COMPARISON OF HUMORAL IMMUNE-RESPONSES ELICITED BY DNA AND PROTEIN VACCINES BASED ON MEROZOITE SURFACE PROTEIN-1 FROM PLASMODIUM-YOELII, A RODENT MALARIA PARASITE

Immunization with DNA vaccines encoding relevant Ags can induce not on ly cell-mediated immune response but also humoral immune responses aga inst pathogenic microorganisms in several animal models. Our previous results demonstrated that, when the C terminus (PyC2) of Plasmodium yo elii merozoite surface protein-1 (MSP-1) a leading vaccine candidate a gainst erythrocytic stages of malaria, was expressed as a fusion prote in (GST-PyC2) with glutathione S-transferase (GST), it elicited Ah-med iated protective immune responses in BALB/c mice. In our present study , we wished to examine the humoral responses to a DNA vaccine (V3) enc oding GST-PyC2, The GST-PyC2 expressed in V3-transfected Cos 7 cells w as recognized by a protective monoclonal Ab to PyC2 (mAb302), although the secreted product had undergone N-linked glycosylation. When BALB/ c mice were immunized with V3 plasmid, anti-PyC2 Abs were successfully induced. These Abs immunoprecipitated native PyMSP-1 protein and comp eted with mAb302 for binding to its epitope at a level similar to thos e elicited by GST;PyC2 protein immunization. However, these Abs had si gnificantly lower titers and avidities, and different isotype profiles and protective capacities against a lethal erythrocytic stage challen ge, than those resulting from immunization with GST-PyC2 protein. Most surprising was the finding that, in contrast to protein immunization, there was no significant increase in the avidity of either GST-specif ic or PyC2-specific IgG Abs during the course of DNA immunization. Thi s suggests that there may be little or no affinity maturation of speci fic Abs during DNA immunization in this system.