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

Immunization of rats against Fasciola hepatica using crude antigens conjugated with Freund's adjuvant or oligodeoxynucleotides

Chronic Fasciola hepatica infection is correlated with the development of a T helper (Th2)-predominant immune response. To determine whether immunostimulatory CpG-containing oligodeoxynucleotides (CpG-ODN) or Freund's complete adjuvant (FCA), known to promote a Th1 (T helper 1) immune responses, could provide protection from F. hepatica infection, total homogenate (TH) of F. hepatica mixed with CpG-ODN or FCA were injected subcutaneously (s.c.) into Wistar rats. A F. hepatica-specific Th1-predominant immune response was induced with CpG-ODN or FCA in lymph nodes of immunized animals. Lymph node cells from TH-CpG-ODN or TH-FCA immunized rats showed increased antigen-specific proliferation with high levels of INFgamma, compared to lymphocytes from rats injected with TH alone. In contrast, these two groups of immunized animals did not modify IL-4 release by draining lymph node cells, when they were subsequently stimulated with TH in vitro. However, a significant reduction in the burden of flukes (76.7%) was only observed in rats immunized with TH-FCA. Conversely, immunization of rats with TH-CpG-ODN did not promote protection against the parasite. Therefore, even though CpG-ODNs and FCA induced Th1 type responses, only FCA provided a significant protection to rats infected with F. hepatica.

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.

Early hepatic cytokine mRNA expression in experimental rat fasciolosis

We studied the development of the cellular response, particularly with respect to Th1 and Th2 cytokine mRNA levels, in rat liver during the first 14 days of experimental infection with Fasciola hepatica. We analysed the panel of cytokines involved in initiation of the inflammatory and immune response. The levels of various mRNAs, particularly those primarily associated with the acute inflammatory response, and those commonly associated with T-cell proliferation and differentiation, were assessed by reverse transcription-polymerase chain reaction (RT-PCR) in liver samples. We also investigated the immune and inflammatory mediators balance in the liver, draining lymph node and spleen, by RT-competitive PCR quantification of mRNA levels for IL-4, IL-10 and IFN-gamma. Our data provide the first evidence that, in the early phase of infection, the inflammatory response in the liver of infected animals is transiently depressed or delayed. A Th0 profile was initially observed in the liver and hepatic lymph node, which developed into a Th2 profile 2 weeks after infection in the liver only. In the spleen, cytokine down-regulation was initiated and maintained during this period, suggesting that the parasite acts differently locally and in the periphery.

Juvenile Fasciola hepatica are resistant to killing in vitro by free radicals compared with larvae of Schistosoma mansoni

Free radicals have previously been shown to kill the immature stages of the trematode, Schistosoma mansoni but their effect on newly excysted juvenile (NEJ) flukes of Fasciola hepatica has not been established. Using acetaldehyde and xanthine oxidase to chemically generate reactive oxygen intermediates (ROI), up to 61% of NEJ were killed but only when exposed to high levels of ROI. At low concentrations of acetaldehyde and xanthine oxidase as sources of reactive oxygen intermediates, only 6-29% of NEJ were killed compared with 70-92% of schistosomula. Incubation with lipopolysaccharide (LPS)-stimulated rat peritoneal lavage cells (PLCs) killed only 7-15% of NEJ whereas 78-87% of schistosomula were killed under the same conditions by a mechanism dependent on the production of reactive nitrogen intermediates. Relative to immature and adult parasites, NEJ expressed 2.5-20-fold lower levels of superoxide dismutase and glutathione S-transferase but no catalase activity was detected. Incubation of NEJ with inhibitors of peroxidases and glutathione metabolism increased the mean killing of NEJ by LPS-stimulated rat PLCs to 40-75%. These results demonstrate that, in comparison to schistosomula of S. mansoni, NEJ of F. hepatica are relatively resistant to killing by free radicals and this resistance could, in part, be due to the activity of oxidant scavenger enzymes of NEJ.

Fasciola hepatica: influence of gender and liver biotransformations on flukicide treatment efficacy of rats infested and cured with either clorsulon/ivermectin or triclabendazole

Two fasciolicide preparations have been compared in 130 rats experimentally infected with Fasciola hepatica. Parasitological, immunological, and biochemical parameters have been followed to monitor the efficacy of the treatments. While Fascinex (triclabendazole) efficiently cured both male and female rats when administered as soon as 4 weeks postinfection, treatment with Ivomec-D (clorsulon + ivermectin) displayed a low efficacy on either male or female rats at this time point (54 and 0%, respectively). Moreover, when administered 8 weeks postinfection, the Ivomec-D treatment proved highly efficient on male rats while it displayed little effect on the female population (100 and 53%, respectively). This unexpected result has been related to an overexpression of a P4503A isoform that is observed only in females that have been treated with Ivomec-D. The influence of this P4503A cytochrome on drug metabolism and the need for the incorporation of both genders in clinical trials are discussed.

Immunological approaches for the control of fasciolosis

The immunological relationship between liver flukes and their mammalian hosts is being unravelled by in vivo and in vitro studies. Vaccine studies in cattle and sheep with purified antigens (fatty acid binding protein, FABP; glutathione S-transferase, GST; cathepsin L, CatL; hemoglobin) have shown that high reductions in worm burdens (31-72%) and egg production (69-98%) can be achieved, raising the realistic possibility that immunological control of Fasciola infection is a commercially achievable goal. Combination vaccines may also be feasible since a cocktail of CatL and hemoglobin elicits a significant 72% protection in cattle. Analysis of immune responses to Fasciola during infection in ruminants suggests that chronic infection correlates with a type 2 helper T cell response, implying that type 1 helper T cell responses are down-regulated in fasciolosis. Recent results studying the resistance of Indonesian Thin Tail (ITT) sheep to F. gigantica have shown that this breed exhibits high innate (or rapidly acquired) resistance to infection and acquires a higher level of resistance after a primary challenge. Initial studies suggest that the resistance of ITT sheep to F. gigantica may be determined by a major gene. Merino sheep also acquire resistance to F. gigantica. In contrast, ITT and Merino sheep do not exhibit resistance to F. hepatica. These results suggest that there are fundamental differences between these two species of Fasciola in the biology of their interaction with the sheep immune system. In vitro studies on immune mechanisms of killing of juvenile fluke have shown that juvenile larvae of F. hepatica are susceptible to antibody-dependent killing by activated rat macrophages in vitro which is mediated by nitric oxide. Future studies on the immune effector mechanisms expressed by resistant sheep which control infection by F. gigantica will lead to new knowledge which may allow the design of more effective vaccines for fasciolosis.

Is the pretreatment effect of low dose Freund's adjuvant on adjuvant arthritis due to an activation of the nervous system?

It has been recently shown that pretreatment with a low dose of Freund's adjuvant decreases the severity of adjuvant arthritis in rats. To study the involvement of the central and peripheral nervous systems in the pretreatment effect, concentrations of substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)- and neuropeptide Y (NPY)-like immunoreactivities (-LI) were measured in the cerebrospinal fluid, plasma and synovial fluid 2 and 24 h after a single s.c. injection of 0.05 mg Freund's adjuvant. Increased concentrations of CGRP-LI were found in the cerebrospinal fluid, plasma and synovial fluid. NPY-LI was decreased in the cerebrospinal fluid while NKA-LI was decreased in plasma. In the synovial fluid, SP-LI was increased at 24 h and NKA-LI was increased at 2 h following treatment. Our results indicate that part of pretreatment effect of low dose of subcutaneous Freund's adjuvant in the rat may be attributed to neurogenic mechanisms.

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.