Pulmonary dendritic cells: playing ball in the BAL?

T he lung is one of the few organs in the human body where there is a continuous and extensive interaction between the environment and our immune system. With every breath we take, numerous airborne particles, both harmless as well as potentially dangerous, are inhaled into the airways. The big challenge for the pulmonary immune system is to discriminate the good from the bad and to react accordingly. Unnecessary action against harmless particles (e.g. innocent antigens, self proteins) should be avoided, while a rapid and strong immune response is needed against potentially dangerous microorganisms. Pulmonary dendritic cells (DCs) are ideally suited to maintain this delicate balance between tolerance and active immune responses [1]. DCs are antigen-presenting cells that play a central role in host immune defence, by linking innate with adaptive immune responses [2]. They recognise danger signals from invading pathogens and are able to prime naı̈ve T-cells and to initiate appropriate T-cell immune responses against these microorganisms [3]. Indeed, T-lymphocytes themselves are actually ‘‘blind’’ to unprocessed antigens, and require the assistance of antigen presenting cells, such as DCs and macrophages, to be able to recognise antigens. Moreover, by the release of inflammatory cytokines, DCs influence the polarisation of the adaptive T-cell response into either a T-helper cell type (Th)1-, Th2or a Tregulatory cell-direction [4]. This enables them not only to initiate active immune responses, but also to control and dampen these immune responses, as well as to avoid unwanted immune responses against self-antigens (autoimmunity).