Malarial pigment hemozoin and the innate inflammatory response

Abstract

Malaria is a deadly infectious disease caused by the intraerythrocytic protozoan parasite Plasmodium. The four species of Plasmodium known to affect humans all produce an inorganic crystal called hemozoin (HZ) during the heme detoxification process. HZ is released from the food vacuole into circulation during erythrocyte lysis, while the released parasites further infect additional naive red blood cells. Once in circulation, HZ is rapidly taken up by circulating monocytes and tissue macrophages, inducing the production of pro-inflammatory mediators, such as interleukin-1β (IL-1β). Over the last few years, it has been reported that HZ, similar to uric acid crystals, asbestos, and silica, is able to trigger IL-1β production via the activation of the NOD-like receptor containing pyrin domain 3 (NLRP3) inflammasome complex. Additionally, recent findings have shown that host factors, such as fibrinogen, have the ability to adhere to free HZ and modify its capacity to activate host immune cells. Although much has been discovered regarding NLRP3 inflammasome induction, the mechanism through which this intracellular multimolecular complex is activated remains unclear. In the present review, the most recent discoveries regarding the capacity of HZ to trigger this innate immune complex as well as the impact of HZ on several other inflammatory signaling pathways will be discussed.

Keywords: hemozoin; inflammasome; macrophages; malaria; signaling.

Figure 1

Synthetic hemozoin analysis. Scanning electron micrographs of rapid crystalline HZ (rcHZ) and Plasmodium falciparum-native HZ. X-ray powder diffraction (XRD) analysis allows the determination of the quality of the crystal. Taken from Ref. (86).

Figure 2

In situ localization of Plasmodium hemozoin and parasitic DNA. Confocal pictures of RBC from Plasmodium chabaudi DK-infected mice. Selected images of schizonts and late trophozoites stages of iRBC. DAPI staining was used to visualize malarial DNA (blue). No staining was required to localize HZ since it autofluorescences (red). Even after merozoite release from the iRBC (see top right panel), malarial DNA was never in contact with hemozoin. Images were taken from Ref. (86).

Figure 3

Inflammatory biomarkers adhering to hemozoin. Sera biomarkers from malaria patients and healthy individuals were detected using Western blotting. The membranes were blotted using antibodies specific for gelsolin, LPS-binding protein (LBP), serum amyloid A (SAA), apolipoprotein E (ApoE), and serum albumin. C1–C8, control; M1–M8, malaria. Figure was taken from the supplemental data of Ref. (116).

Figure 4

Induction of NLRP3 inflammasome complex by the inorganic crystal HZ. HZ induces IL-1β production via the NLRP3/ASC inflammasome: activation of caspase-1 results in the cleavage of pro-IL-1β. Pro-IL-1β expression is resultant of the TLR4- and TNFα-activated NF-κB pathway or the IFNγ-stimulated STAT-1α pathway. The HZ-activated NLRP3 inflammasome is dependent on potassium efflux, ROS generation, HZ internalization, and cathepsin B activation. HZ internalization and the induction of SRC kinase signaling are mediated by lipid rafts. An ITAM-containing receptor could also be the starting point of SRC kinase cascade. HZ is internalized within a phagosome-like vesicle surrounded by LAMP-1. HZ activation of the SRC kinase Lyn leads to Syk phosphorylation. Subsequently, Syk positively modulates cathepsin B activation, which could result in the induction of the NLRP3 inflammasome. HZ is also capable of activating the NLRP3 inflammasome through PI3 kinase. The involvement of malarial DNA, which can adhere to HZ, in the activation of an intracellular receptor and its biological relevance are controversial and only reported for dendritic cells, which are present in limited numbers in the blood. HZ can interact with a large number of inflammatory-related biomarkers found in the circulation of P. falciparum-infected patients. However, the effect of these biomarkers on NLRP3 inflammasome activation is still unknown. Continuous arrows indicate a positive modulation. Dotted arrows indicate a hypothetical effect. Dotted arrows with a question mark indicate an unknown or controversial effect.