Elsevier

Veterinary Parasitology

Volume 208, Issues 1–2, 28 February 2015, Pages 94-100
Veterinary Parasitology

Canine Leishmania vaccines: Still a long way to go

https://doi.org/10.1016/j.vetpar.2015.01.003Get rights and content

Highlights

  • The development of canine Leishmania vaccines is hampered by several factors.

  • An artificial infectious challenge mimicking the natural one is not available.

  • The sample size must necessarily be high because of unpredictable clinical resistance.

  • The slow chronic nature of the disease requires that efficacy studies last for years.

  • Vaccine efficacy studies are both expensive and conflict with animal welfare rules.

Abstract

Dogs are the main reservoir host for zoonotic visceral leishmaniasis, a sand fly-borne disease caused by Leishmania infantum. In endemic areas, “susceptible” dogs suffer from a severe disease characterized by chronic polymorphic viscerocutaneous signs that manifest several months from the exposure, whereas “resistant” dogs can remain subclinically infected for years or lifelong. The protective immune response to Leishmania is cell-mediated; for visceralizing Leishmania species a mixed T helper (Th)1/Th2 response with a dominant Th1 profile is required for protection. The activation of the adaptive immune system in naturally resistant dogs is revealed by parasite-specific lymphoproliferation, delayed-type hypersensitivity, the production of interferon-γ and tumour necrosis factor-α cytokines, and enhanced macrophage leishmanicidal activity via nitric oxide. Hence, an effective canine Leishmania vaccine should induce strong and long-lasting Th1-dominated immunity to control both infection progression and the parasite transmissibility via the vector. Preclinical research in rodent models has evaluated the efficacy of several categories of Leishmania antigens including killed parasites, cell purified fractions, parasite protein components or subunits, single or multiple chimeric recombinant proteins, plasmid DNA and viral particles encoding parasite virulence factors. Promising antigen(s)/adjuvant combinations from each of the above categories have also been tested in dogs; they mostly resulted in limited or no protection in Phase I–II studies (designed to test vaccine safety, immunogenicity and laboratory-induced protection) in which vaccinated dogs were challenged by the artificial intravenous injection of high-load L. infantum promastigotes. The recombinant A2 antigen plus saponin conferred about 40% protection against infection by this challenge system and has been registered in Brazil as a canine vaccine (LeishTec®). An increasing number of efficacy studies have privileged the use of natural challenge consisting in the long-term exposure of vaccinated dogs in endemic settings (Phase III). A 2-year field model including regular assessments by a set of standard diagnostic markers useful for an accurate infection staging has been developed. Again, most of the vaccines tested by this system, which included several antigen categories and adjuvants, failed to protect against infection and disease. Only two vaccines, consisting of parasite purified fractions with saponin derivative adjuvants, showed to confer significant protection against disease and death under natural conditions, and have been registered as canine vaccines: FML-QuilA (Leishmune®) in Brazil, and LiESP/QA-21 (CaniLeish®) in Europe.

Introduction

Protozoa of the genus Leishmania are obligatory intracellular parasites of mammalian macrophages. They are transmitted by the bite of phlebotomine sand flies and are the causative agents of a broad spectrum of human diseases ranging from self-healing cutaneous lesions to severe visceral dissemination. Several nosogeographical entities of leishmaniasis are found worldwide, depending on the parasite species and distribution, the phlebotomine vectors involved, the disease forms caused in humans, and the presence of animal reservoir hosts. Among 15 well-recognized Leishmania species affecting humans and with known reservoirs, 13 are zoonotic (Gramiccia and Gradoni, 2005, World Health Organization, 2010). Zoonotic visceral leishmaniasis is the most widespread entity of leishmaniasis caused by a single parasite species, Leishmania infantum, being endemic in the Mediterranean basin, Middle East, Central Asia and Latin America (Alvar et al., 2012). In this range domestic dogs are the main reservoir host; the parasite is efficiently transmitted to other dogs or humans by the bite of sand fly species of the subgenus Phlebotomus (Larroussius) (e.g. Phlebotomus perniciosus, P. ariasi, P. neglectus, P. tobbi, and others) in most of the Old World, whereas members of the Lutzomyia longipalpis complex are the main vectors in Latin America (Quinnell and Courtenay, 2009). Non-vectorial (e.g. transplacental) transmission in naturally infected dogs has been also demonstrated, although rarely (Boggiatto et al., 2011). The global burden of L. infantum infections in domestic dogs is unknown; in southwestern Europe alone, 947 serosurveys involving more than 500,000 dogs have shown a median seroprevalence of 10% (Franco et al., 2011), which means a much higher infection prevalence if sensitive molecular methods were used (Miró et al., 2008). In this subregion, the recent spread of canine leishmaniasis (CanL) foci beyond traditional endemic ranges has also been documented (Maroli et al., 2008, Dereure et al., 2009).

Besides representing a zoonotic risk for human populations in endemic areas, dogs may suffer from a severe disease characterized by the chronic evolution of polymorphic viscerocutaneous signs that manifest only several months after exposure to infection (Foglia Manzillo et al., 2013). In endemic areas, the infection prevalence appears to be considerably higher than that of apparent clinical illness, its magnitude depending on the presence of optimal conditions for transmission (Baneth et al., 2008). Only a susceptible canine fraction – usually less than 40% of infected dogs – tends to progress towards severe clinical disease, whereas a resistant population does not (Alvar et al., 2004). This latter group appears to be controlling the parasite; however this condition may not be stable so that immunosuppression or intercurrent diseases may permit latent infections to become progressive even years after exposure. Both subclinically infected and diseased dogs can be infectious to phlebotomine vectors, but infectiousness is higher in dogs with overt clinical signs (Courtenay et al., 2014).

In recent years, there have been advances in diagnosis, infection/disease staging, treatment, and prevention of CanL (see reviews by Alvar et al., 2004, Maia and Campino, 2008, Miró et al., 2008, Solano-Gallego et al., 2009, Maroli et al., 2010, Paltrinieri et al., 2010). A major advance in prevention includes evidence that the incidence of zoonotic visceral leishmaniasis, both in humans and dogs, can be significantly reduced by the topical treatment of dogs with synthetic pyrethroids, that have a potent anti-feeding (individual protection) and lethal-by-contact activity (mass protection) against sand flies (Maroli et al., 2001, Mazloumi-Gavgani et al., 2002, Otranto et al., 2013). However, available pyrethroid formulations cannot prevent all potentially infectious sand fly bites and, unlike what was observed in controlled studies, in the hands of owners they have been found not sufficiently effective (Cortes et al., 2012). Hence, the development of effective canine Leishmania vaccine is highly desirable in both fields of veterinary medicine and public health as an additional control measure against the disease.

Section snippets

What we expect from a vaccine to be effective against CanL

The determining factor of the outcome of canine Leishmania infections is the ability of the immune system to manage the parasite efficiently. The protective response against Leishmania is cell-mediated immunity: classically, a T helper (Th)1 response correlates with resistance whereas a Th2 response is associated with susceptibility to infection, revealed by increasing parasite burden and a strong but ineffective humoral response. This dichotomous paradigm comes from experimental models of

Promising Leishmania antigens: from rodent models to dogs

Preclinical research in rodent models (mouse and hamster) has evaluated the efficacy of several generations/categories of Leishmania antigens, and have identified those with promising activity of immune protection when delivered as prophylactic vaccines. Exhaustive lists of such antigens are available from recent reviews (Palatnik-de-Sousa, 2008, Duthie et al., 2012, Mutiso et al., 2013) and include killed parasites, cell purified fractions, parasite protein components or subunits, single or

From artificial to natural challenge for testing Leishmania vaccines in dogs

Because of the role of sand fly saliva and gut components in the pathogenesis and immune control of Leishmania infection (Rohousová and Volf, 2006, Andrade et al., 2007, Rogers, 2012), and because a standard method for laboratory challenge by means of colonized infected sand flies is not available yet (Aslan et al., 2011), an increasing number of efficacy trials have privileged the use of a natural challenge model (Phase III) consisting in the long-term exposure of vaccinated dogs to sand fly

A standardized field model for testing Leishmania vaccines in dogs

The regular assessments performed for each dog through a standard set of diagnostic markers for infection and clinical scoring proposed by Gradoni et al. (2005) have resulted in a highly reproducible model for CanL staging confirmed by subsequent studies and found useful for the interpretation of Leishmania vaccine efficacy (Oliva et al., 2006, Foglia Manzillo et al., 2013). Basically, in a context of a cumulative incidence of 80% of Leishmania infection, three conditions were invariably

Conclusions

Despite advances in Leishmania genomics and proteomics (Myler and Fasel, 2008), modern biotechnology for antigen expression, purification and delivery, and the large availability of murine models in the field of experimental immunology, Leishmania vaccinology still suffers from several bottlenecks that limit the progress towards effective and universal vaccines. Among them, the great genetic diversity of parasites, the difficulty of identifying surrogate markers of resistance in naturally

Conflicts of interest statement

The author knows of no financial or personal conflicts of interest with any person or organisation that could inappropriately influence this work. Funders had no role in study design or the collection, analysis and interpretation of data. Mention of proprietary products does not constitute an endorsement or a recommendation by the author for their use.

Acknowledgments

This study was funded by EU grant FP7-261504 EDENext and is catalogued by the EDENext Steering Committee as EDENext284 (http://www.edenext.eu). The contents of this publication are the sole responsibility of the author and do not necessarily reflect the views of the European Commission.

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