Development of specific surface receptors recognizing mannose-terminal glycoconjugates in cultured monocytes: a possible early marker for differentiation of monocyte into macrophage.

Surface receptors for mannose-terminal glycoconjugates have been reported in various macrophage populations and are thought to be involved in specific binding and internalization of mannose-rich substances. They thereby may serve a function in such phenomena as phagocytosis of yeast and tumor cell recognition. Little is known of mannosyl receptors in blood monocytes. We synthesized a probe by covalently linking D-mannose to bovine serum albumin (BSA). Using this probe in fluoresceinated or latex minibead-derivatized forms, we searched the surface of human monocytes for the presence of mannosyl receptors. 125I-labeled probe was further used to quantify the number of receptors and the kinetics of the binding. Freshly isolated monocytes did not bind the probe, indicating the absence of mannosyl receptors. When placed in a culture system that preserves functional and morphological homogeneity of the cells, surface receptors for D-mannosyl glycoproteins developed within four days, reached a peak after one week, and then remained fairly stable. Binding parameters (Kd, Bmax, and receptor number) also remained stable and were not dissimilar to those reported for macrophages, although the Kd was consistently larger in cultured monocytes. When studied at 37 degrees C, the ligand-receptor complex was internalized through a system of coated pits and vesicles. The development of these receptors before evidence of morphological or functional differentiation suggests that these receptors may constitute an early marker for differentiation of monocytes into macrophages.