Long-lasting protection from LiESAp-MDP vaccine against dog visceral

Abstract

Vaccination against visceral leishmaniasis has received limited attention compared with cutaneous leishmaniasis, although the need for an effective vaccine against visceral leishmaniasis is pressing. Dogs constitute the major reservoir of Leishmania infantum/chagasi responsible for human visceral leishmaniasis. We have recently demonstrated that the combination of naturally excreted/secreted antigens, easily purified from culture supernatant of Leishmania infantum promastigotes (LiESAp) as vaccine antigen in formulation with muramyl dipeptide (MDP) as adjuvant, conferred 100% protection to dogs experimentally infected with L. infantum by inducing in vaccinees a significant, stable and long-lasting Th1-type cell response [Lemesre JL, Holzmuller P, Cavaleyra M, Bras Gonçalves R, Hottin G, Papierok G. Protection against experimental visceral leishmaniasis infection in dogs immunised with purified excreted secreted antigens of L. infantum promastigotes. Vaccine 2005; 23:2825-2840; Holzmuller P, Cavaleyra M, Moreaux J, Kovacic R, Vincendeau P, Papierok G, Lemesre JL. Lymphocytes of dogs immunised with purified excreted secreted antigens of L. infantum co-incubated with Leishmania-infected macrophages produce IFN-gamma resulting in nitric oxide-mediated amastigote apoptosis. Vet. Immunol. Immunopathol. 2005, 106:247-257]. In this report, protection against visceral leishmaniasis is investigated in naturally exposed dogs of endemic areas of the South of France vaccinated with LiESAp/MDP vaccine. A double-blind randomised efficacy field trial was developed on a large-scale dog population composed of vaccinees (n=205) and placebo-treated animals (n=209), which were prospectively studied for a 2-year period. 0f the initial 414 enrolled dogs, 340 (175 controls and 165 vaccinees) were analysed for clinical, serological and parasitological studies at 24 months post-vaccination, after two sand fly seasons. Strong seroconversion disclosed by an L. infantum indirect immunofluorescence antibody test (IFAT) associated with suspicious clinical symptoms, considered an indication that the animals had an established progressive infection, was only observed in the placebo group. The seropositive and/or symptomatic dogs were selected for further examination for possible Leishmania infection by culturing parasites from bone-marrow aspirate. The presence of leishmanial infection was also evaluated by means of the PCR analysis of bone marrow samples in all enrolled dogs prior to vaccination and in all evaluated animals (175 controls and 165 vaccinees) at 24 months post-vaccination. After two transmission cycles completed, the Leishmania infection rate was 0.61% (1/165) in vaccinated dogs and 6.86% (12/175) in the placebo group. The efficacy of the vaccine was calculated to be 92% (P=0.002). A clear difference between the dogs that received vaccine and those that received placebo was also established by the results of their immune status. Increased anti-LiESAp IgG2 reactivity and significant enhanced NO-mediated anti-leishmanial activity of canine macrophages in response to higher IFN-gamma production by T cells were almost exclusively revealed in vaccinees. The LiESAp-MDP vaccine induced a significant, long-lasting and strong protective effect against canine visceral leishmaniasis in the field.