The binding of [H]leucine-labelled pure human interferon α from Namalwa cells to human FL and Daudi cells was studied, and evidence obtained to indicate receptor-mediated internalization of interferon. Cell surface-bound and internalized interferons were quantified separately as trypsin-released and unreleased radioactivities, respectively. At 37 °C, surface-bound interferon reached a maximum after 1 h and then decreased, while internalized interferon reached a maximum after 2 h. At 21 °C, in contrast, surface-bound interferon reached a maximum after 2 h and did not decrease thereafter; no internalization was observed. The same was true at 37 °C in the presence of NaF, indicating dependence of internalization on temperature and energy. In control cultures at 37 °C, internalized interferon, after reaching a maximum, decreased after prolonged incubation, and concomitantly acid-soluble radioactivity appeared in the culture medium. The decrease in internalized interferon and the emergence of degraded interferon were inhibited by the lysosomotropic agents, ammonium chloride and chloroquine. The fate of labelled interferon, bound to the cell surface of FL cells at 21 °C, was studied at 37 °C, and the results indicated that trypsin-unreleased interferon was derived from the surface-bound interferon, and was secreted in part into the culture fluid in a degraded form upon prolonged incubation. The relation of internalization of interferon to its biological activity was studied in three ways. In FL cells, the antiviral activity was not induced when internalization of interferon was entirely blocked (at 21 °C or in the presence of NaF at 37 °C). In Daudi cells, both 2′-5′-oligoadenylate (2–5A) synthetase induction by interferon and internalization of interferon were inhibited completely by diethyldithiocarbamate (DDC), whereas in the case of FL cells, DDC inhibited neither 2–5A synthetase induction nor internalization of interferon. Raji cells, which have an interferon-specific binding site on the cell surface but are insensitive to interferon, were found not to internalize interferon, whereas other Burkitt's lymphoma cells, Daudi and Namalwa, which are sensitive to interferon, did internalize it. These findings suggest (but do not prove) that internalization is required for the establishment of interferon activity.


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