The adenovirus E3/10.4K–14.5K proteins down-modulate the apoptosis receptor Fas/Apo-1 by inducing its internalization

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Abstract
Adenoviruses (Ads) have evolved multiple mechanisms to evade the host immune response. Several of the immunomodulatory Ad proteins are encoded in early transcription unit 3 (E3). The E3/19K protein interferes with antigen presentation and T cell recognition, whereas the E3/10.4K, 14.5K, and 14.7K proteins can protect cells from tumor necrosis factor α-mediated lysis. Here, we describe an additional activity of E3 proteins. Transfectants expressing all E3 proteins of Ad2 exhibit a profound reduction of the apoptosis receptor CD95 (Fas, APO-1) on the cell surface. In contrast, cells expressing only the E3A region have normal Fas levels. Thus, one of the E3B proteins (10.4K, 14.5K, or 14.7K) seems to be responsible for this effect. To identify the E3B products involved, each individual E3B ORF was selectively disrupted. Examination of stable cell lines containing the mutated E3 regions showed that Fas expression is restored when either the 10.4K or the 14.5K ORF is disrupted, whereas mutation of the 14.7K ORF does not rescue Fas expression. Loss of Fas on the cell surface is accompanied by a similar decrease of total Fas levels. However, in the presence of lysosomotropic agents Fas accumulates in endosomal/lysosomal vesicles, indicating that 10.4K–14.5K induce internalization and degradation of Fas. Down-regulation of Fas but not CD40 is also observed during infection and as a consequence, Ad-infected cells are protected from Fas-mediated apoptosis. Thus, the Fas system is implicated in Ad pathogenesis.