Ligula intestinalis (Cestoda: Pseudophyllidea): An ultrastructural study on the cellular response of roach fry, Rutilus rutilus

Ligula intestinalis (Cestoda: Pseudophyllidea): an ideal fish-metazoan parasite model?

Since its use as a model to study metazoan parasite culture and in vitro development, the plerocercoid of the tapeworm, Ligula intestinalis, has served as a useful scientific tool to study a range of biological factors, particularly within its fish intermediate host. From the extensive long-term ecological studies on the interactions between the parasite and cyprinid hosts, to the recent advances made using molecular technology on parasite diversity and speciation, studies on the parasite have, over the last 60 years, led to significant advances in knowledge on host-parasite interactions. The parasite has served as a useful model to study pollution, immunology and parasite ecology and genetics, as well has being the archetypal endocrine disruptor.

The role of serum in leucocyte adherence to the plerocercoid of Ligula intestinalis (Cestoda: Pseudophyllidea)

The role of serum in the adherence of roach (Rutilus rutilus) leucocytes to the plerocercoid of Ligula intestinalis has been investigated in vitro. Roach plerocercoids, either untreated, cultured or killed (fixation in 0·5 % buffered glutaraldehyde or heat at 50°C for 45 mm), were exposed to serum and 4×106 leucocytes obtained from the pronephros of non-infected roach using a discontinuous Percoll gradient. At normal roach serum (NRoS) dilutions of 1:5000 to 1:100, leucocyte adherence was observed on all living parasites but was negligible in killed parasites or in complement-depleted NRoS assays. The number of adherent cells increased with increasing NRoS concentration. Leucocytes bound more avidly in the presence of heat-inactivated immune serum (hi IRoS); at a serum dilution of 1:5000 the percentage of the parasite surface covered by leucocytes was approximately 3 times greater in hi IRoS than in NRoS. Ultrastructural observations on parasites revealed that plerocercoids exposed to cells and hi IRoS had, in some areas, abnormal inclusions in their tegument and lacked a microthrix border and distal cytoplasm. In addition, the efflux of radioactivity from [14C] cycloleucine-labelled worms was greater in parasites incubated in the presence of hi IRos.

Ultrastructural studies on the cellular response of fish hosts following experimental infection with the plerocercoid of Ligula intestinalis (Cestoda: Pseudophyllidea)

With the exception of gudgeon (Gobio gobio) infection of cyprinid fish with the plerocercoid of Ligula intestinalis is associated with a host-tissue response. The nature and specificity of this response has been investigated using transplantation techniques. In roach there is an intense cellular response to implanted Sepharose, Spurr's resin and L. intestinalis, irrespective of donor host species. However, tegument damage only occurs in gudgeon-Ligula and cultured ro&ch-Ligula which has been exposed to gudgeon-plasma. L1 and L2 cell types predominate within the cellular response. Once the host leucocytes have breached the tegument, they migrate over the basal lamina and penetrate into the sub-tegumental region. In gudgeon an intense cellular attack occurs against Sepharose, Spurr's resin, ro&ch-Ligula and cultured gudgeon-Ligula which had been implanted directly or exposed to roach plasma. Tegument damage only occurs in the latter. In contrast, neither direct implantation of gudgeon-Ligula nor cultured roach-Ligula which has been exposed to gudgeon plasma evokes a cellular response. It is suggested that in these two instances, as in natural infections of gudgeon, protective host proteins may be absorbed onto the surface of the parasite.

In vitro polarization of carp leucocytes in response to the blood fluke Sanguinicola inermis Plehn, 1905 (Trematoda: Sanguinicolidae)

An in vitro assay was used to determine the effects of Sanguinicola inermis adults and cercariae on the polarization responses of pronephric leucocytes of carp. Leucocytes were isolated and exposed to live adult flukes or cercariae for up to 48 h. Differences in polarization responses were related to the presence of the parasite, the presence or absence of carp serum and the time of incubation. The mean proportions of cells exhibiting polarization in unstimulated controls ranged from 5 to 30% over the experimental period. Within 15 min of exposure to adults or cercariae, significant increases in mean polarization responses were observed of up to 75% and levels remained higher than control values for over 24 h. Overall, the presence of normal carp serum, either untreated or heat inactivated, did not enhance the polarization responses of leucocytes incubated with only cercariae or adults. However, between 0.25 and 3 h, the presence of carp serum with cercariae significantly enhanced polarization responses when compared with cells incubated with cercariae alone.

Immunolabelling of fish host molecules on the tegumental surface of Ligula intestinalis (Cestoda: Pseudophyllidea)

Immunoblotting, SDS-PAGE and western blotting procedures were used to demonstrate cross-reactivity of a polyclonal anti-carp IgM antibody with components of roach serum. The polyclonal antibody labelled 2 major bands in both immune and normal roach sera corresponding to molecular masses of approximately 90 and 65 kDa. One of these bands (65 kDa) was considered to be heavy chain of fish immunoglobulin whilst the identity of the other remains uncertain. This cross-reaction has been exploited in immunofluorescence and immunogold labelling studies to localize fish host molecules on the tegumental membrane of Ligula intestinalis freshly removed from roach fry. Immunogold studies revealed a low level of host molecules to be associated mainly with the microthrix spines of the tegument with less gold labelling being observed on the microthrix shafts.

Ligula intestinalis (L.) (Cestoda: Pseudophyllidea): polarization of cyprinid leucocytes as an indicator of host- and parasite-derived chemoattractants

Freshly isolated pronephric leucocytes from roach, Rutilus rutilus and gudgeon, Gobio gobio were exposed to extracts of plerocercoids of Ligula intestinalis from these two cyprinid fish. Addition of the extracts or an increase in incubation temperature from 10 to 20 degrees C induced polarization of neutrophils and L1 granulocytes. Cells were transformed from their typical spherical shape to elongate forms possessing a ruffled leading edge. Extracts obtained from gudgeon-Ligula stimulated polarization of both roach and gudgeon leucocytes at 10 and 20 degrees C. In contrast, extracts from roach-Ligula, whilst having little effect at 10 degrees C, suppressed temperature-induced polarization of leucocytes at 20 degrees C. Addition of serum to all the essays enhanced polarization and abolished the roach-Ligula-induced suppression. It is suggested that leucocyte chemoattractants are present in Ligula from roach and gudgeon and only parasites from the former host contain an inhibitor of polarization. In addition, host-derived factors possibly complement, may be involved in leucocyte chemoattraction.

Sequential development of the immune response in rainbow trout [Oncorhynchus mykiss (Walbaum, 1792)] to experimental plerocercoid infections of Diphyllobothrium dendriticum (Nitzsch, 1824)

Development of the inflammatory response of rainbow trout to experimental infections with Diphyllobothrium dendriticum plerocercoids is described using light and electron microscopy. The cellular response to plerocercoids occurred within 2 weeks post-infection (p.i.). This was followed by an increase in leucocyte numbers during weeks 3-6 p.i., with full encapsulation of plerocercoids by week 6 p.i. Neutrophils were the first leucocytes to engage the developing plerocercoid, followed by large influxes of macrophages which transformed into epithelioid cells. With longer times p.i. the accumulation of different leucocyte types increased, and a blood vascular network developed. Full development of the composite cyst was characterized by fibroplasia, particularly at the periphery of the cyst, and the subsequent deposition of a collagenous tissue matrix. Enzyme-linked immunosorbent assay (ELISA) examination of serum samples taken over the 20 week period showed that specific anti-D. dendriticum antibody titres were first detected at 5 weeks p.i. and increased to a maximum by 11 weeks p.i.

Chairman's Concluding Remarks

Although structurally simple, the nervous systems of parasitic helminths are biochemically complex. As well as the classical transmitters, the nervous systems of helminths contain a range of regulatory peptides and so far some 29 different peptides have been described. The only group of compounds thought to be transmitters in free-living organisms which have not yet been shown to be transmitters in helminths are the purines. There is evidence in helminths, as in vertebrates, for the co-localization, in the same cell, of classical transmitters and peptidergic molecules and for the non-neuronal occurrence of some regulatory peptides. Co-localization would effectively increase the number of transmitter types, since compounds could be released in different combinations. How such a selective release might be achieved is unknown. Different regulatory peptides are localized in different parts of the helminth nervous system, but it is quite probable that more than one peptide can occur in the same cell. Whether the relative proportions of the different peptides change under different physiological or developmental conditions is not known. The classical transmitters similarly show a differential distribution. It is presumed that the regulatory peptides in helminths, as in other organisms, are synthesized by post-translational modification of a high molecular weight precursor protein and that the peptides are not recycled but are removed from their site of action by specific peptidases. The latter are located near the receptor sites and may be membrane-bound. In vertebrates the precursor molecules usually contain several different peptides with different biological functions and processing of the precursor often shows tissue-specific variations.

Studies on the host/parasite interface during the development of a pentastomid arthropod parasite in rodent intermediate hosts, with observations on protective surface membranes

The changing structure of the cuticle of the arthropod pentastomid parasite Porocephalus crotali, during growth to the infective stage in mouse and rattlesnake hosts, is described. The outermost cuticulin layer of the cuticle in instars II-VI is elevated to form a dense mat of epicuticular hairs. Since the VI larval cuticle is retained by the infective (VII) nymph as a protective sheath, effectively all stages in mice present a hairy surface to the host and this may constitute a physical barrier to inflammatory cells. The entire surface is overlain by a triple-track 'unit' membrane whose biophysical properties resemble those of a conventional plasma membrane, and there is evidence to suggest that this membrane is susceptible to immune attack. Under natural circumstances, epicuticular hairs entrap secretion, delivered to the cuticle via innumerable minute ducts which communicate with tegumental secretory cells termed subparietal cells (SPC). SPC synthesize lamellate droplets which unfold on the cuticle to constitute a layer of protective polymorphic vesicles. By contrast, infective nymphs in snakes possess a smooth cuticle and SPC membranous secretion is stacked over the entire surface, in sheets up to 20 deep. The function of the lipid and protein components of SPC secretion is discussed.