Soluble Fc gamma R (sFc gamma R): detection in biological fluids and production of a murine recombinant sFc gamma R biologically active in vitro and in vivo.

Soluble forms of receptors for the Fc portion of IgG (sFc gamma R) were detected in biological fluids from mice and humans. In mouse bearing tumors, circulating amounts of sFc gamma R increased concurrently with tumor growth. Tumors secreting IgG2a, IgG2b or IgG3 led to a 5- to 10-fold increase in serum sFc gamma R levels whereas tumors secreting IgG1, IgGA or other types of tumors (non Ig B cell tumors, T cell lymphoma and a melanoma) increased 2- to 3-fold the levels of circulating sFc gamma R. In the human, sFc gamma R were also detected in whole unstimulated saliva. Levels of sFc gamma RII and of sFc gamma RIII were variable and did not seem to depend on the dental status of the individuals. Finally, a murine recombinant sFc gamma R (rsFc gamma R) composed of the two extracellular domains of Fc gamma RII was produced by culture of transfected L cells in bioreactors. The purified rsFc gamma R was found to inhibit antibody production in vitro in anti-SRBC responses and by cultures of small B cells stimulated by anti-IgM antibodies in the presence of IL-4 and IL-5. Moreover, the i.p. injection of this material into adult mice immunized with SRBC led to a decrease of IgG antibody production by splenocytes, as measured by a hemolytic plaque assay, and in serum, as measured by antigen-specific ELISA.