Malarial fever: hemozoin is involved but Toll-free.

T he gospel of Matthew (chapter 8, verse 14) reports that ‘‘Peter’s mother-in-law was sick of the fever,’’ and many commentaries think that this close relative of the apostle living in the Galilee 2,000 years ago was suffering from malaria. The main characteristic of malaria is fever, and periodic fevers have been reported even 3,000 years ago in early Chinese, Chaldean, Hindu, Egyptian, and Greek writings (1, 2). It is likely that some, although surely not all of them, were malarial. Major discoveries in the late 19th century by Laveran (identifying Plasmodium as the pathogen causing malaria) and by Ross (describing the life cycle of the parasite and mosquitoes as the vector) led to advances in partially controlling this devastating disease. It is, however, still affecting 400 million people worldwide, and its complications, such as cerebral malaria caused by Plasmodium falciparum, still have a high mortality rate (3). The pathophysiology of the host–parasite interaction and, particularly, the mechanism of fever induction in malaria until recently has been far from being understood. However, knowledge of the malaria pathology is urgently needed to potentially develop novel intervention strategies to efficiently reduce the high disease burden causing 2 million deaths per year. In this issue of PNAS, Parroche et al. (4) report on a novel mechanism that the host uses to recognize Plasmodium DNA via the Tolllike receptor (TLR) 9, which may be a key step for inducing fever during this disease. These findings reveal an important mechanism of disease pathophysiology that may also apply to other microbial diseases. The protein family of TLRs has been discovered recently and was functionally analyzed by scientists during the last 8 years. Together with the intracellular nodlike receptors (NLRs), they are key recognition molecules for microorganisms including viruses, bacteria, and parasites (5). Engagement of most of the TLRs triggers a signaling pathway leading to the translocation of the transcription factor NFB into the nucleus, followed by the subsequent release of proinflammatory cytokines. Apart from other activities, many of these factors induce the activation of the inducible cyclooxygenase (COX)-2, which up-regulates prostaglandin synthesis, changing the set-point of the thermoregulatory center of the host leading to fever (6). One group of the 11 members of the TLR family is located on the cell surface, recognizing cell wall components of microorganisms, whereas the other group is intracellularly expressed and recognizes nucleic acids, such as single-stranded RNA (TLR7 and TLR8), double-stranded RNA (TLR3), or DNA (TLR9). The innate immune system is the first line of defense potentially controlling microorganisms at an early stage by inducing inflammation and fever. Malaria is a disease leading to an extremely high pathogen burden on one hand and a typically strong febrile response on the other hand. It thus was of great interest to identify the specific molecular mechanisms of the early immune recognition of Plasmodium to understand disease pathophysiology and to potentially develop more successful vaccination and/or antiparasitic strategies. Protozoa, such as P. falciparum causing the most severe form of malaria, contain a glycosylphosphatidylinositol (GPI) anchor on the cell surface, and this molecule is recognized by TLR2 and TLR4 (reviewed in ref. 7). For malaria, however, the GPI anchor–TLR2 interaction is comparably weak; thus, other interactions with the TLR system have been suggested. Recently, Coban et al. (8) showed that hemozoin, the malaria pigment found in large quantities in macrophages during malaria, can stimulate the host via TLR9. These results were quite surprising because TLR9 had been convincingly described as a receptor for DNA, mainly of unmethylated, CpG-containing DNA, frequently found in bacteria. In this publication, ‘‘contaminating’’ DNA was ruled out by DNase treatment and the failure to result in a reduction in TLR9-stimulating capacity of the hemozoin preparations. Although this discrepancy remains unclear, the work of Parroche et al. (4) shows that it is plasmodial DNA instead that stimulates TLR9 of the host and that this activity is clearly DNase-sensitive. The Golenbock group (4), in addition, convincingly shows that hemozoin plays a specific role in presenting the DNA to the intracellular TLR9; however, it cannot stimulate the innate immune system by itself (Fig. 1). The principle of microbial DNA being recognized by the host is fascinating, and the work presented here gives a surprising