Deacetylated sialic acids modulates immune mediated cytotoxicity via the sialic acid-Siglec pathway

Cancers utilize glycans to evade the immune system via the Sialic acid (Sia)-Siglec (Sialic-acid-binding immunoglobulin-like lectins) pathway. Specifically, atypical structural forms of sialic acid bind to inhibitory Siglec receptors on natural killer (NK) cells resulting in the suppression of immune cell mediated cytotoxicity. The mechanism of action that governs the Sia-Siglec pathway in cancers is not understood. Specifically, how deviations from the typical form of Sia mechanistically contribute. Here, we focused on modulating 9-O and 7, 9-O-acetylation of Neu5Ac, via CRISPR-Cas9 gene editing, a functional group that is absent from Sias on many types of cancer cells. The two genes that are responsible for regulating the level of acetylation on Neu5Ac, are Sialic acid acetylesterase (SIAE) and Sialic acid acetyltransferase (CASD1). These genes modulated Siglec binding in colon, lung and a noncancerous kidney cell line. In the absence of SIAE, Neu5Ac is acetylated, engagement of cancer associated Siglecs is reduced while binding was increased when the ability to acetylate was removed via CASD1 knock out. In the absence of SIAE NK mediated cytotoxicity increased in both colon and lung cancer cells. In addition to modulating Siglec binding, SIAE expression modulates the level of Sias in a cell, and the α2-6-linkage of Sias-which is specifically upregulated and associated with cancers. Uncovering how functional group alterations on Neu5Ac contribute mechanistically to both Siglec receptor binding, the Sia-Siglec immune evasion pathway, and the production of cancer associated glycosidic linkages-offers a promising avenue for targeted cancer immune therapies in the future.