We have investigated whether a homospecific nucleus might be necessary in order for a species-specific interferon (IFN) to protect heterokaryons and cybrids against viral infection. We used mouse fibroblast interferon (Mu-IFN-β) and human leukocyte interferon (Hu-IFN-α) to induce resistance to vesicular stomatitis virus (VSV) replication in single cells, polykaryons, or cybrids. We speciated the cells by means of Heochst 33258 fluorescence and counted those displaying rhodamine anti-VSV immunofluorescence to determine the proportion of cells of each type that were infected with VSV. When we fused intact mouse L-929 fibroblasts to intact human HEL fibroblasts with polyethylene glycol (PEG), we found that the single mouse cells and mouse homopolykaryons were protected only by the mouse IFN; that the single human cells and human homopolykaryons were protected only by the human IFN; and that the heteropolykaryons were protected by both IFNs. When we constructed the mouse-human heterokaryons from L-929 cells whose DNA had been crosslinked by treatment with Trioxsalen and near ultraviolet light (320-360 nm), however, they were protected only by the human IFN, not by the mouse IFN. Similar lack of protection by mouse IFN was observed in cybrids made by fusing intact human cells to mouse cytoplasts enucleated by cytochalasin B treatment. We also continuously exposed mouse cells with intact nuclei to cycloheximide (CH) throughout their fusion with PEG, their subsequent brief treatment with IFN, and the termination of the IFN "pulse" with anti-IFN antibody. The CH was then removed, allowing protein synthesis to resume. The fused mouse cells nevertheless resisted VSV infection. This result suggests that the lack of response to mouse IFN of the human-mouse heterokaryons with inactivated mouse nuclei, and of the cybrids, was not due to PEG-induced loss or dysfunction of species-specific IFN receptors in cells incapable of resynthesizing them. Our data support the hypothesis that a homospecific nucleus is needed in order for some species-specific IFNs to protect cells against viral infection and hence that the nucleus is involved in a selective response to IFN.
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