Crystal structure of Trichinella spiralis calreticulin and the structural basis of its complement evasion mechanism involving C1q

Helminths produce calreticulin (CRT) to immunomodulate the host immune system as a survival strategy. However, the structure of helminth-derived CRT and the structural basis of the immune evasion process remains unclarified. Previous study found that the tissue-dwelling helminth Trichinella spiralis produces calreticulin (TsCRT), which binds C1q to inhibit activation of the complement classical pathway. Here, we used x-ray crystallography to resolve the structure of truncated TsCRT (TsCRT Δ ), the first structure of helminth-derived CRT. TsCRT Δ was observed to share the same binding region on C1q with IgG based on the structure and molecular docking, which explains the inhibitory effect of TsCRT on C1q-IgG–initiated classical complement activation. Based on the key residues in TsCRT Δ involved in the binding activity to C1q, a 24 amino acid peptide called P TsCRT was constructed that displayed strong C1q-binding activity and inhibited C1q-IgG–initiated classical complement activation. This study is the first to elucidate the structural basis of the role of TsCRT in immune evasion, providing an approach to develop helminth-derived bifunctional peptides as vaccine target to prevent parasite infections or as a therapeutic agent to treat complement-related autoimmune diseases.