Canine visceral leishmaniosis (CanL) is an emerging disease, expanding in various parts of the world. The infection caused by Leishmania, an intracellular protozoan parasite, can show different clinical manifestations, from asymptomatic or subclinical to symptomatic dogs, in which a wide spectrum of clinical signs is evident. The fact that the parasite replicates in different organs raises the hypothesis that each organ may have a specific immune response. The local immune responses should be evaluated and taken into consideration when developing prophylactic tools. Therefore, phenotypic characterization of peripheral blood, lymph node and bone marrow lymphocyte populations and the expression of class II molecules of major histocompatibility complex (MHCII) were performed in asymptomatic and symptomatic dogs and in dogs that had been diagnosed and treated for leishmaniosis. Our findings showed that blood and bone marrow lymphocytes from symptomatic dogs were highly activated. In bone marrow of asymptomatic and treated dogs, a high frequency of MHCII+ lymphocytes was observed, as well as MHCII monocytes in the treated group. These results show increased expression of MHCII molecules giving evidence for antigenic presentation mainly by lymphocytes. The symptomatic and treated dogs showed an expansion of Cd4(+) T cells subpopulations in lymph nodes, revealing an important contribution of these cells in controlling local parasite replication. This study also underlines the eventual importance of CD3(+)D4(-)CD8(-) (double negative) and CD3(+)D4(+)D8(+) (double positive) T cell subsets in sensing and controlling latent infections and their possible function in the immune dynamics during CanL. The specific cellular immune responses raised in different compartments where the parasite replicates seem to have variable effects on local parasite control, highlighting the complexity of the cellular immune response developed by the dog infected by Leishmania infantum. (c) 2010 Elsevier B.V. All rights reserved.