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Open Access Highly Accessed Research

Opsonization of malaria-infected erythrocytes activates the inflammasome and enhances inflammatory cytokine secretion by human macrophages

Jingling Zhou1, Louise E Ludlow45, Wina Hasang45, Stephen J Rogerson45 and Anthony Jaworowski123*

Author Affiliations

1 Centre for Virology, Burnet Institute, PO Box 2284, Melbourne, Victoria, 3001, Australia

2 Department of Medicine, Monash University, Melbourne, Vic, 3004, Australia

3 Department of Immunology, Monash University, Melbourne, Vic, 3004, Australia

4 Department of Medicine (RMH), Centre for Medical Research, Royal Melbourne Hospital, University of Melbourne, Melbourne, Vic, 3050, Australia

5 Victorian Infectious Diseases Service, Royal Melbourne Hospital, Parkville, Vic, 3050, Australia

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Malaria Journal 2012, 11:343  doi:10.1186/1475-2875-11-343

Published: 9 October 2012

Abstract

Background

Antibody opsonization of Plasmodium falciparum-infected erythrocytes (IE) plays a crucial role in anti-malarial immunity by promoting clearance of blood-stage infection by monocytes and macrophages. The effects of phagocytosis of opsonized IE on macrophage pro-inflammatory cytokine responses are poorly understood.

Methods

Phagocytic clearance, cytokine response and intracellular signalling were measured using IFN-γ-primed human monocyte-derived macrophages (MDM) incubated with opsonized and unopsonized trophozoite-stage CS2 IE, a chondroitin sulphate-binding malaria strain. Cytokine secretion was measured by bead array or ELISA, mRNA using quantitative PCR, and activation of NF-κB by Western blot and electrophoretic mobility shift assay. Data were analysed using the Mann–Whitney U test or the Wilcoxon signed rank test as appropriate.

Results

Unopsonized CS2 IE were not phagocytosed whereas IE opsonized with pooled patient immune serum (PPS) were (Phagocytic index (PI)=18.4, [SE 0.38] n=3). Unopsonized and opsonized IE induced expression of TNF, IL-1β and IL-6 mRNA by MDM and activated NF-κB to a similar extent. Unopsonized IE induced secretion of IL-6 (median= 622 pg/ml [IQR=1,250-240], n=9) but no IL-1β or TNF, whereas PPS-opsonized IE induced secretion of IL-1β (18.6 pg/mL [34.2-14.4]) and TNF (113 pg/ml [421–17.0]) and increased IL-6 secretion (2,195 pg/ml [4,658-1,095]). Opsonized, but not unopsonized, CS2 IE activated caspase-1 cleavage and enzymatic activity in MDM showing that Fc receptor-mediated phagocytosis activates the inflammasome. MDM attached to IgG-coated surfaces however secreted IL-1β in response to unopsonized IE, suggesting that internalization of IE is not absolutely required to activate the inflammasome and stimulate IL-1β secretion.

Conclusions

It is concluded that IL-6 secretion from MDM in response to CS2 IE does not require phagocytosis, whereas secretion of TNF and IL-1β is dependent on Fcγ receptor-mediated phagocytosis; for IL-1β, this occurs by activation of the inflammasome. The data presented in this paper show that generating antibody responses to blood-stage malaria parasites is potentially beneficial both in reducing parasitaemia via Fcγ receptor-dependent macrophage phagocytosis and in generating a robust pro-inflammatory response.

Keywords:
Plasmodium falciparum; Human; Monocyte-derived macrophages; Antibody; Fc gamma receptor; Phagocytosis; Pro-inflammatory cytokines