PLATELETS INDUCE MONOCYTE DIFFERENTIATION IN SERUM-FREE COCULTURE

Terminal maturation of blood monocytes (RIG) in vitro and in vivo into macrophages (MAC) occurs as a result of interactions with various cel l types, To characterize some of the cell-cell connections that may be important for MO differentiation we cocultured hunan MO with lymphocy tes and/or with platelets. We found that intact platelets strongly pro moted MO maturation under serum-free conditions as evident from the ex pression of differentiation-dependent antigens and morphology. To furt her characterize the differentiation-inducing component(s) we prepared membrane and cytosol fractions of platelets. Both fractions could ind uce MO maturation, comparable to intact platelets. Further centrifugat ion of the cytosolic fraction revealed that only the pellet of ultrace ntrifugation, e.g., membrane fragments, could induce MO differentiatio n, Digestion with either trypsin or neuraminidase could only partially inhibit this effect. The same was true for heat-treated fractions, in dicating that this platelet-derived differentiation stimulus is not so lely an intact protein, Next we prepared protein and lipid fractions o f platelets. Treatment of MO with platelet proteins or platelet lipids clearly showed that only the lipid components were able to induce MO maturation. We propose components present in the lipid fraction of pla telet membranes as possible inducers of MO maturation in vitro.