Human lung dendritic cells have an immature phenotype with efficient mannose receptors

Dendritic cells (DC) can be present at distinct stages of differentiation w ithin the immune system. Sallusto and colleagues have recently described an in vitro culture system suitable for analyzing the maturation processes of DC (Sallusto and colleagues, J. Exp. Med. 1994;179:1109-1118). Monocytes c ultured for 6 d in the presence of granulocyte macrophage colony-stimulatin g factor and interleukin-4 develop into immature DC with a high endocytic c apacity but a low capacity to stimulate T cells. When challenged by lipopol ysaccharide, these cells upregulate costimulatory molecules, express CD83, and become mature DC. CCR1 and CCR5 chemokine receptors are highly expresse d on immature DC and downregulated on mature DC. This in vitro system was u sed to characterize human lung DC. Lung DC were shown to express some chara cteristics of in vitro immature DC. These are: (1) low expression of the co stimulatory molecules CD40, CD80, and CD86; (2) poor expression of the diff erentiation marker CD83 and no CD1a; and (3) good capacity to incorporate d extran. Lung DC express moderate levels of CCR1 and CCR5. However, lung DC, like in vitro mature DC, express high levels of major histocompatibility c omplex Class II molecules, show low expression of CD14 and CD64, and are ch aracterized by their high capacity to stimulate allogeneic T cells to proli ferate during mixed leukocyte reactions (MLRs). Although lung DC express lo w levels of CD80 and CD86, the important role of these costimulatory molecu les in inducing high MLR was demonstrated by using blocking antibodies. The refore, while lung DC have overall a phenotype and an endocytic capacity cl ose to in vitro immature DC, they share, like in vitro mature DC, a powerfu l capacity to stimulate T cells.