Alternatively activated antingen presenting cells. Basic concepts and clinical relevance

Lymphocytes do not just act as immunological effector cells, but also play an important role in the regulation of the immune response. They are able t o induce or suppress inflammatory reactions and this balancing function is reflected in the well-known Th1/Th2 concept lymphocytes depend on antigen p resenting cells (APC)for induction of differentiation and specific activati on mediated by antigen capture, processing and presentation. Thus,APC repre sent a link between innate and acquired immunity. In parallel to the Th1/Th 2 dichotomy,APC may be subdivided into (a) pro-inflammatory, classically ac tivated APC such as mature dendritic cells and IFN-gamma -activated effecto r macrophages, and (b) into anti-inflammatory,alternatively activated APC s u ch as IL-10-activated immature dendritic cells and IL-4-induced suppresso r macrophages. Alternatively activated APC may mediate induction and mainte nance of tolerance towards allergens and environmental substances, control the course of inflammatory reactions,and participate in healing processes b y enhancing angiogenesis. Malignant tumors and certain infectious agents ma y misuse alternatively activated APC for their purposes, thereby requiring counter-action by Th1 lymphocytes and classically activated APC. The concep t of alternative activation thus confirms the important role of APC in main taining the balance between induction and suppression of both inflammation and immunity and it opens new perspectives for the development of specific immunotherapeutic approaches.