The sensitization phase of T-cell-mediated immunity.

Many cell-mediated immune responses appear to develop in two phases: A sensitization phase in which unprimed or memory T cells interact with dendritic cells to become active lymphoblasts, and an effector phase in which the T lymphoblasts and other presenting cells interact to eliminate the antigen. Antigen presentation is essential to both phases. Here we review several features that are pertinent to the special sensitization role of dendritic cells. First, dendritic cells from lymphoid tissues, blood, and lymph (lymphoid dendritic cells) express very high levels of class I and II MHC products, and these levels cannot be increased by exposure to cytokines such as immune interferon. Second, dendritic cells efficiently cluster antigen-specific T cells during primary responses. Other presenting cells, like macrophages and B lymphocytes, do not form clusters but do bind to sensitized T lymphoblasts. Dendritic-T-cell binding is not inhibited by mAb to CD4 and LFA-1 antigens. It is suggested that a dendritic-cell-specific molecule is required. Third, it is not yet clear if dendritic cells make a "lymphocyte activating factor." However, IL-1 is not produced, even when dendritic cells are in contact with responding T cells. Fourth, dendritic cells have the capacity to migrate from the tissues and move to T-dependent areas. Epidermal Langerhans cells represent a reservoir of tissue dendritic cells but seem to be immunologically immature. The viability and accessory function of the Langerhans cell greatly depend on a single cytokine, granulocyte-macrophage colony stimulating factor (GM-CSF), leading to the proposal that GM-CSF is critical in mobilizing active dendritic cells at the onset of a cell-mediated immune response.