The capacity of dendritic cells to present protein antigens has been studied with two MHC class II-restricted, myoglobin-specific, T cell clones. Spleen dendritic cells and cultured epidermal Langerhans cells (LC) presented native myoglobin weakly and often not at all. These same populations were powerful stimulators of allogeneic T cells in the primary MLR. Freshly isolated LC were in contrast very active in presenting proteins to T cell clones but were weak stimulators of the MLR. Both fresh and cultured LC could present specific peptide fragments of myoglobin to the clones. These results suggest that dendritic cells in nonlymphoid tissues like skin can act as sentinels for presenting antigens in situ, their accessory function developing in two phases. First antigens are captured and appropriately presented. Further handling of antigen then is downregulated while the cells acquire strong sensitizing activity for the growth and function of resting T lymphocytes. The potent MLR stimulating activity of cultured epidermal LC and lymphoid dendritic cells probably reflects prior handling of antigens leading to the formation of allogeneic MHC-peptide complexes.

3C10 and 1D9 are two related monoclonal antibodies that specifically identify human mononuclear phagocytes in a large number of sites, including blood monocytes, alveolar macrophages, and macrophages in tissue sections of spleen, lymph node, and skin. The antigen persists on monocytes cultured for greater than 4 wk, but it is not found on giant cells. The 3C10-1D9 determinant is carried by a 55 kD polypeptide, is expressed at approximately 40,000 copies per monocyte, and is protease sensitive. The antigen is clearly different from HLA-class II or Ia-like antigens that have been studied with a new monoclonal 9.3F10. The 9.3F10 antigen is found on B cells, dendritic cells and monocytes; is protease resistant, and occurs on a 33-29 kD doublet typical of class II products. The 3C10 monoclonal provides a clear distinction between human mononuclear phagocytes and dendritic cells. First, monocytes and lymphocytes can be eliminated from plastic-adherent mononuclear cells using 3C10, complement, and two previously described cytotoxic antibodies, BA-1 (anti-B cell) and Leu-1 (anti-T cell). As a result, the trace dendritic cell component of blood can be enriched to considerable purity (65-75%) and yield. Second, immunocytochemical staining of tissue sections reveals that 3C10+ macrophages are anatomically segregated from dendritic cells. Large numbers of 3C10+ cells are found in red pulp of spleen and in regions surrounding lymphatic channels of lymph node. However, 3C10+ macrophages are scarce in white pulp of spleen and the lymphocyte-rich cortex of node that are the sites where dendritic cells are localized. 3C10+ cells in skin are found in the dermis, particularly in leprosy infiltrates, but the Langerhans' cells of epidermis are 3C10-. The distinctive localization of macrophages and dendritic cells is consistent with their respective functions as effector and accessory cells in the immune response.

The function of exogenous murine recombinant IL-1 alpha as a T lymphocyte-activating molecule was examined. IL-1 did not induce IL-2 release or responsiveness in purified T cells regardless of their state of activation: unprimed lymphocytes, freshly sensitized lymphocytes, or memory cells derived from the blasts. Nor did IL-1 synergize with mitogens, or with antigens, to stimulate proliferation. For example the combinations of IL-1 plus Ia+ peritoneal macrophages, or IL-1 plus Con A, were less than 5% as effective in triggering T cell growth as a low dose (1%) of dendritic cells. However, when IL-1 was added at the onset of culture, the response to limiting doses of dendritic cells was increased 3- to 10-fold in several systems: the syngeneic and allogeneic MLR, Con A- and periodate-induced polyclonal mitogenesis, and T-dependent antibody formation against foreign red cells. The amplifying effect of IL-1 could be obtained if the dendritic cells but not the responding lymphocytes were exposed to IL-1 before use as accessory cells. Optimal activation of dendritic cells required a dose of 5 U/ml (50 pM) and 18 h of exposure, and was not due to carryover of IL-1 into the lymphocyte culture. IL-2, IL-3, and cachectin/TNF did not amplify dendritic cell function, while IFN-gamma diminished it. The enhanced function of IL-1-treated dendritic cells was due to an enhanced clustering with helper T lymphocytes in the first day of the MLR response. Therefore IL-1 does not seem to act as an activating factor for most peripheral T lymphocytes. Instead, IL-1 enhances the function of accessory dendritic cells and represents the first molecule that has been shown to enhance the immune response at this critical level.

An interleukin 1a (IL-1a) cDNA probe and an IL-l responsive T-cell clone (D1O.G4; half-maximal stimulation, 0.1-1 pM) have been used to study the production of IL-1 by primary murine cell populations, particularly macrophages and dendritic cells. Spleen and peritoneal macrophages produced IL-1 mRNA and released biologically active IL-1 when challenged with lipopolysaccharide (LPS). Induction of IL-1 was evident over a dose range of 0.01-10 ,jg of LPS per ml, and maxmal mRNA levels were maintained from 4 to 20 hr. Several other stimuli did not induce IL-1 in cultured macrophages, including phorbol 12-myristate 13-acetate, r-interferon, Con A, macrophage colony-stimulating factor, IL-3, cachectin, and activated T cells. Activated T cells could markedly reduce the response ofperitoneal macrophages to LPS. When other cell types were compared with macrophages, keratinocytes had high levels of IL-1 mRNA, apparently in response to endogenous LPS. However B and T lymphocytes did not yield detectable IL-1 during proliferative responses to LPS and Con A, respectively, While dendritic cells produced little or no IL-1 when challenged with a battery of stimuli. Therefore, IL-1 may not be required for the potent accessory function of dendritic cells in lymphocyte mitogenesis. The results indicate that macrophages and dendritic cells have different secretory capacities. The macrophage is the principal leukocyte that synthesizes IL-1, and select stimuli increase and decrease the levels of macrophage IL-1 mRNA.

During the primary mixed leukocyte reaction, T lymphocytes of the lyt-2-helper subclass proliferate in response to transplantation antigens on allogeneic dendritic cells. We have isolated populations of antigen-specific proliferating lymphoblasts and recultured them in fresh medium. Within 2 days, the blasts become smaller in size, lose responsiveness to T-cell growth factor or interleukin 2, but retain vigorous reactivity to the original alloantigen. Two new biologic properties of these "memory" lymphocytes have been noted. First, they primarily respond to alloantigen on dendritic cells, whereas freshly sensitized lymphoblasts react to allogeneic dendritic cells, macrophages, and B lymphocytes. Second, the memory lymphocytes quickly aggregate with dendritic cells that are either syngeneic or allogeneic, but not with B cells. The aggregates that form with syngeneic dendritic cells disassemble within hours and do not release interleukin 2 or proliferate. The aggregates that form with allogeneic dendritic cells remain intact, release large amounts of interleukin 2 on the first day of culture, and synthesize DNA on the second day. Therefore, dendritic cells actively cluster memory lymphocytes by an antigen-independent mechanism, and this may underlie the heightened functional activity of each cell type.