Macrophages in efferent lymph of sheep and their role in lectin-induced lymphocyte blastogenesis.

Very low numbers (3.9 +/- 3.0/10(5) cells) of macrophages (cells with intense cytoplasmic staining for nonspecific esterase) were found in seven popliteal lymph samples taken at surgery from 6 sheep. 67% of these cells were phagocytic for latex beads. Dose response and limiting dilution experiments indicated that efferent lymph lymphocytes responded to concanavalin A and pokeweed mitogen when at least 10 macrophages/10(5) cells were present, but macrophages appeared not to be necessary for phytohemagglutinin responses. Further depletion of macrophages from efferent lymph lymphocytes by nylon wool columns almost abolished concanavalin A and pokeweed mitogen responses and reduced phytohemagglutinin responses by 67%. The responses to all mitogens were restored to near normal efferent lymph lymphocyte levels by addition of 1% adherent cells and were elevated to near normal cell levels by 5% adherent cells. These results indicate that macrophages are present in efferent lymph and that, despite their low numbers, they may be of functional significance in regulation of immune responses.