Expression of 9-O- and 7,9-O-Acetyl Modified Sialic Acid in Cells and Their Effects on Influenza Viruses

Barnard, Karen N.; Wasik, Brian R.; LaClair, Justin R.; Buchholz, David W.; Weichert, Wendy S.; Alford-Lawrence, Brynn K.; Aguilar, Hector C.; Parrish, Colin R.

We confirmed that 9-O-and 7,9-O-Ac are expressed on cells in culture and that expression varies between cell lines, as has been previously reported (38) . However, when present these modified Sia made up only 1 to 2% of the total Sia when analyzed by HPLC. Cells showed distinct population heterogeneity in 7,9-O-and 9-O-Ac display and localization that was observed over many passages examined. Previous studies using ICV HEF probes found similar staining patterns on some cell lines and also showed that, after sorting into high and low staining populations, both populations returned to previous levels of heterogeneity within a few passages (11, 38) . Here, we saw that the modified Sia are retained in the Golgi compartments of HEK-293 and A549 cells, although an occasional cell displayed these modified Sia on the surface. It is not clear why these modified Sia are localized in the Golgi body, although perhaps the modifications could block the onward trafficking of glycoproteins to the cell surface. In contrast to the human cell lines, MDCK cells showed many cells expressing 7,9-O-Ac and 9-O-Ac on the cell surface, raising the possibility that trafficking could be cell type or species specific. A summary of the synthesis and localization of 7,9-O-and 9-O-Ac is shown in Fig. 8 .