levels of protectivePyMSP119- andPyMSP8-specific antibodies. Here, we assessed the vaccine potential of MSP8 of the human malaria parasite,Plasmodium falciparum. Distinct fromPyMSP8,P. falciparumMSP8 (PfMSP8) contains an N-terminal asparagine and aspartic acid (Asn/Asp)-rich domain whose function is unknown. Comparative analysis of recombinant full-lengthPfMSP8 and a truncated version devoid of the Asn/Asp-rich domain,PfMSP8(ΔAsn/Asp), showed that both proteins were immunogenic for T cells and B cells. All T cell epitopes utilized mapped within rPfMSP8(ΔAsn/Asp). The dominant B cell epitopes were conformational and common to both rPfMSP8 and rPfMSP8(ΔAsn/Asp). Analysis of nativePfMSP8 expression revealed thatPfMSP8 is present intracellularly in late schizonts and merozoites. Following invasion,PfMSP8 is found distributed on the surface of ring- and trophozoite-stage parasites. Consistent with a low and/or transient expression ofPfMSP8 on the surface of merozoites,PfMSP8-specific rabbit IgG did not inhibit thein vitrogrowth ofP. falciparumblood-stage parasites. These studies suggest that the further development ofPfMSP8 as a malaria vaccine component should focus on the use ofPfMSP8(ΔAsn/Asp) and its conserved, immunogenic T cell epitopes as a fusion partner for protective domains of poor immunogens, includingPfMSP119.

The C-terminal 19-kDa domain of Plasmodium falciparum merozoite surface protein 1 (PfMSP1 19 ) is an established target of protective antibodies. However, clinical trials of PfMSP1 42 , a leading blood-stage vaccine candidate which contains the protective epitopes of PfMSP1 19 , revealed suboptimal immunogenicity and efficacy. Based on proof-of-concept studies in the Plasmodium yoelii murine model, we produced a chimeric vaccine antigen containing recombinant PfMSP1 19 (rPfMSP1 19 ) fused to the N terminus of P. falciparum merozoite surface protein 8 that lacked its low-complexity Asn/Asp-rich domain, rPfMSP8 (⌬Asn/ Asp). Immunization of mice with the chimeric rPfMSP1/8 vaccine elicited strong T cell responses to conserved epitopes associated with the rPfMSP8 (⌬Asn/Asp) fusion partner. While specific for PfMSP8, this T cell response was adequate to provide help for the production of high titers of antibodies to both PfMSP1 19 and rPfMSP8 (⌬Asn/Asp) components. This occurred with formulations adjuvanted with either Quil A or with Montanide ISA 720 plus CpG oligodeoxynucleotide (ODN) and was observed in both inbred and outbred strains of mice. PfMSP1/8-induced antibodies were highly reactive with two major alleles of PfMSP1 19 (FVO and 3D7). Of particular interest, immunization with PfMSP1/8 elicited higher titers of PfMSP1 19 -specific antibodies than a combined formulation of rPfMSP1 42 and rPfMSP8 (⌬Asn/Asp). As a measure of functionality, PfMSP1/8-specific rabbit IgG was shown to potently inhibit the in vitro growth of blood-stage parasites of the FVO and 3D7 strains of P. falciparum. These data support the further testing and evaluation of this chimeric PfMSP1/8 antigen as a component of a multivalent vaccine for P. falciparum malaria.

An efficacious malaria vaccine remains elusive despite concerted efforts. Using the Plasmodium yoelii murine model, we previously reported that immunization with the C-terminal 19 kDa domain of merozoite surface protein 1 (MSP119) fused to full-length MSP8 protected against lethal P. yoelii 17XL, well beyond that achieved by single or combined immunizations with the component antigens. Here, we continue the evaluation of the chimeric PyMSP1/8 vaccine. We show that immunization with rPyMSP1/8 vaccine elicited an MSP8-restricted T cell response that was sufficient to provide help for both PyMSP119 and PyMSP8 specific B cells to produce high and sustained levels of protective antibodies. The enhanced efficacy of immunization with rPyMSP1/8, in comparison to a combined formulation of rPyMSP142 and rPyMSP8, was not due to improved conformation of protective B cell epitopes in the chimeric molecule. Unexpectedly, rPyMSP1/8 vaccine-induced antibody responses were not boosted by exposure to P. yoelii 17XL infected RBCs. However, rPyMSP1/8 immunized and infected mice mounted robust responses to a diverse set of blood-stage antigens. The data support the further development of an MSP1/8 chimeric vaccine but also suggest that vaccines that prime for responses to a diverse set of parasite proteins will be required to maximize vaccine efficacy.