Cellular reactions in the small intestine of rats after infection with Fasciola hepatica

Eosinophil-nerve interactions and neuronal plasticity in rat gut associated lymphoid tissue (GALT) in response to enteric parasitism

Intestinal lymphoid tissues and Peyer's patches (PP) are innervated sites of immune surveillance in the gastrointestinal tract. Following infection with F. hepatica, neuronal hyperplasia and significantly increased eosinophil and mast cell trafficking to colonic PP sites were evident in rat tissues. Nerve-eosinophil associations were significantly elevated in infected colon and colonic PP, as were colonic tissue levels of the circulatory recruitment factors IL-5 and eotaxin. Increased immunoreactivity for neuronal plasticity markers GAP-43 and neural cell adhesion molecule (NCAM) was also found in infected tissues. Such neuronal alterations in the PP during enteric parasitism may have functional consequences on particular or pathogen uptake.

Immunology of the host–parasite relationship in fasciolosis (Fasciola hepaticaandFasciola gigantica)

The protective resolution of liver fluke (Fasciola hepatica and Fasciola gigantica) infection is a dynamic interplay between the host's effector responses and the parasite's defence and immunomodulatory systems. The evidence suggests that the juvenile or immature parasite is the target of protective host immune responses but the effector mechanisms employed vary between hosts. Moreover, F. hepatica and F. gigantica differ in their susceptibility to these killing mechanisms. In the rat, in vitro killing of juvenile F. hepatica involves an antibody-dependent cell cytotoxicity mediated by nitric oxide produced by activated monocytes and (or) macrophages. However, monocytes and (or) macrophages from Indonesian sheep do not produce nitric oxide yet can effectively kill juvenile F. gigantica in vitro and in vivo by a mechanism that is ineffective against F. hepatica. These data show that disease progression or resolution in fasciolosis is determined both by biochemical differences between Fasciola species and by host-dependent factors. Understanding the genetic basis for these differences is a key question for the future. Fasciola hepatica and F. gigantica actively modulate the host immune response, downregulating type 1 responses during infection. It is important to determine whether such modulation of the immune response by Fasciola spp. directly leads to enhanced parasite survival in the various hosts.

Protection against Fasciola hepatica in the intestine is highly correlated with eosinophil and immunoglobulin G1 responses against newly excysted juveniles

Rats were infected with Fasciola hepatica and challenged at regular intervals up to 38 weeks using an ex vivo gut loop, a technique developed in our laboratory. The kinetics of the observed immune responses against F. hepatica in gut tissue and serum were investigated and correlated to protection. Immunohistochemical methods were used to measure the frequency of eosinophils, immunoglobulin (Ig)E-positive cells, and mucosal mast cells in the gut loop, and to determine whether the newly excysted juveniles were coated with IgG antibodies or surrounded by eosinophils, or both. Enzyme-linked immunosorbent assays and a radioimmuno assay were used to measure serum antibody reactive with newly excysted juveniles. Results showed that protection was highly correlated with the frequency of eosinophils and IgE-positive cells in the gut, but was only moderately correlated with the frequency of mucosal mast cells. Newly excysted juveniles taken from rats exhibiting high levels of protection were always coated with IgG antibodies and surrounded by eosinophils. Protection was highly correlated with titers of serum IgG1 antibodies directed against newly excysted juveniles, but was only weakly correlated with titers of serum IgA and IgE antibodies. Because protection was highly correlated with IgG1 in gut tissue and serum, and with eosinophils in gut tissue, we suggest that IgG1 and eosinophils are important in protecting rats against F. hepatica.

Liver pathology and immune response in experimental Fasciola hepatica infections of goats

The relationship between the immune response and the pathogenesis of the disease was studied in different primary and secondary experimental Fasciola hepatica infections of goats. The establishment of the infection, measured as percentage of recovered flukes at the necropsy, was similar in primarily and secondarily infected animals (between 19.7% and 24.3%), but the hepatic damage was much more severe in secondarily infected goats, as revealed by the levels of serum hepatic enzymes GGT and LDH. Primary infection evolves to chronic fasciolosis that did not induce the development of resistance, since goats were highly susceptible to secondary infection, showing severe acute and chronic hepatic lesions that led to the death of some animals in each group. The immune response to the infection was proved by the production of specific IgG antibodies to ESP of F. hepatica and the involvement of CD3+ T lymphocytes and lambda IgG+ plasma cells in the hepatic infiltrate. Secondary infection did not induce any difference in either IgG response or in the cellular composition of the infiltrate of hepatic lesions, although this was much more extended. However, neither antibodies nor cell-mediated response were protective: there was no correlation between IgG levels and fluke burden and there was no evidence of cell-mediated killing of the parasite. This suggests the existence of some immune evasion mechanisms in goat infection with F. hepatica. The parasite may depress the local inflammatory and immune response, as suggested by the scarcity of CD3+ T cells in the infiltrate surrounding acute migratory tunnels. Moreover, in secondary infected goats can be suspected an immunological damage of the liver, since a very severe infiltrate of immune cells replaced wide areas of hepatic parenchyma and an immune-mediated damage of hepatocytes could occur.

Local immune responses in colon from cattle infected with Fasciola hepatica

Colonic mucosae isolated from eight Charolais heifers 14 weeks after oral infection with Fasciola hepatica metacercariae were voltage clamped in Ussing chambers. Antigen challenge provoked a rapid-onset, inward short circuit current in infected but not in control tissue preparations. Chloride secretion accounted for part of the response as the loop diuretic bumetanide attenuated the response to antigen by 89%. The bacterial cell wall component n-formyl-methionyl-leucyl-phenylalanine induced a similar response to antigen and partly desensitised tissues to subsequent antigen challenge, consistent with the involvement of immunocytes in the response to antigen. Infection induced an elevation in the number of tissue eosinophils compared with control colon.

The enigmatic eosinophil: investigation of the biological role of eosinophils in parasitic helminth infection

In many helminth infected hosts the number of eosinophils increases dramatically, often without any concurrent increases in the number of other leukocytes, so that eosinophils become the dominant cell type. Many experimental investigations have shown that the eosinophilia is induced by interleukin-5 (IL-5) but its functional significance remains unclear. Mice genetically deficient in IL-5 (IL-5-/-) have been used to evaluate the functional consequences of the IL-5 dependent eosinophilia in helminth infected hosts. Host pathology and level of infection were determined in IL-5-/- and wild type mice infected with a range of species representative of each major group of helminths. The effects of IL-5 deficiency were very heterogeneous. Of the six species of helminth examined, IL-5 dependent immune responses had no detectable effect in infections with three species, namely the cestodes Mesocestoides corti and Hymenolepis diminuta and the trematode Fasciola hepatica. In contrast, IL-5 dependent immune responses were functionally important in mice infected with three species, notably all nematodes. Damage to the lungs caused by migrating larvae of Toxocara canis was reduced in IL-5-/- mice. Infections of the intestine by adult stages of either Strongyloides ratti or Heligmosomoides polygyrus were more severe in IL-5-/- mice. Adult intestinal nematodes were clearly deleteriously affected by IL- 5 dependent processes since in its presence there were fewer worms which had reduced fecundity and longevity. The implications of these results for the viability of using inhibitors of IL-5 as a therapy for asthma are considered.

Protection of Fasciola hepatica in the gut mucosa of immune rats is associated with infiltrates of eosinophils, IgG1 and IgG2a antibodies around the parasites

We investigated the immune effector mechanisms that underlie protection against F. hepatica in the gut wall of immune rats, using (immuno)histochemistry. In the lamina propria of immune Wistar rats, four weeks after oral infection, frequencies of IgE-positive cells, eosinophils and mucosal mast cells were significantly increased, compared with naïve rats. These factors represent the traditional effector mechanisms against helminths. No significant differences were detected between the two groups in frequencies of IgM-, IgG2a-, IgG1- and IgA- positive cells, CD4- and CD8-positive cells, NK cells, macrophages, neutrophils or goblet cells. Upon challenge of immune rats with F. hepatica in an ex vivo gut segment, NEJs that migrated through the (sub)mucosa were coated with IgG1 and IgG2a antibodies and surrounded by eosinophils. No IgE or IgA antibodies were detected on the parasites. The onset of these immune effector responses, two h after challenge, was related to the expression of protection. These results suggest that NEJs are killed by an eosinophil-mediated cytotoxic response involving IgG antibodies. These antibodies were not produced in the intestine, but infiltrated the gut upon challenge. The observed immune effector responses were not restricted to the site where the primary infection is located, namely the small intestine, but were also detected in the large intestine. The presence of the protective immune mechanisms in two other rat strains demonstrates the pivotal importance of these responses, irrespective the genetic background of the host.

Influence of anti-inflammatory treatments on experimental infection of rats with Fasciola hepatica: changes in serum levels of inflammatory markers during the early stages of fasciolosis

Anti-inflammatory treatments with dexamethasone, indomethacin or aspirin shortly before or immediately after the experimental infection of rats with Fasciola hepatica resulted in an increase in the rats' burden of adult flukes, suggesting that the inflammatory system played an active part in the rats' immune defense against invasive F hepatica. An anti-parasitic effect of cyclosporin A on F hepatica infection in rats was also demonstrated. A weekly study of various inflammatory markers (acute phase proteins, leucocytes and platelets) demonstrated that no inflammatory reaction had occurred after the first two weeks of infection, but that there was a significant increase in acute phase proteins after three and four weeks. These increases indicated that a chronic inflammatory reaction, associated with liver damage and an increasing eosinophilia, had occurred.

Influence of pro-inflammatory treatments on experimental infection of rats with Fasciola hepatica: changes in serum levels of inflammatory markers during the early stages of fasciolosis

The effect of stimulating an inflammatory reaction by injecting rats with either Freund's complete adjuvant or turpentine oil shortly before or after they were infected experimentally with Fasciola hepatica on the establishment of the parasite was studied. Freund's complete adjuvant, given either four hours before or four hours after infection with F hepatica and turpentine oil given four hours before infection resulted in a 40 to 50 per cent decrease in the rats' fluke burden. The study of markers of inflammation (acute phase proteins, leucocytes and platelets) demonstrated that the fluke had a modulating effect on the rat's inflammatory response. During the first few days after infection, F hepatica stimulated a transitory inflammatory reaction, as suggested by changes in the level of acute phase proteins (APP) in the serum. No further change in the APP titre occurred until three weeks after infection, when, together with an increase in the extent of the liver lesions, the change in the titre of APP indicated that a systemic and chronic inflammatory process had taken place.