A succesful fight against viral and bacterial diseases requires a good understanding of virus-host relationships and of the fine mechanisms of protective immune responses. In the case of chronic HIV infection, the final result depends on alterations of the homeostasis of immune cells which, in the absence of a specific therapy, leads almost inevitably, to AIDS. We are focused on the study of cell activation and cell trafficking during acute and chronic HIV infection. To address directly in vivo some of these questions, we use a novel therapeutic tool, called MYP (Myeloid cell purging) which acts via the modulation of cell trafficking receptors on both lymphocytes and monocytes/granulocytes. We are also trying to develop a preventive HIV vaccine based on the use of molecular complexes between viral envelope proteins and host-cell proteins