Trichinella spiralis Calreticulin S-Domain Binds to Human Complement C1q to Interfere With C1q-Mediated Immune Functions.

Shuai Shao,Chunyue Hao,Qinghui Zhuang,Bin Zhan,Xinping Zhu,Limei Zhao,Jingjing Huang,Yi Chen

doi:10.3389/fimmu.2020.572326

Shuai Shao, Chunyue Hao + Show 6 more

Open Access

https://doi.org/10.3389/fimmu.2020.572326

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Abstract

Helminths develop strategies to escape host immune responses that facilitate their survival in the hostile host immune environment. Trichinella spiralis, a tissue-dwelling nematode, has developed a sophisticated strategy to escape complement attack. Our previous study demonstrated that T. spiralis secretes calreticulin (TsCRT) to inhibit host classical complement activation through binding to C1q; however, the C1q binding site in TsCRT and the specific mechanism involved with complement-related immune evasion remains unknown. Using molecular docking modeling and fragment expression, we determined that TsCRT-S, a 153-aa domain of TsCRT, is responsible for C1q binding. Recombinant TsCRT-S protein expressed in Escherichia coli had the same capacity to bind and inhibit human C1q-induced complement and neutrophil activation, as full-length TsCRT. TsCRT-S inhibited neutrophil reactive oxygen species and elastase release by binding to C1q and reduced neutrophil killing of newborn T. spiralis larvae. Binding of TsCRT-S to C1q also inhibited formation of neutrophil extracellular traps (NETs), which are involved in autoimmune pathologies and have yet to be therapeutically targeted. These findings provide evidence that the TsCRT-S fragment, rather than the full-length TsCRT, is a potential target for vaccine or therapeutic development for trichinellosis, as well as for complement-related autoimmune disease therapies.

Highlights

Trichinellosis, caused by the parasitic nematode, Trichinella spiralis, is a serious food-borne parasitic zoonosis affecting more than 11 million people worldwide [1]
Treatment with rTsCRT-S or rTsCRT alone, without C1q, yielded no obvious fluorescence detection on cells. This inhibition was detected by immunofluorescence using an anti-C1q antibody (Figure 5B). These results indicate that the binding of rTsCRT-S or rTsCRT to C1q can inhibit C1q binding to the C1q receptor on neutrophillike dHL60 cells, possibly through competitive binding to C1q sites that bind to dHL60
The results showed that C1q alone attracted dHL60 migration through the inserted membrane; C1q-induced migration was significantly inhibited after C1q was incubated with different concentrations of rTsCRT-S (Figure 6A). rTsCRT significantly inhibited C1q-induced neutrophil cell migration

Summary

Introduction

Trichinellosis, caused by the parasitic nematode, Trichinella spiralis, is a serious food-borne parasitic zoonosis affecting more than 11 million people worldwide [1]. TsCRT is a calreticulin protein secreted by T. spiralis that has important roles in immunomodulation of the host immune system, including direct binding to the complement component, C1q [5], which is the initiator of the classical complement activation pathway, triggering a multistep activation cascade on binding to an immune complex (IC) [6]. T. spiralis is a tissue-dwelling nematode, and its larvae invade intestinal epithelial cells and migrate to striated muscle, where they form an infective encysted ML. During this stage, the nematode is vulnerable to the host immune response, complement attack, and TsCRT is a multifunctional protein expressed by the nematode to bind to C1q and inactivate complement-mediated damage [5]. Like the intracellular protozoa Trypanosoma cruzi, filarial parasite Brugia malayi, hookworm Necator americanus, and nematode Haemonchus contortus produce calreticulin to bind with C1q and inhibit classical complement activation on the parasite surface, thereby avoiding opsonization, immune stimulation, and lytic effects, as an immune evasion strategy [9,10,11,12,13]

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