Dendritic cells (DCs) are antigen-presenting cells with a unique ability to induce primary immune responses. DCs capture and transfer information from the outside world to the cells of the adaptive immune system. DCs are not only critical for the induction of primary immune responses, but may also be important for the induction of immunological tolerance, as well as for the regulation of the type of T cell-mediated immune response. Although our understanding of DC biology is still in its infancy, we are now beginning to use DC-based immunotherapy protocols to elicit immunity against cancer and infectious diseases.

DCs (dendritic cells) function as sentinels of the immune system. They traffic from the blood to the tissues where, while immature, they capture antigens. They then leave the tissues and move to the draining lymphoid organs where, converted into mature DC, they prime naive T cells. This suggestive link between DC traffic pattern and functions led us to investigate the chemokine responsiveness of DCs during their development and maturation. DCs were differentiated either from CD34+ hematopoietic progenitor cells (HPCs) cultured with granulocyte/macrophage colony–stimulating factor (GM-CSF) plus tumor necrosis factor (TNF)-α or from monocytes cultured with GM-CSF plus interleukin 4. Immature DCs derived from CD34+ HPCs migrate most vigorously in response to macrophage inflammatory protein (MIP)-3α, but also to MIP-1α and RANTES (regulated on activation, normal T cell expressed and secreted). Upon maturation, induced by either TNF-α, lipopolysaccharide, or CD40L, DCs lose their response to these three chemokines when they acquire a sustained responsiveness to a single other chemokine, MIP-3β. CC chemokine receptor (CCR)6 and CCR7 are the only known receptors for MIP-3α and MIP-3β, respectively. The observation that CCR6 mRNA expression decreases progressively as DCs mature, whereas CCR7 mRNA expression is sharply upregulated, provides a likely explanation for the changes in chemokine responsiveness. Similarly, MIP-3β responsiveness and CCR7 expression are induced upon maturation of monocyte- derived DCs. Furthermore, the chemotactic response to MIP-3β is also acquired by CD11c+ DCs isolated from blood after spontaneous maturation. Finally, detection by in situ hybridization of MIP-3α mRNA only within inflamed epithelial crypts of tonsils, and of MIP-3β mRNA specifically in T cell–rich areas, suggests a role for MIP-3α/CCR6 in recruitment of immature DCs at site of injury and for MIP-3β/CCR7 in accumulation of antigen-loaded mature DCs in T cell–rich areas.

Dendritic cells comprise a system of highly efficient antigen-presenting cells which initiate immune responses such as the sensitization of T cells restricted by major histocompatibility complex molecules, the rejection of organ transplants and the formation of T-cell-dependent antibodies. Dendritic cells are found in many non-lymphoid tissues, such as skin (Langerhans cells) and mucosa, and they migrate after antigen capture through the afferent lymph or the bloodstream to lymphoid organs, where they efficiently present antigen to T cells. Dendritic cells are difficult to isolate and, although they originate from bone marrow their site of maturation and the conditions that direct their growth and differentiation are still poorly characterized. Granulocyte macrophage-colony stimulating factor (GM-CSF) favours the outgrowth of dendritic cells from mouse peripheral blood. Here we extend this finding to man and demonstrate that cooperation between GM-CSF and tumour necrosis factor-alpha (TNF-alpha) is crucial for the generation of human dendritic/Langerhans cells from CD34+ haematopoietic progenitors. The availability of large numbers of these cells should now facilitate the understanding of their role in immunological regulation and disorder.

Sllmm~'yDendritic cells, the professional antigen-presenting cells (APC) involved in T cell priming, express CD40, a molecule which triggering plays a key role in B cell growth and differentiation as well as monocyte activation. Herein we demonstrate that dendritic Langerhans cells (D-I.c) generated by culturing cord blood CD34 + progenitor cens with granulocyte/macrophage colony-stimulating and tumor necrosis factor oe (TNF-c~) express functional CD40 at a density higher than that found on B cells. Culturing D-IX on CD40-ligand (CD40L) transfected L cells allowed D-IX survival as 50 • 15% of seeded cells were recovered after 4 d while only 5% survived over control L cells. CD40 activation induced important morphological changes with a reduction of cytoplasmic content and a remarkable increase of dendrite development as well as an altered phenotype. In particular, CD40 triggering induced maintenance of high levds of major histocompatibility complex class II antigens and upregulation of accessory molecules such as CD58, CD80 (B7-1) and CD86 (B7-2). CD40 engagement also seems to turn on D-IX maturation as illustrated by upregulation of CD25, a molecule usually expressed on interdigitating dendritic cells of secondary lymphoid organs. Finally, CD40 activated D-IX secreted a limited set of cytokines (TNF-c~, IL-8, and macrophage inflammatory protein 1,', [MIP-loc]) whereas a similar activation induced dutriated monocytes to secrete IL-lot, IL-1/5, IL-6, IL-8, IL-10, TNF-~x, and MIP-lo~. As D-Ix activated T cells upregulated CD40L, it is likely that CD40 activation olD-ix observed herein with a fibroblast cell line stably expressing CD40L, mimics physiological interactiom between dendritic cells and T cells.T he CD40 antigen (1, for review) was identified by monodonal antibodies reacting with carcinomas and B cells (2) and showing costimulatory effects for B lymphocytes (3). It is a 50-kD glycoprotein which bdongs to the TNF receptor superfamily (4). Cross-linking of CD40, in conjunction with IL-4, was found to induce B cells to undergo long-term growth, as well as isotype switching, whereas addition of IL-10 results in B cell differentiation as well as isotype switch (5-8). The use of a CD40-~ fusion protein allowed the isolation of a cDNA encoding for a CD40 ligand (CD40L) 1, a new member of the TNF superfamily mainly expressed on activated T cells (9). Interaction between CD40 and CD40L has now been shown in vitro to be essential during T cell-de. pendent B cell activation (10,11). In vivo studies in mice have demonstrated that an antibody to CD40L can inhibit primary and secondary antibody production and establish- Functional CD40 molecules were found to be expressed on cells other than mature B cells. In particular, upon CD40 cross-llnklng, human progenitor B lymphocytes express CD23 and proliferate in response to . Thymic epithelial cells secrete GM-CSF in response to CD40 engagement (19). Finally, monocytes express high leveh of CD40 after eq, osure to IFN-% IL-3, and GM-CSF and CD40 cross-llnklng induces cyto...