9- O -acetylation of sialic acid is a common modification that plays important roles in host-pathogen interactions. CASD1 has been described as a sialate- O -acetyltransferase and has been shown to be essential for 9- O -acetylation of sialic acid in some cell lines in vitro . In this study, we used knockout mice to confirm that CASD1 is indeed responsible for 9- O -acetylation of sialic acids in vivo . We observed a complete loss of 9- O -acetylation of sialic acids on the surface of myeloid, erythroid and CD4 ⁺ T cells in Casd1 -deficient mice. Although 9- O -acetylation of sialic acids on multiple hematopoietic lineages was lost, there were no obvious defects in hematopoiesis. Interestingly, red blood cells from Casd1 -deficient mice also lost reactivity to TER-119, a rat monoclonal antibody that is widely used to mark the murine erythroid lineage. The sialic acid glyco-epitope recognized by TER-119 on red blood cells was sensitive to the sialic acid O -acetyl esterase activity of the hemagglutinin esterase from bovine coronavirus but not to the corresponding enzyme from the influenza C virus. During erythrocyte development TER-119 ⁺ Ery-A and Ery-B cells could be stained by catalytically inactive bovine coronavirus hemaggutinin-esterase but not by the inactive influenza C hemagglutinin esterase, while TER-119 ⁺ Ery-C stage cells and mature erythrocytes were recognized by both virolectins. These results suggest that throughout murine erythrocyte development, cells of the erythroid lineage express a glycoconjugate bearing a modified 7,9-di- O -acetyl form of sialic acid, that is recognized specifically by the bovine coronavirus lectin and not by the influenza C hemagglutinin, and this modified sialic acid moiety is a component of the TER-119 epitope. As erythrocytes mature, the surface of Ery-C cells and mature erythrocytes also acquires a distinct CASD1-dependent 9- O -acetyl sialic acid moiety that can be recognized by virolectins from both influenza C and bovine coronavirus that are specific for 9- O -acetyl sialic acid.

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