Identification of fibronectins in peritoneal macrophages during the phagocytosis of Brucella. An immunocytochemical study by electron microscopy.

Fibronectin is a high-molecular-weight molecule whose opsonic properties favor the endocytosis of bacteria and particulate material. Using immunolabeling with anti-fibronectin IgGs coupled with peroxidase, we have looked for its localization in peritoneal macrophages from antigenically stimulated and nonstimulated mice. Ultrastructural location of fibronectin in macrophages sheds light on several points. It confirms that it is synthesized and secreted by mouse peritoneal macrophages. In addition, we found significant variability among the cells of a given population, and also between populations from stimulated and nonstimulated animals. Under certain conditions macrophages secrete an extracellular matrix whose aspect varies with the physiological conditions, but which generally consist of fibronectins either on a fibrillar network connecting the macrophages to one another or in an agglutinating extracellular cement. Endocytosis of Brucella by macrophages is accompanied by secretion of fibronectin inside endocytic vacuoles; this secretion contributes to the formation of an electron-dense material closely surrounding phagocytosed bacteria.