Sustained parasite burden in the spleen of Leishmania infantum-infected BALB/c mice is accompanied by expression of MCP-1 transcripts and lack of protection against challenge

We analyzed differential responses of spleen and liver, major organ targets for viscerotropic Leishmania species, to experimental infection and examin ed if resistance to challenge was organ-specific, In liver, parasites were spontaneously cleared and iNOS trancripts expression paralleled that of ama stigote-load, In the spleen, amastigote multiplication was only partly cont rolled, and iNOS transcripts expression was transient. Total numbers of spl een cells, B cells, and T cells were decreased, while F4/80(+) and Mac1(+): cells were conserved. Expression of splenic MCP-1 transcripts remained cons tant, indicating its possible contribution to immigration of Leishmania hos t cells and to sustained parasite load. Spleen cells produced both, Th1- an d Th2-type cytokines and Th2-type response was dominant, compatible with th e sustained MCP-1 expression. Challenge experiments showed that in contrast to the liver, where initial infection conferred a progressively establishe d immunity, in the spleen there was no induced:protection against reinfecti on. Organ-specific resistance against challenge could be important for desi gning antileishmanial vaccines.