Evidence for the recycling nature of the fibronectin receptor of macrophages

Plasma fibronectin (pFN) has been shown to mediate phagocytosis of several types of artificial particles and tissue debris by macrophages. In the present investigation some of the dynamic aspects of this receptor‐mediated cellular process have been studied. Plasma fibronectin did not bind specifically to fibronectin (FN)‐receptors of rat peritoneal macrophages at either 4°C or 37°C. On the other hand, pFN aggregated on the surface of gelatin‐coated latex beads (gLtx) and 125 I‐labeled pFN covalently coupled to latex beads (pFN * ‐Ltx) bound strongly to macrophages at both temperatures. Both of these particles were also internalized at 37°C. Treatment of macrophages by chymotrypsin, thermolysin, or trypsin in a protein‐free tissue culture medium did not affect either of the above reactions; however, pronase treatment strongly reduced both the binding and internalization of the pFN‐coated particles. The pronase‐treated macrophage monolayers in time regained their ability to bind and internalize pFN‐gLtx * when incubated in fresh tissue culture medium. Such recovery, however, did not take place when the medium contained cycloheximide. On the other hand, phagocytosis of pFN‐gLtx * was not affected directly by cycloheximide with untreated macrophages; this suggests that the FN‐receptor recycles during sustained phagocytosis. This assumption was substantiated by the observations that some of the established lysosomotropic amines—i.e., chloroquine, dansylcadaverine, and dimethyldansylcadaverine—caused total inhibition of internalization without affecting the binding of particles to macrophages. Furthermore, chloroquine protected the FN‐receptors against destruction by pronase. Together these results suggest that macrophage receptors for FN are protein, present both on the cell surface and intracellularly, and recycle between the plasma membrane and intracellular sites during phagocytosis.