Histidine-rich protein 2: a new pathogenic factor of Plasmodium falciparum malaria

Plasmodium falciparum causes serious malaria symptoms; when this protozoan parasite infects human erythrocytes, it produces and secretes large amounts of histidine–rich protein 2 (PfHRP2) into human blood. Thus, PfHRP2 is a well–known diagnostic marker for malaria infection. Here, however, we also identified PfHRP2 as a pathogenic factor produced by P. falciparum. PfHRP2 showed cell penetration and cytotoxicity against various human cells. In particular, PfHRP2 showed significant cytotoxicity over 5 days at the same concentration as in P. falciparum–infected patients′ blood (90–100 nM). This result is consistent with the mortality rate of P. falciparum malaria, which increases rapidly in untreated cases for 3–7 days. In addition, the cell penetration and cytotoxicity of PfHRP2 increased 2.5– and 2.6–fold, respectively, in the absence of serum, which suggests that low serum protein concentrations (occurring during malnutrition, for example) increase the risk of adverse effects from PfHRP2 (consistent with malnutrition increasing the lethality of malaria infection). We also showed that PfHRP2 bound to Ca2+ ions, localized to intracellular lysosomes, increased lysosomal Ca2+ levels, and inhibited the basal level of autophagy by inhibiting autolysosome formation. Furthermore, the Ca2+–dependent cytotoxicity of PfHRP2 was suppressed by the metal ion chelator ethylenediaminetetraacetic acid (EDTA). In summary, our findings suggest that PfHRP2 acts as a pathogenic factor in P. falciparum–infected patients and is associated with the exacerbation of malaria. Furthermore, EDTA is a promising candidate as a therapeutic agent for the suppression of PfHRP2 pathogenicity. Overall, this study provides new insights into P. falciparum malaria pathogenesis and treatment.