DIFFERENTIAL INHIBITION OF THE FAS-MEDIATED AND GRANULE-MEDIATED CYTOLYSIS PATHWAYS BY THE ORTHOPOXVIRUS CYTOKINE RESPONSE MODIFIER A SPI-2AND SPI-1 PROTEIN/

Cytotoxic T lymphocytes are important effecters of antiviral immunity, and they induce target cell death either by secretion of cytoplasmic granules containing perforin and granzymes or by signaling through the Pas cell surface antigen. Although it is not known whether the granul e-mediated and Pas-mediated cytolytic mechanisms share common componen ts, proteinase activity has been implicated as an important feature of both pathways. The orthopoxviruses cowpox virus and rabbitpox virus e ach encode three members of the serpin family of proteinase inhibitors , designated SPI-1, SPI-2, and SPI-3. Of these, SPI-2 (also referred t o as cytokine response modifier A in cowpox virus) has been shown to i nhibit the proteolytic activity of both members of the interleukin Ip converting enzyme family and granzyme B. We report here that cells inf ected with cowpox or rabbitpox viruses exhibit resistance to cytolysis by either cytolytic mechanism, Whereas mutation of the cytokine respo nse modifier A/SPI-2 gene was necessary to relieve inhibition of Fas-m ediated cytolysis, in some cell types mutation of SPI-1, in addition t o cytokine response modifier A/SPI-2, was necessary to completely abro gate inhibition, In contrast, viral inhibition of granule-mediated kil ling was unaffected by mutation of cytokine response modifier A/SPI-2 alone, and it was relieved only when both the cytokine response modifi er A/SPI-2 and SPI-1 genes were inactivated, These results suggest tha t an interleukin 1 beta converting enzyme-like enzymatic activity is i nvolved in both killing mechanisms and indicate that two viral protein s, SPI-1 and cytokine response modifier A/SPI-2, are necessary to inhi bit both cytolysis path-ways.