Date Published: April 30, 2008
Publisher: Public Library of Science
Author(s): Judith E. Allen, Ohene Adjei, Odile Bain, Achim Hoerauf, Wolfgang H. Hoffmann, Benjamin L. Makepeace, Hartwig Schulz-Key, Vincent N. Tanya, Alexander J. Trees, Samuel Wanji, David W. Taylor, Sara Lustigman
Abstract: River blindness is a seriously debilitating disease caused by the filarial parasite Onchocerca volvulus, which infects millions in Africa as well as in South and Central America. Research has been hampered by a lack of good animal models, as the parasite can only develop fully in humans and some primates. This review highlights the development of two animal model systems that have allowed significant advances in recent years and hold promise for the future. Experimental findings with Litomosoides sigmodontis in mice and Onchocerca ochengi in cattle are placed in the context of how these models can advance our ability to control the human disease.
Partial Text: Infection with Onchocerca volvulus, a filarial nematode, can lead to debilitating skin disease and blindness (river blindness). Adult worms live in subcutaneous nodules; however, the pathology of onchocerciasis is primarily associated with death of microfilariae larvae in the skin and eyes (Figures 1 and 2). It is estimated that 37 million people are infected with O. volvulus, over 99% of whom live in West and Central Africa, although there are significant foci in South and Central America. Early attempts at control of onchocerciasis relied on treatment of water courses with insecticides to kill the larvae (larviciding) of the blackfly (Simulium spp.) vectors. Using this approach for over 25 years, the WHO/UNDP Onchocerciasis Control Programme (OCP) reduced the burden of disease in savannah regions of West Africa ,. In 1987, ivermectin (Mectizan, Merck & Co.) was introduced for mass treatment of onchocerciasis either alone or in combination with larviciding. The OCP closed in December 2002, and control of onchocerciasis now relies on community-based treatment with ivermectin implemented through the African Programme for Onchocerciasis Control (APOC) . The Onchocerciasis Elimination Programme for the Americas similarly distributes Mectizan twice a year in its target countries of Brazil, Colombia, Ecuador, Guatemala, Mexico, and Venezuela .
In the L. sigmodontis model, innate responses at the inoculation site are associated with destruction of a majority of L3s in the subcutaneous tissue within 2 days post-infection. However, about one-third of L3 larvae avoid this attack by entering lymphatic vessels ,, a strategy characteristic of many human filariae ,. The number of larvae that survive this early stage varies depending on sex and strain of the host , but is unaffected by the size of the initial inoculum . From Day 4 post-inoculation, surviving L3 begin to appear in the pleural cavity of L. sigmodontis–infected mice. Differences in the pattern of development of the parasites in resistant C57BL/6 and susceptible BALB/c mice appear early and get progressively more apparent . By 30 days post-infection, about one-third of the population in C57BL/6 mice are still at the L4 stage; this contrasts with <15% in susceptible BALB/c mice . Furthermore, worms recovered from the C57BL/6 mice are smaller than those from BALB/c mice. Analysis of cytokine production at this time shows mixed T helper cell type 1 (Th1)-Th2 response in the C57BL/6 mice reminiscent of that observed in putative immune human patients . In BALB/c mice, the cytokine response is more biased towards Th2 (see Box 1). Although a clearer picture of how mammalian hosts can kill filarial nematodes is emerging, in a successful infection these mechanisms fail. Human studies have long since demonstrated that filarial parasites induce a state of hypo-responsiveness in the host that is associated with the presence of circulating microfilaria . Both the L. sigmodontis and O. ochengi models can mimic this, with Th1 and Th2 cytokines down-regulated coincident with the onset of patency ,. Intrinsic defects in T cell responses in human filarial infection are linked with expression of the T cell–inhibiting receptor, CTLA-4 , and neutralisation of CTLA-4 in mice results in enhanced L. sigmodontis killing . In addition to this intrinsic T cell hypo-responsiveness, T cell responses in humans can be dampened by suppressive antigen-presenting cells . Both mechanisms are operative in the L. sigmodontis model where macrophages that block proliferation of T cells are present at the site of infection prior to patency but become apparent in the draining lymph nodes only following patency . Studies in susceptible BALB/c mice have now directly demonstrated that L. sigmodontis survival is dependent on the induction of a regulatory T cell population that induces hypo-responsiveness . This corroborates the data from human field studies demonstrating that T regulatory (Treg) cells can be isolated from onchocerciasis patients , and generalised onchocerciasis is associated with antigen-specific Treg cells that can be found in nodules . These studies demonstrate the utility of the L. sigmodontis model to reveal details of protective and regulatory mechanisms that can help explain observations made in human infections. The ability of irradiated L3 to generate protection in naïve animals challenged experimentally with normal larvae has been demonstrated in numerous models of filariasis ,, including both the L. sigmodontis,, and O. ochengi models . The protective efficacy of irradiated L3 has been successfully translated into a field trial using O. ochengi in cattle in which significantly lower worm burdens were observed in vaccinated animals compared to controls after almost 2 years of continuous exposure to intense natural challenge from infected Simulium. This success contrasts with the failure of cattle to develop immunity after drug-abbreviated infections. When naïve, infection-free calves were exposed to sustained and intensive levels of natural challenge, monthly or 3-monthly prophylaxis with macrocyclic lactones completely prevented the development of adult worms. However, when chemotherapy ended but exposure continued, these animals were found to be more susceptible to infection than previously unexposed controls  both in terms of adult numbers and microfilarial levels. Similarly, following successful macrofilaricidal treatment of pre-existing patent infections with melarsomine  or oxytetracycline , cattle were fully susceptible to re-infection. These data suggest that parasite death is an insufficient stimulus for the induction of protective immunity and highlight the importance of defining the mechanisms by which irradiated L3 induce protection. Control of onchocerciasis in Africa relies on mass distribution of microfilaricidal ivermectin. Given the impossibility of onchocerciasis eradication with ivermectin alone  and rising concerns about resistance to this drug –, there is a more pressing need to identify complementary therapy using existing drugs. The development of a new drug, apart from the enormous costs, would take 15 years or more to be completed. Ten years ago, the mechanisms by which filarial nematodes are killed by the mammalian host were largely unknown. Although fine detail of these processes remain to be determined, the animal models have now allowed us to determine conclusively that Th2 responses drive protective immunity against L3 larvae as well as the microfilarial stage. Bigger weaponry that includes a Th1 pro-inflammatory component may be needed to tackle the adult stage. However, in successful infections all these mechanisms fail because of the ability of the parasite to initiate regulatory pathways. Bypassing this regulation may be the key to development of a vaccine and future disease control. This will require a thorough understanding of how the parasite induces regulation and identification of the targets and processes that mediate a protective but non-pathological response. In the meantime, the prospect of developing new drug regimes using antibiotics to complement ivermectin treatment and to achieve a macrofilaricidal activity may mitigate against problems of emerging drug resistance and offer new therapy in cases where ivermectin is contra-indicated. Source: http://doi.org/10.1371/journal.pntd.0000217