Developmentally regulated expression and secretion of a polymorphic antigen by Onchocerca infective-stage larvae.

In order to analyse the developmental biology of Onchocerca spp. with a view to identifying molecules with specialised functions, we have devised a novel method for labelling proteins synthesised by larvae during growth in the vectors. Pulse labelling of Onchocerca lienalis by micro-injections of [35S]methionine into blackflies have revealed a major acidic protein of 23 kDa which is developmentally expressed almost exclusively by infective, third-stage larvae. The protein appears to be antigenically conserved between O. lienalis and Onchocerca volvulus, but exhibits size polymorphisms both among species and among individual organisms. It continues to be elaborated after terminal differentiation of the parasite in flies, but not by post-infective larvae entering the phase of development in the vertebrate host. A shift in temperature from 26 degrees C to 37 degrees C triggers secretion of the 23-kDa molecule as a discrete event 24-72 h after transmission. The labelling technique has been successfully employed with filarial species that develop in mosquitoes, and in principle should be widely applicable to the study of endoparasite gene expression within arthropods.