Specific Interferon Genes Are Expressed in Individual Cells in the Peritoneum and Bone Marrow of Normal Mice

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Abstract
The use of a highly sensitive method of in situ hybridization capable of detecting one copy of IFN mRNA per cell showed that from 20–50% of the cells from the peritoneum and bone marrow of both normal pathogen-free and axenic mice exhibited grain counts significantly greater than background levels following hybridization with riboprobes specific for the mouse interferon-α (IFN-α), IFN-β, or IFN-γ genes. Labeling was shown to be specific, as the labeled probe was displaced by a 200-fold excess of the specific unlabeled probe but not by a 200-fold excess of an unrelated probe. Grain counts were reduced to background levels when cells were pretreated with ribonuclease prior to in situ hybridization. The extent of labeling with either IFN-α or IFN-β-specific probes increased following i.v. inoculation of mice with the IFN-inducer Newcastle disease virus (NDV) whereas the degree of labeling observed with a probe specific for β-actin remained unchanged. No significant differences were observed in the number of bone marrow or peritoneal ccells that expressed IFN-α or IFN-β mRNA from either high (C57B1/6) or low (BALB/c) IFN-producing strains of mice. The majority of IFN-α and IFN-β-containing cells from both the bone marrow and peritoneum of normal pathogen-free and axenic mice resembled monocytes morphologically, whereas the majority of IFN-γ mRNA-containing cells resembled small lymphocytes. In addition, in the bone marrow a number of large cells wich resembled megacaryocytes were found to express high levels of IFN-α mRNA. Nuclear run-on assays showed that IFN-α and IFN-β genes were actively transcribed in both bone marrow and peritoneal cells from normal and axenic mice. Low levels of de novo IFN-γ RNA synthesis were detected in the nuclei of peritoneal cells only. The expression of IFN genes in individual cells in the tissues of normal animals may constitute a basis for the regulation of both homeostasis and host defense against virus infection and neoplastic cells.

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