When immunoglobulin G (IgG) was incubated with Spirometra mansoni plerocercoid (sparganum), it was cleaved into Fab and Fc fragments. Fab/c fragments were also hydrolysed. The digestion was accelerated by dithiothreitol (DTT), indicating that cleavage of IgG heavy chain was due to a cysteine protease secreted into the medium. The responsible enzyme, of Mr 27 (± 0·8) kDa, was purified by a series of thiopropyl affinity, Sephacryl S-300 HR and DEAE-anion exchange chromatographies, either from worm extracts or from excretory–secretory products (ESP). The purified, thiol-dependent protease showed an optimal activity at pH 5·7 with 0·1 M sodium acetate but was active over the pH range 4·5–8·0. Its activity was inhibited completely by 10−5 M L-trans-epoxysuccinylleucylamido(4-guanidino) butane (E-64) and 1 mM iodoacetamide (IAA), but by only 53% using the specific cathepsin L inhibitor, Z-Phe-Phe-CHN2 (5 × 10−5 M). Partial NH2-terminal amino acid sequence was Leu-Pro-Asp-Ser-Val-Asn-Trp-Arg-Glu-Gly-Ala-Val-Thr-Ala-Val which showed 80% homology to human cathepsin S. Immunoblot analysis showed that sera from infected patients exhibited IgE antibody reaction. It is proposed that cleavage of immunoglobulin by an excreted–secreted, cathepsin S-like, allergenic protease is a mechanism of immune evasion used by the sparganum.
Prevalence of Spirometra mansoni in dogs, cats, and frogs and its medical relevance in Guangzhou, China
