Human cytomegalovirus strain-specific differences in protein expression of type I IFN pathway proteins do not impact virus replication

The type I IFN response is crucial for cells to restrict viral replication during infection. Many viruses, including human cytomegalovirus (HCMV), have evolved mechanisms to antagonize the type I IFN response. We have previously observed an increase in protein expression of certain IFN-stimulated genes when comparing the high-passage HCMV strain AD169 to the low-passage strain HCMV Merlin, suggesting that AD169 is defective in its ability to inhibit type I IFN function. To better understand HCMV interaction with the type I IFN response, we examined expression of cellular and viral proteins expressed in Merlin- and AD169-infected cells associated with IFN production and signalling. HCMV IFN antagonists expressed by both viruses had differences in amino acids throughout their protein sequences, although analysis using AlphaFold revealed that there was likely to be no obvious differences in the overall structure of these proteins. Analysis of quantitative mass spectrometry datasets showed modest differences in the expression of cellular IFN-associated proteins between strains. Contrary to previously reported data, we found no obvious loss of IRF3 expression, though this may be due to experimental differences between studies. These data revealed that multiplicity of infection was an important factor in IRF3 expression. We found little or no statistical difference in the production of IFN-<i>β</i> RNA between Merlin- and AD169-infected cells in reverse transcriptase quantitative PCR assays and little or no statistical difference in replication of AD169 and Merlin in virus replication assays. Overall, these data suggest that different strains of HCMV have different, albeit modest, abilities to influence the expression of type I IFN pathway proteins during infection. However, this had no overall impact on the ability of different strains to produce a type I IFN or to replicate.