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Infection and Immunity, September 2005, p. 5936-5944, Vol. 73, No. 9
0019-9567/05/$08.00+0     doi:10.1128/IAI.73.9.5936-5944.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Merozoite Surface Protein 1 of Plasmodium vivax Induces a Protective Response against Plasmodium cynomolgi Challenge in Rhesus Monkeys

Department of Immunology, Walter Reed Army Institute of Research, Silver Spring, Maryland 20910,¹ Division of Microbiology,² Division of Parasitology,³ Division of Lab Animals, Central Drug Research Institute, Lucknow 226001, India⁴

Received 11 March 2005/ Returned for modification 22 April 2005/ Accepted 11 May 2005

The 42-kDa fragment of the merozoite surface protein 1 (MSP-1₄₂) is a leading candidate for the development of a vaccine to control malaria. We previously reported a method for the production of Plasmodium vivax MSP-1₄₂ (PvMSP-1₄₂) as a soluble protein (S. Dutta, L. W. Ware, A. Barbosa, C. F. Ockenhouse, and D. E. Lanar, Infect. Immun. 69:5464-5470, 2001). We report here a process to manufacture the same PvMSP-1₄₂ protein but as an insoluble inclusion body-derived protein which was then refolded in vitro. We compared the immunogenicity and protective efficacy of the soluble and refolded forms of PvMSP-1₄₂ protein by using a heterologous but closely related P. cynomolgi-rhesus monkey challenge model. As comparative controls we also expressed, purified, and immunized rhesus with the soluble and refolded forms of the P. cynomolgi MSP-1₄₂ (PcMSP-1₄₂) proteins. All proteins induced equally high-titer, cross-reacting antibodies. Upon challenge with P. cynomolgi, none of the MSP-1₄₂-vaccinated groups demonstrated sterile protection or a delay in the prepatent period. However, following an initial rise in parasitemia, all MSP-1-vaccinated animals had significantly lower parasite burdens as indicated by lower cumulative parasitemia, lower peak parasitemia, lower secondary peak parasitemia, and lower average daily parasitemia compared to the adjuvant control group (P < 0.05). Except the soluble PcMSP-1₄₂ group, monkeys in all other groups had fewer numbers of days with parasitemia of >10,000 parasites mm^–3. Interestingly, there was no significant difference in the level of partial protection observed in the homologous and heterologous groups in this challenge model. The soluble and refolded forms of PcMSP-1₄₂ and PvMSP-1₄₂ proteins also appeared to have a similar partially protective effect.

CDRI communication no. 6711.

Editor: W. A. Petri, Jr.

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