Siglec cis-ligands and their roles in the immune system.

The article focuses on the role and regulation of sialic acid-binding immunoglobulin-like lectins (Siglecs), a family of membrane molecules primarily expressed on immune cells, emphasizing their interactions with cis-ligands—sialylated glycans on the same cell surface. It highlights that inhibitory Siglecs, such as CD22 (Siglec-2), modulate immune cell signaling through interactions with cis-ligands, which can differentially regulate B-cell receptor (BCR) signaling during antigen engagement and tonic signaling in resting B cells. The article reviews methods for identifying cis-ligands, including in situ labeling and proximity assays, and discusses how cis-ligand interactions influence Siglec clustering, phosphorylation, and downstream signaling, contributing to immune homeostasis and quality control of signaling-competent B cells. It also notes that while trans-ligand interactions of Siglecs are implicated in self-tolerance and pathogen evasion, the biological significance of cis-ligand interactions is less understood but appears crucial for maintaining immune cell function.