Proteases from Schistosoma mansoni cercariae cleave IgE at solvent exposed interdomain regions.

Parasitic infections, including schistosomiasis, are associated with high titres of specific and non-specific IgE antibody, and many reports show an in vitro role for IgE in parasite killing. Despite an active immune response, schistosomes survive for long periods in the human bloodstream, implying that the parasite is able to overcome or evade the IgE response mounted against it. One such mechanism is through cleavage of IgE into non-functional fragments by potent parasite derived enzymes. Using domain swap antibodies, recombinant Fcepsilon, and C-terminally tagged Cepsilon4 domains, we have narrowed down the principal cleavage sites to the Cepsilon2/Cepsilon3 and Cepsilon3/Cepsilon4 interdomain region of the IgE-Fc. Two serine proteases, one chymotrypsin-like and the second trypsin-like, have been proposed to be involved. Inhibition assays using selective inhibitors confirmed that both proteases contribute to Fc cleavage, although the chymotrypsin-like enzyme makes the greater contribution. Protein sequencing of IgE fragments cleaved by highly pure preparations of the chymotrypsin-like enzyme revealed that cleavage also occurred post Lys residues within kappa light chain dimers (LELK/GA). Related sequences are found in myosin, thrombospondin, collagen and actin-related proteins; macromolecules present in the skin and through which cercariae must penetrate to initiate an infection. Chemical knockout experiments using specific inhibitors and chromogenic substrates allowed us to show that the trypsin-like enzyme was responsible for light chain cleavage. The finding that pathogenic proteases can cleave the Fc of IgE may provide a useful biochemical tool for the further analysis of IgE structure. Indeed, the finding may raise new possibilities for treatment of IgE-mediated allergic reactions mediated through Fcepsilon-receptors.

IgE-tailpiece associates with α-1-antitrypsin (A1AT) to protect IgE from proteolysis without compromising its ability to interact with FcεRI.

Several splice variants of IgE exist in human plasma, including a variant called IgE-tailpiece (IgE-tp) that differs from classical IgE by the replacement of two carboxy-terminal amino acids with eight novel residues that include an ultimate cysteine. To date, the role of the secreted IgE-tp isoform in human immunity is unknown. We show that levels of IgE-tp are raised in helminth-infected donors, and that both the classical form of IgE (IgE-c) and IgE-tp interact with polymers of the serine protease inhibitor alpha-1-antitrypsin (A1AT). The association of IgE-tp with A1AT polymers in plasma protects the antibody from serine protease-mediated degradation, without affecting the functional interaction of IgE-tp with important receptors, including FcεR1. That polymers of A1AT protect IgE from degradation by helminth proteases may explain why these common and normally non-disease causing polymorphic variants of A1AT have been retained by natural selection. The observation that IgE can be complexed with polymeric forms of A1AT may therefore have important consequences for our understanding of the pathophysiology of pulmonary diseases that arise either as a consequence of A1AT-deficiency or through IgE-mediated type 1 hypersensitivity responses.