Suppressive activity of cell-free ascites obtained from murine tumors. Soluble Fc receptor as a possible active agent.

It has been shown that cell-free ascites (CFA) from Ehrlich carcinoma, AKR leukemia, and CBA thymoma, which all had strong suppressive activity in vivo, inhibit EA-rosette formation, complement-dependent hemolysis, and agglutinate EA-complexes. Since these are the characteristics of soluble FcR the data suggest the presence of FcR in CFA tested. To test whether the suppressive activity observed in vivo was due to the soluble FcR and FcR-like fraction was absorbed out from CFA or Ehrlich carcinoma by means of antigen-antibody complexes. The isolated FcR-like fraction was strongly suppressive in vivo. Additional support for the proposition that soluble FcR might be one of the mediators of suppression in vivo was gained by testing cell-free supernatants (CFS) of cell-line cells known to shed off FcR into the medium. It could be shown that CFS of highly FcR positive K-562 cells inhibited PFC stronger than did the CFS of weakly FcR positive Raji cells.