Protective immunity linked with a distinct developmental stage of a filarial parasite.

Repeated low dose infections of the jird Meriones unguiculatus, with the filarial parasite Acanthocheilonema viteae cause a substantial reduction of the total worm burden, suggesting a parasite-driven immune mechanism that controls super-infections. Quantitative recovery of parasites from tissues of triple infected jirds reveals that the larvae derived from a subsequent challenge infection are inactivated or severely impaired several days after transmission, precisely during their molt from the L3 to the L4 stage. Moreover, only larvae undergoing the molt from L3 to L4 stages are capable of stimulating an immune response directed against the challenge infection, indicating that protective Ag are produced during the molting period. Consistent with this, inactivated L3 or live L4 do not produce the same effect. In contrast to susceptible animals, immune jirds elicit high serum antibody titers against molting Ag. Indirect fluorescence antibody-binding tests with sera from protected jirds reveal specific labeling of the surface of molting L3 and not other larval stages, implying a stage-specific elimination process. The identification of molting L3 as a natural target for host immune mechanisms, emphasizes the central importance of this larval stage for future efforts aimed toward the development of a filarial vaccine.

Changes in the surface composition after transmission of Acanthocheilonema viteae third stage larvae into the jird.

This study describes the dynamics and the biochemical nature of changes in the surface of the filarial nematode Acanthocheilonema viteae after its transmission into the vertebrate host. Vector-derived third-stage larvae (mL3) were inoculated into naive Meriones unguiculatus and recovered from the tissues at different times post-infection until their moult to fourth-stage larvae (L4). Surface-specific labelling with fluoresceinated lectins revealed that the larvae are covered by a carbohydrate envelope. Although the mL3 envelope was strongly reduced one day after transmission, new surface carbohydrates appeared until the onset of moulting, some of which could also be identified on the surface of L4. In general, surface carbohydrates were partially shed by moving larvae, suggesting a loose association of these components in the epicuticle. The fate of cuticular lipids and proteins of L3 and L4 was monitored by external 125I-labelling and differential extraction of the components. Thin-layer chromatography of surface-labelled lipids revealed only minor changes 1 day after parasite transmission. Afterwards the number of lipids accessible to label decreased further until moulting was complete. Two-dimensional sodium dodecyl sulphate-polyacrylamide gel electrophoresis of surface-labelled proteins showed a consistent surface exposure of mL3 specific proteins until 1 day post-infection. Thereafter, the composition of surface-labelled proteins changed rapidly, resembling that of the L4 as early as several days before moulting. During this period individual differences in the composition of surface proteins were evident.

Recovery, distribution, and development of Acanthocheilonema viteae third- and early fourth-stage larvae in adult jirds.

Living third- and fourth-stage larvae (L3 and L4) of Acanthocheilonema viteae were recovered quantitatively from adult Meriones unguiculatus within the first 10 days after subcutaneous inoculation of 60 arthropod-derived larvae (mL3). The average recovery of the inoculated larvae was about one third (28.5%), and the majority (87.7%) were found in muscular tissues. Seventy-two hours after inoculation, larvae could be isolated from all body locations, although the majority still was found near the site of inoculation. Morphological and biometrical data indicated that, at least until molting, the development of the larval population was not synchronous, with molting occurring over a period of 48 hr on days 7 and 8 postinoculation. The stomatal rings of postinvasive L3's and L4's were distinguishable structurally and could be used as stage-specific determinants. Immediately after infection, L3's showed a linear growth in diameter; rapid longitudinal growth started after the molt, leading to a doubling in the length of L4's within 4 days. The time course of shedding was reconstructed in detail using isolated L3/L4 intermediates.