Novel molecular mechanisms of dendritic cell-induced T cell activation.

In this study we have re-examined the molecular mechanisms involved in activation of T cells by dendritic cells (DC). Human peripheral blood DC (PBDC) were derived by 2 h adhesion followed by 7 day culture in a combination of granulocyte macrophage colony stimulating factor and IL-4, and depletion of residual T and B cells. These PBDC were used to induce autologous T cell proliferation in a CD3-dependent response, and antibodies against CD11a/18 and CD86 were used as control inhibitors of accessory function. Antibodies against five of the cell surface molecules that we have recently identified on the surface of DC, CD13, CD87, CD98, CD147 and CD148, and an antibody which recognizes a molecule that has not as yet been identified, all inhibited the CD3-induced T cell proliferation. These findings were observed not only when antibodies were present throughout the culture, but also when they were prepulsed on to the surface of the DC, suggesting the inhibition was mediated via the antigen-presenting cells rather than the T cell. The same set of antibodies also inhibited an allospecific mixed lymphocyte reaction, confirming that the inhibitory effect was not dependent on the use of a CD3 antibody as the stimulating agent. All the antibodies of known specificity inhibited both CD4 and CD8 T cells equally. Unlike CD87, CD98 and CD147 antibodies, which inhibited activation of both CD45RA (naive) T cells and CD45RO (memory) T cells, CD13 and CD148 appeared to be involved in activation of naive cells only. The molecules identified in this study have not previously been demonstrated to play a role as accessory molecules on DC, the cells that are pivotal for immune induction. Therefore they may provide new potential targets for modulation of the immune response at the APC level.

Molecular characterization of U937-dependent T-cell co-stimulation.

U937 cells provide a co-stimulatory signal for CD3-mediated T-cell activation which is independent of the CD28/CD80/CD86 interaction. This study set out to identify which molecules contribute to this co-stimulatory activity. Monoclonal antibodies (mAb) to the known accessory molecules CD11a, CD18, CD54 and CD45, all inhibited T-cell proliferation. Although CD11a/18 mAb inhibited U937/T-cell cluster formation as well as proliferation, CD45 enhanced the size of the clusters formed, suggesting that this was not the only mechanism of inhibition. The alternative co-stimulatory pathway provided by U937 cells preferentially stimulated a response in the CD18+ T-cell population, and this reflected the reduced sensitivity of CD8+ T cells to CD28-mediated activation. Monoclonal antibodies to three molecules, CD53, CD98 and CD147, also inhibited U937-dependent T-cell proliferation. The mAb to CD98 and CD147 were inhibitory when prepulsed on to the U937 cells, suggesting an effect mediated by these molecules on the antigen-presenting cell.

From sentinel to messenger: an extended phenotypic analysis of the monocyte to dendritic cell transition.

The transitional stages in the relationship between sentinel monocytes and messenger dendritic cells that are active in adaptive immunity, are, as yet, unclear. To explore these events, 2-hr adherent peripheral blood mononuclear cells were used either as monocytes, or cultured for 7 days with granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4) to generate dendritic cells, and the phenotypic features and relationship of the two cell populations was investigated using an extensive panel of monoclonal antibodies (mAbs). The features of the shift from monocyte to dendritic cell were also examined by daily phenotyping during the 7-day culture period. Twenty-five mAbs, most of which recognized known CD molecules, bound both monocytes and dendritic cells equally, whereas 19 mAbs exhibited differential staining. Four molecules not previously reported on dendritic cells were documented: CD87, CD98, CD147 and CD148. Seven cell-surface molecules (HLA-DQ, CD1a, CD13, CD30, CD43, CD63 and CD86) were expressed either at very low levels or not at all on monocytes, but had a strikingly increased expression on dendritic cells, suggesting a role in antigen presentation. The kinetics of monocyte to dendritic cell transition revealed a rapid activation phase within the first 24 hr, with a considerable increase in expression of the activation markers HLA-DR, CD13, CD14 and CD98; this was followed by a down-regulation of CD14 and a more gradual development of the other dendritic cell features over the remaining 6 days, with steady increases in CD1a, CD18, CD43, CD86, HLA-DR and HLA-DQ. Thus, these studies have demonstrated four novel components of the dendritic cell, and have documented the dynamic multistep nature of the process whereby an antigen-presenting dendritic cell phenotype may emerge from a monocyte precursor.