A synthetic <i>Antrodia cinnamomea</i> galactomannan library by frameshift unveils a potent immunoregulatory octasaccharide domain

We report the design, synthesis, and immunological evaluation of an <i>Antrodia cinnamomea</i> galactomannan library consisting of 11 oligosaccharides, with chain lengths ranging from 4 to 24 sugars. These oligosaccharides maintain a consistent composition of 75% mannose and 25% galactose, mirroring the key structural features of natural galactomannan. Notably, the library includes five tetrasaccharides and four octasaccharides, exhibiting all possible frameshift patterns of <i>A. cinnamomea</i> galactomannan, offering valuable materials for structure-activity relationship studies to explore the role of linking modes and domain effect. The synthesis of this library was achieved through a highly effective stereoselective α-galactosylation approach. Immunological evaluations revealed that octasaccharide <b>4</b> and its two subunit tetrasaccharides <b>7</b> and <b>10</b> exhibited potent immunoregulatory activities, demonstrating a notable domain effect. Preliminary mechanistic studies unveiled that these oligosaccharides exert their effects by suppressing mitogen-activated protein kinase signaling pathway. In addition, octasaccharide <b>4</b> uniquely attenuated nuclear factor κB pathway, highlighting its compound-specific mechanism and demonstrating a 1 + 1 > 2 effect.