Cloning and characterization of a potentially protective Chitinase-like Recombinant Antigen from Wuchereria bancrofti

Abstract

While there is no direct evidence demonstrating the existence of protective immunity to Wuchereria bancrofti infection in humans, the presence of individuals, in populations in areas where infection is endemic, with no clinical evidence of past or current infection despite appreciable exposure to the infective larvae, suggests that protective immunity to filarial parasites may occur naturally. Earlier work indicated that such putatively immune individuals generated antibodies to a 43-kDa antigen from larval extracts of the related filarial parasite Brugia malayi that was recognized by only 8% of the infected population. With rabbit antiserum raised against this 43-kDa antigen, this current study identified a recombinant clone, WbN43, with an insert size of 2.3 kb, from a W. bancrofti genomic expression library. The recombinant fusion protein was differentially recognized by the putatively immune individuals but not by the infected patients. The coding sequence (684 bp) from the 5' end had significant sequence similarity to chitinases from Serratia marcescens, BaciUlus circulans, Streptomyces plicatus, and B. malayi. Peptide sequencing of the expressed product also defined a chitinase-like sequence. Molecular characterization indicated WbN43 to be a low-copy-number gene, with expression predominantly in infective larvae and microfilariae but not in adult parasites.The presence, in areas where lymphatic filariasis is endemic, of a variable proportion of the adult population without any clinical manifestations of infection despite life-long exposure to the infective third-stage (L3) larvae, has long been an observation suggestive of the existence of protective immunity. Since it is impossible to determine resistance to reinfection directly, the study of such immunity in human populations has been particularly difficult because of the inability to distinguish between truly immune individuals and those considered concomitantly immune (infected but resistant to reinfection).