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Dendritic cells (DC) form a diverse system of antigen presenting cells (APC) responsible for orchestrating the body's response to infection. As the only cell type capable of eliciting effector functions among antigen-naive T cells, DC act at the very heart of the immune response, either driving immunity or securing a state of self-tolerance. Although DC hold promise as targets for immune intervention, the mechanisms responsible for defining the balance between these opposing forces currently remain obscure. While techniques for analysing differential gene expression have begun to identify novel DC-specific genes that may be involved, an approach to elucidating their function is presently lacking, due, primarily, to the resistance of primary DC to genetic modification. Given the propensity for genetic manipulation of embryonic stem (ES) cells and the potential for their directed differentiation in vitro, we have defined the culture conditions and growth factors required for their commitment to the DC lineage. Here we describe the derivation of long-term cultures of cells displaying the characteristics of immature DC, including the capacity to acquire and process foreign antigen for presentation to T cells. Exposure to inflammatory stimuli induces the terminal differentiation of these precursors causing them to adopt the morphology and surface phenotype of mature DC, capable of stimulating primary responses among allogeneic T cells. These findings pave the way for the rational design of DC lines competent to function in immunity, in which candidate genes have been over-expressed or functionally ablated. Exploration of the unique properties of such mutant DC may help define novel targets for intervention in the pathogenesis of immunologically-related disease.A cardinal feature of the adaptive immune system is its inherent specificity for antigen, permitting the mounting of an immune response, designed to exploit the vulnerability of particular micro-organisms. Such specificity is achieved by the generation of vast repertoires of T and B lymphocytes, whose surface receptors for antigen show unparalleled conformational diversity: conservative estimates suggest the potential for synthesizing as many as 10(9) structural variants of both the T-cell receptor (TCR) and immunoglobulin molecules by the random rearrangement of the multiple gene segments that encode them. Only by expressing these receptors in a clonal fashion, may the adaptive immune system meet the challenge posed by the immense diversity of potential pathogens.Although the evolutionary advantage conferred on higher vertebrates by adaptive immunity undoubtedly lies in its specificity, such advances create a number of logistic difficulties. Firstly, the probability that rare T cells, among such a vast repertoire, will encounter their cognate antigen purely by chance, is vanishingly small, requiring the intervention of a dedicated antigen presenting cell (APC) capable of delivering the components of pathogens to the relevant clones. Secondly, any system for generating diversity which relies on random gene rearrangement risks the emergence of cells with greater specificity for self proteins than foreign antigens, requiring their subsequent removal, functional silencing or active regulation in order to maintain self-tolerance. Both of these dilemmas have been resolved, in evolutionary terms, by a system of cells with unique properties: the dendritic cells. |
