Hyaluronan (HA) production by dendritic cells (DCs) is known to promote antigen presentation and to augment T-cell activation and proliferation. We hypothesized that pericellular HA can function as intercellular 'glue' directly mediating T cell-DC binding. Using primary human cells, we observed HA-dependent binding between T cells and DCs, which was abrogated upon pre-treatment of the DCs with 4-methylumbelliferone (4-MU), an agent which blocks HA synthesis. Furthermore, T cells regulate HA production by DCs via T cell-derived cytokines in a T helper (Th) subset-specific manner, as demonstrated by the observation that cell-culture supernatants from Th1 but not Th2 clones promote HA production. Similar effects were seen upon the addition of exogenous Th1 cytokines, IL-2, interferon gamma (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha). The critical factors which determined the extent of DC-T cell binding in this system were the nature of the pre-treatment the DCs received and their capacity to synthesize HA, as T-cell clones which were pre-treated with monensin, added to block cytokine secretion, bound equivalently irrespective of their Th subset. These data support the existence of a feedforward loop wherein T-cell cytokines influence DC production of HA, which in turn affects the extent of DC-T cell binding. We also document the presence of focal deposits of HA at the immune synapse between T-cells and APC and on dendritic processes thought to be important in antigen presentation. These data point to a pivotal role for HA in DC-T cell interactions at the IS.