C.C. Santana et al. Parasite Immunology © 2008 The Authors Journal compilation © 2008 Blackwell Publishing Ltd, Parasite Immunology, 30, 515–524 subverted by a parasite to favour its survival and growth (31). In fact, interlukin-10 and other cytokines produced by granuloma cells may provide conditions for the survival of Leishmania (12) and other microorganisms (32,33). The fact that granulomas were only present in animals with positive spleen culture in our study supports the view that these granu- lomas may reﬂect a state of inefﬁcient control of Leishmania infection by the host immune system. The increase in the plasma cell population in lymphoid organs is a common ﬁnding in leishmaniasis (7,27,34), and it may be present at relatively early stages of the disease (35). In all of the infected groups examined in this study, a signif- icant increase in the plasma cell population was observed to replace other cell populations in areas of the red pulp. A progressive replacement of T-cell areas in the white pulp was also observed in some animals (data not shown). The deter- minants of the changes in number or distribution of plasma cells in VL are poorly understood. However, in visceral leishmaniasis, several factors like the well-characterized polyclonal B-cell activation (36,37) and production of cytokines [e.g. IFN-γ, IL10 (38– 41), and IL6 (42,43)] and chemokines (e.g. CXCL12) may contribute to plasma cell differentiation and retention in the red pulp (44). Further studies are necessary to deﬁne the mechanisms involved in the plasma cell replacement of normal cell populations in the spleen, and future work should also examine the potential contribution of cell population replacement to both the increase of polyclonal immunoglobulin levels observed in VL (45) and disease development. Lymphoid hyperplasia, atrophy, and disorganization were all present in the infected dogs examined in this study. Hyperplasia was common in animals from the infected and potentially resistant to VL group, and atrophy and lym- phoid tissue disorganization were most frequently present in the animals from the infected and potentially susceptible to VL group. Hyperplasia followed by atrophy of the lymphoid follicles may occur in the spleen during the course of VL (7,34,35). However, such atrophy and hyperplasia are not uniform in all compartments of the white pulp. As shown in Figure 2, marginal zone hyperplasia may occur with a variable degree of follicle atrophy. This ﬁnding, together with the effacement of the boundaries between usually distinct areas of the lymphoid tissue, indicates that structural disorganization is present in the white pulp, as suggested by Veress et al. (1977). The loss of the architectural structure of splenic tissue has been the subject of recent studies performed in mice infected with Leishmania (20) or other pathogens (21). A variety of changes, including the loss of cell popula- tions and the impairment of cell migration to the different splenic compartments, have been observed in murine models of visceral leishmaniasis induced by TNF and IL-10 (20,46,47). We also observed that disorganization of the splenic lymphoid 522 tissue was more frequent in animals with emaciation, even for those animals without evidence of Leishmania infection, than it was in animals without evidence of undernourishment. In these emaciated animals, weight loss may result from insufﬁcient access to food, which, in its turn, may be associated with other infections (9,48,49). For instance, studies on experimental models have shown that viral infections may also cause splenic lymphoid tissue disorganization (21). A factor common to many infections, such as high levels of TNF (unfortunately not tested in this study), may account both for emaciation and for lymphoid tissue disorganization (20,50). Nevertheless, the fact that the intensity of white pulp disorganization was higher in animals with splenic parasitism than in those without indicates that an active Leishmania infection may play a causal role in a spleen disorganization process. Further studies are necessary to clarify the mecha- nisms involved in the structural disorganization of the splenic tissue in canine leishmaniasis and assess the potential role of such splenic changes in mediating increased susceptibility to bacterial infections in the late stages of the disease. In conclusion, our study shows that natural infection by L. chagasi is associated with splenic architecture disruption, which is characterized by disorganization of normal lymphoid tissue, loss of normal spleen leucocyte diversity via replace- ment of leucocytes by plasma cells, and eventual atrophy of the lymphoid tissue. Studies examining the redistribution of the main cell populations, the proﬁle of cytokine expression in the spleen, and the potential association of splenic alterations with increased susceptibility to bacterial infection in these animals are currently underway. ACKNOWLEDGEMENTS Authors would like to thank Raimundo Celestino for his assistance with the statistical analysis and representation of the data presented in the manuscript. This work was supported by the Brazilian Ministry of Science and Technology (Brazilian National Research Council – CNPq, Grant Nos 7500320060 and 7628720064], [RENORBIO Program: BNB 737 and FINEP 3773/05], [INOVABIO Program: FINEP, Project 381], and FAPESB (Grant No. APR0434/2005). REFERENCES 1 Desjeux P. The increase in risk factors for leishmaniasis worldwide. Trans R Soc Trop Med Hyg 2001; 95 (3): 239–243. 2 Badaro R, Jones TC, Lorenco R, et al. A prospective study of visceral leishmaniasis in an endemic area of Brazil. J Infect Dis 1986; 154 (4): 639–649. 3 Caldas AJ, Silva DR, Pereira CC, et al. [Leishmania (Leishmania) chagasi infection in children from an endemic area of visceral leishmaniasis in the Sao Luis Island-MA, Brazil]. Rev Soc Bras Med Trop 2001; 34 (5): 445 – 451.