Digestion and resynthesis of receptors binding IgG-sensitized erythrocytes on rat macrophages.

The proteinase sensitivity of rat macrophage Fc receptors (FcR) binding rabbit IgG-sensitized sheep erythrocytes (EA) was analysed by incubating alveolar macrophages with trypsin, alpha-chymotrypsin, pronase, granulocytic elastase and inhibitors of protein biosynthesis. Even under conditions at which enzymes alone did not bring about any receptor degrading effect, a reduction of cycloheximide. After a temporary blockade of protein biosynthesis by cycloheximide of macrophages which had lost their FcR through treatment with pronase, a rapid reappearance of EA-binding activity could be observed when the cells were washed and incubated in medium without drug or enzyme. On the other hand, the blockade induced by actinomycin D could not easily be reversed. The rate of reappearance of EA-binding activity of alveolar macrophages (AM psi) was faster than that of peritoneal macrophages (PM psi). Our results may suggest an alternative explanation for the apparent resistance of receptors to proteolytic digestion. The finally observed result may be caused by re-expression of FcR--either newly synthesized or from an intracellular pool--as well as by an inaccessibility of the receptor to the enzyme.