Cell-to-cell signals between T lymphocytes and antigen-presenting cells strictly regulate the development of the immune response. It has clearly emerged that among these signals few cell surface receptor-ligand pairs, such as CD40 and its ligand, CD154, are mandatory for the induction of lymphocyte activation. The early observation that mutations of CD154 gene are responsible for a human severe immunodeficiency primed an impressive number of studies aimed to functionally characterize this receptorial system in view of therapeutically exploiting its properties. Indeed, various approaches aimed to disrupt natural CD40-CD154 interaction were highly effective in the prevention and treatment of several experimental models of autoimmune disease and transplant rejection. In parallel, abnormalities of this pathway were constantly found in several immunologically-mediated human diseases. Furthermore, a number of studies have dissected the role of CD40 and its ligand in the immune response against various microbial and viral pathogens. Since these molecules are often expressed by tumor cells, it is not surprising that great efforts have been made to address their function also in the development of cancer. Most recent data strongly suggest an involvement of endothelial CD40 in the vascular processes that lead to atherogenesis. This review focuses on the most significant advances in the understanding of the molecular regulatory events involving CD40 and its ligand in experimental and human disease.

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