Subcutaneous Immunization with Unaltered Axenic Malaria Parasite Liver Stages Induces Sterile Protection against Infectious Sporozoite Challenge

Abstract

Host cell-free, axenic development of liver stages (LS) of the malaria parasite has been demonstrated. Here we explored axenic liver stages as a novel live whole parasite malaria vaccine platform, which is unaltered and not prone to human-error, compared to the immunization with live-attenuated sporozoites that must be done intravenously. We show that in contrast to live sporozoites, axenic LS are not infectious to the immunized host. Subcutaneous immunizations of mice with Plasmodium yoelii axenic LS, developed from wild-type (WT) sporozoites or WT sporozoites expressing enhanced-GFP, conferred sterile protection against P. yoelii infectious sporozoite challenge. Thus, axenic liver stages of P. falciparum and P. vivax might constitute an attractive alternative to live sporozoite immunization.

Keywords: Plasmodium yoelii; axenic liver stage; malaria; subcutaneous immunization; vaccine.

Figure 1

Axenic transformation of eGFP expressing P. yoelii p230p(−) salivary gland sporozoites into LS. In-vitro host cell free cultured salivary gland sporozoites completely transform into early LS after 24 hrs of incubation. To follow the cell free transformation of salivary gland sporozoites into early exoerythrocytic forms, we used eGFP expressing Pyp230p(−) and followed the transformation by confocal microscopy of cultured sporozoites at 6 hrs intervals for 24 hrs. At 28th of Axenic LS culture we could not find any good GFP images. Scale bars 2 µm.