Taeniid eggs contain an infective larval form of the parasite, known as the oncosphere, which has been found to be highly susceptible to attack by the host's immune system and this fact has been exploited in the development of highly effective vaccines. Relatively little is known about the structure of taeniid oncospheres and the localization of host-protective antigens within or on the oncosphere. Here, we briefly review the current state of knowledge of the structure of the oncosphere and present preliminary data on the localization of a host-protective antigen within the oncospheres of Taenia ovis. The precise localization of the antigens, in the context of a detailed knowledge of the ultrastructure of the parasite, may reveal the immune mechanisms by which the taeniid parasites are killed by vaccine-induced immune responses, which, in turn, may provide clues about how vaccines could be developed against other parasitic helminths.

Rabbits were vaccinated once, by subcutaneous and intradermal injection, with sonicates of oncospheres (TpO) or conditioned media from in vitro maintained mature metacestodes (TpMcES) of Taenia pisiformis. Extracts were either incorporated into or mixed with unilammelar liposomes (reverse phase evaporative vesicles) or emulsified in Freund's Incomplete Adjuvant (FIA). Control groups received liposomes or FIA without antigen, or antigen preparation without adjuvant. Rabbits were challenged orally two weeks after vaccination with approximately 1500 eggs of T. pisiformis and necropsied eight weeks after challenge. A mean of 155 cysts was recovered from seven control rabbits. A 67% reduction in peritoneal cyst numbers was obtained in TpO–IFA vaccinated rabbits compared to 75% for the TpO–liposome entrapped group. The highest level of protection (86%) was obtained when TpO was mixed with but not entrapped in liposomes. Only 32% and 39% reduction in peritoneal cyst numbers was obtained after immunizing with the TpMcES preparation in liposomes or IFA respectively, however >85% of peritoneal metacestodes were dead (necrotic or calcified) and suggests a different immune response than occurs after vaccination with oncosphere extracts. Specific anti-oncospheral or anti-metacestode ES antibody (IgG) responses at two weeks post vaccination were similar in rabbits immunized with liposome or IFA associated extracts.

The carbohydrate constituents of whole eggs and fractions derived from the eggs of Hymenolepis diminuta were partly characterized. Whole eggs contained 9·6 ng of carbohydrate (phenol-sulphuric acid-positive material) per egg, and 3·6 ng of glucose. Analysis of hydrolysed eggs by HPLC demonstrated the presence of glucose, galactose, glycerol, N-acetylgalactosamine, and 2-deoxyribose. Isolated egg shells and the KOH-stable, ethanol-precipitable fraction (putative glycogen) of eggs contained 26 and 76%, respectively, of the total carbohydrate associated with eggs. Glucose and galactose were present in both of these samples, but only glucose was present in the KOH-stable, ethanol-precipitable fraction isolated from chemically deshelled eggs; thus, the source of the galactose in the KOH-stable, ethanol-precipitable fraction isolated from whole eggs was the egg-shells. These data confirm earlier reports that the shells contain carbohydrate; the data demonstrate further that only two monosaccharides (glucose and galactose) are present in the shells, and that they are present as high molecular weight polymers. The ethanol-soluble fraction of eggs contained 2·5% of the total carbohydrate in eggs, and glucose accounted for < 1% of the ethanol-soluble carbohydrate. Analysis of the ethanol-soluble fraction by HPLC demonstrated that the predominant monosaccharides were glycerol and mannose, with smaller quantities of 2-deoxyribose and 2-deoxyglucose. The absence of a ‘free pool’ of glucose in eggs, and the presence of large amounts of glycerol and mannose, suggest that the pathways of intermediary carbohydrate metabolism in eggs might be very different from those in adult tapeworms.

When incubated in vitro for 24 h, intact eggs, chemically shelled eggs (obtained by treating intact eggs with NaOCl), activated larvae (eggs in which the outer shell and inner envelope were removed), and oncospheres (activated larvae treated with papain to remove the embryophore) absorb and metabolize radioactive glucose. Intact eggs, which are covered by the impermeable shell, absorb only small amounts of exogenous radioactive glucose, while chemically shelled eggs, activated larvae, and oncospheres absorb much larger amounts. Only very small amounts of the exogenous glucose are incorporated into the ethanol-precipitable carbohydrate fraction (which would include glycogen) by any of the preparations of eggs/larvae. However, the glucose is incorporated into higher molecular weight end-products that are liberated into the incubation medium. There is a temporal shift in the ability of activated larvae and oncospheres to metabolize exogenous glucose. Activated larvae and oncospheres absorb but do not metabolize glucose during the first 8 h post-activation. Between 8 and 16 h post-activation, however, virtually all of the absorbed glucose is metabolized into higher molecular weight end-products that are liberated into the incubation media. This temporal shift suggests that activation of oncospheres and cysticercoid morphogenesis are accompanied by distinct changes in carbohydrate metabolism.

When incubated in vitro for 24 h, oncospheres of Hymenolepis diminuta absorb and metabolize radioactive glucose. Between 0 and 12 h post-activation, oncospheres absorb glucose, but glucose is neither metabolized into other carbohydrates nor incorporated into the ethanol-precipitable fraction (which would contain glycogen). Between 12 and 24 h post-activation glucose is incorporated into a number of higher molecular weight carbohydrates that are demonstrable in ethanol extracts of the larvae, as well as the incubation media. Furthermore, measurable amounts of radioactivity are incorporated into the ethanol-precipitable carbohydrate fraction of oncospheres. To determine if these temporal changes in carbohydrate metabolism occurred spontaneously following activation, oncospheres were pre-incubated for 12 h (0–12 h post-activation) in the absence or presence of glucose, and then transferred to media containing radioactive glucose for an additional 12 h (12–24 h post-activation). In these latter experiments, glucose absorption and metabolism between 12 and 24 h post-activation were virtually identical to glucose metabolism in oncospheres that were incubated in radioactive glucose for 0–12 h immediately following activation. Thus, these data do not support the hypothesis that the temporal shift in carbohydrate metabolism occurs spontaneously.

When Small-Ear-Miniature pigs subcutaneously injected once with frozen oncospheres of Taiwan Taenia, Korea Taenia, T. saginata or T. solium emulsified with Freund's complete adjuvant (1·6 × 104/0·4 ml) were challenged orally with 1·6 × 104 viable eggs of Taiwan Taenia 41 days later, they all showed strong resistance compared with pigs vaccinated with T. solium. Most pigs (5/8) of the former 3 groups harboured no cysticerci. The number of cysticerci was 5·5±9·1 (mean±S.D.), whereas pigs of the T. solium group and control group harboured 160±30·1 and 661±637·2 cysticerci, respectively. All cysticerci recovered from vaccinated pigs and most cysticerci in control pigs were degenerated or calcified at 36–55 days after oral egg challenge. These results strongly suggest that oncospheres of Taiwan Taenia and Korea Taenia are very similar to T. saginata in their immunogenicity in pigs.