Fasciola hepatica is refractory to complement killing by preventing attachment of mannose binding lectin (MBL) and inhibiting MBL-associated serine proteases (MASPs) with serpins.

Carolina De Marco Verissimo,József Dobó,John P Dalton,Krystyna Cwiklinski,Heather L Jewhurst,Péter Gál,Patrick J Skelly

doi:10.1371/journal.ppat.1010226

Abstract

The complement system is a first-line innate host immune defence against invading pathogens. It is activated via three pathways, termed Classical, Lectin and Alternative, which are mediated by antibodies, carbohydrate arrays or microbial liposaccharides, respectively. The three complement pathways converge in the formation of C3-convertase followed by the assembly of a lethal pore-like structure, the membrane attack complex (MAC), on the pathogen surface. We found that the infectious stage of the helminth parasite Fasciola hepatica, the newly excysted juvenile (NEJ), is resistant to the damaging effects of complement. Despite being coated with mannosylated proteins, the main initiator of the Lectin pathway, the mannose binding lectin (MBL), does not bind to the surface of live NEJ. In addition, we found that recombinantly expressed serine protease inhibitors secreted by NEJ (rFhSrp1 and rFhSrp2) selectively prevent activation of the complement via the Lectin pathway. Our experiments demonstrate that rFhSrp1 and rFhSrp2 inhibit native and recombinant MBL-associated serine proteases (MASPs), impairing the primary step that mediates C3b and C4b deposition on the NEJ surface. Indeed, immunofluorescence studies show that MBL, C3b, C4b or MAC are not deposited on the surface of NEJ incubated in normal human serum. Taken together, our findings uncover new means by which a helminth parasite prevents the activation of the Lectin complement pathway to become refractory to killing via this host response, in spite of presenting an assortment of glycans on their surface.

Highlights

The complement system is the frontline immune defence against invading microorganisms and parasites [1, 2]
We discovered that the infective stage of F. hepatica, the newly excysted juvenile (NEJ), is resistant to the damaging effects of complement by using unique means that consists of: (1) preventing binding of the main initiator of this cascade, the mannose binding lectin (MBL), to the parasite surface; and (2) secreting two protease inhibitors, FhSrp1 and FhSrp2, that inactivate the main serine proteases involved in the Lectin pathway activation, namely MBL-associated serine proteases (MASPs)
We calculated that the incubation of F. hepatica NEJ with normal human sera (NHS) for 1 hr reduced membrane attack complex (MAC) formation via the Lectin pathway >97% when compared to control NHS incubated in the same conditions

Summary

Introduction

The complement system is the frontline immune defence against invading microorganisms and parasites [1, 2]. The three pathways that activate the complement system, namely the Classical, Lectin and Alternative, consist of more than 35 plasma and membrane-associated proteins organized in a well-balanced network. Even though the complement system is present in tissues, fluids and blood, it often fails to kill protozoa and helminth parasites within these host compartments [1, 4], suggesting that parasites have developed effective mechanisms to evade or subvert this system. It is not surprising, that attention has been focused on uncovering parasite-specific mechanisms and molecules involved in this complement escape. Helminth parasites of the genus Schistosoma and Echinococcus, and protozoans of the genus Trypanosoma and Leishmania have been shown to avoid complement attack or complement mediated responses mainly by (1) avoiding recognition by complement activators, e.g., antibodies and mannosebinding lectins, (2) varying or changing their surface components, and (3) expressing regulators of complement activation as secreted or membrane-associated products [1, 4, 6,7,8,9,10,11]

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