INTEGUMENT ULTRASTRUCTURE OF ESTRUS-OVIS (L) (DIPTERA, OESTRIDAE) LARVAE - HOST IMMUNE-RESPONSE TO VARIOUS CUTICULAR COMPONENTS

The nasal bot fly, Oestrus ovis, was investigated to establish which s pecific cuticular component is most immunogenic to infested sheep and how larval cuticle attains a protective role, if any, against the host immune system. To accomplish these goals, larval cuticle was extracte d by a variety of agents and tested against immune sera from infested sheep and experimentally immunized rabbits, The cuticle substructure r emaining after extraction was examined to localize various immunogenic components. O. ovis larval integument comprises an inner cellular lay er, the epidermis, and an overlying cuticle layer. In 3rd instar larva e, the cuticle comprises 2 additional layers: the procuticle with nume rous pore canals and the epicuticle which includes the wax canals. Thr ee additional layers, altogether comprising the cuticulin layer, are p resent external to the epicuticle. The epicuticle is completed by appo sition of an amorphous electrondense material extending for up to 1 mu m in thickness, When fixed with ruthenium red, cuticle becomes heavil y stained all along the epicuticular surface in larvae of all developm ental stages, However, in 3rd instar larvae, ruthenium red deposits ar e restricted to the cuticulin layer alone. By gel electrophoresis, 3rd instar larval cuticle is shown to contain a number of polypeptides ra nging in molecular weight from 180 to 4.5 kDa. The number and relative concentration of low molecular weight polypeptides was shown to vary in relation to the extraction media employed. Cuticular fragments exam ined after extraction exhibit an altered ultrastructure. When tested b y immunoblotting, the cuticular polypeptides most reactive against she ep antisera are in the range of 180-56 kDa. A similar reaction was als o detected with sera from rabbits infested experimentally with O. ovis larvae. Results are interpreted in relation to differential polypepti de distribution within the larval cuticle and to accessibility of the host immune system. (C) 1997 Australian Society for Parasitology.