Cell surface events which may initiate lysosomal enzyme secretion by human monocytes

Freshly isolated and subsequently matured human monocytes secreted lysosomal hexosaminidase in response to exposure to IgG‐Sepharose, but not certain derivatized control Sepharoses. The cells bound selectively to the surface of IgG‐Sepharose (and not the control Sepharoses) but because of the large size of the particles, could not ingest them. Since the soluble IgG was covalently linked to the Sepharose and free soluble IgG was not an inducer of secretion, the secretion was thus induced directly at the cell surface. Zymosan, a yeast cell wall particle which contains a mannan, was also able to induce secretion in the various monocyte stages under study. It could even bind to the cell surface of fresh monocytes which lacked the receptor for mannose‐terminated glycoproteins, and induce secretion in these cells. The mannose receptor appeared as monocytes matured, and the Ir number on the surface was increased by the action of lymphokines. Although zymosan‐induced secretion could be inhibited by mannose and certain other sugars, these seemed to have some complex metabolic effects in human monocytes (which previous work with mouse macrophages has not revealed). Thus, it was not possible to demonstrate whether zymosan could initiate secretion directly by interaction at the monocyte surface mannose glycoprotein receptor.