Down-regulation of the surface expression of class I MHC antigens by human cytomegalovirus

Class I major histocompatibility complex (MHC) antigens of higher eukaryotes play a crucial role in the recognition of human cytomegalovirus (HCMV)-infected cells by cytotoxic T lymphocytes. In the present study, we have demonstrated that HCMV infection resulted in a marked reduction in the surface expression of class I MHC antigens on human embryonic lung fibroblasts (HEL). Even when HCMV-infected HEL were cultured in the presence of 9-(1,3-dihydroxy-2-propoxymethyl)-guanine (DHPG), the reduction was observed to the same extent, indicating that HCMV DNA synthesis was not required for this phenomenon. However, immunoprecipitation studies have shown that there was no significant reduction in the synthesis of either the heavy chain or the light chain of class I MHC antigens after HCMV infection. In addition, Western blotting studies have revealed that the total amount of the antigens was almost unaltered after HCMV infection. These results suggest that the reduction in the cell surface expression of class I MHC antigens is due to some defect occurring post-translationally, most likely at the level of the correct complex formation and/or the intracellular transport of class I MHC antigens.