Effective suppression of class I major histocompatibility complex expression by the US11 or ICP47 genes can be limited by cell type or interferon-gamma exposure.

An impediment encountered in many viral-based gene therapy clinical trials has been the rapid destruction of the transgene by the host's immune response. The processing and presentation of antigens through the class I major histocompatibility complex (MHC) pathway is the initial specific response to viral infection. Disruption of the class I MHC pathway by herpes simplex virus (HSV) or the human cytomegalovirus (HCMV) results in a decrease of the CD8(+) cytotoxic T lymphocyte (CTL) response and prolongs survival of infected cells in the host. Two viral immune suppression genes that interfere with the class I MHC presentation pathway, the HSV type I ICP47 gene and HCMV US11 gene, were cloned and each incorporated into a retroviral vector. HSV ICP47 and HCMV US11 transgenes were expressed in multiple cells lines and compared for their abilities to reduce antigen presentation on the cell surface by class I MHC. Retroviral supernatants were used to transduce human, canine, and rat cell lines. Fluorescence-activated cell sorter (FACS) analysis of US11- and ICP47-transduced cell lines demonstrated substantial reductions in class I MHC cell surface expression in most cell lines except in rodent cells where ICP47 is nonfunctional. The decrease in the level of class I MHC expression for ICP47 transduced cell lines ranged from 31-98% relative to negative controls. US11 decreased class I cell surface MHC by 67-96%. When both ICP47 and US11 are expressed in human cells, a further reduction of class I MHC was observed. Next, human A375 melanoma cells were tested to determine if the resulting reduction in cell surface class I MHC would reduce in vitro cytotoxicity by CTL. A375 cells expressing either ICP47 or US11 demonstrated a twofold to threefold reduction of specific lysis by primed CD8(+) CTL. These data clearly establish an ability to convey immune protection to human cells by viral genes. However, further analysis demonstrated that interferon (IFN)-gamma could reverse part or all of the downregulation of class I MHC induced by the ICP47 or US11 genes. The ICP47 and US11 genes, when expressed in target cells, decrease class I MHC presentation and as such might be used in strategies to create local immunosuppression against transgenes or allografts.