Plant Hormone Salicylic Acid Produced by a Malaria Parasite Controls Host Immunity and Cerebral Malaria Outcome

<div><p>The apicomplexan parasite <i>Toxoplasma gondii</i> produces the plant hormone abscisic acid, but it is unclear if phytohormones are produced by the malaria parasite <i>Plasmodium</i> spp., the most important parasite of this phylum. Here, we report detection of salicylic acid, an immune-related phytohormone of land plants, in <i>P</i>. <i>berghei</i> ANKA and <i>T</i>. <i>gondii</i> cell lysates. However, addition of salicylic acid to <i>P</i>. <i>falciparum</i> and <i>T</i>. <i>gondii</i> culture had no effect. We transfected <i>P</i>. <i>falciparum</i> 3D7 with the <i>nahG</i> gene, which encodes a salicylic acid-degrading enzyme isolated from plant-infecting <i>Pseudomonas</i> sp., and established a salicylic acid-deficient mutant. The mutant had a significantly decreased concentration of parasite-synthesized prostaglandin E₂, which potentially modulates host immunity as an adaptive evolution of <i>Plasmodium</i> spp. To investigate the function of salicylic acid and prostaglandin E₂ on host immunity, we established <i>P</i>. <i>berghei</i> ANKA mutants expressing <i>nahG</i>. C57BL/6 mice infected with <i>nahG</i> transfectants developed enhanced cerebral malaria, as assessed by Evans blue leakage and brain histological observation. The <i>nahG</i>-transfectant also significantly increased the mortality rate of mice. Prostaglandin E₂ reduced the brain symptoms by induction of T helper-2 cytokines. As expected, T helper-1 cytokines including interferon-γ and interleukin-2 were significantly elevated by infection with the <i>nahG</i> transfectant. Thus, salicylic acid of <i>Plasmodium</i> spp. may be a new pathogenic factor of this threatening parasite and may modulate immune function via parasite-produced prostaglandin E₂.</p></div

Identification of salicylic acid from <i>Plasmodium berghei</i> ANKA.

<p><i>P</i>. <i>berghei</i> ANKA was purified from infected mice blood, and salicylic acid (SA) was extracted, and analyzed by LC-triple TOF mass spectrometry. (A) Structural formula of SA. (B) LC chromatogram of SA standard (control) and <i>P</i>. <i>berghei</i> ANKA sample. (C) Fragmentation analysis of peaks in (B) (colored in aqua). Collision energy was 20 eV.</p

Parasite SA influences the cerebral malaria outcome.

<p>(A-C) Histological observation of infected mouse cerebellums. (A) Cerebellum infected by <i>nahG</i>-expressing parasites. Note the sequestrated leukocytes in microvessels. The inset image shows a higher magnification of the boxed portion. Phagocytized hemozoin is observed (arrowhead). (B) Brain of a mouse infected with <i>gfp</i>-expressing parasites. Slight microbleeding was observed, but no sequestrated vessels were found. (C) Brain of an uninfected control. Sections were stained by hematoxylin and eosin. (D) Evans blue leakage analysis of the severity of cerebral malaria. Photographs of brains from mice infected with <i>nahG</i>- (left upper) and <i>gfp-</i> (left middle) expressing parasites and uninfected controls (left bottom), and quantification of dye leakage (right). Mice (n = 5) were sacrificed at 6 days post-infection. Solid line, p<0.01; dashed line, p<0.05. C57BL/6 mice at 6 days post-infection were used for all experiments. Bar: 50 μm.</p