[Alternative activation of antigen-presenting cells: 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 to induce or suppress inflammatory reactions and this balancing function is reflected in the well-known Th1/Th2 concept. Lymphocytes depend on antigen presenting cells (APC) for induction of differentiation and specific activation mediated by antigen capture, processing and presentation. Thus, APC represent a link between innate and acquired immunity. In parallel to the Th1/Th2 dichotomy, APC may be subdivided into (a) pro-inflammatory, classically activated APC such as mature dendritic cells and IFN-gamma-activated effector macrophages, and (b) into anti-inflammatory, alternatively activated APC such as IL-10-activated immature dendritic cells and IL-4-induced suppressor macrophages. Alternatively activated APC may mediate induction and maintenance of tolerance towards allergens and environmental substances, control the course of inflammatory reactions, and participate in healing processes by enhancing angiogenesis. Malignant tumors and certain infectious agents may misuse alternatively activated APC for their purposes, thereby requiring counter-action by Th1 lymphocytes and classically activated APC. The concept of alternative activation thus confirms the important role of APC in maintaining the balance between induction and suppression of both inflammation and immunity and it opens new perspectives for the development of specific immunotherapeutic approaches.