Antigen-independent binding of T-cells by dendritic cells and alveolar macrophages in the rat.

The antigen-independent binding of CD4+ T-lymphoblasts by alveolar and peritoneal macrophages and splenic dendritic cells (DC) was compared. DC formed clusters with T-lymphoblasts within 30 min at 37 degrees C, whereas alveolar and peritoneal macrophages did not. Antigen-independent binding developed between macrophages and CD4+ blasts by 4 h at 37 degrees C. Binding by alveolar macrophages was trypsin sensitive, magnesium dependent, serum independent, and cold insensitive, whereas binding by DC required serum and was inhibited by cold. Cluster formation (cell aggregates greater than 250 microns 2) by macrophages and CD4+ blasts was increased by interferon-gamma and phorbol esters, but diminished by lipopolysaccharide. However, each of these factors increased cluster formation by blasts with DC. Efforts to promote antigen-independent binding of T cells by Ia+ macrophages did not alter their poor accessory cell capacities. The role of cluster formation in accessory cell activities was examined. Inhibitors of DC clustering, including trypsin, paraformaldehyde, and tunicamycin, abrogated the ability of DC to support antigen presentation and lectin-mediated proliferation. It is concluded that rapid antigen-independent binding to T-cells is a distinct property that is restricted to DC. Exposure to LPS may down regulate nonproductive binding of T-cells to alveolar macrophages. Our data further suggest that accessory cell activities in the rat are not a function of alveolar macrophages and may be limited to specialized Ia+ cells of dendritic lineage.