Trends in Parasitology

Trends in Parasitology

Volume 29, Issue 1, January 2013, Pages 26-34
Journal home page for Trends in Parasitology

Review
Monocytes and macrophages in malaria: protection or pathology?

https://doi.org/10.1016/j.pt.2012.10.002Get rights and content

Recruitment and activation of monocytes and macrophages are essential for clearance of malaria infection, but these have also been associated with adverse clinical outcomes. In this review we discuss recent discoveries on how distinct molecular interactions between monocytes, macrophages, and malaria parasites may alter the balance between protection and pathology in malaria-infected individuals. The immunopathology of severe malaria often originates from excessive immune activation by parasites. The involvement of monocytes and macrophages in these events is highlighted, and priorities for future research to clarify the roles of these cells in malaria are proposed. Knowledge of the factors influencing the balance between protection and pathology can assist in the design of therapeutics aimed at modulating monocyte and macrophage functions to improve outcomes.

Section snippets

Monocytes, macrophages, and malaria

Whether an individual develops asymptomatic, mild, or severe malaria is partly dependent upon the balance between protective immunity and immunopathology. However, the factors that drive host immunity towards either protection or pathology, contributing to differential susceptibility and response to malaria, are not well understood. A comprehensive understanding of how host immune components interact with Plasmodium spp. malaria parasites may assist improved management and the design of novel

Monocyte/mac activation by parasite components in malaria erythrocytic stages

Various parasite components can activate monocytes/macs, either directly or indirectly by first activating other host immune factors (Figure 1). After erythrocyte invasion by merozoites, major transformations are induced on the IE surface, including the insertion of parasite proteins [5] that can interact with receptors on the monocyte/mac surface 6, 7. Whole IEs, or microparticles from their plasma membrane (shown in mouse model) [8], are potent immunogens that can directly promote macrophage

Phagocytosis

Phagocytosis of IEs by monocytes/macs is an effective way of reducing parasitaemia. However, efficiencies of IE uptake may differ among individuals, resulting in varying degrees of protection and susceptibility to malaria. An important factor regulating the efficiency of monocyte/mac phagocytosis is the availability of opsonins. Opsonins, such as complement fragments and antibodies, bind to IEs to enhance their interaction with monocyte/mac phagocytic receptors (Figure 1). Complement-mediated

Immunopathology associated with monocytes/macs in malaria infection

Individuals who cannot effectively clear erythrocytic-stage malaria infection are at risk of progressing to severe disease. Recent studies suggesting a role for monocytes/macs in causing immunopathology in these individuals are discussed below, in the context of different manifestations of severe malaria.

Concluding remarks

Recent studies have demonstrated that monocytes/macs are involved in both protection and immunopathology during malaria infection. The level of protection afforded by monocytes/macs through phagocytosis, ADCI, and cytokine production may differ between individuals, depending upon various parasite and host genetic factors. Monocytes/macs can also contribute to severe malaria manifestations through accumulation in the microvasculature and via various effector responses that may disrupt host

Acknowledgements

C.C. is supported by a Melbourne International Research Scholarship (University of Melbourne), a Melbourne International Fee Remission Scholarship (University of Melbourne) and a Nossal Institute for Global Health Travel Scholarship (The Nossal Institute for Global Health). J.A.H. is a Senior Principal Research Fellow of the National Health and Medical Research Council (NHMRC). This work was supported in part by Project Grants from the Australian NHMRC to S.J.R. (70318) and P.B. (1003384).

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