Leishmaniasis is a group of vector-borne diseases caused by obligate intracellular protozoan parasites of the genus Leishmania. In both dogs and humans Leishmania infantum infection is more prevalent than disease, as infection often does not equate with clinical disease. In order to differentiate infection vs. progressive disease for better disease prognostication, we temporally evaluated humoral and cellular immunologic parameters of naturally infected dogs. The work presented here describes for the first time the temporal immune response to natural autochthonous L. infantum infection in Foxhounds within the United States. Several key changes in immunological parameters should be considered to differentiate infection versus clinical disease, including a dramatic rise in IgG production, loss of antigen-specific PBMC proliferation, and IFN-gamma production, concomitant IL-10 production and consistent detection of parasite kinetoplast DNA in whole blood. This clinical presentation and immuno-dysregulation mirrors that observed in human patients indicating that this animal model will be very useful for testing immunomodulatory anti-IL-10 or other therapies.

The murine model of cutaneous leishmaniasis using L. amazonensis infection provides an excellent model to study host-Leishmania interactions that may lead to a non-productive immune response. Dendritic cells (DC) are professional antigen presenting cells, critical for induction and regulation of T cell immune responses against pathogens. Initiation of productive immune responses against Leishmania depends on the successful transition of DC from an immature to a mature phenotype, characterized by high CD40 surface expression and IL-12 production. This productive immune response is frequently seen in response to L. major infection. Characterization of draining lymph node CD11c+ DC from L. amazonensis or L. major promastigote-infected mice revealed that by 7 days post-infection, CD11c+ cells from the draining lymph node of L. amazonensis promastigote-infected mice have significantly reduced CD40 surface expression, and there was a significant decrease in the number of IL-12p40-producing cells. Analysis of MAP kinase signaling revealed that infection of DC in vitro with L. amazonensis resulted in increased ERK phosphorylation. Inhibition of ERK in vitro or in vivo led to enhanced DC surface CD40 expression and in vitro increased IL-12p40 production. Peak ERK1/2 phosphorylation was observed 4 hours post-infection in vitro, when parasites are primarily present within intracellular phagolysosome-based parasitophorous vacuoles. Intracellular activation of ERK1/2 from phagolysosomes requires the recruitment and association of ERK1/2-specific scaffolding proteins, including p14/MP1 and MORG1, on the surface of this intracellular compartment. Here we identify scaffolding module co-localization, specifically the endosomal ERK-related proteins MP1 and MORG1 with L. amazonensis-containing, LAMP2 positive-phagolysosomes in DC. This is the first time pathogen ERK1/2 activation is correlated to intracellular-induced ERK activation and identifies novel signaling cascade components which interact with the L. amazonensis parasitophorous vacuole.