To determine whether the appearance of interferon (IFN) and the modulation of humoral responses observed following injection of irradiated tumor cells were mediated by suppressor cells, the effects of in vivo injection of I-Js specific antibodies were studied. We found that anti-I-J-containing, as well as normal ascites fluids, obtained after repeated ip injection of complete Freund's adjuvant, contain a factor which (a) induces the appearance of serum IFN, (b) enhances the response to SRBC, and (c) suppresses the response to TNP-Ficoll, when injected 1 day before antigen. This effect is not immunologically specific, is probably not caused by intact Ig, and does not appear to be mediated by T cells. Although the nature of the factor(s) responsible for the observed results has not been fully clarified, we report these findings now as a cautionary note for the interpretation of studies where in vivo injection of unfractionated ascites fluids containing monoclonal antibodies are used.
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