We have found that interleukin-4 (IL-4) abrogated constitutive beta interferon (IFN-j) synthesis in mouse macrophages. Analysis of endogenous IFN mRNA by reverse transcription-PCR clearly documented the presence of IFN-0 but not of IFN-a transcripts in RNA from bone marrow-derived macrophages (BMM). Culture of bone marrow cells for 7 days in the presence of macrophage colony-stimulating factor plus IL-4 or treatment of mature BMM with IL-4 for 3 to 4 days resulted in pronounced inhibition of

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... -I transcript levels. As a consequence, BMM became highly permissive to viral infection and induction of the IFN-responsive 2',5'-oligoadenylate synthetase was inhibited. In view of the evidence for low-level constitutive secretion of IFN in vivo, it is possible that IL-4 influences the host's antiviral defense system. nuclei from BMM after 10 days of culture in M-CSF alone or after treatment with IL-4 (2 ng/ml) from day 6 to day