Dendritic cells in Leishmania major-immune mice harbor persistent parasites and mediate an antigen-specific T cell immune response.

Upon infection with Leishmania major, a cause of human cutaneous leishmaniasis, mice of resistant strains are able to control the infection, with lesions resolving spontaneously. A long-lasting cell-mediated immunity protects them from reinfection. Nevertheless, small numbers of viable parasites persist in the lymph nodes of these mice. We have recently documented that, in addition to macrophages, epidermal Langerhans cells can ingest L. major. Furthermore, Langerhans cells have the unique ability to transport viable parasites from the infected skin to the draining lymph node for presentation to antigen-specific T cells and initiation of the cellular immune response. During migration, Langerhans cells develop into dendritic cells. In the present study, we analyzed whether dendritic cells support the persistence of parasites in immune hosts. Immunohistological studies and assays in vitro showed that in the lymph nodes of mice that have recovered from infection with L. major, both macrophages and dendritic cells harbor viable parasites. However, only dendritic cells were able to induce a vigorous T-cell immune response to L. major in vitro in the absence of exogenous antigen. Tracking experiments conducted in vivo suggested that the infected dendritic cells in the lymph nodes are derived from Langerhans cells that have emigrated from the skin. The data demonstrate that L. major-infected dendritic cells and macrophages in lymph nodes of immune animals represent long-term host cells, but only dendritic cells have the ability to present endogenous parasite antigen to T cells. Long-term infected dendritic cells may thus allow the sustained stimulation of a population of parasite-specific T cells, protecting the mice from reinfection. Our results favor the hypothesis that the persistence of antigen supports the maintenance of T cell memory and that dendritic cells are critically involved in this process.