The IgA-Binding Lectin Jacalin Induces Complement Activation by Inhibition of -Inactivator Function

Abstract

Jacalin. a D‐galaetose‐specific lectin from jackfruit, interacts with human IgA and one or two other serum proteins. Incubation ol jaealin with fresh human serum was shown to result in activation of the complement system. Therefore the mechanism of complement activation by jacalin was studied. Jacalin was extracted from jackfruit seeds (crude preparation) and purified to homogeneity by affinity chromatogtaphy on lgA‐Sepharose to yield a pure preparation of jacatin. Both crude and pure Jacalin were able to activate complement, accompanied by conversion of C3. Consumption of C1, C4 and C1 ‐inacuvator(Cl ‐In) indicated involvement of the classical pathway. Aggregated IgG (AlgGl caused partial (38%) and jacalin induced complete consumption of Cl ‐ln functional activity, It was found upon Ouchterlony analysis that jacalin forms a precipitalien line with purified Cl ‐In. In addition binding of ¹²⁵I‐C1 ‐In to jacalin‐Scpharose was observed, and this binding was inhibitable by either secretory IgA or D‐galactose. Next to binding of jacalin to Cl ‐In. jacalin was also shown to inhibit the functional activity of Cl ‐In. These results indicate that jacalin induces complement activation by inhibition of Cl ‐In function and thereby facilitates the activation of precursor Cl in either the absence or presence of low amounts of Cl aetivators.