Mutant/mutation
The mutant, Pb-PfM19, expresses a mutated form of merozoite surface protein 1, precursor (MSP1). The P. berghei MSP-1 19 kD C-terminal region is replaced with the P. falciparum MSP-1 19 kD C-terminal region. The transfection vector was designed to integrate into P. berghei MSP-1 and replace the endogenous sequences encoding epidermal growth factor (EGF) domains 1 and 2, and the GPI recognition sequence, with the corresponding P. falciparum (D10 line) sequence.
A second chimeric line, Pb-PbM19, with a homologous P. berghei MSP-1₁₉ replacement, was also constructed to generate a control transfectant.

Protein (function)
MSP1 is an attractive target for antibody therapy. Animals actively or passively immunized against MSP1 can be protected against parasite challenge, and mAbs to MSP1 can block parasite invasion of erythrocytes in vitro. MSP1 undergoes dual proteolytic processing. It is initially cleaved (primary processing) into multiple fragments that form a protein complex on the merozoite surface. Then, at erythrocyte invasion, the protein is cleaved again (secondary processing) and shed from the surface, except for a C-terminal 19-kDa polypeptide (MSP119) that comprises two epidermal growth factor (EGF)-like domains and which is carried into the newly invaded erythrocyte.

Phenotype
To determine whether the mutants Pb-PfM19 and Pb-PbM19 lines expressed the expected MSP-119 domains, Western blot analysis was performed on late stage parasite extracts using specific anti–MSP-119 antibodies. P. falciparum MSP-119 antibodies recognized both MSP-119 and an 200-kD band corresponding to full-length MSP-1 in Pb-PfM19 parasites but not in Pb-PbM19 parasites. In contrast, antibodies specific for P. berghei MSP-119 only recognized MSP-119 and full-length MSP-1 in wild-type P. berghei and the control line, Pb-PbM19. This indicates that P. falciparum MSP-119 can be correctly expressed and processed in P. berghei and that the endogenous MSP-119 gene is no longer expressed in Pb-PfM19 parasites. The localization of MSP-119 in P. berghei lines was also assessed by double-labeling IFA. Characteristic merozoite surface labeling was observed in both chimera lines, with Pb-PfM19 parasites reacting only with the P. falciparum–specific monoclonal antibody 4H9/19, whereas P. berghei wild-type and Pb-PbM19 chimeric parasites reacted only with rabbit anti–P. berghei MSP-119 antibodies. This indicates that the appropriate MSP-119 domain is correctly localized in both Pb-PbM19 and Pb-PfM19 parasite lines.

Mice made semi-immune to mutant Pb-PfM19 line generate high levels of merozoite inhibitory antibodies that are specific for P. falciparum MSP-119. Protection from homologous blood stage challenge in these mice correlated with levels of P. falciparum MSP-119–specific inhibitory antibodies, but not with titres of total MSP-119–specific immunoglobulins. The results indicate that merozoite inhibitory antibodies generated in response to infection can play a significant role in suppressing parasitemia in vivo.

Additional information
The rodent malaria parasite line that expresses P. falciparum MSP-119 in place of its own domain can be used as a simple and robust model to analyse the protective role of inhibitory antibodies in vivo. In this study  it was shown that the level of MSP-119–specific invasion inhibitory antibodies generated in mice that had been repeatedly exposed to this chimeric parasite line correlates with the ability of these animals to control a subsequent blood stage infection. The availability of this novel rodent malaria model might be used as an alternative to nonhuman primates for assessing and monitoring P. falciparum MSP-119–based vaccines.

Other mutants
RMgm-330: Another mutant in which the P. berghei MSP-1 19 kD C-terminal region is replaced with the P. falciparum MSP-1 19 kD C-terminal region.