Increased levels of soluble low‐affinity Feγ receptors (IgG‐binding factors) in the sera of tumour‐bearing mice

SUMMARY Soluble forms of low affinity Feγ receptors (FcyR), also called IgG‐binding factors (IgG‐BF), have been shown to play a regulatory role in immune responses. By using an immunodot assay with the anti‐mouse FcyR MoAb, 2.4G2, the levels of IgG‐BF have been measured in the sera of mice bearing syngeneic tumours of lymphoid or non‐lymphoid origin or in mice injected with high doses of murine IgG. These sera contained large amounts of IgG‐BF as compared with controls. In the case of mice bearing IgG2a‐ or IgG2b‐secreting hybridomas or lymphomas, serum IgG‐BF increased progressively with tumour size and serum monoclonal IgG concentration, reaching 4–12 times the normal levels. A less than three‐fold increase was found in mice bearing an IgG1‐secreting hybridoma or tumours which do not secrete IgG (IgA‐secreting hybridoma, non‐immunoglobulin‐secreting lymphoid tumours or melanoma) or in mice injected with 9 mg of monoclonal IgG2a. The enhancement of serum IgG‐BF levels was independent of the expression of FcγR by the tumour cells, suggesting that the majority of IgG‐BF secreted in response to tumours was produced by the host rather than by the tumour. The increased production of IgG‐BF may participate in the control of tumour growth and in the modulation of the host immune responses in tumour‐bearing animals.