Evidence of the existence of a factor that induces C3 receptors on bone marrow cells.

The existence of a molecule responsible for the induction of C3R receptors (C3R) on murine bone marrow cells (C3R inducer, C3RI) is demonstrated in lung conditioned media. The inducer of Fc receptors on murine bone marrow cells (FcRI) and the colony-forming factor MGI (macrophage and granulocyte inducer) are also identified from the same source. The three inducing factors are shown by molecular sieving chromatography and by isoelectric focusing in granulated gel to be three different molecules. The molecular weights estimated were of 35,000, 22,000, and 10,500 for C3RI, MGI, and FcRI, respectively, with isoelectric pH values of 3.9, 6.6, and 7.6 for the same molecules. The C3RI is assumed to be of protein nature, because the molecule lost its biologic activity after trypsin or heat treatment. In the two techniques used, the three peaks of activity were completely separated, suggesting that the induction of C3 or Fc receptors and colony formation are mediated by different molecules and thus regulated independently at the cellular level.