Endocytosis of integrin alpha 5 beta 1 (fibronectin receptor) of mouse peritoneal macrophages in vitro: an immunoelectron microscopic study.

Localization and dynamic aspects of fibronectin receptor (alpha 5 beta 1 integrin) were investigated by immunoelectron microscopy on freshly isolated mouse peritoneal macrophages and after cultivation in vitro. This receptor has only been characterized biochemically. Identity of macrophages was proved by binding of the antibody against the F4/80 specific macrophage marker. Demonstration of fibronectin receptor and mouse macrophage marker on the plasma membrane revealed that these glycoproteins were rapidly endocytosed within the first minutes after isolation. Endocytotic uptake of these glycoproteins was demonstrated by double immunolabeling and secondary labeling with gold particles of different size. Label was found in endocytotic and lysosome-like vesicles, but not in coated vesicles. During further cultivation the macrophage marker reappeared again on the plasma membrane after 3 hr, but the fibronectin receptor was not demonstrable. Treatment with cytochalasin B in culture and incubation at 4 degrees C resulted in strongly labeled plasma membrane and inhibition of internalization. In contrast to macrophages, alpha 5 beta 1 integrin was demonstrable on marmoset skin fibroblasts even after 3 days in culture. In this cell type, endocytotic uptake was solely detectable in coated vesicles. These results suggest a high endocytotic activity of macrophages after isolation leading to a rapid disappearance of both glycoproteins, whereby, in contrast to the fibronectin receptor, only the macrophage marker is recycled again. These data indicate a change in plasma membrane (receptor content) after isolation and cultivation of macrophages and cell-specific differences in vitro.