Histamine, immune cells and autoimmunity.

Histamine is one ofthe most versatile biogenic amines with multiple roles during the immune response and in allergic disorders. With four distinct G protein-coupled receptors (H1R, HER, H3R and H4R), intracellular histamine binding sites (most likely members of the cytochrome P450 family) as well as a membrane transporter (Organic Cation Transporter; OCT3) expressed in various immunocompetent cells, it can entertain a complex network of interactions. These signaling pathways are expressed differentially, depending on the stage of differentiation or activation of target cells, thus adding a further degree of complexity to the system. For this reason, published data are sometimes conflicting and varying according to the particular cell type or responses analyzed and the experimental approaches used. On the other hand, histamine is generated by several cells during the immune response, not only through release of intracellular stores in mast cells or basophils in response to IgE-dependent or -independent stimuli, but also through neosynthesis catalyzed by histidine decarboxylase (HDC) in a number ofhematopoietic cells that secrete the amine immediately without prior storage. These features enable histamine to tune the fine balance between immunity and tolerance by affecting dendritic cells, immunoregulatory cells, T-cell polarization and cytokine production, making the way for new pharmacological strategies to control immune reactivity during immune disorders, such as autoimmunity.