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Canine visceral leishmaniasis (CVL) is known to affect the cellular immunity of infected dogs, through impairing lymphoproliferation and microbicidal mechanisms. This study examined heme oxygenase-1 (HO-1) and its metabolites, oxidative stress and IL-10 levels in CVL and investigated correlations between these parameters. Additionally, the effects of HO-1 inhibition on the lymphoproliferative response and cytokine production in lymph node cells (LNCs) from infected dogs were evaluated. Forty-four dogs, 24 controls and 20 dogs with CVL were selected. Plasma and splenic levels of HO-1, haptoglobin, soluble CD163 receptor, ferritin and IL-10 were determined using capture ELISA. The HO-1 levels and relative gene expression in peripheral blood and bone marrow mononuclear cells were also determined. LNCs proliferation was evaluated with an HO-1 activator and with an HO-1 inhibitor, in the presence of the Leishmania infantum soluble antigen (SAgL), using flow cytometry. HO-1, IL-2, IFN-gamma and IL-10 were also determined in these cultures using capture ELISA. Infected dogs presented oxidative stress and increased HO-1 levels and relative gene expression, with correlation between oxidative stress and HO-1. The substances from heme metabolism and IL-10 were also elevated in the plasma and spleens of infected dogs. IL-10 and HO-1 levels were positively correlated with one another. Inhibition of HO-1 increased LNCs proliferation and decreased IL-10 and IL-2 production in the presence of SAgL. The increased HO-1 metabolism observed in CVL is probably associated with oxidative stress and increased IL-10, which could be one of the mechanisms responsible for inhibition of the lymphoproliferative response in sick dogs.

Dogs infected with Leishmania infantum have a reduced number of T lymphocytes. PD-1 (Programmed cell death 1) a new member of the B7-CD28 family that is expressed by immune cells, and its binding to PD-L1 (CD274) or PD-L2 (CD273) induces the deactivation or apoptosis of T cells. This study aimed to evaluate the expression of PD-1 and its ligands, as well as blocking in the induction of apoptosis in T lymphocytes, TNF-α, IL-4 and nitric oxide production by leucokocytes from PBMC and spleen and the parasite load in dogs with visceral leishmaniasis (VL). Our results showed that the expression of PD1 and its ligands was increased in CD3(+) T cells and CD21(+) B lymphocytes within the peripheral blood and splenic mononuclear cells of dogs with VL. In peripheral blood monocytes, only PD-1 ligands exhibited increased expression; however, in spleen macrophages, increased expression of both PD-1 and its ligands was observed. Levels of apoptosis in peripheral blood and splenic T lymphocytes were higher in dogs with VL compared to healthy dogs. Blocking monoclonal antibodies to PD-1 and its ligands in the culture of mononuclear cells from the peripheral blood and spleen decreased the amount of CD3(+) T lymphocyte apoptosis. The concentration of nitric oxide, TNF-α and IL-4 increased in the culture supernatants of peripheral blood mononuclear cells treated with a blocking monoclonal antibody against PD-1. The TNF-α concentration increased in the culture supernatants of splenic cells following all treatments with antibodies blocking PD-1 and its ligands; however, the amount of IL-4 increased only in the presence of a PD-1 blocking agent. Treatment with a PD-1 blocking monoclonal antibody in the mononuclear peripheral blood of dogs with VL reduced the parasite burden while increased TNF-α. We conclude that in canine visceral leishmaniasis, PD-1 and its ligands are involved in the induction of T lymphocyte apoptosis and in regulating the production of nitric oxide, TNF-α, and IL-4, as well as the parasitic load.

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