A new ELISA assay demonstrates sex differences in the concentration of serum polysialic acid

Male and female immune systems are strikingly different and yet little is known about sex differences in immune glycans, though glycans play central roles in regulating the immune response. Polysialic acid (polySia) occurs on the majority of leukocytes and is a potent immunomodulatory glycan which enables cell migration and serves as an immune checkpoint. Due to widespread influence of polySia on the immune system, we aimed to characterize its levels in serum, its presence on specific proteins, and differences in the amounts of polySia in male and female serum. However, polySia is difficult to quantify and detect on specific proteins, which makes it challenging to elucidate the molecular details of polySia function. We developed a sandwich ELISA that allows for the quantification of polySia as well as specific polysialylated proteins in complex mixtures without any pretreatment or harsh conditions. The assay is quick, linear, and robust under a wide variety of conditions and gave a limit of detection of approximately 0.2 ng polySia per mL of serum. We then quantified polySia and polysialylated CD56 in human and mouse serum. These studies strongly support our hypothesis of differences in glycosylation between the sexes as significantly less polySia was observed in female samples than in male samples.