Toll-like receptor 9 mediates innate immune activation by the malaria pigment hemozoin

Abstract

Malaria parasites within red blood cells digest host hemoglobin into a hydrophobic heme polymer, known as hemozoin (HZ), which is subsequently released into the blood stream and then captured by and concentrated in the reticulo-endothelial system. Accumulating evidence suggests that HZ is immunologically active, but the molecular mechanism(s) through which HZ modulates the innate immune system has not been elucidated. This work demonstrates that HZ purified from Plasmodium falciparum is a novel non-DNA ligand for Toll-like receptor (TLR)9. HZ activated innate immune responses in vivo and in vitro, resulting in the production of cytokines, chemokines, and up-regulation of costimulatory molecules. Such responses were severely impaired in TLR9−/− and myeloid differentiation factor 88 (MyD88)−/−, but not in TLR2, TLR4, TLR7, or Toll/interleukin 1 receptor domain–containing adaptor-inducing interferon β−/− mice. Synthetic HZ, which is free of the other contaminants, also activated innate immune responses in vivo in a TLR9-dependent manner. Chloroquine (CQ), an antimalarial drug, abrogated HZ-induced cytokine production. These data suggest that TLR9-mediated, MyD88-dependent, and CQ-sensitive innate immune activation by HZ may play an important role in malaria parasite–host interactions.

Inflammatory cytokine and humoral responses to Plasmodium falciparum glycosylphosphatidylinositols correlates with malaria immunity and pathogenesis

“…Several studies demonstrated that malaria infection induces activation of Toll‐like receptor (TLR1, TLR2 TLR4, and TLR9) and involvement of anti‐TLR2 and anti‐TLR4 antibodies 36, 37, 38. Krishnegowda et al have confirmed that GPIs are recognized mainly by TLR2 and to a lesser extent by TLR4 10.…”

Mast cell‐mediated immune responses through IgE antibody and Toll‐like receptor 4 by malarial peroxiredoxin

“…These results provide the first evidence that malarial Prx associates directly with TLR4. Recently, glycosyl phosphatidylinositol (GPI) [9] and hemozoin derived from Plasmodium parasites [10,11] have been identified as the ligands for TLR2 and TLR9, respectively. However, regarding TLR4 ligands in malarial studies, so far, there is no reported evidence.…”

“…However, we could not observe significant differences in parasitemia between TLR4 -/-and WT (data not shown). Since recent reported evidences indicate that glycosyl phosphatidylinositol (GPI) [9] and hemozoin derived from Plasmodium parasites [10,11] participate in innate immune responses in murine malaria, these findings might suggest that other malarial molecules also will be involved in the induction of innate resistance in murine malaria. To clarify whether malarial Prx induces a complete protective immunity in malaria or not, further experiments will be needed.…”

Effect of malaria components on blood mononuclear cells involved in immune response

“…These findings suggest that activation of TLRs by parasites such as GPIs and hemozoin transmits signals by intracellular pathway [25,37] , leading to activation of transcription factor NF-κB, which in turn propagates signals to the nucleus to regulate the expression of pro-inflammatory cytokines [26] . Consequently, during malaria infection, increased levels of phospho-NF-κB and nuclear translocation of NF-κB in the host immune cells are stimulated by parasite products which subsequently release and contribute to inflammatory mediators.…”