Major histocompatibility complex class II- fetal skin dendritic cells are potent accessory cells of polyclonal T-cell responses

Whereas dendritic cells (DC) and Langerhans cells (LC) isolated from organs of adult individuals express surface major histocompatibility complex (MHC ) class II antigens, DC lines generated from fetal murine skin, while capab le of activating naive, allogeneic CD8(+) T cells in a MHC class I-restrict ed fashion, do not exhibit anti-MHC class II surface reactivity and fail to stimulate the proliferation of naive, allogeneic CD4(+) T cells. To test w hether the CD45(+) MHC class I+ CD80(+) DC line 80/1 expresses incompetent, or fails to transcribe, MHC class II molecules, we performed biochemical a nd molecular studies using Western blot and polymerase chain reaction analy sis. We found that 80/1 DC express MHC class II molecules neither at the pr otein nor at the transcriptional level. Ultrastructural examination of thes e cells revealed the presence of a LC-like morphology with indented nuclei, active cytoplasm, intermediate filaments and dendritic processes. In contr ast to adult LC, no LC-specific cytoplasmic organelles (Birbeck granules) w ere present. Functionally, 80/1 DC in the presence, but not in the absence, of concanavalin A and anti-T-cell receptor monoclonal antibodies stimulate d a vigorous proliferative response of naive CD4(+) and CD8(+) T cells. Fur thermore, we found that the anti-CD3-induced stimulation of naive CD4(+) an d CD8(+) T cells was critically dependent on the expression of Fc gamma R o n 80/1 DC and that the requirement for co-stimulation depends on the intens ity of T-cell receptor signalling.