Identification of paramyosin as a binding protein for calgranulin C in experimental helminthic keratitis

PURPOSE. Calgranulin C (CaGC) is a protein released by activated neutrophils and involved in host defense against filarial infections. This study involved the identification of binding protein(s) of the helminth Brugia malayi to CaGC and the ability of binding complexes to induce keratitis. METHODS. Parasitic extracts prepared from B. malayi microfilariae and adult worms were incubated with recombinant CaGC protein. Parasite binding protein-CaGC complex was isolated by affinity chromatography. A B. malayi microfilariae cDNA library was immunoscreened with antisera from rats immunized with the isolated parasitic CaGC-binding protein. All positive clones contained paramyosin sequences. Paramyosin was thus considered the major CaGC-binding protein in the parasite. To delineate the binding of CaGC to native and recombinant paramyosin, ¹²⁵I-CaGC was used as a binding tracer in SDS-PAGE analysis to identify a CaGC-binding complex. To determine whether the complex of CaGC and its binding protein could induce keratitis mimicking the onchocercal human corneal disease, BALB/c mice preimmunized with the binding complex were challenged with intracorneal binding complex or live Brugia microfilariae In addition, splenocytes harvested from the same animals were assessed for their ability to elicit cellular immune responses to the binding complex by [³H]thymidine assay. RESULTS. In vitro binding of CaGC to paramyosin was confirmed using recombinant paramyosin and ¹²⁵I-CaGC. Test animals showed development of severe keratitis that mimicked, clinically and histopathologically, the human onchocercal corneal disease, demonstrating the antigenic specificity of the paramyosin-CaGG- binding complex. CONCLUSIONS. Paramyosin is identified as a CaGC-binding protein in B. malayi.